Imaging of the Acromioclavicular Joint: Anatomy, Function, Pathologic Features, and Treatment
Abstract
The acromioclavicular joint is an important component of the shoulder girdle; it links the axial skeleton with the upper limb. This joint, a planar diarthrodial articulation between the clavicle and the acromion, contains a meniscus-like fibrous disk that is prone to degeneration. The acromioclavicular capsule and ligaments stabilize the joint in the horizontal direction, while the coracoclavicular ligament complex provides vertical stability. Dynamic stability is afforded by the deltoid and trapezius muscles during clavicular and scapular motion. The acromioclavicular joint is susceptible to a broad spectrum of pathologic entities, traumatic and degenerative disorders being the most common. Acromioclavicular joint injury typically affects young adult males and can be categorized by using the Rockwood classification system as one of six types on the basis of the direction and degree of osseous displacement seen on conventional radiographs. MRI enables the radiologist to more accurately assess the regional soft-tissue structures in the setting of high-grade acromioclavicular separation, helping to guide the surgeon’s selection of the appropriate management. Involvement of the acromioclavicular joint and its stabilizing ligaments is also important for understanding and classifying distal clavicle fractures. Other pathologic processes encountered at this joint include degenerative disorders; overuse syndromes; and, less commonly, inflammatory arthritides, infection, metabolic disorders, and developmental malformations. Treatment options for acromioclavicular dysfunction include conservative measures, resection arthroplasty for recalcitrant symptoms, and surgical reconstruction techniques for stabilization after major trauma.
© RSNA, 2020
SA-CME LEARNING OBJECTIVES
After completing this journal-based SA-CME activity, participants will be able to:
■ Describe the anatomy and function of the static and dynamic stabilizers of the AC joint.
■ Recognize the imaging findings of various pathologic processes that affect the AC region, including acute injury of the joint and distal clavicle, distal clavicle osteolysis, and degenerative disorders.
■ Discuss the treatment options and surgical techniques used to manage AC disease and the most common associated complications.
Introduction
The acromioclavicular (AC) joint links the axial skeleton to the upper extremity, functioning in concert with the rest of the shoulder girdle to ensure fluid arm motion (1). Despite its small size and relatively limited range of motion, the AC joint is a common source of shoulder symptoms that are overlooked or attributed to the rotator cuff and glenohumeral joint, which receive greater attention from radiologists and have greater emphasis in the imaging literature. Although radiologists encounter degenerative and traumatic disorders of the AC joint regularly, they may be less familiar with the anatomy and range of pathologic processes that affect this “other joint” at the shoulder than they are with those involving the glenohumeral articulation (2). This review is intended to enhance the radiologist’s understanding of the structure and function of the AC joint and illustrate the broad range of pathologic conditions that can affect this articulation and its supporting structures.
Anatomy and Biomechanics
The AC joint is part of the shoulder girdle, a complex of structures that connect the upper limb to the axial skeleton and work in concert to coordinate movement of the upper extremity. The shoulder girdle includes the clavicle and scapula, plus five mobile regions where the movements of these bones take place during arm motion.
Figure 1. Coronal three-dimensional CT image highlights the five mobile regions of the shoulder girdle. The AC joint (A) connects the clavicle to the scapula and suspends the upper arm. Synchronous motion at the AC, glenohumeral (C), and sternoclavicular (E) joints occurs as the arm moves. The two nonarticular gliding zones of the shoulder girdle include the subacromial (B) and scapulothoracic (D) spaces.
Clavicle
The clavicle is an S-shaped bone that has a large anteriorly convex medial curve that protects the neurovascular structures and a smaller anteriorly concave lateral curve (4,5). The larger medial end of the clavicle articulates with the sternum and is rigidly bound to the ribs; the midportion is tubular; the lateral third flares gently to articulate with the acromion and serves as a rigid base of attachment for the AC joint stabilizers (5,6). The clavicle functions as a strut that prevents the shoulder from collapsing into the body, stabilizes the shoulder and thus prevents it from falling away from the body, and suspends the scapula and arm (5,7). The clavicle is the only long bone that lies horizontally and undergoes intramembranous ossification at its primary ossification center, which is the first region to ossify in the fetus (7–9). The peripheral epiphyses of the clavicle undergo endochondral ossification, with the medial physis being the final physis to complete closure, when a person is aged 20–25 years (4). The peculiar embryologic development of the clavicle is associated with well-recognized deformations of ossification, although the precise pathophysiologic features of these deformations are a subject of debate (7,8). Congenital pseudoarthrosis is more common in females and typically unilateral, on the side contralateral to the aortic arch (7,9). Cleidocranial dysostosis is a rare hereditary condition in which both clavicles are partially (90% of cases) or completely (10% of cases) absent (4,7).
Acromion
The acromion is an anterior continuation of the lateral scapular spine and serves as the attachment site for the coracoacromial ligament and midportion of the deltoid muscle. The superior surface of the acromion is subcutaneous and vulnerable to injury. The lateral surface, where the deltoid attaches, is thick and roughened, whereas the inferior surface, which contributes to the coracoacromial arch, is smooth (10). The medial border includes the small ovoid facet, where the acromion articulates with the distal clavicle (10,11) (Fig 2). Ossification of the scapular body and spine commences in utero, whereas the acromion ossifies during childhood and adolescence (12). Although the number and size of the ossification centers vary, these centers typically coalesce into four zones: the preacromion, meso-acromion, meta-acromion, and basiacromion (12,13). Fusion between these zones occurs when an individual is aged 15–25 years, usually from the lateral to medial aspect and the posterior to anterior aspect (13,14). Failed fusion leads to an os acromionale (discussed later).
Figure 2. Normal anatomy of clavicle, acromion, and AC joint. Three-dimensional CT image of the right shoulder girdle in the axial plane shows the osseous and articular anatomy of the clavicle and scapula. Note the gentle S-shaped curvature of the clavicle (arrowheads) when it is viewed from above, with a thinner midportion and more bulbous articular ends. The acromion (arrow) is the anterolateral extension of the scapular spine that articulates with the lateral clavicular end to form the AC joint. The coracoid (*) projects anteriorly from the scapular body and serves as the attachment site for the coracoclavicular (CC) ligaments.
AC Joint
The AC joint is a planar diarthrodial joint between the distal clavicle and anteromedial acromion (11,15). These bones articulate at cartilage-covered facets that are initially hyaline but convert to fibrocartilage with age (5). The shapes of the facets are variable, with a convex clavicular facet articulating with a flattened or concave acromial facet being the most common (15). The joint plane is typically slanted 20°–30° but varies from being nearly vertical to being nearly horizontal, with the clavicle overriding the acromion (5) (Fig 3). The AC joint is small, averaging 9 mm in height and 19 mm in length (16). The width of the joint space is 1–7 mm in males and 1–6 mm in females and declines with age (17).
Figure 3. Photograph of a coronal pathologic section through the right shoulder of an adult cadaver demonstrates the normal obliquity of the AC joint, which is typically slanted from the superolateral to anteromedial aspect. Note the relatively thicker capsular tissues of the superior capsule, where there is a small remnant of the articular disk (arrow). The thin superficial fasciae of the deltoid and trapezius muscles merge to form the deltotrapezial fascia, which blends imperceptibly with the superior articular capsule (arrowheads). There is mild AC osteoarthrosis, with narrowing and small osteophytes at the inferior margin of the joint. (Case courtesy of Donald Resnick, MD, University of California, San Diego, Calif.)
AC joint movements are commanded by the regional muscles. In the biomechanics literature, translational (straight line along an axis) and rotational (spinning around an axis) movements at a joint are described as taking place in the horizontal, vertical, or axial planes (18). In reality, these movements are complex, taking place simultaneously in multiple planes that may be oblique to the cardinal axes (1). The principal movements that take place at the shoulder girdle during arm abduction are elevation, posterior rotation, and retraction of the clavicle and upward rotation, internal rotation, and posterior tilting of the scapula (1). Articular movement is restrained by supporting static and dynamic stabilizers. Although the role of each stabilizer is to constrain movement principally in one direction, each stabilizer typically has more than one function. The contributions of the stabilizing structures vary with the direction and degree of displacement, with some stabilizers functioning maximally when displacement is small while others function when displacement is large (18).
Static Stabilizers
AC Joint Capsule and Ligaments.—The AC capsule and ligaments stabilize the AC joint, principally to restrain horizontal translation, affording three times more control in the anteroposterior (AP) direction than in the vertical direction (18,19). These structures also constrain posterior rotation of the clavicle and contribute to vertical stability at small degrees of displacement (18,19). The AC ligaments are capsular thickenings that are traditionally described as four structures: the superior, anterior, inferior, and posterior ligaments (20). These ligaments vary in thickness from 1 to 5 mm, with the inner margin of their osseous insertions located 3–4 mm from the joint line while the peripheral fibers insert farther distally (20). The superior ligament is the most robust and is the principal ligament maintaining horizontal stability. Nakazawa et al (21) recommend that the AC ligaments be considered as just two structures, a substantial superoposterior bundle responsible for resisting posterior clavicular translation and a thin anteroinferior bundle that assists in limiting anterior displacement (21,22).
The AC joint contains a flexible meniscus-like fibrocartilaginous disk that is continuous with the capsule to cushion the joint and correct for bone incongruence; however, this disk does not appear to be necessary (15). It degenerates as early as the 2nd decade of life and is typically nonfunctional in adults (23). This disk can appear complete, meniscoid, fragmented, or absent, depending on its state of degeneration (23). In young adults, the disk is typically meniscoid, continuous with the superior ligament and attached most firmly at the acromion (24). Disk resorption, tearing, superior extrusion, and mineralization, principally due to the deposition of calcium pyrophosphate dehydrate crystals, become apparent with aging (25).
Coracoid Ligaments.—The coracoid process (ie, coracoid), referred to as the “lighthouse of the shoulder,” serves as an anatomic anchor for multiple structures and an important surgical landmark, as the brachial plexus and axillary vessels are intimate with its medial border (26). The pectoralis minor, coracobrachialis, and biceps short-head tendons insert near the tip of the coracoid, while the coracoacromial and coracoclavicular (CC) ligaments originate along its hook and base.
The ligaments form an inverted triangle, with its apex at the coracoid and its base at the inferior surface of the distal third of the clavicle (Fig 4) (27). The coracoacromial ligament is a triangular fibrous structure extending from the lateral coracoid to insert broadly onto the acromion. Although some of its fibers imbricate with the inferior AC capsule, this ligament is not a significant articular stabilizer (3,28). The CC ligament, consisting of the trapezoid and conoid ligaments, is the principal vertical stabilizer of the AC joint and substitutes as its principal horizontal stabilizer when the capsule is disrupted (18,22). The conoid and trapezoid ligaments are anatomically and functionally distinct. The trapezoid ligament is larger and roughly quadrilateral in shape, and it inserts onto the distal clavicle along the trapezoidal line (trapezoidal ridge), spanning a region 1.5–3.0 cm from the joint line (29,30). The conoid ligament is conical in shape, nearly vertical, and located posteromedial to the trapezoid ligament. Its fibers twist as they extend superiorly to insert at the conoid tubercle and adjacent clavicle, spanning 3–5 cm medial to the joint line (29,30). The trapezoid ligament restrains posterior clavicular displacement and AC compression, while the conoid ligament is principally responsible for vertical stability, restraining superior migration of the clavicle (3,18).
Figure 4. Normal anatomy of the ligaments arising from the coracoid process in a 38-year-old man with a labral anomaly. Shallow oblique coronal T1-weighted MR arthrogram of the right shoulder shows normal ligaments arising from the coracoid. The coracoacromial ligament (curved arrow), located the farthest laterally, extends at a shallow angle toward the acromion. Medial to the coracoacromial ligament, the trapezoid portion of the CC ligament (arrowhead) inserts onto the inferior margin of the lateral clavicle. The conoid ligament (straight arrow) is posteromedial to the trapezoid and has vertically oriented fibers that attach to the posterior margin of the inferior clavicle at a distance from the joint.
The conoid process is a common variant whereby the tubercle is replaced by a large bony protuberance. It is most commonly found in populations originating from East Asia (31,32). It is typically asymptomatic, commonly bilateral, smoothly marginated, and not associated with malalignment (Fig 5) (31,32). An additional thin cordlike structure extending from the coracoid to the medial clavicle and subclavius sheath has been described recently. This structure, named the medial CC ligament, may have a role in supplementing stabilization (33). Others consider this structure to be a thickening of the CC fascia rather than a true ligament (34). We are not aware of reports describing traumatic injury to this structure and do not know whether such disruption affects the injury grade.
Figure 5. Bilateral conoid processes in a 65-year-old man of Asian descent. AP bilateral radiograph of the AC joints shows a large flat-top conoid process (arrow) arising from the inferior left clavicle and articulating with the corocoid. Such articulations can be fibrocartilaginous or form a true synovial joint. Note that the smooth margins of the protuberance show cortical and marrow continuity with the underlying clavicles such that the protuberance can be differentiated from posttraumatic ossification, which is more irregular and typically unilateral. A smaller conoid process is seen on the right side.
Dynamic Stabilizers
The deltoid and trapezius muscles are dynamic stabilizers of the AC joint, although their precise contributions are not fully understood, as muscle activity is challenging to recreate in vitro (5,35,36). The trapezius is a large flattened muscle originating from the occiput, nuchal ligament, and C7–T12 spinous processes. Its upper fibers insert onto the posterior edge of the distal clavicle, its middle fibers insert onto the acromion and lateral scapular spine, and its lower fibers insert at the medial scapular spine.
The deltoid has a broad C-shaped origin, arising from the anterior margin of the distal third of the clavicle, acromion, and lateral scapular spine, and tapering distally to insert onto the lateral humeral shaft (37). The deltoid and trapezius muscles insert onto bone by way of muscular and aponeurotic fascial fibers (35). The deltotrapezial fascia refers to the collagenous region derived from the superficial aponeuroses of these muscles that bridge the gap between their attachments (Fig 3) (5,37,38). The deltotrapezial fascia is adherent to the periosteum of the clavicle and acromion and imbricates with the superior AC ligament, reinforcing the articulation (5,38).
Imaging Techniques
Radiographs are the initial and typically the only imaging modality used for evaluation of suspected AC pathologic entities (39,40). The AC joint is seen on standard AP shoulder radiographs, but it is variably angulated and typically overpenetrated (41). The AP angulated Zanca view is preferred for assessment of the AC joint and distal clavicle (42).
Figure 6. Normal Zanca radiographic view of the left clavicle and AC joint in a 34-year-old woman with a history of psoriasis and sternoclavicular pain. Obtaining a Zanca view with 10°–15° cephalad beam angulation eliminates overlap between the clavicle and scapula and improves visualization of the AC joint.
In acute trauma, upright panoramic Zanca radiographic views that include the contralateral side are widely used. Bilateral views provide a reference for the normal articular configuration and the AC and CC distances, improving diagnostic accuracy (39,44). Accuracy is further enhanced by measuring and comparing the two sides rather than relying on visual inspection, with the CC distance being the most reliable (41,44). Stress views are obtained by either suspending or having the patient hold 10–15-lb weights; the two methods yield similar results (45).
While the stated rationale for stress views is that they increase sensitivity and unmask higher grade injury, typically by upgrading a type I injury to a type II injury, there is controversy regarding whether this upgrading has any utility given the current trend to manage the majority of AC injuries conservatively (39,46–48). In one large study (47), the use of weights unmasked CC widening and led to a higher injury grade in 4% of cases. In addition, paradoxical CC narrowing with weights is reported in one-third of healthy volunteers and in 10% of AC injuries (41,47). The majority of experienced shoulder surgeons report that they do not require weighted views (49,50).
The role of advanced imaging in AC injury is limited to select cases. US is used to assess alignment and ligamentous integrity at rest and during dynamic stress, but it has low accuracy in depicting concomitant osteochondral and labral injuries (Fig 7) (6,39,51). CT is useful for assessment of fractures; however, it has accuracy similar to that of radiography in grading traumatic AC malalignment (6,52,53). However, the inability to accurately assess concomitant soft-tissue injury and the use of ionizing radiation with CT have led to a preference for use of MRI for the 5%–10% of patients with high-grade AC injury requiring advanced imaging for surgical planning. Assessment of high-grade injuries is performed most accurately with MRI, which is the reference standard for delineation of the regional stabilizers and enables assessment of the bones, rotator cuff, glenohumeral articulation, and neurovascular structures (38,39,48,54).
Figure 7a. Grade III AC joint injury in a 23-year-old woman after a car accident. (a) AP bilateral Zanca-view radiograph obtained with weights shows vertical diastasis of the left AC joint, with clavicular elevation relative to the acromion, and asymmetric widening of the left CC distance. (b) Long-axis US image of the left AC joint shows superior displacement of the clavicle (*) relative to the acromion (arrowhead), with effusion distending the joint and elevating the superior capsule (arrow). The inferior capsular region normally is not visible with US, and thus assessment of inferior AC disease is limited.
Figure 7b. Grade III AC joint injury in a 23-year-old woman after a car accident. (a) AP bilateral Zanca-view radiograph obtained with weights shows vertical diastasis of the left AC joint, with clavicular elevation relative to the acromion, and asymmetric widening of the left CC distance. (b) Long-axis US image of the left AC joint shows superior displacement of the clavicle (*) relative to the acromion (arrowhead), with effusion distending the joint and elevating the superior capsule (arrow). The inferior capsular region normally is not visible with US, and thus assessment of inferior AC disease is limited.
Acute AC Joint Injury
There is a paucity of reliable information on the incidence of AC joint injury (55). It is widely quoted that 9%–12% of shoulder injuries involve the AC joint; however, the origin of this statistic appears to be a book chapter from 1958 that actually describes a lower percentage (56). A review of all shoulder injuries seen at one urban hospital for more than 1 year revealed that 4% of shoulder injuries occurred at the AC joint, with humeral fracture, clavicle fracture, and glenohumeral dislocation occurring far more frequently (57). It is likely that hospital data are underestimations of the incidence of low-grade AC injury, as such patients are typically assessed in a primary care setting or do not seek medical treatment (58).
The majority of AC injuries occur in young adults between the ages of 20 and 40 years. Males are affected at least five times more frequently than females (55,58). In young male athletes involved in contact sports who are evaluated in a primary care setting, up to 40%–50% of shoulder injuries involve the AC joint (59).Injury Mechanisms
The most common mechanism of AC injury is a direct blow to the superior acromion, with the arm adducted at the side. This type of injury is typically due to a fall onto the shoulder during contact sports, skiing, or cycling (60,61). The force drives the acromion and clavicle inferiorly, but the robust ligamentous constraints of the sternoclavicular joint restrict clavicular movement, resulting in dissociation between the acromion and clavicle and soft-tissue injury at the AC joint (60). Less commonly, AC injuries are due to indirect trauma from a fall onto the outstretched hand or elbow, driving the humerus against the acromion (61,62). Indirect trauma disrupts the AC ligaments but typically spares the CC ligaments, resulting in a lower-grade injury (38). Associated injuries to the clavicle, glenohumeral joint, rotator cuff, and superior labrum occur in 20% of AC separations (61).
Clinical Presentation
Patients with AC joint injury present with superior shoulder pain and swelling, holding the arm in an adducted supported position to alleviate symptoms (59,61). The clinical diagnosis is made by palpating the region for joint tenderness, applying manual stress to identify instability, and eliciting pain by performing dynamic maneuvers such as the cross-body test, whereby the affected arm is elevated 90° and then adducted toward the contralateral side (63). The O’Brien active compression test, with which the examiner forces the 90° flexed and 10° adducted arm downward in internal and external rotation, is another provocative maneuver that aids in differentiating AC from labral disease (64).
Seventy percent to 90% of injuries are low grade and not associated with a visible deformity (55,58). In higher-grade injuries, deformity becomes apparent, with tenting of the skin by the distal clavicle, which is accentuated when the patient is sitting or standing (61). Rockwood (60) considered clavicular tenting to be the result of inferior sagging of the scapula, whereas radiologists describe the deformity in terms of clavicular elevation (58,60). Azar et al (65) compared the positions of the clavicle and scapula relative to spinal landmarks and noted that both displacements take place, with clavicle elevation being more common and scapular depression limited to severe injuries.
Systems for Classifying AC Injury
Cadenat divided AC injuries into incomplete or complete types and is credited with deducing that such injuries progress in a predictable sequence with increasing force, with the AC ligaments tearing initially and tearing of the CC ligaments and then deltoid and trapezius attachments following (59,66). Subsequently, Tossy and Allman described similar three-part grading systems based on the extent of damage to the AC and CC ligaments (40). Rockwood (60) appreciated that these systems were focused on vertical instability only and in 1984 proposed a comprehensive system in which the direction and degree of clavicular displacement relative to the scapula on AP and axillary radiographs were used to deduce the extent of ligamentous and myofascial injury (Fig 8) (58,60,66). One limitation of the Rockwood classification is that only soft-tissue ligamentous injury is considered when classifying AC joint injury. Goss (67) and others (58) described the superior shoulder suspensory complex, considering the bone and soft structures of the shoulder a ring akin to the pelvis, with double breaks in the ring producing instability. This classification is useful when there are combined osseous and ligamentous injuries.
Figure 8. Drawings illustrate the six types of AC joint injury recognized in the Rockwood classification.
Rockwood Classification
The Rockwood classification (60) remains the most widely used system for classifying AC injuries (Table). This classification forms the basis for selecting management.
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Type I.—A Rockwood type I injury is a low-grade sprain of the AC capsule. There is local tenderness and swelling without instability or deformity, and alignment remains normal on radiographs (16). Capsular thickening and joint fluid may be recognized at US and MRI, but these findings are nonspecific and require correlation with patient age and history (54).
Type II.—With a Rockwood type II injury, the AC capsule is completely torn, resulting in dynamic horizontal instability that can be palpated by moving the clavicle anteriorly and posteriorly (62). Radiographs typically show AC joint widening greater than 7 mm with overlying swelling (41) (Fig 9). A few millimeters of vertical offset (<25% acromial height) between the inferior margins of the clavicle and acromion may be present, but this finding is nonspecific unless it is asymmetric, as there can be similar mild offset in 20% of skeletally healthy individuals (41). The CC ligaments are sprained but functional, maintaining the normal CC distance of 10–13 mm (60). MRI depicts tearing of the AC capsule and variable edema within the CC ligaments, although the CC fibers remain largely intact (Fig 10).
Figure 9. Type II AC injury in a 31-year-old male pedestrian who was struck by a car. AP radiograph of the right shoulder shows a widened AC joint measuring 9 mm. There is no elevation of the clavicle, and the CC distance is normal. There is mild overlying soft-tissue edema.
Figure 10a. Type II injury of the AC joint in a 24-year-old man who was referred for MR arthrography of the left shoulder owing to a suspected labral tear 1 month after an injury. (a) Sagittal T1-weighted MR image shows thickening and irregularity of the CC ligament fibers (arrow). (b) Coronal proton-density–weighted fat-suppressed MR image shows edema of the trapezoid component of the CC ligament (arrows), with intact fibers more medially at the conoid (arrowhead). The joint capsule was injured, but alignment was normal. This case illustrates the difficulty in recognizing AC joint injury at MR arthrographic studies, which typically include a limited range of sequences.
Figure 10b. Type II injury of the AC joint in a 24-year-old man who was referred for MR arthrography of the left shoulder owing to a suspected labral tear 1 month after an injury. (a) Sagittal T1-weighted MR image shows thickening and irregularity of the CC ligament fibers (arrow). (b) Coronal proton-density–weighted fat-suppressed MR image shows edema of the trapezoid component of the CC ligament (arrows), with intact fibers more medially at the conoid (arrowhead). The joint capsule was injured, but alignment was normal. This case illustrates the difficulty in recognizing AC joint injury at MR arthrographic studies, which typically include a limited range of sequences.
Type III.—Tearing of the CC ligaments, the hallmark characteristic of Rockwood type III injuries, allows the clavicle and scapula to separate in the vertical axis, widening the CC distance to more than 13 mm (41,60). On bilateral upright radiographs, the CC distance is typically 25%–100% or 5 mm greater than this distance on the contralateral side. The diagnosis should not be based on AC offset in the absence of CC widening, particularly on weighted views, as the clavicle normally can sublux up to 50% of acromial height with stress at the joint line (43). The identification of CC ligament injury is important for management, as type III separation typically takes considerably longer to heal than do lower-grade injuries.
Although radiography is specific for diagnosing type III injury, the integrity of the CC ligaments can be assessed more sensitively with MRI. A potential limitation is failure to include the medial insertion of the conoid ligament on the sagittal images due to its far medial position (38,54) (Fig 11). The ligaments are easiest to separate on coronal oblique MR images obtained at a shallower angle than is typical, orienting the sections parallel to the distal clavicle (38,68). They normally appear as smooth, striated, low-signal-intensity bands on T1-weighted or proton-density–weighted MR images without fat saturation. Fluid-sensitive MRI sequences enable detection of edema and ligament disruption after injury (68).
Figure 11. CC ligament tearing in type III AC injury in a 31-year-old man who was injured after a fall onto his shoulder while playing basketball. Sagittal proton-density–weighted fat-suppressed MR image shows tearing of the CC ligaments, with fiber disruption and ligament edema (arrowheads). There is also injury to the trapezius muscle (arrow) near its posterior clavicular attachment and overlying soft- tissue swelling. In our experience, soft-tissue injury extending from the posterior AC capsule into the distal trapezial insertion is a common finding of type III injury. The clavicular and acromial attachments of the deltoid muscle remain intact.
Type IV.—The rare Rockwood type IV separation is a horizontal-plane injury with posterior displacement of the clavicle and palpable AP instability, although examination is challenging in acute injury (64). The clavicle may become incarcerated in the trapezius muscle or button-holed through the fascia of this muscle, rendering the dislocation irreducible (Fig 12) (60). The AC ligaments are torn, and the CC ligaments are horizontally oriented and either partially or completely torn (66). The findings on AP radiographs appear deceptively similar to type II injury, without CC diastasis. Posterior displacement is assessed on the axillary view, although the criteria for malalignment remain poorly defined (39,64,69) (Fig 13). A mild step-off between the anterior margins of the clavicle and acromion is not pathologic. An offset of greater than 2 mm between the anterior clavicle and the acromion is present in 40% of specimens, typically with anterior acromial overhanging (70). In addition, physiologic posterior clavicular displacement peaks at 90° abduction, which coincides with the arm position on the axillary view (36). Alternative projections and indexes proposed for the diagnosis of type IV injury include the Alexander view (y-axis view with patient shrugging the shoulder to stress the joint), pseudodynamic axillary views (arm in 60° flexed, neutral, and 60° extended positions), Vaisman index (quantitative ratio of AC and CC distances), glenoacromioclavicular angle (calculated on axillary view), and cross-sectional CT and MRI (39,43,71,72). There are limited data on the reliability and accuracy of these methods for evaluation and diagnosis of type IV injury, given its rarity (39,52,64).
Figure 12a. Type IV AC joint injury with button-holing of the clavicle through the trapezius muscle. Coronal (a) and sagittal (b) T1-weighted MR images of the right shoulder in a 50-year-old man after a vehicular accident show that the bulbous distal end of the clavicle (*) has perforated through the posterosuperior surface fascia of the trapezius muscle (T in a).
Figure 12b. Type IV AC joint injury with button-holing of the clavicle through the trapezius muscle. Coronal (a) and sagittal (b) T1-weighted MR images of the right shoulder in a 50-year-old man after a vehicular accident show that the bulbous distal end of the clavicle (*) has perforated through the posterosuperior surface fascia of the trapezius muscle (T in a).
Figure 13a. Type IV injury in a 33-year-old man after an assault. (a) AP radiograph of both clavicles shows swelling over the right AC joint (arrow), which is minimally wider than the left but still within the normal range, with a normal CC distance. (b) Axillary view shows greater than 1-cm posterior displacement of the anterior clavicular margin (arrowhead) relative to the anterior edge of the acromion (arrow). (c) The horizontal misalignment at the AC joint is more clearly evident on the axial three-dimensional CT reconstruction. The patient underwent surgical reconstruction.
Figure 13b. Type IV injury in a 33-year-old man after an assault. (a) AP radiograph of both clavicles shows swelling over the right AC joint (arrow), which is minimally wider than the left but still within the normal range, with a normal CC distance. (b) Axillary view shows greater than 1-cm posterior displacement of the anterior clavicular margin (arrowhead) relative to the anterior edge of the acromion (arrow). (c) The horizontal misalignment at the AC joint is more clearly evident on the axial three-dimensional CT reconstruction. The patient underwent surgical reconstruction.
Figure 13c. Type IV injury in a 33-year-old man after an assault. (a) AP radiograph of both clavicles shows swelling over the right AC joint (arrow), which is minimally wider than the left but still within the normal range, with a normal CC distance. (b) Axillary view shows greater than 1-cm posterior displacement of the anterior clavicular margin (arrowhead) relative to the anterior edge of the acromion (arrow). (c) The horizontal misalignment at the AC joint is more clearly evident on the axial three-dimensional CT reconstruction. The patient underwent surgical reconstruction.
Type V.—Rockwood type V injury is a more severe version of type III, with greater disruption of the deltotrapezial fascia attached to the lateral end of the clavicle, enabling the clavicle to assume a subcutaneous position (58,60). Some resources suggest that the presence of deltotrapezial fascia tearing differentiates type V from type III injury. However, Rockwood (60) described deltotrapezial injury as a feature of both injuries, stating that with type III injuries, the periosteal sleeve with muscle attachments can be separated from the outer clavicle (2,60). He described the tearing in type V injury as more extensive, with the deltoid and trapezius muscles detached from the distal half of the clavicle (60).
The radiographic hallmark of a type V injury is wider separation of the CC distance, typically 100%–300% greater than the normal width of the separation (60) (Fig 14). MR images of such injuries are increasingly being obtained for surgical planning (39). Osseous separation may not be as apparent because the patient is supine, but the ligamentous and muscular stabilizers are well seen. The main finding that indicates a type V injury is extensive disruption affecting the deltoid and trapezius muscles. Tearing of the muscles and fascia from their clavicular and acromial attachments is best appreciated in the sagittal plane. However, understanding the full extent of this injury requires assessment in all imaging planes (Fig 15). We are not aware of MRI criteria that define the extent of muscle tearing required to distinguish type V from type III injury, and we find the extent of tearing of the anterior deltoid muscle from the clavicle to be the most predictive of high-grade injury. The deltoid muscle normally acts as a clavicular depressor, counteracting the elevation produced by the sternocleidomastoid and trapezius muscles (5,35). When the clavicular attachments of this muscle are disrupted, the elevators are unopposed, enabling superior clavicular migration while the remaining shoulder girdle sags inferiorly, resulting in wide diastasis of the CC space (58,65).
Figure 14. Type V AC injury in a 41-year-old male security guard with pain and deformity after rolling down a 50-ft ravine during an assault. AP bilateral upright radiograph obtained with weighting shows marked widening of the left CC interval (black bracket) and vertical offset at the AC joint (white bracket). Grade V injury results in greater separation of the CC distance than does grade III injury, with the separation typically being greater than double the width of the normal size owing to unopposed action of the sternocleidomastoid following deltotrapezial tearing. This high-grade injury involves superior displacement of the clavicle and inferior depression of the scapula.
Figure 15a. Deltotrapezial muscle complex tearing in the setting of type V injury in a 29-year-old man with high-grade AC separation on radiographs, with concern for labral disease. (a) Sagittal proton-density–weighted fat-suppressed MR image shows complete tearing of the CC ligaments (*) and injury of the trapezius muscle (arrowheads), with intramuscular edema and architectural distortion indicating muscle tearing. (b) More lateral sagittal MR image shows avulsion of the anterior deltoid muscle from the clavicle (arrow), superior capsular and deltotrapezial fascia injury (arrowhead), and stripping of the deltotrapezial fascia at the clavicle with overlying edema at the clavicular surface. There is marked elevation of the clavicle relative to the acromion.
Figure 15b. Deltotrapezial muscle complex tearing in the setting of type V injury in a 29-year-old man with high-grade AC separation on radiographs, with concern for labral disease. (a) Sagittal proton-density–weighted fat-suppressed MR image shows complete tearing of the CC ligaments (*) and injury of the trapezius muscle (arrowheads), with intramuscular edema and architectural distortion indicating muscle tearing. (b) More lateral sagittal MR image shows avulsion of the anterior deltoid muscle from the clavicle (arrow), superior capsular and deltotrapezial fascia injury (arrowhead), and stripping of the deltotrapezial fascia at the clavicle with overlying edema at the clavicular surface. There is marked elevation of the clavicle relative to the acromion.
Type VI.—With a Rockwood type VI injury, the clavicle is displaced inferior to the acromion. This injury type is exceedingly rare and typically associated with other injuries, and it requires surgical reduction (61,73). The suggested mechanism of injury is a severe blow to the superior clavicle, with the arm abducted and the scapula retracted (60). With type VIa, the clavicle is displaced below the acromion (Fig 16). With type VIb, the clavicle lies below the coracoid behind the conjoint tendon (60,73).
Figure 16a. Grade VI AC injury associated with fracture-dislocation of the sternoclavicular joint in a 17-year-old adolescent boy injured during a high-impact dirt bike accident. (a) AP Zanca-view radiograph shows inferior displacement of the distal end of the clavicle, which rests below the acromial articular facet. Inferior clavicular subluxation at the AC joint is difficult to reduce without surgery because torn capsular tissue in the interval hinders reduction. (b) Coronal reconstruction from CT of the thorax shows a fracture of the inferior clavicle, with superior displacement of the medial clavicle relative to the sternum at the sternoclavicular joint (arrows).
Figure 16b. Grade VI AC injury associated with fracture-dislocation of the sternoclavicular joint in a 17-year-old adolescent boy injured during a high-impact dirt bike accident. (a) AP Zanca-view radiograph shows inferior displacement of the distal end of the clavicle, which rests below the acromial articular facet. Inferior clavicular subluxation at the AC joint is difficult to reduce without surgery because torn capsular tissue in the interval hinders reduction. (b) Coronal reconstruction from CT of the thorax shows a fracture of the inferior clavicle, with superior displacement of the medial clavicle relative to the sternum at the sternoclavicular joint (arrows).
Limitations of the Rockwood Classification.—Although the Rockwood classification has been the dogma for AC injury classification and management for decades, it has recognized limitations. Clinically, injury type correlates poorly with pain, range of motion, and functional deficit, and the patient’s functional demands when selecting treatment are not considered in the classification (74). Diagnostically, the Rockwood classification relies on radiographic alignment, which can vary considerably depending on the image projection and patient position. Even with optimal radiographs, there are concerns regarding the reliability and reproducibility of injury categorization (22,74). When Ng et al (40) asked 15 shoulder surgeons to classify 24 Zanca views of AC injury, mean interobserver agreement was 64.6% and there were no cases in which all surgeons agreed on the injury type. At subsequent reassessment, the mean intraobserver agreement was 59.4% (40).
Cho et al (52) reported low interobserver agreement (κ =. 214) and only moderate intraob-server agreement (κ =. 474) with use of AP and axial radiographs. Agreement between readers regarding horizontal displacement on the axillary view is particularly poor (69).
The Rockwood system is used to predict which ligaments are injured on the basis of osseous malalignment. With MRI, the ligaments can now be directly evaluated, allowing validation of the classification. High concordance between the Rockwood type and MRI findings has been reported in a few studies (22,68). In contrast, Nemec et al (75) evaluated a large group of patients with type I–IV injuries and noted discordant findings in 52% of cases. In their study, with MRI, the injury was reclassified as a higher type in 11% of cases and as a lower type in 36% of cases (75). On the basis of their findings, they proposed an expanded classification to better account for partial tears that are not recognized in the Rockwood classification (75). Partial tearing of the AC capsule, representing injury that is intermediate between types I and II, typically shows peeling off of the superior AC ligament at the clavicle (24) (Fig 17). Partial tearing of the CC complex, constituting injury that is intermediate between types II and III, typically involves the trapezoid portion of this complex and may account for the poor outcome in some patients who are managed as though they have a type II injury (4,76). Injuries intermediate between types III and IV, with superior and posterior clavicular displacement and invagination of the elevated clavicle into the trapezius, also are common (Fig 18).
Figure 17. Rockwood type I–II injury in a 26-year-old man. Sagittal proton-density–weighted fat-suppressed MR image shows soft-tissue swelling overlying the AC joint, with frank tearing of the superior AC ligament, which is avulsed from its clavicular attachment (arrowhead). The inferior AC ligament (arrow) remains intact, and joint alignment is normal.
Figure 18a. Superior and posterior clavicular displacement with intramuscular invagination in a 48-year-old man who fell off a bicycle. (a) AP radiograph of the right shoulder shows elevation of the clavicle relative to the acromion (arrow) and widening of the CC distance, compatible with a Rockwood type III injury. (b) Sagittal proton-density–weighted fat-suppressed MR image shows displacement of the distal clavicle into the trapezius muscle, with linear edema at the site of invagination (arrow). The CC ligaments (arrowheads) are edematous and torn, with loss of normal fiber architecture. This unstable injury required surgical reduction, and the AC joint was fixated with a hook plate.
Figure 18b. Superior and posterior clavicular displacement with intramuscular invagination in a 48-year-old man who fell off a bicycle. (a) AP radiograph of the right shoulder shows elevation of the clavicle relative to the acromion (arrow) and widening of the CC distance, compatible with a Rockwood type III injury. (b) Sagittal proton-density–weighted fat-suppressed MR image shows displacement of the distal clavicle into the trapezius muscle, with linear edema at the site of invagination (arrow). The CC ligaments (arrowheads) are edematous and torn, with loss of normal fiber architecture. This unstable injury required surgical reduction, and the AC joint was fixated with a hook plate.
Fractures Related to the AC Joint
Coracoid Fracture
Certain fractures of the coracoid and clavicle have clinical, anatomic, and imaging features that overlap with AC joint injury. The characteristic coracoid fracture associated with AC separation occurs when the coracoid is avulsed by intact CC ligaments during a high-grade injury due to bone rather than ligamentous failure (Fig 19). This fracture is most common in adolescents before physeal closure, typically occurs near the coracoid base, and is challenging to see on radiographs unless it is displaced (77). In this injury, which is considered equivalent to a Rockwood type III AC separation, the CC distance remains normal as the coracoid migrates superiorly in conjunction with the clavicle (Fig 20) (38).
Figure 19a. Coracoid fracture in a 79-year-old woman with an AC injury from a fall. (a) Sagittal T1-weighted MR image of the right shoulder shows a nondisplaced fracture near the coracoid base (arrowhead), as well as thickening and irregularity of the AC capsule (arrows). (b) Corresponding proton-density–weighted fat-suppressed MR image shows linear fluid (arrowhead) within the fracture. The AC junction is edematous, with disrupted ligaments and surrounding edema (straight arrows). There is minor thickening and edema of the CC ligaments, but they remain intact and attached to the avulsed coracoid fragment (curved arrow).
Figure 19b. Coracoid fracture in a 79-year-old woman with an AC injury from a fall. (a) Sagittal T1-weighted MR image of the right shoulder shows a nondisplaced fracture near the coracoid base (arrowhead), as well as thickening and irregularity of the AC capsule (arrows). (b) Corresponding proton-density–weighted fat-suppressed MR image shows linear fluid (arrowhead) within the fracture. The AC junction is edematous, with disrupted ligaments and surrounding edema (straight arrows). There is minor thickening and edema of the CC ligaments, but they remain intact and attached to the avulsed coracoid fragment (curved arrow).
Figure 20. Coracoid fracture in grade III–equivalent AC separation in a 29-year-old man injured in a motorcycle accident. AP upright bilateral radiograph shows widening and vertical malalignment of the right AC joint, with elevation of the clavicle (arrow) relative to the acromion. The fractured coracoid process (*) is elevated along with the clavicle, so the CC distance remains normal.
Distal Clavicle Fractures
Fractures of the clavicle are classified into distal (15%–20% of cases), midshaft (80%–85% of cases), and medial (<5% of cases) types (78). These fractures have a bimodal age distribution, occurring in young adults or, more commonly, in elderly women who have osteoporosis, typically after a fall (79). The widely used Neer classification of distal clavicle fractures is based principally on the location of the fracture relative to the AC joint and CC ligaments (Fig 21) (79,80). A Neer type I fracture is a stable extra-articular injury lateral to the CC ligaments. Type II fractures are located more medially and prone to superior clavicular displacement. In type IIA, the fracture is medial to both CC ligaments, whereas in type IIB, the fracture is between the trapezoid and conoid ligaments, with tearing of the latter (79,80). Neer type III fracture involves the clavicular end and disrupts the articular surface of this end (Fig 22). Type IV is an uncommon pediatric injury consisting of a Salter I fracture of the distal clavicle accompanied by avulsion of the clavicular periosteum (Fig 23). The periosteal sleeve remains attached to the coracoid while the shaft displaces superiorly, simulating an AC separation (81). Type V is comminuted and unstable, with only a horizontal cortical fragment from the inferior clavicle attached to the CC ligaments, compromising the stability of both clavicular ends (79,80) (Fig 24).
Figure 21. Drawing illustrates the Neer classification of distal clavicle fractures. Type I and type III fractures are stabilized by the AC capsule and CC ligaments and typically undisplaced. Type II fractures are associated with some CC ligament disruption and are unstable, with high rates of displacement and nonunion. Distinguishing type IIA from type IIB can be challenging on radiographs and is based on the location and obliquity of the fracture. Type V fractures also are unstable, with only a free-floating cortical fragment attached to the CC ligaments. Type IV injuries are limited to children.
Figure 22. Neer type III fracture of the distal clavicle in a 13-year-old boy who was injured during skateboarding. Axial CT image shows an acute, minimally displaced intra-articular fracture (arrowhead) of the distal left clavicle involving the articular facet. These fractures heal but may result in secondary osteoarthrosis. The acromion has not ossified at this stage of development.
Figure 23a. Coracoid fracture and periosteal sleeve injury of the clavicle in a 14-year-old boy. (a) Sagittal proton-density–weighted fat-suppressed MR image at the level of the coracoid process shows an undisplaced fracture of the coracoid process with a large amount of surrounding edema. The CC ligaments (arrow) are intact. (b) Sagittal proton-density–weighted fat-suppressed MR image from the same series but at the level of the joint shows AC joint effusion and elevation of the clavicle relative to the acromion, with stripping of the periosteum from the clavicular undersurface (arrowhead). (c) Corresponding sagittal T1-weighted MR image shows that the distal clavicle and anterior acromion have not yet ossified. Note the multiple marrow-containing ossification centers in the immature acromion.
Figure 23b. Coracoid fracture and periosteal sleeve injury of the clavicle in a 14-year-old boy. (a) Sagittal proton-density–weighted fat-suppressed MR image at the level of the coracoid process shows an undisplaced fracture of the coracoid process with a large amount of surrounding edema. The CC ligaments (arrow) are intact. (b) Sagittal proton-density–weighted fat-suppressed MR image from the same series but at the level of the joint shows AC joint effusion and elevation of the clavicle relative to the acromion, with stripping of the periosteum from the clavicular undersurface (arrowhead). (c) Corresponding sagittal T1-weighted MR image shows that the distal clavicle and anterior acromion have not yet ossified. Note the multiple marrow-containing ossification centers in the immature acromion.
Figure 23c. Coracoid fracture and periosteal sleeve injury of the clavicle in a 14-year-old boy. (a) Sagittal proton-density–weighted fat-suppressed MR image at the level of the coracoid process shows an undisplaced fracture of the coracoid process with a large amount of surrounding edema. The CC ligaments (arrow) are intact. (b) Sagittal proton-density–weighted fat-suppressed MR image from the same series but at the level of the joint shows AC joint effusion and elevation of the clavicle relative to the acromion, with stripping of the periosteum from the clavicular undersurface (arrowhead). (c) Corresponding sagittal T1-weighted MR image shows that the distal clavicle and anterior acromion have not yet ossified. Note the multiple marrow-containing ossification centers in the immature acromion.
Figure 24. Neer type V distal clavicle fracture in a 32-year-old man injured in a motorcycle accident. AP radiograph of the right shoulder shows a comminuted fracture of the inferior clavicle. Note the free-floating horizontal bone fragment (*) arising from the inferior clavicle, which includes the insertion sites of the CC ligaments.
Whereas CT is used for diagnosis of medial clavicle fractures and assessment of suspected nonunion, conventional radiography is typically sufficient for assessment of midshaft and distal clavicle fractures. Neer type I and type III fractures are managed conservatively, but type III may be complicated by premature osteoarthrosis (80). Type IV fractures are also managed conservatively, as bone readily fills in the fracture gap in children. Fractures proximal to, between, or disrupting the CC ligaments (types IIA, IIB, and V) are unstable and complicated by nonunion or delayed union in 10%–44% of cases, particularly when the fractures are widely displaced (80). These fractures are managed surgically, typically by means of clavicular plating with optional CC stabilization (80).
Bipolar Injury
Rarely, simultaneous injuries occur at the lateral and medial clavicles, producing a bipolar injury with an unstable floating clavicle (82). The lateral injury can be a distal clavicle fracture or an AC dislocation. The medial injury can be a medial clavicle fracture or a sternoclavicular dislocation (see Fig 16). The most common bipolar injury pattern is anterior dislocation of the sternoclavicular joint combined with a distal clavicle fracture (82). This injury is often missed because of unfamiliarity with this pattern and the presence of concomitant rib fractures, hemothorax, and pneuomothorax, which distracts attention from the articular injuries (Fig 25).
Figure 25. Bipolar injury of the clavicle in a 66-year-old man with persistent pain and shoulder girdle instability 5 months after a fall from a horse. Three-dimensional volume-rendered CT image of the right upper thoracic cage shows type IV dislocation at the right AC joint with posterior migration of the clavicle (arrow). There is also an incompletely united fracture of the medial clavicle involving the sternoclavicular joint (arrowhead). The patient also had multiple rib fractures, and the AC and medial clavicular injuries were overlooked at the time of the initial injury.
Distal Clavicle Osteolysis and Acromial Overuse
Distal Clavicle Osteolysis
Distal clavicle osteolysis is characterized by painful bone resorption that manifests in two distinct forms (83,84). The posttraumatic form typically appears several months after an acute injury, although it may develop within weeks or several years after an acute injury (85). It is estimated that fewer than 10% of patients with an AC injury subsequently develop distal clavicle osteolysis. The overuse form of distal clavicle osteolysis is a stress-induced injury without instability that is seen in laborers and in athletes involved in weight-lifting or throwing sports (84). Despite their clinical differences, the histopathologic and imaging findings of these two forms are virtually identical (83,85). Histopathologic analysis reveals synovial inflammation and fibrosis, and articular cartilage degeneration, with subchondral microfractures in 50% of resected specimens (86).
Distal clavicle osteolysis is most common in young adults, with a strong male predominance (87). Symptoms, which are bilateral in 20% of cases, start with joint aching exacerbated by activity, pain radiating to the arm and neck, and difficulty sleeping on the affected side (84,86). Radiographs initially show normal findings but subsequently show cortical irregularity, “flame-shaped” bony resorption, subchondral cysts, and ultimately erosions involving the distal 0.5–3.0 cm of the clavicle (Fig 26) (85). MRI is more sensitive, depicting intense clavicular edema early in the course of the disease (85,87). Effusion, capsular edema, subchondral cysts, and acromial edema also may be present (87). Kassarjian et al (87) found a subchondral low-signal-intensity vertical line in 86% of cases and suggested that it represented a fatigue fracture (Fig 27). In our experience, fracture lines are less common and are difficult to distinguish from the edges of confluent subchondral cysts.
Figure 26. Posttraumatic osteolysis of the distal clavicle in a 47-year-old man with intense pain following an injury 5 weeks previously. AP radiograph of the right shoulder shows cystic changes and erosions at the distal clavicle (arrowheads). Alignment is normal, but there is overlying soft-tissue swelling (arrows). These imaging findings suggest that the patient sustained a low-grade AC capsular injury leading to osteolysis.
Figure 27. Osteolysis of the distal clavicle in a 41-year-old man. Coronal short tau inversion-recovery MR image shows intense marrow edema at the distal clavicle. There is a vague low-signal-intensity subchondral line (arrow) within the edema, compatible with an early fatigue fracture. While overuse injury can affect the clavicle or acromion, clavicular involvement is more common owing to preferential force concentration at the clavicular end, especially in more vertically oriented joints.
Differential Diagnosis of Clavicular Osteolysis
Septic arthritis of the AC joint is rare, occurring principally in individuals who are immunocompromised, have diabetes, or abuse intravenous drugs (88,89). The diagnosis is frequently delayed because the infection is incorrectly ascribed to the glenohumeral joint (89). Radiographs show swelling, osteopenia, and joint widening; bone erosion is typically delayed and better seen with CT (89). MRI depicts effusion and capsular swelling, with soft-tissue, muscle, and marrow edema (Fig 28). Differentiating septic arthritis from osteoarthrosis is challenging in the absence of significant erosion, adjacent pyomyositis, or soft-tissue abscess (88). US shows effusion and capsular distention and is useful for guiding joint aspiration. Early imaging-guided diagnostic aspiration and surgical débridement are recommended because the small size of the joint predisposes it to extravasation, resulting in adjacent osteomyelitis and/or pyomyositis.
Figure 28a. Subacute septic arthritis of the AC joint in a 25-year-old man, an intravenous drug abuser, who presented after 6–8 weeks of atraumatic right shoulder pain. (a) AP radiograph shows AC joint widening, fragments within the joint, and irregular erosion of the distal clavicle, with periosteal new bone formation (arrows). (b) Corresponding coronal T2-weighted fat-suppressed MR image shows effusion, capsular thickening, and irregularity of the bone margin (arrowhead). Note the edema (arrow) within the overlying soft tissues and trapezius muscle (*). Cultures from surgical débridement were positive for Staphylococcus saccharolyticus, an uncommon slowly growing anaerobe. Organisms more commonly associated with AC septic arthritis include Staphylococcus aureus and other aerobic gram-positive cocci.
Figure 28b. Subacute septic arthritis of the AC joint in a 25-year-old man, an intravenous drug abuser, who presented after 6–8 weeks of atraumatic right shoulder pain. (a) AP radiograph shows AC joint widening, fragments within the joint, and irregular erosion of the distal clavicle, with periosteal new bone formation (arrows). (b) Corresponding coronal T2-weighted fat-suppressed MR image shows effusion, capsular thickening, and irregularity of the bone margin (arrowhead). Note the edema (arrow) within the overlying soft tissues and trapezius muscle (*). Cultures from surgical débridement were positive for Staphylococcus saccharolyticus, an uncommon slowly growing anaerobe. Organisms more commonly associated with AC septic arthritis include Staphylococcus aureus and other aerobic gram-positive cocci.
Rheumatoid arthritis is a disabling inflammatory arthritis affecting 1% of the population. Disease-modifying drugs have altered the course of disease such that the classic finding of severe symmetric AC erosion emphasized in the older literature is now infrequent. Erosions are caused by synovitis at the joint and adjacent subacromial bursa. They initially are most pronounced inferiorly but ultimately progress to destroy the joint, leaving the bone ends thin and tapered (Fig 29) (90,91).
Figure 29. AC erosion in chronic rheumatoid arthritis in a 68-year-old man. AP radiograph of the right shoulder shows widening of the AC joint with tapered erosion of the distal clavicle (arrowhead), simulating prior resection. The acromion is eroded by a high-riding humeral head related to chronic rotator cuff tearing. The patient has profound osteopenia, with severe erosive changes at the glenohumeral joint (*).
Hyperparathyroidism is a metabolic disorder marked by accelerated bone resorption that is most commonly seen in its secondary form in patients with chronic renal failure (92). Typical sites of resorption include the phalangeal subperiosteal cortex and the sacroiliac and AC subchondral surfaces (92). Clavicular erosions are bilaterally symmetric and may be associated with subligamentous erosion at the CC ligament insertion (Fig 30) (93). Ancillary findings around the AC joint include acromial, rib, and coracoid resorption; brown tumors; and soft-tissue mineralization (Fig 31) (93).
Figure 30. Bilateral clavicle erosion in chronic renal failure with secondary hyperparathyroidism in a 30-year-old woman with a right dialysis catheter. AP radiograph of the upper chest shows bilaterally symmetric erosive changes at the distal clavicles, with loss of the cortical border at the clavicular ends. There is also subtle irregularity of the inferior clavicles at the CC ligament insertions and multiple masslike areas of rib enlargement compatible with chronic brown tumors.
Figure 31a. Tumor calcinosis eroding the AC joint in a 44-year-old woman with chronic renal failure who was on long-term dialysis. (a) AP radiograph of the right shoulder shows erosive changes of the distal clavicle (arrowhead) and widening of the joint space. There is a large amount of amorphous soft-tissue mineralization within and around the joint, compatible with tumoral calcinosis secondary to renal disease. (b) Axial proton-density–weighted fat-suppressed MR image shows heterogeneous, predominantly low-signal-intensity material within the joint (*) with articular widening and bone erosion, and surrounding soft-tissue edema.
Figure 31b. Tumor calcinosis eroding the AC joint in a 44-year-old woman with chronic renal failure who was on long-term dialysis. (a) AP radiograph of the right shoulder shows erosive changes of the distal clavicle (arrowhead) and widening of the joint space. There is a large amount of amorphous soft-tissue mineralization within and around the joint, compatible with tumoral calcinosis secondary to renal disease. (b) Axial proton-density–weighted fat-suppressed MR image shows heterogeneous, predominantly low-signal-intensity material within the joint (*) with articular widening and bone erosion, and surrounding soft-tissue edema.
Primary and secondary neoplasms affecting the clavicle can lead to osteolysis; however, tumors are relatively uncommon in this region. The diagnosis is made on the basis of medical history, imaging characteristics, and histopathologic sampling. Rarely, osteolysis is caused by Gorham disease, a nonneoplastic disorder of unknown cause that is marked by vascular and lymphatic proliferation (94,95). The disorder starts in one bone, most commonly at the pelvis, shoulder, or mandible, and spreads relentlessly into contiguous tissues, with advanced disease showing striking bone dissolution (Fig 32) (94,95).
Figure 32. Gorham disease of the thorax eroding both clavicles in a 34-year-old man with a 15-year history of Gorham disease, also known as vanishing bone disease. AP radiograph of the upper thorax shows osseous destruction involving both clavicles, multiple upper ribs, and the left scapula, with inferior drooping of both shoulders. There is blunting of the left costophrenic angle from previous chylothorax, and a tracheostomy tube. This degree of involvement of the clavicles and upper ribs can lead to collapse of the chest and visceral involvement of the pleura and mediastinum.
Acromial Apophysiolysis and Os Acromionale
Although stress-induced injury of the acromion is rare in adults, in one large study (96) involving patients aged 15–25 years who were examined with MRI, acromial abnormalities were found in 2.6% of cases related to overuse and primarily affected baseball pitchers who were throwing more than 100 pitches per week. Acromial apophysiolysis is a stress injury that causes pain and marrow edema along the curvilinear elongated peripheral contour of the unfused growth centers in the immature skeleton (14,97) (Fig 33). It is associated with disordered maturation and leads to an os acromionale in 31% of affected patients (96). Os acromionale is diagnosed only when the acromial ossification centers fail to fuse by age 25 years (98). It is present in 2%–8% of the population and bilateral in one-third of cases (13,99).
Figure 33a. Acromial ossification centers and apophysiolysis. (a) Drawing illustrates the acromial ossification centers from the anterior to posterior aspect: The preacromion (PA), mesoacromion (MSA), meta-acromion (MTA), and basiacromion (BA) are depicted. (b) Axial proton-density–weighted fat-suppressed MR image in an active 16-year-old boy, a soccer player, shows an immature acromion with marrow edema centered at the junction of the immature mesoacromial and meta-acromial growth centers. Note the normal curvilinear and slightly lobulated contours at the periphery of the immature ossification centers (arrowheads). Failed fusion at this synchondrosis is the most common cause of an os acromionale.
Figure 33b. Acromial ossification centers and apophysiolysis. (a) Drawing illustrates the acromial ossification centers from the anterior to posterior aspect: The preacromion (PA), mesoacromion (MSA), meta-acromion (MTA), and basiacromion (BA) are depicted. (b) Axial proton-density–weighted fat-suppressed MR image in an active 16-year-old boy, a soccer player, shows an immature acromion with marrow edema centered at the junction of the immature mesoacromial and meta-acromial growth centers. Note the normal curvilinear and slightly lobulated contours at the periphery of the immature ossification centers (arrowheads). Failed fusion at this synchondrosis is the most common cause of an os acromionale.
The most common type of os acromionale is related to failed fusion between the mesoacromion and meta-acromion, resulting in a coronal cleft at the posterior margin of the AC joint (10,99,100). The majority of cases are asymptomatic, although some of these disorders cause pain related to an unstable synchondrosis or an impingement-like syndrome during overhead activity (101). If symptoms do not respond to conservative measures, surgical options include screw fixation of the synchondrosis or resection (13,100).
Degenerative Disorders of the AC Joint
AC Osteoarthrosis
Osteoarthrosis is the most common cause of AC symptoms in adults, causing local articular pain exacerbated by movement or symptoms related to impingement of adjacent tissues (102). Primary osteoarthrosis is due to cumulative degeneration with aging, whereas secondary osteoarthrosis is most commonly caused by previous injury (102).
Figure 34. Coronal high-spatial-resolution radiograph of a sectioned specimen from an adult cadaver shows the four cardinal findings of osteoarthrosis: joint space loss, osteophytes (arrow), subchondral sclerosis, and cysts. Note the vacuum in the joint and the complete absence of the intra-articular disk. There is a small amount of mineralization (arrowhead) at the rotator cuff insertion. The glenohumeral cartilage appears normal. (Case courtesy of Donald Resnick, MD, University of California, San Diego, Calif.)
Figure 35a. AC osteoarthrosis with marrow edema in a 34-year-old man with shoulder pain of 3 months’ duration, related to moderate osteoarthrosis of the AC joint. (a) Coronal proton-density–weighted fat-suppressed MR image shows marrow edema at the clavicle and acromion. Note the thickened superior capsule (arrow) and small remnant of the meniscus (arrowhead) at the superior joint. (b) Sagittal T1-weighted MR image shows inferior osteophytes and capsular hypertrophy from the AC joint mildly indenting the supraspinatus muscle (arrowheads).
Figure 35b. AC osteoarthrosis with marrow edema in a 34-year-old man with shoulder pain of 3 months’ duration, related to moderate osteoarthrosis of the AC joint. (a) Coronal proton-density–weighted fat-suppressed MR image shows marrow edema at the clavicle and acromion. Note the thickened superior capsule (arrow) and small remnant of the meniscus (arrowhead) at the superior joint. (b) Sagittal T1-weighted MR image shows inferior osteophytes and capsular hypertrophy from the AC joint mildly indenting the supraspinatus muscle (arrowheads).
Impingement
Shoulder impingement syndromes are divided into external and internal types. External impingement is more common, typically affecting the subacromial space (98). This space is bounded by the humeral head inferiorly and the coracoacromial arch and composed of the acromial undersurface, coracoacromial ligament, and coracoid superiorly (107,108). Neer (107) considered subacromial impingement to be the principal cause of rotator cuff tearing (107,109). Bigliani et al (110) categorized acromial undersurface morphology as flat (32% of cases), curved (42% of cases), or hooked (26% of cases), suggesting that the latter two types are associated with impingement and rotator cuff tears (99,110). Morphology categorization has poor interobserver agreement, and there is controversy regarding whether it correlates with rotator cuff disease (111,112).
The diagnosis of subacromial impingement is clinical, based on a painful arch of motion at 70°–120° arm abduction (107). Imaging findings include enthesopathy at the coracoacromial ligament attachment, ligament thickening, and AC arthrosis. A large anterior acromial enthesophyte is the most specific finding, but it develops late in the process (Fig 36) (112). Less commonly, enthesopathy produces a “keeled acromion,” with roughening of the inferior acromion related to a broad undersurface attachment; this finding also correlates with rotator cuff tear (113). Impingement at the joint is caused by inferiorly directed capsular hypertrophy and osteophytes that indent the rotator cuff and reduce clearance for the greater tuberosity during arm elevation (109,111). AC joint disorders and acromial enthesopathy frequently coexist.
Figure 36. Large subacromial enthesophyte causing subacromial impingement in a 46-year-old man with shoulder pain during arm elevation. Lateral outlet-view radiograph shows a large triangular bone excrescence arising from the anterior acromion and coursing along the plane of the coracoacromial ligament, with its apex directed toward the coracoid process (arrowhead). Enthesopathy and subacromial morphology are best assessed on the outlet view, a lateral scapular y-axis view obtained with a 10°–15° caudal tilt of the beam along the axis of the scapular spine.
AC Joint Cyst
Uncommonly, degenerative disease results in the formation of a cyst above the AC joint that manifests as a mass that can be easily mistaken for a neoplasm (114). The events leading to cyst formation start with a large rotator cuff tear that produces a high-riding humerus that abuts and erodes the inferior AC capsule (114,115). The capsular defect allows the ingress of glenohumeral effusion into the AC joint via the subacromial bursa, producing the “geyser sign,” a finding initially described at arthrography (Fig 37) (115). Subsequently, chronic distention leads to supraclavicular cyst formation by way of gross distention of the superior capsule or fluid dehiscence beyond the capsule. At radiography, the mass is nonspecific, although adjacent findings of advanced cuff arthropathy can suggest the diagnosis (Fig 38) (116). MRI demonstrates the cuff and capsular defects that allow continuity of fluid between the glenohumeral joint and the cyst (114).
Figure 37a. Geyser sign, with articular fluid entering the AC joint, in a 68-year-old man with a full-thickness rotator cuff tear. (a) Coronal proton-density–weighted fat-suppressed MR image of the right shoulder shows fluid (arrowheads) in the AC joint communicating with the glenohumeral joint and subacromial bursa. (b) Corresponding long-axis US image shows intra-articular fluid (arrowheads) extending superiorly between the acromion (a) and clavicle (c) and distending the thickened superior joint capsule (arrows).
Figure 37b. Geyser sign, with articular fluid entering the AC joint, in a 68-year-old man with a full-thickness rotator cuff tear. (a) Coronal proton-density–weighted fat-suppressed MR image of the right shoulder shows fluid (arrowheads) in the AC joint communicating with the glenohumeral joint and subacromial bursa. (b) Corresponding long-axis US image shows intra-articular fluid (arrowheads) extending superiorly between the acromion (a) and clavicle (c) and distending the thickened superior joint capsule (arrows).
Figure 38a. AC cyst due to rotator cuff arthropathy in a 70-year-old man. (a) AP radiograph of the left shoulder shows a large rounded soft-tissue mass superior to a degenerated AC joint. There is advanced glenohumeral arthrosis with bone remodeling. The AC space is widened from effusion, and there is erosion of the acromial articular surface. The humerus is not high riding on this radiograph. (b) Coronal T2-weighted fat-suppressed MR image obtained on an outside low-magnetic-field MRI system shows a massive cuff tear with fluid (arrows) tracking from the joint into the AC cyst (*).
Figure 38b. AC cyst due to rotator cuff arthropathy in a 70-year-old man. (a) AP radiograph of the left shoulder shows a large rounded soft-tissue mass superior to a degenerated AC joint. There is advanced glenohumeral arthrosis with bone remodeling. The AC space is widened from effusion, and there is erosion of the acromial articular surface. The humerus is not high riding on this radiograph. (b) Coronal T2-weighted fat-suppressed MR image obtained on an outside low-magnetic-field MRI system shows a massive cuff tear with fluid (arrows) tracking from the joint into the AC cyst (*).
Imaging characteristics vary, from homogeneous nonenhancing fluid to a complex collection with internal septa, hemorrhage, and debris (116). While the majority of AC cysts are associated with chronic rotator cuff arthropathy, they occasionally result from chronic AC effusion in the setting of intact tendons (Fig 39) (117). Although US can enable evaluation of the mass and guide cyst aspiration, such cysts tend to recur if the underlying disease is not addressed (114).
Figure 39a. AC joint cyst in a 72-year-old man who reported having a mass 6 months after acromioplasty and rotator cuff reconstruction. (a) Axial proton-density–weighted fat-suppressed MR image shows AC arthrosis and irregularity of the anterior capsule, with a well-defined mass (*) seen anteromedial to the joint. (b) Sagittal T1-weighted fat-suppressed MR image obtained after intravenous administration of gadolinium-based contrast material shows a thin rim of enhancement at the periphery of the mass, with central nonenhancing fluid (*). Note the enhancement of the thickened irregular capsule at the adjacent AC joint, subacromial bursa, and glenohumeral joint (arrowheads). No preoperative imaging studies were available to determine whether the cyst was caused by the cuff tear or arose primarily from the joint.
Figure 39b. AC joint cyst in a 72-year-old man who reported having a mass 6 months after acromioplasty and rotator cuff reconstruction. (a) Axial proton-density–weighted fat-suppressed MR image shows AC arthrosis and irregularity of the anterior capsule, with a well-defined mass (*) seen anteromedial to the joint. (b) Sagittal T1-weighted fat-suppressed MR image obtained after intravenous administration of gadolinium-based contrast material shows a thin rim of enhancement at the periphery of the mass, with central nonenhancing fluid (*). Note the enhancement of the thickened irregular capsule at the adjacent AC joint, subacromial bursa, and glenohumeral joint (arrowheads). No preoperative imaging studies were available to determine whether the cyst was caused by the cuff tear or arose primarily from the joint.
Treatment
Conservative Management
The principal treatment goal in the setting of AC arthrosis is pain control. Initial management consists of rest, activity modification, thermal compresses, and oral analgesics (102,118). Intra-articular corticosteroid injections are used when these methods are insufficient and sometimes in combination with a local anesthetic agent to confirm that the joint is the source of the symptoms (102). Response rates to corticosteroids are variable, long-term efficacy is unclear, and there is a risk of soft-tissue damage when the injectate is inadvertently injected outside the joint (102,118,119). Fewer than 40% of blind injections are successful, so imaging guidance is recommended (119).
Resection Arthroplasty
Resection of the distal clavicle is performed for refractory osteoarthrosis, distal clavicle osteolysis, impingement (typically in conjunction with acromioplasty), and chronic injuries that are not amenable to reconstruction (58). The procedure can be open (Mumford procedure) (Fig 40) or arthroscopic. Arthroscopic resection is preferred because the recovery is faster and there is less tissue damage, particularly when a subacromial approach, preserving the superior capsule, is used (120). Resection is kept to a minimum to avoid capsular damage and instability. Removing 3–4 mm of bone preserves the capsule; however, because this may not provide adequate decompression, the resection is typically wider. Although the wider resection violates capsular tissues, it is generally well tolerated (Fig 40) (20,22). Fifteen millimeters of the clavicle can be resected before the trapezoid ligament is breached (20).
Figure 40a. Mumford resection of the distal clavicle for AC osteoarthrosis in a 61-year-old man. (a) Preoperative AP radiograph of the right shoulder shows narrowing of the joint (arrowhead) and osteophytes. Peripheral beads mark the edge of a wedge-shaped filter that is used to equalize tissue penetration and improve visualization of the AC joint. (b) AP radiograph obtained after surgical resection shows loss of the normal bulbous end of the truncated clavicle.
Figure 40b. Mumford resection of the distal clavicle for AC osteoarthrosis in a 61-year-old man. (a) Preoperative AP radiograph of the right shoulder shows narrowing of the joint (arrowhead) and osteophytes. Peripheral beads mark the edge of a wedge-shaped filter that is used to equalize tissue penetration and improve visualization of the AC joint. (b) AP radiograph obtained after surgical resection shows loss of the normal bulbous end of the truncated clavicle.
Trauma Management
The goal of AC injury management is a pain-free shoulder, with restoration of motion and strength that enables a return to full activity (22,73). The choice of treatment depends on the acuity and grade of the injury in conjunction with the patient’s age, overall health, and functional needs (16,22,56). Rockwood type I and type II injuries can be managed conservatively with a brief period of sling immobilization. Injury types IV, V, and VI are routinely managed surgically (58,121). Open injuries with penetration of the distal clavicle through the integument require emergent surgical reduction to minimize the risk of infection and tissue necrosis (Fig 41).
Figure 41a. Open AC injury in a 33-year-old man who fell on his side in a motorcycle accident. (a) AP radiograph of the shoulder shows dislocation of the AC joint, with subtle locules of gas within the joint and adjacent to the distal clavicle (arrows). (b) Axial gradient-echo MR image clearly shows susceptibility artifacts from the gas around the AC joint and within the trapezius muscle (arrowheads), which was perforated by the dislocated clavicle. There is also avulsion and stripping of the trapezius muscle from the distal clavicle (arrow).
Figure 41b. Open AC injury in a 33-year-old man who fell on his side in a motorcycle accident. (a) AP radiograph of the shoulder shows dislocation of the AC joint, with subtle locules of gas within the joint and adjacent to the distal clavicle (arrows). (b) Axial gradient-echo MR image clearly shows susceptibility artifacts from the gas around the AC joint and within the trapezius muscle (arrowheads), which was perforated by the dislocated clavicle. There is also avulsion and stripping of the trapezius muscle from the distal clavicle (arrow).
The treatment for Rockwood type III injuries remains controversial, although there is a growing trend toward conservative management (16,22,58,73). Rockwood type III appears to include a heterogeneous group of injuries that vary in stability and prognosis (50,121). The International Society of Arthroscopy, Knee Surgery and Orthopedic Sports Medicine recommends subdividing these injuries into two categories after 3 weeks of conservative management (50). With type IIIA injuries, the AC joint feels stable, there is no clavicular overriding on the Alexander view, and there is no scapular dysfunction; thus, continuation of conservative treatment is allowed. Conversely, type IIIB injuries exhibit persistent instability, clavicular overriding, and scapular dysfunction, and thus surgical reconstruction is appropriate (50).
More than 60 surgical procedures for AC injury are described, but there is no standard technique (16,56,64,122). Surgery is directed toward realignment, augmented with temporary fixation to allow the injured ligaments to heal or with reconstruction to repair or, more commonly, recreate the torn tissues.
Figure 42a. Failed Bosworth screw fixation in a 31-year-old man with an unstable grade III AC separation from a snowboarding accident. (a) Postoperative AP radiograph of the right shoulder shows a vertically oriented Bosworth screw fixating the clavicle to the coracoid process. (b) Follow-up radiograph obtained 1 month later shows the screw head is no longer flush with the washer (arrow), with interval superior subluxation of the clavicle relative to the acromion at the joint line and widening of the CC distance, indicating loss of fixation. Note the ossification developing in the injured CC ligaments (arrowheads); this is commonly seen in subacute and chronic high-grade injuries.
Figure 42b. Failed Bosworth screw fixation in a 31-year-old man with an unstable grade III AC separation from a snowboarding accident. (a) Postoperative AP radiograph of the right shoulder shows a vertically oriented Bosworth screw fixating the clavicle to the coracoid process. (b) Follow-up radiograph obtained 1 month later shows the screw head is no longer flush with the washer (arrow), with interval superior subluxation of the clavicle relative to the acromion at the joint line and widening of the CC distance, indicating loss of fixation. Note the ossification developing in the injured CC ligaments (arrowheads); this is commonly seen in subacute and chronic high-grade injuries.
The classic Weaver-Dunn procedure, consisting of distal clavicle resection combined with coracoacromial ligament transfer from the acromion to the residual clavicle, was the first reconstruction method to gain widespread acceptance (58,122). Despite modifications designed to improve its strength, this reconstruction method was weak, resulting in symptomatic laxity in more than 30% of patients (16). Subsequently, fixation with use of a hook plate inserted deep to the acromion and superficial to the clavicle to prevent superior clavicular migration became popular. This plate is still used, most often for fixating unstable distal clavicle fractures with CC diastasis (Fig 43). The bulky plate has been associated with acromial erosion and impingement and is typically removed after the tissues have healed (123).
Figure 43a. Unstable distal clavicle fracture treated with hook-plate fixation in a 58-year-old woman who was injured during a high-energy fall from a height. (a) Upright AP radiograph of the left shoulder shows a comminuted unstable Neer type IIB fracture of the clavicle, with superior displacement of the shaft and widening of the CC distance. The patient also sustained multiple lateral rib fractures. (b) Close-up postoperative AP radiograph obtained after placement of a hook plate across the AC joint shows restoration of the normal alignment. Hook-plate fixation maintains the three-plane articular alignment, holding the clavicle in place while allowing the CC ligaments to heal.
Figure 43b. Unstable distal clavicle fracture treated with hook-plate fixation in a 58-year-old woman who was injured during a high-energy fall from a height. (a) Upright AP radiograph of the left shoulder shows a comminuted unstable Neer type IIB fracture of the clavicle, with superior displacement of the shaft and widening of the CC distance. The patient also sustained multiple lateral rib fractures. (b) Close-up postoperative AP radiograph obtained after placement of a hook plate across the AC joint shows restoration of the normal alignment. Hook-plate fixation maintains the three-plane articular alignment, holding the clavicle in place while allowing the CC ligaments to heal.
With current techniques, the focus is reconstruction of the CC ligaments, with a variety of constructs used to replicate their function and restore vertical stability. There is no consensus regarding the indications for supplemental AC reconstruction for horizontal stabilization (64). Reconstruction can be performed by using open, mini-open, or purely arthroscopic procedures (59). The risk of bleeding, infection, and neurologic injury is higher during large open procedures (59). CC reconstructions can be anatomic, whereby the trapezoid and conoid functions are reproduced independently, or nonanatomic. The nonanatomic method is technically easier and requires less drilling in bone (16). Currently, CC reconstruction in which fiberwire strands are passed from the coracoid through single or dual tunnels to the clavicle, with the wire entry sites augmented with endobuttons to prevent bone erosion, is popular (59).
Implanted materials such as tendon graft, polydioxanone, and fiberwire may not be visible on radiographs unless they are anchored by metallic devices (Fig 44) (73). Reconstruction implants can be complicated by device failure, irritation from surrounding soft tissues, and coracoid or clavicle fractures at the drilling sites (Fig 45) (61,122). Loss of reduction is the most common complication, occurring in 20% of reconstructions, regardless of injury acuity (121).
Figure 44a. Anatomic CC reconstruction in a 32-year-old woman 1 month after surgery for high-grade AC injury. (a) AP radiograph of the right shoulder shows two vertically oriented drill tunnels (arrowheads) at the distal clavicle without any radiopaque hardware. There is 2 mm of vertical offset at the bottom of the AC joint and a normal CC distance. (b) Coronal T1-weighted MR image shows two clavicular tunnels, with anatomic reconstruction of the trapezoid and conoid ligaments performed by using strands of low-signal-intensity surgical material (arrows) extending from the clavicle to the coracoid process.
Figure 44b. Anatomic CC reconstruction in a 32-year-old woman 1 month after surgery for high-grade AC injury. (a) AP radiograph of the right shoulder shows two vertically oriented drill tunnels (arrowheads) at the distal clavicle without any radiopaque hardware. There is 2 mm of vertical offset at the bottom of the AC joint and a normal CC distance. (b) Coronal T1-weighted MR image shows two clavicular tunnels, with anatomic reconstruction of the trapezoid and conoid ligaments performed by using strands of low-signal-intensity surgical material (arrows) extending from the clavicle to the coracoid process.
Figure 45. Distal clavicle fracture after minor trauma in a 50-year-old man who previously underwent CC reconstruction. Y-axis radiographic view of the left shoulder shows endobuttons (arrows) at the clavicle and coracoid process from a previous CC reconstruction performed with use of braided fiberwire. There is a comminuted fracture of the distal clavicle (arrowhead) at the drilling sites for the clavicular tunnel. The risk of fracture increases with the number and size of the tunnels used for reconstruction.
Conclusion
The AC joint is part of the shoulder girdle complex that enables smooth coordinated motion of the upper extremity. This articulation can be affected by a spectrum of pathologic entities, with traumatic and degenerative disorders being the most common. An understanding of the anatomy, mechanics, common diseases, surgical classifications, and imaging findings of disorders that affect the AC joint enables accurate diagnosis.
Acknowledgment
The authors give special thanks to Judy Ann D. Tamayo, Quezon City, Philippines, for preparing the graphic illustrations.
Recipient of a Magna Cum Laude award for an education exhibit at the 2019 RSNA Annual Meeting.
For this journal-based SA-CME activity, the authors, editor, and reviewers have disclosed no relevant relationships.
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Article History
Received: Mar 15 2020Revision requested: Apr 14 2020
Revision received: Apr 24 2020
Accepted: Apr 28 2020
Published online: Aug 07 2020
Published in print: Sept 2020
