Trauma/Emergency RadiologyFree Access

Traumatic Hip Dislocation: What the Orthopedic Surgeon Wants to Know

Abstract

Hip dislocation is an important orthopedic emergency usually seen in young patients who have experienced high-energy trauma, often resulting in significant long-term morbidity. Rapid identification and reduction is critical, as prolonged dislocation increases the risk of developing avascular necrosis of the femoral head, and posttraumatic osteoarthritis is a common complication, even in the absence of associated fractures. Identification and timely management of hip dislocation are highly dependent on imaging, both at presentation and after attempted reduction. It is imperative for the radiologist to understand imaging features that guide management of hip dislocation to ensure timely identification, characterization, and communication of clinically relevant results. Although the importance of prompt identification of hip dislocation is universally recognized, the significance of imaging features that guide correct management and are thought to prevent complications is less emphasized in the radiology literature. In this article, the authors review the anatomy of the hip, common injury mechanisms for various types of dislocations, and imaging findings for associated injuries. They review the most commonly used classification systems and propose a simplified checklist approach to hip dislocation to aid rapid interpretation and communication of the most clinically relevant imaging features to the treating orthopedic surgeon.

^©RSNA, 2017

SA-CME LEARNING OBJECTIVES

After completing this journal-based SA-CME activity, participants will be able to:

■ Describe the most pertinent imaging findings in traumatic hip dislocation, both on initial radiographs and at postreduction CT, to guide management by the orthopedic team.
■ Discuss potential pitfalls in imaging of traumatic hip dislocation.
■ Explain the important treatment implications of several key imaging findings of traumatic hip dislocation.

Introduction

Traumatic hip dislocation is a true emergency that requires immediate orthopedic evaluation and reduction (1). Hip dislocations are serious injuries that are associated with significant long-term morbidity, most notably avascular necrosis and posttraumatic osteoarthritis (2). They typically occur in young patients in the setting of high-energy trauma and are increasing in incidence, predominantly because of motor vehicle crashes, which cause between 62% and 93% of all hip dislocations (3). In contrast, fractures of the femoral neck are more common in older patients (4), unless there is concomitant systemic disease in younger patients (5). Patients with traumatic hip dislocation should undergo a complete evaluation by the trauma service because of the high prevalence of additional injuries, especially in the setting of a motor vehicle crash. In patients with hip dislocation resulting from a motor vehicle crash, there is a reported prevalence of nonorthopedic injuries in 67% of cases, including 24% with closed-head injuries, 21% with craniofacial fractures, 21% with thoracic injuries, and 15% with abdominal injuries (6).

Given the inherent stability of the hip due to its osseous, labral, ligamentous, and muscular anatomy, dislocations require significant force and are typically associated with fractures of the acetabulum or femoral head (7). Dislocations without additional fractures may be less common than initially thought, owing to the widespread adoption of computed tomography (CT) and the resulting ability to visualize small fractures that may not have been evident at earlier radiographic investigations (8). Of historical interest, the first major malpractice case in the United States was a case of hip dislocation occurring in 1821, which is said to have influenced the current malpractice environment (9).

Multiple and somewhat redundant classification systems exist to characterize hip dislocations. In general, neither radiologists nor orthopedic surgeons routinely classify hip dislocations by any one system, although the Pipkin classification of femoral head fractures is routinely used if a femoral head fracture is present. Nevertheless, familiarity with the most-used classification systems allows a better understanding of findings that most affect patient care.

Treatment of patients with hip dislocation is performed in two stages. Initially, the goal is to perform rapid reduction of the hip. The second stage is focused on definitive management.

At initial radiography in the setting of trauma, the radiologist must identify and characterize the dislocation and communicate any features that contraindicate rapid closed reduction. At postreduction CT, the radiologist must appropriately recognize complicating features that necessitate further surgical intervention by the orthopedic surgeon.

In this article, we review the anatomy of the hip and illustrate the classification systems for hip dislocation and associated femoral head fractures. Using a checklist approach, we highlight key imaging findings that guide treatment. Finally, we review pitfalls encountered at patient presentation and imaging to prevent misinterpretation and delays in clinical management.

Osseous, Ligamentous, and Vascular Anatomy of the Hip

The hip is one of the most stable joints of the body, with extensive passive and active support. Approximately 82% of the articular surface of the femoral head is covered by the bony acetabulum when the hip is in the neutral position (10). The fibrocartilaginous labrum further adds to the passive stability of the hip. In addition, the hip is covered by a thick capsule of longitudinally oriented fibers, which condense into three discrete ligaments (Fig 1) (11,12). The iliofemoral ligament (or the Y-shaped or Bigelow ligament) consists of two bands and is the primary static stabilizer of the hip in terminal extension (using the inferior band) and external rotation (using the superior band) (13). The pubofemoral ligament covers the anterior inferior aspect of the hip, and the ischiofemoral ligament covers the posterior aspect. The ligamentum teres femoris arises from the fovea of the femoral head and inserts onto the transverse acetabular ligament (14), which is a noncartilaginous continuation of the labrum at the inferior aspect of the hip. The function of the ligamentum teres in the stability of the hip is controversial, but isolated injuries may be implicated as a source of hip pain (15). It is the experience of the authors that the ligamentum teres is invariably torn in patients who sustain a hip dislocation, although there are rare reports of an intact or only partially torn ligamentum teres demonstrated at arthroscopy performed after dislocation (16).

Figure 1a. Ligaments of the hip. **(a)** Drawing of the anterior hip shows the iliofemoral (red) and pubofemoral (blue) ligaments, with the iliofemoral ligament comprising the superior and inferior bands. **(b)** Drawing of the posterior hip shows the ischiofemoral ligament (green) and the posterior aspect of the iliofemoral ligament (red).

Figure 1b. Ligaments of the hip. **(a)** Drawing of the anterior hip shows the iliofemoral (red) and pubofemoral (blue) ligaments, with the iliofemoral ligament comprising the superior and inferior bands. **(b)** Drawing of the posterior hip shows the ischiofemoral ligament (green) and the posterior aspect of the iliofemoral ligament (red).

The vascular anatomy of the hip (Fig 2) is clinically important because of the direct correlation between the rate of occurrence of avascular necrosis and the time to reduction of a hip dislocation. The main blood supply to the weight-bearing femoral head is provided by the medial femoral circumflex artery (17), which arises from the profunda femoris artery.

The extracapsular deep branch of the medial femoral circumflex artery branches to provide two to four subsynovial retinacular arteries, which can completely perfuse the femoral head. The lateral circumflex femoral artery provides a relatively insignificant contribution to the femoral head blood supply. Similarly, the artery of the ligamentum teres (or foveolar artery) is a small-caliber artery arising from the obturator artery that perfuses the perifoveal region only and is typically vestigial with an insignificant contribution in adults. Avascular necrosis of the femoral head due to nontraumatic causes is thought to be caused by obstruction of the intraosseous branches of the medial femoral circumflex artery, whereas trauma may directly rupture, compress, or kink its extraosseous branches (17). In traumatic hip dislocations and fractures, the femoral neck is usually intact and thus the subsynovial terminal branches also remain intact, although the extracapsular deep branch of the medial femoral circumflex artery is susceptible to injury (17).

Figure 2. Drawing shows the vascular supply of the femoral head. The main contribution to the femoral head blood supply is the medial femoral circumflex artery *(MFCA)*, with the extrasynovial portion being the most susceptible to traumatic injury.

Radiographic Anatomy of the Hip

Several contour lines seen at standard anteroposterior (AP) pelvic radiography aid in systematic interpretation (Fig 3) (18). Both the iliopectineal line (comprising the anterior column of the acetabulum) and the ilioischial line (comprising the posterior column) should be continuous and smooth. Special attention should be paid to the posterior wall of the acetabulum, as posterior wall fracture is the most commonly associated acetabular fracture pattern in hip dislocation. Shenton line disruption can be seen in hip dislocation or in femoral neck fractures without dislocation. If a femoral head fracture is present, its location should be described as involving the femoral head below or above the fovea capitis.

Figure 3a. Annotated radiographs show the normal anatomy of the hip. **(a)** The iliopectineal line comprises the anterior column of the acetabulum, and the ilioischial line is the posterior column. The anterior and posterior walls of the acetabulum can be visualized through the femoral head. The medial wall of the acetabulum is contiguous with the roof. **(b)** Of particular importance to hip dislocation is the fovea capitis, which is an important landmark to classify femoral head fractures.

Figure 3b. Annotated radiographs show the normal anatomy of the hip. **(a)** The iliopectineal line comprises the anterior column of the acetabulum, and the ilioischial line is the posterior column. The anterior and posterior walls of the acetabulum can be visualized through the femoral head. The medial wall of the acetabulum is contiguous with the roof. **(b)** Of particular importance to hip dislocation is the fovea capitis, which is an important landmark to classify femoral head fractures.

Mechanisms of Injury in Hip Dislocation

The most common mechanism of posterior hip dislocation is a posteriorly directed force against a flexed knee, with the hip flexed and adducted, such as from a dashboard injury in a motor vehicle crash. The force is transmitted through the femur and causes the posterior aspect of the femoral head to make an impact upon the posterior wall of the acetabulum. Hip dislocations in sporting activities are rare, representing only 2%–5% of hip dislocations, but may occur in high-velocity sports such as football and rugby (19–21).

The position of the leg at the time of the trauma determines if the hip dislocates with or without fracture of the acetabulum or femoral head. Specifically, if the hip is flexed and adducted, then the femoral head is most likely to dislocate posteriorly without fracture. However, a more extended and abducted position directs the axial load into the hip joint and increases the risk of fracturing the posterior acetabulum or femoral head (22). With progressively greater degrees of abduction, isolated acetabular fracture without dislocation may result. The mechanism of anterior dislocation is due to the rarer combination of extreme abduction, extension, and external rotation (23).

Some anatomic variations may predispose an individual to posterior hip dislocation. It has been observed that despite the millions of car crashes that occur annually in the United States (24), hip dislocation remains relatively uncommon, suggesting that there may need to be an unlikely confluence of external force mechanism and osseous anatomy to dislocate the hip (25). In contrast, fractures of the femoral shaft and tibia are much more common than hip dislocations that involve direct trauma to the knee region. The two anatomic variations of the hip that have been described as predisposing a person to hip dislocation are decreased femoral anteversion (25) and femoroacetabular impingement (26,27). Decreased femoral anteversion is equivalent to relative internal rotation of the femur, and forcible internal rotation of the femur with a lever mechanism has been shown to produce posterior dislocation in cadavers (25). Persons with femoroacetabular impingement may be predisposed to traumatic posterior dislocation, especially in cases of lower-velocity trauma, such as from sporting activities, with the proposed mechanism of levering of the femoral head being due to restricted hip flexion and internal rotation (26).

Initial Radiographic Evaluation of Hip Dislocation

The initial imaging evaluation of the acutely traumatized patient with suspected hip dislocation is typically an AP radiograph of the pelvis. Additional pelvic radiographs, including lateral, oblique, or Judet views, are not routinely obtained. It is therefore important to be able to deduce the direction of dislocation at frontal imaging. Interpretation must be performed promptly, as rapid reduction of the hip is essential. Urgent closed reduction, most commonly performed in the emergency department under sedation, is imperative to reduce the risk of avascular necrosis. To aid in interpretation of the initial radiograph, we suggest using a checklist approach (Fig 4a).

It is especially important to note the presence of a femoral neck fracture, which contraindicates closed reduction because of the risk for fracture displacement. The presence of a femoral neck fracture (or suspicion for an occult femoral neck fracture) necessitates urgent CT followed by surgical intervention.

Figure 4a. **(a)** Flowchart demonstrates the integration of imaging and management in patients with hip dislocation. Checklists are suggested for interpreting the initial pelvic radiograph and subsequent postreduction CT scan. **(b)** Sample postreduction CT report is shown for a hypothetical patient with successful reduction of a hip dislocation.

Figure 4b. **(a)** Flowchart demonstrates the integration of imaging and management in patients with hip dislocation. Checklists are suggested for interpreting the initial pelvic radiograph and subsequent postreduction CT scan. **(b)** Sample postreduction CT report is shown for a hypothetical patient with successful reduction of a hip dislocation.

The femoral head may dislocate in a posterior or anterior direction depending on the mechanism of injury; the vast majority (approximately 90%) are posterior in direction (3). In posterior hip dislocation, the femoral head is typically positioned superolateral to the acetabulum, with the hip in flexion, internal rotation, and adduction (Fig 5). The lesser trochanter is often obscured by the medial femoral cortex owing to internal rotation of the femur. Because of the AP radiographic technique, the posteriorly dislocated femoral head may appear to be smaller than the contralateral femoral head, due to magnification effect.

Figure 5a. **(a)** Drawing demonstrates the typical position of the femur in a posterior dislocation. **(b)** Posterior hip dislocation in a 19-year-old man with a basketball injury who struck the foot of another player with the knee in extension. Frontal radiograph of the pelvis shows that the femoral head (orange arrow) is typically positioned superolateral to the acetabulum, with the hip in flexion, internal rotation, and adduction. Internal rotation of the femur causes decreased conspicuity of the lesser trochanter (yellow arrow). The dislocated femoral head may appear smaller than the normally positioned femoral head owing to AP radiographic technique and magnification effect. Note the tiny posterior acetabular wall fracture (red arrow) in this case.

Figure 5b. **(a)** Drawing demonstrates the typical position of the femur in a posterior dislocation. **(b)** Posterior hip dislocation in a 19-year-old man with a basketball injury who struck the foot of another player with the knee in extension. Frontal radiograph of the pelvis shows that the femoral head (orange arrow) is typically positioned superolateral to the acetabulum, with the hip in flexion, internal rotation, and adduction. Internal rotation of the femur causes decreased conspicuity of the lesser trochanter (yellow arrow). The dislocated femoral head may appear smaller than the normally positioned femoral head owing to AP radiographic technique and magnification effect. Note the tiny posterior acetabular wall fracture (red arrow) in this case.

In contrast, in anterior hip dislocation the femoral head is most commonly positioned inferomedial to the acetabulum, and the hip is typically in marked external rotation, with smaller degrees of flexion and abduction (Fig 6). External rotation of the femur causes the lesser trochanter to be seen en face in anterior dislocations, with greater conspicuity than the contralateral side. In contrast to posterior hip dislocation, the anteriorly dislocated femoral head may appear larger than the normally positioned femoral head, owing to magnification effect from its position being closer to the x-ray source, although this has been reported to be an unreliable sign in anterior hip dislocations (28,29).

Figure 6a. **(a)** Drawing demonstrates the typical position of the femur in an anterior dislocation. **(b)** Anterior dislocation in a 25-year-old woman who was on a sled that crashed into a tree. Frontal radiograph of the pelvis shows the femoral head (orange arrow) typically positioned inferomedial to the acetabulum, with the hip in marked external rotation, with milder flexion and abduction. Anterior superior dislocation of the hip is rare in comparison. External rotation of the femur causes increased conspicuity of the lesser trochanter (yellow arrow). The dislocated femoral head may appear larger than the normally positioned femoral head owing to AP technique and magnification effect.

Figure 6b. **(a)** Drawing demonstrates the typical position of the femur in an anterior dislocation. **(b)** Anterior dislocation in a 25-year-old woman who was on a sled that crashed into a tree. Frontal radiograph of the pelvis shows the femoral head (orange arrow) typically positioned inferomedial to the acetabulum, with the hip in marked external rotation, with milder flexion and abduction. Anterior superior dislocation of the hip is rare in comparison. External rotation of the femur causes increased conspicuity of the lesser trochanter (yellow arrow). The dislocated femoral head may appear larger than the normally positioned femoral head owing to AP technique and magnification effect.

A rare type of anterior dislocation is anterior superior dislocation, with only a few small case series and case reports in the literature (23). The more common anterior inferior dislocations are usually easily recognized on a standard AP radiograph of the pelvis. However, the radiographic appearance of the rare anterior superior hip dislocation may be less straightforward and may mimic posterior hip dislocation (28). One helpful clue to differentiate anterior superior hip dislocation from posterior dislocation is the appearance of the lesser trochanter.

With posterior hip dislocation, the hip is typically in internal rotation and the lesser trochanter is less conspicuous or completely obscured by the femoral shaft. In anterior dislocation, the femur is typically held in external rotation, allowing nearly the entire cross-sectional diameter of the lesser trochanter to be visualized.

Postreduction Imaging of Hip Dislocation

After reduction is attempted, CT is performed to assess joint congruence, evaluate for intra-articular fragments, and search for associated bone and soft-tissue injuries.

CT is clearly superior to radiography for enabling detection of abnormalities about the hip (30), including small acetabular fractures, intra-articular fragments, step and gap deformities of the acetabulum (31), fracture and impaction injuries of the femoral head, and residual articular incongruities (32). Radiographs are not adequate to show small intra-articular bone fragments that can be seen reliably at CT (33). Evaluation of pure chondral fragments at CT is difficult, but they may be identified by using narrow soft-tissue window settings (34), or their presence may be inferred if there is any incongruity of the hip joint compared with the contralateral hip joint.

A comprehensive checklist approach (Fig 4a) and sample dictation (Fig 4b) to aid in interpretation of the CT study are suggested. Specific bone findings to include in the search pattern include the presence of posterior or medial acetabular fracture, femoral head fracture, and intra-articular osseous fragments. The congruence of the joint must be carefully analyzed, as noncongruent joints require additional surgery. Additional fractures, such as of the femoral neck, greater trochanter, or pelvis, should also be assessed. The soft tissues should also be carefully scrutinized, with particular attention being paid to the sciatic nerve, the vessels, and any hematomas.

The role of magnetic resonance (MR) imaging in the evaluation of patients with hip dislocation is currently evolving. Muscular injuries, sciatic nerve injuries, displaced or incarcerated labral tears, femoral head contusions, and intra-articular chondral fragments can be best seen at MR imaging (35). Although MR imaging is highly sensitive for detection of pure chondral fragments, it may be less sensitive than is CT for detection of small intra-articular osseous fragments (36). At our institution, MR imaging is not routinely performed in these cases, and any future role for MR imaging would likely be complementary to that of CT. There may also be some role for MR imaging in helping to predict the possible development of avascular necrosis, although previous studies in this regard have not shown a consistent benefit for MR imaging (37).

Classification of Hip Dislocation

There have been several proposed classification systems of posterior hip fractures and/or dislocations in the past 66 years, including Thompson and Epstein in 1951 (38), Stewart and Milford in 1954 (39), and Levin in 1992 (40). The Thompson-Epstein and Stewart-Milford classifications are the most frequently described in the orthopedic literature (10). Both are based on the presence and severity of acetabular fracture and the presence of femoral head fracture. The Stewart-Milford classification incorporates stability of the hip joint. The Levin classification is less commonly used in the literature but includes important information about the success of the initial reduction attempt as well as postreduction joint congruence. These classifications overlap substantially and are seldom used in clinical practice at our institution. None of the classification schemes can reliably predict functional outcome and prognosis on the basis of the initial clinical and imaging findings, and several additional important patterns of injury can be seen that are not included in these classification systems (41). A summary of the Thompson-Epstein and Levin classifications is provided for reference (Fig 7), although we focus here on describing the key imaging findings that predict and guide management with the most relevant classifications given in parentheses, rather than focusing on any single classification system. The Pipkin classification of femoral head fractures is separately applied and is also discussed (42).

Figure 7. Summary of the Levin and Thompson-Epstein classifications of posterior hip fractures and dislocations.

An additional Epstein classification exists for anterior dislocations (43), but some authors advocate using the more established classifications for posterior dislocation, prefixed by “anterior” (13). We do not routinely classify anterior dislocations by any system. Instead, we simply describe any concomitant injuries.

Posterior Dislocation with No or Insignificant Acetabular Fracture

A posterior hip dislocation with either no fracture or an insignificant acetabular fracture (Levin 1, Thompson-Epstein 1) (Fig 8) is typically treated with closed reduction only. An insignificant acetabular fracture does not affect hip joint stability. It has been reported that the size of acetabular fracture that is likely stable and therefore seen as insignificant is less than 20% involvement of the posterior wall, whereas more than 40%–50% involvement is considered to be probably unstable (44,45). However, it is not always obvious what size or location of acetabular fracture would more likely lead to instability. Occasionally even small (less than 20% posterior wall involvement) fractures are unstable, particularly those that involve the superior aspect of the posterior wall (46) or a Bankart-like avulsion of the posterior acetabular rim (47). To test stability after successful hip reduction, the orthopedic surgeon obtains fluoroscopic stress views, with the patient under general anesthesia. The decision to perform fluoroscopic evaluation of stability is dependent on the imaging findings and the surgeon’s discretion. In the authors’ opinion, the radiologist should describe the size and approximate percentage of the acetabular fracture if even a tiny or small fracture is present.

Figure 8a. **(a)** Drawing shows the typical appearance of posterior hip dislocation without acetabular fracture. **(b, c)** Posterior dislocation with insignificant acetabular fracture in a 19-year-old man (same patient as in Fig 5) with a basketball injury who struck the foot of another player with the knee in extension. Frontal radiograph of the right hip **(b)** demonstrates a posterior dislocation. There is a tiny posterior acetabular fracture (arrow) seen on the radiograph and on the subsequent postreduction CT image **(c)**, which does not require fixation and is considered an insignificant acetabular injury.

Figure 8b. **(a)** Drawing shows the typical appearance of posterior hip dislocation without acetabular fracture. **(b, c)** Posterior dislocation with insignificant acetabular fracture in a 19-year-old man (same patient as in Fig 5) with a basketball injury who struck the foot of another player with the knee in extension. Frontal radiograph of the right hip **(b)** demonstrates a posterior dislocation. There is a tiny posterior acetabular fracture (arrow) seen on the radiograph and on the subsequent postreduction CT image **(c)**, which does not require fixation and is considered an insignificant acetabular injury.

Figure 8c. **(a)** Drawing shows the typical appearance of posterior hip dislocation without acetabular fracture. **(b, c)** Posterior dislocation with insignificant acetabular fracture in a 19-year-old man (same patient as in Fig 5) with a basketball injury who struck the foot of another player with the knee in extension. Frontal radiograph of the right hip **(b)** demonstrates a posterior dislocation. There is a tiny posterior acetabular fracture (arrow) seen on the radiograph and on the subsequent postreduction CT image **(c)**, which does not require fixation and is considered an insignificant acetabular injury.

Posterior Dislocation Not Initially Reducible

If the surgeon is unable to initially reduce a posterior dislocation (Levin 2), then urgent CT and definitive surgical management should be undertaken. Multiple attempts at closed reduction should not be performed, because of the risk for iatrogenic femoral neck fracture. The radiologist can usually deduce the presence of an irreducible dislocation if initial radiographs show a dislocation and subsequent CT images show a persistent dislocation, as the orthopedist will almost always attempt to reduce the fracture before CT. In these patients, careful scrutiny of the CT images following attempted reduction is especially important to evaluate for intra-articular osseous fragments that widen the joint. Other reasons for failure of closed reduction that may not be evident at CT include a displaced labral tear and herniation (buttonholing) of the femoral head through a traumatic tear of the capsule (48,49).

Posterior Dislocation with a Nonconcentric Joint after Reduction

The presence of any perceptible incongruity of the joint at postreduction imaging (Levin 3) (Fig 9) requires further surgical management. It is imperative for the radiologist to carefully evaluate the patient for any asymmetry in the joint space after reduction and, if asymmetry is present, to search for intra-articular osteochondral fragments by using both bone and soft-tissue windows. An additional reason for an incongruent joint after reduction is a displaced labral tear, which is not typically evident at CT. As the adequacy of reduction determines the long-term outcome (50), management of an asymmetric joint is with either arthrotomy (open surgery) or arthroscopy (51–53) to remove the fracture fragments, fix the labral tear if necessary, and attain a perfectly congruent hip joint. MR imaging may be helpful in evaluating the patient for chondral fragments or a displaced labral tear.

Figure 9a. **(a)** Drawing shows posterior dislocation with a nonconcentric joint after reduction (Levin 3). **(b–d)** Posterior hip dislocation in a 19-year-old woman who was in a car-versus-tree motor vehicle crash. **(b)** Initial frontal radiograph demonstrates the dislocation. **(c)** Postreduction radiograph shows anatomic reduction but subtle widening (yellow arrows) of the medial joint space. **(d)** Postreduction axial CT image demonstrates osseous fracture fragments (red arrows) in the joint space, which necessitated surgical removal.

Figure 9b. **(a)** Drawing shows posterior dislocation with a nonconcentric joint after reduction (Levin 3). **(b–d)** Posterior hip dislocation in a 19-year-old woman who was in a car-versus-tree motor vehicle crash. **(b)** Initial frontal radiograph demonstrates the dislocation. **(c)** Postreduction radiograph shows anatomic reduction but subtle widening (yellow arrows) of the medial joint space. **(d)** Postreduction axial CT image demonstrates osseous fracture fragments (red arrows) in the joint space, which necessitated surgical removal.

Figure 9c. **(a)** Drawing shows posterior dislocation with a nonconcentric joint after reduction (Levin 3). **(b–d)** Posterior hip dislocation in a 19-year-old woman who was in a car-versus-tree motor vehicle crash. **(b)** Initial frontal radiograph demonstrates the dislocation. **(c)** Postreduction radiograph shows anatomic reduction but subtle widening (yellow arrows) of the medial joint space. **(d)** Postreduction axial CT image demonstrates osseous fracture fragments (red arrows) in the joint space, which necessitated surgical removal.

Figure 9d. **(a)** Drawing shows posterior dislocation with a nonconcentric joint after reduction (Levin 3). **(b–d)** Posterior hip dislocation in a 19-year-old woman who was in a car-versus-tree motor vehicle crash. **(b)** Initial frontal radiograph demonstrates the dislocation. **(c)** Postreduction radiograph shows anatomic reduction but subtle widening (yellow arrows) of the medial joint space. **(d)** Postreduction axial CT image demonstrates osseous fracture fragments (red arrows) in the joint space, which necessitated surgical removal.

Posterior Dislocation with Acetabular Fracture Requiring Fixation

A posterior dislocation with an unstable acetabular fracture (Levin 4; Thompson-Epstein 2, 3, or 4) requires definitive surgical management after closed reduction, typically involving open reduction and internal fixation of the acetabular fracture. Following closed reduction, skeletal traction is often used to maintain the reduction, in anticipation of definitive surgical management.

Three common patterns of unstable acetabular fracture are a single dominant posterior acetabular wall fracture fragment (Thompson-Epstein 2) (Fig 10), a comminuted posterior acetabular wall fracture (Thompson-Epstein 3) (Fig 11), and a medial acetabular wall/floor fracture (Thompson-Epstein 4) (Fig 12). An additional fracture pattern that the radiologist should recognize is marginal impaction of the acetabulum (Fig 13), which describes rotation and impaction of the subchondral bone into the underlying cancellous bone (54,55). Marginal impaction is typically treated with elevation and bone grafting and, when unidentified at imaging or surgery, can lead to imperfect reduction and an increased occurrence of osteoarthritis. Three-dimensional reformation of CT scans may be helpful in assessing fracture morphology and percentage loss of the posterior acetabular wall (56). Acetabular injury is not limited to the articular surface, as plastic deformation and impaction of the extra-articular retroacetabular surface have also been described, which may prevent anatomic reduction if left untreated (57).

Figure 10a. **(a)** Drawing shows a posterior dislocation and a single dominant posterior acetabular wall fracture fragment (Thompson-Epstein 2) requiring fixation. **(b, c)** Initial pelvic radiograph **(b)** and postreduction axial CT image **(c)** in a 27-year-old man in a car-versus-tree motor vehicle crash show a posterior dislocation with a dominant posterior acetabular fracture fragment (arrow).

Figure 10b. **(a)** Drawing shows a posterior dislocation and a single dominant posterior acetabular wall fracture fragment (Thompson-Epstein 2) requiring fixation. **(b, c)** Initial pelvic radiograph **(b)** and postreduction axial CT image **(c)** in a 27-year-old man in a car-versus-tree motor vehicle crash show a posterior dislocation with a dominant posterior acetabular fracture fragment (arrow).

Figure 10c. **(a)** Drawing shows a posterior dislocation and a single dominant posterior acetabular wall fracture fragment (Thompson-Epstein 2) requiring fixation. **(b, c)** Initial pelvic radiograph **(b)** and postreduction axial CT image **(c)** in a 27-year-old man in a car-versus-tree motor vehicle crash show a posterior dislocation with a dominant posterior acetabular fracture fragment (arrow).

Figure 11a. **(a)** Drawing shows a posterior dislocation and a comminuted posterior acetabular wall fracture (Thompson-Epstein 3). **(b, c)** Posterior dislocation and fracture of the left hip in a 24-year-old man who was tackled while playing football. **(b)** Initial pelvic radiograph demonstrates a cortical irregularity (arrows) of the acetabulum. **(c)** Coronal CT image after attempted reduction shows a comminuted posterior acetabular wall fracture with fracture fragments (arrows) superior to and within the joint.

Figure 11b. **(a)** Drawing shows a posterior dislocation and a comminuted posterior acetabular wall fracture (Thompson-Epstein 3). **(b, c)** Posterior dislocation and fracture of the left hip in a 24-year-old man who was tackled while playing football. **(b)** Initial pelvic radiograph demonstrates a cortical irregularity (arrows) of the acetabulum. **(c)** Coronal CT image after attempted reduction shows a comminuted posterior acetabular wall fracture with fracture fragments (arrows) superior to and within the joint.

Figure 11c. **(a)** Drawing shows a posterior dislocation and a comminuted posterior acetabular wall fracture (Thompson-Epstein 3). **(b, c)** Posterior dislocation and fracture of the left hip in a 24-year-old man who was tackled while playing football. **(b)** Initial pelvic radiograph demonstrates a cortical irregularity (arrows) of the acetabulum. **(c)** Coronal CT image after attempted reduction shows a comminuted posterior acetabular wall fracture with fracture fragments (arrows) superior to and within the joint.

Figure 12a. **(a)** Drawing shows a posterior dislocation and a medial acetabular wall and floor fracture (Thompson-Epstein 4). **(b)** Initial pelvic radiograph in a 59-year-old man who tripped on wires and fell from standing, landing on his right side, demonstrates a fracture through the entire medial acetabular wall (arrow). **(c)** Posterior dislocation is more clearly evident on a lateral radiograph, which demonstrates an empty acetabular fossa (arrows) and posterior dislocation of the femoral head. **(d)** Axial postreduction CT image demonstrates comminution of the medial acetabulum (arrows).

Figure 12b. **(a)** Drawing shows a posterior dislocation and a medial acetabular wall and floor fracture (Thompson-Epstein 4). **(b)** Initial pelvic radiograph in a 59-year-old man who tripped on wires and fell from standing, landing on his right side, demonstrates a fracture through the entire medial acetabular wall (arrow). **(c)** Posterior dislocation is more clearly evident on a lateral radiograph, which demonstrates an empty acetabular fossa (arrows) and posterior dislocation of the femoral head. **(d)** Axial postreduction CT image demonstrates comminution of the medial acetabulum (arrows).

Figure 12c. **(a)** Drawing shows a posterior dislocation and a medial acetabular wall and floor fracture (Thompson-Epstein 4). **(b)** Initial pelvic radiograph in a 59-year-old man who tripped on wires and fell from standing, landing on his right side, demonstrates a fracture through the entire medial acetabular wall (arrow). **(c)** Posterior dislocation is more clearly evident on a lateral radiograph, which demonstrates an empty acetabular fossa (arrows) and posterior dislocation of the femoral head. **(d)** Axial postreduction CT image demonstrates comminution of the medial acetabulum (arrows).

Figure 12d. **(a)** Drawing shows a posterior dislocation and a medial acetabular wall and floor fracture (Thompson-Epstein 4). **(b)** Initial pelvic radiograph in a 59-year-old man who tripped on wires and fell from standing, landing on his right side, demonstrates a fracture through the entire medial acetabular wall (arrow). **(c)** Posterior dislocation is more clearly evident on a lateral radiograph, which demonstrates an empty acetabular fossa (arrows) and posterior dislocation of the femoral head. **(d)** Axial postreduction CT image demonstrates comminution of the medial acetabulum (arrows).

Figure 13a. Marginal impaction in a 25-year-old man who was in a motor vehicle crash. **(a)** Axial postreduction CT image demonstrates marginal impaction (yellow arrows) of the inferior posterior wall, where subchondral sclerosis (red arrow) represents impacted trabeculae. **(b)** More superior CT image also demonstrates a large posterior wall fracture (blue arrow). In addition to fixation of the large posterior wall fracture, the marginal impaction was treated with loosening of the impacted fragments with an osteotome and elevation with bone cement.

Figure 13b. Marginal impaction in a 25-year-old man who was in a motor vehicle crash. **(a)** Axial postreduction CT image demonstrates marginal impaction (yellow arrows) of the inferior posterior wall, where subchondral sclerosis (red arrow) represents impacted trabeculae. **(b)** More superior CT image also demonstrates a large posterior wall fracture (blue arrow). In addition to fixation of the large posterior wall fracture, the marginal impaction was treated with loosening of the impacted fragments with an osteotome and elevation with bone cement.

Posterior Dislocation with Femoral Head Fracture

The management of posterior hip dislocation with femoral head fracture (Levin 5, Thompson-Epstein 5) is dependent on fracture involvement of the weight-bearing dome of the femoral head. If the fracture does not involve the weight-bearing dome, then surgical treatment is not required in all cases. The fovea capitis, the focal concavity of the medial femoral head where the ligamentum teres originates, is the anatomic landmark that demarcates the weight-bearing dome (above the fovea capitis) from the non–weight-bearing portion (below the fovea capitis) of the femoral head. The Pipkin classification (42) (Fig 14) is often used to characterize the location of the fracture as inferior to the fovea (Pipkin 1) (Fig 15) or involving the weight-bearing dome (Pipkin 2) (Fig 16). The Pipkin 3 and 4 classifications are less clinically useful because involvement of the weight-bearing dome is ambiguous. A Pipkin 3 fracture (Fig 17) is a femoral head fracture (either Pipkin 1 or 2) with a concomitant femoral neck fracture. A Pipkin 4 fracture (Fig 18) is a femoral head fracture (Pipkin 1, 2, or 3) with a concomitant acetabular fracture.

Figure 14. Summary of the Pipkin classification of femoral head fractures.

Figure 15a. **(a)** Drawing shows a Pipkin 1 fracture. Arrow = fovea. **(b)** Pipkin 1 fracture in a 17-year-old female adolescent who was in an motor vehicle crash. Coronal CT image demonstrates a small fracture (arrow) of the right anteroinferomedial femoral head, located inferior to the fovea (which is positioned more posteriorly than in a).

Figure 15b. **(a)** Drawing shows a Pipkin 1 fracture. Arrow = fovea. **(b)** Pipkin 1 fracture in a 17-year-old female adolescent who was in an motor vehicle crash. Coronal CT image demonstrates a small fracture (arrow) of the right anteroinferomedial femoral head, located inferior to the fovea (which is positioned more posteriorly than in a).

Figure 16a. **(a)** Drawing shows a Pipkin 2 fracture. **(b, c)** Pipkin 2 fracture in a 27-year-old man who was tackled on his left side while playing football. Coronal **(b)** and axial **(c)** CT images show a fracture of the femoral head above the level of the fovea capitis (arrow in b). The hip was initially posteriorly dislocated but reduced at the time of CT.

Figure 16b. **(a)** Drawing shows a Pipkin 2 fracture. **(b, c)** Pipkin 2 fracture in a 27-year-old man who was tackled on his left side while playing football. Coronal **(b)** and axial **(c)** CT images show a fracture of the femoral head above the level of the fovea capitis (arrow in b). The hip was initially posteriorly dislocated but reduced at the time of CT.

Figure 16c. **(a)** Drawing shows a Pipkin 2 fracture. **(b, c)** Pipkin 2 fracture in a 27-year-old man who was tackled on his left side while playing football. Coronal **(b)** and axial **(c)** CT images show a fracture of the femoral head above the level of the fovea capitis (arrow in b). The hip was initially posteriorly dislocated but reduced at the time of CT.

Figure 17a. **(a)** Drawing shows a Pipkin 3 fracture. **(b, c)** Pipkin 3 fracture in a 52-year-old man who was in a motorcycle-versus-automobile crash. Coronal **(b)** and axial **(c)** CT images demonstrate a highly comminuted fracture and a posterior dislocation of the femoral head (yellow arrows), with a fracture (red arrow in b) of the femoral neck.

Figure 17b. **(a)** Drawing shows a Pipkin 3 fracture. **(b, c)** Pipkin 3 fracture in a 52-year-old man who was in a motorcycle-versus-automobile crash. Coronal **(b)** and axial **(c)** CT images demonstrate a highly comminuted fracture and a posterior dislocation of the femoral head (yellow arrows), with a fracture (red arrow in b) of the femoral neck.

Figure 17c. **(a)** Drawing shows a Pipkin 3 fracture. **(b, c)** Pipkin 3 fracture in a 52-year-old man who was in a motorcycle-versus-automobile crash. Coronal **(b)** and axial **(c)** CT images demonstrate a highly comminuted fracture and a posterior dislocation of the femoral head (yellow arrows), with a fracture (red arrow in b) of the femoral neck.

Figure 18a. **(a)** Drawing shows a Pipkin 4 fracture. **(b, c)** Coronal **(b)** and axial **(c)** CT images in a 24-year-old man in an motor vehicle crash, who swerved to avoid a truck but hit a tree, demonstrate a fracture of the inferior femoral head (yellow arrow in b) and a fracture of the posterior acetabulum (red arrow). Note the tiny intra-articular fragment seen on the coronal image (blue arrow in b).

Figure 18b. **(a)** Drawing shows a Pipkin 4 fracture. **(b, c)** Coronal **(b)** and axial **(c)** CT images in a 24-year-old man in an motor vehicle crash, who swerved to avoid a truck but hit a tree, demonstrate a fracture of the inferior femoral head (yellow arrow in b) and a fracture of the posterior acetabulum (red arrow). Note the tiny intra-articular fragment seen on the coronal image (blue arrow in b).

Figure 18c. **(a)** Drawing shows a Pipkin 4 fracture. **(b, c)** Coronal **(b)** and axial **(c)** CT images in a 24-year-old man in an motor vehicle crash, who swerved to avoid a truck but hit a tree, demonstrate a fracture of the inferior femoral head (yellow arrow in b) and a fracture of the posterior acetabulum (red arrow). Note the tiny intra-articular fragment seen on the coronal image (blue arrow in b).

The main goal in treatment of femoral head fractures is anatomic reduction of the fracture, particularly in injuries that extend into the weight-bearing portion of the femoral head (22). However, these are rare injuries and the optimal management of femoral head fractures is controversial owing to the small number of cases seen at any one institution (58). Nonsurgical management may be appropriate for nondisplaced fractures not involving the weight-bearing dome (Pipkin 1; below the fovea capitis), although surgical treatment options can include excision of fracture fragments, open reduction internal fixation of the femoral head, and arthroplasty (59). The prognosis for complete functional recovery for a Pipkin 1 or 2 injury is usually good, for a Pipkin 4 injury it is dependent on the size and morphology of the acetabular fracture, and for Pipkin 3 injuries it is generally poor (60).

An important pattern of femoral head injury not included in the Pipkin classification but nonetheless essential for the radiologist to describe is impaction injury to the femoral head (61,62). This spectrum of injury can be considered analogous to a Hill-Sachs lesion of the shoulder and may range from cartilage impaction without separation of the fracture fragment to subchondral signal changes only evident at MR imaging (35). The size, severity, and location of such an impaction injury leads to highly variable management options ranging from conservative management (Fig 19) to arthroplasty (Fig 20).

Figure 19a. Femoral head impaction injury in a 57-year-old woman who was in a car that crashed into a tree. **(a)** Frontal pelvic radiograph demonstrates a reduced dislocation with a posterior acetabular fracture (yellow arrow). There is subtle flattening (red arrow) of the inferomedial femoral head. **(b)** Coronal CT image allows confirmation of the impaction injury of the inferomedial femoral head (red arrow). The patient was treated surgically for the acetabular fracture, but if the femoral head impaction were an isolated injury it likely would have been treated conservatively.

Figure 19b. Femoral head impaction injury in a 57-year-old woman who was in a car that crashed into a tree. **(a)** Frontal pelvic radiograph demonstrates a reduced dislocation with a posterior acetabular fracture (yellow arrow). There is subtle flattening (red arrow) of the inferomedial femoral head. **(b)** Coronal CT image allows confirmation of the impaction injury of the inferomedial femoral head (red arrow). The patient was treated surgically for the acetabular fracture, but if the femoral head impaction were an isolated injury it likely would have been treated conservatively.

Figure 20a. Femoral head impaction injury requiring arthroplasty in a 65-year-old woman who tripped and fell from a standing height. **(a)** Frontal pelvic radiograph demonstrates posterior subluxation of the right femoral head and a posterior acetabular fracture, which is difficult to characterize at radiography. **(b)** Coronal CT image better demonstrates the posterior acetabular fracture (yellow arrow). **(c)** Sagittal CT image shows that the femoral head is perched onto the posterior acetabulum with impaction (red arrow) of the articular surface of the femoral head. **(d)** Surgical specimen after resection of the femoral head demonstrates extensive impaction injury (arrows). **(e)** Radiograph shows the acetabular reconstruction and total hip arthroplasty.

Figure 20b. Femoral head impaction injury requiring arthroplasty in a 65-year-old woman who tripped and fell from a standing height. **(a)** Frontal pelvic radiograph demonstrates posterior subluxation of the right femoral head and a posterior acetabular fracture, which is difficult to characterize at radiography. **(b)** Coronal CT image better demonstrates the posterior acetabular fracture (yellow arrow). **(c)** Sagittal CT image shows that the femoral head is perched onto the posterior acetabulum with impaction (red arrow) of the articular surface of the femoral head. **(d)** Surgical specimen after resection of the femoral head demonstrates extensive impaction injury (arrows). **(e)** Radiograph shows the acetabular reconstruction and total hip arthroplasty.

Figure 20c. Femoral head impaction injury requiring arthroplasty in a 65-year-old woman who tripped and fell from a standing height. **(a)** Frontal pelvic radiograph demonstrates posterior subluxation of the right femoral head and a posterior acetabular fracture, which is difficult to characterize at radiography. **(b)** Coronal CT image better demonstrates the posterior acetabular fracture (yellow arrow). **(c)** Sagittal CT image shows that the femoral head is perched onto the posterior acetabulum with impaction (red arrow) of the articular surface of the femoral head. **(d)** Surgical specimen after resection of the femoral head demonstrates extensive impaction injury (arrows). **(e)** Radiograph shows the acetabular reconstruction and total hip arthroplasty.

Figure 20d. Femoral head impaction injury requiring arthroplasty in a 65-year-old woman who tripped and fell from a standing height. **(a)** Frontal pelvic radiograph demonstrates posterior subluxation of the right femoral head and a posterior acetabular fracture, which is difficult to characterize at radiography. **(b)** Coronal CT image better demonstrates the posterior acetabular fracture (yellow arrow). **(c)** Sagittal CT image shows that the femoral head is perched onto the posterior acetabulum with impaction (red arrow) of the articular surface of the femoral head. **(d)** Surgical specimen after resection of the femoral head demonstrates extensive impaction injury (arrows). **(e)** Radiograph shows the acetabular reconstruction and total hip arthroplasty.

Figure 20e. Femoral head impaction injury requiring arthroplasty in a 65-year-old woman who tripped and fell from a standing height. **(a)** Frontal pelvic radiograph demonstrates posterior subluxation of the right femoral head and a posterior acetabular fracture, which is difficult to characterize at radiography. **(b)** Coronal CT image better demonstrates the posterior acetabular fracture (yellow arrow). **(c)** Sagittal CT image shows that the femoral head is perched onto the posterior acetabulum with impaction (red arrow) of the articular surface of the femoral head. **(d)** Surgical specimen after resection of the femoral head demonstrates extensive impaction injury (arrows). **(e)** Radiograph shows the acetabular reconstruction and total hip arthroplasty.

Classification of Anterior Dislocation

Anterior hip dislocations (Figs 21–23) are much less common than posterior hip dislocations, accounting for approximately 10% of hip dislocations (63). Additional osseous injuries commonly include impaction injuries of the femoral head in 35% of cases (29). Anterior hip dislocations can be inferior or superior. Anterior inferior hip dislocation (obturator dislocation) (Figs 21, 22) is by far the most common direction of anterior dislocation. Anterior superior hip dislocation (Fig 23) is rare, with only a few case reports in the literature (23,28), and is also called pubic or iliac dislocation, depending on the position of the femoral head. As previously discussed, anterior superior hip dislocation may mimic posterior dislocation on a frontal radiograph, except that the hip is not internally rotated, so that the lesser trochanter remains clearly visible.

Figure 21a. Anterior hip dislocation without fracture in a 25-year-old woman (same patient as in Fig 6b) who was on a sled that crashed into a tree. **(a)** Initial pelvic radiograph demonstrates an anterior inferior (obturator) dislocation, with a characteristic inferomedial position (yellow arrow) of the femoral head, and prominence of the lesser trochanter (red arrow) due to external rotation. The right femoral head appears subtly larger than the normally located left femoral head owing to its position closer to the x-ray source. **(b)** Postreduction CT image shows anatomic reduction with a congruent joint. Note the small focus of gas (blue arrow) abutting the anterolateral aspect of the femoral head. The presence of intra-articular gas in the setting of hip trauma implies a prior dislocation.

Figure 21b. Anterior hip dislocation without fracture in a 25-year-old woman (same patient as in Fig 6b) who was on a sled that crashed into a tree. **(a)** Initial pelvic radiograph demonstrates an anterior inferior (obturator) dislocation, with a characteristic inferomedial position (yellow arrow) of the femoral head, and prominence of the lesser trochanter (red arrow) due to external rotation. The right femoral head appears subtly larger than the normally located left femoral head owing to its position closer to the x-ray source. **(b)** Postreduction CT image shows anatomic reduction with a congruent joint. Note the small focus of gas (blue arrow) abutting the anterolateral aspect of the femoral head. The presence of intra-articular gas in the setting of hip trauma implies a prior dislocation.

Figure 22a. Anterior hip dislocation with associated fractures. **(a)** Initial pelvic radiograph in a 29-year-old woman who fell from a first-floor window shows an anterior inferior (obturator) dislocation of the left hip (yellow arrow) and marked widening of the pubic symphysis (red arrow), in keeping with an anterior-posterior compression pelvic injury. **(b, c)** Subsequent postreduction axial **(b)**and coronal **(c)** CT images demonstrate right pubic and left acetabular fractures (green arrows in b) and a vertically oriented sacral fracture (blue arrows in c).

Figure 22b. Anterior hip dislocation with associated fractures. **(a)** Initial pelvic radiograph in a 29-year-old woman who fell from a first-floor window shows an anterior inferior (obturator) dislocation of the left hip (yellow arrow) and marked widening of the pubic symphysis (red arrow), in keeping with an anterior-posterior compression pelvic injury. **(b, c)** Subsequent postreduction axial **(b)**and coronal **(c)** CT images demonstrate right pubic and left acetabular fractures (green arrows in b) and a vertically oriented sacral fracture (blue arrows in c).

Figure 22c. Anterior hip dislocation with associated fractures. **(a)** Initial pelvic radiograph in a 29-year-old woman who fell from a first-floor window shows an anterior inferior (obturator) dislocation of the left hip (yellow arrow) and marked widening of the pubic symphysis (red arrow), in keeping with an anterior-posterior compression pelvic injury. **(b, c)** Subsequent postreduction axial **(b)**and coronal **(c)** CT images demonstrate right pubic and left acetabular fractures (green arrows in b) and a vertically oriented sacral fracture (blue arrows in c).

Figure 23a. Anterior superior hip dislocation in a 45-year-old woman who was in a car struck by a semitrailer in a motor vehicle crash. **(a)** Frontal projection CT topogram demonstrates the anterior superior dislocation, where the femoral head (yellow arrows) is dislocated superiorly. In contrast to a posterior dislocation, the femur is in external rotation, and the lesser trochanter (red arrow) is clearly visible. **(b)** Axial CT image demonstrates the anterior direction of the dislocation. **(c)** Three-dimensional volume-rendered reconstruction in a sagittal orientation shows the anterior superior dislocation of the femoral head (yellow arrow) and a displaced fracture of the greater trochanter (blue arrow).

Figure 23b. Anterior superior hip dislocation in a 45-year-old woman who was in a car struck by a semitrailer in a motor vehicle crash. **(a)** Frontal projection CT topogram demonstrates the anterior superior dislocation, where the femoral head (yellow arrows) is dislocated superiorly. In contrast to a posterior dislocation, the femur is in external rotation, and the lesser trochanter (red arrow) is clearly visible. **(b)** Axial CT image demonstrates the anterior direction of the dislocation. **(c)** Three-dimensional volume-rendered reconstruction in a sagittal orientation shows the anterior superior dislocation of the femoral head (yellow arrow) and a displaced fracture of the greater trochanter (blue arrow).

Figure 23c. Anterior superior hip dislocation in a 45-year-old woman who was in a car struck by a semitrailer in a motor vehicle crash. **(a)** Frontal projection CT topogram demonstrates the anterior superior dislocation, where the femoral head (yellow arrows) is dislocated superiorly. In contrast to a posterior dislocation, the femur is in external rotation, and the lesser trochanter (red arrow) is clearly visible. **(b)** Axial CT image demonstrates the anterior direction of the dislocation. **(c)** Three-dimensional volume-rendered reconstruction in a sagittal orientation shows the anterior superior dislocation of the femoral head (yellow arrow) and a displaced fracture of the greater trochanter (blue arrow).

Although a classification of anterior dislocation has been described by Epstein, this classification scheme is not in widespread use given the relative rarity of these injuries and the fact that associated fractures are uncommon.

Central Dislocation: A Misnomer

The term central dislocation is considered an outdated phrase that is no longer relevant to the classification of hip injuries (13). A central dislocation (Fig 24) refers to medial displacement of the femoral head due to a displaced acetabular fracture. In these cases, it is best to describe the acetabular fracture and the femoral head as protruding medially into the pelvis.

Figure 24a. Medial acetabular fracture with medial displacement of the femur in a 75-year-old man who tripped and fell from standing. **(a)** Pelvic radiograph demonstrates medial displacement of the femoral head (yellow arrows) into the pelvic cavity due to a displaced medial acetabular fracture. **(b)** Coronal CT image demonstrates the articular surface gap (red arrow) of the acetabular roof and medial displacement of the femoral head. This pattern of injury is not considered a hip dislocation, and the description of “central dislocation” is no longer considered appropriate or useful.

Figure 24b. Medial acetabular fracture with medial displacement of the femur in a 75-year-old man who tripped and fell from standing. **(a)** Pelvic radiograph demonstrates medial displacement of the femoral head (yellow arrows) into the pelvic cavity due to a displaced medial acetabular fracture. **(b)** Coronal CT image demonstrates the articular surface gap (red arrow) of the acetabular roof and medial displacement of the femoral head. This pattern of injury is not considered a hip dislocation, and the description of “central dislocation” is no longer considered appropriate or useful.

Pitfalls in Imaging Hip Dislocations

There are several circumstances in which the clinical and imaging signs of hip dislocation may be subtle. Two potential pitfalls in the imaging of hip dislocations are hip dislocations that do not exhibit the typical varus angulation and internal rotation (Fig 25) and hip dislocations that have spontaneously reduced before imaging (Fig 26). Cases of hip dislocation where the femur does not exhibit the typical varus angulation and internal rotation can be difficult to diagnose, as findings on the initial pelvic radiographs can appear nearly normal at first glance. However, closer scrutiny may reveal loss of the joint space on the affected side, typically with the top of the femoral head projecting superior to the acetabular roof.

Figure 25a. Pitfall in imaging of hip dislocation: Posterior hip dislocation without typical varus angulation and internal rotation in a 46-year-old man who flipped over the handlebars while riding an off-road motorcycle. **(a)** Initial pelvic radiograph demonstrates absence of the left hip joint space, with the superior aspect of the femoral head overlapping the acetabular roof (yellow arrows), disruption of the Shenton line (dashed lines; compare with the right side that demonstrates a continuous Shenton line), and an acetabular fracture fragment projecting superior to the hip (red arrow). In addition, the left femoral head appears smaller than the right femoral head owing to magnification effect. **(b)** Axial CT image allows confirmation of the posterior dislocation with a fat-fluid level (green arrow) in the acetabular fossa.

Figure 25b. Pitfall in imaging of hip dislocation: Posterior hip dislocation without typical varus angulation and internal rotation in a 46-year-old man who flipped over the handlebars while riding an off-road motorcycle. **(a)** Initial pelvic radiograph demonstrates absence of the left hip joint space, with the superior aspect of the femoral head overlapping the acetabular roof (yellow arrows), disruption of the Shenton line (dashed lines; compare with the right side that demonstrates a continuous Shenton line), and an acetabular fracture fragment projecting superior to the hip (red arrow). In addition, the left femoral head appears smaller than the right femoral head owing to magnification effect. **(b)** Axial CT image allows confirmation of the posterior dislocation with a fat-fluid level (green arrow) in the acetabular fossa.

Figure 26a. Pitfall in imaging of hip dislocation: Spontaneously reduced dislocation in a 34-year-old man who was the unrestrained driver of a truck in a motor vehicle collision. **(a)** Initial coronal CT image demonstrates a small osteochondral fracture (yellow arrow) of the medial right femoral head just above the level of the fovea. **(b)** Axial CT image demonstrates a fracture (red arrow) of the right posterior acetabulum. The findings are consistent with a Pipkin 4, Levin 5 fracture dislocation that has spontaneously reduced. Surgical fixation of the acetabulum was performed because of instability of the hip.

Figure 26b. Pitfall in imaging of hip dislocation: Spontaneously reduced dislocation in a 34-year-old man who was the unrestrained driver of a truck in a motor vehicle collision. **(a)** Initial coronal CT image demonstrates a small osteochondral fracture (yellow arrow) of the medial right femoral head just above the level of the fovea. **(b)** Axial CT image demonstrates a fracture (red arrow) of the right posterior acetabulum. The findings are consistent with a Pipkin 4, Levin 5 fracture dislocation that has spontaneously reduced. Surgical fixation of the acetabulum was performed because of instability of the hip.

It is important to recognize a spontaneously reduced hip dislocation, as the treatment algorithm is identical to a hip dislocation reduced by the orthopedist. The presence of an intracapsular gas bubble in the hip joint (Fig 21) (in the absence of penetrating trauma or joint aspiration) is considered a reliable indicator of hip dislocation, thought to be due to release of intracapsular nitrogen produced by forcible distraction and resulting vacuum phenomenon (64). Another clue is an isolated posterior acetabular wall fracture. In these cases, it is especially important to carefully scrutinize the joint space for any asymmetry or intra-articular debris, which may warrant surgery. If there is debris in the weight-bearing portion of the hip joint, many orthopedic surgeons will place the patient in skeletal traction to prevent any further chondral injury while awaiting definitive management in the operating room.

As previously discussed, another pitfall is the rare anterior superior hip dislocation, where the femoral head is positioned superior and lateral to the acetabulum, thereby mimicking a posterior dislocation on a frontal radiograph of the pelvis. The appearance of the lesser trochanter may be the only clue to differentiate between these two directions of dislocation. The lesser trochanter in an anterior superior hip dislocation remains clearly visible, whereas the lesser trochanter is usually obscured in a posterior hip dislocation owing to internal rotation of the femur. This distinction is important, as the reduction maneuver for an anterior dislocation differs from that for a posterior dislocation.

Associated Osseous Injuries

In addition to fractures of the femoral head and acetabulum, proximal femur fractures associated with hip dislocation (Fig 27) include femoral neck and greater trochanteric fractures. Femoral neck fractures greatly increase the risk for avascular necrosis, and closed reduction is contraindicated in the presence of a femoral neck fracture owing to the risk of displacement. In addition to fractures of and around the hip, radiologists should be aware of a high prevalence of knee injuries associated with posterior hip dislocation due to the mechanism of force transmission through the flexed knee. Up to a 93% prevalence of ipsilateral knee injuries has been reported at MR imaging; these include effusion, contusion, and meniscal tears (65).

Figure 27a. Proximal femur fractures in two patients with hip dislocation. **(a)** Right femoral neck fracture in a 52-year-old man who was in a motorcycle-versus–sport utility vehicle crash. Frontal radiograph of the right hip demonstrates a posterior hip dislocation with a large posterior acetabular fracture (yellow arrow). There is a mildly displaced femoral neck fracture (red arrow) through the lesser trochanter. **(b)** Greater trochanter fracture in a 45-year-old woman who was in a semitrailer crash. Frontal radiograph of the right hip demonstrates a posterior hip dislocation with internal rotation of the femoral head and a laterally displaced greater trochanter fracture (arrow).

Figure 27b. Proximal femur fractures in two patients with hip dislocation. **(a)** Right femoral neck fracture in a 52-year-old man who was in a motorcycle-versus–sport utility vehicle crash. Frontal radiograph of the right hip demonstrates a posterior hip dislocation with a large posterior acetabular fracture (yellow arrow). There is a mildly displaced femoral neck fracture (red arrow) through the lesser trochanter. **(b)** Greater trochanter fracture in a 45-year-old woman who was in a semitrailer crash. Frontal radiograph of the right hip demonstrates a posterior hip dislocation with internal rotation of the femoral head and a laterally displaced greater trochanter fracture (arrow).

Associated Soft-Tissue Injuries

Soft-tissue injuries associated with hip dislocation include nerve injury, labral and ligamentous tears, muscle tears, and hematomas. Nerve injuries occur in approximately 10% of hip dislocations in adults and 5% of those in children (66). The three most significant nerves around the hip are the femoral nerve anteriorly, the lateral femoral cutaneous nerve located subcutaneously medial to the anterior superior iliac spine, and the sciatic nerve posteriorly.

The sciatic nerve is the most commonly injured nerve in posterior hip dislocations. The sciatic nerve includes the L4–S3 nerve roots and divides into tibial and peroneal branches before exiting the pelvis in most patients at the greater sciatic notch anterior to the piriformis muscle (66). Of the two branches, the peroneal branch is more susceptible to traumatic injury, possibly because of its fixed tethering at the fibular neck and sciatic notch. The mechanism of acute neural injury is usually due to stretching and compression of the nerve by the posteriorly dislocated femoral head, with acute laceration significantly less common in the absence of a penetrating injury. The rate of occurrence of major sciatic nerve injury (Fig 28) is correlated with the length of time that a hip remains dislocated, with a higher rate of occurrence in patients with prolonged dislocation (67). At least partial recovery of nerve function has been shown to occur in 60%–70% of patients (67). Injuries of the femoral nerve (68) and lateral femoral cutaneous nerve due to hip dislocation are exceedingly rare. Late neurologic sequelae may be due to encasement by heterotopic ossification (69).

Figure 28a. Sciatic nerve injury in an 18-year-old man who was an unrestrained driver ejected in a motor vehicle rollover. **(a)** Initial pelvic radiograph demonstrates a right posterior hip dislocation without acetabular fracture, which was subsequently reduced. **(b)** Axial CT image (soft-tissue window) demonstrates a congruent hip joint, but there is hematoma formation posteriorly with effacement of the fat plane (yellow arrows) between the quadratus femoris and gluteus maximus muscles, along the expected course of the sciatic nerve. **(c, d)** Axial fat-suppressed T2-weighted MR image **(c)** demonstrates expansion of the right sciatic nerve with surrounding edema (red arrow). The blue arrow indicates the normal left sciatic nerve. A more inferior image **(d)** shows a tear of the obturator externus muscle (green arrows).

Figure 28b. Sciatic nerve injury in an 18-year-old man who was an unrestrained driver ejected in a motor vehicle rollover. **(a)** Initial pelvic radiograph demonstrates a right posterior hip dislocation without acetabular fracture, which was subsequently reduced. **(b)** Axial CT image (soft-tissue window) demonstrates a congruent hip joint, but there is hematoma formation posteriorly with effacement of the fat plane (yellow arrows) between the quadratus femoris and gluteus maximus muscles, along the expected course of the sciatic nerve. **(c, d)** Axial fat-suppressed T2-weighted MR image **(c)** demonstrates expansion of the right sciatic nerve with surrounding edema (red arrow). The blue arrow indicates the normal left sciatic nerve. A more inferior image **(d)** shows a tear of the obturator externus muscle (green arrows).

Figure 28c. Sciatic nerve injury in an 18-year-old man who was an unrestrained driver ejected in a motor vehicle rollover. **(a)** Initial pelvic radiograph demonstrates a right posterior hip dislocation without acetabular fracture, which was subsequently reduced. **(b)** Axial CT image (soft-tissue window) demonstrates a congruent hip joint, but there is hematoma formation posteriorly with effacement of the fat plane (yellow arrows) between the quadratus femoris and gluteus maximus muscles, along the expected course of the sciatic nerve. **(c, d)** Axial fat-suppressed T2-weighted MR image **(c)** demonstrates expansion of the right sciatic nerve with surrounding edema (red arrow). The blue arrow indicates the normal left sciatic nerve. A more inferior image **(d)** shows a tear of the obturator externus muscle (green arrows).

Figure 28d. Sciatic nerve injury in an 18-year-old man who was an unrestrained driver ejected in a motor vehicle rollover. **(a)** Initial pelvic radiograph demonstrates a right posterior hip dislocation without acetabular fracture, which was subsequently reduced. **(b)** Axial CT image (soft-tissue window) demonstrates a congruent hip joint, but there is hematoma formation posteriorly with effacement of the fat plane (yellow arrows) between the quadratus femoris and gluteus maximus muscles, along the expected course of the sciatic nerve. **(c, d)** Axial fat-suppressed T2-weighted MR image **(c)** demonstrates expansion of the right sciatic nerve with surrounding edema (red arrow). The blue arrow indicates the normal left sciatic nerve. A more inferior image **(d)** shows a tear of the obturator externus muscle (green arrows).

Long-term Complications of Hip Dislocation

The most common long-term complication of hip dislocation is posttraumatic osteoarthritis, with the rate depending on the severity of the acetabular fracture. The rate of occurrence of posttraumatic osteoarthritis ranges from 24% for simple dislocations to up to 88% in patients with complex acetabular fractures (70).

Avascular necrosis (Fig 29) is the second most common complication and is thought to be due to a combination of disruption of the blood supply at the time of dislocation, arterial vasospasm, and/or compromised venous outflow. The time to reduction directly influences the incidence of avascular necrosis, which ranges from 4.8% in hips reduced within 6 hours to 52.9% in hips reduced more than 6 hours after the injury (60). The odds ratio to develop avascular necrosis in dislocations reduced after 12 hours have elapsed is 5.6 compared with those reduced before 12 hours (71). It is important for the radiologist to be alert to subtle findings of dislocation on the AP radiograph of the pelvis obtained as part of the trauma series, as hip dislocation seen at initial trauma CT or radiography should be communicated urgently to reduce this risk of vascular compromise. The type of injury also influences the risk for avascular necrosis, with a significantly higher incidence of avascular necrosis in higher-grade injuries (60).

Figure 29a. Avascular necrosis as a late complication of posterior hip dislocation in a 38-year-old man found down with facial lacerations. **(a, b)** Initial pelvic radiograph **(a)** shows a left posterior hip dislocation without acetabular fracture. Initial attempts at reduction were unsuccessful (Levin 2), with persistent dislocation seen on an axial CT image **(b)**. There is hemorrhage in the acetabular fossa (yellow arrow in b) and a focus of intra-articular gas (red arrow in b), which is a common finding in dislocation. The hip was successfully reduced (not shown) under general anesthesia. **(c)** Follow-up radiograph 18 months after the initial injury demonstrates subchondral sclerosis, articular surface irregularity, and osteophyte formation consistent with avascular necrosis and osteoarthritic changes.

Figure 29b. Avascular necrosis as a late complication of posterior hip dislocation in a 38-year-old man found down with facial lacerations. **(a, b)** Initial pelvic radiograph **(a)** shows a left posterior hip dislocation without acetabular fracture. Initial attempts at reduction were unsuccessful (Levin 2), with persistent dislocation seen on an axial CT image **(b)**. There is hemorrhage in the acetabular fossa (yellow arrow in b) and a focus of intra-articular gas (red arrow in b), which is a common finding in dislocation. The hip was successfully reduced (not shown) under general anesthesia. **(c)** Follow-up radiograph 18 months after the initial injury demonstrates subchondral sclerosis, articular surface irregularity, and osteophyte formation consistent with avascular necrosis and osteoarthritic changes.

Figure 29c. Avascular necrosis as a late complication of posterior hip dislocation in a 38-year-old man found down with facial lacerations. **(a, b)** Initial pelvic radiograph **(a)** shows a left posterior hip dislocation without acetabular fracture. Initial attempts at reduction were unsuccessful (Levin 2), with persistent dislocation seen on an axial CT image **(b)**. There is hemorrhage in the acetabular fossa (yellow arrow in b) and a focus of intra-articular gas (red arrow in b), which is a common finding in dislocation. The hip was successfully reduced (not shown) under general anesthesia. **(c)** Follow-up radiograph 18 months after the initial injury demonstrates subchondral sclerosis, articular surface irregularity, and osteophyte formation consistent with avascular necrosis and osteoarthritic changes.

Early diagnosis of avascular necrosis is challenging. Although MR imaging is able to depict abnormal subchondral marrow signal, this finding is nonspecific and is more likely than not to be transient (37). Similarly, although single photon emission computed tomography (SPECT) with technetium 99m medronate can demonstrate a transient global decrease of femoral head blood flow in the early injury period in 10% of patients with hip dislocation, this pattern of decreased blood flow does not correlate with the incidence of avascular necrosis (72). There may be a role for contrast material–enhanced MR imaging in the prediction of avascular necrosis, although this has so far demonstrated mixed results in the setting of femoral neck fractures (73).

Heterotopic ossification may occur in up to 32% of patients with posterior dislocation and associated acetabular wall fractures, despite the routine use of indomethacin as prophylaxis (50). Heterotopic ossification associated with hip dislocation is usually low grade when it occurs, and the radiographic findings of heterotopic ossification can often be overestimated (74). Clinically significant heterotopic ossification is rare (75). Sciatic nerve entrapment by heterotopic ossification following a hip fracture dislocation has been reported (69).

Conclusion

The hip is a highly stable joint owing to its osseous, labral, ligamentous, and soft-tissue anatomy, usually requiring high-energy trauma to dislocate. Hip dislocations represent a spectrum of injuries, all of which are orthopedic emergencies with a relatively high rate of morbidity, predominantly due to posttraumatic osteoarthritis and avascular necrosis. Treatment by the orthopedic team is performed in two stages. At the initial stage, the goal is to perform rapid closed reduction of the hip. The second stage is focused on definitive management. Time from injury to reduction has been shown to be highly correlated with the risk for developing avascular necrosis. Evaluation of hip dislocation requires not only radiographic detection of subtle findings of dislocation and urgent communication to the treating physician, but also the inference of potential associated secondary osseous and soft-tissue injuries that could place the patient at risk for early osteoarthritis. The key features that guide management can be succinctly described by using a checklist approach.

Recipient of a Cum Laude award for an education exhibit at the 2016 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: Feb 15 2017
Revision requested: May 18 2017
Revision received: May 25 2017
Accepted: June 28 2017
Published online: Nov 13 2017
Published in print: Nov 2017

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Abbreviations
Abbreviation:
AP
anteroposterior

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Abbreviation:
AP	anteroposterior