Damaging the femoral head mandates destruction of its protective soft-tissue envelope first, which is most often accomplished through forceful dislocation of the hip joint. The vast majority of hip dislocations occur from high-energy motor vehicle accidents. Other mechanisms include falls, pedestrians struck by motor vehicles, industrial accidents, and athletic injuries [2].

Posterior dislocations outnumber anterior dislocations by approximately 9 to 1 [4, 6]. The typical mechanism for a posterior dislocation is a deceleration accident in which the patient’s knee strikes the dashboard with both the knee and hip flexed. By vector analysis, Letournel demonstrated that more flexion and adduction of the hip during application of a longitudinal force through the femur increases the likelihood of pure dislocation [7].

Minimal adduction or internal rotation predisposes to fracture dislocation, which may occur together with a posterior wall fracture or a shearing injury of the femoral head. As the head impacts against the posterior wall, a fragment of the femoral head remains in the acetabulum, and the intact portion of the head connected to the femoral neck dislocates posteriorly.

The concept that the position of the femoral head at impact plays a large role in the type of injury was supported by Upadhyay and colleagues, who studied the effect of femoral anteversion in patients with hip dislocations and fracture dislocations [8, 9]. They saw that decreased anteversion of the femoral neck results in a more posterior position of the femoral head, similar to internal rotation, both tending to produce pure dislocation. In contrast, increased femoral neck anteversion and less internal rotation led to fracture dislocation.

The less common anterior dislocations are a result of hyperabduction and extension. This mechanism may be present in deceleration injuries in which the occupant is in a relaxed position during impact with the legs flexed, abducted, and externally rotated. This is a typical leg position in motorcycle accidents where the legs are frequently hyperabducted. Using cadavers, Pringle et al were able to cause anterior hip dislocations by hyperabduction and external rotation [5]. The degree of hip flexion determined the type of anterior dislocation, with extension leading to a superior pubic dislocation and flexion resulting in inferior obturator dislocation.

Femoro-acetabular impingement, either from decreased femoral head-neck offset (cam- type), or a deep acetabulum (pincer type), may be a risk factor of hip dislocation [10]. Insufficiency and stress fractures of the femoral head may occur, and their mechanism is often less comprehensible than high-energy trauma. They usually occur in patients with osteopenia, but also in healthy adults starting or intensifying exercise (e.g., in military recruits). They are reported as “subchondral impaction” or “insufficiency” fractures, but represent a significant injury to the femoral head [7, 11, 12].

Patients with a hip dislocation and/or femoral head fracture typically sustain multiple injuries (including intra-abdominal, head, and chest trauma) that require inpatient management. Marymont et al showed that posterior hip dislocations may even signal thoracic aortic injuries because of abrupt deceleration [13]. Despite typical clinical findings, such as extremity deformation, the diagnosis of hip dislocation may be delayed due to life-threatening injuries.

Common accompanying skeletal injuries comprise femoral head, neck, or shaft fractures, acetabular fractures, pelvic fractures, and knee, ankle, and foot injuries. Knee injuries, including posterior dislocation, cruciate ligament injuries, and patellar fractures, are associated with posterior hip dislocations due to direct dashboard impact (Fig. 4.1).

Tabuenca et al identified major knee injuries in 46 out of 187 (25%) patients with hip dislocations and femoral head fractures [14]. Seven of these injuries were not diagnosed during the initial hospital stay. Associated injuries dictate treatment in most cases of hip dislocation. Among them, undisplaced femoral neck fractures represent a major diagnostic pitfall. High-resolution computed tomography (HRCT) with fine cuts (2 mm) is needed to rule out occult femoral neck fractures before attempting closed reduction. In case of fracture lines at the level of the femoral neck, initial internal fixation must be considered.

Similarly, associated pelvic ring fractures may prohibit counter-traction, necessitating open reduction of the dislocation. Injuries to the knee are likely to be detected by careful clinical examination and conventional radiography.

Associated fractures of the hip itself, such as acetabular wall fractures and femoral head fractures, may require surgical intervention even if the hip dislocation can be reduced in a closed fashion. Femoral head fractures or intra-articular fragments may hinder closed reduction of the hip. Acetabular wall fractures may lead to instability − even after sufficient reduction − and then require fixation. Determining hip stability in the presence of a posterior wall fracture is important.

In the scenario of interest, hip dislocations may be easily missed simply because other, potentially life-threatening injuries demand attention by the trauma surgeon in charge. Thus, no caregiver can be blamed for overseeing a hip dislocation and/or femoral head fracture in patients with multiple trauma. Whole-body MDCT has emerged as the imaging standard in most industrial countries and is likely to reveal unsuspected hip dislocations and femoral head fractures.

Still, clinical examination is valuable, and hip dislocations may occasionally be detected simply by the position of the patient’s legs. Typically, the involved leg appears shortened and excessively rotated, either externally rotated in case of anterior dislocation, or internally rotated in case of posterior dislocation. If hip dislocation is suspected, palpation of all long bones and joints (specifically the knee) of the affected extremity and the pelvis (stability testing), along with a meticulous neurologic and vascular examination, are key. Documenting pre-reduction function of the sciatic nerve is important in posterior dislocations, as the nerve can be injured by reduction. Careful testing of all branches is required. For example, impaired foot eversion may indicate peroneal branch lesions. Posterior dislocations are associated with posterior knee dislocations (posterior cruciate ligament rupture). Anterior dislocations may injure the femoral vessels, necessitating a careful assessment of distal pulses and duplex ultrasound.

The first imaging available is usually the anteroposterior (AP) pelvis radiograph . This is usually taken as part of the initial trauma workup and helps direct treatment. The diagnosis of hip dislocation should be apparent on this single radiographic view (Fig. 4.2).

The key to the diagnosis on the plain AP pelvis is the loss of congruence of the femoral head with the roof of the acetabulum. On a true AP view, the head will appear larger than the contralateral head if the dislocation is anterior, and smaller if posterior. The most common finding, in the case of a posterior dislocation, is a small head that is overlapping the roof of the acetabulum. In an anterior dislocation, the head may appear medial to or inferior to the acetabulum.

It is critical that the initial radiograph be of good quality and carefully inspected for associated injuries before a reduction is attempted. In particular, associated femoral neck fractures, which may be nondisplaced, must not be overlooked. Likewise, associated femoral head fractures are usually visible as a retained fragment in the joint (Fig. 4.3). Acetabular fractures and pelvic ring injuries are also visible on the plain AP radiograph . Additional radiographic assessment is not usually indicated before attempts at reduction unless a femoral neck fracture cannot be ruled out or there is a clinical suspicion of a femur, knee, or tibial injury that will affect the ability to use the extremity to manipulate the hip. In such cases, bi-planar radiographs of all questionable areas must be obtained.

The patient with a hip dislocation (including those with a femoral head fracture) has, in most of the cases, sustained a major trauma and will be subject to modern trauma management, which consists of an initial pan-CT-scan including angiography as a keystone of diagnostics (Fig. 4.3) [15]. Here, all relevant injuries can be detected within the first minutes of the patient’s arrival at the trauma center.

A concomitant non-displaced femoral neck fracture and other adjacent injuries can be identified and have to direct the treatment. In the case of an unreducible hip, the CT scan has to be analyzed to identify the obstacle that prevents the femoral head from moving back into the acetabulum.

After reduction, five standard views of the pelvis should be obtained. These include the ap pelvis both Judet (45° oblique) views, and an inlet and outlet of the pelvis. Evaluation of the X-rays should focus on the concentric reduction of the hip. The use of the contralateral hip is necessary to answer this question. Using the relationship of the femoral head to the acetabular roof on each view, the congruency of the hip is evaluated by comparing it to the contralateral side. Any incongruency or widening of the joint space may indicate a loose body inbetween femoral head and the acetabulum.

After reduction of the hip, a CT scan with a minimum of 2 mm cuts through the hip is the diagnostic standard. The scan is more sensitive in detecting small, intra-articular fragments, femoral head fractures, femoral head impaction injuries, acetabular fractures, and joint incongruity. Hougaard et al reported six cases of minor acetabular fractures, and six cases of retained intra-articular fragments visualized on CT and not visible on plain radiographs after closed reduction of posterior hip dislocations [16]. The congruence of the hip is also easily evaluated using CT. The head should be in the center of the subchondral ring of the acetabulum as it becomes visible, appearing as a bulls eye. Impaction injuries and femoral head fractures are much more easily seen on the post-reduction CT. The quality of the reduction of femoral head fractures is also apparent and determines treatment. Besides the importance of meticulous diagnostics, the CT scan plays a major role in planning the operative intervention, when necessary, in cases of concomitant fracture, irreducible dislocation, or incongruent reduction. The location, size, and number of free intra-articular fragments and the location, The location and size of an acetabular fracture as well as the size and location of a femoral head fragment must be identified and will affect the treatment plan.

MRI is helpful in the evaluation of a traumatic osteonecrosis of the hip. MRI changes of AVN may not be present before 6 to 8 weeks. MRI studies can also help define soft tissue injuries following hip dislocations. Apart from its predictive values of AVN in the acute setting, MRI is the optimal study for evaluation of the soft tissues such as the external rotator tendons, the labrum, and cartilage. The traumatized hip from a dislocation will likely have an effusion, which will help identify any abnormalities of the labrum or capsule.