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CATEGORY	DETAILS
RISK FACTORS	High-energy trauma (motor vehicle accidents, especially dashboard injuries), contact sports, prior hip arthroplasty, acetabular dysplasia
ETIOLOGY	Traumatic force transmitted along the femoral shaft while the hip is in a flexed and adducted position (posterior) or extended and abducted position (anterior)
PATIENT PRESENTATION	Severe hip pain with inability to bear weight or move the affected extremity following a high-energy mechanism
CLASSIC PHYSICAL EXAM	Posterior: hip is flexed, adducted, and internally rotated; limb appears shortened. Anterior: hip is extended, abducted, and externally rotated
KEY DIAGNOSTIC RESULTS	AP pelvis radiograph confirms direction of dislocation; CT scan reveals associated acetabular fractures and intra-articular fragments
MANAGEMENT	Emergent closed reduction within 6 hours to reduce risk of avascular necrosis (AVN); post-reduction CT; operative fixation if unstable or associated fractures present
KEYWORDS	"Dashboard injury," "flexed, adducted, internally rotated," "limb shortening," "posterior dislocation," "sciatic nerve palsy," "avascular necrosis"

1. Pathophysiology

The hip is an inherently stable ball-and-socket joint formed by the femoral head articulating within the deep acetabulum, reinforced by a strong fibrous capsule and surrounding musculature. Dislocation therefore requires a substantial force to overcome this stability, which is why nearly all traumatic hip dislocations result from high-energy mechanisms such as motor vehicle collisions or falls from significant height.

Posterior dislocation accounts for approximately 90% of all traumatic hip dislocations. The classic mechanism is a "dashboard injury," where a seated passenger's knee strikes the dashboard during a frontal collision. With the hip in a position of flexion and adduction, the axial force is driven posteriorly along the femoral shaft, pushing the femoral head out of the acetabulum and behind it. This explains the characteristic posture: the leg appears shortened, flexed, adducted, and internally rotated because the femoral head now sits posterior and superior to the acetabulum, pulling the limb into that position. The sciatic nerve courses directly behind the hip joint, which is why posterior dislocations carry a risk of sciatic nerve injury (reported in 10-20% of cases), most commonly affecting the peroneal division, resulting in foot drop.

Anterior dislocation is far less common (approximately 10% of cases) and occurs when a force is applied to an abducted and externally rotated hip, or from a blow to the back while the hip is extended. The femoral head displaces anteriorly and inferiorly, and the limb appears extended, abducted, and externally rotated. The femoral artery, vein, and nerve are at risk in anterior dislocations due to their anterior anatomic relationship to the hip joint, though vascular injury remains uncommon.

The most feared long-term complication of any hip dislocation is avascular necrosis (AVN) of the femoral head. The blood supply to the femoral head is tenuous, relying primarily on the medial femoral circumflex artery (a branch of the profunda femoris). Dislocation disrupts or kinks this blood supply. The risk of AVN rises dramatically when reduction is delayed beyond 6 hours, which is why hip dislocation is treated as an orthopedic emergency.

Associated acetabular fractures occur in roughly 50-70% of posterior dislocations. Loose bony fragments may become trapped within the joint, preventing concentric reduction and causing accelerated cartilage damage if not addressed.

2. Classification and Clinical Manifestation

Thompson-Epstein Classification (Posterior Hip Dislocation)

TYPE	DESCRIPTION
Type I	Simple dislocation without fracture, or with a minor posterior wall fragment
Type II	Dislocation with a single large posterior acetabular wall fragment
Type III	Dislocation with comminuted posterior wall fracture
Type IV	Dislocation with acetabular floor fracture
Type V	Dislocation with femoral head fracture

Anterior Dislocation Subtypes

SUBTYPE	POSITION OF FEMORAL HEAD	CLINICAL NOTE
Superior (pubic)	Femoral head lies near the pubic symphysis or anterior superior iliac spine	Less common; hip held in extension, abduction, external rotation
Inferior (obturator)	Femoral head lies in the obturator foramen region	More common anterior subtype; hip held in flexion, abduction, external rotation

Clinical Manifestation by Direction

FEATURE	POSTERIOR DISLOCATION	ANTERIOR DISLOCATION
Limb position	Flexed, adducted, internally rotated	Extended (or flexed in obturator type), abducted, externally rotated
Limb length	Shortened	Variable; may appear lengthened
At-risk neurovascular structure	Sciatic nerve (peroneal division)	Femoral nerve, femoral artery and vein
Associated fracture	Posterior acetabular wall, femoral head	Femoral head (indentation fracture)
Frequency	~90% of traumatic hip dislocations	~10% of traumatic hip dislocations

3. Diagnostic Workup

TEST	ROLE	KEY FINDINGS
AP pelvis radiograph	Best initial test	Femoral head position relative to acetabulum confirms dislocation and direction; assess for obvious fractures
Cross-table lateral radiograph	Supplementary initial imaging	Confirms anterior vs. posterior displacement
CT scan of the pelvis	Most accurate test (post-reduction and pre-reduction if fracture suspected)	Detects acetabular fractures, femoral head fractures, intra-articular loose bodies, and confirms concentricity of reduction
Neurovascular examination	Mandatory before and after any reduction attempt	Document sciatic nerve function (posterior) or femoral nerve function (anterior); check dorsalis pedis and posterior tibial pulses

The best initial test in any patient with suspected hip dislocation after trauma is an AP pelvis radiograph. This is obtained rapidly and will show the femoral head displaced out of the acetabulum. In a posterior dislocation, the femoral head appears superiorly and laterally displaced, and because it is closer to the film, it appears smaller than the contralateral side. In an anterior dislocation, the femoral head appears inferiorly and medially displaced and appears larger (closer to the X-ray source). An associated fracture line of the acetabulum or femoral head may also be visible.

However, plain radiographs frequently miss small acetabular wall fractures and intra-articular fragments. Therefore, a CT scan is the most accurate test and should be obtained. The timing of CT depends on clinical urgency: if no fracture is apparent on initial films and the joint is reducible, most protocols call for emergent closed reduction first, followed by a post-reduction CT scan to confirm concentricity of the reduction and identify occult fractures or loose bodies. If the initial radiograph already shows a large acetabular fracture or femoral head fracture, CT may be obtained before reduction to guide operative planning, though reduction should not be excessively delayed for imaging.

A thorough neurovascular examination must be documented before and after every reduction attempt. For posterior dislocations, test the sciatic nerve by checking ankle dorsiflexion and plantarflexion, as well as sensation along the posterior leg and foot. For anterior dislocations, assess femoral nerve function (quadriceps strength, patellar reflex) and palpate distal pulses.

MRI is not part of the acute workup but may be used weeks to months later if AVN is clinically suspected despite normal radiographs.

4. Management and Treatment

PHASE	INTERVENTION	DETAILS
Acute stabilization	Airway, breathing, circulation (trauma protocol)	Assess for polytrauma; hip dislocation often accompanies other life-threatening injuries
Emergent reduction	Closed reduction under procedural sedation	Goal: reduce within 6 hours of injury to minimize AVN risk
Reduction techniques	Allis maneuver (posterior), Stimson gravity technique, or Whistler technique	Perform with adequate sedation and muscle relaxation; inline traction along the axis of deformity
Post-reduction imaging	AP pelvis radiograph + CT scan	Confirm concentric reduction; identify fractures or loose bodies
Stable, concentric reduction	Protected weight-bearing for 4-6 weeks; physical therapy	Non-weight-bearing or toe-touch weight-bearing; progressive rehabilitation
Non-concentric reduction or associated fracture	Open reduction and internal fixation (ORIF)	Surgical exploration to remove loose bodies and fix fractures; posterior wall fractures involving >40% of the wall generally require fixation
Post-reduction instability	Operative stabilization	If the hip redislocates during gentle ROM testing under anesthesia
Long-term follow-up	Serial radiographs and clinical assessment for AVN	AVN may develop up to 2 years after injury; monitor with imaging if symptomatic

Acute management begins with the standard trauma evaluation. Hip dislocation is frequently part of polytrauma (motor vehicle crashes), so life-threatening injuries to the chest, abdomen, and pelvis must be addressed simultaneously. However, once the patient is hemodynamically stabilized, closed reduction of the hip is an orthopedic emergency and should be performed as soon as possible, ideally within 6 hours.

The Allis maneuver is the most commonly tested closed reduction technique for posterior dislocation. The patient is supine, the clinician applies longitudinal traction on the flexed hip and knee (hip at 90 degrees of flexion) while an assistant stabilizes the pelvis. Gentle internal and external rotation may be applied to disengage the femoral head, which then reduces with a palpable "clunk." The Stimson technique (patient prone, hip and knee flexed over the edge of the bed with gravity-assisted traction) is an alternative. Adequate procedural sedation with agents such as propofol, etomidate, or ketamine and sufficient muscle relaxation are critical for successful reduction and to prevent iatrogenic femoral neck fracture.

For anterior dislocations, reduction involves longitudinal traction with the hip in its position of deformity, followed by gentle internal rotation and adduction to guide the femoral head back into the acetabulum.

After successful closed reduction, an AP pelvis film is obtained to confirm joint congruency, followed by a CT scan to evaluate for:

Intra-articular loose bodies (bone or cartilage fragments)
Acetabular wall fractures
Femoral head fractures (Pipkin classification)

If the CT reveals loose bodies within the joint space or a non-concentric reduction, the patient requires open surgical exploration to remove fragments and restore joint congruency. Posterior wall fractures involving greater than approximately 40% of the posterior wall are associated with recurrent instability and typically require ORIF.

Post-reduction, the patient is kept non-weight-bearing or toe-touch weight-bearing for 4 to 6 weeks, followed by progressive weight-bearing and rehabilitation. Patients should be counseled about the risk of AVN, which may develop months to years later, particularly when reduction was delayed. Follow-up imaging at 3, 6, 12, and 24 months is recommended by most guidelines.

Contraindications to closed reduction include an associated femoral neck fracture (attempting closed reduction risks displacing the fracture and further compromising blood supply). If a femoral neck fracture is identified on initial imaging, the patient should proceed directly to the operating room for open reduction.

5. Differential Diagnosis and Distractors

DIFFERENTIAL	WHY IT IS SIMILAR	KEY DISCRIMINATOR
Femoral neck fracture	Severe hip pain, inability to bear weight, shortened and externally rotated limb after trauma	Femoral neck fracture classically presents with the limb in external rotation and shortening without the adduction/flexion posture of a posterior dislocation; radiograph shows fracture line across the femoral neck, not displacement of the head from the acetabulum
Intertrochanteric fracture	Hip pain after a fall, shortened and externally rotated extremity	Typically occurs in elderly patients with osteoporosis after low-energy falls; radiograph shows fracture between the greater and lesser trochanters, femoral head remains in the acetabulum
Acetabular fracture without dislocation	High-energy trauma, severe hip pain, may have limited range of motion	Hip remains located on radiograph; CT shows fracture lines through acetabular columns or walls without femoral head displacement
Slipped capital femoral epiphysis (SCFE)	Hip pain and altered limb positioning in a young patient	Occurs in obese adolescents with insidious onset (unless acute slip); frog-leg lateral view shows posterior displacement of the epiphysis on the metaphysis; no history of high-energy trauma
Septic arthritis of the hip	Severe hip pain, refusal to bear weight, limb held in flexion and external rotation	Patient is typically febrile with elevated inflammatory markers (ESR, CRP, WBC); no traumatic mechanism; joint aspiration shows purulent fluid with >50,000 WBC/mm$^3$
Prosthetic hip dislocation	Hip pain and deformity after total hip arthroplasty	History of prior hip replacement; occurs with lower-energy mechanisms (sitting in a low chair, crossing legs); radiograph shows prosthetic components out of alignment

6. Traps and High-Yield Pearls

The most common way students lose points on hip dislocation questions is by confusing the limb position of posterior versus anterior dislocation. The exam relies heavily on positional descriptors to tell you the direction: a limb that is "flexed, adducted, and internally rotated" is posterior, while "abducted and externally rotated" is anterior. Mixing these up leads to selecting the wrong associated nerve injury, the wrong reduction technique, and the wrong anticipated complication.

A second frequent trap involves timing. Students sometimes choose advanced imaging (CT or MRI) as the next best step before reduction. In most clinical vignettes, if the diagnosis is apparent on plain film and no femoral neck fracture is present, the next best step is emergent closed reduction, not further imaging. The CT comes after reduction to evaluate for occult fractures and loose bodies. The only exception is when initial films suggest a concomitant femoral neck fracture, in which case closed reduction is contraindicated and the patient goes directly to the operating room.

Another high-yield pearl: always check the sciatic nerve before and after reducing a posterior dislocation. If a vignette describes a posterior hip dislocation with foot drop or loss of sensation on the dorsum of the foot, the answer is peroneal division of the sciatic nerve injury. This is the most commonly injured nerve, not the tibial division.

Finally, remember the 6-hour rule. The risk of AVN of the femoral head increases dramatically when reduction is delayed beyond 6 hours. Any vignette that emphasizes a time delay is testing whether you recognize this urgency and its consequence. AVN may not manifest clinically for months to years, so a follow-up question asking about the "most likely long-term complication" after a delayed reduction is almost always AVN.

The core competency being tested is the ability to recognize the direction of dislocation from the clinical presentation, prioritize emergent reduction over additional imaging, identify the at-risk neurovascular structures, and understand the time-sensitive nature of the injury.

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