Step 1 topics → Musculoskeletal, Skin, and Connective Tissue → Hip Dislocation

Hip Dislocation

USMLE Step 1 trap: Confuses limb positioning of anterior vs posterior hip dislocation. Anterior dislocation presents with extension, abduction, and external rotation; posterior dislocation presents with flexion, adduction, and internal rotation.

Hip dislocation is a high-energy injury most commonly seen after motor vehicle accidents — the classic 'dashboard injury' drives the femoral head posteriorly out of the acetabulum. Posterior dislocation accounts for roughly 90% of cases, so the exam leans heavily on it, but USMLE Step 1 loves to test whether you can distinguish anterior from posterior by limb position alone. The concept also connects directly to two major complications: avascular necrosis (AVN) of the femoral head and sciatic nerve injury, both of which the exam tests through application and clinical reasoning rather than pure recall.

What makes this topic tricky is that students memorize the wrong positioning for anterior dislocation, or they learn the complications without understanding the mechanism linking them to direction of dislocation. USMLE Step 1 will hand you a vignette describing a trauma patient with a specific leg position and ask you to identify the dislocation type, name the at-risk nerve, or pick the correct next step. If your mental models are backwards — especially on anterior vs. posterior positioning — you'll get these wrong even if you've seen the topic before.

The third layer is management timing. AVN is the most feared long-term complication, and the exam expects you to know that reduction must happen within 6 hours. Students who treat this as a 'schedule it soon' situation rather than a true emergency will miss questions that hinge on that specific window. Build your understanding around the mechanism: posterior dislocation = femoral head pushed back = stretches posterior structures including sciatic nerve and the end-arterial supply to the femoral head.

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Common misconceptions

Common mistake
Wrong: Anterior hip dislocation presents with the leg in flexion, adduction, and internal rotation.
Right: Anterior dislocation presents with extension, abduction, and external rotation; posterior dislocation presents with flexion, adduction, and internal rotation.
Anterior dislocation forces the femoral head forward and inferiorly, which levers the leg into extension, abduction, and external rotation — think of the hip opening outward. Posterior dislocation does the opposite: the femoral head is driven back, pulling the leg into flexion, adduction, and internal rotation, like a knee-to-dashboard position. If you keep mixing these up, anchor it to mechanism: posterior force on a flexed knee → posterior dislocation → leg stays flexed and adducted.
Common mistake
Wrong: AVN risk after hip dislocation is unrelated to time of reduction.
Right: AVN risk increases significantly if reduction is delayed beyond 6 hours, making emergent reduction the priority.
The femoral head receives its blood supply from end-arteries that travel along the capsule and are stretched or kinked during dislocation. The longer the head sits out of the socket, the longer those vessels are compromised. Data shows AVN risk rises dramatically after 6 hours of delay, which is why hip dislocation is treated as an orthopedic emergency requiring immediate closed reduction, not elective scheduling. On USMLE Step 1, if a question asks about the next best step in management, 'emergent reduction' is almost always the answer.
Common mistake
Wrong: Sciatic nerve injury is associated with anterior hip dislocation.
Right: Sciatic nerve injury (especially the peroneal division) is associated with posterior hip dislocation due to the nerve's posterior course.
The sciatic nerve runs posterior to the hip joint, so it is the nerve at risk when the femoral head is driven backward in a posterior dislocation. The peroneal division of the sciatic nerve is especially vulnerable because it has less slack and a more fixed course. Anterior dislocation pushes the femoral head toward the femoral neurovascular bundle (femoral nerve, artery, vein), not the sciatic nerve. Always pair the direction of dislocation with the anatomical structure in that direction.
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What the exam tests

  1. Given a trauma vignette, identify whether the described limb position (flexion/adduction/internal rotation vs. extension/abduction/external rotation) corresponds to posterior or anterior hip dislocation.
  2. Recognize that emergent closed reduction within 6 hours is the priority management step because delay beyond this window sharply increases the risk of avascular necrosis of the femoral head.
  3. Identify AVN of the femoral head, sciatic nerve injury (particularly the peroneal division), and post-traumatic arthritis as the major complications of hip dislocation, and know which dislocation type is associated with sciatic nerve injury.

Can you avoid these mistakes?

A 28-year-old man is brought in after a head-on collision. His right leg is held in flexion, adduction, and internal rotation and he cannot actively move it. What type of hip dislocation does he have, and which nerve is most at risk?
An ER attending says the orthopedic team can reduce the dislocated hip in the morning since the OR is busy. What is the specific complication risk that makes this decision dangerous, and what is the critical time window?
A patient with a hip dislocation reduced at 8 hours post-injury returns 18 months later with groin pain and limited range of motion. X-ray shows collapse of the femoral head. What is the diagnosis, and why did the delayed reduction contribute to it?
You are shown two images: one leg in extension, abduction, and external rotation; another in flexion, adduction, and internal rotation. Which represents anterior dislocation, which represents posterior, and what neurovascular structure is at risk in each?

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