Which finding best distinguishes a posterior hip dislocation from an anterior hip dislocation?

## Posterior vs Anterior Hip Dislocation: Clinical Differentiation ### Comparison Table | Feature | Posterior Dislocation | Anterior Dislocation | |---------|----------------------|----------------------| | **Frequency** | 90–95% of all hip dislocations | 5–10% of all hip dislocations | | **Mechanism** | Dashboard injury, flexed hip trauma, fall on flexed knee | Forceful abduction + external rotation, motor vehicle accident | | **Hip position** | Flexion, adduction, internal rotation (classic "dashboard" posture) | Extension, abduction, external rotation | | **Limb appearance** | Shortened, internally rotated, adducted | Lengthened, externally rotated, abducted | | **Associated fractures** | Posterior wall acetabular fracture (50–70%) | Anterior wall acetabular fracture | | **Nerve injury** | Sciatic nerve (~10–15%) | Femoral nerve (rare) | | **Avascular necrosis risk** | High (if delayed reduction) | Lower | ### Key Point: **Hip position is the single best clinical discriminator.** Posterior dislocations present with the classic **flexion–adduction–internal rotation** posture ("dashboard" position). Anterior dislocations present with **extension–abduction–external rotation** ("frog-leg" position). ### Clinical Pearl: The posterior dislocation's flexed, adducted, internally rotated position is so characteristic that it is pathognomonic and visible on inspection alone. This posture results from the mechanism: a blow to the flexed knee drives the femoral head posteriorly out of the acetabulum. ### High-Yield: **Posterior = Flexion + ADduction + Internal rotation (FAD-IR)** **Anterior = Extension + ABduction + External rotation (EAB-ER)** ### Mnemonic: **"FADI" for posterior** — **F**lexion, **AD**duction, **I**nternal rotation **"EABE" for anterior** — **E**xtension, **AB**duction, **E**xternal rotation