Correct Answer: D. Posterior hip dislocation
Posterior hip dislocation is the most common type of hip dislocation (90% of cases) and classically presents with the limb in flexion, adduction, and medial rotation—the exact position described in this trauma case. This position occurs because the posterior capsule and muscles (especially the piriformis and obturators) pull the femoral head posteriorly and medially when disrupted. The shortening of the limb is due to the femoral head being displaced posteriorly relative to the acetabulum, effectively reducing the functional length of the limb. Posterior dislocations typically result from high-energy trauma (RTA, fall from height) with the hip in flexion—a common mechanism in Indian road traffic accidents. The clinical triad of flexion-adduction-medial rotation is pathognomonic and distinguishes posterior dislocation from other hip injuries. Early recognition is critical because posterior dislocation carries a high risk of associated femoral head fractures (Pipkin fractures) and posterior wall acetabular fractures, which can compromise the blood supply to the femoral head and lead to avascular necrosis if reduction is delayed beyond 6–12 hours.
Why the other options are wrong
A. Anterior hip dislocation — Anterior dislocation (10% of hip dislocations) presents with the limb in flexion, abduction, and external rotation—the opposite of what is described here. The patient would have the hip abducted and externally rotated, not adducted and medially rotated. Anterior dislocations result from different trauma mechanisms (e.g., hyperabduction or external rotation forces) and are much rarer in RTA scenarios. B. Intertrochanteric fracture — Intertrochanteric fractures present with external rotation and abduction of the limb, not the flexion-adduction-medial rotation seen here. While both cause limb shortening, the position is distinctly different. Intertrochanteric fractures typically occur in elderly patients with osteoporosis from simple falls, whereas this young trauma patient with the classic dislocation position makes fracture unlikely. C. Transcervical fracture — Transcervical (femoral neck) fractures also present with external rotation and abduction, not the medial rotation and adduction described. The position of the limb in transcervical fractures is determined by the unopposed action of hip flexors and external rotators, producing a different clinical picture. Additionally, transcervical fractures are more common in elderly patients and do not typically present with the dramatic positional deformity seen in posterior dislocation.
High-Yield Facts
- Posterior hip dislocation accounts for 90% of all hip dislocations and presents with the classic triad of flexion, adduction, and medial rotation.
- Flexion-adduction-medial rotation position is pathognomonic for posterior dislocation; anterior dislocation shows flexion-abduction-external rotation.
- Pipkin fracture (femoral head fracture associated with posterior dislocation) occurs in 5–10% of posterior dislocations and significantly increases AVN risk.
- Time to reduction is critical: reduction within 6–12 hours reduces AVN risk; delays >24 hours substantially increase morbidity.
- Associated injuries in posterior dislocation include posterior wall acetabular fractures (25–50%), sciatic nerve injury (10%), and femoral head fractures.
- Mechanism in RTA: high-energy trauma with hip in flexion (dashboard injury, crush injury) is the classic cause of posterior dislocation in Indian trauma settings.
Mnemonics
FAMED for Posterior Dislocation Flexion, Adduction, Medial rotation, Evident shortening, Displacement posteriorly. Use this when you see a flexed, adducted, medially rotated hip after trauma—think posterior dislocation immediately. Anterior = ABduction + External rotation Anterior dislocation = ABduction + External rotation. Opposite of posterior. Quick way to differentiate the two types by limb position alone.
NBE Trap
NBE may pair intertrochanteric or transcervical fractures with RTA to distract from the specific limb position. The key discriminator is the flexion-adduction-medial rotation posture—students who focus only on "trauma + shortening" without noting the exact position may incorrectly choose a fracture instead of dislocation.
Clinical Pearl
In Indian emergency departments, posterior hip dislocation from RTA is a surgical emergency. The classic "frog-leg" appearance (flexion-adduction-medial rotation) is unmistakable on clinical exam and should trigger immediate imaging and urgent orthopedic consultation to reduce within 6–12 hours and prevent avascular necrosis of the femoral head—a devastating complication in young, working-age patients.
_Reference: Bailey & Love Ch. 37 (Hip and Pelvis); Rockwood & Green's Fractures in Adults (Hip Dislocation section)_