Correct Answer: D. Posterior dislocation of the hip
Posterior dislocation of the hip is the most common type of hip dislocation (90% of cases), typically occurring after high-energy trauma such as road traffic accidents. The classic mechanism is a dashboard injury where the knee strikes the dashboard with the hip flexed and adducted, driving the femoral head posteriorly out of the acetabulum. On pelvic radiographs, the key diagnostic features are: (1) the femoral head appears smaller and higher than the contralateral side due to internal rotation and posterior displacement, (2) the femur is held in flexion, adduction, and internal rotation (the pathognomonic "frog-leg" or "pigeon-toe" position), and (3) the femoral head is displaced posteriorly relative to the acetabulum. This is a true orthopedic emergency requiring urgent reduction within 6–12 hours to prevent avascular necrosis of the femoral head, which occurs in up to 10–15% of cases even with prompt reduction. The mechanism in RTA with dashboard impact is classic for posterior dislocation, making this the most likely diagnosis in this trauma scenario.
Why the other options are wrong
A. Central dislocation of the hip — Central dislocation (or central fracture-dislocation) occurs when the femoral head is driven medially into the pelvis through the acetabulum, typically from a lateral pelvic impact or crush injury. This is not a true dislocation but rather a fracture of the acetabulum with medial displacement of the femoral head. The radiographic appearance shows the femoral head within or medial to the pelvic ring, not posteriorly displaced as in this case. Dashboard RTA typically causes posterior, not central, dislocation. B. Anterior dislocation of the hip — Anterior dislocation accounts for only 10% of hip dislocations and occurs with hip flexion and external rotation (opposite mechanism to posterior). On radiographs, the femoral head appears larger (less internal rotation), is positioned anteriorly and inferiorly relative to the acetabulum, and the limb is held in flexion and external rotation. The classic mechanism is a fall on a flexed, externally rotated hip or a dashboard injury with the hip extended—neither matches the typical RTA dashboard impact described here. C. Fracture shaft of femur — Femoral shaft fractures present with gross limb deformity, shortening, and external rotation of the entire limb. On radiographs, there is a clear fracture line through the femoral diaphysis with proximal and distal fragment displacement. While RTA can cause femoral shaft fractures, the clinical and radiographic presentation differs markedly from hip dislocation. The absence of a visible fracture line and the specific positioning of the femoral head relative to the acetabulum rule out this diagnosis.
High-Yield Facts
- Posterior hip dislocation accounts for 90% of all hip dislocations and is the classic injury from dashboard impact in RTA.
- Pathognomonic position: flexion, adduction, and internal rotation (femur appears shortened and internally rotated on radiograph).
- Avascular necrosis risk increases with delay in reduction; reduction must be attempted within 6–12 hours to minimize AVN (occurs in 10–15% even with prompt reduction).
- Mechanism in RTA: knee strikes dashboard with hip flexed and adducted, driving femoral head posteriorly out of acetabulum.
- Radiographic sign: femoral head appears smaller and higher than contralateral side due to internal rotation and posterior displacement relative to acetabulum.
- Associated injuries: posterior wall acetabular fractures (25–50% of cases), sciatic nerve injury (10–15%), and femoral head fractures (Pipkin fractures).
Mnemonics
FADI for Posterior Hip Dislocation Flexion, Adduction, Displacement (posterior), Internal rotation. This is the position of the limb and the direction of femoral head movement in posterior dislocation. Dashboard = Posterior Dislocation In RTA, dashboard impact with flexed hip and adducted knee → posterior dislocation (90% of all hip dislocations). Remember: Dashboard = Distal (posterior).
NBE Trap
NBE may pair anterior dislocation with external rotation to trap students who memorize position without understanding mechanism. In RTA with dashboard impact, the hip is flexed and adducted (not extended and externally rotated), making posterior—not anterior—dislocation the answer.
Clinical Pearl
In Indian emergency departments, posterior hip dislocation from RTA is a common orthopedic emergency. Delayed reduction (>12 hours) significantly increases AVN risk; therefore, immediate reduction under anesthesia is the priority, followed by imaging to rule out associated acetabular or femoral head fractures before mobilization.
_Reference: Bailey & Love Ch. 35 (Hip Joint Injuries); Rockwood & Green's Fractures in Adults, Ch. 51 (Hip Dislocation)_