Posterior dislocations of the hip, although uncommon, are the most common direction of dislocation for this joint, outnumbering anterior dislocations 9:1 2.
It most frequently occurs in the setting of significant trauma, given the large amount of force required. The most common scenario is in motor vehicle accidents (MVA) where the hip is flexed, and the knee is pushed backwards by an impact on the dashboard.
In the clinical examination, the affected lower extremity is usually shortened, adducted, and internally rotated.
AP pelvic x-rays will usually be sufficient for the diagnosis, although associated acetabular fractures will require CT to fully characterise.
Anterior and posterior dislocations may appear similar as both demonstrate loss of the normal joint congruency between the femoral head and the acetabulum. However, in a posterior dislocation, the femoral head is usually displaced posterior, superior, and slightly lateral to the acetabulum and also internally rotated hence the lesser trochanter is often obscured on the AP view. In a well centred AP film the posteriorly dislocated femoral head will appear smaller than the contralateral hip, and vice versa, on account of geometric magnification 2.
- 1. Doherty Gerard M. and Lawrence W. Way. Current surgical diagnosis & treatment. McGraw-Hill Professional, 2006. ISBN: 007142315X, 9780071423151.
- 2. Rockwood Charles A. and David P. Green. Rockwood and Green's fractures in adults. Lippincott Williams & Wilkins, 2006. ISBN: 0781746361, 9780781746366.