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Posterior dislocation of the hip

Posterior dislocations of the hip, although uncommon, are the most common direction of dislocation for this joint, outnumbering anterior dislocations 9:1 2.

Pathology

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 impact on the dashboard.

Posterior dislocation may have associated fractures of the posterior lip of the acetabulum and/or injury to the acetabular labrum.

Radiographic features

Plain film 

AP 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, in so far as both demonstrate loss of the normal joint congruency between the femoral head and the acetabulum. In a well centered AP film the posteriorly dislocated femoral head will appear smaller than the contralateral hip, and vice versa, on account of geometric magnification 2.

See also

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