Computed tomography evaluation of stability in posterior fracture dislocation of the hip.

Measurements of the percentage of remaining posterior acetabulum on computed tomography (CT) scan (the Acetabular Fracture Index) in posterior fracture dislocations of the hip were evaluated to determine the stability of the joint. All hips with less than 34% of the remaining posterior acetabulum were unstable. Hips with greater than 55% were stable. Between these values, hips were either stable or unstable. A statistical analysis demonstrated highly significant differences in the average remaining posterior acetabulum between the stable and unstable group. These findings were based on a review of 26 patients with posterior fracture dislocations of the hip (Epstein Type I-IV injuries) combined with CT scan analysis. The clinical status of hip stability was correlated with the Acetabular Fracture Index, and this provided the basis for the study. A simple linear measurement of the remaining posterior acetabulum on CT (the Approximate Acetabular Fracture Index) can be done easily by a physician, and this closely approximates the true remaining acetabular arc. Seven of ten unstable hips in 31 Epstein Type I-V patients showed femoral head subluxation of 0.5 mm or more on CT scan, whereas none of the 21 stable hips had demonstrable subluxation. Risk analysis provided a means of predicting hip stability for individual patients.