THE EFFECT OF VARIABLE SIZE POSTERIOR WALL ACETABULAR FRACTURES ON CONTACT CHARACTERISTICS OF THE HIP-JOINT

The indications for open reduction and internal fixation of posterior wall acetabular fractures associated with a clinically stable hip join t are unclear, In previous work a large posterior wall defect (27% art icular surface area) resulted in significant alteration of load transm ission across the hip; specifically, there was a transition from evenl y distributed loading along the acetabular articular surface to loadin g concentrated mainly in the superior portion of the articular surface during simulated single leg stance. However, the majority of posterio r wall fractures involve a smaller amount of the articular surface, Po sterior wall acetabular fractures not associated with instability of t he hip are commonly treated nonoperatively. This practice does not acc ount for the size of the posterior wall fracture. To study the biomech anical consequences of variably sized articular defects, a laboratory experiment was conducted evaluating three progressively larger posteri or wall defects of the acetabulum during simulated single leg stance u sing superlow Fuji prescale film (Itochu International, New York): (a) 1/3 articular surface width through a 50 degrees are along the poster ior wall of the acetabulum, (b) 2/3, and (c) 3/3 articular width defec ts through the same 50 degrees are along the posterior wall of the ace tabulum. In the intact acetabulum, 48% of the total articular contact was located in the superior acetabulum, Twenty-eight percent of articu lar contact was in the anterior wall region of the acetabulum and 24% in the posterior wall region, After the 1/3 width posterior wall defec t, 64% of the articular contact was located in the superior acetabulum (p = 0.0011), The 2/3 width posterior wall defect resulted in 71% of articular contact area being located in the superior acetabulum (p = 0 .0006), After the 3/3 width posterior wall defect, 77% of articular co ntact was located in the superior acetabulum, significantly greater th an the intact condition (p < 0.0001) and 1/3 width defect (p = 0.0222) , The total absolute contact areas for all defect conditions were sign ificantly less than the intact condition, The results of this study re confirm the observation that posterior wall fractures of the acetabulu m significantly alter the articular contact characteristics in the hip during single leg stance. The relationship between defect size and ch anges in joint contact showed that the smallest defect resulted in the greatest alteration in joint contact areas, whereas larger defects re sulted in minor increments of change in contact area. This finding is of concern because the clinical practice of managing acetabular fractu res nonoperatively if the hip joint is stable is based on the supposit ion that the joint retains enough integrity to function without undue risk of late posttraumatic osteoarthritis. A better understanding of t he natural history of stable posterior wall acetabular fractures is ne eded to ascertain whether some of these fractures merit operative repa ir.