Retroacetabular osteolysis: when to operate?

Polyethylene liner exchange for retroacetabular osteolysis should be done before the shell becomes loose. The purpose of this study was to determine the radiographic quantity of osteolysis that will predict impending loosening of the cementless shell. Between 1992 and 2002, 46 cementless shells were revised at our institution for aseptic osteolysis. Radiographs and a computer-assisted technique were used to quantify osteolysis. Implant stability was confirmed intraoperatively. Of 26 stable and 20 loose shells, the average area of osteolysis on anteroposterior radiographs showed no significant difference, whereas lateral radiographs showed a difference. The percentage of shell circumference with associated osteolysis seen on anteroposterior and lateral radiographs showed a significant difference. Diagnostic criterion of 50% shell circumference associated with osteolysis on lateral films has a sensitivity of 0.84 and a specificity of 0.54, and on anteroposterior views, a sensitivity of 1.0 and a specificity 0.27 for predicting shell loosening. Percent of shell circumference with surrounding osteolysis seems to be more predictive of loosening than the area of osteolysis. When 50% of the shell circumference has osteolysis evident on anteroposterior or lateral radiographs, but preferably anteroposterior radiographs, liner exchange should be considered so that the exchange procedure is still possible, rather than allowing the osteolysis to increase and compromise shell fixation.