Measurement of polyethylene wear in metal-backed acetabular cups. II. Clinical application.

The new 3-dimensional technique was applied to the radiographs of 141 patients who had received Porous Coated Anatomic total hip prostheses. Values were obtained for the position of the acetabular cup, the 3-dimensional distance and direction of femoral head displacement, and the minimum volume of polyethylene debris produced. Mean age of the patients at the time of replacement was 61 years old; mean followup was 5.6 years (range, 4-7.2 years). Change in the position of the femoral head between initial and long-term followup films was assumed to represent polyethylene wear, and formulas were used to calculate the minimal volume of polyethylene debris produced. The overall rate of 3-dimensional femoral head displacement was 0.264 mm per year, almost twice that usually quoted in the current literature for 2-dimensional linear wear. A significant contribution to this value was made by anterior and posterior displacement. Two-dimensional femoral head displacement (measured in the plane of anteroposterior radiographs) on the same patients was 0.149 mm per year. The mean minimum volume of polyethylene debris produced after 5.6 years was 0.448 cm3 (range, 0.00-2.83 cm3), giving a mean rate of 0.079 cm3 polyethylene debris produced each year. Thirteen patients in this series had radiologic osteolysis and a significantly greater femoral head displacement and polyethylene volumetric wear than those with no osteolysis. Patients with a 32-mm femoral head diameter and a polyethylene linear < 1 cm thick had a significantly greater amount of polyethylene wear. This series gives previously unavailable data on 3-dimensional femoral head displacement and is the first report that correlates the minimum volume of polyethylene wear produced with radiologic osteolysis.