Effect of Acetabular Modularity on Polyethylene Wear and Osteolysis in Total Hip Arthroplasty

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Abstract
Debris from polyethylene wear causes osteolysis. In this study, we examined the effect of acetabular liner modularity on polyethylene wear and osteolysis. We compared forty-one hips (thirty-nine patients) treated with a nonmodular, porous-coated acetabular component with a matched group of forty-one hips (forty patients) treated with a modular acetabular component. The groups were matched by patient gender and age, type of polyethylene material, method of polyethylene sterilization, femoral head size and manufacturer, and stem manufacturer. The mean follow-up period was 5.3 years (range, 3.8 to 6.8 years) for the nonmodular group and 5.5 years (range, 3.8 to 8.0 years) for the modular group. Using serial radiographs and a computer-assisted method, we measured two-dimensional head penetration into the polyethylene liner. Temporal head-penetration data and linear regression analysis were used to calculate the true wear rates. The nonmodular acetabular components demonstrated a lower, but not a significantly lower, mean true wear rate than did the modular components (0.11 compared with 0.16 mm/yr, p = 0.22), and they were associated with a significantly lower rate of osteolysis (2% compared with 22%, p = 0.01). In addition, the true wear rates of the nonmodular components were less variable than those of the modular components. The 95% confidence interval for the wear rates of the nonmodular components (0.08 to 0.13 mm/yr) was nearly half that of the modular group (0.11 to 0.20 mm/yr). The lower and more consistent true wear rates of the nonmodular components could be attributed to the fact that these cups were designed to have greater liner-shell conformity, greater liner thickness, and less liner-shell micromotion than modular components. These design factors could have favorably altered the stress distribution throughout the liner and could have thereby decreased wear. Although nonmodular components may present a partial solution to the problems of wear and osteolysis, they pose a disadvantage when a failed liner in a bone-ingrown acetabular component needs to be revised.