A new method to make 2-D wear measurements less sensitive to projection differences of cemented THAs

Abstract

Wear curves from individual patients often show unexplained irregular wear curves or impossible values (negative wear). We postulated errors of two-dimensional wear measurements are mainly the result of radiographic projection differences. We tested a new method that makes two-dimensional wear measurements less sensitive for radiograph projection differences of cemented THAs. The measurement errors that occur when radiographically projecting a three-dimensional THA were modeled. Based on the model, we developed a method to reduce the errors, thus approximating three-dimensional linear wear values, which are less sensitive for projection differences. An error analysis was performed by virtually simulating 144 wear measurements under varying conditions with and without application of the correction: the mean absolute error was reduced from 1.8 mm (range, 0-4.51 mm) to 0.11 mm (range, 0-0.27 mm). For clinical validation, radiostereometric analysis was performed on 47 patients to determine the true wear at 1, 2, and 5 years. Subsequently, wear was measured on conventional radiographs with and without the correction: the overall occurrence of errors greater than 0.2 mm was reduced from 35% to 15%. Wear measurements are less sensitive to differences in two-dimensional projection of the THA when using the correction method.

Fig. 1

The raw 2-D measurement method in this study uses the change in distance between two reference points to determine linear wear. The center of the metal ring (a) and the center of the femoral head (b) are used as reference points.

Fig. 2A–B

(A) The true wear values are plotted against the wear values as measured using plain radiographs without application of the correction method. (B) The true wear values are plotted against the wear values as measured using plain radiographs after application of the correction method. The data cloud is now closer to the diagonal as a consequence of better correlation between these values in comparison with the uncorrected values in (A).

Fig. 3

The magnitude of wear measurement errors is clearly diminished by application of the correction method. It eliminates errors in the highest category (> 0.6 mm) but also lowers the prevalence of errors of lesser magnitude.