Abstract
Introduction
RSA is widely accepted as a precise method to asses wear and migration early in the postoperative period. In traditional RSA, one segment defines both the acetabular shell and the polyethylene liner. However, inserting beads into the liner permits employment of the shell and liner as two separate segments, thus enabling distinct analysis of the precision of three measurement methods in determining wear and acetabular shell migration. The purpose of this in vivo follow-up study was to determine if assigning the shell and liner as one combined, or two individual segments affected the precision of RSA measurements of wear and shell stability.
Methods
The UmRSA program was used to analyze the double examinations of 51 hips to determine if there was a difference in precision among 3 measurement methods: the shell only, the liner only, and the shell + liner combined segment. Tantalum beads were inserted into the liner and pelvic bone surrounding the shell intraoperatively for the purpose of RSA. Polyethylene wear was measured using point motion of the center of the head with respect to 3 different segments: 1) liner only, 2) the shell only and, 3) shell + liner segment. Cup stability was measured by segment motion comparing the stable pelvic segment to 1) the liner segment, 2) the shell only segment, and 3) the shell + liner segment. The Wilcoxon paired signed-ranks test was used to determine differences in condition number and bead counts among the 3 measurement methods (p ≤0.05).
Results
The 95% confidence interval, calculated from double examinations, established the precision of each method. The shell + liner and liner only methods had a precision of 0.03mm when measuring both wear and shell migration. The shell only method precision was 0.07mm when measuring wear and 0.08mm when measuring shell migration, making it the least desirable method. In both the wear and migration analyses, the shell + liner condition number was significantly lower and the bead count was significantly higher than those of the shell only and liner only methods, indicating a superior RSA analysis on all counts compared to the shell only and liner only methods.
Discussion
Insertion of beads in the polyethylene improves the precision of wear and shell migration measurements. A greater dispersion and number of beads when combining the liner with the shell generated more reliable results in both analyses by engaging a larger portion of the radiograph. The liner beads also allow measurement of cup rotation of the shell + liner segment, which is not possible when using the shell segment alone, due to the 2D nature of the program's algorithm to detect the edge of the cup. As the prediction of implant survivorship in the early postoperative period relies heavily upon RSA, it is crucial to use the most precise system to monitor these implants and the shell+ liner method meets that standard.
