COMPARISON OF SUPINE VERSUS STANDING RSA EXAMS FOR IN VIVO WEAR MEASUREMENT OF TOTAL KNEE ARTHROPLASTIES

Purpose

The purpose of this study was to examine the influence of weight-bearing on the measurement of in vivo wear of total knee replacements using model-based RSA at 1 and 2 years following surgery.

Methods

Model-based RSA radiographs were collected for 106 patients who underwent primary TKR at a single institution. Supine RSA radiographs were obtained post-operatively and at 6-, 12-, and 24-months. Standing (weight-bearing) RSA radiographs were obtained at 12-months (n=45) and 24-months (n=48). All patients received the same knee design with a fixed, conventional PE insert of either a cruciate retaining or posterior stabilized design. Ethics approval for this study was obtained.

In order to assess in vivo wear, a highly accurate 3-dimensional virtual model of each in vivoTKA was developed. Coordinate data from RSA radiographs (mbRSA v3.41, RSACore) were applied to digital implant models to reconstruct each patient's replaced knee joint in a virtual environment (Geomagic Studio, 3D Systems). Wear was assessed volumetrically (digital model overlap) on medial and lateral condyles separately, across each follow-up. Annual rate of wear was calculated for each patient as the slope of the linear best fit between wear and time-point. The influence of weight-bearing was assessed as the difference in annual wear rate between standing and supine exams. Age, BMI, and Oxford-12 knee improvement were measured against wear rates to determine correlations.

Results

Weight bearing wear measurement was most consistent and prevalent in the medial condyle with 0–4% of calculated wear rates being negative compared to 29–39% negative wear rates for the lateral condyle. For the medial condyle, standing exams revealed higher mean wear rates at 1 and 2 years; supine, 16.3 mm³/yr (SD: 27.8) and 11.2 mm³/yr (SD: 18.5) versus standing, 51.3 mm³/yr (SD: 55.9) and 32.7 mm³/yr (SD: 31.7). The addition of weight-bearing increased the measured volume of wear for 78% of patients at 1 year (Avg: 32.4 mm³/yr) and 71% of patients at 2 years (Avg: 48.9 mm³/yr). There were no significant (95% CI) correlations between patient demographics and wear rates.

Discussion and Conclusion

This study demonstrated TKA wear to occur at a rate of approximately 10 mm³/year and 39 mm³/year in patients imaged supine versus standing, respectively, averaged over 2 years of clinical follow-up. In an effort to eliminate the effect of PE creep and deformation, wear was also calculated between 12 and 24 months as 9.3 mm³ (standing examinations), This value is comparable to wear rates obtained from retrieved TKRs. Weight-bearing exams produced better wear data with fewer negative wear rates and reduced variance. Limitations of this study include: supine patient imaging performed at post-op, no knee flexion performed, and unknown patient activity level. Strengths of this study include: large sample size of a single TKR system, linear regression of wear measurements and no requirement for implanted RSA beads with this method. Based on these results, in vivo volumetric wear of total knee replacement polyethylene can be reliably measured using model-based RSA and weight-bearing examinations in the short- to mid–term.

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