Abstract
Summary
In this study we validate that weight-bearing images are needed for accurate polyethylene liner wear measurement in total knee prostheses by measuring the difference in minimum joint space width between weight-bearing and non-weight-bearing RSA views.
Introduction
Recent studies show that Model-based Roentgen Stereophotogrammetric Analysis is superior to the conventional in vivo measurements of polyethylene liner wear in total knee prostheses. Although it is generally postulated that weight-bearing (standing) views are required to detect liner wear, most RSA images are acquired in non-weight-bearing (supine) view for practical reasons. Therefore, it would be of interest to know if supine views would be sufficient for measuring TKA liner wear, defined as a change in minimum joint space width (mJSW). As a difference in mJSW between weight-bearing and non-weight-bearing RSA images has never been validated, the aim of this study is to compare the outcome of in vivo measurements of mJSW in total knee prosthesis when conducted with weight-bearing and non-weight-bearing RSA views.
Method
We selected the first 16 patients with a Triathlon total knee prosthesis from a clinical study for which subsequently a weight-bearing eand a non-weight-bearing RSA image pair were acquired at one year follow up. For both images the mJSW and the corresponding contact point locations in terms of mediolateral (ML) and anterioposterior (AP) coordinates were measured. In addition, the ML stability was scored to assess knee laxity. The size of the mJSW difference between non-weight-bearing and weight-bearing views was determined and a regression analysis was conducted to investigate the effect of knee laxity to this difference, while correcting for differences between the contact point locations.
Results
On average, the measured mJSW was 0.22 mm larger in the non-weight-bearing views (T-test, p < 0.05). The standard deviation of the difference was 0.22 mm. The regression analysis showed that a difference in the ML position of the contact location was an important covariate (beta = 0.255±0.054, Wald 95% CI). 13 patients had a high ML stability (< 5 deg) and three had a medium stability (5–10 deg). The regression analysis showed that on average patients with medium ML stability had a 0.17 mm larger difference in mJSW than patients with a high ML stability.
Conclusion
The study shows that the mJSW is larger in non-weight-bearing views. The differences found are clinically relevant, as wear rates in practice are as small as 0.1 mm per year. Hereby it is validated that weight-bearing RSA images are more capable of detecting the mJSW in total knee prostheses. The size of the difference in mJSW between the views seems to be in relation with the knee laxity.
