Accuracy and repeatability of Roentgen stereophotogrammetric analysis (RSA) for measuring knee laxity in longitudinal studies

Roentgen stereophotogrammetric analysis (RSA) can be used to assess tempora l changes in anterior-posterior (A-P) knee laxity. However, the accuracy an d precision of RSA is dependent on many factors and should be independently evaluated for a particular application. The objective of this study was to evaluate the use of RSA for measuring A-P knee laxity. The specific aims w ere to assess the variation or "noise" inherent to RSA, to determine the re producibility of RSA for repeated A-P laxity testing, and to assess the acc uracy of these measurements. Two experiments were performed. The first expe riment utilized three rigid models of the tibiofemoral joint to assess the noise and to compare digitization errors of two independent examiners. No d ifferences were found in the kinematic outputs of the RSA due to examiner. repeated trials, or the model used. In a second experiment, A-P laxity valu es between the A-P shear load limits of +/- 60 N of fi ve cadaver goat knee s were measured to assess the error associated with repeated testing. The R SA laxity values were also compared to those obtained from a custom designe d linkage system. The mean A-P laxity values with the knee 30 degrees, 60 d egrees. and 90 degrees of flexion for the ACL-intact goat knee (+/- 95% con fidence interval) were 0.8 (+/-0.25), 0.9 (+/-0.29), and 0.4 (+/-0.22) mm, respectively. In the ACL-deficient knee, the A-P laxity values increased by an order of magnitude to 8.8 (+/-1.39), 7.6 (+/-1.32), and 3.1 (+/-1.20) m m, respectively. No significant differences were found between the A-P laxi ty values measured by RSA and the independent measurement technique. A high ly significant linear relationship (r(2) = 0.83) was also found between the se techniques. This study Suggests that the RSA method is an accurate and p recise means to measure A-P knee laxity for repeated testing over time. (C) 2001 Elsevier Science Ltd. All rights reserved.