Clinical Interpretation of Skeletal Motion During Instrumented Knee Arthrometry

To assess the relationship between anterior cruciate ligament (ACL) injury status and clinical instability, 13 patients with suspected ACL tears were evaluated bilaterally for knee instability with subjective clinical examinations, instrumented arthrometry (using the KT- 1000 knee ligament arthrometer) and computed tomography (CT) imaging. Knee laxity during the instrumented and CT examinations was assessed in three positions of leg orientation; 15° external rotation (EXT), 15° internal rotation (INT), and saggital-neutral (NEU). All subjective evaluations predicted knee injury status well (rϕ = 0.97 to 1.0). Anterior displacement was significantly greater in the injured knees than in the comparison knees (5.51 ± 2.64mm and 2.37 ± 1.28mm for injured and non-injured comparison knees, respectively). Anterior displacement was greatest when the knee was in· the EXT position regardless of injury status. No main effects were seen for injury status on tibial, femoral or total articular rotation. Correlations of measurements of anterior displacement obtained with CT and the KT-1000 ranged from r = 0.63 to r = 0.52 depending of the orientation of the tibia. Correlations of subjective and instrumented measurements ranged from r = 0.74 to r = 0.34. Tibial motion was found to be a fraction of the rotation measured at the foot. The ratio of tibial motion to foot-referenced motion was 0.31 and 0.40 for EXT and INT, respectively. The best instrumented predictor of injury status was a composite Anterior Rotary Score (ARS) which was a combination of anterior displacement and tibial rotation measures. When regressed against injury status, the ARS correlated well (r = 0.83, r2 = 0.69). The data show that ACL injury status can be assessed confidently using subjective and instrumented evaluation techniques. When performing instrumented evaluation, the orientation of the tibia will significantly impact measures of joint displacement. Our data suggest that leg rotation referenced from the foot is not analogus to tibial rotation and thus should not be called tibial rotation. Using the CT procedure it is possible to describe knee instability using a composite score which accounts for linear and rotary motion.