Simultaneous measurements of sagittal knee laxity with an external device and radiostereometric analysis.

We obtained simultaneous measurements of sagittal knee laxity in 12 consecutive patients after reconstruction of the anterior cruciate ligament (ACL), using the Stryker laxity tester and radiostereometric analysis (RSA). The mean anteroposterior (AP) displacement when a 90 N load was applied in both directions was 5.3 +/- 2.7 mm with RSA and 9.8 +/- 1.6 mm with the external device (p < 0.001). The corresponding measurements at a load of 180 N were 5.7 +/- 2.4 mm and 13.8 +/- 3.7 mm, respectively (p < 0.001). More than 50% of the sagittal knee movement, as measured by the external device at a load of 180 N, was not true femorotibial displacement of the joint but was due to soft-tissue deformation.