In vitro laxity-testers for knee joints of mice

The knee joints of mice can be used as a model for studying the effects of interventions on knee laxity. The goal of this study was to quantify knee joint laxity in vitro. Three devices were developed: a positioning- and cementing device, an anterior-posterior (AP) laxity tester and a varus-valgus (VV) laxity tester. The positioning- and cementing device was used to position the joint in a reproducible way and to attach clamping pins to the proximal femur and distal tibia using PMMA. The clamping pins were used to fix the joint to the AP- and VV-testers. In both testers the load was applied by means of a spindle-actuated spring while load and displacements were measured simultaneously. The load-displacement data were used to calculate displacement and compliance parameters. The performance of the testers was evaluated by testing 5 normal knee joints of 5 mice. Total AP-translation at ±0.8 N was 0.43 (± 0.16 S.D.) mm with compliances of 0.14 (±0.05 S.D.) mm N−1 and 0.12 (±0.05 S.D.) mm N−1 at 0.8 N posterior and anterior force, respectively. Total VV-rotation at ±4 Nmm was 17.2 (±2.6 S.D.) degrees with compliances of 0.9° Nmm−1 (±0.2° N mm−1 S.D.) and 1.0° Nmm−1 (±0.4° Nmm−1 S.D.) at 4 Nmm valgus and varus moment, respectively. The contributions of the deformations of the bones and the fixtures to the rotations were negligible in the VV-test. In the AP-test they account for approximately 0.07 (±0.03 S.D.) mm of the total AP-translation. This will not affect the utilization of the device for comparative analysis. It is concluded that in vitro evaluation of AP- and VV-laxity in knees of mice is feasible with sufficient accuracy for evaluation of changes after ligament damage.