Methodology to identify subject-specific dynamic laxity tests to stretch individual parts of knee ligaments

The mechanical properties of ligaments are important for multiple applications and are often estimated from laxity tests. However, the typical laxity tests are not optimized for this application and, a potential exists to develop better laxity tests in this respect. Therefore, the purpose of this study was to develop a methodology to identify optimal, dynamic laxity tests that isolate the stretch of the individual ligaments from each other. To this end, we applied an existing rigid body-based knee model and a dataset of ∼100.000 random samples of applied forces (0-150 N), moments (0-10 Nm) and knee flexion angles (0-90°) through Monte Carlo Simulations. For each modelled ligament bundle, we identified ten load cases; one producing the highest force and nine equally spaced between the maximal and zero force, where the maximal force in all other ligament bundles were minimized. We compared these novel laxity tests to standard internal/external and varus/valgus laxity tests using an isolation metric. We found that no laxity test could stretch the anterior part of the posterior cruciate and medial cruciate ligaments (PCL and MCL), whereas for all other ligaments, except the posterior PCL, the new laxity tests isolated the ligament stretch 28 % to 450 % better than standard tests. From our study, we conclude that it is possible to define better laxity tests than currently exist and these may be highly relevant for determination of mechanical properties of ligaments in vivo. Future studies should generalize our results and translate them to modern laxity measurements technologies.