Bi-unicondylar knee replacement laxity with changes to simulated soft tissue constraints.

Unicondylar knee replacement systems have been shown to perform comparably to total knee replacements, while being much less surgically invasive. Proper ligament balancing, as well as knee laxity, has been shown to play an important role in optimizing kinematic behavior of these implant systems and improving long-term survival of the implant. This study investigates the effect of different simulated ligament laxity conditions of the anterior cruciate ligament and the posterior cruciate ligament on the resulting anteroposterior and mediolateral contact kinematics for medial and lateral pairs of UKR implants with flat and symmetric ultrahigh-molecular-weight polyethylene inserts during force-controlled ISO-14243-1 knee testing simulation. A novel method of capturing the tibiofemoral lowest point contact path was used to calculate the shear plane lowest point contact path kinematics in both the anteroposterior and the mediolateral directions. The results illustrated that multiple clinically relevant soft tissue configurations produce statistically different measured knee kinematics in unicondylar knee replacement systems than is seen in accepted "standard" knee simulator protocols with 95% confidence interval. The observed kinematic differences in anteroposterior and mediolateral movement from what was observed using standard wear testing protocols could aid in the development of unicondylar knee replacement design enhancements that are resistant to varying soft tissue deficiencies.