Summary
A new biomechanical model for ACL injury and a potential prevention strategy.
Abstract
Background
ACL injury preventative measures, including training programs and knee-braces, are controversial and have not been shown to universally decrease injury risk. In the present work, we introduce and validate the feasibility of an implantable knee brace to prevent ACL injury by inhibiting motions that would endanger the ACL, without impeding normal motion.
Materials And Methods
The objective of this work was to identify potential insertion points for an extra-articular implant that would remain slack during healthy knee motion but engage before ACL rupture during motions known to cause isolated ACL injury. A KUKA robot arm (KR210; KUKA Robotics Corp.) was used to simulate kinematically-constrained 6 degree-of-freedom (DOF) motion profiles on human cadaver knees. Each knee was subjected to previously published 6-DOF kinematic profiles i) recorded during the stance phase of healthy walking gait and ii) derived from experiments that produced isolated ACL injuries. ACL strain and joint reaction forces were measured on the specimen during these “healthy” and “injury” profiles. After simulating the injury profile, isolated ACL injury was confirmed using clinical stability tests. Potential insertion points for the implant were identified using kinematic modeling in OpenSim, an open source musculoskeletal modeling platform, based on the difference in the maximum distance between points during simulated healthy and injury motions. Points from this model were marked on the ACL-injured knee specimen. A dummy implant was then attached across pairwise combinations of these points, and the distance between points was directly measured during each motion. Points that showed the greatest difference in distance between healthy and injury motions were considered most promising.
Results
ACL strain recorded during the stance phase of simulated walking gait showed a high level of agreement with published data of in vivo ACL strain data, as estimated using biplanar fluoroscopy. Peak ACL strain during walking averaged 2.88±0.39 percent. The injury-inducing motions reproduced in this study caused isolated ACL injury, as confirmed via simulated clinical stability testing. Peak ACL strains and out-of-plane joint reaction loads recorded during injury motion were higher than during healthy motion. Several combinations of insertion points were identified for which the surface distance between points increased more during injury motion than healthy motion. Promising points were generally located in the posterior-lateral region of the lateral femoral condyle and the anterior-lateral region of the tibial plateau. These model predictions were validated on the physical knee specimens. The maximum difference in distance between the healthy and injury motions was 3.6+/-0.55SD mm; this distance represents the margin for error in length of a rigid implant that would remain slack during healthy motion, but would engage to prevent injury.
Conclusions
Our 6-DOF kinematically-constrained method of reproducing motion profiles demonstrated similar ACL strain values to those in the published literature. We were able to reproduce an isolated ACL rupture, and identify and test insertion locations for an implanted knee brace. The proposed implant has the potential to prevent ACL injury by shielding the ACL from injurious strain.