Automated Quasi-static In Vitro Knee Joint Simulator: Construction and Validation

Abstract

Anterior cruciate ligaments (ACL) are among the most common reported ligament injuries in athletes. This injury has been linked to changes in joint stability, neuromuscular activity and contact mechanics. In vitro simulators have proven to provide valuable insights on the potential effects of muscle activity on joint stability. The University of Ottawa Knee Simulate (UOKS) is a mechanical load driven quasi-static apparatus that provides the framework to explore pressure changes in knee compartments and the resultant kinematics in response to six individual muscle loads applied around an unconstrained joint. The main objective of the present work was to develop an automated loading mechanism for the UOKS. Furthermore, the secondary objective was to evaluate the accuracy, precision, reliability and validity of the newly automated system. Six transmission units were designed, built and tested as part of the development of the automated mechanism adapted to the UOKS. Load cells were used to obtain real time feedback of the load created by the transmission units. A software controller was programmed using LABVIEW to control these transmissions as a graphic user interface (GUI). Each transmission was tested independently and compared to an external master load cell. Furthermore, four cadaveric knee joints were mounted and suspended inside the UOKS for experimentation. Seven different loading conditions were tested with the ACL intact and after the ACL was severed. Pressure and kinematic data were recorded to correlate the changes in these variables due to changes in loading conditions simulated by the UOKS. The controlled loading experiment of the automated mechanism showed the accuracy of the controller to be within +/-1N, and multiple trials showed the system’s capability to produce loads. Additionally, the results showed the controller to have an ICC of 0.99 between the load produced and the target load. The comparison between feedback load cells and the master load cell displayed unique results regarding each transmission and the loads being applied. The results showed overestimations and underestimations with unique load difference trends for each transmission. Nevertheless, the relative difference measured by the master load cell was below 10% in all the transmissions. This study demonstrated that the controller was accurate and reliable when producing loads. On the other hand, the results of the integration analysis showed the presence of losses in the system when transmitting the loads from the actuators to the front of the UOKS. These losses were different for each transmission, likely due to the independence of each pulley system. Nevertheless, the automated loading mechanism proved to be a valid replacement for the original static loading mechanism previously used by the UOKS and is capable of reliably simulating six independent loading conditions.