DESIGN AND VALIDATION OF A SMART KNEE FIXTURE FOR MEASURING KNEE BALANCING

Introduction Evaluation of post-operative soft tissue balancing outcomes after Total Knee Arthroplasty (TKA) and other procedures can be measured by stability tests, with Anterior-Posterior (AP) drawer tests and Varus-Valgus (VV) ligamentous laxity tests being particularly important. AP stability can be quantified using a KT1000 device; however there is no standard way of measuring VV stability. The VV test relies on subjective force application and perception of laxity. Therefore we sought to develop and validate a device and method for quantifying knee balancing by analyzing VV stability. Materials and Methods Our team developed a Smart Knee Fixture to measure VV angular changes using two dielectric elastomer stretch sensors, placed strategically over the medial and lateral collateral ligaments (see Figure 1). The brace is secured in position with the leg in full extension and the sensors locked with pre-tension. Therefore, contraction and elongation of either sensor is measured and the VV angular deviation of the long axis of the femur relative to that of the tibia is derived and displayed in real time using custom software. EMG muscle activity was previously investigated to confirm there is no resistive activity during the VV test obstructing ligamentous evaluations. The device was validated in two ways: A bilateral lower body cadaver specimen, secured in a custom test rig, was used to compare the Smart Knee Fixture9s readings to those measured from an optical surgical navigation system. Abduction and adduction force was gradually applied as varus and valgus moments with a wireless hand-held dynamometer up to 50N (19.8Nm) at 0 and 15° flexion. Two male volunteers were used to compare the Smart Knee Fixture9s readings to those measured from fluoroscopic images. An arthroscopic distal thigh leg immobilizer was used to prevent rotation and lateral movements of the thigh when moments were applied at the malleoli. A C-arm Fluoroscope was then positioned focusing on the center of the joint. The tests were performed at full extension, 10 and 20° of flexion and force was gradually applied to 50N. Results and Discussion R values were calculated to validate the Smart Knee Fixture9s accuracy. Excellent correlation was observed between the Smart Knee Fixture and the gold standard of navigation (see Figure 2). The R values were 0.9909 and 0.9966. Correlation was also observed between Smart Knee Fixture and the measured fluoroscopic angular changes. The R values were 0.9118 and 0.7529. Conclusions The strong R values allow us to conclude that the Smart Knee Fixture can potentially be used to accurately measure VV angular changes in a clinical setting and hence provide a quantified measure of coronal plane soft tissue balance. Clinical studies are underway to compare TKA patient outcomes to balancing measured by the Smart Knee Fixture. This information should further define balancing goals at the time of surgery. We also envisage broader applications to early detection of ligamentous injury associated with sporting activities, such as multiple ligamentous knee injuries in teenage females.