One of the main reconstructive objectives in a primary TKA is to develop a well balanced knee by focusing on establishing flexion symmetry during the procedure using external femoral rotation. Current surgical techniques rely on anatomic or “boney” landmarks, including the posterior condylar axis, AP axis and Transepicondylar axis (TEA), to accomplish this objective. Variability in using these anatomic references has been sighted in published studies on the subject and clinical complications associated with joint instability continue to surface in the literature. A main reason for this variability is the fact that functional ligament and soft tissue support is not interpreted early enough in the procedure when using anatomic landmarks. This can make correcting flexion symmetry challenging later in the procedure given soft tissue releases vary in their end result. To address this issue, an electronic force sensing technology has been used to balance the flexion space for the past 24 months. This simple reproducible technology utilises a soft tissue force sensing device to develop flexion symmetry by creating balanced external femoral rotation relative to the proximal tibial resection. The sensor and adjustable femoral trial-like device enable balancing the relative forces in the medial and lateral femoral-tibial compartments in the knee to establish symmetry in flexion before the implant trials are placed. This step is performed early in the procedure before the posterior femoral condyles are resected, a technique delivering reliable results covering a broad range of deformities. Since the sensor relies on relative force values cued from the medial pillar, the question of “how tight is just right” needs to be answered to further optimise outcomes.

The current study involves 50 posterior stabilised rotating platform primary knees. Data on operative variables was collected and tibial spacers of different thicknesses (matching implant system thicknesses) were introduced into the balanced flexion space to register relative force values. The thickest tibial spacer creating balanced function and ROM was recorded first and the next thinner spacer recorded for comparison. Oxford Knee scores were then collected at two weeks, six weeks and three months following the primary knee reconstruction and associated to the relative force value of the tibial insert implanted to develop functional feedback on “how tight is just right.” Early experience seems to indicate the tighter tibial insert is the better choice based on the characteristics of the knee design used in this study.

The results of this study have shed important light on tibial insert selection related to functional outcomes. Expanded study on this subject would greatly benefit future surgeons and patients alike.