Prior to 2009, there had been no precedent for dynamic load bearing sensors in total knee arthroplasty (TKA), and it is now possible to track quantified tibiofemoral loading and center of load* location through the range of motion. As a consequence, arthroplasty surgeons have recently been confronted with the question: “What do these numbers mean?”
In order to successfully integrate any new tool into surgical workflow, it’s necessary to understand how best to use it in pursuit of maximizing patient outcomes. By extension, being able to measure medial and lateral forces at the tibiofemoral interface is only valuable if the surgeon understands the range of loading data that best encapsulates healthy knee kinematics. In balancing soft tissues to reconstruct the native non-diseased knee, patients are expected to experience a reduction in pain, return to function, and low likelihood of instability-related complications. However, a surgeon that has been trained to “feel” ligament balance may not find quantified soft-tissue data to be immediately intuitive. Fortunately, years of biomechanical and clinical research has begun to carve out a definition for TKA ligament balance that demonstrates the highest levels of clinical efficacy.
Currently, the working definition of balance, which has been applied to several knee designs, is a mediolateral differential of ≤ 15 lbf.1-3 This definition of balance was first established after the evaluation of preliminary multicenter study results. Surgeons were asked to perform the TKA in the way they normally would and to indicate when they felt the knee was balanced. After confirmation from the surgeon, operating room staff activated the sensor. Each surgeon placed the sensor into the tibial tray and recorded the mediolateral loading values and contact point location* at 10°, 45° and 90° of flexion. Patient reported outcomes scores were administered at the pre-operative interval and post-operatively at 6 weeks, 6 months, and annual intervals to 3 years. When 6-month and 1-year outcomes were analyzed, a statistical trend emerged with respect to patients within 15 lbf. of mediolateral differential, through the range of motion: these patients exhibited higher function, less pain, more satisfaction and greater activity levels than those outside of the 15 lbf. Range.1-3
In order to understand the variables that contributed to these results, extensive regression analyses were executed with the inclusion of all potential confounding variables – demographic, anatomical, and otherwise. Consistently, for every outcomes measure collected, the only independent variable that significantly contributed to the favorable trend in patients with 15 lbf. of mediolateral differential was the state of “balance” (either within 15 lbf., or outside of 15 lbf.).
At the release of these results, independent surgeon investigators began to explore the implications of using this same definition of balance. Continued research is currently being conducted to further investigate intraoperative loading values and improved patient outcomes.
*For reference only
Dr. Kenneth A. Gustke is a paid consultant to OrthoSensor, Inc.
1 Gustke K, et al. A New Method for Defining Balance: Promising Short-Term Outcomes of Sensor-Guided TKA. J Arthroplasty. 2014 May;29(5):955-60
2 Gustke KA, et al. Primary TKA Patients with Quantifiably Balanced Soft-Tissue Achieve Significant Clinical Gains Sooner than Unbalanced Patients. Adv Orthop. 2014:628695
3 Gustke K, et al. Increased Patient Satisfaction After Total Knee Replacement Using Sensor-Guided Technology. Bone Joint J. 2014 Oct;96-B(10):1333-8