Ensuring a Stable Total Knee Arthroplasty: Sensor-Guided Techniques Used to Quantify Sagittal Plane Balance

Introduction Flexion instability of the knee accounts for, up to, 22% of reported revisions following TKA. It can present in the early post-operative phase or present— secondary to a rupture of the PCL— in the late post-operative phase. While most reports of instability occur in conjunction with cruciate retaining implants, instability in a posterior-stabilized knee is not uncommon. Due to the prevalence of revision due to instability, the purpose of constructing the following techniques is to utilize intraoperative sensors to quantify flexion gap stability. Methods 500 posterior cruciate-retaining TKAs were performed between September 2012 and April 2013, by four collaborating surgeons. All surgeons used the same implant system, compatible with a microelectronic tibial insert with which to receive real-time feedback of femoral contact points and joint kinetics. Intraoperative kinematic data, as reported on-screen by the VERASENSE™ knee application, displayed similar loading patterns consistent with identifiable sagittal plane abnormalities. These abnormalities were classified as: “Balanced Flexion Gap,” “Flexion Instability” and “Tight Flexion Gap.” All abnormalities were addressed with the techniques described herein. Results Balanced Flexion Gap Flexion balance was achieved when femoral contact points were within the mid-posterior third (Figure 1) of the tibial insert, symmetrical rollback was seen through ROM, intercompartmental loads were balanced, and central contact points displayed less than 10 mm of excursion across the bearing surface during a posterior drawer test. Flexion Instability The femoral contact point tracking option dynamically displayed the relative motion of distal femur to the proximal tibia during the posterior drawer test, and through range of motion. Excessive excursion of the femoral contact points across the bearing surface, and femoral contact points translating through the anterior third of the tibial trial, was an indication of laxity in the PCL. Surgical correction requires use of a thicker tibial insert, anterior-constrained insert, or a posterior-stabilized knee design (Figure 2). Tight Flexion Gap Excessive tension in the PCL was displayed during surgery via femoral contact points and excessive high pressures in the posterior compartment during flexion. When a posterior drawer test was applied no excursion of the femoral tibia contact point was seen. Excessively high loading in the posteromedial compartment was corrected through recession of the PCL using an 19 gauge needle or 11 blade. Additional tibial slope was added when excessive loads were seen in both compartments (Figure 3). Discussion Flexion gap instability, or excessive PCL tension, is a common error resulting in poor patient outcomes and early revision surgery. The techniques described, utilized intraoperative sensor data to address sagittal plane abnormalities in a quantified manner. By using technology to guide the surgeon through appropriate sagittal plane correction, the subtleties in soft-tissue imbalance or suboptimal bone cuts can be accounted for, which otherwise may be overlooked by traditional methods of subjective surgeon “feel.” Longer clinical follow-up of these patients will be necessary to track the outcomes associated with quantifiable sagittal plane balance.