Abstract
Introduction
Knee instability, stiffness, and soft-tissue imbalance are causes of aseptic revision and patient dissatisfaction following total knee arthroplasty (TKA). Surgical techniques that ensure optimal ligament balance throughout the range of motion may help reduce TKA revision for instability and improve outcomes. We evaluated a novel tibial-cut first gap balancing technique where a computer-controlled tensioner is used to dynamically apply a varying degree of distraction force in real-time as the knee is taken through a range of motion. Femoral bone cuts can then be planned while visualizing the predicted knee implant laxity throughout the arc of flexion.
Surgical Technique Description
After registering the mechanical axes and morphology of the tibia and femur using computer navigation, the tibial resection was performed and a robotic tensioning tool was inserted into the knee prior to cutting the femur. The tool was programmed to apply equal loads in the medial and lateral compartments of the knee, but to dynamically vary the distraction force in each compartment as the knee is flexed with a higher force being applied in extension and a progressively lower force applied though mid-flexion up to 90° of flexion. The tension and predictive femoral gaps between the tibial cut and the femoral component in real-time was determined based on the planned 3D position and size of the femoral implant and the acquired pre-resection gaps (figure 1). Femoral resections were then performed using a robotic cutting guide and the trial components were inserted.
Methods
The technique was evaluated by three experienced knee arthroplasty surgeons on 4 cadaver knees (3 torso-to-toe specimens, Pre-operative deformity range: 4° varus − 6° valgus; Extension lag: 0° – 13°; BMI 23.4 – 32.6; Age 68 – 85yr). An applied targeted load of 80N in extension and 50N in flexion was used in each of the four knees. These force values were determined in a prior cadaver study aimed at determining what magnitude of applied load corresponded to an optimally rated knee tension and stability. The femoral component was planned in each of the four knees to have symmetric gaps at 0° and 90° of flexion. The overall balance of the knee was assessed clinically by each surgeon using a varus/valgus stress test with the trial components inserted. No soft-tissue releases were performed other than a standard medial release during initial exposure of the knee. The following scale was used to rate the final knee stability achieved: 1 – too loose; 2 – slightly loose; 3 – ideal; 4 slightly tight; 5 – too tight.
Results
‘Ideal' balance was achieved in three out of the four knees tested (table 1). In two of the four knees the final inserted thickness selected was 1mm thicker than the planned insert thickness.
Conclusions
Our preliminary cadaver results suggest that it is possible to achieve a balanced knee by incorporating dynamic ligament tensioning and gap data throughout flexion into the femoral planning process using a robotic tensioning tool.
For figures/tables, please contact authors directly.
