ACCURACY OF BALANCING AT TOTAL KNEE SURGERY USING AN INSTRUMENTED TIBIAL TRIAL

INTRODUCTION Important surgical requirements for optimal function are accurate bone cut alignments and soft tissue balancing. From an unbalanced state, balancing can be achieved by Surgical Corrections including soft tissue releases, bone cut modifications, and changing tibial insert thickness. Surgical balancing can now be quantified using an instrumented tibial trial, but the procedures and results need further investigation. Our major purpose was to determine the initial balancing after making the bone cuts, and the final accuracy of balancing after Surgical Corrections. A related purpose was to determine the number and effectiveness of different Corrections in achieving balancing. METHODS During 101 surgeries of a PCL-retaining TKA, screen capture software recorded the video feed of surgery, angular data from the navigation system, and lateral and medial contact forces from the instrumented tibial trial. Initial bone cuts were made using navigation based on measured resection. The instrumented tibial trial measured the magnitudes and locations of the contact forces on the lateral and medial sides throughout flexion. The Heel Push Test (Walker 2014) determined the initial balancing, defined as a ratio of the medial/total force at 0, 30, 60 and 90 degrees flexion. A balanced knee with equal lateral and medial forces would show a value of 0.5. Surgical Corrections were then performed with the goal of achieving balancing. The most common Corrections were soft tissue releases (total 63 incidences), including MCL, postero-lateral corner, postero-medial corner; and increasing/decreasing tibial insert thicknesses (34 incidences). RESULTS After the bone cuts and inserting the trials, the average medial/total force ratio was 0.49 +/− 0.27. After final balancing, the ratio was 0.52 +/− 0.14 (Figure 1). The initial data was scattered between 0.0 (lateral force only) and 1.0 (medial force only). The final data showed a clear narrowing of the range of imbalance. The different Surgical Corrections achieved an improvement of the medial/total ratio between 0.11–0.18. A 2mm insert increase increased the total force by 106 Newtons. A final medial/total ratio between 0.35–0.65 was achieved in 80% of cases from 0–30 degrees; 77% from 0–90 degrees. In 84% of the cases, 0–2 Corrections were required. The average total force on the condyles from 0–90 degrees was 290.5+/−166.8 Newtons initially and 215.3+/−86.3 Newtons after balancing, the large range due to patient variations in ligament stiffness. DISCUSSION Acceptable balancing was achieved in the majority of cases with only 0–2 Surgical Corrections. The sensitivity of the balancing values to the Surgical Corrections was consistent with a previous study showing that changes of 2mm or 2 degrees could correct most imbalanced states (Walker 2014), related to collateral stiffness being in the region of 50N/mm (Wilson 2012, Robinson 2005). With only 2 Surgical Corrections needed in the majority of cases, no additional time was needed compared with qualitative methods. The acceptable level in our study from 0.35–0.65 medial/total force ratio was based on what could readily be achieved, consistent with a previous study (Gustke 2014). An IRB study is now underway to determine the ideal balancing ratio and the effect on functional outcome.