Tibiofemoral dynamic stressed gap laxities correlate with compartment load measurements in robotic arm-assisted total knee arthroplasty

Aims It is unknown whether gap laxities measured in robotic arm-assisted total knee arthroplasty (TKA) correlate to load sensor measurements. The aim of this study was to determine whether symmetry of the maximum medial and lateral gaps in extension and flexion was predictive of knee balance in extension and flexion respectively using different maximum thresholds of intercompartmental load difference (ICLD) to define balance. Methods A prospective cohort study of 165 patients undergoing functionally-aligned TKA was performed (176 TKAs). With trial components in situ, medial and lateral extension and flexion gaps were measured using robotic navigation while applying valgus and varus forces. The ICLD between medial and lateral compartments was measured in extension and flexion with the load sensor. The null hypothesis was that stressed gap symmetry would not correlate directly with sensor-defined soft tissue balance. Results In TKAs with a stressed medial-lateral gap difference of <= 1 mm, 147 (89%) had an ICLD of <= 15 lb in extension, and 112 (84%) had an ICLD of <= 15 lb in flexion; 157 (95%) had an ICLD <= 30 lb in extension, and 126 (94%) had an IUD <= 30 lb in flexion; and 165 (100%) had an ICLD <= 60 lb in extension, and 133 (99%) had an ICLD <= 60 lb in flexion. With a 0 mm difference between the medial and lateral stressed gaps, 103 (91%) of TKA had an ICLD <= 15 lb in extension, decreasing to 155 (88%) when the difference between the medial and lateral stressed extension gaps increased to +/- 3 mm. In flexion, 47 (77%) had an ICLD <= 15 lb with a medial-lateral gap difference of 0 mm, increasing to 147 (84%) at +/- 3 mm. Conclusion This study found a strong relationship between intercompartmental loads and gap symmetry in extension and flexion measured with prostheses in situ. The results suggest that ICLD and medial-lateral gap difference provide similar assessment of soft-tissue balance in robotic arm-assisted TKA.