Grading medial compartment tightness among varus osteoarthritic knees during image-free robot-assisted total knee arthroplasty for optimized pre-resection gap balancing.

Aims: Robot-assisted total knee arthroplasty (RA-TKA) allows for calibrated pre-resection gap balancing (PRGB). Complete PRGB is considered inappropriate as some gap tightness resolves spontaneously during surgery. The aims of this study were to 1) assess the degree of medial tightness that spontaneously resolves during the course of surgery; 2) assess the magnitude of medial tightness beyond which extensive soft-tissue releases (STRs) can be anticipated; and 3) develop a grading system for preoperative medial tightness based on anticipated STRs to optimize PRGB, among varus osteoarthritic knees during image-free RA-TKA.

Methods: Overall, 300 patients who underwent mechanically aligned image-free RA-TKA for varus osteoarthritis were studied. In part 1 of the study, 100 patients were assessed with the aim of developing a grading system for medial compartment tightness. Grades 1, 2, and 3 of medial compartment tightness were planned such that they represented patient sub-groups who required the highest proportions of routine-only, moderate, and extensive STRs, respectively. In part 2 of the study, the grading system was applied to 200 patients to evaluate its ability to predict the extent of STRs required.

Results: Medial compartment tightness of < 4 mm (grade 1), 4 to 7 mm (grade 2), or ≥ 7 mm (grade 3) in extension or flexion was associated with routine-only, moderate, or extensive STRs, respectively, in ≥ 80% of patients in both parts of the study (p < 0.001). Almost perfect agreement was noted between grades of preoperative medial tightness and STRs in part 1 (weighted ƙ = 0.843 and 0.874, for extension and flexion tightness, respectively) and part 2 of the study (weighted ƙ = 0.815 and 0.866, for extension and flexion tightness, respectively).

Conclusion: The proposed grading system for medial compartment tightness is easily applicable and optimizes PRGB. Further research is required on its applicability to alternative alignment strategies.