Development of a ligament balance-based technology for total knee arthroplasty
Background: Although total knee arthroplasty (TKA) yields implant survival rates of 90–95% at ten years, up to 25% of patients remain dissatisfied, particularly with regard to functional outcomes. Whereas considerable effort has been devoted to implant design and bony resection accuracy, soft tissue management has historically relied on subjective, surgeon-dependent techniques. This paper presents the rationale, design and validation of a ligament balance-based technology (Newton, Exactech, Gainesville, FL, USA) developed to objectively measure intra-articular gaps under a controlled distraction force throughout the full arc of flexion and to inform intra-operative femoral resection planning.
Methods: The Newton distractor combines two antagonistic mechanical actuators delivering a constant intra-articular distraction force (90 N per compartment) and integrates with a computer-assisted surgery platform (AdvitaGPS, Blue-Ortho, Meylan, France) to acquire alignment, sizing and gap data. The system was assessed through a stepwise programme comprising bench testing of force constancy, cadaveric studies of inter- and intra-rater reliability and of targeted balance achievement, evaluation of the surgeon learning curve using cumulative sum (CUSUM) analysis on 450 cases, comparison of planned versus achieved balance in 273 procedures, and a clinical comparison of 100 Newton-instrumented TKAs versus 100 control TKAs performed with a conventional tensioner.
Results: Bench testing confirmed maintenance of a near-constant distraction force regardless of gap, implant size or batch. Inter- and intra-rater reliability of gap acquisition were superior with the instrumented method (mean intraclass correlation coefficient 0.72 and 0.66) compared with the manual method (0.35 and 0.41). Junior surgeons in particular achieved improved medio-lateral balance. The mean learning curve was 6.4 ± 2.8 cases. Achieved balance closely matched the surgical plan (unsigned differential 1–2 mm). Clinically, the Newton group demonstrated significantly greater improvements in Knee Society Scores and shorter hospital stays than the control group.
Conclusions: This integrated ligament balance-based technology provides reproducible intra-operative measurements, a short learning curve and improved early clinical outcomes, supporting personalised TKA planning and offering a foundation for future artificial intelligence-driven developments.
