Total knee replacement designs have differing stability under novel robotic testing method in vitro.

purposeThis study implemented a novel robotic test method to quantify the effect of three distinct total knee arthroplasty (TKA) designs on knee kinematics and stability. It was hypothesised that the implant geometries would affect stability and rollback, with differences between the native and replaced knees, as well as between implant designs.

methodsEight fresh-frozen cadaveric knees were tested across the arc of flexion-extension under 710 N compressive load, combined with either no anterior-posterior (AP) tibial force, 90 N anterior or 90 N posterior drawer force using a robotic actuator. The same testing protocol was used post-TKA using three distinct implant designs (gradually reducing femoral condylar radius medially stabilised, multi-radius medially conforming and single-radius symmetrical), matched to the same bone cuts. Laxity and rollback were analysed using statistical parametric mapping and implant designs were compared to the intact knee and each other.

resultsNo significant differences in AP laxity were found between the intact knee (4.7 ± 0.7 mm), gradually reducing radius (6.3 ± 1.3 mm) and multi-radius designs (5.7 ± 1.1 mm). The single-radius implant showed significantly larger average AP laxity envelope (11.6 ± 2.3 mm) than the intact knee, the multi-radius design and the gradually reducing radius design and was more variable between knees. The rollback among the intact knee and TKAs were not significantly different: gradual radius 81% of native, multi-radius 85% and single-radius 90%.

conclusionsSignificant differences of AP laxity were found between the pre- and post TKA knee and between implant designs. Rollback did not differ significantly. Implanted knee behaviour also showed differences of sensitivity to cadaveric specimen and implantation variation among the prosthesis designs. CLINICAL RELEVANCE: Instability post-TKA remains an issue for good patient outcomes. Robotic testing of implanted knees shows the effects of implant design on knee stability and motion, potentially improving outcomes by providing the surgeon with objective data on which to base their choice of TKA. LEVEL OF EVIDENCE: N/A. Controlled laboratory study.