Background: One of the remaining issues in total knee arthroplasty (TKA) is achieving sufficient rotational arc during deep flexion range of the knee for specific postures such as the 'seiza' or cross-legged sitting. This study aimed to evaluate whether there was a change in the actual in vivo rotational arc during deep flexion range before and after a design change of polyethylene (PE) inserts.Methods: In 50 posterior-stabilised TKA cases, knee kinematics, including rotational movement, were measured intraoperatively using an image-free navigation system to compare a newly designed PE insert with reduced the posterior lip with a conventional PE insert. Femoral-tibial rotational angles at 30 degrees, 45 degrees, 60 degrees, 90 degrees, 120 degrees, and 130 degrees knee flexion were evaluated. Varus/valgus instability, knee range of motion, and femoral rollback were also measured. Obtained parameters were compared between new and conventional PE inserts. The independent factors associated with rotational arc during deep flexion range (120 degrees and 130 degrees knee flexion) were analysed using multivariate analysis.Results: The newly designed PE insert demonstrated a significant increase in the rotational arc at 120 degrees (22.9 +/- 8.7 degrees vs. 30.1 +/- 11.9 degrees, P < 0.001) and 130 degrees (24.3 +/- 9.5 degrees vs. 32.5 +/- 12.4 degrees, P < 0.001) knee flexion compared with that with the conventionally designed posterior-stabilised insert. Multivariate analysis demonstrated that using the newly designed PE insert was an independent predictor of improved rotational arc during deep flexion range: regression coefficient was 11.2 (95% confidence interval 7.1-15.3, P < 0.001).Conclusion: The design change, which reduced the posterior lip of the PE insert, contributed to improved rotational arc in 120 degrees and 130 degrees deep flexion ranges.
