The posterior cruciate-stabilized (PS) implant is a widely utilized option in total knee arthroplasty (TKA) that replaces the posterior cruciate ligament (PCL) with the post-cam mechanism. The ultracongruent (UC) implant was recently introduced as an alternative to PS design which avoids removal of bone required for the post-cam mechanism and incorporates a polyethylene insert with prominent anterior and posterior edges touted to allow PCL sacrifice. Clear consensus regarding superiority of the PS or UC implants in TKA has not been elucidated. Accordingly, this study compared outcomes between PS and UC TKA in the largest single-center study to date.
A retrospective review identified all patients undergoing unilateral or bilateral TKA with a PS or UC implant from August 2011 to March 2020 at the authors’ institution. Age, sex, body mass index, extension, flexion, and Knee Society Knee (KSS-K) and Function Score (KSS-F) were recorded.
This study included 712 PS and 481 UC implants used in TKA, with no significant differences in patient demographics. Post-operatively, UC patients achieved knee flexion =120° less often (63.3% vs. 82.6%, p<0.001) and demonstrated a significantly lower KSS-K (91.0±8.5° vs. 91.9±10.0, p<0.001) and KSS-F (76.5±21.0° vs. 80.6±21.2°, p<0.001) compared to that of PS patients. PS patients had a greater improvement in flexion angle (5.0±15.1° vs. 1.6±15.3°, p<0.001) and KSS-F (27.9±22.9 vs. 22.7±24.5, p<0.001) compared to that of UC patients. Risk factors for post-operative flexion <120° were pre-operative flexion <120° (OR: 3.038, CI: 2.306-4.002; p<0.001) and UC implant (OR: 2.643, CI: 1.922-3.636; p<0.001) versus PS.
PS implants demonstrate favorable clinical and functional outcomes compared to UC implants in TKA and may be considered for patients with pre-operative flexion <120°. This can be attributed to the ability of the PS post-cam to reproduce the natural femoral rollback between the distal femur and tibial plateau, facilitating greater range of motion. Conversely, the less favorable outcomes in the UC group may be attributed to decreased femoral rollback. Further kinematic studies with UC implants should be performed in the future.