Mid-term outcomes of ultra-congruent total knee arthroplasty: a comparison with the posterior-stabilized implants

Background: Total knee arthroplasty (TKA) is an effective treatment for knee osteoarthritis, yet one in five patients remain dissatisfied after surgery. Implant design is thought to be a contributing factor to this dissatisfaction. Posterior-stabilized (PS) TKA rely on a cam-post mechanism for stability, but this design has been associated with specific complications. Ultra-congruent (UC) TKA rely on highly-conforming polyethylene instead. The aim of this study was to describe and compare the mid-term functional, clinical and radiological outcomes of the UC and PS TKA, in order to determine whether UC implants can be recommended as a safe alternative to the well-established PS implants. Methods : This retrospective study included 70 primary TKAs (36 UC, 34 PS) performed at the Adults B department, of the Mohamed Kassab Institute of orthopedics, between 2013 and 2020, with a minimum follow-up of 3 years (mean 4.7 years). Pre- and postoperative assessment included functional, clinical and radiological outcomes of the Knee Society Score (KSS). Operative time and hospital stay were noted, as well as complications and overall satisfaction. Results : Preoperative demographic, functional, clinical and radiographic parameters were comparable between groups. Operative time was shorter with UC (116.4 vs 122.9 minutes; p=0.03). Postoperatively, both designs showed significant improvements in pain, range of motion, limb alignment, KSS knee and function scores (all p<0.001). Functional outcomes were similar for UC and PS, except for better stair-climbing in Charnley A patients with UC (p=0.037). Pain, range of motion and KSS scores did not differ significantly. PS knees showed better anteroposterior stability (p=0.045). There was a trend to fewer radiolucent lines with UC (p=0.08). Overall satisfaction was high and comparable. Conclusion : At mid-term follow-up, UC TKA provided functional, clinical and radiological outcomes equivalent to PS TKA, with less bone resection, shorter operative time, better stair climbing in Charnley A patients and a tendency toward fewer tibial radiolucent lines, at the cost of a small increase in anteroposterior laxity that remained within acceptable limits. UC inserts can be recommended as a safe and effective alternative to PS designs.