Kinematical comparison between ultra-congruent and posterior-stabilized total knee arthroplasty: dynamic RSA study

Introduction: In primary total knee arthroplasty (TKA), the ultra-congruent (UC) design has been proposed as a possible alternative to posterior-stabilized (PS) design. Although the claimed advantages were the achievement of the same stability while sparing femoral bone stock for further revision and the reduction of the intraoperative complications, kinematical analyses showed conflicting results. The aim was to compare the in-vivo kinematics of PS and UC TKA. Material and methods: A cohort of 16 randomly selected patients (8 PS Persona Zimmer, 8 UC Persona Zimmer; mean age 70.0 ± 7.3 years old) was evaluated through dynamic radiostereometric analysis (RSA) at a minimum of 9 months after TKA, during the execution of a sit-to-stand motor task. The anteroposterior (AP) translation of the femoral component and the AP translation of the Low Point (tibio-femoral contact point) of medial and lateral femoral compartments were compared through Student’s t test (p < 0.05). Results: A significantly greater anterior translation of the femoral component was found for the PS group compared to the UC group, mainly to almost complete extension (p=0.017). Both groups showed a significantly greater anterior translation of the Low Point of the lateral compartment with respect to the medial one (PS: p=0.012, UC: p= 0.018). This was consistent with a medial-pivot pattern. Furthermore, a comparison of the anterior translation of the medial compartment between the PS group and the UC group was statistically significant and showed a greater excursion in the PS group (p= 0.001). The same pattern was observed for the lateral compartment (p=0.006). Interpretation and Conclusions: The TKA designs evaluated showed comparable in-vivo kinematics in terms of medial pivot pattern, but differences in terms of absolute AP translation. Further studies on larger samples are needed to understand the correlation between in-vivo implant kinematics and functional outcomes at long-term follow-up.