No kinematical difference between ultra‐congruent and medial‐congruent total knee arthroplasty when implanted with mechanical alignment: an in vivo dynamic RSA study

Purpose To explore in vivo kinematical behavior of the same total knee arthroplasty (TKA) cruciate‐retaining (CR) femoral design with either medial‐congruent (MC) or ultra‐congruent (UC) inlay using model‐based dynamic radiostereometric analysis (RSA). The hypothesis was that there would be comparable kinematics between the two groups. Methods A cohort of 16 randomly selected patients (8 MC Persona Zimmer, 8 UC Persona Zimmer) was evaluated through dynamic radiostereometric analysis (RSA) at a minimum of 9 months after TKA, during the execution of a sit‐to‐stand. The antero‐posterior (AP) translation of the femoral component and the AP translation of the low point of medial and lateral femoral compartments were compared through Student’s t test ( p  < 0.05). Results Both groups showed a medial pivot behavior, with a significantly greater anterior translation of the Low Point of the lateral compartment with respect to the medial compartment (MC medial range: 2.4 ± 2.4 mm; MC lateral range: 7.7 ± 3.0 mm; p  < 0.001 – UC medial range: 3.3 ± 3.3 mm; UC lateral range: 8.0 ± 3.2 mm; p  < 0.001). A statistically significant greater degree of flexion was clinically recorded at follow‐up visit in the MC group respect to the UC group (126° vs 101°— p  = 0.003). Conclusion The present study did not show difference in the medial pivot behavior between ultra‐congruent and medial‐congruent total knee arthroplasty when implanted with mechanical alignment; however, the MC group demonstrated a greater degree of flexion. The MC design examined is a valid alternative to the UC design, allowing to achieve a screw‐home movement restoration combined with a high flexion. Level of evidence IV.