Purpose: Limited or absent axial rotation of the mobile insert of total knee prostheses could lead to high contact stresses and stresses at the bone-implant interface, which in turn might lead to implant loosening. The aim of this study was to assess knee kinematics and muscle activation and their possible change over time in patients with a highly congruent, mobile-bearing total knee prosthesis.
Methods: A prospective series of 11 rheumatoid arthritis patients was included to participate in this fluoroscopic and EMG study; only 7 patients completed the study. Kinematic evaluations took place 7 months, 1 and 2 years post-operatively. Repeated measurements ANOVA and linear mixed-effects model for longitudinal data were used to compare the differences between the follow-ups.
Results: There are no significant changes in axial rotations between follow-up moments for the femoral component as well as the mobile insert. The insert remained mobile and followed the femoral component from 0° until approximately 60° of knee flexion. Diverging and reversed axial rotations and translations were seen during the dynamic motions.
Conclusions: Knee kinematics and muscle activation do not appear to change in the first 2 post-operative years. Reversed and divergent axial rotations with increasing knee flexion indicate that as soon as the congruency decreases, the femoral component is no longer forced in a certain position by the insert and moves to a self-imposed position. At lower knee flexion angles, the femoral component might be obstructed by the highly congruent insert and therefore might not be able to move freely.
Level of evidence: Therapeutic study, Level IV.