Does a medial congruent bearing in total knee arthroplasty compromise fixation? A randomized controlled trial

Medially congruent (MC) bearings aim at promoting medial pivoting after total knee arthroplasty (TKA), as the congruency provides further constraint for the medial femoral compartment. However, this design difference could alter intra-articular force distribution, potentially compromising fixation of the tibia implant. The aim of this study was to compare migration, measured with radiostereometric analysis (RSA), of an MC to a more traditional cruciate retaining (CR) TKA system. Secondary aims were to compare patient-perceived treatment outcome and number of complications between the treatment groups.

Sixty patients undergoing TKA were randomized to an MC or CR bearing and had follow up visits after 3 months, 1 and 2 years. Primary outcome was tibia implant migration, measured by maximal total point motion (MTPM) with model-based RSA, 2 years post-surgery. Secondary outcomes were tibia MTPM, change in patient-reported outcome measurements (PROMs) and number of complications registered at all follow up visits.

Primary outcome was available for 52 patients (27 MC patients, 25 CR patients). We found no difference in tibia MTPM between the MC and CR groups 2 years post-surgery. Median (interquartile range) MTPM was 0.60 (0.39-0.97) mm and 0.48 (0.32-0.78) mm in the MC and CR group, respectively (P = 0.167). There were no between-group differences in improvement in PROMs and no between-group differences in number of complications.

We found no compromising of tibia implant fixation in TKA by choosing an MC bearing, when compared with a CR bearing. PROMs and complication rates suggest comparable treatment results with both types of bearings in TKA.