Biomechanical Effects of Different Varus and Valgus Alignments in Medial Unicompartmental Knee Arthroplasty

Background: Medial unicompartmental tibial components are not always positioned following neutral mechanical alignment and a tibial varus alignment of 3° has been suggested based on several clinical follow-up studies. However, no biomechanical justification is currently available to confirm the suitability of different alignment positions.

Methods: This study aims at quantifying the effects on bone stresses, load distribution, ligament strains, and polyethylene insert stress distribution induced by a possible varus/valgus alignment in medial unicompartmental knee arthroplasty, ranging from 6° of varus to 6° of valgus, developing and using a validated patient-specific finite element model.

Results: Results demonstrate that both neutral mechanical and 3° of varus alignment induce lower stress distributions than valgus or a higher varus alignment for which higher values, up to 40%, are achieved for the polyethylene stress. When a unicompartmental knee arthroplasty is implanted, a mismatch in the stiffness of the joint is introduced, changing the load distribution from medial to lateral for all configurations with respect to the native configuration. However, slight differences are noticeable among the different configurations with a maximum of 190 N and 90 N for the lateral and the medial side, respectively.

Conclusion: Neutral mechanical or 3° of varus alignment present similar biomechanical outputs in the bone, collateral ligament strain, and on the polyethylene insert. A 6° varus alignment or changes in valgus alignment were always associated with more detrimental effects.