Influence of component alignment on outcome for unicompartmental knee replacement

This study's aim was to assess the effect of component mal-alignment on outcome of Oxford Unicompartmental Knee Replacement (UKR). Two hundred and eleven knees implanted with a medial UKR, using a minimally invasive approach, were followed up at a minimum of 4 years. Sagittal and frontal plane femoral and tibial component alignments were determined from antero-posterior and lateral radiographs. The cohort was divided into alignment groups which represented consecutive 2.5 degrees intervals over the range of measured values for femoral varus/valgus, femoral flexion/extension, tibial varus/valgus and tibial tilt. The Oxford Knee Score (OKS) and incidence of radiolucency (RL) were compared between alignment groups for each alignment parameter. In 98% of cases the femoral components were positioned between 10 degrees varus and 10 degrees valgus; all femoral components were within +/-10 degrees flexion. In 92% of cases the tibial components were positioned between +/-5 degrees varus/valgus and superior/inferior tilt (neutral tilt being 7 degrees). Within these ranges there were no significant differences in OKS or RL between the alignment groups; nor were there any differences between those at the extremes of component alignment compared to those in the inner ranges of alignment. We conclude that, because of the spherical femoral component, the Oxford UKR is tolerant to femoral mal-alignment of 10 degrees and tibial mal-alignment of 5 degrees.