Early migration of the cemented tibial component of unicompartmental knee arthroplasty: a radiostereometry study.

Purpose: Unicompartmental knee arthroplasty has good clinical results but high revision rates. A unicompartmental knee arthroplasty design features an all-polyethylene and conforming tibial component, and we hypothesized that this may put at risk its fixation. Implant-to-bone micromotion was measured together with relevant clinical outcomes. Methods: The migration of the tibial component in twenty patients was measured at 3, 6, 12 and 24 months, using standard radiostereometry, along with the relevant clinical outcomes using the IKS scoring system. Results: The eighteen arthroplasties at 24 months were found successful, with very good functional (mean 87.7; SD 15.4) and knee scores (mean 94.8; SD 10.1). The means and the standard deviations of the maximum total point motion (MTPM) for the four follow-ups were, respectively, as small as 0.4 ± 0.1 mm, 0.6 ± 0.2 mm, 0.6 ± 0.3 mm and 0.7 ± 0.3 mm, an average over all patients but one. In this knee, these were 1.6, 2.1, 2.4 and 2.2 mm, therefore not at high risk of aseptic loosening according to the literature. Only one knee was found at this risk, having the MTPM from 12 to 24 months of 0.5 mm, and the component moving and sinking medially, and rising laterally. Conclusion: At 2-year follow-up, a successful implant-to-bone fixation can be achieved in conforming all-polyethylene cemented tibial component together with excellent clinical outcomes. Level of evidence: Prognostic studies, Level I