Low tibial baseplate migration 1year after unrestricted kinematically aligned total knee arthroplasty using a medial conforming implant design.

Abstract

Varus alignment of the tibial baseplate and limb > 3° might adversely affect baseplate fixation after total knee arthroplasty (TKA), especially for unrestricted kinematically aligned (KA) TKA which aligns a majority of baseplates in varus. The purposes of this study were to determine whether baseplate migration at 1year (1) was significantly less than a stability limit of 0.5mm, (2) increased over time, and (3) was related to varus alignment of the baseplate and limb after unrestricted KA TKA. Thirty-five patients underwent unrestricted KA TKA using a fixed-bearing, cemented, medial conforming tibial insert with posterior cruciate ligament retention. Using model-based radiostereometric analysis, maximum total point motion (MTPM) (i.e., largest displacement on the baseplate) was computed at 6weeks, 3months, 6months, and 1year postoperatively relative to the day of surgery. Baseplate and limb alignment were measured postoperatively on long-leg CT scanograms. At 1year, mean MTPM of 0.35mm was significantly less than the 0.5mm stability limit (p = 0.0002). Mean MTPM did not increase from 6weeks to 1year (p = 0.3047). Notably, 89% (31/35) of tibial baseplates and 46% (16/35) of limbs were > 3° varus. Baseplate and limb alignment had no relationship to MTPM at 1year (|r|≤ 0.173, p ≥ 0.3276). Low and non-progressive tibial baseplate migration 1year after unrestricted KA TKA with a medial conforming design should allay any concern that unrestricted KA TKA increases risk of baseplate looseningdue to varus alignment of the baseplate and limb. Level II, therapeutic prospective cohort study.