A new cementless tibial baseplate in kinematically aligned total knee arthroplasty has low migration and stabilizes at 3 months: A cohort study using radiostereometric analysis.

PURPOSE: Introduction of new components in total knee arthroplasty (TKA) should be assessed for long-term stability. The primary objective was to determine whether migration of a new cementless tibial baseplate in kinematically aligned (KA) TKA is below the 6-month threshold where baseplates have a low incidence of long-term loosening. Other objectives were to determine when stability occurred, whether stability was associated with varus angles in knee and limb alignments, and whether a recut of the tibia affected migration. METHODS: New cementless components were implanted in 45 patients, of which 38% had a tibial recut. Baseplate migration was assessed using model-based radiostereometric analysis with radiographs obtained post-operatively and at 6 weeks, 3 months and 6 months. Mean maximum total point motion (MTPM) at 6 months was compared to a 6-month threshold where baseplates have low incidence of long-term loosening. An equivalence analysis determined when stability occurred. Correlation analyses related MTPM at 6 months to the medial proximal tibial angle (MPTA) and the hip-knee-ankle angle (HKAA). A two-sample t test determined whether mean MTPM differed between recut and non-recut groups. RESULTS: Mean 6-month MTPM of 0.93 mm was significantly below the 6-month stability threshold of 1.15 mm (p = 0.015), and stability was achieved by 3 months. There was no significant relationship between MTPM and MPTA (ρ = 0.050, p = 0.77) or HKAA (ρ = 0.060, p = 0.72). Mean MTPM did not differ between recut (0.85 ± 0.56 mm) and non-recut (0.97 ± 0.64 mm) groups (p = 0.57). CONCLUSION: The new cementless tibial baseplate has migration significantly below the 6-month threshold where baseplates have a low incidence of long-term loosening and migration stabilizes early at 3 months. Varus alignment of the baseplate and the limb as well as tibial recuts were not associated with increased migration. LEVEL OF EVIDENCE: N/A.