Favorable midterm clinical results of medial unicompartmental knee arthroplasty guided by coronal limb alignment using an image-free navigation system

Medial unicompartmental knee arthroplasty results in good postoperative motion and high patient satisfaction. However, inaccurate bone resection during manual procedures can cause tibial fractures, implant subsidence, or lateral compartment osteoarthritis. To improve accuracy, we performed medial unicompartmental knee arthroplasty using an image-free navigation system, with a target postoperative alignment of 2°–5° varus. This study evaluated midterm clinical outcomes and safety. This single-center retrospective case series included 144 consecutive knees (25 in men and 119 in women; mean age, 72.2 years) that underwent medial unicompartmental knee arthroplasty for varus osteoarthritis between 2011 and 2021, with at least three years of follow-up (mean, 63.1 months). A fixed-bearing implant (Triathlon PKR; Stryker) was used in all cases. The navigation system guided tibial and femoral resections to achieve a hip–knee–ankle angle of 2°–5° varus, while minimizing tibial resection. Pre- and postoperative outcomes were compared for range of motion, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), ability to sit in the seiza position, implant survival, and complications. Mean knee extension improved from − 5.7° to − 0.6°, flexion from 141.6° to 145.9°, and the mean WOMAC score improved from 41.5 ± 9.9 to 4.9 ± 3.5. The number of patients able to sit in the seiza position increased significantly. Four knees underwent lateral unicompartmental knee arthroplasty for progression of lateral osteoarthritis, and one was converted to total knee arthroplasty for infection. No tibial fractures or implant subsidence occurred. The Kaplan–Meier implant survival rate, with failure defined as reoperation for any cause, was 96.5%. Alignment-guided medial unicompartmental knee arthroplasty targeting 2°–5° varus using image-free navigation achieved favorable midterm outcomes without major complications. Level IV.