Fixed-bearing medial unicompartmental knee arthroplasty restores pre-arthritic coronal alignment and achieves satisfactory functional outcomes: a retrospective study

Abstract Purpose This study aimed to (1) analyze the Coronal Plane Alignment of the Knee (CPAK) classification in patients undergoing unicompartmental knee arthroplasty (UKA), (2) assess whether UKA can restore pre-arthritic coronal alignment by utilizing the arithmetic hip-knee-ankle angle (aHKA), and (3) evaluate the relationship between patient-reported outcome measures (PROMs) and postoperative alignment following UKA. Methods We retrospectively analyzed 152 consecutive patients who underwent fixed-bearing medial UKA. A radiological analysis was conducted using an EOS imaging system. Postoperative alignment was classified as neutral (varus ≤ 3°), mild varus (3° < varus ≤ 7°), or marked varus (varus > 7°) based on the mechanical hip-knee-ankle angle (mHKA). The aHKA was calculated and the CPAK classification was used to categorize knee phenotypes. The PROMs were obtained both before and after the surgery. Results A total of 152 knees from 113 patients were included with a mean two-year follow-up. The mean difference between the postoperative mHKA and the estimated aHKA was 0.80° (90% CI 0.35 to 1.24; P = 0.003). Postoperatively, 63 (41.45%) of the 152 knees were in neutral group, 54 (35.53%) mild varus, and 35 (23.03%) marked varus. The neutrally aligned cohort did not exhibit significantly higher scores in range of motion, VAS, HSS, WOMAC, or FJS-12 scores compared to the mild or marked varus cohort (P = 0.205, 0.118, 0.076, 0.140, and 0.788, respectively) during the short-term follow-up. Conclusion Non-robotically assisted, fixed-bearing medial UKA can restore pre-arthritic coronal alignment and achieve satisfactory PROMs. Fixed-bearing medial UKA aims to restore the pre-arthritic alignment rather than achieving neutral mechanical alignment.