Changes in knee phenotype in coronal plane alignment after medial unicompartmental knee arthroplasty did not affect clinical outcomes

Background Studies on total knee arthroplasty (TKA) demonstrated increased positive patient-reported outcomes when the coronal plane alignment of the knee (CPAK) phenotype was preserved. However, reports on outcomes related to changes in CPAK following unicompartmental knee arthroplasty (UKA) remain limited. Methods One hundred forty-five patients undergoing unilateral medial UKAs with pre- and postoperative full-body anteroposterior standing radiographs were enrolled in the study. Angles in the coronal plane alignment, including medial proximal tibial angle (MPTA), lateral distal femoral angle (LDFA), arithmetic hip–knee–ankle angle (aHKA), and joint line obliquity (JLO), were measured. Postoperatively, patients were grouped into restored and altered CPAK groups. With a minimum 1-year follow-up (FU), patient-reported outcome measures, including the Forgotten Joint Score-12 (FJS-12), Oxford Knee Score (OKS), and 12-Item Knee Injury and Osteoarthritis Outcome Score (KOOS-12), were evaluated and compared between the two groups. Results Among 145 patients, 76 with true anteroposterior radiographs completed the final FU. The mean age, BMI, and FU time were 69.5 years, 27.19 kg/m2, and 23.2 months, respectively. The two most common preoperative CPAK phenotypes were type I (52.6%) and type II (32.9%). With no differences in preoperative MPTA, LDFA, Kellgren–Lawrence (KL), and Ahlbäck grade, 35 patients had their CPAK restored, and 41 had altered CPAK. The postoperative aHKA was not overcorrected toward valgus alignment (aHKA > 2 degrees) in either group, and similarly significant improvements were observed in FJS-12, OKS, and KOOS-12 scores from baseline. The altered CPAK group demonstrated significantly higher MPTA and JLO values than the restored CPAK group, corresponding to a shift in CPAK phenotype, most commonly from type I to IV or VII and from type II to VII or VIII. In addition, clinical outcomes of 65 patients with unsuitable radiographs for CPAK evaluation were similar to those of included patients in the final analysis. Conclusion Both restored CPAK and altered CPAK patients after medial UKA showed similar improvements in short-term patient-reported outcomes at a mean 23-month FU. Mid- to long-term clinical outcomes in patients with changes in knee phenotype should be evaluated.