Functional Alignment Reduces Residual Lateral Laxity and Improves Patient Satisfaction Compared With Mechanical Alignment Using a 3° Medially Inclined Joint Line Implant in Constitutional Varus Knees

Purpose This study compared postoperative laxity and clinical outcomes between mechanically aligned total knee arthroplasty (MA-TKA) and functional alignment TKA (FA-TKA) using a 3° medially inclined joint line implant in patients with Coronal Plane Alignment of the Knee (CPAK) type 1. Methods This retrospective cohort study included patients with preoperative CPAK type 1 who underwent primary unilateral TKA for osteoarthritis with ≥12 months follow-up. All procedure used an implant with a 3° medially inclined joint line and a bi-cruciate substituting design, performed using manual MA-TKA (2017–2021) or robotic-assisted FA-TKA (2022–2024). Among 155 eligible patients, 44 matched pairs were generated using propensity matching for age, sex, follow-up duration and preoperative Forgotten Joint Score-12 (FJS-12). Radiographic parameters including the arithmetic hip–knee–ankle angle (aHKA) and joint line obliquity (JLO), were measured pre- and postoperatively. Varus–valgus stress radiographs at 1 year quantified medial and lateral joint opening. Patient-reported outcomes included FJS-12, Knee Society Score (KSS), and Knee Injury and Osteoarthritis Outcome Scores (KOOS). Results At 1 year, lateral opening under varus stress was smaller after FA-TKA (4.8 ± 1.9 vs 6.0 ± 2.1; p =0.03), whereas medial opening under valgus stress did not differ. FA-TKA produced a more varus postoperative aHKA (−2.4 ± 1.8 vs −1.4 ± 2.6; p = 0.03) and required less coronal correction (ΔaHKA: 3.0 ± 2.8 vs 4.6 ± 3.3; p = 0.01). Greater coronal correction correlated with larger lateral opening (r = 0.39; p = 0.0006). FA-TKA demonstrated greater improvement in KSS satisfaction scores (18.9 ± 8.2 vs 15.2 ± 8.3; p = 0.04). FJS-12 scores were higher after FA-TKA; however, not statisticaly significant. Conclusion In CPAK type 1 knees, robotic-assisted FA-TKA better preserved coronal alignment and reduced postoperative lateral laxity compared with manual MA-TKA, resulting in higher patient satisfaction at 1 year.