Limited Femoral Joint-Line Alteration and Preserved Extension Lateral Laxity Enhanced Early Outcomes in Restricted...

Restricted kinematic alignment (rKA) total knee arthroplasty (TKA) seeks to restore native alignment while limiting excessive correction. The influence of alignment change magnitude and intraoperative gap characteristics on early patient-reported outcome measures (PROMs) after robotic rKA TKA remains unclear.

This retrospective cohort study included 69 robotic-arm-assisted cruciate-retaining TKAs performed using an rKA protocol. Intraoperative medial and lateral extension and flexion gaps-extension medial gap, extension lateral gap, flexion medial gap (FMG), and flexion lateral gap-were recorded automatically. Preoperative and postoperative hip-knee-ankle (HKA) angle, lateral distal femoral angle (LDFA), and medial proximal tibial angle were measured using a validated deep learning model. Multivariable analyses of covariance evaluated associations between demographics, baseline alignment, alignment changes, gap characteristics, and PROMs at one and two years.

All PROMs improved postoperatively. Smaller changes in HKA and LDFA were independently associated with higher two-year Knee Society Knee Scores (P = 0.001 and P = 0.013). Greater ELG was associated with a higher two-year Forgotten Joint Score (P = 0.001), whereas greater FMG was associated with lower two-year Western Ontario and McMaster Universities Osteoarthritis Index pain and stiffness scores (P = 0.011 and P = 0.002). Receiver operating characteristic analyses identified changes in LDFA less than 2.3° (area under the curve [AUC] 0.622) and changes in HKA less than 8.4° (AUC 0.541) as thresholds associated with higher Knee Society Knee Score. An ELG ≥ 3.0 mm predicted higher Forgotten Joint Score (AUC 0.658), and FMG ≥ 1.5 mm predicted lower Western Ontario and McMaster Universities Osteoarthritis Index stiffness (AUC 0.716). No revisions occurred.

In robotic cruciate-retaining rKA TKA, minimizing femoral joint-line alteration and preserving controlled lateral extension laxity, along with mild medial flexion laxity, were associated with modestly improved early outcomes. These findings are exploratory and require validation in larger cohorts with longer follow-up.