Kinematic Alignment Using Conventional Mechanical-Alignment Instruments with 3D-Printed Tibial Recut Guide Versus Manual Mechanical Alignment in Total Knee Arthroplasty: An IPTW-Adjusted Comparative Cohort Study

Background/Objectives: Unrestricted kinematic alignment (KA) may produce excessive tibial varus in some knees. We evaluated a modified KA technique using conventional mechanical instruments combined with a standardized 3D-printed 2° tibial recut guide that retained calipered femoral resurfacing while selectively restricting tibial varus, comparing it with historical manual mechanical alignment (MA) controls. Methods: This single-centre comparative cohort study included 120 knees from 104 patients (66 consecutive prospective KA knees and 54 historical MA knees). Inverse probability of treatment weighting targeting the average treatment effect on the treated adjusted for age, sex, body mass index, ASA class, and preoperative hip–knee–ankle angle. Treatment-effect inference used patient-level cluster-robust standard errors across 104 patient clusters. Outcomes included the 2011 Objective Knee Society Score (Objective KSS), patient-reported outcomes, and postoperative coronal alignment. Results: The recut guide was used in 37/66 KA knees (56.1%). KA was associated with a higher Objective KSS (ATT +15.47; 95% CI 9.07–21.87; p < 0.001), driven mainly by alignment (+11.93; 95% CI 5.93–17.94; p < 0.001) and stability (+2.16; 95% CI 0.54–3.79; p = 0.009), while the Objective KSS ROM component did not differ significantly. Patient-reported outcomes were not significantly different, including WOMAC (ATT −4.16; 95% CI −8.33 to 0.02; p = 0.051), OKS (ATT −1.89; 95% CI −3.88 to 0.10; p = 0.062), FJS (ATT +10.70; 95% CI −1.31 to 22.71; p = 0.081), and total KSS (ATT +5.17; 95% CI −2.36 to 12.70; p = 0.179). KA showed less residual varus HKA (−2.03° vs. −4.61°), fewer postoperative HKA outliers beyond ±6° (6.1% vs. 38.9%), higher FMA (91.20° vs. 89.26°), and a more horizontal joint line, whereas TMA did not differ significantly (p = 0.331). Conclusions: Modified KA with selective tibial varus restriction was associated with a higher postoperative Objective KSS and more favorable coronal alignment than historical manual MA, while patient-reported outcomes were largely comparable. These findings support the technical feasibility of the approach but do not establish causal or clinical superiority over contemporary MA.