Mechanical Versus Kinematic Alignment in Total Knee Arthroplasty: Relationship Between Postoperative Alignment and Short-Term Functional Outcomes

Background/Objectives: Mechanical alignment (MA) has traditionally been the reference alignment strategy in total knee arthroplasty (TKA), whereas kinematic alignment (KA) has emerged as an individualized alternative intended to approximate patient-specific knee anatomy. However, the relationship between postoperative mechanical-axis alignment and functional recovery remains incompletely understood. This study aimed to compare short-term functional outcomes and postoperative mechanical-axis alignment between MA and KA following primary TKA and to investigate the relationship between measured postoperative alignment parameters and functional outcomes. Methods: This single-center retrospective comparative cohort study included 120 patients who underwent primary unilateral TKA for end-stage knee osteoarthritis (KOA) between January 2022 and December 2024; 51 underwent MA and 69 underwent KA. Radiographic assessment included the hip–knee–ankle (HKA) angle, mechanical-axis deviation (MAD), residual varus deformity, and radiographic knee extension deficit. Functional outcomes were assessed using the Knee injury and Osteoarthritis Outcome Score (KOOS), the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and the Knee Society Score (KSS). Preoperative, postoperative, and change-from-baseline (Δ) functional scores were evaluated. Analysis of covariance (ANCOVA) was used to assess associations between alignment strategy and postoperative functional outcomes after adjustment for baseline functional scores and relevant covariates. Associations between postoperative radiographic parameters and functional outcomes were additionally assessed. Results: Both alignment groups demonstrated substantial preoperative-to-postoperative functional improvement. MA achieved postoperative HKA alignment closer to the neutral mechanical axis than KA (179.63 ± 1.23° vs. 178.24 ± 2.32°; p < 0.001; FDR-adjusted p < 0.001). Absolute HKA deviation from neutral was lower with MA (0.97 ± 0.83° vs. 2.42 ± 1.62°; FDR-adjusted p < 0.001), as was postoperative absolute MAD (5.19 ± 4.53 mm vs. 7.86 ± 6.76 mm; p = 0.011; FDR-adjusted p = 0.018). KA was associated with a higher unadjusted postoperative KOOS score than MA (84.04 ± 12.33 vs. 77.12 ± 15.65; p = 0.010), whereas postoperative WOMAC (p = 0.206) and KSS (p = 0.843) did not differ significantly between groups. Between-group differences in change from baseline were not statistically significant for KOOS (p = 0.227), WOMAC (p = 0.130), or KSS (p = 0.427). After multivariable adjustment, no statistically significant between-group associations were observed for postoperative KOOS (adjusted β = 2.42; 95% CI, −0.93 to 5.77; p = 0.155), WOMAC (adjusted β = −2.80; 95% CI, −6.73 to 1.14; p = 0.162), or KSS (adjusted β = 1.09; 95% CI, −2.05 to 4.24; p = 0.492). Associations between the postoperative radiographic parameters and functional outcomes were negligible to weak, with none remaining statistically significant after FDR correction. Conclusions: Both MA and KA were associated with substantial short-term functional improvement, with no significant between-group differences in the magnitude of improvement from baseline. MA achieved postoperative alignment closer to the neutral mechanical axis, while the higher unadjusted postoperative KOOS score observed with KA was not statistically significant after adjustment for baseline characteristics and potential confounders. No significant postoperative differences in WOMAC or KSS were observed. The generally weak associations between measured postoperative mechanical-axis parameters and functional outcomes suggest that mechanical-axis alignment alone may not adequately explain short-term functional recovery. Because patient-specific coronal alignment parameters and achievement of the individualized KA target were not quantitatively assessed, the present findings cannot determine whether restoration of patient-specific anatomy was associated with functional outcomes.