Postural coronal alignment shift after total knee arthroplasty diminishes with age and becomes negligible in elderly patients.

Purpose: To investigate whether patient age moderates the change in coronal hip-knee-ankle (HKA) alignment between supine and standing positions following total knee arthroplasty (TKA).

Methods: In this cross-sectional study, 113 consecutive patients after primary TKA for osteoarthritis underwent standardized full-length radiographs in both supine and standing positions. The postural shift (ΔHKA) was defined as standing minus supine alignment. Associations between age and ΔHKA were examined using a linear mixed-effects model with restricted cubic splines, adjusting for body mass index (BMI), sex, preoperative alignment and time since surgery. Equivalence testing (±1° margin) was performed in patients aged ≥60 years.

Results: Younger patients demonstrated a standing varus drift of approximately 2-3°, whereas the effect progressively diminished with advancing age. The age-position interaction was significant (p = 0.011), while BMI, sex, preoperative alignment and time since surgery were not. In patients ≥60 years, the mean ΔHKA was -0.01° (90% confidence interval = -0.55° to +0.53°), meeting statistical equivalence to 0 (p = 0.008). Collateral ligament laxity on stress radiography showed no association with ΔHKA.

Conclusion: Postural coronal alignment change after TKA is strongly age dependent. Younger patients show a measurable varus drift, whereas older patients demonstrate negligible change within radiographic error. Age should be considered when tailoring intraoperative alignment strategies to ensure durable functional outcomes.

Level of evidence: Level III.