Achieving a neutral hip-to-calcaneus axis in kinematically aligned total knee arthroplasty equalizes coronal hindfoot pressure balance at initial ground contact in the gait cycle

Aims Evaluating plantar pressure distribution and coronal lower limb alignment, including the hindfoot, using the hip-to-calcaneus axis, known as the ground mechanical axis (GA), is valuable in total knee arthroplasty (TKA). This study aimed to compare postoperative changes in plantar pressure distribution and lower limb alignment between mechanically aligned TKA (MA-TKA) and ground kinematically aligned TKA (gKA-TKA), targeting neutral GA. Methods After 1:1 propensity score matching, 35 pairs of patients with end-stage osteoarthritis of the knee, who underwent gKA-TKA and MA-TKA with similar preoperative disability between September 2019 and March 2022, were compared. Plantar pressure distribution during walking and unipedal stance was measured preoperatively and one year postoperatively using a pressure plate. The hip-knee-calcaneus angle (HKC) (positive values = valgus) and the percentage of the GA passing position at the knee joint (%GA; medial edge, 0%; lateral edge, 100%) were assessed on long-leg radiographs in unipedal stance. Results MA-TKA showed a medial loading pattern in the hindfoot. gKA-TKA exhibited a nearly equal pressure distribution in the coronal plane of the hindfoot. Postoperative centre-of-pressure path length during unipedal stance was significantly better for gKA-TKA than MA-TKA (p = 0.043). Furthermore, mean HKC angle and mean %GA of the MA-TKA were 2.0° (SD 2.4°) and 54.2% (SD 9.0%), respectively; those of the gKA-TKA were -0.2° (SD 1.0°) and 49.6% (SD 5.0%), respectively. Significant correlations were observed between the HKC angle (r = 0.391 (95% CI 0.172 to 0.573)) or %GA (r = 0.343 (95% CI 0.117 to 0.535)) and the hindfoot plantar pressure pattern in the coronal plane, indicating that postoperative lower limb valgus deformity in the hip-to-calcaneus axis results in a medial loading pattern. Conclusion gKA-TKA can provide more neutral weightbearing in the GA, more equal coronal hindfoot pressure during walking, and improved stability in unipedal stance compared with MA-TKA. Cite this article: Bone Joint J 2025;107-B(6):604–614.