Investigating the Effect of Center of Rotation of Angulation (CORA) Location on Varus Knee Joint Mechanics.

Objectives: Malalignment of the lower limb, such as varus deformity, can impair function by altering joint biomechanics and gait. However, the effect of the center of rotation of angulation (CORA) location on joint mechanics in varus deformity remains unclear. Methods: This study proposed an approach to enhance the accuracy and efficiency of joint kinetics and kinematics estimation during gait by incorporating precise CORA positioning along the femur. Simulations were performed using inverse kinematics and dynamics for a 10° varus deformity. The deformities were modeled with CORA located at fractions ranging from 1/6 to 5/6 of femoral length (measured from the hip to the knee joint), and were compared with a baseline representing normal lower-limb alignment. Results: The findings indicated that varus deformity had a minimal effect on the subtalar angle but substantially altered the subtalar moment during stance. CORA placement near the hip joint increased both ankle plantar-/dorsiflexion and knee flexion/extension angles. Proximal CORA positioning also influenced joint reaction forces, producing higher forces at the ankle and reduced forces at the hip. While hip abduction/adduction moments showed only minor changes, hip rotation moments varied considerably across CORA locations, particularly during stance. Conclusion: Incorporating CORA into deformity modeling enables more accurate simulation of pathological alignment, providing detailed insights into joint kinetics and kinematics. Such information can help surgeons better understand patient-specific conditions and develop more precise surgical plans.