Dynamic coronal plane knee alignment: Femoral anatomy determines kinematic curve morphology, tibial anatomy determines curve position.

Abstract

A single hip-knee-ankle angle (HKA) angle does not reflect the biomechanics of native, arthritic or prosthetic knees. Since HKA varies throughout flexion, dynamic coronal alignment is best represented by a kinematic curve plotting HKA against the range of motion. This study aimed to evaluate the relationship between kinematic HKA curves and the bony morphology of the distal femur and proximal tibia. We hypothesised that variations in distal femoral and proximal tibial anatomy are associated with distinct patterns of dynamic coronal alignment. This was an experimental study using a non-weight-bearing articulated surgical education bone model including hemipelvis, femur and tibia. Articular surfaces and bony landmarks were registered with a computer navigation system. Using medial opening wedge femoral and tibial osteotomies and a rotational femoral osteotomy, 70 morphotypes were created by altering distal femoral angle (DFA), proximal tibial angle (PTA) and femoral axial angle (FAA). For each configuration, a coronal kinematic curve was recorded from 0° to 120° of flexion. Five curve morphotypes were identified: straight, drift, inverse drift, C-shaped and inverse C-shaped. DFA and FAA differed significantly among morphotypes (p < 0.001), whereas PTA had no effect (p = 0.084). Paired comparisons confirmed significant differences in DFA and FAA across curve types. In this sawbone model, dynamic coronal plane alignment curve morphology was determined by distal femoral coronal and torsional anatomy, while tibial anatomy shifted the curve position without altering morphology. Restoring the pre-arthritic curve in TKA requires restoring DFA and FAA, whereas achieving a neutral straight curve requires individualised FAA adjustment. Consistently producing a neutral straight curve is not possible without computer-assisted or robotic surgery. These findings require validation in cadaveric or clinical studies but may guide surgical strategies aiming to reproduce native knee kinematics. N/A.