The impact of systematic displacement of the lateral knee marker on gait kinematics using the virtual knee alignment device and the Plug-in Gait model

BACKGROUND: Marker-based motion capture is widely used in clinical gait analysis, but errors due to incorrect marker placement can impact kinematic results. When using the Plug-in Gait model (PiG, Vicon, Oxford, UK) the virtual Knee Alignment Device (vKAD, Prophysics, Kloten, Switzerland) is designed to automatically correct marker displacements on the lateral shank and femur. However, the impact of lateral knee marker displacement on gait kinematics using vKAD has not been studied.

RESEARCH QUESTION: Does systematic lateral knee marker displacement have a significant effect on the maximum angles of the knee (sagittal, frontal, transverse) and hip joint (transverse) during swing phase when using PiG and vKAD?

METHODS: Twelve healthy adults (27.9 ± 7 years, 173.1 ± 9 cm, 68.9 ± 7 kg) underwent gait analysis using PiG and vKAD in five marker placement conditions of the lateral knee marker on both legs. Conditions included the correct anatomical position (reference) and systematic displacements of 1 cm and 2 cm anterior/posterior to the reference. A two-way repeated ANOVA compared maximum joint angles of the knee (sagittal, frontal, transverse) and hip (transverse) during the swing phase between conditions.

RESULTS: All selected joint angles were significantly affected by the marker placement conditions. A 2 cm displacement to the reference position resulted in the following kinematic deviations: hip internal rotation 7°/-11°, knee flexion 2°/-4°, knee varus/adduction 7°/-8°, and knee internal rotation 1°/-4°.

SIGNIFICANCE: Even a 1 cm anterior displacement of the lateral knee marker led to an unphysiological knee varus range of motion in the swing phase, overestimation hip internal rotation. These findings have practical implications for researchers and clinicians as they can serve to optimize the knee joint axis and enhancing accuracy of instrumental gait analysis. Improved accuracy, particularly in hip rotation, is crucial for clinicans working with patients like those with cerebral palsy.