Evaluating Knee Exoskeleton Design Based on Movement With Respect to Underlying Bone Structure Using MRI

Knee braces and exoskeletons can increase stability, provide support, and functionally assist ambulation. Researchers have studied knee kinematics for decades and have used that knowledge to produce knee exoskeletons with a variety of kinematic designs inspired by the anatomical knee. Traditionally lower limb and knee exoskeletons have been evaluated using optical motion capture, force plates, inertial sensors, and simulation. There have been few, if any, studies that capture both knee brace position and underlying bone position in live human subjects. We developed an approach to evaluate the relative motion between a knee brace and the underlying bony anatomy using MRI. We performed a study on four healthy subjects to test this approach using a pin joint, representative of many standard braces and commercial exoskeletons, and a prototype bio-inspired knee brace that more closely matches the kinematics of a live human knee. We report the Euclidean distance, displacement with respect to the anatomical coordinate systems, and the orientation of the brace with respect to the femur and tibia using data from three static knee positions. The average Euclidean distance of brace shift with respect to the tibia and the femur, for the pin joint and the bio joint, ranged from
$17.0mm \pm 6.3mm$
to
$23.8mm \pm 12.0mm$
. Our results suggest that brace movement resulting in alignment errors are likely due to soft tissue compliance, and variability of knee brace donning, rather than differences in the chosen knee brace design kinematics. MRI presents limitations including static and non-weightbearing positioning of the subject, as well as constrained body positioning resulting in reduced range of motion at the knee. Our evaluation approach can be used to supplement but not replace traditional evaluation methods to assess their knee exoskeleton designs and to study how the skin motion and initial alignment effects interact with their kinematic design.