Sagittal plane position of the functional joint centre of prosthetic foot/ankle mechanisms

The use of motion analysis techniques in amputee rehabilitation often utilizes kinematic data from the prosthetic limb. A problem with methods currently used is that the joint positions of the prosthetic ankle are assumed to be in the same position as that of an intact ankle. The aim of this study was to identify both traditional anatomical joint centres as well as functional joint centres in a selection of commonly used prosthetic feet. These coordinates were then compared across feet and compared to the contralateral intact ankle joint.

Six prosthetic feet were fit to a unilateral trans-tibial amputee on two separate occasions. The subject's intact limb was used as a control. Three-dimensional kinematics were collected to determine the sagittal position of the functional joint centre for the feet investigated.

None of the prosthetic feet had a functional joint centre that was within the 95% CI for that of an intact ankle (both x- and y-coordinate position), nor any of the other prosthetic feet investigated. The repeatability of the method was found to be adequate, with 95% CI of the difference (test-retest) of the prosthetic feet similar to that for the intact ankle and within clinically accepted levels of variability.

The motion of the prosthetic feet tested is clearly different from that of an intact ankle. Kinematic methods that assume ankle constraints based on an intact ankle are subject to systematic error as this does not reflect the real motion of the prosthetic foot.