What could lead to lateral rotation of the foot at heel strike in TF prosthetics?

Lateral rotation of the foot at heel strike in transfemoral (TF) prosthetics can occur due to various mechanical and alignment issues. A stiff plantarflexion (PF) bumper or heel wedge is likely to restrict the normal range of motion at the ankle during loading response, which is the phase when the foot first strikes the ground. If the PF bumper is too stiff, it does not allow enough dorsiflexion to occur. Consequently, when the heel strikes, the ankle cannot move into its natural pronation or alignment, leading to a compensatory lateral rotation of the foot to accommodate the rigidity of the bumper.

This compensation can also be exacerbated if the foot is poorly aligned with respect to the socket or limb, which could also lead to similar rotational issues. However, the stiffness of the bumper or wedge is a direct mechanical influence that restricts the movement needed for proper alignment at heel strike.

Other factors like excessive pre-flexion of the socket, improper foot alignment, or weak knee control can also affect the gait mechanics, but the root cause of lateral rotation at heel strike due to the inflexible nature of the PF bumper is particularly significant in creating the observed rotation.