PROSTHETIC BIOMECHANICS
Prosthetic alignment alters gait by manipulating the position of LE and prosthetic joints with respect to ground reaction forces (GRF).
In normal gait,
from initial contact through loading response,
- ground reaction (GRF) originates at the heel, passing posteriorly to the ankle and knee joints.
- This produces knee flexion and ankle plantar flexion, which are controlled respectively by the knee extensors and ankle dorsiflexors.
From midstance though preswing,
- The GRF moves anterior to the ankle, producing ankle DF that must be controlled by the plantar flexors.
- The ankle gradually dorsiflexes (under eccentric PF control), then plantar flexes (owing to concentric PF activity).
- Forward tibial inclination permits the heel to rise and the GRF to fall behind the knee, inducing preswing knee flexion.
In gait with a prosthetic foot,
- The shape and density of the prosthetic heel (e.g. cushion on a SACH foot or PF bumper on a single-axis foot) allows the front of the foot to settle on the floor during loading response (in "pseudoplantar flexion").
- A relatively rigid prosthetic ankle or DF bumper controls movement of GRF over foot through midstance.
- Length of keel structure determines timing of heel rise.
- The location of the trochanter-knee-ankle (TKA) line determines "tradeoff" during stance between:
- inherent knee stability and
- voluntary control of prosthetic knee.
The way a prosthesis behaves during gait depends on the relative length of what prosthetists call the heel and toe levers.
- "Heel lever": roughly the perpendicular distance from heel cushion to center of socket.
- "Toe lever": roughly the perpendicular distance from center of socket to end of keel.
Changes in these levers produce predictable changes in an amputee's gait pattern:
SHORTENING the heel lever:
- locates the GRF more anteriorly with respect to the knee during loading response and midstance,
thus producing knee extension earlier in the gait cycle.
- increases the toe lever,
which sustains knee extension later in the gait cycle. If the toe lever is too long, the individual may be unable to initiate preswing knee flexion.
LENGTHENING the heel lever:
- locates the GRF more posteriorly with respect to the knee at initial contact (heel strike),
thus producing a larger knee flexion moment during loading response.
- decreases the toe lever,
which causes the knee to flex earlier in midstance or terminal stance. "Drop off" results if the toe lever is too short and the knee flexes before the person is ready to accept weight on the opposite leg.
The prosthetist can alter heel and toe levers in predictable ways by changing the relationship between the socket and foot.
Socket and foot alignment also influence forces and pressures that the socket places on the residual limb.