Socket and foot alignment influences forces and pressures
between prosthetic socket and residual limb

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SAGITTAL PLANE ALIGNMENT

	Typical alignment for below-knee socket An appropriately short heel lever permits GRF to move anterior of knee joint early in stance phase.

	In this position, the GRF places an extension moment on the socket-RL interface, and causes the socket to "rotate" on the RL in a direction that concentrates pressure: proximally and anteriorly on the pressure-tolerant patellar tendon distally and posteriorly on pressure tolerant muscle bulk.

	If the prosthesis' heel lever is too long, the GRF is also located more posteriorly.

	In this position, the GRF places a flexion moment on the socket-RL interface, and causes the socket to "rotate" on the RL in a direction that concentrates pressure: distally and anteriorly on a pressure-sensitive region of the tibia proximally and posteriorly

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FRONTAL PLANE ALIGNMENT

	Typical alignment for below-knee socket By "insetting" the foot, the prosthetist causes the GRF to be located medial to the interface between the RL and the socket.

	In this position, the GRF places a varus moment on the socket-RL interface, and causes the socket to "rotate" on the RL in a direction that concentrates pressure: proximally and medially, on the pressure-tolerant medial tibial flare distally and laterally on the pressure-tolerant lateral distal fibula.

	If the prosthetic foot is "outset" or aligned more laterally, the GRF is also located more laterally during midstance.

	In this position, the GRF places a valgus moment on the socket-RL interface, and causes the socket to "rotate" on the RL in a direction that concentrates pressure: distally and medially proximally and laterally on the sensitive fibular head.

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Prosthetic Biomechanics
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Last updated 8-30-2006 ©Dave Thompson, P.T., Ph.D.