Lower-limb Prosthetic Alignment

Why is alignment important?

• Optimal distribution of pressure of socket against stump
• Optimal stability
• Optimal Control
• Cosmesis
• Energy efficiency

Standard Prosthetic Alignment

B/K - sagittal plane

• 12-14° of flexion
• tends to load the pressure-tolerant patellar tendon distally and muscle bulk posteriorly
• increases effective heel lever
• encourages knee flexion between heel strike and foot flat
• reduces length of keel encouraging knee flexion from midstance to heel off
• places the quadriceps muscle under slight stretch at heel strike, which improves stability and control
• encourages "roll over" between mid stance and heel off
• limits recurvatum (hyperextension) forces during mid-stance to terminal stance phase of gait
• socket anterior
• may be useful to minimize the tendency for knee hyperextension at heel strike
• assists roll over between mid stance and heel off
• Problems
• insufficient flexion
• concentrates pressure distally and anteriorly on pressure-sensitive region of the tibia proximally and posteriorly
• excessive knee flexion (at heel strike) may result from
• heel too firm (knee flexes when the heel is fully compressed)
• foot too posterior
• foot too dorsiflexed
• interface too flexed
• knee hyperextension at heel off
• foot too anterior or plantarflexed
• socket too extended or posterior
• heel cushion too soft (knee flexes when the heel is fully compressed)
• keel too long
• forward placement of socket
• causes increased knee entension
• patellar pain
• causes knee to buckle into flexion at heel strike
• posterior translation
• tends to keep knee in hyperextension during stance phase

Summary of sagittal alignment Heel & Toe levers

	More Stability Reduced Heel Lever Increased Toe Lever	More Control Increased Heel Lever Reduced Toe Lever
Ankle angle	plantarflexed	dorsiflexed
Socket translation wrt foot	more posterior	more anterior
Socket angulation	more extended	more flexed
GRF in early stance with respect to the knee	more anterior	more posterior
External knee moment during loading	extensor (i.e. needs less quads activity)	more flexor (i.e. needs more quads activity)
Problem	if toe lever is too long, the amputee may not be able to initiate knee flexion	"Drop off" results if the toe lever is too short and the knee flexes too early

Non-level gait

Frontal plane

• initial foot placement is 12 mm inset in relation to midpoint of pylon-socket connector
• helps load pressure tolerant areas (medial tibial condyle, lateral fibular surface)
• narrows base of support
• smooths horizontal displacement of center of gravity
• improves efficiency
• Problems
• outset knee
• GRF is also located more laterally during midstance
• places a valgus (abductor) moment on the socket-stump, concentrating pressure distally and medially, proximally and laterally on the sensitive fibular head
• excessive varus (adductor) moment may be caused by
• foot positioned too medially
• there is laxity of the lateral collateral ligament
• insufficient varus moment may be exhibited by
• foot positioned too laterally
• loading of pressure sensitive areas (such as a load on the fibular head)
• base of support too wide
• excessive medial shoe wear
• socket too medial (equivalent to outset foot)
• valgus (abductor) knee strain
• socket too lateral (equivalent to inset foot)
• varus (adductor) strain on knee
• increased pressure distal-laterally and proximal-medially

Summary of frontal plane alignment

A/K Alignment

Sagittal

• socket usually flexed
• plumb line from the greater trochanter directly through the axis of the knee (Trochanter-Knee-Ankle, TKA line) usually optimal
• placing knee center of rotation
• posterior provides better stability
• anterior allows better control

Frontal plane

• aligned so that a varus moment is generated about hip joint during midstance (lateral wall placed in adduction);
• ischium is considered a fulcrum & foot is aligned directly under or up to 1 inch lateral to ischium (never inset)

Transverse plane

• typically the socket is placed in 5 deg external rotation

Summary

Other problems

 

Problem	Prosthesis Length	Socket	Knee	Foot	Other
Excessive heel rise			inadequate friction, extension assist too weak
Rotation of foot at heel-strike				Heel cushion too hard
Foot slap				Heel cushion too soft
Lateral whip (knee in, heel out)			Knee axis medially rotated or valgus		muscle weakness
Medial whip (heel in, knee out)			Knee axis laterally rotated or varus		muscle weakness
Terminal knee snap			insufficient knee flexion
Unequal cadence		painful	too anterior, insufficient friction		muscle weakness
Unequal arm swing		painful			poor balance
Pistoning (stump moves inside socket) in swing					ineffective suspension
Pistoning in stance					poor socket fit or volume changes in stump (may require a change in stump sock thickness)