Early-Stance Knee Instability in a Transfemoral Prosthesis with Hip Flexion Contracture
Ms K, 34, had a traumatic transfemoral amputation 6 months ago. Residual limb length is 22 cm from the ischium. She uses an ischial containment socket, a single-axis knee with adjustable friction and extension aid, and a solid-ankle cushioned-heel foot. Thomas test shows an 8° hip flexion contracture; knee extension and hip abduction strength are normal. On gait observation she leans her trunk forward at initial contact, hyperextends her lumbar spine through midstance, and reports that the knee 'buckles' whenever she tries a longer step on the prosthetic side. She has stopped lengthening her step and now walks slowly with a rollator. The prosthetist's bench notes record socket flexion of 3°, the foot set with its heel 4 cm anterior to the knee axis projection, and knee friction at the lowest setting. Her residual limb is healthy with no volume change over 6 weeks.
1. Using ground reaction force reasoning, explain why this alignment produces early-stance knee instability and how each of the three recorded settings contributes.
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In early stance the knee is stable only if the ground reaction force passes anterior to the knee axis, creating an external extension moment. Anything that moves that force line posterior to the knee axis converts stance into a flexion moment, and no amount of patient effort can fully compensate for it.
- Foot 4 cm anterior to the knee axis: the foot is the point where the force enters the prosthesis. Moving it forward rotates the whole line of action backwards relative to the knee, so a heel-strike force lands behind the axis and the knee flexes. This is the dominant error.
- Knee friction at the lowest setting: friction and the extension aid resist flexion as the knee is loaded. With both minimised there is nothing to absorb the transient flexion moment before the foot is flat and the force line moves forward.
- Socket flexion of only 3° with an 8° contracture: the socket does not sit the femur in the flexion it actually holds, so the patient must lean forward and hyperextend the lumbar spine to bring her centre of mass over the foot. That postural compensation is the excessive lordosis you see at midstance.
So the three settings act together: a posterior force line, minimal resistance to flexion, and a postural compensation that masks both. The long step fails because a longer step increases the heel-strike moment arm and the flexion moment with it.
2. List the alignment and component changes you would make at the next visit, with the rationale and the order in which you would make them.
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Change one variable at a time and re-observe, otherwise you cannot attribute the improvement.
- Foot position first. Move the foot posterior in small increments (5 mm at a time) until the heel-strike force line sits comfortably anterior to the knee axis on the bench check, then verify dynamically. This addresses the primary cause.
- Knee friction and extension aid. Raise friction and the extension aid enough to control heel strike without making swing flexion sluggish. Test swing clearance afterwards, because too much extension aid causes a stiff, circumducted swing.
- Socket flexion. With an 8° contracture, do not chase the contracture with the socket. A small increase to about 5° is usually tolerable and reduces the forward lean; beyond that you simply transfer the problem into lumbar hyperextension.
- Knee unit. If the above is not enough for a community ambulator, consider a stance-control or microprocessor knee, but only after the alignment is correct - a stance-control knee fitted to a poor force line will still perform badly.
- Contracture management in parallel. Refer for sustained stretching and hip flexor work; alignment can accommodate a contracture but cannot resolve it.
Use the bench knee-stability check plus a marked 10-metre walk before and after each change, and record step length and the patient's confidence rating.
3. The patient asks you to shorten the prosthesis so she can walk more easily and to 'set the knee stiff'. How do you respond, and what are the consequences of each request?
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Both requests come from a real problem and deserve a respectful explanation, but neither should be granted as asked.
Shortening. A prosthesis made short by, say, 1–2 cm will reduce the apparent effect of the flexion contracture and may feel easier, but it does so by making her vault over the sound limb and by increasing the pelvic drop and lateral lean on the prosthetic side. It also blocks rehabilitation: her contracture remains, her gait pattern is trained around a compensator, and the shorter limb will be difficult to lengthen later without retraining. Correct length should be maintained and the contracture treated.
Stiffening the knee. A locked or heavily stiffened knee reduces the buckling sensation but converts a stumble into a fall risk by removing the knee's ability to absorb load and to shorten in swing, and it substantially raises energy cost. Friction and extension aid should be tuned to control heel strike, not used to freeze the joint. If instability persists after correct alignment, a stance-control knee is the structural answer.
What to offer instead: explain the force-line mechanism in plain terms, show her the improvement from the foot-position change immediately, set a short-term goal such as a 100-metre walk with a cane, and agree a review date. Record that shortening and stiffening were discussed and declined, with reasons.