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Lower limb prosthetics — sockets, feet, knees and gait deviations
Most gait deviations with a prosthesis come from three sources — prosthesis length, alignment of the components and socket fit — so in every question about a gait deviation, first ask whether the prosthesis is too long, too short or wrongly aligned.
Why it matters in the exam
Transtibial socket: in a PTB socket weight is borne on pressure-tolerant areas — the patellar tendon, the medial tibial flare and the calf muscles — and pressure is kept off sensitive areas: the fibular head, the distal tibia and the tibial crest. A total surface bearing (TSB) socket with a liner is common today. Suspension: a sleeve, a locking pin liner, vacuum or suction. Transfemoral socket: a quadrilateral socket in which the ischium "sits" on a shelf, or an ischial containment socket that holds the femur in adduction better.
Feet: SACH (a soft cushion heel without joints) — cheap, durable, suited to a fixed cadence; single axis; multiaxial for uneven surfaces; energy-storing for active users. Knees: a manual locking knee — the most stable, for a weak or older patient; a weight-activated stance-control knee; a polycentric ("four-bar") knee — stable in stance and shortening in swing, so it suits a long residual limb and knee disarticulation; a hydraulic or microprocessor knee for variable cadence. Stance stability of the prosthetic knee depends on its axis lying behind the trochanter-knee-ankle line; an axis too far forward makes the knee buckle.
Deviations after transtibial amputation: a heel too hard or a foot too far forward/dorsiflexed — excessive knee flexion in early stance; a heel too soft or a plantar-flexed foot — knee hyperextension. Deviations after transfemoral amputation: rising on the toes of the sound leg (vaulting) — prosthesis too long, poor suspension or high knee friction; circumduction — prosthesis too long or a locked knee; lateral trunk bending towards the prosthesis — prosthesis too short, weak hip abductors or an unsupportive lateral wall; medial or lateral heel "whip" — wrong rotation of the knee axis or socket; terminal impact at the end of swing — low knee friction.
Key points
- Gait deviations: length, alignment, socket.
- PTB: bear weight on the patellar tendon, not on the fibular head.
- Locked knee — most stable; polycentric — for a long residual limb.
- Long — vaulting and circumduction; short — lateral lean.
- Hard heel — knee flexion; soft heel — hyperextension.
Summary table
| Deviation | Common cause | Correction |
|---|---|---|
| Vaulting (transfemoral) | Prosthesis too long, poor suspension, high knee friction | Shorten, improve suspension, reduce friction |
| Lateral trunk bending | Prosthesis too short, weak abductors, weak lateral wall | Lengthen, strengthen, correct the socket |
| Prosthetic knee buckling | Knee axis too far forward, hard heel | Move axis back, soften heel |
| Excessive knee flexion (transtibial) | Hard heel, dorsiflexed or anterior foot | Soften heel, realign foot |
| Knee hyperextension (transtibial) | Soft heel, plantar-flexed foot | Stiffen heel, realign foot |
A free exam question
מה מ?הבאים מהווה חיסרון של תותבת מיואלקטרית (the question as examined, in Hebrew)
- א. התותבת תלויה בכוח הפיזי של האדם
- ב. התותבת מתאימה רק לקטיעה מעל המרפק
- ג. לתותבת יש כבלים חיצוניים
- ד. התותבת כבדת משקל
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Sources
- Atlas of Amputations and Limb Deficiencies (AAOS), 4th edition (2016) — Transtibial and transfemoral prosthetic components and alignment
- Braddom's Physical Medicine and Rehabilitation, 6th edition (2021) — Lower Limb Prosthetics
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.