Choosing an Ankle-Foot Orthosis in Combined Dorsiflexor and Quadriceps Weakness
A 71-year-old man with Charcot-Marie-Tooth disease has right lower-extremity strength of 3/5 in tibialis anterior, 4/5 in triceps surae, and 4/5 in quadriceps, with ankle dorsiflexion limited to −5° (5° of plantar flexion) when the knee is extended. He has a steppage gait and reports that his right knee 'collapses' in mid-stance without pain. He is fitted with a posterior leaf-spring AFO; on his first walk in the clinic the knee buckles in mid-stance and he nearly falls. The orthotist suggests simply stiffening the plastic and adding a metatarsal pad.
1. Explain biomechanically why the posterior leaf-spring AFO did not prevent knee buckling, using the ground reaction force and the external knee moment.
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In stance, the ground reaction force (GRF) normally passes behind the knee, creating an external flexion moment that the quadriceps must counteract.
- A posterior leaf-spring AFO is deliberately compliant in the sagittal plane. It resists dorsiflexion only near end range and stores energy as the tibia advances, but it does not block plantar flexion at heel strike and does not stop tibial advancement in mid-stance.
- As the tibia advances over the foot, the knee moves anteriorly relative to the GRF, lengthening the flexion moment arm and increasing the external flexion moment.
- With 4/5 quadriceps - and with a pre-existing plantar-flexion posture that already places the tibia forward - the patient has no reserve to generate the needed extension moment, so the knee buckles.
A solid-ankle AFO with a plantar-flexion stop at neutral blocks tibial advancement, so the GRF is effectively carried anterior to the knee and the net moment favors extension. The trade-off is real: that same extension moment can drive genu recurvatum in a patient with plantar-flexor spasticity or weak hip extensors, so the device must be checked at heel strike, not just at mid-stance.
2. Specify the design changes you would request and the fitting checks you would perform before he leaves the clinic.
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Design:
- Solid-ankle (rigid) AFO with the plantar-flexion stop set at 0° (a few degrees of dorsiflexion only if tolerated without heel lift).
- Full-length footplate to the toes to lengthen the forefoot lever and increase the extension moment at the knee.
- Proximal trim lines and calf band height set to distribute pressure and control the tibia; a firm heel counter in the shoe.
- If quadriceps strength is at or below 3/5, or if recurvatum appears, move to a knee-ankle-foot orthosis with a knee joint and extension stop (offset joint for stance control) rather than pushing a rigid AFO to do a job it cannot do.
Fitting checks:
- Watch heel strike for knee hyperextension or heel lift; watch mid-stance for tibial advancement and buckling; watch pre-swing for toe clearance.
- Skin inspection at the tibial crest, fibular head, malleoli, and dorsum of the foot.
- Check the contralateral shoe for a lift if there is a leg-length difference and confirm shoe fit; a rigid AFO in a soft shoe is a recipe for failure.
3. He has only 5° of plantar flexion and cannot reach neutral. How should the orthosis accommodate this, and why is forcing the ankle to neutral a mistake?
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Do not force the ankle to neutral in the device. With a fixed plantar-flexion contracture, forcing neutral either lifts the heel out of the shoe or pistons the tibia upward inside the shell, producing shear and pressure at the heel and dorsum of the foot - and it does nothing to improve the contracture.
- Bank the AFO in the available position so the foot sits fully seated in the shell.
- Add a heel lift to the shoe (or build a rocker into the sole) so the tibia is tilted posteriorly relative to the foot. This reduces the effective dorsiflexion demand during stance and counters the forward tibial lean that was worsening the flexion moment.
- If the contracture is flexible, pursue serial casting or a stretching program so the orthosis can be re-set closer to neutral later; if it is rigid, accept it and design around it.
The principle: an orthosis should match the patient's available range and then modify the moment arms, not fight the deformity with leverage that shows up as skin breakdown.