Orthoses for Neurological Ankles
An orthosis is a device that supports residual function. This is in contrast to a prosthesis which is a device that replaces function. Orthoses provide support to prevent joint movement past normal or target range.
Basic Principles
They can be used in a wide variety of clinical scenarios, including:
- Patients who cannot ambulate to help with correct ankle positioning to allow for standing and help with standing transfers
- Prevent further loss of range of movement for example in bed-bound patients and in high muscle tone
- Patients with loss of range of movement by applying a sustained stretch
When prescribing an orthosis, functional aspects such as abnormal movement, posture, and range of motion are more important than the underlying disease process. It is also important to consider aspects of gait and the specific phase or phases of impairment.
Swing Phase Problem
Disorders of swing phase include foot drop due to dorsiflexor weakness (e.g. peroneal neuropathy) or spasticity of the plantarflexors. With the latter the orthosis needs to be more precisely fit as greater forces are required with or without heel wedges. For the former there are a variety of options
- DIY or over the counter solution can work well e.g. a simple elastic foot lifter which can be easily adjusted and can still allow for driving.
- Posterior leaf spring (PLS)
- Carbon fibre or silicone AFO
- Functional electrical stimulation.This may be a more comfortable option for those with upper but not lower motor neuron cause of foot drop. The peroneal nerve is stimulated with this device providing dorsiflexion. The timing is controlled by a switch under the heel; when the heel presses the switch the stimulation is temporarily stopped to allow the foot to drop. It also doesn't work well when there is significant spasticity. It also doesn't help with problems during the stance phase.
Stance Phase Problem
A custom AFO is usually the best option for problems in stance phase where there is loss of normal passive range of motion, increased muscle tone, or poor control of ankle and knee movement. The amount of surface area covered is proportional to the amount of force needed. This is why AFOs need to be custom molded, otherwise they won't achieve sufficient force and/or they will be uncomfortable.
In patients with oedema the ankle will change in size constantly and the custom AFO won't change with it. The only real option here is an external caliper. If good control of the oedema can be achieved through medication or compression stockings then an AFO may be able to be used some of the time.
For foot slap, such as in tibialis anterior tendinopathy, there is uncontrolled plantar flexion at initial heel contact, but the ankle is then stable. Options here include a rocker bottom shoe or simple plastic or carbon fibre AFO. The rocker sole moves the fulcrum of initial contact anteriorly towards the action of tibialis anterior, thereby reduces the moment of rotation. Before purchasing expensive rocker shoes a darco wedge shoe can be trialed to see if this approach will help.
For patients with minor calf shortening but a stable ankle who are uncomfortable while standing due to resultant knee hyperextension and calf discomfort, heel wedges can be trialed.
Mixed Problems
In patients with poor ankle control where there is variable foot drop during swing phase and poor control but adequate power during stance phase, compression may help. This may work through improving propioception. Tubigrip can be trialed and if it helps then there is the option for custom lycra stockings (e.g. DM orthotics).
If the patient has weak plantar flexors along with poor ankle stability, then the options are a strong stiff carbon fibre AFO, or moulded AFO if there is loss of normal range.
In patients with weak proximal and distal muscles, a strong stiff carbon fibre AFO is used by aligning it in a way to use the ground reaction force to keep the shin upright and help with optimal knee alignment, as well as possibly helping with the toe-off phase by providing an extra spring. This can only be used if the ankle rests in neutral position, otherwise a moulded AFO is required.
Surgery is the only option in patients with fixed extreme plantarflexoin +/- inversion from upper motor neuron lesions. In less extreme situations serial casting, botulinum toxin, surgery, and moulded AFOs with heel build up are all possible options.
