Definition
An orthosis is an external device used to support, align, prevent or correct deformity, improve function or modify mechanical loading.
Mechanical principles
Orthoses work by:
- controlling motion
- redistributing pressure
- changing the line of action of the ground-reaction force
- supporting a weak segment
- limiting painful movement
- applying a three-point pressure system
- increasing the effective lever arm of a corrective force
Three-point pressure principle
A corrective force is applied at the site of deformity with two counterforces acting in the opposite direction on either side.
The effectiveness depends on:
- force magnitude
- lever-arm length
- fit
- tissue tolerance
- time worn
Many corrective orthoses use a three-point force system: a primary corrective force opposed by two counterforces. The effectiveness depends on lever arms, fit, tissue tolerance and whether the deformity is flexible.
A rigid deformity may need accommodation rather than attempted correction.
Axial load redistribution
Total-contact designs can spread load across a larger surface area and reduce focal pressure.
Examples include selected:
- fracture braces
- Charcot restraint orthotic walker designs
- custom foot orthoses
Ground-reaction force
An orthosis can alter joint moments by shifting the point or line of action of the ground-reaction force.
This is the basis of many:
- ankle-foot orthoses
- knee braces
- shoe wedges
- heel modifications
Foot orthoses
Common functions:
- accommodate deformity
- redistribute plantar pressure
- alter hindfoot or forefoot mechanics
- support an arch
- reduce painful motion
Rigid correction should not be forced onto a fixed deformity.
Foot orthoses may:
- redistribute plantar pressure
- support a flexible arch
- accommodate deformity
- alter hindfoot or forefoot loading
- offload a painful area
In neuropathic feet, pressure relief and total-contact principles are particularly important because pain cannot be relied upon as a warning signal.
Shoe modifications
Rocker sole
Moves the rollover point and can reduce the need for motion at a painful or stiff forefoot or ankle.
Heel and sole modifications
Can alter loading and ground-reaction moments. The exact design should match the deformity and gait problem.
Ankle-foot orthosis
An AFO can:
- control plantarflexion
- assist dorsiflexion
- control inversion or eversion
- influence knee position through ground-reaction mechanics
- improve toe clearance during swing
Design options range from flexible dynamic devices to rigid or ground-reaction AFOs.
Principles of prescription
Define:
- pathology
- flexible versus fixed deformity
- muscle power
- joint range
- skin and sensation
- gait problem
- functional goal
Then choose the simplest device that achieves the goal without creating a new problem.
What an orthosis can do
An orthosis can:
- support a weak segment
- limit unwanted movement
- permit selected movement
- redistribute pressure
- correct or accommodate deformity
- improve function
- reduce pain
- protect healing tissue
The prescription should therefore state the biomechanical objective, not simply the device name.
Ankle-foot orthoses
An AFO may help manage:
- foot drop
- ankle instability
- spastic equinus
- mediolateral instability
- selected crouch or knee-control problems depending on design
A posterior leaf-spring AFO assists swing-phase dorsiflexion but offers limited stance control. More rigid or articulated designs alter ankle motion and can influence knee moments.
Ground-reaction concepts
Because the ground-reaction force passes in relation to joint centres, changing ankle position or orthotic stiffness can influence the external moment at the knee.
This is why an AFO is not merely an ankle device; it can alter proximal gait mechanics.
Spinal orthoses
Spinal braces may be used for:
- selected fractures
- postoperative support
- deformity management
- symptomatic support in specific conditions
Their mechanical control varies by design, and patient adherence is often as important as theoretical rigidity.
Practical prescription
Specify:
- diagnosis
- functional problem
- joint position to control
- desired range of motion
- areas requiring offloading
- skin or sensory risk
- footwear requirements
- duration and review plan
Complications
Possible problems include:
- pressure injury
- skin irritation
- muscle deconditioning with unnecessary prolonged immobilisation
- joint stiffness
- poor adherence
- worsening of proximal or distal mechanics
FRCS synthesis
The examiner wants to hear the biomechanical purpose of the orthosis. Describe the force or motion you want to control, then choose a device capable of producing that effect safely.