orthosis

Picture an ankle that turns over too easily, a wrist that flops when the muscles are weak, or a knee that buckles under weight. Often the part is not lost, just unsupported or out of line. An orthosis is an external device worn on the body that supports, aligns, protects, or improves the function of a moving part, without replacing it. The cloth wrist support sold in pharmacies and the moulded plastic brace inside a shoe are both, in different degrees, orthoses.

The word comes from the Greek for straightening, and that is the core idea: an orthosis works on a body part that is still present. It can hold a joint in a safe position, stop an unwanted movement, take load off a painful or healing structure, or substitute for muscles that no longer fire. A device is named for the joints it crosses (an ankle-foot orthosis spans the ankle and foot, a knee-ankle-foot orthosis adds the knee), so the name tells a clinician exactly what region it controls. This is different from a prosthesis, which replaces a part of the body that is missing.

Orthoses matter because so much of disability is a gap between what a body part can do and what a task demands. A person with foot drop after a stroke trips because the toes catch; a simple plastic brace holds the foot up and the tripping stops. The device does not heal the nerve or the stroke; it bridges the gap so the person can walk safely now. Used well, an orthosis frees a person to move; used carelessly (too rigid, ill-fitting, worn longer than needed), it can cause pressure sores, weaken muscles from disuse, or stiffen a joint.