An Ankle Foot Orthosis (AFO) is an external medical device designed to support or correct the foot and ankle joint. This custom or prefabricated brace is worn around the lower leg and foot, extending from below the knee to the base of the foot. The primary clinical goal of an AFO is to improve mobility by influencing the mechanics of walking. By managing alignment and controlling unwanted motion, the device enables a safer, more efficient gait pattern, enhancing functional independence.
Mechanical Function and Core Components
The fundamental function of an AFO is to control the foot and ankle using the biomechanical three-point pressure system. This system applies pressure at three distinct points to counteract unwanted movement, maintaining the joint in a corrected or neutral alignment. The orthosis controls motion in multiple planes, managing the degree of dorsiflexion (lifting the foot) and plantarflexion (pointing the foot down) at the ankle. It also provides medial and lateral stability, preventing excessive inversion (inward rolling) or eversion (outward rolling) during weight-bearing activities.
The structure of a typical AFO consists of three essential components. The foot plate sits inside the shoe, acting as a foundation to control the foot’s position and transfer forces up the leg. An upright, often called a shank shell or strut, extends up the calf, providing the leverage necessary to influence the ankle joint. A calf cuff or strap secures the orthosis around the calf, ensuring the device remains in place and applies corrective pressures. These devices are fabricated from lightweight yet rigid materials, such as polypropylene thermoplastics or high-strength carbon fiber.
Conditions That Require an AFO
An AFO is prescribed to address impairments causing muscle weakness, joint instability, or abnormal gait patterns. The most frequent indication is neurological deficit, where damage to the nervous system affects the muscles responsible for lifting the foot. “Drop foot,” characterized by weakness of the dorsiflexor muscles, makes it difficult to clear the toes during the swing phase of walking, leading to tripping and falls.
Patients recovering from a stroke or managing progressive neurological disorders like Multiple Sclerosis (MS) or Cerebral Palsy (CP) often rely on AFOs to compensate for muscle weakness and spasticity. In CP, an AFO can help manage increased muscle tone that causes toe-walking or an equinus (pointed toe) deformity. AFOs are also used to treat musculoskeletal issues involving joint degeneration or collapse, such as severe arthritis or a Charcot joint, by immobilizing the unstable area. Temporary use may be indicated following lower extremity trauma or surgery to limit the range of motion and protect healing tissues during rehabilitation.
Major Categories of Ankle Foot Orthoses
The design of AFOs is specialized, with four major categories providing distinct levels of control and mobility.
Solid Ankle Foot Orthosis (SAFO)
The SAFO offers the highest degree of stability, completely restricting all movement at the ankle joint. This design is indicated for individuals with severe spasticity, pronounced ankle instability, or total loss of ankle control, ensuring the ankle is held rigidly at a fixed angle to maximize support during walking.
Articulated or Hinged AFO
This less restrictive option incorporates a mechanical joint at the ankle level. This design allows for controlled movement, often permitting dorsiflexion while blocking excessive plantarflexion. Articulated designs are frequently used to promote a more natural walking pattern while providing medial and lateral ankle stability.
Posterior Leaf Spring (PLS) AFO
The PLS AFO is characterized by a thinner, more flexible plastic or carbon fiber strut positioned behind the calf, extending down to the foot plate. This flexibility offers minimal side-to-side stability but provides a passive dorsiflexion assist. The PLS flexes under load during the stance phase of gait and then recoils to lift the forefoot during the swing phase, making it a suitable choice for mild to moderate isolated foot drop.
Ground Reaction AFO (GRAFO)
The GRAFO is utilized when control of the knee joint is also required, often in conjunction with ankle support. It features an anterior shell that applies a backward force against the shin, harnessing the ground reaction force. This prevents excessive knee flexion or hyperextension during walking and is commonly prescribed to manage a crouch gait pattern or instability caused by quadriceps muscle weakness.