Ankle-Foot Orthoses (AFOs) are external support devices worn around the lower leg, ankle, and foot. These orthopedic tools are among the most frequently prescribed lower limb braces used in rehabilitation and long-term care. An AFO encompasses the foot, ankle, and leg, typically finishing just below the knee. The purpose of these devices is to manage alignment, increase mobility, and provide stability for individuals experiencing muscle weakness, joint instability, or abnormal muscle tone. They function by externally controlling the ankle joint’s range of motion and stabilizing the foot during standing and walking.
Core Biomechanical Functions
The primary role of an AFO is to address biomechanical deficiencies by influencing the forces and movements of the lower limb during walking and standing. AFOs work by applying a three-point force system to the limb, where two forces are applied in one direction and a single opposing force is applied between them. This mechanical leverage controls joint moments to restrict excessive motion or position the limb into a more functional alignment.
One function is stabilizing the ankle and subtalar joints, preventing excessive inward (inversion) or outward (eversion) rolling of the foot. Controlling this rotational motion is performed by applying forces proximal and distal to the ankle joint. This stability is paramount to proper weight distribution during the stance phase of gait.
AFOs are frequently used to assist with foot clearance during the swing phase of walking, a problem known as “drop foot.” When the muscles that lift the foot (dorsiflexors) are weak, an AFO holds the foot at a neutral or slightly upward angle, preventing the toes from dragging on the ground. AFOs also control knee alignment by managing the forward movement of the shin bone (tibia) over the foot during mid-stance. By influencing the ankle position, an AFO can prevent the knee from collapsing into excessive flexion or hyperextending (recurvatum).
Common Structural Variations
Ankle-Foot Orthoses are categorized into structural variations based on the degree of movement they allow at the ankle and the materials used in their construction.
Solid AFOs
Solid AFOs offer the maximum degree of support and stability because their design completely restricts movement at the ankle joint. These devices are typically fabricated from rigid thermoplastics and are used when a patient requires extensive control over both the ankle and the knee.
Articulated or Hinged AFOs
Articulated or Hinged AFOs incorporate a mechanical joint at the ankle, which allows for controlled movement, most often dorsiflexion and plantarflexion. This design is selected for individuals who need stability in the side-to-side planes but can benefit from allowing the foot to move naturally. The hinge often includes a stop mechanism to limit movement in one direction, such as preventing the foot from pointing down excessively.
Ground Reaction AFOs (GRAFOs)
GRAFOs are designed specifically to control knee instability by leveraging the forces generated between the foot and the ground. It features an anterior shell that extends up the shin to the front of the knee, applying a force that pushes the knee backward, which helps to prevent the knee from buckling. While rigid thermoplastics like polypropylene are common materials, lightweight carbon fiber is increasingly used in designs intended to store and return energy during gait to enhance walking efficiency.
The Prescription and Fitting Process
The process of obtaining an AFO begins with a comprehensive assessment by a medical specialist, such as a physiatrist or orthopedic surgeon. This physician provides a detailed prescription that specifies the type of AFO required, the necessary materials, and the specific biomechanical goals, based on the patient’s diagnosis and functional limitations. The patient is then referred to a certified orthotist, a professional trained to design, fabricate, and fit orthotic devices.
The orthotist conducts an evaluation that includes observing the patient’s walking pattern and assessing muscle strength and joint range of motion. For custom-made devices, the orthotist creates a precise impression of the limb, traditionally using plaster casting or three-dimensional scanning. This mold is used to fabricate a device that conforms exactly to the patient’s anatomy to ensure optimal fit and pressure distribution.
The final stage involves the fitting appointment, where the orthotist adjusts the device and its straps to ensure comfort and effectiveness. The patient receives instruction on how to wear the AFO, including using appropriate footwear and a gradual break-in period to allow the skin to adjust to the new pressures. Regular follow-up appointments are scheduled to review the device, make minor adjustments, and confirm that the AFO continues to meet the patient’s functional needs.