The biomechanical principles of AFOs are founded on the functional anatomy of the ankle-foot complex. Dorsiflexion and plantarflexion of the ankle are multiplanar (i.e., movement occurs in all three planes of motion) as the talus rotates through the mortise of the ankle in both open chain movement, and when the mortise moves over a relatively fixed (weight-bearing) talus in a closed chain movement.24 The proximal surface of the mortise is shaped by the syndesmosis (fibrous articulation) between the distal tibia and fibula. The medial wall of the talocrural joint is formed by the medial malleolus, a downward extension of the tibia. The lateral wall, formed by the lateral malleolus of the fibula, is both longer and shifted posteriorly. Because of the shape of the articular surfaces of the talus in its mortise and the offset position of the malleoli, the axis of the ankle joint is slightly oblique, running in an anteromedial to posterolateral direction. As a result, dorsiflexion is accompanied by some degree of forefoot pronation and abduction along with hind foot valgus. Plantarflexion is accompanied by forefoot supination with adduction and hind foot varus. Note that during stance, movement occurs in closed chain situation because the foot is fixed on the ground by weight-bearing forces, so that the mortise rolls over the head of the talus.

If an AFO has mechanical ankle joints, the axis of the mechanical joints should be aligned as closely as possible to the obliquely oriented anatomical axis of motion (Figure 9-3). The mechanical joint heads are placed at approximately midline of the malleoli in the sagittal plane. This strategy reduces likelihood of abnormal torque and shearing between the orthosis and limb as the individual wearing the orthosis walks. When there is incongruence between the anatomical and mechanical axes, excessive motion of the limb within the orthoses is likely, and action of the mechanical ankle joint is compromised.

The solid-ankle AFO (SAFO), also known as a rigid AFO, is typically fabricated from relatively thick thermoplastic and aims to hold the ankle and foot in as close to biomechanically neutral position (zero degrees of ankle dorsiflexion, subtalar and calcaneal neutral, with a balanced forefoot) as possible given the individual’s functional anatomy.25 The anteroposterior trim line of the SAFO falls at or near the midline of the medial and lateral malleolus (Figure 9-4). Medial and lateral corrugations may be incorporated into the shell of the orthosis to provide additional strength when hypertonicity or excessive weight creates loading forces that require a stronger orthosis. The proximal border is typically trimmed to fall 1.5 inches below the apex of the head of the fibula. The footplate it either trimmed just short of the plantar metatarsal heads or can be lengthened beyond the metatarsal distally into a toe plate if hypertonicity is a concern.²⁶ Some orthotists recommend the addition of a tone-inhibiting bar if hypertonicity is severe.²⁷ There is a tradeoff to consider if the footplate is lengthened: It is much easier to fit into and put on shoes with a shorter footplate; assistance may be necessary to don shoes when the SAFO has a full toeplate.

SAFO Control Systems

There are four distinct control systems incorporated into the SAFO design (Figure 9-5). To resist plantarflexion during swing phase, there is a fulcrum force applied at the anterior ankle (by strapping or by the shoe’s laces or Velcro closure) opposed by a distal counterforce upward under the metatarsal heads and a proximal counterforce at the posterior proximal surface of the AFO. To resist dorsiflexion during stance phase, there is an upward and inward compressive force at the posterior heel, opposed by a distal downward counterforce delivered by the shoe, and a proximal force applied by the anterior closure straps just below the knee. It is important to note that the locked ankle created by an AFO generates an extensor moment at the knee during stance. In this way, a SAFO can substitute for impaired motor control or muscle performance of knee extensors for persons with stroke, cerebral palsy, or other neuromotor dysfunction.

To resist varus and inversion of the foot, a medially directed force is applied just above and below the lateral malleolus with laterally directed counterforces at the proximal medial tibia and the medial foot. To resist valgus and eversion of the foot, there is a laterally directed force applied above and below the medial malleolus, with medially directed counterforces just below the fibular head proximally and at the lateral foot distally.

The degree of control for the foot is also influenced by the position of the trim lines of the foot section. When there is midtarsal joint deformity with forefoot abduction or adduction to contend with, trim lines are adjusted to capture the shafts of the first and fifth metatarsals. If there is too much subtalar valgus, the height of the medial wall is increased, and a flange might be placed proximal to the medial malleolus. These strategies provide greater surface area for distribution of corrective forces applied by the orthosis, so that the patient is more comfortable with the external stability it provides. If knee hyperextension at midstance is a problem for individuals with impaired motor control, the orthotist might fabricate the SAFO set in just a couple of degrees of ankle dorsiflexion, rather than neutral, to minimize excessive extension moment and preserve knee joint health over time. A Gillette modification can be added to the outer medial or lateral surface of the heel cup to influence excessive valgus or varus moment at the knee joint during stance. A medial or lateral post (similar to those used in a biomechanical foot orthosis) can be incorporated into the foot section to equalize forefoot to rearfoot relationships or to enhance biomechanical effects on the knee.

All static AFO designs use moments that result from the GRF to provide stability during stance. An orthosis that has evolved from the basic SAFO design to better address impaired motor control of the knee and weakness of the quadriceps is the anterior floor reaction orthosis (FRO)36^,37 (Figure 9-6). The FRO is fabricated to hold the ankle in a few degrees of plantarflexion. This restricts the ability of the tibia to roll forward over the foot in the second/ankle rocker of gait, creating an extensor moment that stabilizes the knee during stance (Figure 9-7). If stability is also needed in late stance phase, a stiff toeplate can be added to reinforce the extension moment at the knee. The FRO and SAFO use the same force systems to control foot and ankle position. Whether the FRO is fabricated in a single piece (see Figure 9-7A) or as a SAFO with the addition of a thermoplastic anterior shell (see Figure 9-7B), padding is added where the FRO contacts the proximal surfaces of the tibia to make the extra extension force delivered by the orthosis more tolerable to the wearer.

The FRO is often used for children with cerebral palsy who demonstrate “crouch gait,” or for children with myelomeningocele and persons with postpolio syndrome who have paralysis or weakness at the knee and ankle.36–38 To be efficacious, the FRO relies on a GRF vector that passes anterior to the anatomical knee joint. As knee and hip flexion contractures approach or exceed 10 degrees, the GRF vector nears or passes posterior to the anatomical knee joint, and the FRO is less effective in stabilizing the knee during stance.³⁶ The FRO is contraindicated for persons with notable recurvatum during stance as well as those with cruciate ligament insufficiency; in these circumstances, the extra knee extension moment can further damage joint structure. While the rigid control of ankle and knee enhances mechanical stability in stance, these external restrictions imposed by the FRO may compromise efficacy of postural responses. In those with balance impairment, an ambulatory assistive device (e.g., a cane, Lofstrand crutches, or rolling walker) may be needed for safety, especially if FROs are worn on both limbs.

The weight-relieving AFO, also known as a patellar tendon–bearing AFO (PTB-AFO), incorporates the intimate fit and load-bearing characteristics of a PTB prosthetic socket into a SAFO or traditional metal double upright AFO (discussed later) as a means of offloading or unloading weight-bearing forces during stance phase for individuals with painful, unstable, or recently repaired ankle or foot (Figure 9-8).^39–42 The anterior shell of the AFO is modified to accept weight-bearing forces via the medial tibial flare and patellar tendon bar, along with total contact around the upper calf. As with a PTB prosthetic socket, the proximal portion of the PTB-AFO is set in approximately 10 degrees of knee flexion (with respect to vertical) to load some of the body weight onto the anterior shell at the medial tibial flare and patellar tendon bar during stance. This axial force is then transmitted to the ground through the medial and lateral walls of the thermoplastic orthosis, which may be reinforced with metal uprights or through the medial and lateral uprights of a traditional double upright AFO. This strategy effectively reduces axial loading of the ankle and foot during gait.

The weight-relieving AFO is used for persons with Charcot arthropathy, plantar neuropathic ulcers, slowly healing fractures, severe rheumatoid or osteoarthritis of the ankle, surgically repaired ankles after fracture, synovial debridement in hemophilia, and other circumstances that require protection of the foot and ankle when non–weight bearing is not possible. A tibial fracture orthosis and the Charcot restraint orthotic walker (CROW) are adaptations of the weight-relieving AFO; both encase the entire lower limb to provide maximal reduction of axial loading while healing takes place.^43,44 Individuals wearing a weight-relieving AFO must have normal anatomical structure of the knee, adequate muscle performance and motor control of the quadriceps muscles for stability in early stance, and sufficient skin integrity to tolerate the loading forces applied by this orthotic design.

In the 1970s, researchers at the University of California Biomechanics Laboratory developed a custom-molded shoe insert, now known as the UCBL orthosis, as an orthotic intervention for subtalar joint instability (Figure 9-9).⁴⁵ The UCBL controls flexible calcaneal deformities (rearfoot valgus or varus) as well as transverse plane deformities of the midtarsal joints (forefoot abduction or adduction) by “grabbing” the calcaneus and supporting the midfoot with high medial and lateral trim lines; it realigns the calcaneus, improving the angle of pull of the Achilles tendon, providing a more stable foundation for the articular surfaces of the talus, navicular, and cuboid bones.⁴⁶ The UCBL is also used to improve functional alignment of children and adolescents with flexible pes planus, a longitudinal arch deformity.⁴⁷ The Gillette modification, an external post positioned either on the medial or lateral border of the heel cup, can be used to apply additional rotatory moments to the calcaneus during weight bearing. In addition to being useful when there is flexible structural malalignment of the rear and midfoot, the UCBL shoe insert may improve foot performance during stance for persons with hypotonicity. It is important to recognize that the UCBL orthosis acts primarily at subtalar and midfoot tarsal joints during weight bearing; it would not be appropriate for persons with swing phase clearance issues, which require the trim line of the orthosis placed above the ankle joint.

The dynamic ankle-foot orthosis (DAFO), also known as a flexible supramalleolar orthosis (SMO), is a custom-molded orthosis that has evolved from the UCBL shoe insert to better address sagittal plane control of the ankle and foot during stance and to facilitate foot clearance in swing.48 Custom-molded from relatively thin thermoplastic, its proximal trim lines are just superior to the ankle joint, and its distal trim lines encase more of the forefoot than the UCBL (Figure 9-10). By limiting movement of the midfoot and forefoot and by holding the foot in functional position, the DAFO provides a stable base for more effective motor performance and postural control during standing and ambulation.^49,50 The DAFO is designed to redistribute plantar pressures of spastic equinovarus from the anterior foot to the heel, perhaps by reducing overall stimulation of reflexes that would otherwise reinforce extensor hypertonicity in children with mild to moderate diplegic cerebral palsy.⁵¹ By providing a stable base during stance phase, the DAFO improves swing limb clearance, stride length, cadence, and self-selected walking speed of children with diplegic cerebral palsy, nearly to that of age-matched peers without neuromotor impairment.^50,52 Parents of children using DAFOs report improved upright posture as well as enhanced ability of their children to participate in play activities, based on perceived changes in stability, agility, and speed of movement when wearing the DAFO.⁴⁹ The major complaint about the DAFO was that there was a need to frequently change socks because confinement of the foot within the plastic DAFO shell and shoe contributed to sweaty feet.

The Posterior Leaf Spring (PLS) AFO is a dynamic thermoplastic AFO designed to accomplish two things53:

The PLS is one of the group of AFOs that provide dorsiflexion assistance. In contrast to the SAFO, medial and lateral trim lines are located well posterior to the midline of both malleoli so that the orthosis is flexible at the anatomical ankle joint (Figure 9-11).⁵⁴ The degree of flexibility is determined by the thickness of the thermoplastic material used to construct the orthosis and width of the posterior upright in the distal third of the orthosis.⁵⁴ In custom-molded PLS orthosis, the orthotist tailors the stiffness of the orthosis using the trim line pattern that will best support the weight of the foot during swing as well as the individual’s needs for stability in stance.⁵⁴

Once the heel contacts the ground at initial contact and loading response begins, the flexible plastic of the PLS serves as a proxy for impaired or absent eccentric activity of the tibialis anterior muscle, slowing but not stopping the foot’s descent toward the ground.53 As stance phase continues, the flexible PSL allows the tibia to roll forward over the weight-bearing foot to accomplish a smooth second/ankle rocker of midstance to terminal stance. As the foot leaves the ground in initial swing, the PLS is able to hold the ankle at the desired neutral 90-degree position, which assists swing clearance and keeps the toes elevated so that next initial contact will be made at the heel. Compared to the stiffness of the SAFO, which makes walking up and down inclines even more challenging than walking on level surfaces, the flexibility inherent in the PLS provides a notable functional advantage.^55–57

Owing to its narrow posterior upright and relatively shallow heel cup, a PSL is not as effective in stabilizing the calcaneus and talus during stance as are the SAFO, DAFO, or UCBL designs. This makes it less effective in controlling mediolateral foot position, especially for persons with flexible deformities of the forefoot, midfoot, or rearfoot.⁵⁸ In these circumstances, the orthotist may opt to place medial and lateral trim lines somewhere between the mid-malleolar position of a SAFO and the narrower PLS to provide additional stability. This modification is sometimes referred to as a semisolid AFO. The modification provides better control of ankle motion, but at the cost of limiting mobility during the second/ankle rocker of gait.⁵⁹ Note that the flexibility of a PLS and a semisolid AFO make them inappropriate for individuals with significant equinovarus or spasticity of the lower extremity: A high level of abnormal tone will overpower the control systems in these designs.

The thermoplastic hinged (articulating) ankle-foot orthosis (HAFO) allows sagittal plane motion at the ankle by incorporating a mechanical ankle joint between the foot and calf sections of the orthosis. This variation of the SAFO was designed primarily to allow the tibia to roll over the weight-bearing foot during stance, for a smooth second/ankle rocker, at the same time holding the foot in optimal alignment to control the impact of tone-related equinovarus forces throughout the gait cycle (Figure 9-15). In children with cerebral palsy and in adults after stroke, these orthoses reduce energy cost of walking (compared to barefoot), as well as improve stride length, cadence, and walking speed.^74–76 As compared with the SAFO, the HAFO also improves mobility in many functional activities, such as rising from the floor (an important component of play for children), ascending and descending stairs, and walking up or down inclines.^56–58 There is some indication that HAFOs lessen the magnitude of abnormal muscle activation associated with spasticity in children with cerebral palsy.⁷⁷ HAFOs appear to reduce risk of falls in adults with longstanding hemiplegia after stroke, improve static postural control, and have less negative impact on dynamic postural control in standing than SAFOs.^78,79 For some children with moderate to severe spastic diplegic cerebral palsy, however, the mobility provided by a HAFO compromises stability in early stance, reinforcing crouch gait pattern, reducing walking speed, and increasing energy cost as compared to a SAFO.⁷⁴ Multiple perspectives on function, as well as the desired impact on quality of gait, must be considered when deciding which orthosis is best.

Hinged AFOs can be fabricated to allow free motion at the ankle, to allow limited range of motion (i.e., allow dorsiflexion and stop plantarflexion, or the inverse), or to provide some assistance to dorsiflexion, depending on what orthotic ankle joint option motion best meets the individual’s needs and maximizes the resources they bring to the task of walking. A variety of mechanical thermoplastic joints are commercially available (Figure 9-16). Those with true articulations (e.g., the Oklahoma joint) have a single axis of motion that should be aligned as closely as possible to the anatomic ankle joint; other orthotic joints (e.g., the Gillette joint) are flexible and nonarticulating.

Traditional metal orthotic ankle joints have been incorporated into thermoplastic HAFOs as well; These designs are referred to as hybrid orthoses. Simple single axis joints provide mediolateral stability without restriction of available dorsiflexion or plantarflexion; a motion stop can be incorporated if there is need to limit plantarflexion beyond neutral (for stability in early stance) or dorsiflexion (to limit weight bearing on the forefoot in later stance) as the individual’s needs dictate. A bi-channel adjustable ankle joint (also referred to as a double action ankle joint or a double Klenzak joint) (Figure 9-17) can be used to provide assistance and/or limit motion based on an individual’s capabilities and needs for support. If motion assistance is desired, a coil spring is placed in the channel, and a screw is used to adjust compression until the desired level of assistance is achieved. If motion is to be blocked, a solid steel pin is inserted instead of the spring, to stop motion beyond a particular point in the range of motion. Because of its versatility and adjustability, the double action ankle joint is often chosen when change in a patient’s functional status (improvement or deterioration) is anticipated. Orthotic ankle joints such as the Camber Axis Hinge® allow the orthotist to adjust the available range of motion from none (as in a SAFO) through an array of limited anteroposterior stop settings. This would be an advantage when motor control around the ankle and knee is expected to improve over time, for example, in individuals who are recovering from an acute stroke.