Commonly Used Types and Recent Development of Ankle-Foot Orthosis: A Narrative Review

2.1. Typical Plastic AFO

Plastic AFO (PAFO) is mainly made of thermoplastics such as polypropylene and is one of the most widely used orthosis in clinical practice owing to its numerous advantages such as its relatively low cost, good aesthetics, being easy to clean, and easy desorption [ 11 12 ]. PAFO is customized to the patient’s body, as it is generally produced by casting the patient’s lower extremity to make a positive plaster model. By placing thermoformed plastic to cover the positive plaster model, it produces the orthosis in the exact shape of the model. PAFO commonly consists of a shank shell, foot plate, and Velcro strap, with hinges on ankle joints as needed [ 13 14 ]. PAFO can be classified according to the presence of hinges, mainly as solid ankle types without hinges and hinged ankle types with additional hinges. If the purpose is to solely keep the ankle in a neutral position, a solid ankle type is applied, and the trimline is placed in front of the ankle bone to control the medial and lateral stabilities of the ankle ( Figure 1 A) [ 15 ].

Solid AFO (SAFO) is predominantly applied to completely limit the ankle joint movement in patients with foot drop, weak dorsiflexion and/or plantarflexion, ligament injury about the ankle, mild knee instability, and valgus/varus [ 16 17 ]. Posterior leaf spring orthosis (PLSO) is a SAFO, but unlike the conventional SAFO, PLSO has a characteristic trimline located behind the ankle and has a leaf-shaped corrugation near the ankle ( Figure 1 B). The leaf-like creases are intended to strengthen the part of the ankle with the most amount of movement and repeated loadings. The creases act as a spring in the ankle that allows slight dorsiflexion in the mid and terminal stances, and this elasticity can also marginally assist the push-off function in the terminal stance. PLSO is used in the presence of motor weakness in the ankle dorsiflexor caused by conditions such as cerebral palsy and stroke. Owing to its greater elasticity and flexibility than those of regular SAFO, PLSO is suitable for patients with mild cramps or who are more active and have better balance than those for whom SAFO is used [ 18 20 ]. In addition, as the ankle trimline extends further to the front of the ankle joint, the effectiveness in controlling the instability of the ankle increases. However, the PLSO does not contribute significantly to ankle stability, as the trimline is behind the ankle. Therefore, PLSO has a limitation in controlling valgus/varus [ 20 21 ].

The hinged AFO (HAFO) is used when ankle movement is permitted but movement restrictions to a certain extent is required. HAFO is produced by using hinges to connect two pieces, the shank and foot shells, and the hinges are commonly located on the malleolus side ( Figure 1 C). The hinge on the HAFO allows a certain degree of dorsiflexion that makes it easier for users to walk on uneven surfaces or to climb stairs. It also increases ankle dorsiflexion in the terminal stance and increases ankle plantar flexion during the pre-swing phase, which helps users walk naturally [ 22 ]. The commonly used hinged types of PAFO include the overlap, Oklahoma, and Gillette joints ( Figure 2 ). The overlap joint limits plantarflexion by overlapping the shank and foot shells, and fixing it in with a rivet. The Oklahoma joint connects a separate shank shell with the foot shell, which creates a space between the shank shell and the back of the foot shell to allow plantarflexion until the two pieces meet. The plantarflexion can also be completely limited by fitting the shells at 90° without space in between. The Gillette joint, like the Oklahoma joint, connects a separate shank shell with the foot shell, allowing both plantarflexion and dorsiflexion. HAFO is widely used in children with spastic diplegia and patients with spastic hemiplegia after stroke, as it can stretch the ankle plantar flexor to reduce stiffness and reduce disorganized muscle-response patterns. It is also used in the presence of low muscle tone (hypotonia), high muscle tone (hypertonia), flexible pronation or supination, sagittal and/or frontal plane weakness, excessive plantarflexion, and genu recurvatum. However, it should be applied into patients with sufficient control of their knee joints and should not be used for patients with severe mediolateral instability of the ankle [ 22 24 ].

The patellar tendon bearing AFO (PTB-AFO), unlike other PAFOs, has an additional anterior shell to support weight with the patellar tendon, which helps to reduce the weight load on the heel, ankle, and sole, and therefore reduces pain in each of the mentioned areas ( Figure 1 D) [ 25 26 ]. This is used in cases that require the pressure on the foot to be minimized, such as ulcers, calcanectomy, plantar skin graft, severe foot/ankle trauma, and fractures.

In 2007, Balaban et al. [ 1 ] measured the walking parameters and angle of the ankle during walking in 11 children with hemiplegic cerebral palsy according to the presence or absence of a hinged AFO made with a plantarflexion stop at 0° with no dorsiflexion stop. As a result, with a hinged AFO and bare feet, respectively, the mean velocities were 0.89 and 0.70 m/s; single support averages, 0.38 and 0.36 s; double support averages, 0.18 and 0.23 s; and stride lengths, 0.90 and 0.73 m, which demonstrated significant differences in all four components. Furthermore, the angle of ankle dorsiflexion at initial contact was 4.79° with a hinged AFO and −4.28° with bare feet, and the ankle dorsiflexion at mid stance averaged at 11.59° and 5.81°, respectively, which also showed significant differences in the two components. With such results, walking function has been confirmed to improve when hinged AFO is used, as compared with walking barefoot. Abe et al. [ 27 ] in 2009 evaluated walking function using an 8-m walk test and functional ambulation category (FAC) with or without using a plastic AFO in 16 hemiplegic stroke patients. As a result, in the patients who used and did not use orthoses, respectively, the stride lengths were 65.7 ± 13.6 and 56.9 ± 13.6 cm; affected-side step lengths, 34.0 ± 10.0 and 30.4 ± 9.4 cm; step widths, 29.8 ± 4.4 and 28.2 ± 5.0 cm; velocities, 22.9 ± 6.8 and 18.1 ± 8.1 m/min; and cadences, 73.3 ± 15.8 and 66.8 ± 21.0 step/min, which all demonstrated significant differences. The FAC score was 3 points in 9 patients (56.3%), 4 points in 7 patients (43.8%), and 5 points in none (0%) of the patients prior to wearing the orthosis. However, after using the orthosis, the FAC score was 3 points in 1 patient (6.3%), 4 points in 5 patients (31.3%), and 5 points in 10 patients (62.5%), which reported significant improvement in walking ability.