Foot orthoses

Foot orthoses (FOs) are the foundation for lower limb management. Not only are they suitable for treating many of the basic problems encountered in daily practice, but each and every more proximal orthosis (e.g., ankle–foot orthosis [AFO], knee–ankle–foot orthosis [KAFO], hip–knee–ankle–foot orthosis [HKAFO]) is first and foremost an FO. In general, the FO portion should be used to manage as many deficits as possible, relying on more proximal designs only when an FO alone is insufficient. This approach is the most cost effective and enhances patient acceptance because most FOs are “invisibly” contained within the footwear.

Conceptually FOs usually are divided into three broad categories³³:

Accommodative FOs typically are used to “cradle” and protect rigidly deformed, insensate, or dysvascular feet. They can be made from a variety of soft or flexible materials. Minor complaints, such as mild metatarsal pain, may respond to over-the-counter (OTC) or prefabricated inserts of resilient materials. Significant problems most often are managed with custom-molded FOs.

Intermediate FOs made of semirigid materials are popular clinically because they can be fabricated from multiple layers of materials of slightly different densities to provide graduated degrees of control, thereby enhancing client acceptance. OTC, prefabricated, or modular intermediate FOs may suffice for mild problems such as metatarsalgia without ulceration. These somewhat generic devices also may be suitable for a clinical trial to verify the effectiveness of orthotic management in a given case. Chronic conditions as well as moderate and severe problems, including all feet with a history of ulceration or sensory limitation, typically require custom-molded devices to ensure the most meticulous fit.

Corrective FOs made of rigid materials can be extremely difficult to fit successfully and require meticulous attention to detail.² A weaning period of several weeks is commonly recommended for client acceptance. Most orthotists and pedorthists reserve the use of rigid materials for easily correctable, flexible deformities such as mild ankle valgus, and for subtle control of a nearly normal foot with slight biomechanical deficits, such as slight excess pronation in a distance runner.

Remember that the foot is a dynamically changing organ during the gait cycle, whereas the FO is a relatively static device. A thorough understanding of foot and ankle biomechanics, in addition to careful follow-up and adjustments, is essential for long-term success with this modality. Few hypotheses regarding the use of FOs have been investigated in controlled studies, so much of the variance encountered is based on clinical experience and judgment.

Ankle–foot orthoses

Ankle–foot orthoses (AFOs) can be designed with sufficient mechanical lever arms to effectively control the ankle complex and to influence the knee joint indirectly, making them applicable for the management of more extensive disabilities than can be managed with FOs. Successful use of electronic AFOs based upon electrical stimulation of weak or paralyzed muscles has been reported with increasing frequency in recent decades.¹³ In the United States, commercial versions of orthoses that incorporate neuromuscular electrical stimulation (NMES) recently have become clinically available.³¹

When a mechanical AFO is desired, the clinician must choose from among metal alloy devices, plastic devices, and a hybrid assembly incorporating both materials. In most cases, such technical decisions are best left to the orthotist and patient, who can take the time to fully discuss the various advantages and disadvantages of each approach. In general, plastic or hybrid plastic/metal systems predominate in North America because of the greater degree of client acceptance and circumferential control they offer.¹⁸ Carbon fiber composite AFOs that are thinner, lighter, and stronger than thermoplastic devices are now available. Composite AFOs have been very well accepted by patients and offer improvements in range of ankle motion and push-off characteristics.⁷ Unfortunately, funding restrictions continue to prevent many good candidates from having access to the most modern orthotic technology and effectively discourage the development of more advanced future orthoses.

The older-style metal and leather orthoses usually are reserved for selected applications, including the following:

Minimal contact designs are sometimes best for persons with fluctuating edema and for heat-sensitive individuals who cannot tolerate the more intimately fitting plastic contours.²³ Some clinicians prefer metal systems for growing children because of their adjustability for growth, but many others believe that plastic or hybrid devices are just as versatile in this regard.²⁴ One of the most significant factors in material selection is client (or parent) preference, which largely determines acceptance and therefore should predominate over all moot points in the final decision.

Functionally, all orthotic devices including AFOs must achieve one or more fundamental goals, such as⁴ (1) control of motion, (2) correction of deformity, and (3) compensation for weakness. Figure 26-1 illustrates one common application of control of motion: a rigid, plastic AFO with the ankle locked in slight dorsiflexion and a well-padded anterior proximal segment to stabilize the tibia. This design, first reported in 1969 by the Israeli orthotist Saltiel, is often colloquially termed a floor reaction AFO²⁸ because the extended, rigid forefoot section accentuates the knee extension moment at midstance and thereby prevents tibial collapse that otherwise would result from weak or absent gastrocnemius–soleus strength. This illustrates one of the critical treatment principles of orthotic management: The orthosis may indirectly affect remote body segments, and this characteristic can be used therapeutically. The sagittal plane biomechanics of this AFO at midstance are shown in Figure 26-2.

Fig. 26-1 Plastic AFO, with a rigid ankle set to accommodate various heel heights, uses floor reaction forces to reduce knee flexion in the latter half of the stance phase.

Fig. 26-2 Biomechanics of the floor reaction AFO, from the original illustration by Saltiel. In late stance, the extended forefoot plate contacts the floor and creates a knee extension moment via the well-padded anterior contours.

Orthoses can be rationally and succinctly prescribed based on the biomechanical function desired.¹⁹ The plastic floor reaction AFO with ankle locked in slight dorsiflexion usually is applicable when the patient has a paralyzed ankle–foot complex but good or better quadriceps and balance. Examples of pathologies that could result in this clinical picture include myelodysplasia, spinal cord injury, peripheral nerve injury, poliomyelitis, and gastrocnemius–soleus trauma or dysfunction.

One of the more common lower limb deficits is a flaccid equinus, which may result from many causes, including Charcot-Marie-Tooth disease, cerebrovascular accidents with mild residual symptoms, muscular dystrophy, and peroneal palsies of various types. The orthotic options to compensate for pretibial compartment weakness or paralysis definitively (Fig. 26-3) include the following:

Fig. 26-3 Matrix depicting different potential orthotic solutions for treatment of flaccid equinus.

A functionally based prescription, such as “orthosis to compensate for weakened pretibial musculature,” ensures that the orthotist considers all available alternatives before determining the optimum solution for a particular patient.²³ A more restrictive prescription, such as “prefabricated plastic AFO to provide dorsiflexion assist,” is appropriate only if that is the sole desirable solution—and all others are automatically ruled out.

The first alternative, which in some instances might be supplied by the 2-inch heels typical of cowboy footwear, prevents the plantarflexed forefoot from dragging in swing phase because the boot stabilizes the flaccid foot, and the heel height lengthens both legs sufficiently for clearance of a plantarflexed foot. This simple option may be acceptable in some instances as long as the affected leg has sufficient mediolateral ankle stability.

The next two choices are essentially spring-loaded metal devices attached to the client’s shoes. Use of a caliper box and removable stirrups with the dorsiflexion assist AFO allows shoe interchange (as does the spring wire AFO), but the cost and inconvenience of modifying all shoes should not be underestimated. The chief advantage of the metal systems is the limited contact with the leg, which must be balanced against the bulk, weight, and maintenance required for such mechanical solutions.

The fourth option (flexible plastic AFO) may be either prefabricated or custom molded. The device can be made from thermoplastics, carbon fiber composites, or silicone elastomer. In general, prefabricated solutions are applicable for short-term use by patients who retain protective sensation (e.g., mild iatrogenic palsy from cast application) as long as the limb contours are within normal limits. Deformed limbs, insensate limbs, and definitive long-term applications generally require custom-made devices to ensure long-term comfort and success.

The fifth option (NMES) recently has become commercially available in the United States. Although the biomechanical result (increased toe clearance during swing) is similar among all AFO designs, potential additional benefits from electronic AFOs³⁰ include the following: