Chapter 4 – Orthoses for the Foot and Ankle




Chapter 4 Orthoses for the Foot and Ankle


James Brousil and Andrew “Fred” Robinson



Introduction


The treatment of the foot and ankle differs from many other areas of orthopedic practice, in that a large proportion of patients are treated non-operatively, many with orthoses. An orthosis is defined as “an externally applied device used to modify the structural and functional characteristics of the neuromuscular and skeletal systems.”


The reason for using an orthosis will fall into one or more of the following categories:




  • pain relief



  • deformity management



  • restriction or promotion of movement through a joint



  • to correct the imbalance of neuromuscular structures



  • to compensate for abnormalities of body-segment shape or volume



  • to protect tissues during healing.


In prescribing an orthosis the diagnosis and goal of treatment must be clear. Specific considerations should be documented in the prescription, such as weightbearing limitations, recommended movement restrictions, and neurovascular pathology. The prescription for an orthosis needs tailoring to the patient and their practical needs, to ensure an appropriate, wearable device is produced. Detailed prescription will help ensure that the orthosis is acceptable to the patient, corrects the abnormality, and fits the limb to which it is applied.


The orthotist will evaluate the patient, specify the device, and make the orthosis. The orthotist should also educate the patient and, if necessary, fit or adjust the device.



Terminology


The terminology for orthoses is defined by the International Organisation for Standardisation (ISO), based in Geneva. ISO-8549 defines the vocabulary for prostheses and orthoses, these definitions facilitate communication and limit the use of eponyms, which can introduce error and variability.


Orthoses are described by the part of the body they pertain to. Relevant devices to the scope of this chapter are: foot orthosis (FO), ankle–foot orthosis (AFO), knee–ankle–foot orthosis (KAFO), and hip–knee–ankle–foot orthosis (HKAFO).



Materials


The materials used in the manufacture of an orthosis determine its properties and function. The materials may be classified in many ways. Rigid (for example metal or plastic) or soft (for example leather, cork, or cellular plastics) is one way. In the end soft and hard materials are often combined to produce a supportive appliance (hard), which is comfortable (soft).



Rigid Materials



Metals

Steel struts were the basis for the first generation of orthoses. Steel is rigid and may be used in combination with leather strapping to correct deformity. Steel can be contoured to the shape of the limb, and joints can be fashioned to allow movement. The struts are linked to the shoe and the addition of springs and movement stops can be used to address motor deficits and instability. Nevertheless steel’s stiffness risks damaging the soft tissues and it is therefore used with a soft interface, such as leather.



Plastics and Composites

Two main families of plastics are in common use in the fabrication of orthoses: thermosetting and thermoforming. The two groups behave differently with heat and their method of manufacture differs.



Thermosetting Plastics and Composites

Thermosetting plastics are formed from syrup-like resins composed of long chain, synthetic organic polymers. The two in most common use are polyester (cheap) and epoxy (expensive). Thermosetting plastics are worked in their liquid phase. They may be poured into molds to form complex shapes or laminated to form rigid structures. The result is a tough plastic, which has a high strain modulus, but which is relatively brittle and will fatigue and crack under extreme deformation.


Depending on the plastic and the desired properties of the finished product, thermosetting plastics may be cured at room temperature or using heat and pressure. In the case of epoxy, a nitrile hardener is added to cross-link it and form a rigid structure. Once curing is complete, strong, permanent bonds form between the linear-chain polymers. Once these strong covalent bonds are formed, they cannot be undone by heat. Instead the plastic will be destroyed if its glass transition temperature is exceeded. An appropriate analogy is with that of boiling an egg. Once the albumin protein is denatured by heat a solid structure is formed, and this process cannot be reversed.


The properties of these plastics may be enhanced significantly by the addition of glass, carbon, or aramid fibers to the resin base. The fibers can either be added as sheets or mixed into the liquid plastic prior to setting. The addition of glass increases tensile strength by up to 90%. Carbon fibers are lighter, improve the stiffness to weight ratio, but are more expensive. Aramid fibers added to a thermosetting plastic form Kevlar®. All of these composite materials are brittle and lack yield capacity. This may result in dramatic failure when stressed to failure, for instance during impact loading.


Epoxy and polyester are the dominant materials used in fashioning rigid orthoses:




  • Polyester is quick setting but unpleasant to work with as a result of its odor. It can be toxic in confined spaces. It is versatile and may be manipulated to alter its flexibility. This is usually achieved by the addition of styrene to the mix, which increases its elastic deformation. A ratio of 60:40 of polyester:styrene is used for most foot orthoses. The resultant plastic is more flexible than epoxy, but will flex and fail upon excessive loading. It is permeable to moisture.



  • Epoxy is more expensive but produces a better performing plastic when cured. Its bonding strength is four times that of polyester and it is less prone to cracking and fatigue. It is also impervious to moisture. Epoxy is used in most high-pressure lamination plastics; it may also be formed by vacuum or direct pressure molding.



Thermoforming Plastics and Composites

Thermoforming plastics are long-chain polymers, resembling strands of spaghetti. They are formed by an addition polymerization reaction. This requires heat, pressure, and a catalyst. There are no discrete bonds between the strands. They are held in apposition by temperature-sensitive, weak electrostatic attraction. These bonds become weaker with heating and form again upon cooling. Prolonged application of force to these plastics causes the molecules to slide over each other, a deformation process known as “creep.”


The macroscopic organization of these long-chain polymers has an effect upon the properties of the plastic. Organization of the plastic may be amorphous (loosely packed) or crystalline (tightly packed). Molecules that have few side chains will lie in close apposition to each other, resulting in stronger intermolecular forces. Polymer strands held apart by side chains form weaker intermolecular bonds, leading to loosely associated amorphous zones within the plastic.


Composite plastics, which contain both crystalline and amorphous elements, are in common use. As with the thermosetting plastics, the addition of glass, carbon, or aramid fibers greatly improves the performance of the finished composite.




  • Amorphous plastics include acrylonitrile butadine styrene (ABS), polystyrene, polycarbonate, polyethermide, and acrylic. Amorphous plastics tend to be heat moldable over a broad range of temperatures making them easy to thermoform. When cured, the polymer chains of amorphous plastics are held together by weak electric bonds, heating allows these long-chain molecules to slide over each other during the molding process. There is a window when the material is pliable and heat moldable, after this it melts. If the plastic melts, the long-chain polymers will break and the plastic degrades. Amorphous plastics tend to set clear and can be bonded to other plastics using solvents or adhesives. They are prone to creep and stress fatigue. Their main application is in the structural components of orthoses in which bending and shear stress are not involved, for example in the base layer of an in-shoe orthosis.



  • Common crystalline plastics include polyethylene, polypropylene, polyether ether ketone (PEEK), and nylon. These plastics have much sharper melting points, making heat molding more difficult. They tend to be opaque when set and are difficult to bond to other plastics. They display excellent resistance to fatigue and cracking. They are used for structural orthoses that are required to resist bending and shear forces. Polyethylene and polypropylene are commonly used in AFOs. Polyethylene is available in two formats: ortholene and subortholene. Subortholene is more extensively used in sports bracing as it is more flexible, but also prone to plastic fatigue. Polypropylene is more rigid and durable than polyethylene and is more commonly used for corrective AFOs. Its thermoformable properties allow adjustment of the orthosis post manufacture.



Soft Materials



Leather

Soft materials dissipate force and accommodate deformity. Leather is still widely used, and should not be forgotten. It is soft, comfortable, breathable, readily available, and can be easily worked.



Cork

Cork is flexible and moldable. It is often used as the base layer of in-shoe orthoses. It is also used for posting and wedging hindfoot orthoses. Cork is impervious to moisture, and rigid enough to be corrective when added to an orthosis. It is used in the welting process of shoe production to fill the void between the upper and sole. Cork is commercially available in heat moldable forms, which are utilized in custom foot orthoses.



Synthetics

Soft deformable plastics are divided into open- and closed-cell foams. Open-cell foams dissipate heat and are less resistant to deformation than closed-cell materials. They allow moisture evaporation and are more durable. Polyurethane is an open-celled plastic used in the middle and base layers of cushioning insoles and training shoes. It has excellent deformation memory but poor shear resistance, and consequently is not used as an interface layer. Its most commonly available commercial form is Poron®.


Closed-cell foams are characterized by discrete, non-communicating cells. Closed-cell foams are prone to “compression settling” under load. This property is harnessed to accommodate bony prominences, for example when fitting a total contact insole in a neuropathic foot.


The most common closed-cell foam in use is a cross-linked polyethylene compound, commercially available as Plastazote®. The foam consists of nitrogen bubbles in a polyethylene base. It is both soft and deformable and is commonly used in total contact orthoses. Three densities are produced and color coded accordingly – in order of increasing density – pink, white, and black. Plastazote® is heat moldable and will adhere to other plastics if heated above its yield temperature. Aliplast®, Pelite®, and ethylene vinyl acetate (EVA) are similar polyethylene foams, all of which are available in differing grades of stiffness. These foams are used in prosthetic socket fabrication and may be formed to the shape of the limb by direct molding. Ethylene vinyl acetate is the most commonly used cushioning component used in athletic shoes, as a result of its combination of shock absorption and support.


Viscoelastic materials, such as Sorbothane®, are unique among the soft materials in that they display rate-dependent resistance to compression. This shock-absorbing property is utilized in custom foot orthoses. Sorbothane® cannot be heat formed or molded to accommodate or correct foot shape but can be shaped, or contoured, by sublayering it with harder materials.


Neoprene is a polymer of chloroprene and has a wide range of applications. It forms an ideal top layer for foot orthoses. It is impervious to moisture and resists shear stress. Neoprene is available as an open-cell variant, which has superior heat-dissipation properties, but at the expense of its cushioning ability.



Manufacture


Fabrication techniques have changed to reflect changes in material technology. The first generation of orthoses were made of leather straps and steel struts. These have now largely been replaced by moldable, bespoke devices constructed from lightweight thermoplastic materials, often reinforced with carbon fiber. Nevertheless, the techniques required to fit modern devices continue to rely on the original principles of fitting, established by “brace makers.”


Orthoses may be prefabricated or custom made. Prefabricated orthoses include most of the simple in-shoe devices such as heel cups, metatarsal bars, and semi-rigid contoured orthoses. These can often be bought over the counter and are less expensive.


Custom made orthoses are molded to the limb to which they are applied. The first step in custom orthosis manufacture is the creation of a positive model of the limb to be braced. Two techniques are in common use to form this model: casting and foam-block molding.



Casting


The mold for the positive model is usually made from a complete below-knee plaster of Paris or synthetic cast. The steps are:




  1. 1. Apply a single layer of stockingette around the limb, with a length of tubing down the anterior portion of the limb to allow cast removal without damaging the skin beneath.



  2. 2. Mark joint axis and bony prominences on the stockingette. These marks transfer to the plaster.



  3. 3. Apply a layer of plaster.



  4. 4. Remove the cast with shears and then reseal it to form a negative mold of the limb.


The mold is then filled with liquid plaster, creating a positive model of the limb. The positive model can be used as a template around which a heat formable plastic can be applied.



Foam-Block Molding


The creation of a model of the foot is undertaken in a block of low-density polyurethane. The patient’s weight is used to create an impression of the foot (Figure 4.1). This negative space is in turn filled with liquid plaster, to create a model of the foot. This technique is commonly used in the production of total contact insoles, which are used for patients with deformed feet, for example diabetics with Charcot neuroarthropathy. It is less reliable when fitting a corrective orthosis, as an impression of the foot in neutral alignment is not usually achieved. This is because the foot is usually partially loaded during acquisition of the impression.





Figure 4.1 Creating a foot impression in foam. The foot is placed onto a preformed polyurethane block to create a negative impression of the foot.



Lamination and Vacuum Molding


If the orthosis is to be predominantly made of plastic, vacuum and lamination molding are usually the techniques of choice. Both techniques require a positive model of the limb and will result in a custom-made device, bespoke to accommodate the foot, or limb, shape.



Vacuum Molding

Thermoplastic sheeting may be vacuum formed to a shape using negative pressure to draw a heated, moldable sheet against the contours of a model. Reinforcements may be incorporated to provide extra rigidity in key areas. Corrugating the thermoplastic sheet can also reinforce the orthosis. Extra layers may be added to improve durability, for example on the sole of an AFO. These layers are usually added during molding of the initial shell layer. Once the orthosis has been formed, it is finished with the removal of prominent surfaces or ridges.



Lamination Molding

Lamination is a production method by which beneficial properties may be imparted into a finished plastic by layering the plastic with additional or “base” materials. Commonly used bases include nylon, Dacron® fibers, glassfiber, boron, and aramid.


The lamination process uses a polymer (thermosetting or thermoplastic) in its workable state, and a base material applied in alternate layers over a positive mold. The layers adhere to each other during production, as they are added while the polymers are still in a workable state. For example, if epoxy is used, the base material is draped over a positive mold and saturated in resin prior to the application of a further layer. Resin can be layered one on top of the other.


Lamination requires the application of pressure. Three variants are in common use:




  • High-pressure lamination


This method uses a thermosetting plastic applied over a positive mold using a press capable of exerting between 7 and 14 MPa to the material. A highly durable plastic is produced. Specialized machinery is required, usually in a factory setting.




  • Low-pressure lamination


This technique uses a vacuum to exert pressure on the laminated plastic and can be performed in the hospital workshop. Local production speeds up the production process.




  • Contact-pressure lamination


This technique is used to fashion multiple bespoke sections of an orthosis or to adjust an existing device by creating an addition. Pressures of approximately 1 MPa are used to manually apply layers over a positive model.


Recent advances in technology now allow models to be produced from CT/MRI data without the need for a molding visit to the orthotist. It is possible to use 3D printing to produce positive models of a limb. The printed model of the limb is then used in construction of the orthosis.


In building a metallic orthosis, which is rare for the foot and ankle, the positive model is used to ensure the contours and hinges are at the correct level.




Table 4.1 A summary of common conditions and recommended orthotic prescriptions






































































Disease Goals of treatment Prescribed device
Hallux rigidus/turf toe Reduce the moment through the first MTPJ Morton’s extension
Rocker bottom shoe
Metatarsalgia Redistribution of plantar pressures Metatarsal dome or bar
Midfoot arthritis Reduce bending moment across midfoot joint Arch support and rocker-sole shoe
Acquired flat foot – correctable Pain relief, offloading eccentric force across medial hindfoot Plaster or walker boot (acute phase)
Rigid correcting through shoe orthosis
Arizona brace
Acquired flat foot – stiff Pain relief, realignment of ankle, redistribution of plantar pressures Custom-molded through-shoe accommodative orthosis
Cavovarus foot Relief of high-pressure areas under the first MTPJ and base of fifth metatarsal Through-shoe semi-rigid accommodating orthosis
Ankle arthritis Pain relief, restriction of sagittal plane movement across ankle Lace-up leather or neoprene brace extending to the mid-calf
Heel-pad pain Cushioning at heel strike, pain relief Cushioned heel cups
Plantar fasciitis Cushioning of heel pad Cushioned heel cups
Insertional Achilles tendonopathy Cushioning the heel and reducing the working length of the Achilles Semi-rigid heel cups
Ligamentous ankle instability Preventing extraphysiological varus across the ankle Neoprene stirrup with added metal or plastic inserts
Flaccid instability across ankle due to paralysis (polio, nerve injury, etc.) Holding ankle alignment through each phase of gait Ankle–foot orthosis
Neuropathic foot with deformity Redistribute plantar pressures Total-contact insole


Orthoses


Virtually all diseases of the foot are treatable using an orthosis. These devices may be either in-shoe orthoses or ankle–foot orthoses, the indications for which differ depending upon the disease being treated.


Both foot orthoses and AFOs can be utilized in correcting flexible deformities within the foot. Foot orthoses are small, and more cosmetically acceptable, but achieve less correction than an AFO.



Foot Orthoses


The foot orthosis provides, and may modify, the interface between the foot and the floor. Both corrective and accommodative foot orthoses have been shown to be clinically beneficial12.


In-shoe orthoses are commonly manufactured in three layers:




  1. 1. Top layer – compressible open- or closed-cell, e.g., Poron®



  2. 2. Middle layer – often compressible polyurethane



  3. 3. Base layer – firm non-compressible layer made of cork, dense foam, or thin plastic.



Assessment and Evaluation

While the measurement of pressures under the foot is possible, careful examination of the foot, weight bearing and non-weightbearing, gives the majority of the information. The deformity, its correctability, and assessment of the range of movement in each of the major motion segments of the foot should be recorded. Callosity develops under the sole of the foot in areas of high pressure. It is the callused areas that will need offloading by the orthosis. It is also important to confirm that the foot is sensate – hard insoles under an insensate foot may well lead to ulceration.


Following assessment the orthosis is prescribed. Foot orthoses can either be soft, semi-rigid, or rigid. Soft, accommodative insoles are designed to support and cushion the foot, without correction of deformity. An example is the total-contact insole, which is often used in diabetics with insensate feet and is made of three layers (Figure 4.2). The cushioning upper layer is usually made from a closed-cell polyethylene. This rests upon shock-absorbing polyurethane, which in turn sits upon a tough ethylene vinyl acetate layer that is placed in the shoe. Cut-outs are contraindicated in the neuropathic foot as pressure is generated around the cut-out, with the risk of ulceration. Similarly, patients with degenerative disease will find corrective, hard insoles painful and also need soft, accommodative insoles.





Figure 4.2 A total-contact insole, composed of three layers.


On the other hand, sportsmen usually need corrective insoles, for example to correct a flat foot. Such insoles will need to be more rigid and will consequently be harder.


Foot orthoses are also classified by their length – hindfoot, full-length, three-quarter, and forefoot.



Hindfoot Orthoses

Simple heel orthoses can usually be bought off the shelf. Their commonest indication is plantar fasciitis, where soft inserts are used to cushion heel strike. Simple prefabricated off-the-shelf heel inserts, for example made of silicon rubber, associated with an Achilles stretching program, have been shown to be more effective than a customized polypropylene insole in the treatment of plantar fasciitis3.


A simple in-shoe heel raise can also be used to treat insertional Achilles tendinopathy by reducing the motion segment46. A heel raise can also be used to address leg length discrepancies of up to 1.5 cm, raises greater than this need to be built into the shoe.


More complex, corrective insoles can be divided into two principal groups: cavovarus and planovalgus. The cavovarus foot is stiff with reduced shock absorption, and increased pressure under the heel and first and fifth metatarsal heads. Manoli described successful management of 92% of patients with mild cavovarus feet with an orthosis with a small heel raise, to accommodate tightness of the calf, a recess under the first ray, and a laterally based forefoot wedge to correct the hindfoot varus7. The medial arch of this orthosis is low.


At the opposite end of the spectrum is the physiological flat foot, which is supple and more evenly distributes the pressure. In later life, flat foot may develop, or worsen, as a result of tibialis posterior tendon insufficiency, or degenerative change in the tarsometatarsal joint. In asymptomatic patients with physiological pes planus, orthoses are not indicated8. There is no objective evidence that orthoses alter the shape of the foot, in the short or long term. The beneficial effect seen in patients with flexible deformities is thought to result from a favorable alteration in kinetics, by modifying the position of the joints of the foot and the shoe–orthosis interface910.


The correctable, painful flat foot can often be successfully treated with orthoses1112. The orthosis has a medial hindfoot post to correct the valgus and an arch support to raise and support the midfoot. The most powerful example of this device is the University of California Biomechanics Laboratory (UCBL) insert (Figure 4.3). This is a corrective, in-shoe device, fashioned from injection-molded polypropylene. It has a molded heel cup, which controls the subtalar joint. The UCBL insert has fallen out of favor recently, as they are bulky and difficult to fit into shoes and can be uncomfortable.





Figure 4.3 The University of California Biomechanics Laboratory (UCBL) insole is used to correct flexible deformities. Note the raised flange around the heel and sides of the device, which control the foot position, but add bulk.



Forefoot Orthoses

Forefoot orthoses are mostly used in the treatment of metatarsalgia. Metatarsalgia is a symptom, with a number of etiologies. Not all metatarsalgia, for example the synovitis of inflammatory arthropathy, will be helped by insoles. In other cases, for example a Morton’s neuroma, an orthosis can be used to offload the forefoot. Offloading can be achieved by using a metatarsal dome for a single ray, or a metatarsal bar across the width of the forefoot to offload multiple metatarsals (Figure 4.4). The bar, or dome, is placed just proximal to the pathology and redistributes the pressure.


Sep 15, 2020 | Posted by in ORTHOPEDIC | Comments Off on Chapter 4 – Orthoses for the Foot and Ankle
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