Depending on the clinical treatment goals and functional requirements, the materials used in the construction of orthoses and prostheses have different properties. Clinical goals of orthoses include relieving pain, controlling deformities, preventing excessive range of joint motion, increasing range of joint motion, compensating for abnormalities, protecting tissues, and promoting healing. On the other hand, the purpose of prostheses is to replace an absent or deficient limb. These objectives are achieved through functional requirements, such as preventing, reducing, or stabilizing a deformity; limiting the mobility of a joint; or reducing or redistributing a load on specific tissues. Thus, some materials used for the construction of prosthetic and orthotic devices are flexible to adapt to the body and absorb the energy generated during the gait cycle, or rigid to control the bending generated by the loads. ^1,14-16 Similarly, prosthetic and orthotic devices must be safe and durable, and have sufficient strength to withstand the maximum stresses expected from being subjected to normal pressure by the action of weight and gait, which generate tensile and compressive stress, in addition to tangential stress that occurs when prosthetic and orthotic interfaces and the affected limbs interact. ^5,16-19

Materials with high tensile strength improve the suspension and adjustment of the devices by reducing pistoning or slipping during the gait cycle. However, it is important to clarify that this phenomenon depends not only on the mechanical characteristics, but also on the presence of friction in the contact area, which depend, in turn, on the adhesion and deformation properties of the materials. ^7,8,19 Moreover, the stiffness of the material should be considered, as it depends on the state of the soft tissues in the affected limb: individuals with little bone end covering require coatings with low compression stiffness, while those with abundant soft tissue can achieve a better sense of control from interfaces with a little more stiffness. ⁷

These parts must allow for shape variability and must adapt to fluctuations in the volume of the lower limbs, while maintaining their overall structure, thickness, complete contact with the skin and tolerance to pressures generated during the gait cycle. ^8,20 Thus, the prosthetic and orthotic sockets and interfaces must allow for the damping of the stresses generated and adequate adjustment to correctly transfer the pressures when using them, taking into account the clinical requirements of each patient.

External orthoses and prosthetic rigid sockets are developed from thermostable or thermoplastic polymers. The former are hard and rigid polymers, even at high temperatures, and it is not possible to melt them with heat, which makes their manufacturing processes complex. ^21-23 The latter have a high molecular weight and can be plasticized by pressure or temperature ²³; among the most commonly used of this type are high and low-density polyethylene (PE) and polypropylene (PP). ²²

PE is characterized by its flexibility, ease of vacuum forming, and low weight, so it is commonly used in orthoses and artificial limbs requiring flexibility. ^5,23 PP is the lightest of the plastics used in orthopedics and is characterized by its high tensile strength, stiffness, and hardness; however, it is sensitive to deformation, and its surface can easily deteriorate with heat. PP is commonly used in the manufacture of sockets for prostheses, splints, and orthoses requiring high rigidity ^5,23; thermoforming has replaced some traditional materials in prosthetic and socket practice, reducing processing and manufacturing times. ^5,21,23

Orthoses and prosthetic sockets have been built from composite materials, in other words, they have a matrix and one or more reinforcements joined by an interface to improve the properties of the parts. ^23,24 The matrices are the body of the material and must guarantee the stability of the pieces in the environment. ^23,24 Some of the most common for rigid sockets are unsaturated polyester resins, epoxy and vinylester. The reinforcing agents fundamentally define the mechanical properties of the composite material and can be particles or fibers.

Arun & Kanagaraj ²² enhanced the performance of prosthetic sockets made from epoxy resins, smooth glass fabric and between 2 and 10 bales of elastic fabric by incorporating 0.3% (w/w) of carbon particles called multi-walled carbon nanotubes. Flexural strength and thermal conductivity were found to increase by 11.38±1.5% and 29.8±1.3%, respectively, in compounds with 4 to 10 layers of elastic tissue; flexural modulus in compounds with 2 to 10 layers of elastic fiber increased by 4.5±1.4% compared to epoxy resin compounds without nanotube reinforcement. This also reduces the weight and heat generated by rigid prosthetic interfaces.

In contrast, fibers are more used in sockets for prostheses and orthoses, as they considerably increase mechanical properties such as strength, stiffness, hardness, and dimensional stability of polymeric compounds. ⁵ Fiber arrangement and the variation in the fraction of fibers volume allow varying the characteristics of the materials. ^5,24 Aramid fibers are so resistant that they can surpass steel or aluminum; however, they are not ideally compatible with matrices like glass or carbon fibers. Fiberglass reinforcement improves most of the mechanical properties of plastics, so they have been used in prostheses with polypropylene and high-density polyethylene matrices; still, a decrease in the mechanical performance of the device and patient comfort has been reported. ²⁴ Carbon fiber, on the other hand, considerably increases the mechanical properties of thermoplastic composites, adds stiffness and high resistance to the material (up to 35 000 psi), and reduces the weight of the reinforced material. ^23-26 However, unlike some plastics, carbon fiber cannot be reheated to allow reshaping and, therefore, its adaptation processes must be accurate. ²⁷

Soft interfaces for prosthetic and orthotic devices have been developed from shock-absorbing foams and interface systems designed to provide filling and cushioning in the affected limbs. Cushioning foams can be designed from urethane and can be rigid, soft or elastic and extremely vary in density according to their cells, so they are widely used for the keel of prosthetic feet and internal coatings of orthosis and prosthetics with hypoallergenic properties. ²¹ Also, latex, polyurethane and polyethylene cushioning foams -such as plastazote and pelite- have low weight and good capacity to recover and support compression loads. These foams are generally used along with denser foams to achieve smoother interfaces and slower load compression. ⁵

For lower limb prostheses, it is common to find liners for the patient's residual limb. These are mostly made from silicone gels, silicone thermoplastic elastomers (TPE) and some additives. ^7,20 Silicone gel interfaces contain slightly crosslinked polysiloxane networks, with high polydimethylsiloxane (PDMS) free fluid content, thus allowing liquid extension under load. ⁷ However, they have lower compressive strength, shear and tensile stiffness than silicone elastomer liners, which are quite crosslinked and, unlike gels, contain little PDMS-free fluid, allowing them to keep their liquids under pressure. ⁷ The viscoelastic nature of silicone allows the flow of high pressures towards low-pressure areas under heavy loads and increased walking performance by reducing the use of additional aids and improving the suspension of prostheses. ^21,28,29 Figure 2 presents a summary of the polymers used for external lower limb prosthetic and orthotic devices.

Figure 2 Source: Own elaboration.

Datta et al.³⁰ have reported greater control of prostheses and decreased abrasion and skin irritation in transtibial amputees using silicone interfaces. The positive effects of the use of silicone liners can be attributed to the way the material adheres to the stump and its properties. ²⁹ This type of liner has an excellent shape memory and a minimum hardness of 30, average hardness of 40, and high hardness of 40 to 50 on the Shore 00 scale. In addition, they do not require much thickness to absorb the efforts of a prosthetic socket, and it is possible to produce them from a catalyst and a resin, but its shape cannot be modified. ²⁰

Sanders et al.³¹ subjected 15 types of commercial liners to compression, friction, shear and tension tests. Silicone gel liners were the softest during compression trials, suggesting that they may be the most appropriate for cushioning bony prominences, but had less resistance to compression, shear, traction and stiffness than silicone elastomer.

The use of prostheses and orthoses requires a suitable fit with the affected residual limb or stump; in this process, it is also necessary to exert forces that correct or stabilize a segment of the body over a long period of time. Nevertheless, a device that does not fit correctly to the conditions of the body or residual limb, that has insufficient resistance, or is used for long periods of time, can cause trauma to the limbs and induce skin problems. ^32-34

Skin problems can occur throughout the life cycle of the devices and are caused by various circumstances. These disorders include those induced by pressure, friction, shear, tension and stress. ³⁵ Friction between the skin and the device can cause tissue ischemia and skin breakdown, while shear injuries can cause damage to the deep layers of the skin. ³⁵ Moreover, mechanical stress on the skin in contact with orthoses and prostheses results in the destruction of the dermis and in lesions that stimulate tissue proliferation. ^12,13,35,36 For example, prosthetic liners must be adhered to the skin to promote proper suspension; however, these components pull a small amount from the limb in each phase of the oscillation, generating vertical movements called pistoning, which are produced by the elasticity of the material and cause damage to subcutaneous tissues. ³⁷ It has also been hypothesized that hidradenitis suppurativa is caused by mechanical stress accompanied by a warm and humid occlusive microclimate in people with lower limb amputation, which also favors the presence of bacteria. ³⁵

Another circumstance that affects the skin of users is related to the tight fit that generates excessive pressure on the skin, especially in bony protuberances or irregular residual limbs. ³⁷ Sbano et al.³⁸ reported a case of dermatitis clinically similar to verrucous hyperplasia on the residual limb of a patient with lower limb prosthesis due to the rigid suction socket of the device; the authors associated the presence of that disorder to the type of socket and the poor adaptation of the residual limb to the prosthesis as a result of the need for adaptation of the skin by the technical aid.

Furthermore, the heat generated by the prosthesis or orthosis socket causes a tendency for the skin to perspire, so the increase of humidity causes injuries and infections. ³⁹ Folliculitis and similar inflammatory conditions may be more recurrent in people with hairy extremities or sensitive skin. ⁴⁰ Other problems are also caused by hypersensitivity to the material, lack of hygiene, or presence of bacteria or fungi. ³⁸ Some of the bacteria species that usually populate residual limbs or affected legs when using prosthetic and orthotic devices are Staphylococcus epidermidis and Staphylococcus aureus. Moreover, the constant interaction of the skin with orthoses and prostheses also provides an ideal environment for the growth of dermatophytes and Candida albicans.^36,41 Also, due to the presence of microorganisms, pre-existing skin disorders may recur or cause allergic reactions caused by ointments used for skin care. ^21,22 These problems can be exacerbated by the device over time. ³⁶ Table 3 shows the skin problems associated with the use of external lower limb orthoses and prostheses, and their main causes.

Table 3

Source: Own elaboration based on ^{12,13,35,36,38,39,41-46}.

In the mid- 1980s, Össur Kristinsson developed the Icelandic roll on silicone socket (ICEROSS) with the aim of offering more comfortable sockets that would provide better suspension and alleviate the skin problems associated with the previous designs. This type of socket has become a standard for the treatment of transtibial amputees. ⁴⁷ However, in 2008, Hall et al.⁴⁷ conducted a study to determine the prevalence of skin problems in people using this type of coating, and found that 90.9% of subjects reported problems in their medical history and 78% during the study, regardless of the cause of amputation. Common problems included excessive sweating, itchy skin, and redness.

Literature shows that, since 2000, there has been little research on the incidence of skin problems associated with the use of external lower limb prostheses and orthoses. In 2001, Dillingham et al.⁴⁸ found that about a quarter of the amputees surveyed reported skin irritation problems, wounds, or constant pain in the residual limb; Hagberg & Brånemark ⁴⁹, when studying individuals who used transtibial prosthesis, found that the most frequent problems that led to a decrease in quality of life were heat and sweating in the prosthetic socket (72%), factors that favor the presence of skin problems.

In 2005, Dudek et al.³⁶ reported the frequency of skin problems in 745 lower limb amputees using prostheses through a 6-year retrospective review, finding that 40.7% of the cases had at least one skin problem. Similarly, Meulenbelt et al.⁵⁰ identified the determinants of skin problems in lower limbs or prosthesis of amputees through a survey in 2009; out of 805 people, 63% reported at least one skin problem, and the biggest determinant was the use of antibacterial soap, followed by smoking and frequent washing of the residual limb.

The following year, Ghoseiri & Bahramian ⁵¹ evaluated the satisfaction of 172 women and 121 men wearing prostheses and orthoses in Iran. When asked whether the skin was free of irritation and abrasion, the result yielded an average of 2.0 with a standard deviation of 1.1, where the overall satisfaction score ranged from 0 to 100 (the lowest and highest possible score, respectively). ⁵¹ That same year, Meulenbelt et al.¹³ reported a cross-sectional survey consisting of a questionnaire and a clinical evaluation in order to estimate the prevalence of skin problems in the residual limb of patients; of 124 subjects, 36% reported having skin problems, including cold skin and excessive perspiration, skin problems that usually trigger pain, affecting people's performance. ³⁷ Furthermore, between 2014 and 2015 a study was conducted in Colombia with 150 people who used lower limb prostheses, finding that 78% of the sample had presented some discomfort or condition in the residual limb, having irritation as the painful condition with the highest incidence. ⁵²

These studies show that skin conditions are frequent complications in people who use prosthetic and orthotic devices (Table 4), with prostheses being the most common cause of these conditions. The risk of skin problems differ from one device to another and depend on each individual. ⁴⁰

Table 4

SP: skin problems; P: people; O: observed by specialists; R: reported by patients; M: males; F: females