One aspect of the orthotic industry that can often be overwhelming and confusing to newcomers is the range of materials available. In addition to the many different types and varieties of actual materials, there are also a myriad of trade names to contend with.
For more information, please visit low ankle carbon fiber foot factory.
Before looking at some of the more popular materials used for foot orthotics, let us first examine some of the design criteria that go into their development. How you design the orthotic will strongly influence the materials chosen. In essence, the design is a series of decisions and selections that leads to the final product.
Usually there are several disparate objectives that need to be met in order to realize the optimum outcome. We demonstrate our value and worth as a profession in skillfully balancing these competing demands. First, we must follow the physician’s prescription, or involve the physician in a conversation to help develop the best device possible. Second, we want to heal the patient. This must be done as we consider the patient’s specific diagnosis and the relevant biomechanics. Thirdly, we should always aim to satisfy the customer while not compromising our standards of care. Unfortunately many of us know the challenges patients can present: hoping for a low-cost and speedy miracle that will not take up any room in the shoe. Other considerations can include longevity of the device and speed of delivery.
Over the years I have developed a flowchart to guide me through the design process (figure 1). Although the first step has little to do with a physician’s prescription or biomechanical exam, the entire success of the orthotic may well depend on the patient’s weight, shoe style, and lifestyle.
The patient’s weight is a critical piece of information that a lab must have if it is to make a correct material choice. For example, 4mm Subortholen® may be rigid for a lightweight 135-pound athlete, but it will be flimsy for a 250-pound construction worker with a size 13 shoe.
The shoe styles that a person wears will influence the type of orthotic that will work best for him or her. If the patient is a professional and experiences general foot pain on a daily basis, then the orthotics will need to fit dressier shoes. Likewise, if the patient is very active and is strictly using them during training, then the orthotic may only need to fit into a sneaker. Well-fitting shoes with a firm counter, a sensible heel height (1/2 inch to 1 inch), and a removable inlay will help to ensure the proper use and success of the orthotics. Frequently it is necessary to educate the patient about the combined benefits of orthotics plus good shoes.
In general, foot orthotics fall into one of two broad categories: functional or accommodative. Functional orthotics seek to control the subtalar joint (STJ) and foot biomechanics, while accommodative orthotics minimize changes to foot function while providing relief and/or protection to specific areas of the foot.
Functional foot orthotics are usually made from thinner, firmer materials. Subortholen, polypropylene, copolymer, and the carbon graphite composites are all good choices for functional devices. Usually they will incorporate a deep heel cup and a good medial longitudinal arch. Among other diagnoses, functional devices are used to treat pronation, plantar fasciitis, and heel spur syndrome.
Accommodative devices tend to be made from less rigid materials such as EVAs, Thermocorks®, Neoprene, Plastazote®, etc. Although a little more bulky, they are usually molded to the entire plantar surface of the foot, providing comfort. In general, accommodative orthotics are a good choice for patients with diabetes, early Charcot joint disease, or any form of neuropathy. In addition, they are often the better choice for patients who present with a rigid foot structure or limited range of motion, e.g. cavus foot type, clubfoot, or post-polio syndrome.
Knowing the type of device required, you can now choose the most suitable base material for its manufacture. The flowchart shows that there is overlap across the categories. The final choice may depend on practitioner preference, material availability, or a patient’s previous experience. There is a brief description of the major material groups used for the orthotic base in table 1.
This is a broad sketch of base material choices for foot orthotic manufacture. There are even more options when it comes to padding, cushions, and top covers-foams, gels, and laminates to name just a few. In addition, recent technology has introduced the metallic elements silver and copper into top cover and sock materials. Knowledge of the full array of material choices allows the practitioner to design and develop the ideal orthotic for each patient’s needs.
Thermoplastics
Materials that soften when heated and harden when cooled. There are several groups of plastics used in the orthotic industry, and they are sold in many different thicknesses, strengths, and colors.
Polypropylene
A plastic with a low specific gravity and high stiffness. This combination of light weight and high strength makes it ideal for manufacturing rigid foot orthotics although any notch or groove on the finished shell can create a stress point that may eventually crack.
Subortholen Family
Officially known as high-molecular-weight, high-density polyethylene (HMW-HDPE), Subortholen is a wax-like, inert, flexible, and tough polymer. These characteristics ensure a high melt strength and deep draw without thinning. It is also easily cold-formed; i.e., hammered, allowing for adjustments after the heating and vacuum process.
Acrylic
Rohadur, Polydur, and Plexidur are some of the more common trade names for this class of material. Made from methyl methacrylate polymers, these were among the first of the man-made (synthetic) materials used for rigid orthotics. They were prone to cracking. The search for alternatives took on urgency when it was discovered that the Rohadur production process was carcinogenic.
Composite Carbon Fibers
Combining acrylic plastic with carbon fibers creates a rigid sheet material. Known by various trade names such as Carboplast, Graphite, and the TL-series, the “carbons” are good for thin, functional orthotics. They are a little more difficult to work with, requiring a higher softening temperature, faster vacuuming, and complete accuracy during the “pull,” as they do not re-work easily.
Cork
This natural material can be combined with rubber binders to create an excellent thermo-formable sheet. Thermocork comes in many weights and thicknesses and vacuums well to provide a firm but forgiving orthotic, which is easily adjusted with a sanding wheel.
Leather
This was the original material used for “arch supports.” Shoemakers took sole leather and wet-molded it to casts. These devices typically had high medial flanges to support the midfoot, and relatively low heel cups. Leather laminates are still used today when patients want good support but cannot tolerate firmer plastics. Their bulk and weight usually necessitates an extra-depth shoe, work boot, or sneaker.
Polyethylene Foams
This is a very broad category of materials that are in widespread use. Cross-linked polyethylenes (CL-PE) include the trade names Plastazote®, Pelite, Aliplast®, Dermaplast®, XPE, and Nickelplast™. These closed-cell foams are ideal for total-contact, pressure-reducing orthotics although some are subject to compression with continued wear.
Ethyl-vinyl cetates (EVAs), crepes/neoprenes, and more recently silicones are other groups of man-made materials that are ideal for making accommodative foot molds.
For a fuller discussion of these materials and their chemistry, please refer to Foot Orthoses and Materials by Bob Schwartz, CPed (Eneslow, New York, New York).
Séamus Kennedy, BEng (Mech), CPed, is president and co-owner of Hersco Ortho Labs, New York. He can be contacted via e-mail at seamus@hersco.com or by visiting www.hersco.com.
A total of 242 papers were found. 75 of these complied with the inclusion criteria and were included in the review. Some of these papers covered more than one macro-topic specified in Material and Methods. The number of papers covering each topic follows:
a)
16 papers addressed and collected patient geometry of the shank and foot;
b)
The company is the world’s best lower limb prosthesis for sale supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
14 papers reported AFO customization criteria other than those based on foot and leg morphology;
c)
19 papers reported the production techniques;
d)
16 papers investigated characterization of mechanical properties;
e)
33 papers reported the functional evaluation of patients/subjects.
A summary of the main results from the literature review on each topic is summed up in the following subsections.
a)
Scanning technologies and geometry acquisition
Custom AFOs are traditionally modelled by hand by the orthotist via thermal molding on models of the patient’s foot and leg. Traditionally, the plaster model is obtained by filling the negative impression of the patient’s cast with liquid plaster. The custom AFO is then manufactured over the positive model. This process, however, is time-consuming and highly operator-dependent. Therefore, in the last 10 years, new technologies to obtain a 3D digital replica of the patient’s geometry have been used to create a solid model of the foot and leg: laser-based scanners [5,6,7,8,9,10] (6 out of 16 studies); structured-light scanners [11,12,13] (3/16); computer tomography [5, 14,15,16] (4/16); 3D coordinate digitizer to acquire landmark positions [17, 18] (2/16), and photogrammetry [19] (1/16). According to recent reviews [20, 21], 3D scanning, computer tomography and optical motion capture systems all represent valid and reliable alternatives to traditional casting methods to obtain a solid model of the patient’s foot and leg geometry.
b)
Customization criteria
According to the present review, PD-AFOs are usually customized on the patient’s lower limb morphology. Few studies used a commercial customizable PD-AFO — the modular Intrepid Dynamic Exoskeletal Orthosis (IDEO) — featuring a posterior strut, the stiffness of which can be customized to the patient’s ankle ROM, the type and level of activities, body mass and load carriage requirements [22,23,24,25]. A similar modular design featuring a variable stiffness rod in relation to the patient’s degree of impairment was proposed [26]. However, no indications are provided on the weight and the direction (towards stiffer or more compliant) of each parameter on the strut rigidity. AFO stiffness optimization based on the minimization of knee angle and energy cost of walking was reported for children with cerebral palsy [27, 28]. A combination of the following parameters has also been used as input data to set the stiffness of the custom AFOs: the patient’s prior experience; visual observations of patient’s gait; body weight; muscle strength; severity of ankle deformity [29,30,31,32,33]. Only one study customized the AFO stiffness according to the natural ankle pseudo-stiffness [34]. The majority of the studies optimized the stiffness of the calf shell. Only one study reported the effect of footplate stiffness on ankle joint power in gait [35].
iii)
Production techniques
Additive manufacturing is becoming widely used in orthopaedics, since it allows to obtain complex shaped devices made with a number of different materials [20]. The present review, in agreement with two recent studies [36, 37], has shown that most 3D-printed PD-AFOs are manufactured via Selective Laser Sintering (SLS) [5, 6, 8, 14, 15, 18, 25, 26, 38,39,40,41,42] and Fused Deposition Modeling (FDM) – also known as Fused Filament Fabrication (FFF) – [10, 15, 17, 43, 44]. SLS works with a high-power laser to sinter polymer powders, while FDM adds melted thermoplastic filaments in consecutive stratified layers to create the object. Stereolithography (SLA) [7, 11] and Multi Jet Fusion (MJF) [11] are less frequently used to produce custom AFOs. In SLA, a UV laser induces polymerization of a photopolymer to obtain the object; in MJF, a fusing agent is deposited on layers of heated powder where the particles are fused together.
iv)
Mechanical testing
This section is reporting only studies related to the experimental analysis of custom-made PD-AFOs. Whenever the AFO type was not clearly defined as “dynamic”, it was decided to include only the manuscripts which reported the force/deformation properties, providing evidence of a dynamic behavior of the orthosis. Three review studies were found which reported stiffness values for a variety of AFOs — custom and off-the-shelf — and the testing methods [3, 45, 46]. Most of these studies investigated the stiffness properties in plantar-dorsiflexion in the range 20 deg plantar- to 30 deg dorsiflexion. Only one study assessed the AFO’s mechanical properties outside the sagittal plane [47].
Most studies assessed the stiffness properties of the strut component, i.e. the long, flexible part of the calf shell [17, 41, 47,48,49,50,51]. Fewer studies investigated the mechanical properties of other components, such as the foot plate [50], or isolated parts of the AFO [52]. Displacements during AFO deflection were assessed in two studies [49, 53], while only one study performed a fatigue test [44]. A few papers [17, 49, 52] reported the mechanical testing of dynamic AFOs which were customized on a healthy subject’s leg or on other geometrical models of the lower limb and not for drop-foot patients were included in this review. In general, the AFO foot plate is fixed, and bending moments/forces or displacements are applied to the calf shell, simulating ankle dorsiflexion. The reported bending stiffness of the strut, in terms of resistance to dorsiflexion moment, ranged between 0.12 and 8.9 N*m/deg across these studies [15, 17, 33, 41, 48,49,50]. The energy absorbed/released by custom AFOs during gait has been seldom addressed in the literature [29, 54].
Custom PD-AFOs have also been tested in-silico via FEA [17, 42, 48, 52,53,54]. Boundary conditions were generally consistent with those used for the experimental mechanical tests, when present. In addition to stiffness [17, 42], FEA allowed to estimate the maximum Von Mises stresses [52, 55] and displacements [53] of the analyzed AFOs. Only one study assessed the maximum Von Mises stress against the material yielding [52], and reported the safety factor of each component in simulated jogging and downhill walking tasks.
e)
Functional evaluation
Table 1 sums up the outcome of the literature review in relation to the functional evaluation of custom dynamic AFOs. Thirty-three papers published from 1999 to 2021 were retrieved and found relevant to the topic. In terms of populations investigated, custom AFOs were used for post-stroke patients (n = 6) [11, 34, 57, 58, 62, 64], for generic drop-foot and muscles weakness (n = 13) [8, 24, 30,31,32,33, 39, 40, 44, 50, 51, 56, 59], for lower limb reconstruction (n = 4) [22, 23, 25, 60], for cerebral palsy (n = 4) [27, 28, 61, 66], for Charcot–Marie–Tooth (n = 1) [29], in children with hemiplegia (n = 2) [63, 65], and in normal/healthy subjects (n = 3) [7, 35, 43]. Posterior Leaf Spring (PLS) are the most common types of AFOs functionally evaluated and were compared to solid and hinged AFOs, and/or to shod/barefoot conditions. Carbon-fiber was found to be the most used material; plastic (nylon and polyamide) and thermoplastic (polypropylene and polyurethane) were also used due to their favorable manufacturing process and compatibility with current 3D printing technology. In terms of functional evaluation, gait analysis during walking at comfortable speed was by far the most common motor task investigated. Three studies reported on stair ascent/descent, and two studies reported on walking over an inclined ramp or treadmill. In one study, the AFOs were evaluated in a static balance test. Spatio-temporal parameters and lower limb joint kinematics and kinetics (mainly in the sagittal plane) were usually recorded and analyzed. Two studies also reported on surface EMG of the main lower limb muscles. Six studies reported on other qualitative scores such as comfort or ease of use (donning and removing). In terms of spatio-temporal parameters, while it is difficult to compare the functional outcome of PD-AFOs customized and produced for different populations with ankle weakness, 8 studies reported improved gait velocity and stride length in custom AFOs with respect to solid AFOs or shod/barefoot conditions. Due to the flexibility of the calf shell, custom PD-AFOs can absorb and release energy during walking. The two studies that assessed this parameter reported a reduction in the energy cost of walking while wearing the optimal stiffness AFOs with respect to other AFOs.
Table 1 Literature review with respect to the papers reporting on the functional evaluation of custom PD-AFOs. For each paper, when present, it is reported the AFO type(s), the customization criteria, the materials, the functional data/parameters, and the main outcome. Comfort assessment or other subjective scores are also reportedFull size table
Are you interested in learning more about Prosthetic Parts Manufacturer? Contact us today to secure an expert consultation!