February 2017

3D PRINTING OF FOOT ORTHOSES: Clinical feasibility and cost-benefit analyses

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Several technical issues currently limit the cost effectiveness of 3D-printing custom foot orthoses, but these will likely be addressed in the near future with the emergence of larger and faster 3D printers.

By Steven Hoeffner, PhD; Timothy Pruett; Breanne Przestrzelski, MS; Brian Kaluf, CP; Nikki Hooks, CO; Katelyn Ragland; Shannon Hall; Kyle Walker; Dan Ballard, CPed; and John DesJardins, PhD

During the 2016 Olympic Games in Rio de Janeiro, many of the athletes competed wearing custom 3D-printed athletic shoes, and researchers are developing new applications for 3D printing technology every day. 3D printing technology holds great potential for creating custom devices that interface with the human body, including custom foot orthoses. But the feasibility and cost-benefit ratio associated with investing in these novel technologies and equipment must be better understood for 3D printing technology to bridge the gap between R&D and clinical practice.

The Orthotix research group at Clemson University in South Carolina has focused on the feasibility of 3D printing while innovating new approaches to create custom foot orthoses for persons with diabetes. Students at Clemson have demonstrated the structural viability of the rapid fabrication of custom foot orthoses using 3D-printed ultraviolet (UV)-cured resin materials.1 However, the cost to fabricate orthoses using UV-cured resin printers is significantly more than the cost of traditional orthoses. Orthoses printed with 3D printing technology cost about $300 to $400 per pair compared with $60 to $80 for traditional orthoses.1

3D printing of custom orthoses has advantages over traditional orthotic manufacturing: 3D printing can produce custom shapes and geometries not possible through traditional fabrication techniques, devices can be made more quickly and are easier to modify and reproduce, and a permanent digital record is generated for all orthoses and any changes. The cost, however, will need to be lower for this device fabrication approach to be viable.

3D printing continues to evolve, and it should be possible, now or in the near future, to fabricate custom orthoses using lower-cost 3D printers and print materials. An understanding of the possibilities and limitations of additive manufacturing, including fabrication costs, can help lower extremity practitioners make more informed decisions when contemplating 3D printing of custom prosthetic and orthotic devices.

Identifying 3D printing needs

Two key criteria for 3D printing of custom foot orthoses include build box size (ie, the volume available for printing, measured in length, width, and height, that is the maximum size of an object a 3D printer can produce) and the hardness of the materials used for fabrication. A relatively large build box (12″ long x 4″ wide x 1″ high) is required to print foot orthoses that will fit in shoes up to men’s size 11. This constraint eliminates many commercially available printers and can significantly affect printer cost. In addition, a print-material hardness of Shore A85 or less is needed.2 Softer areas can be produced by printing in a pattern that introduces voids, or areas without print material, into the print product.

Material strength and other properties will also need to be considered, but acceptable tests and values have yet to be defined. Other qualifiers could include flexural modulus, tear resistance, toughness, impact strength, impact resistance, abrasion resistance, and UV resistance.

3D printers are designed to print certain types of materials, but within a given print technology, there can be a range of material properties; for example, fused filament fabrication (FFF) materials can be acrylonitrile butadiene styrene (ABS) or polylactic acid (PLA). Several print technology options are available for 3D printing of custom foot orthoses, and more options may be available soon.

Currently available options

Printing using UV-cured resins. An inkjet print head jets proprietary photopolymer materials in thin layers onto a build tray, layer by layer. Each layer is cured by UV light. Gel-like support material is removed by water jetting. (Because of the nature of certain types of 3D printing and the geometry of the part being printed, structural support of the part may be required during the printing process.) These systems can currently produce a material with a Shore A hardness of 26 and higher.3

Printing using thermoplastic filaments. FFF printers use a rigid thermoplastic filament that is fed through a heated extruder head. Once cooled and set, the materials are typically fairly rigid or hard. The filament must have some rigidity so it does not buckle when being fed into the heated extruder head, and so the feeding rollers or gears can grip the filament. This limits how soft the filament can be, typically a Shore A hardness in the mid 80s, though new materials continue to come out. For example, we recently purchased and successfully printed a material with a reported Shore A hardness of 40.4 Another recent material is part rubber-elastomeric polymer and part polyvinyl alcohol (PVA).5 The PVA dissolves during a one- to four-day water soak, and the remaining material has a Shore A hardness of 40 to 60.

Printing using thermoplastic elastomers. Using selective laser sintering (SLS), plastic beads are fused together by heat from a high-power laser to form a solid 3D part. These systems can currently print materials with a Shore A of 45 to 75.6

Printing using high-performance polyurethane elastomers. Using stereolithography (SLA) printing, a vat of liquid photopolymer resin is cured by UV laser, solidifying the pattern layer by layer to create the solid 3D model. This process is similar to UV-cured resin printing, but a key difference is that laser printing occurs layer by layer, which can be much faster than the line-by-line process of UV-cured printing. The process includes a postprinting curing step that appears to improve the properties of the final product. These systems can currently print materials with a Shore A hardness of 55 to 60.7

Options that may soon be available

Printing using low Shore A thermoplastic elastomer (TPE) materials. Modifying the print head from a filament line feed to a miniscrew extruder (FFF-SE) allows a 3D printer to use pellets of very soft thermo­plastic elastomer materials.8 An advantage of this approach is that potentially any TPE material could be used (eliminating the need to use proprietary materials), and these printed materials have superior mechanical properties, such as tensile strength and elongation to break, compared with UV-light cured photopolymers. This type of printer is not yet commercially available, but it could be within the next three years. In early trials,7 materials with a Shore A hardness as low as 5 were printed.

Printing with silicone rubber. Using subsurface catalyzation (SSC) printing,9 a catalyst is injected into a bed of silicone oil, cross linker (which controls the product’s softness), and thickener. The catalyst causes localized curing, forming the room temperature vulcanization (RTV) silicone product. The resolution is low, but may be adequate for orthoses. The raw materials are inexpensive. The printer is not commercially available, but could be within three to five years. This technology can produce very soft materials (Shore A hardness of 10 and less).

Costs of printing foot orthoses

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Printers using one of the technologies/print materials identified above can likely print foot orthoses of acceptable build box size and material hardness (other criteria such as durability are currently being evaluated and will be added as this application area matures). Costs associated with the following 3D printing options were analyzed:

  • UV-cured printer10 producing photopolymer material11
  • FFF printers12, 13 producing TPE14 and PVA5
  • SLA printer15 producing polyurethane elastomer7
  • SLS printer16 producing thermoplastic elastomers5
  • SSC printer9 producing RTV silicone
  • Custom FFF-SE8 producing TPEs

Table 1 provides a summary of representative printers and print materials from each of these technologies and includes printer cost, printer build box size, and the cost of print materials and some material properties. Printer costs were obtained from a 2015 Wohlers Associates report on 3D printing.17 For the printers not yet commercially available, $125,000 was assumed for the FFF-SE printer and $175,000 for the SSC printer. Build box size determined the number of orthoses that could be printed per run (typically three, and sometimes two single orthoses).

The information used to calculate the cost to print foot orthoses includes the cost of the printer, annual maintenance contract, consumables (print material and, if needed, support material), electricity, and labor. A pair of orthoses require 600 g of UV-cured print material and 600 g of UV-cured support material. Densities were used to estimate the amount of print material required when using other print technologies. Electrical consumption was obtained from printer specifications. Pre- and postprinting labor was estimated to be .2 hours at $55 an hour for all printers. Excluded were lower-cost items that had minimal impact on overall cost, such as solutions that dissolve support material, FFF build sheets, injector tips or print heads, and other replacement parts).

A seven-year life span was assumed for all printers. Washer and oven cost (for postprocessing on some print technologies) was set at $3000. These costs, along with the annual maintenance contract (6% of printer purchase cost), were amortized over the seven-year printer life. A 10% contingency was added to the final total cost.

Cost for a pair of orthoses was calculated as:

Printer cost + (printer cost x .06 x 7)+ (washer or oven cost)
_________________________________________
(Total # pairs of orthoses printed over 7 years)

+ (labor+ materials+ electrical) per pair

The results, shown in Figure 1, provide a convenient way to compare the cost of printing materials using different printers as well as the costs of in-house printing, third-party printing, and traditional foot orthoses. The denominator (total pairs of orthoses printed over seven years) varied; all other values were fixed.

Included in Figure 1 are:

  • The cost curves for the six different print technologies/materials (two different printers and materials are shown for the FFF technology) as a function of the total number of pairs of orthoses printed over the seven-year printer life;
  • The equivalent number of pairs of orthoses printed per week (blue text and vertical dashed lines);
  • The number of traditional pairs of orthoses produced by Upstate Pedorthic Services, a regional pedorthic practice (blue text box);
  • The cost a clinician pays for a traditional pair of orthoses ($60-$80, gray horizontal shaded band);
  • The cost a clinician would pay to for a pair of 3D custom orthoses made by a company that provides 3D printing services (green horizontal dotted lines, two different printer technologies/ print
    materials); and
  • Maximum printer capacities (the point at which the curves end)

Currently, only the FFF technologies are price-competitive with traditional foot orthosis manufacturing (horizontal gray shaded band). In addition, FFF printers are low cost, materials are moderate to low cost, many of the print materials appear to be durable, printers and materials are widely available, and percent infill can be varied to change the softness of the printed material. In the longer term, FFF, FFF-SE, and SSC all appear promising from a cost perspective.

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For any volume greater than a couple of pairs of orthoses per week, it is more cost effective to 3D print the orthoses in house than to have them made by a 3D print service (horizontal green dotted lines). This assumes the 3D printer employee can continue to bill for other services with his remaining time.

Print speed is not a significant cost discriminator. Print speed is determined by multiple factors: print technology, bed size, support material needs, and in some cases, even software.16 Printer speed for similar technologies (eg, those that print line by line) is mainly determined by bed size. And which print technology is fastest can change as conditions change.19 In general, print speeds for most technologies (except for the SLA printer, which can be up to 25 times faster) are all relatively comparable. For example, it takes eight hours to print three orthoses using a UV-cured resin printer and 12 hours to print the same orthoses using an FFF printer. Although print speed determines the ultimate limit on the number of orthoses that can be printed, Figure 1 indicates unit cost is determined more by the amortized cost of the printer and the cost of print materials than by print speed (indicated by the total number of pairs of orthoses printed).

A single printer may not be adequate to meet demand. One printer (with the possible exception of the SLA printer) could not print all of the 70 pairs of orthoses per week prepared by Upstate Pedorthic Services. The UV-cured resin printer can generate about 45% of this amount, the FFF printers, 30%. Having two or more printers may be an option, especially with the low-cost FFF printers.

Table 2 summarizes the amortized and fixed costs of 3D printing foot orthoses. The table indicates that, based on the cost of materials alone, the UV-cured, SLA, and SLS technologies are currently all more expensive than traditional orthotic manufacturing. Proprietary technologies and print materials always command a premium; as a result, at this time, custom foot orthoses made using these technologies are $300 to $400 a pair when the costs of labor, supplies, and consumables are included. In contrast, the technologies that use nonproprietary print materials show significant promise from a cost perspective, especially FFF.

Summary

There are several materials (and different associated 3D printer technologies) that could be used to 3D print custom foot orthoses. This list will continue to grow as the field develops and matures. Some of the technologies presented here are not yet commercially available, but could be in the next few years. At this time, only FFF technologies have the potential to be price-competitive with the cost of traditionally manufactured custom foot orthoses.

Build box size is a significant factor in determining which printers are acceptable. For many of the available printers, the build box is too small for printing orthoses.

The ability to use nonproprietary materials helps significantly decrease the cost of 3D printing foot orthoses. The FFF technology, printers, and print materials are especially attractive.

The cost of 3D-printed orthoses is determined mainly by amortized cost of printer and the cost of print materials, and less by printer speed. For anything more than a couple pairs of orthoses per week it should be more cost effective to print them in house than to have them made by a 3D print service.

Conclusion

After examining factors that influence the cost-benefit ratio and feasibility associated with new 3D printing equipment, it is apparent there are still several technical limitations to printing custom foot orthoses. These limitations may be overcome, however, and the associated costs reduced, as the technology becomes more common and as future innovations produce larger and faster 3D printers.

Steven Hoeffner, PhD, is the owner of Hoeffner Consulting in Easley, SC, and has experience applying different 3D printing technologies. Timothy Pruett is a member of the Machining and Technical Services staff, and Breanne Przestrzelski, MS, is a PhD student in the bioengineering program at Clemson University in South Carolina. Brian Kalu, CP, is the clinical outcomes and research director at Ability Prosthetics and Orthotics in Exton, PA. Nikki Hooks, CO, is an orthotist and board-eligible prosthetist at Ability Prosthetics and Orthotics, and is on the board of directors for the Orthotic and Prosthetic Activities Foundation in Charlotte, NC. Katelyn Ragland, Shannon Hall, and Kyle Walker are undergraduate bioengineering students at Clemson University. Dan Ballard, CPed, is a pedorthist at Upstate Pedorthic Services in Greer, SC. John DesJardins, PhD, is the Robert B. and Susan B. Hambright Leadership Associate Professor of Bioengineering at Clemson University.

REFERENCES
  1. Timothy Pruett and Nikki Hooks,CO, in an email communication, August 19, 2016.
  2. Przestrzelski BT, Stanley S, Walker K, et al. Novel additive-manufactured foot orthotic achieves comfort equivalent to the traditional standard. Presented at the Orthopaedic Research Society Annual Conference, Orlando, FL, March 2016.
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