3D Orthotic Printing: Fad Or Game Changer?

Could 3D printing reduce costs, increase orthotic efficacy and reinvent the industry with innovative designs? This author combines a closer look at the research with insights from his practical experience to compare orthoses derived from 3D printing with traditionally manufactured devices.

Podiatric physicians have employed foot orthoses for over 100 years in the treatment of pathologies of the foot and lower extremity. During this time, the methods of construction for orthoses and the materials from which they are manufactured have evolved, reflecting the available technologies and advances in materials science of the day.

Early pioneers in this field used steel and leather to construct their foot or-thoses. In 1896, Whitman advocated taking a plaster impression of the patient’s foot and then casting a positive model from iron to form an “anvil” over which one could beat and temper sheet steel to construct the orthosis.¹ Ogden received a patent in 1949 that detailed a method of directly molding “doped” cellulose acetate to the foot while a mechanical alignment device held the foot in a “corrected position.”² In 1950, Levy described the use of a “rubber butter” compound made from ground cork, leather dust and latex, which he spread directly onto a cast of the foot in order to manufacture insoles.³

It was Root who began to experiment with the use of vacuum-molded thermo-plastics to manufacture foot orthoses in the late 1950s and this led to the global foot orthotic industry we know today.⁴ While researchers described the “layering-up” of composite materials for foot orthoses in 1969, the manufacturing techniques pioneered by Root over half a century ago are probably still the most prevalent methods of manufacture within the foot orthotic industry today.⁵ In this industry, globally registered revenues were around $2.5 billion in 2014, a number estimated to grow to $3.5 billion by 2020.⁶

Computer-aided design and manufacturing (CAD-CAM) of foot orthoses first emerged in 1989.⁷ This technology allowed one to digitize a model of the foot and enabled the design of the foot orthoses to a point where the lab could then directly mill the orthoses from a block of plastic through the use of a computerized, numerically controlled milling machine. The exploitation of CAD-CAM in foot orthoses manufacturing has been something of a “slow burner” with the widespread use of this technology only recently gaining in popularity as equipment costs have reduced.

Direct milling, along with vacuum forming of foot orthoses, involves the removal of material to form the finished product. Accordingly, these methods of construction are called “subtractive manufacturing.” One of the major criticisms of subtractive manufacturing is the amount of waste product it produces. While the waste product can be recycled, it is a relatively resource heavy method of manufacturing in terms of volume of raw material to finished product. Also, the machinery is relatively noisy during operation and does not necessarily lend itself well to in-office production of a wide variety of foot orthoses.

However, CAD-CAM does offer a number of advantages over the traditional plaster model and vacuum forming. For example, it offers the potential for reduced staffing levels and the reduction in space required at the manufacturing facility, including the space to store all of the plaster casts.

Three-dimensional (3D) printing or additive manufacturing (I will use the terms synonymously here) emerged in the early 1980s. Additive manufacturing refers to the process of the creation of 3D objects by the laying down and building up of successive layers of a material to create the object. Different methods of additive manufacture exist. The most common technologies employed at present for the production of foot orthoses are selective laser sintering and fused deposition modeling.

Over the last few years, 3D printing has been promoted as an alternative to the subtractive manufacturing methods for foot orthoses with a number of claims regarding the advantages of this manufacturing process. Is this method of manufacture of foot orthoses destined to become pervasive? In order to become pervasive, additive manufacturing needs to offer clear advantages over more established methods of manufacturing. Do the claims of those touting this 3D technology in the hope of securing their share of the global foot orthotic market really stand up to scrutiny?

Do 3D Printed Foot Orthoses Cost Less?

While certain companies make claims on the Internet of 3D-printed insoles available for the price of just a few dollars, published academic studies reveal a different picture.

Pillari and colleagues reported that the patients in their trial self-reported paying an average price of $106 for their existing orthoses, excluding clinical costs.⁸ The authors estimated the design and manufacturing costs for producing a pair of additive manufacturing orthoses using selective laser sintering to be in the order of $65. This price for the additive manufacturing orthoses did not include the profit that would need to be added by the manufacturing laboratory.

Telfer and coworkers estimated the cost of the 3D-printed orthoses employed in their study to be approximately $63 per pair.⁹ The authors concluded that: “the cost estimates for the (selective laser sintering) devices manufactured for this study are still above those normally quoted for traditionally manufactured devices.”

As the saying goes, time is money. Cotoros and colleagues reported a production time of six hours per insole while Telfer and coworkers reported a manufacturing time of around 5.5 hours per pair of foot orthoses.^9,10 Clearly, this is much slower than the established subtractive manufacturing techniques for foot orthoses. However, these two studies were published in 2011 and 2012 respectively. While these studies are relatively recent in academic terms, 3D printing technology has moved on significantly since this time. Accordingly, the aforementioned production times noted in these studies probably do not accurately reflect those achievable with more modern equipment. Marketing materials are now promoting declarations of a production time of one hour per pair.¹¹

Perhaps we can glean a more accurate analysis of costs for commercial scale production of foot orthoses using additive manufacturing from a series of published works, all of which come from the same research team.^12-15 Published between 2013 and 2016, these reports offer far more detailed analyses that include factors such as the purchase cost of equipment and its depreciation, etc.

The first two papers provided cost analyses using selective laser sintering and fused deposition modeling techniques respectively.^12,13 From their analysis of selective laser sintering, Jumani and colleagues concluded that “material and machine cost of additive manufacturing techniques in general and selective laser sintering (e)specially (are) still higher in comparison to traditional orthoses fabrication techniques.” With regard to fused deposition modeling, they concluded: “currently the cost of using fused deposition modeling technique for fabrication of custom-foot orthoses is exceeding the fabrication cost through traditional fabrication techniques.”

The third paper, consisting of a cost and lead-time analysis, compared additive manufacturing for foot orthoses with traditional methods of manufacturing. In this study, Jumani and coworkers reported the cost of a pair of 3D-printed orthoses to be approximately $243 in comparison to what the authors described as “the present market value” of traditionally produced foot orthoses of between $195 to $259 per pair.¹⁴ It is unclear if the authors consider “production cost” and “present market value” to be one and the same thing, but even if they are, the use of 3D printing did not appear to offer a cost reduction in this study.

More recently, Jumani and colleagues published another cost-modeling study that reported an increased fabrication cost of $374.41 per pair of foot orthoses when employing 3D printing technology.¹⁵ While the authors reported an advantage of reduced product lead time with 3D printing, the fabrication cost here appears somewhat excessive.

The weight of evidence provided by the published literature appears to suggest that additive manufacturing of foot orthoses currently offers no clear advantages in terms of production cost or production time in comparison to traditional techniques. However, this seems to be at odds with the current marketing claims for 3D orthoses. It is possible that the published scientific literature does not reflect the current state of the art due to the time lag in the publication processes and a high volume production model being assumed in such studies.

Does 3D Printing Produce More Efficacious Orthoses Than Traditional Manufacturing Techniques?

Pallari and colleagues performed a small trial comparing selective laser sintering foot orthoses to the preexisting, traditionally manufactured devices for seven individuals with rheumatoid arthritis.⁸ One of the interesting findings from the trial was that both patients and dispensing clinicians often perceived the selective laser sintering devices as being “too hard” although one should note that researchers applied no cushioning topcovers to the selective laser sintering devices within the trial.

In a single-blind, randomized study of 13 pairs of matched recreational runners, the study participants either received a “personalized” insole or a control insole with both types of insoles being produced with additive manufacture.¹⁶ The personalized insoles were designed and manufactured from the scans of the feet to mirror the exact plantar geometry of their feet from the heel to the base of the metatarsal heads without any other corrections. The control insoles were manufactured from scans of the shoes’ original insoles. Both types of insoles were manufactured with selective laser sintering from the same thickness of nylon material.

Those who had the personalized insoles had fewer reported discomfort issues in comparison to the control patients and the patients with the personalized insoles subjectively reported a better fit.¹⁶ However, some patients reported the material used to print both types of insoles was “too rigid” and created discomfort in the medial longitudinal arch area of the foot. The study authors also noted small kinematic differences with a 1 to 2 degree reduction in maximum foot eversion with the personalized devices in comparison to the controls.

While this study demonstrates that customized foot orthoses may have benefits over a control insole, it does not show any advantage of additive manufacturing over traditional methods of manufacturing of custom foot orthoses.¹⁶

A 2014 study compared three types of foot orthoses produced from the same plaster cast model of each patient’s feet in a cohort of patients with rheumatoid arthritis.¹⁷ The study focused on “standard foot orthoses” manufactured from polypropylene and based on a subtalar joint neutral model. It is unclear from the paper whether this was via vacuum forming or direct milling. The authors suggest that the design features for the orthoses were “prescribed according to standard clinical protocols,” although it is unclear exactly what these protocols may have been.

Employing an algorithm based on the results of gait analyses with a high degree of instrumentation, the authors constructed two further pairs of orthoses using additive manufacturing.¹⁷ The manufacturing of one set of orthoses used a high-end selective laser sintering machine and a nylon-based material while the second set of orthoses used fused deposition modeling on a low-cost machine and polylactide material.

Patients reported that both selective laser sintering and fused deposition mod-eling devices provide “better” control of rearfoot motion and forefoot pressure redistribution in comparison to the standard devices. They also noted the orthoses produced with additive manufacturing provided “equivalent or better” patient-reported outcomes when tested against the standard orthoses.

The key in this study is that researchers applied different design rules, features and materials in the construction of the standard devices versus the orthoses that were created with additive manufacturing. The authors claim that the stiffness of the standard orthoses and the selective laser sintering devices were similar, but the fused deposition modeling devices were approximately half as stiff based on the mechanical properties of the construction materials. However, since the design protocol was different for the standard devices, it seems reasonable to assume that the geometry of the standard devices was different from the orthoses created with additive manufacturing. Clearly then, the point-to-point load/deformation characteristics could have differed between the devices due to differences in material and the finished geometry of the devices. This would have resulted in differences in the point-to-point reaction forces acting on the plantar foot with the different orthoses in-situ.^18,19

Based on the results of this study, we cannot conclude that there is any superiority when it comes to additive manufacturing versus subtractive manufacturing. Rather, any benefit reported seems likely to be based in the design process itself, which, for the additive manufacturing devices employed within this study, involved the use of technologies not currently financially viable to the vast majority of clinicians.

While the aforementioned studies have demonstrated the feasibility of additive manufac-turing application for foot orthoses, due to the methodologies researchers em-ployed within these studies, we cannot apply them to demonstrate any advantage in clinical efficacy of additive manufacturing over subtractive manufacturing per se. At present, we simply do not have any adequately designed studies to inform the knowledge base in this regard.

We need studies of identical foot orthoses that differ only in terms of manufac-turing (subtractive manufacturing versus additive manufacturing) in order to make a judgment on whether one manufacturing technique is superior to another in terms of clinical efficacy. To date, no such studies exist.

Does 3D Printing Offer Potential Design Innovations For Foot Orthoses?

Ignoring the changes in the materials that the foot orthoses are constructed from, their physical form has changed little since the days of Root or even Whitman.^1,4 One may argue that a foot is still a foot and this has not changed recently either.

However, the design of foot orthoses has shown little evolution other than sub-tle “tweaks” (inverted pours, Kirby skives, forefoot extensions, etc.), despite the advances in manufacturing techniques provided by CAD-CAM and additive man-ufacturing. Foot orthoses still typically consist of a curved shell, roughly match-ing the contours of the plantar foot or attempting to match that of some idealized “normal foot” (whatever that may be) with or without a block(s) of material beneath the heel and/or forefoot. One could argue that this design of foot orthoses has a proven record of efficacy so why should it even need to change?²⁰

The reality is that foot orthoses design perhaps doesn’t need to be changed and the design may have already reached a pinnacle of evolution. However, additive manufacturing has the potential to allow the inclusion of design features (such as voids within the structure of the orthoses) that other methods of manufacturing are incapable of achieving.^9,20 To date, this does not appear to have been widely exploited. Indeed, while Telfer and colleagues presented a 3D printed device with some innovative features, there is no obvious reason why the design of this particular orthosis could not have occurred with subtractive manufacturing.⁹

At present, most commercially available 3D printed orthoses either resemble traditionally manufactured orthoses or have a full-thickness lattice structure within the shell. The rationale for the use of lattice structures is that they allow for a decrease in weight, a reduction in materials and variation in the load/deformation characteristics of the devices to be engineered into the devices in order to manipulate the magnitude and location of reaction force at the foot–orthosis interface. Lattice structures may do these things well but one could argue that the same influences on reaction forces could also occur with subtractive manufacturing, either to form a similar lattice design or by manipulation of the local shell thicknesses.

Moreover, since most orthotic laboratories do not have access to kinetic data for each patient, the lattice design is simply guesswork by the additive manufacturing orthotic companies and not tailored to the individual patient. Accordingly, the lattice structures are analogous to a prefabricated device in this regard.

While additive manufacturing could undoubtedly produce innovative foot orthoses design features that subtractive manufacturing could not reproduce, there is little evidence of this to date. Furthermore, there is currently no evidence that any new innovation in design features that only additive manufacturing could achieve should result in better clinical outcomes.

Can 3D Printing Really Disrupt The Status Quo And Change The Industry?

There is no doubt that the cost of 3D printing technology has rapidly decreased in recent times. Small fused deposition modeling printers designed for home use are now appearing on the market at just a few hundred dollars. Provided that such printers have a sufficient resolution, build area and can accommodate suitable materials, there is no reason why such printers could not produce foot orthoses. Indeed, researchers have already demonstrated that low-cost fused deposition modeling can manufacture clinically efficacious devices.¹⁷

One of the most interesting aspects of additive manufacturing is its potential to impact on the existing production process and business model with orthoses. The traditional process has the clinician obtaining a model of the patient’s foot and sending it to the laboratory either via the postal service or as a digital file via the Internet. The laboratory then designs the foot orthoses, manufactures the foot orthoses and physically ships the product back to the dispensing clinician.

As each laboratory is usually producing foot orthoses for many clinicians, labs must have the capacity to manufacture large volumes of their products in as short a time period as possible to make this model cost-effective. This requires large-scale, relatively expensive equipment and/or relatively high staffing levels. The laboratories must hold large quantities of stock and rent/purchase premises of adequate size to hold this inventory.
Additive manufacturing appears to offer an alternative to the traditional production process/business model because it lends itself so well to in-office production of foot orthoses. Individually, the vast majority of clinicians do not dispense high volumes of foot orthoses. Accordingly, the large scale, superfast production currently required by the laboratories is unnecessary to most clinicians. Modern 3D printers operate quietly, produce little waste and can run with minimal human intervention.

Thus, an alternative production process is possible. The clinician obtains a model of the patient’s foot and sends this to the lab. The lab designs the foot orthoses and sends the manufacturing data file to the clinician via the Internet. The clinician subsequently uses 3D printing to create the devices in his or her own office. Provided that one could send the model of the patient’s foot as a digital file, this alternative process would allow true globalization of the industry with a clinician in Europe easily being able to exploit the expertise of a laboratory in North America (or anywhere in the world) to design the orthoses or vice versa.

The role of the orthotic laboratory could subsequently change from one of a design and construction facility to focus purely as a design agency. This should result in large-scale cost savings to the orthotic laboratories as they will simply require a suite of computers with operators able to use the software to design the foot orthoses and prepare the manufacturing files, along with administrative staffing. The laboratories will not need to purchase manufacturing equipment or materials. Adopting this model would mean a potential reduction in inventory. With all of these savings and the clinician taking over the responsibility for the manufacturing of the foot orthoses, the cost per pair of foot orthoses could be markedly reduced.

Of course, employing CAD-CAM with additive manufacturing for both the design and manufacturing of foot orthoses could easily move into the clinician’s office, negating the need for the laboratories. This is an option currently being marketed but requires clinicians or someone within their staff having the skills and time required to use the design software. Note that office-based CAD-CAM with subtractive manufacturing has been available for a number of years but has failed to become pervasive.
 
In Summary

At present, there are probably few if any advantages of additive manufacturing over subtractive manufacturing when manufacturing foot orthoses on an industrial scale. However, it is clear that the capabilities of relatively low-cost equipment are improving at an extraordinary rate and that small desktop-based printers designed for home use are already capable of producing efficacious foot orthoses with or without innovation in the orthoses designs.

To exploit 3D printing technology to its full potential, a change in the existing business model and production processes of the foot orthoses industry will probably be required. Such change could provide cost savings and the true globalization of the industry. To a large extent, the future of additive manufacturing for foot orthoses is dependent upon the willingness to embrace change on behalf of both the clinicians and the foot orthotic laboratories.

Dr. Spooner is in private practice at Peninsula Podiatry in Plymouth, United Kingdom.

References

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