Insight into Digital Design and Orthopaedic 3D Printing

blank blank Oct 11, 2021

This article describes the status and nature of the additive manufacturing process currently adopted by Korthotics including a description of the digital process workflow. Additionally, special advantages will be described including technological advancements, material-based benefits, and regulatory considerations.

Introduction

3D printing – also called additive manufacturing (AM) – is considered to be the next industrial revolution. The dream of being able to produce high-precision, complex-feature, fully custom-made orthopaedic devices at the touch of a button has become a reality for the few who have dared to adventure into unfamiliar territory. Unfortunately, many orthopaedic technicians using conventional methods fail to realize that they are being left behind by the rapid pace of technological development. However, it should be pointed out that other medical device production developments that were once hailed as revolutionary failed to meet their expectations. For example, industrialized central fabrication services, particularly in the wake of the COVID-19 pandemic. 

The objective of this article is to describe the specific advantages of AM as well as to describe technical, material, and regulatory risks and benefits, thus informing the greater orthopaedic community and promoting open discussion. This paper will also present examples of what a digital supply chain can look like in health settings, particularly within Australia.

Other health professions have already benefited from fully digitized processes. For example, as early as 2009 custom foot orthoses have been produced and shipped to podiatrists all over Australia. This shows that 3D printed custom-made mass-produced medical devices are already feasible and profitable, and are pushing companies using conventional manufacturing methods out of the market.

Additive Manufacturing

“3D printing” is a term used for the fabrication of three-dimensional objects that are generated from a CAD model by adding layers of material. However, various additive methods differ with respect to physical principles, technological challenges, and inherent weaknesses within the finished product. AM characteristics can be classified based on: 

  • Adhesion mechanism (polymerisation, separation, chemical reaction, etc.),
  • Type (metal, polymer) and form (wire, powder, paste, fluid, etc.),
  • Energy use (heat, laser, electron beam, etc.).

Selective laser sintering (SLS) is a thermal method in which targeted polymer powder particles are melted and bonded with a laser beam. The process is repeated layer by layer until the 3D object has been formed. Thermoplastic materials primarily used are nylon (polyamide PA11 and PA12), polystyrene, and polycarbonate. Other methods of additive manufacturing are material extrusion (MEX)(commonly known as fused deposition modelling (FDM)) and multi jet fusion (MJF). 

AM differs greatly from subtractive manufacturing which involves removing material, and formative fabrication, which forms a container from a desired shape. AM does not require product-specific tools and can be fabricated in any spatial orientation. Most advantageous is that the overall cost of the part is not connected to its complexity or degree of customization. The clinician has unprecedented freedom to produce complex shapes integrating high functionality with efficient use of the material in a single manufacturing process. Perceived disadvantages such as limited construction size and relatively lengthy manufacture period per piece are negligible given the limitations of other custom medical technologies. However, limitations exist such as the limited number of additive materials available with the required mechanical properties and the difficulty of ensuring quality control in a heavily regulated medical device market. It is anticipated that AM processes will need to be carefully curated to adapt to regulatory guidelines and safety requirements [1].

Not the Holy Grail

If we look at the products presented at trade shows, congresses, and the internet, two extremes are highly evident. On the one hand, there are attractive devices designed by concept designers, which are often deficient in biomechanical aspects regarding force transmission, congruence, pressure considerations, etc. In most cases, the most attractive design is useless since the required functionality is not met. On the other hand, orthopaedic technical copies of traditional designs intended for conventional fabrication techniques, have functional characteristics worse than the original. This may be because the available additive materials cannot yet compete with the mechanical properties of conventional materials. Also, by copying an old design the opportunity to improve and innovate (such as with numerical optimisation) is lost. In both cases, it shows that there is often little understanding of how AM and functional properties correlate with the biomechanical effect of the orthopaedic device. 

Devices made using AM should be designed with AM characteristics in mind and the possibilities of producing complex structures must be taken into consideration in the design phase. The designer must have a fundamental understanding of the biomechanical mechanisms of action and the functional parameters of the devices. Orthopaedic devices are not necessarily better, less expensive, and produced more quickly simply because they are made using AM. 

Additive manufacturing in orthotics

Despite the barriers, AM has had success in some orthopaedic device settings. Selective design choices can lead to greater acceptance and optimized properties. Adapting the appearance to suit a patient’s tastes has been shown to correlate to the acceptance of the device. Also, there have been initial scientific studies that suggest that involving the patient in the design process can have lasting effects on acceptance [2]. These results can also be transferred to other kinds of orthoses. An example of this is the orthotic cosmetic cover from Korthotics (Fig. 1), for which a wide range of different patterns are offered.

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Figure 1 3D-printed orthotic cosmetic cover.

Areas of use that have seen advantages of AM are:

  • Additive manufacturing of previously custom-made devices (foot orthosis, supramalleolar orthosis, ankle-foot orthosis, knee-ankle-foot orthosis, cranial helmets, etc.),
  • Customized AM as a substitute for standardized fabrication, i.e., devices that are prefabricated according to a size system (prosthetic cosmetic covers, etc.),
  • Customized AM of system components (carbon fibre struts, prosthetic hands, etc.) which previously were industrially produced.

The first group has shown some success from the potential for customization offered by AM despite these devices already being custom-made. For example, in a systematic review by Wojciehowski et al. it was concluded that 3D printed devices are comparable with conventionally produced lower limb devices [3]. These products must, however, still meet strict requirements for fit and functionality. 
A major challenge when using AM in orthotics is that, despite improvements in recent years, optical 3D scanning techniques and CAD modeling software have yet to comprehend the full scope of conventional orthopaedic manufacturing. Typically, during the plaster casting process, the clinician would put targeted pressure on the tissue and thus estimate the tissue properties and compress the tissue if required to achieve the optimal shape for the transfer of force. While 3D scanning and CAD modeling have yet to find a comparable solution to this process, a promising approach is to have the patient semi-weight bearing when scanned. This accommodates for soft-tissue compression (Fig.2). Standard software can easily perform alignment adjustments in any plane the clinician desires but rarely do these programs allow adjustments according to human biomechanics. Unless the designer performing the adjustment is well-versed in human biomechanics it is difficult to be certain that the shape changes are anatomically accurate. One example of a CAD modeling program that is showing promise is MediACE3D developed by RealDimension (Korea) which has built-in features that can analyze and correct anatomical adjustments according to the patient’s clinical needs (Fig. 3). However, more research is needed to verify results despite promising outcomes.

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Figure 2 3D-scanning patient foot.
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Figure 3 Inbuilt anatomical alignment tool

As a replacement for standardized orthopaedic devices, AM devices, with their customisation possibilities, have the advantage of a better fit to the individual patient anatomy and thus the possibility of optimising the transfer of loads, reducing pressure points, and improving wearer comfort. An example of this is the 3D-printed “ROKband” from Headstart Medical (Fig. 4), with which good patient compliance and improved comfort has been reported. The disadvantages are the higher cost associated with expensive manufacturing equipment. Additionally, clinical studies are needed to quantify the extent of the anticipated positive effects. Without a convincing cost-benefit analysis, it will be difficult to obtain reimbursement from insurers. However, it can still be profitable for manufacturers to take this path. A recent randomized controlled study was published that strongly suggests the advantages of a custom-made device for plantar fasciitis compared with prefabricated devices [4].

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Figure 4 3D-printed cranial shaping helmet.

For example, customized AM of industrially produced components such as carbon fiber struts has the advantage of adapting the functional properties to the needs and activities of the patients. This is especially important for pediatric devices. However, again, clinical studies are needed to be able to quantify the extent of the anticipated positive effects (Fig.5). At the same time, on-demand production minimizes waste by-products and the management of unnecessary stock levels.

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Figure 5 3D-printed carbon fibre composite struts.

Overall, AM manufacturing is primarily used when customisation leads to verifiable advantages for the patient or makes it possible to reduce costs and/or improve fabrication processes. 

Regulatory challenges

The major obstacle to the success of AM medical technology is primarily regulatory in nature. Technically speaking, every individual adaptation is a design change, the safety of which must be confirmed with appropriate risk management steps and product testing. There is no doubt that this process may not be feasible due to high costs and time delays. Currently there are no regulations that allow a practical way of ensuring the safety of custom-made 3D-printed medical devices (or conventional devices for that matter).

The concept of custom fabrication plays a key role in current regulatory considerations of 3D-printed medical devices because custom fabrication makes it possible to have a simplified conformity assessment without the involvement of designated site and specified TGA marking. However, it is debatable whether the modified definition of custom fabrication in the TGA even applies to AM medical devices. The TGA assumes industrial production with all the entailing regulatory consequences if more than five custom fabrications of one type (e.g., ankle-foot orthoses) per year are produced. Suitable safety methods for assessing custom 3D printed devices will be needed that take all the innovative aspects of 3D printing – and existing relevant regulatory requirements – into account. They will also need to be feasible for smaller and mid-sized orthopaedic companies to undertake. An example of this has been demonstrated at Liverpool Hospital Brain Injury Unit located in NSW, Australia. Patients requiring post-craniectomy protective helmets require devices that not only conform to the patient head shape but also protect the surgical site. The alternative to conventionally made protective helmets was to 3D print custom-fitting helmets (Fig. 6) that have been designed to reduce force impact for the wearer and minimize weight. When compared to conventional medical-grade protective helmets the 3D printed helmets were able to exceed safety and comfort parameters (Fig. 7).

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Figure 6 3D-printed cranial protective helmet.
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Figure 7 Cranial protective helmet impact testing.

However, a particular challenge is managing the time and cost of regulatory processes when there is a separation between development and production and collaboration is dependent on external service providers. Rarely do providers willingly undertake comprehensive validation processes because of the small quantities produced. 

Digital process workflow

AM is a single component in the complete digital process workflow [Fig. 8]. To make full use of the advantages in AM a process that combines digital shape capture, design, and simulation processes are required. Most companies are already using digital solutions for various process steps, particularly for shape capture, but a complete digitized process is yet to be mainstream. The digital process workflow as shown in figure 8 takes all the aspects of 3D printing and existing relevant regulations such as TGA guidelines [5] into account. While this workflow is currently being implemented in some form by a few invested parties, a fully automatic parametric optimization of the product design, based on the patient data with background simulation and load testing is yet to be realized. Currently, numerical simulation with validated measurement technology is being utilized for compliance with required conformity tests [Fig. 9]. When the platform operator compares the simulation results the admissibility of the modification is assessed to ensure the structural strength of the device. This minimizes the number of time-consuming and expensive physical tests and removes the onus of responsibility for the integrity of the device from the clinician fitting it. The feasibility of this approach has already been demonstrated through small research-orientated organizations. 

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Figure 8. digital process workflow.
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Figure 9 Finite element analysis (FEA) of static ankle-foot orthosis.

However, the major challenge is to unify the specifications of existing standards under a single cohesive platform that will allow the comparison of conventional devices to AM-produced devices. Until a common standard of specification for conventional devices is produced, 3D printed standards can only be assessed and classed on their own.  

Conclusion

In the first issue of Prosthetics and Orthotics International, Dr. Fishman stated that the study of materials sciences and mechanics are necessary for the design of orthopaedic devices, and that a qualified practitioner requires knowledge of biomechanics, anatomy, kinesiology, and pathomechanics when fitting a device to a patient [6]. Safari et al, further suggests that new fields of study such as engineering, computer modeling, AM, and artificial intelligence have begun to enhance our understanding of designing and fabricating orthopaedic devices [7]. There are multiple benchmarks that need to be achieved for AM to reach a critical mass market in technical orthopaedics. These include increasing the number of available materials; clarifying and adapting regulatory requirements; specifying production standards; and establishing manageable digital workflows. However, the considerable advantages of already functioning processes point to the increasing use of AM in orthopaedics today. Naturally, these will evolve as AM technology – and education in its usage – become more freely available.

References:

[1] American Board for Certification in Orthotics, Prosthetics, and Pedorthics (ABC). The Provision of 3D Printed Orthoses and Prostheses Should be administered by a Certified/Licensed Orthotist/Prosthetist. 07.07.2021. https://cdn.ymaws.com/aaop.site-ym.com/resource/resmgr/files/policy_statements/statements/07-2021_PS_3D_Printing.pdf

[2] Papagelopoulos PJ et al. Three-dimensional Technologies in Orthopedics. Orthopedics, 2018, 41 (1): 12-20

[3] Wojciechowski E et al. Feasibility of designing, manufacturing and delivering 3D printed ankle-foot orthoses: a systematic review. Journal of Foot and Ankle Research, 2019, (12): 11. https://doi.org/10.1186/s13047-019-0321-6

[4] Xu, R., Wang, Z., Ma, T., Ren, Z., & Jin, H. (2019). Effect of 3D printing individualized ankle-foot orthosis on plantar biomechanics and pain in patients with plantar fasciitis: A randomized controlled trial. Medical science monitor: international medical journal of experimental and clinical research, 25, 1392.

[5] Therapeutic Goods Administration (TGA). Personalized medical devices (including 3D-printed devices): Regulatory changes for custom-made medical devices. Version 3.0, June 2021. https://www.tga.gov.au/sites/default/files/personalized-medical-devices-including-3d-printed-devices.pdf 

[6] Fishman, S. (1977). Education in prosthetics and orthotics. Prosthetics and orthotics international, 1(1), 52-55.

[7] Safari, R. (2020). Lower limb prosthetic interfaces: Clinical and technological advancement and potential future direction. Prosthetics and Orthotics International, 44(6), 384-401.

About the Author:

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Mr. Victor Phan, Orthotist (MPO, BHs, BIndDes). Graduated from LaTrobe University Bachelor of Health Sciences and Master of Prosthetics and Orthotics. My interests include orthopedics and associated technologies (3D printing/additive manufacturing, simulation, 3D scanning) as well as integration of innovative technologies (design for additive manufacturing (DfAM), computer-aided-design (CAD), computer-aided-manufacturing (CAM)). Currently holds a position of clinical orthotist at Korthotics Pty Ltd, an orthotics manufacturer based in Sydney, Australia. Guest lectures for the Royal Australasion College of Physicians and University of the Sunshine Coast, consulting in clinical orthotics and orthotic related CAD/CAM. 

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