Manufacturing for Orthotic and Prosthetic Devices: History
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Contributor:
Chowdhury Sakib-Uz-Zaman
,
Mohammad Abu Hasan Khondoker

The conventional manufacturing methods for fabricating orthotic and prosthetic (O&P) devices have been in practice for a very long time. O&P service providers have started exploring different advanced manufacturing techniques. 

  • prosthetics
  • orthotics
  • prosthesis
  • orthosis

1. Introduction

In 2017, it was reported that from 2006 to 2012, there was a total of 44,430 lower limb amputations in Canada (~7400 per year), and the average age-adjusted rate of lower limb amputation was 22.9 per 100,000 individuals [1]. Although the need is progressively increasing, the O&P industry is not still fully utilizing the emerging new technologies which have great potential to improve the quality of life of those needing orthotic and prosthetic (O&P) devices [2].
Prostheses are devices that restore the functionality of a missing body part, whereas orthoses are assistive devices that restore the stability and mobility of a body part of those with neuromuscular and/or musculoskeletal impairments [3]. Personalized O&P devices with better fit and proper alignment are found to be crucial for users’ satisfaction when treating patients [4]. Also, providing the patient with a comfortable O&P device within 4 weeks after amputation or impairments significantly decreases the user abandonment of the devices [5]. Therefore, a rapid fabrication technique and comfortable fit of the O&P devices are two key factors that can significantly improve patients’ life. The currently practised conventional manufacturing process of O&P devices requires a significant amount of time to ensure a custom fit and involves a considerable number of manual operations and design iterations producing large amounts of waste [6][7]. However, they have been empirically tested and proven to provide patients with safe, reliable, and lasting O&P devices. Recently, many researchers as well as O&P practitioners have started evaluating advanced manufacturing technologies for producing high-performance O&P devices., Several different types of personalized foot orthoses have been created using AM technologies. These orthoses have been compared to traditionally fabricated items utilizing gait analysis and subjective assessments of fit and comfort [8][9][10][11]. These findings not only demonstrate that the AM approach to customizing foot and ankle-foot orthoses is feasible, but they also point to the significant clinical potential for this method [7]. Similarly, prosthetic sockets produced using AM techniques demonstrated enhanced comfort, increased step symmetry, and comparable lower extremity joint performance when compared to the definitive prosthetic devices [7][12][13]. However, there are still clinical, economical, and technological hurdles to overcome before AM can be applied on a large scale in a service system for the production of customized orthoses and prostheses [14][15].

2. Conventional Manufacturing Processes of O&P Devices

Laura Trevelin, (RTP)c (June 2022) provided a vivid description of the conventional manufacturing of prostheses which starts with wrapping the residual limb with plaster bandages and making a cast. After the cast dries, it is taken off, which serves as the negative mold, which is then filled with plaster to get the positive mold. Then, the positive mold is modified manually to adjust the amount and location of pressures on the limb. The appropriate suspension method and componentry are decided based on the patient’s need. Then the diagnostic ‘check’ socket (to check the fit) is fabricated using clear and heat moldable thermoplastics. Once it is assembled with the componentry chosen in the correct standard neutral alignment, the “check” socket is tested with a patient to evaluate its alignment and fit.
Alternatively, instead of taking the plaster cast of the residual limbs, the shape of the limb can also be captured by using a 3D scanner. Then, computer-aided design (CAD) software is used to relieve the pressure-sensitive areas and reinforce the areas where needed. Once the design is finalized, a positive model is carved out of foam e.g., expanded polystyrene (EPS), and then the check socket is made from that carved model using the steps described above.
The patient wears the check device for 4 to 6 weeks to check the fit and alignment and assess comfortability. If the fit and alignment are appropriate, a copy of the socket with an impression material (e.g., Jeltrate alginate) is taken and refilled with plaster to have a mold for the final socket. Alternatively, the check socket can be filled with plaster and cut off. If the fit is not appropriate, the check socket would be refilled with plaster, and further and more drastic changes to the shape would be made. In some cases, the suspension method could be changed altogether. Then another check socket would be created by the technician and more fittings would be required. The back and forth of the initial fitting is what takes the longest.
Once the final socket shape is tested, approved, and copied; and the final plaster mold is completely dry, it would be laminated by the technician. Using stretchable polyvinyl alcohol (PVA), bag the plaster cast is covered so that it acts as a separator. Then layers of ‘nyglass’ (a blend of nylon and fiberglass), fiberglass, and carbon fiber are added based on the empirical experience of the prosthetist that is appropriate for the patients, taking into consideration their weight, activity level, and socket suspension type. After the layers are applied, a final PVA bag funnel is put over it and the bags are sealed together with tape hooked up to a vacuum hose to draw all the air out between the layers. Acrylic resin is added and carefully coated all over the socket, taking care not to cause any wrinkles to the materials or create air pockets. The funnel is sealed off and left to be cured for 1 h. Then a cast saw is used to cut the trim lines off, break the plaster out, trim and grind the edges. The device is then reassembled onto the original setup, taking care to preserve the alignment. The patient still may need small adjustments in the alignment and fit every once in a while, especially for new amputees whose shape changes quite a bit in the first 2 years, and sometimes they need to make a whole new socket within 6 months to a year post initial fit.
The conventional manufacturing process of orthoses is somewhat similar to that of prostheses, with the exception that, unlike prostheses, no “check” device is made in orthoses. The procedure begins with taking measurements of the body component. Next is the molding process, in which the impression of the orthosis is taken using a casting method. After that, liquid plaster is poured into the negative mold to create a positive mold. Drying the mold might take anywhere from two to forty-eight hours. The positive mold must then be corrected and polished. Once the positive form is dried and smooth, the negative mold is removed. The model is then stapled to the Plastazote or plastaform foam, and a thermoplastic (typically, polypropylene (PP), polyethylene (PE) or other co-polymers) is pulled over it [16].
After that, the thermoplastic is heated up in the oven. The elevated temperature softens the thermoplastic material so that it may be molded over the model by the technician. The mold is detached from the model and uneven edges are smoothed off. This is the point at which the orthotic device is ready for the first fitting. Adjustments are usually required, which may be time-consuming. If significant levels of adjustments are anticipated, a completely new orthosis may be required. In such circumstances, the procedure might have to be restarted from the beginning [16].

3. Additive Manufacturing for O&P Devices

Additive Manufacturing (AM), also known as 3D printing has emerged as one of the core components of the fourth industrial revolution, Industry 4.0 [17]. In AM technology, a part is manufactured by selectively adding material in 3D volume resulting in minimal waste [18][19]. This digital but toolless manufacturing technique also allows rapid design iteration, low-cost, lightweight parts and extensive personalization which are particularly crucial for O&P devices [2]. Although clinicians’ knowledge about patient care is invaluable, to remain competitive they need to adopt advanced manufacturing [20]. Among other AM processes, Fused Deposition Modeling (FDM), Selective Laser Sintering (SLS), Material Jetting (MJ) and Vat Photopolymerization (VPP) are the four categories of AM techniques that can be used to 3D print polymer materials suitable for O&P devices [21][22]. While the direct write nature of FDM technology allows printing O&P devices without any size restrictions, the inherent anisotropy and poor inter-layer adhesion resulting from this technology often limit its suitability for safe O&P devices [23][24]. On the contrary, SLS results in isotropic material properties, however, its limited material choice often restricts the use of this AM technology in the field [25]. Many researchers and O&P practitioners have utilized AM technologies to print O&P devices and evaluated the performance of AM-processed O&P devices against the performance of conventionally manufactured O&P devices. For instance, Pousett et al. investigated the structural strength of transtibial sockets fabricated using conventional manufacturing and FDM [26].
Design for additive manufacturing (DfAM) can play a significant role in improving performance and solving problems for O&P devices. It encourages clinicians to explore new design solutions to take advantage of the unique capabilities of AM. Three advantages of using DfAM have been reported: customization, cost-effectiveness, and repeatability [27]. It has been found that DfAM improves cost-effectiveness, shortens production time, lowers wastage, and reduces human involvement when fabricating highly customized devices, compared to conventional manufacturing [28]. Interestingly, intricate designs do not affect the cost of production as the cost in AM is mainly determined by the volume of the part rather than its complexity [29]. Topologically optimized O&P devices, enabled by DfAM, were demonstrated to reduce material volume while increasing mechanical robustness [29]. The true design freedom of infill or lattice parameters, offered by DfAM, allows heterogeneous builds of O&P devices realizing a range of spatial Shore hardness is unique [30]. Finally, through DfAM the design data is captured and stored in a digital environment which permits scalable repeatability with minimal effort [27].

This entry is adapted from the peer-reviewed paper 10.3390/polym15061506

References

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