Rapid Prototyping for Prosthetics

Did you know that according to a study by the World Health Organization from 2018, 30 million people are in dire need of prosthetics, but 75 percent of developing nations cannot cater to produce prosthetics?

Image by splotramienny from Pixabay

A prosthesis is a man-made limb for those individuals who have either faced an amputation due to an accident or were born with a natural deficiency. Amputations are also caused by several traumatic and medical conditions like diabetes, tumors or blood disease.

According to WHO, about 0.5 percent of the global population requires prosthetics whereas only 1 in 10 people can afford them. The reason for the lack of reach of these devices is the cost which can range from a few hundred dollars to over a hundred thousand dollars. Not only is this massive capital cost involved but also there is a regular need for expensive maintenance and in the case of growing children, there is a need for replacement as well. To counteract with this dilemma, the use of additive manufacturing and rapid prototype manufacturing is trending. This article will discuss how these methods aid the prosthetics industry in greater detail as the article progresses.

Counteracting the Dilemma

The core issue which lies in the prosthetics industry is the dire shortage of machining services and trained personnel. The main challenge in the prosthetics industry lies in the fact that there are no standard products that need to be repeatedly manufactured. Every case can vary in design and application. Therefore, manufacturing techniques that follow mass production principles are not viable. This is where rapid prototyping comes handy. It allows for review of design and amendments until a perfect prototype for the patient has been achieved.

Moreover, there is a major dilemma of massive capital costs involved. In this regard, it is worth quoting that according to a case study report by Day and Riley in 2018, they reported an average cost reduction of 56 percent when 3D printing and prototype manufacturing are incorporated to manufacture an assistive device for a person with finger amputation. Along with this, they also reported massive weight and environmental waste reductions. Let us now discuss major design considerations for prosthetics.

Vital Design Considerations for Prosthetics

In prosthetic devices, the prosthetic socket is the most critical part of the design process. It is the part that covers the remaining limb. There are some major considerations for this part including:

• Biomechanical principles need to be accounted for which include tissue mechanics and biomechanical forces which will act on the artificial limb.
• Optimal strength to weight ratio is required according to the application. For instance, in the design of a prosthetic socket for a lower limb, the weight of the socket must be as minimal as possible. This allows for less fatigue for the wearer while ensuring that the weight of the body is appropriately transferred to the prosthetic device through the socket.
• Range maximization of motion for prosthetics is a vital consideration. Furthermore, proper ventilation of the electronic device, stability and the suspension system, all need to be optimized for long term viability.

The Workflow

• Obtain an image of the patient’s relevant body parts to ensure that the custom-made prosthetic socket is appropriately manufactured. The imaging can be done using 3D scanning of the individual or a cast of their body.
• The file generated by the 3D scanning is then uploaded to a software package which modifies the image. Biomechanical principles are preloaded into the software and it uses them to apply certain modifications. These modifications are to ensure proper fit, comfort and efficient transmission of forces from the body to the prosthetic device.
• The modified image is then converted in a CAD software and converted to STL file format for 3D printing.
• For the fabrication phase, there are two ways. The first way is that the prosthetic company entirely relies on the rapid prototyping service provider. The service provider will only be supplied with the raw scanned images and specifications of the required prosthesis. They will then modify, use prototype manufacturing and perform post-machining alterations if required on their behalf. The second way is that the prosthetist modifies the file themselves, and then send it to the machining service provider.
• Additive manufacturing techniques are then followed to fabricate the final and finished product. Amongst various techniques, Fused Deposition Modeling (FDM) is the most widely used due to its cost efficiency and ability to produce parts quickly. However, at places where accuracy is needed and flexible joints are to be manufactured, Selective Laser Sintering (SLS) is used.
• Material considerations for prosthetics is a wide and in-depth topic. However, the most widely used material for low-cost applications or non-critical components is PLA filament which is a thermoplastic. For parts that require durability, Nylon is a good option. For the most critical parts like sockets where flexibility is crucial, materials like urethane and TPE filaments are really useful. For any medically critical components, only medical grade materials can be used. 

Ensuring Conformity of the Prosthetics

Prosthetic devices are subjected to a diverse set of loading and environmental conditions. For instance, in the case of lower limb prostheses, two types of loads superpose i.e. static loads and cyclic loads. As a result, during natural walking, the prostheses bears up to 6 times the body weight. Similarly, upper limb prostheses must bear massive lifting moments.

Keeping these facts in consideration, a variety of ISO standards are followed to ensure conformity of prosthetics to international and medical standards. ISO 10328:2016(E) encompasses all the structural strength standards for lower limb prosthetic devices. It ensures conformity of a prosthetic device concerning the endurance of static and cyclic loads over its lifespan. It also dictates various testing methods which include static loading, torsional loading and cyclic loading tests for each part of the device. For instance, knee joints are particularly tested for torsional and cyclic loading under maximum flex condition. There are additional standards that cover the manufacturing and testing of prosthetic sockets.

Last but not the least, in today’s modern era, finite element analysis (FEA) is a handy tool when it comes to testing before manufacturing. It helps prototype manufacturer to save material and manufacturing costs. It also plays a major role in conditions where the components are so expensive to manufacture that destructive testing is out of the scope. Moreover, simulation results from FEA help to conduct risk assessment studies when providing a unique prosthesis.

We really hope that this article helped shed light on the rising use of prototype manufacturing in the prosthetics industry.