Prosthetics: designed to fit – and fast

Elly Earls catches up with Elaine Figgins, of the University of Strathclyde’s National Centre for Prosthetics and Orthotics, and Anna Reyes-Potts, of Trulife, a manufacturer of orthotic and lower limb prosthetic devices, to find out how CNC machining and rapid prototyping are giving patients quicker access to a wider choice of prosthetic devices than ever before.


Not only do prosthetic devices need to be carefully customised for each individual patient depending on their weight, activity levels and gait; factors ranging from increased muscle tone to unexpected weight gain mean that patients are in a constant state of flux throughout their lives. This throws up manufacturing challenges aplenty but, just as patients will never stay the same from one year to the next, prosthetics manufacturing technologies continue to develop, resulting increasingly in reduced patient waiting times and better quality of care.

"We've seen huge advances both in the materials used and the manufacturing itself over the last 15-20 years," says Elaine Figgins, principal teaching fellow at the National Centre for Prosthetics and Orthotics at the University of Strathclyde, UK.

"The technology of the designs is getting better and the tools and equipment available are always being upgraded."

"The technology of the designs is getting better and the tools and equipment available are always being upgraded," adds Anna Reyes-Potts, vice-president of operations, orthotics and prosthetics at medical device manufacturer Trulife. "Although the technology has been out there for the last 20 years, manufacturing is now getting much faster."

CNC machining

One of the key technologies used by Trulife is computer numerically controlled (CNC) machining. This involves the engineer providing an electronic drawing or 'solid model' of a part, which is then converted into a program using software such as SolidWorks. "This allows you to see on the screen what the solid finished product is going to look like," says Reyes-Potts. "Ten to 15 years ago we did everything with line drawings, but it was much harder to create a design using 2D lines. Solid modelling allows you to create and view the drawing in 3D, which speeds up the design process."

The 3D electronic drawing is then sent to the machine shop, where it is converted to machine code. "We have some really talented machinists on staff, who know exactly which tools and cutters to use based on the electronic model and the material being used," Reyes-Potts notes. "They programme the tool paths, the feeds and speeds, so every single time we make a particular part it will be exactly the same to a tolerance of +/-0.005 of an inch, for example. The cutting tools do go through wear and tear, so although you'll always get a range, once the machine deviates out of the range of 0.005 of an inch, we call for an insert change or offset."

These parts are then sold either through distribution or directly to the medical practitioner. "After a licensed, certified prosthetist has seen the patient, they put together the limb in a customised manner," Reyes-Potts explains. "Every patient is different, and their alignment and their weight are important for prosthetic devices. Each prosthetist has to carefully select the appropriate products to give to each patient."

As well as being used to create the standard components that go into a prosthetic limb, CNC machining can be used to manufacture completely individualised parts, such as sockets, which need to fit each patient perfectly. In this situation, the prosthetist would take a digital image of the patient's limb, design the prosthetic socket using a computer program, and send the design to an in-house milling machine, which could then construct a model to use in the fabrication of the socket. "If the patient was seen in the morning, they could be fitted later on that day with a check socket," says Figgins. "The turnaround should be that quick. The trouble is that, in practice, you would have a whole queue of patients waiting so it would be unlikely to actually be the same day."

The speed of the process when compared with conventional casting methods is impressive nonetheless. However, the cost of high-tech milling machines often discourages healthcare facilities from investing. "You have to think of it as a long-term investment, rather than an immediate, short-term investment," Figgins says. "If a company or department were to invest, it would pay for itself in the long term by reducing patient waiting times."

"If a company or department were to invest, it would pay for itself in the long term by reducing patient waiting times."

Figgins also suggests the idea of having a central milling machine for a group of local facilities to share. "This certainly happens in the Glasgow area and it means the prosthetic can be returned to the department promptly rather than being shipped half way across the country," she notes.For Figgins, the industry is currently split. "Certain companies and departments have embraced CNC machining wholeheartedly but others haven't put in that initial investment," she notes. While these facilities probably do an excellent job with conventional casting methods, the clinician time spent on creating the prosthesis using a plaster of Paris cast is significantly greater than if the patient were to be scanned digitally.

Rapid prototyping

Another technology coming to the fore is rapid prototyping. This involves using a rapid prototyping machine, which adds and bonds materials in layers to form a 3D model directly from computer-aided design (CAD) data sources, and means that geometrically complex, intricate designs complete with a range of different components can be created without an elaborate machine set-up.

Although this is an expensive option for manufacturers, it is being embraced across the prosthetics manufacturing industry. "We have been doing it for a while," Reyes-Potts notes. "The materials and technologies for making rapid prototypes are getting better; now you can actually fit check the prototype, which could never have been done in the past. It means you can go to the initial test phases much more quickly." For Figgins, the next step is an increase in the number of materials that can be used within rapid prototyping. "If you could lay out the different materials all at the same time, you could effectively make the socket and the prosthesis all in one go," she remarks. "But that's probably in the next 20-30 years."

The range of materials being used to manufacture prosthetics has certainly begun to increase. "Silicone has come on board when it comes to interface materials," says Figgins. "There is also a lot more use of different microprocessor technologies and a lot of different titaniums and carbon fibres. In the future, combining some of the interface materials with strengthening materials would be an interesting concept, and that's something researchers are working on now. People want the skin-tissue interface materials to be made from silicone for its comfort, but you also need structural components within that, such as carbon fibres and titaniums."

The patient comes first

The latest manufacturing technologies essentially mean that prosthetic devices can be made at a much faster pace, something that has an obvious impact on patient care. "Because of these new technologies, designers and engineers can be more creative and responsive to market needs," Reyes-Potts remarks. "They can output their designs and provide them to market more quickly, and as a result medical practitioners have not just better devices, but a much wider variety to choose from."

"In the past it might have taken three years, now you can do it in a year, depending on the complexity of the product."

Trulife is currently working on several new products, including prosthetic feet, and the advanced manufacturing technologies being used to design and manufacture them allow the company to juggle these projects effectively. "It means we can perform the evaluation of our prototypes fairly easily," Reyes-Potts notes. "It really speeds up the design process. Whereas in the past it might have taken three years, now you can do it in a year, depending on the complexity of the product, of course."

From reduced overall project lengths to shorter patient waiting times in healthcare facilities, the use of the most advanced technologies by both manufacturers and prosthetists has served to increase productivity industry wide. And with research continuing on both technologies and materials in this ever-evolving field, the speed of the tricky prosthetics manufacturing process is only set to accelerate further.


Medical Device Developments
This article was first published in our sister publication Medical Device Developments.