Introduction

The manufacture of a conventional artificial limb usually involves the combination of pre-manufactured and custom fabricated components. The most common example of this is the endoskeletal trans tibial prosthesis consisting of

Standards of Manufacture

Pre-manufactured components

Pre- manufactured components are increasingly being assessed by International Standards of which the most relevant is ISO 10328. There are however several international benchmarks and it can be difficult to ascertain which is relevant or what compliance to a standard means. REHAB Tech have created the TechGUIDE, which in the case of lower limb prosthetic components outlines which components have met a baseline minimum standard.

(http://www.monash.edu.au/rehabtech/pub/techgui.htm)

Custom fabricated components

The difficulty arises in the custom fabrication area There are certainly applicable standards such as the ISO (000 series, however few fabrication centres world-wide comply with these. There are examples however of predefined standards of practice in the custom fabrication area both from independent sources and from material suppliers and the difficulty is in the area of compliance. How can you know that a particular socket was constructed using the correct lamination guidelines?

(http://www.monash.edu.au/rehabtech/info.htm#manufac)

Education

It is the responsibility of Rehabilitation engineers not only to investigate these areas, but also to provide the education to Prosthetists in the areas of material sciences and mechanical structures.

We encourage and run such a course to all Health professional involved in Mechanical devices known as the Component Inspection Course. At the completion of the course participants have:

• A knowledge of basic structural mechanical theory and its application in the field of prosthetics and orthotics
• An understanding of concern and an ability to recognise areas of potential failure.
• The ability to conduct basic non-destructive investigative tests to assist in the decision of continued use of components

USING ADVANCED MANUFACTURING TECHNIQUES

CADCAM

The application of CADCAM in the prosthetics field can either allow for reasonably conventional manufacture after the modification of the socket or it can introduce the possibility of some advanced manufacturing methods. The ability to define the socket, cosmesis and alignment in digital format allows for the investigation of a variety of manipulations and manufacturing techniques that are not available using the conventional positive model (plaster or otherwise) as the definitive information storage mechanism.

METHOD

Using the Seattle ShapeMaker™ CADCAM system a conventional cast was taken and digitised to allow on screen modification.

CONVENTIONAL SOCKET

Once the socket modifications have been completed on screen a carver then carves out the "modified" cast and a socket is completed using any conventional technique.

RAPID PROTOTYPE SOCKET

Once the socket modifications have been completed on screen using a Seattle ShapeMaker™ CAD system, the socket carve file is emailed to IRIS who convert the file to a three dimensional surface. Therefore there is information about the inside/outside and wall thickness and is sent electronically to the Rapid prototyping machine which creates what has been designed on screen using a technique called Fused Deposition Modelling. This means layers of molten plastic are continuously applied at a rate and direction driven by the computer to complete a 3D Model, in this case a socket. A complete plastic socket is thus produced. The initial trial socket took approximately 10 hours to construct and is made of nylon.

The ability to construct a socket using rapid three-dimensional prototyping encouraged the investigation of the process of manufacture of the trans tibial prosthesis. As this manufacturing technique is an "end product" technique, the aim was be able to define as much about the completed prosthesis as possible. By defining the device (and defining it digitally) it can be manipulated and manufactured in a variety of ways or locations.

INCORPORATING ALL PROSTHESIS INFORMATION INTO ONE DIGITAL FILE

The final prosthesis consists of the correctly fitting socket, alignment characteristics and cosmetic finishing. Conventional prosthetic CAD/CAM (using ShapeMakerä in this case) is sufficient to define the socket characteristics. This same system can be used to define the cosmesis as the reverse of the unaffected limb. Following a trial walk, the final socket shape and alignment are determined. Alignment is simply defined geometrically.

Consequently, following a trial walk, all aspects of the final structural design of the prosthesis can be defined digitally.

A process has been established whereby the socket, cosmesis and alignment information are mathematically merged to produce a " best fit". Having verified this a program was developed that would merge the files from CAD/CAM information. This is not specific to ShapeMakerä as it was also trialed with the BioSculptorä unit - The file format could be manipulated and shared between the two systems. The final file included all the information for a complete prosthesis.

OPTIONS CREATED USING ADVANCED TECHNIQUES

A digital file containing all the prosthesis information allows flexibility in the manufacturing process. Examples investigated include:

• The foam cover (carved by the computer-aided carver) can be made separate to the socket. (see figure 1)
• An exoskeletal shell can also be constructed separately which would inherently accept the socket. (see figure 2)
• A rapid prototype prosthesis can be constructed as one unit

Figure 1

Figure 2

However, it is also possible to bring the file into a conventional CAD package and model the prosthesis structurally, defining all the critical loads and requirements for different material constructions.

INVESTIGATION OF THE IMPROVEMENT OF "CLINICAL" PROVISION OF PROSTHESES*

Clinical Delivery

The use of Rapid three dimensional prototyping technology and in particular Fused Deposition Modelling was investigated for the production of a trans tibial prosthetic socket and how this would effect the clinical delivery of the device. It was found that although it was quite feasible to produce and trial the socket, it had no significant impact on the key aspects of speed, quality and design of the socket.

It was found that the process by which the device was being delivered made the technological construction method used irrelevant. Process has subsequently been investigated and a process outline that provides a significant saving in cost and time as well as improved quality has been suggested. This process has been trialed in a conventional way, however has not been used to deliver a prosthesis using Rapid 3D.

TRIAL

A socket has been fabricated and is currently on trial. There does not appear to be any significant deviation in geometry between what has been designed and what is constructed using FDM.

By redigitising the final model a comparison of the original design and 3D-prototype model could be made. Variations of between 1 - 3% are within the variation of digitising a model. The FDM machine has an accuracy or resolution of up to 1/10th of a millimetre

In terms of fitting and aligning the socket there was no variation to the conventional process.

From this point the process of completing a prosthesis is identical to the conventional process.

Figure 3- Trial Rapid 3D socket being fitted

Conclusion for Clinical delivery

The use of FDM Rapid 3D technology had no effect on the design or modification process of the prosthetic socket. Nor did the FDM socket have an altered outcome from that which the prosthetist would have outlined.

-Design of prosthesis

The construction of the socket can currently take anywhere from 7 to 24 hours depending on factors and high resolution, wall thickness and wall construction. The prosthetic trans tibial socket would typically be at the lower end of this time scale.

-Speed of delivery

The clinical delivery however is heavily dependent on the organisational setup. If sockets are typically fitted at time of casting, then 7 hours is not a realistic time to wait. If a second appointment is routine, the process is identical. New FDM technology is promising build times of around 3 hours, however these have yet to be trialed.

Quality:

-Design process.

Definition of wall thickness and strength requirements can contribute to the overall quality of the construction as opposed to "cover-all factor of safety" techniques dictated by conventional construction

-Provision –

There are no specific advantages such as reduced visits, less time, patient education or information using this process as opposed to the conventional technique

-Fit

The fit of the socket remained consistent with the manufacture. This is certainly reproducible, however as the socket is constructed independently of the liner (thus limiting the technique to patients using preformed liners

-Cosmesis

The cosmesis of the socket is not unlike other plastic sockets. It has a white appearance and on close inspection the layers of plastic can be seen and felt.

Discussion of advanced manufacturing technology and influence on clinical delivery.

Overall, the FDM socket offers little discernible difference to a conventionally manufactured plastic socket in terms of the clinical provision of the prosthesis to the client if incorporated into conventional stages of delivery. The technology must be considered as part of the process and it is that process that needs to be reviewed.

As with CADCAM technology, Rapid 3D prototyping adds 'process benefits', ie. changes or advantages to the way something is designed, assembled or delivered.

By slotting in the technology to replace a conventional step we are negating the process advantage and, at best, doing "the same thing in a different way".

The entire process needs to be reviewed to fully optimise the use of any new technology that may impact the way prosthetic services are produced and delivered.

Each step is reviewed and re-judged and may not necessarily either be required or performed at the stage that it is conventionally done.

These are best reviewed in a way that shows no bias to the particular technology to be employed.

For example:

If we know we will use alignable modular components the alignment may occur either after a check socket OR after the definitive socket manufacture.

Cosmesis can be commenced only AFTER the prosthesis has been completed in terms of final socket and components.

If Rapid 3 dimensional prototyping allows us to construct a socket according to our individual design, then it follows that we can construct a socket with an adaptor to suit the component to be used OR a socket with an appropriately angled length of pylon to suit the chosen foot. With this type of construction we can also influence the design (strength ) criteria of the construction with an accuracy that is impossible using conventional techniques.

The process is really made of two separate stages:

Design Stage (includes)

Manufacture Stage (includes)

Figure 4 - Merging cosmetic and socket information

Bill Contoyannis

Manager/ Rehabilitation Engineer

REHAB Tech

c/o C.G.M.C.

260-294 Kooyong Road

Caulfield 3162

Melbourne AUSTRALIA

bill.contoyannis@eng.monash.edu.au

Further Acknowledgments:

* Cumbo J.¹, Nagarajah R.³, Costello B.C.³, Dempster B.³, Gray S.J.⁴, Brown T.I.H.²

Prosthetic Technology Research Collaboration involving:

¹REHAB Tech,

²Monash University Centre for Biomedical Engineering,

³IRIS - Swinburne University,

⁴Alfred Hospital Plastic Surgical Unit.

Melbourne, Australia

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