Rapid manufacturing applied to human prosthetics

Written by: Tom Shelley | Published:

Tom Shelley reports on moves by the medical industry to use rapid prototyping technology to improve amputee care

The medical sector is applying rapid prototyping technology to produce more comfortable sockets for use with artificial limbs. The result is a better interface between prosthetic legs and residual limbs. Leading medical professionals and engineers in the field say CADCAM technology will play a key part in improving amputee care.

The present method to manufacture what is termed a transfemoral prosthesis socket starts by taking a cast of the stump. This is then used to produce a first check socket. This is then tested by the patient and modified according to feedback. After this, a second cast is made in order to produce a final socket that comprises of an inner, soft, thermoformable layer within an outer structure made of a composite material.

A method being pioneered at INAIL Centro Protesi - the Italian Workers' Compensation Authority - is using CADCAM and rapid prototyping technology developed under the European Union Custom-Fit programme.

This uses a laser scanner to capture the exact shape of the stump. A stereolithography machine then uses the data to produce a check socket. Once this has been tested, the modified shape is rescanned and the final socket is designed using an automated CAD tool developed by rapid prototyping CAD software specialist Materialise, another Custom-Fit partner.

Then, using technology developed by De Montfort University the final socket is made from a process called Plastic Powder Printing (PPP). This involves laser printing thermoplastic toners derived from standard engineering polymers such as high and low density polyethylene, polypropylene, and polystyrene, which are subsequently fused using infrared radiation to form the layers.

Head of PPP development at De Montfort University is Professor David Wimpenny. He says: "PPP can be used with multiple materials and products. It is able to vary the material density and is capable of printing up to 2000 layers per minute with a [equivalent] resolutions of up to 2400 dots per inch." This allows the sockets to be made with an incredible accuracy.

It is estimated that the new procedure should reduce the time to make a socket from around 25 days to 18 days and save about €2,000 per patient. However, what is really needed is the ability to go from a laser scan to producing the final socket in one go.

Prosthetic and Orthotic company Chas Blatchford in Basingstoke has been developing a technology to do just this for some time. Professor Saeed Zahedi, technical director at Chas Blatchford says: "We have done lots of research and made samples. We are now trying to bring some key partnerships together."
He explained these included Bournemouth University, which is carrying out research to help identify the right stress pattern in artificial limbs, and a company in Switzerland that has a database of socket shapes.

Finding the best shape for a socket is not as simple as matching the shape of the socket to the shape of the stump. Since the stump and the internals of the socket are both deformable under load the stress pattern changes according to whether the patient is standing or walking.

The Italian technical university, Politecnico di Milano, is working on a virtual simulation tool for the Custom-Fit project. The aim is to be able to model a patient's stump, including the dynamic behaviour of the muscle and bone.

Chas Blatchford already uses CADCAM to produce custom foot orthoses, or inserts for shoes. The company claims that this offers 100% repeatability, because patient information is held on a computer database. This also saves clinical time and production lead times.

It is only a matter of time before the simulation and modeling problems are solved, and patients are able to be fitted with comfortable prostheses, as soon as they are fit enough to use them.

* Sockets for leg prostheses are being experimentally produced by rapid prototyping techniques as opposed to taking a cast, producing a first attempt at a socket, modifying it, and making another cast before producing the final article.
* Present state of the art technology still requires making two sockets. The move now is to only have to make just one, which requires not only matching shapes, but takes into account the deformation of the patient's stump being loaded/unloaded in a variety of typical day-to-day conditions.

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