‘Prosthetics have been used throughout history,’ explains Professor Gordon Blunn of the Institute of Orthopaedics and Musculo-Skeletal Science at University College London. But these prosthetics had always been conceived in a similar fashion, until Professor Blunn and his colleague Dr Catherine Pendegrass who invented the Intra Osseous Transcutaneous Prostheses – known as ITAP for short. Unlike previous prosthetics, this device is attached directly to the bone, which alleviates many problems associated with the previous model of the prosthetic.
As Professor Blunn notes, the prosthetic as an aid or replacement for a limb have been used throughout history. In 2012 researchers at the University of Manchester copied wooden toes that were discovered on the bodies of Egyptian mummies buried 3,000 years ago. Tests were conducted at the Gait Laboratory at Salford University's Centre for Rehabilitation and Human Performance Research and it was discovered that these wooden toes weren’t purely cosmetic, they were aids to walking.
There have been some developments in the traditional stump-socket technology over time. ‘Technology has been used to develop the external socket device,’ says Professor Blunn. ‘For example you can buy computer-controlled legs which actually go on to the femur.’ They help prevent you from falling. There are also hands that can contract and open, wrists that rotate and elbows that can flex and bend. ‘Those things are controlled by electrodes that are placed over the residual muscles on the stump,’ explains Blunn, ‘and they detect muscle contraction and that sends a signal to the motor or actuator to open and close.’
With these variations on the traditional prosthetic the key is that there are parts of the body which can control more supple prosthetic devices. One development in this field is called Targeted Muscle Reinnervation where residual nerves from the stump are taken and put into other surface muscles used to detect signals when they contract. For example, explains Blunn, ‘the idea is that the residual nerve in the arm is plugged in to the pectoral muscles on the chest and you can get quite a number of signals, up to 16 in some cases via muscle contraction of the chest. The problem is you have to then wear a waistcoat and plug the electrodes in.’ It is also quite complicated learning to contract and co-ordinate muscles for a specific movement.
Model of deer antlers
What ITAP does is to get rid of the socket by attaching the prosthetic to the skeleton, anchoring the artificial limb directly onto the implant, ‘that of course transmits loads directly on the bone which is of course where the loads are supposed to go to,’ says Blunn. They have known for a long time how to integrate bone and prosthetic, but it’s taken a while to develop. ‘The key is the seal at the skin interface,’ explains Blunn, ‘and we started working on this about ten years now. We used natural analogues of bone passing through the skin of which there are not many, such as deer antlers. We looked at the morphology of bone as it passed through the skin and we tried to copy that on our ITAP prosthesis.'
As for next steps, the team are continuing to develop the device looking to the skin interface to enable better integration. ‘We have a grant from the Wellcome Trust to make the skin interface using a technique called Selective Laser Sintering which prints 3D shapes in titanium metal. That allows the structure, especially at the skin interface, to be more porous which means it can integrate better with the skin.’