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Advances in Artificial Limbs

In an unassuming lab in Chicago, under the watchful eye of Dr Todd Kuiken, a series of experiments is about to get under way that could revolutionise how we view prosthetic arms. The subject of these experiments is a middle aged man named Jesse Sullivan, who has had the ill fortune of losing both his arms in an accident. Attached to his shoulders is a contraption that clearly belongs on a Terminator set, with electrodes, wires and motors protruding out of its plastic casing. It is in fact one of the world’s most advanced artificial arms. Known as the Boston Digital Limb, it still remains in the experimental stages, but it is able to interpret Jesse’s thoughts and produce upper limb movement as he desires.

A few decades ago the idea of an articulate mechanical limb which was controlled by thought would have seemed completely unfathomable, worthy only of science fiction lore. Yet in the past 20 years rapid advances in so called ‘bionic limb technology’, have resulted in the world’s first commercially available bionic hand, the iLimb.

Made by a company called TouchBionics who are based in Livingstone, near Edinburgh, it uses the electrical signals from the remaining muscles in the amputee’s forearm to control the bionic hand. These electrical signals are very similar to the ones produced by the heart that a doctor is able to analyse using an ECG machine. Although the amputee has lost the hand, the nerves from his brain still fire neural signals to the muscles in the forearm which normally control the hand. Each muscle has a different function - for example the muscles in the back of the forearm are involved in extending the fingers, while the muscles in the front flex the fingers. The bionic limb is able to pick up these minute electrical impulses through electrodes on the surface of the skin. It then interprets the signal and through a series of motor and gears causes the fingers to extend and flex. It has a thumb that can be moved to various positions to provide a range of grips.

The iLimb may be very sophisticated and advanced, but it only represents the first step on the ladder. With all due respect to its inventors, it is a very basic model. After all, it can only elicit two individual movements – opening and closing the fingers –- much far fewer movements than a normal hand.  Although it may suffice in a few basic day to day situations, it cannot be used by above-elbow (trans-humeral) amputees and has limited wrist movement – a serious hindrance to an amputee’s dexterity and quality of life. The search for more advanced and dexterous prosthetics continues and a major impetus for more sophisticated prosthetic technology was the 2nd Gulf War.

It is often the case that there is a surge of interest in prosthetic technology following a major war or conflict. The 2nd Gulf War is no different. The advances in surgical techniques and intensive care have meant that many of the soldiers that would have died from their injuries in previous wars are able to return home and make a full recovery, albeit with horrendous disabilities and needing rehabilitation.

Spurred on by defence organisations and veteran welfare groups, scientists have been working tirelessly to improve the functionality of artificial limbs. The most promising advance is an elegant technique known as targeted muscle re-innervation (TMR). First trialled less than 2 years ago, it has proved to be a cornerstone in prosthetic technology. TMR is a surgical procedure where the remnants of the nerves used to control the forearm and hand are carefully transferred to the muscles of the chest. The nerves are very fragile and require the most delicate of touches. Unsurprisingly, despite the best efforts of the surgeons, there have been a number of failures. Happily however, there have been many successes as well and one of the first patients to undergo a successful TMR operation was Jesse Sullivan.

Now, when Jesse thinks about closing or opening the hand or moving his elbow, it is the chest muscles, not the arm or forearm muscles that contract. As the nerve signal reaches the chest muscle, it produces an electrical impulse at the chest, which can be picked up by  electrodes placed on the chest skin, similar to those on the iLimb., 

Once Jesse was fitted with the Boston Digital Arm (BDA), it could to read and interpret the EMG signals and produce the desired movement. Jesse was given around 20 hours of training to allow his brain and nerves to adapt to the machine. He is now able to carry out many of the activities of daily living that we take for granted. Even simple things like picking up a pint glass and putting on a cap that were previously impossible are now carried out with relative ease.

Using TMR has significant advantages over the previous models such as the iLimb – thanks to the chest muscles acting as amplifiers, it produces a much stronger, cleaner signal, allowing for astonishing degrees of accuracy and dexterity. In addition to being able to open and close the hand like the iLimb, Jesse is able can move his elbows, and articulate his wrists like any normal individual as well as rotate, flex and extend his shoulders, giving him a much larger and more natural repertoire of movements. Amazingly, all these movements can be produced just by subconscious thought, a truly remarkable feat of engineering.

TMR and the Boston Digital Arms have been greeted with fanfare and hailed as a stunning success at many international exhibitions. However, it is unfair to expect advanced artificial limbs such as the BDA to become commercially available in the near future.  After all, artificial limb research is still in its infancy and there are many hindrances and obstacle that have to be addressed first.

Firstly, such a delicate operation is inherently risky, carrying a high risk-benefit ratio and only a handful of surgeons in the world have the necessary skills and expertise to carry out such an operation. This however should not be a major difficulty – surgical techniques are constantly advancing and it will only be a matter of time before surgeons have refined their technique to offer safe and reliable TMR operations.

A significant stumbling block concerns the engineering aspects of the BDA. It is simply too heavy and cumbersome to be used in daily life over long periods of time. The whole device weighs close to 8kgs and even the strongest amputee would struggle to use it for more than a few hours before feeling exhausted. Scientists have been experimenting with lighter materials like carbon fibre and alloys to bring the weight of the device down as well as superior supporting braces. If they are to be widely accepted and used, the limbs must resemble normal arms. This means modifying the shape and by  experimenting with prosthetic skin, the limbs can be made to look more lifelike. Scientists have had remarkable success with the skin. The latest prototypes look and feel so similar to the real thing, it is difficult to tell them apart, even close up. Surely, it is only a matter of time before lighter, more ergonomical and aesthetically pleasing devices are engineered.

The BDA is very good at replicating the movements of the upper limb, but what it cannot do is offer sensory input to the amputee. If Jesse is blindfolded, he is unable differentiate between a sponge and a brick. In the last few months, scientists have begun work on using pressure and temperature sensors which are placed on the tips of the fingers and are linked to sensory nerves in the arm. The nerves feed back to the brain which can be trained to interpret the signals. Essentially, it is the reverse of TMR and offers the next advance in artificial limb technology.
But the most significant hurdle is the cost of building, maintaining and repairing the artificial limbs. Naturally, avant-garde technology such as the BDA is extremely expensive and is only possible thanks to the millions poured into artificial limb research by scientific organisations and governments. But if a fully functioning artificial arm were to become commercially available, the manufacturers should be able to produce high-end, state of the art technology that is reliable and safe to use while still turning over a handsome profit – clearly a Herculean task. However, any potential manufacturer should take heed of the fact that there is strong niche for such technology, and many armed forces would be willing to pay attractive sums for contracts to help rehabilitate their wounded veterans.

There are some who believe that artificial limbs will never be as advanced as nature’s own. This may well be the case, but engineers and doctors are working ceaselessly to optimise the functions of artificial limbs to provide amputees with a real alternative. A functioning limb will give amputees greater freedom and may allow them to return to work. This in turn would greatly improve the amputee’s quality of life. It is hard to believe that less than 20 years ago, all an amputee could expect was plastic mould with no functional value. We have certainly come a long way since then. and We can only speculate what will happen in the next decade, but I am sure than within our lifetime, sophisticated and commercially available artificial arms will enhance and transform the lives of amputees for the better.

Rishikesan Ramaesh is a 3rd year medical student


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