By Bishr Albadri
Despite great progress in synthetic biology and prosthetic design, overcoming the detrimental effects associated with the loss of a limb remains a challenge. The repercussions of losing a limb are not sufficiently compensated by many of today’s prosthetics due to their locomotive restrictions. This review details the intricacies of applying a prosthetic limb, the origins of the prosthetic which leads onto the bioengineering behind the modern-day prosthetic, using the upper-limb (sometimes dubbed the “bionic arm”) as an adequate example. The impact of prosthetics on society, and the psychological implications that people experience when resorting to prosthetics will also be explored.
The word “prosthetic” denotes an artificial body part such as a limb, heart or even breast implant. One of the first prosthetics known to exist is a magnificent artificial big toe for the right foot, originating from ancient Egypt and found to originate from 600BC. It is composed of a material called cartonnage, which is a paper mache of linen soaked with animal glue and coated with tinted plasters. This, although not an artificial limb, has laid many of the foundations for modern-day prosthetics of upper and lower limbs. The understanding that prosthetics require very particular mechanics and our understanding on supporting missing parts of the human body was birthed from ancient times (Finch et al., 2012).
Applying a prosthetic today is more advanced than ever before and requires very precise procedures to ensure it is safely accepted by the body. There are 2 surgical procedures considered after the amputation of a limb and the application of a prosthetic; TMR (Targeted Muscle Reinnervation) and TSR (Targeted Sensory Reinnervation). Both procedures allow for natural sensory recall from the prosthetic through surgical nerve-transfer procedures manipulating motor and sensory neurons respectively. The residual nerve-stubs (the remaining nerves present post-amputation) are surgically anastomosed into neighbouring muscle fibres in order to create independently controlled nerve-to-muscle units. If the upper-limb is used as the main example, then these muscle fibres act as biological amplifiers of motor commands used to control the bionic arm (although applies to every limb prosthetic). This is highly beneficial through its provision of a physiologically appropriate environment for the regeneration of axons and nerves into the targeted muscle, alongside improving the potential of intuitive motion control. The distinctive feature of this type of prosthetic is the transfer of information into the somatosensory system, allowing for sensory input of 4 types: Proprioception (location), Thermoception (temperature regulation), Nociception (pain) and Hygrosensation (humidity). Surgery must, however, occur very soon after amputation to ensure that the nerves are left unscarred and at their greatest length before reinnervation. Additionally, the reliance on unscarred and relatively undamaged nerves results in fewer patients being strong candidates for this procedure, and highlights the importance of careful analysis of individual suitability to these interventional treatments (Dumanian, 2015; Weir, 2004; Marasco, 2013).
Another concept to appreciate when applying the prosthetic is how it connects to the central nervous system. This connection is identical to the connection used in myoelectric prosthesis whereby “bipolar differential electromyographic (EMG) electrodes” are implanted into a neighbouring bone in the patient. Post-surgery, the EMG signals sent from the CNS are recognised by electrodes in the residual limb and relayed into the prosthetic as a form of data input. Using a complex algorithm, the intramuscular EMG signals of neighbouring muscles can be read by a microcomputer within the prosthetic and converted into logical instructions for the execution of the intended movement. Using the example of an elbow disarticulation, the activation of the muscle fibres connecting to the residual nerves of the flexor compartment of the forearm (located in the bicep) would release an electrical pattern which would be recognised by the electrodes and consequently result in electrical activity in the stump in order to flex the bionic arm and move a finger. This is also replicated if the finger was shook by an external stimulus. The TSR surgical procedure means that the user would be able to sense which finger was shook and by how much force it was moved, through the sensory nerves that were rerouted to their bicep. This is known as transfer sensation and is the distinct difference between this particular form of prosthetic and other types (Mavroidis et al., 2002).
The action of executing a movement is highly complex. In order to execute locomotion as the userintends, there are some key motor functions to explore. A single motor powers each subassembly (e.g. a finger in this particular hand prosthetic); the motors are controlled by a microprocessor, monitoring the position of the assembly via the action of accelerometers. This allows for manipulation of several contact points as different amounts of force can be applied in order to balance an object. This makes for a reliable, precise and intuitive control of the prosthetic by the user, as the motors can be manipulated and positioned in such a way to ensure optimisation of weight distribution. These motorised digits allow for a natural mimicking of gripping patterns, including auto grip functions, to prevent the slipping of objects. These auto grip functions work by incorporating the Coulomb model of friction, implying that the tangential force of lift at each contact point is limited to the coefficient of friction x grip force. Hence, the coefficient of friction between the digits (known by the device) is used to decide the amount of force required to hold an object without it slipping, giving the illusion of a natural grip. However, in reality, more force would be applied due to the coefficient of friction for the prosthetic fingers being lower than the associated coefficient for the human hand. It also allows for fine motor control, which aids in tasks that require higher levels of precision (writing or playing the violin); motors can apply different amounts of force at each contact point in order to balance/stabilise objects, as necessary, and so increase the capabilities of an individual requiring prosthetic aid (Kulkarni & Uddanwadiker, 2015).
Amputation can be considered a traumatic experience to many, resulting in multiple physical, psychological and social sequelae. It remains clear that individuals who are missing a limb face extensive hardships in their day to day endeavours, many of which may not yet be treatable with current prosthetic capabilities. Of course, we can appreciate that there are inevitable psychological benefits of being able to perform an activity of massive importance in their life once again (e.g. a simple task like typing for a programmer or running for a sprinter). The United Kingdom has approximately 5000 new major amputations a year according to the amputee statistical database for the United Kingdom (NASDAB, 2009). It also found that 67% of the British public report feeling uncomfortable talking to people with disabilities according to a survey done by Scope in 2014. Over 40 million people watched the London 2012 Paralympics and post-viewing, most viewers said they were more comfortable talking about the topic of disabilities. This is clear evidence that innovation in the field of prosthetics has a strong influence upon the perception of physical disabilities within society and with the accelerating growth of modern bioengineering, the societal, medical and psychological issues will gradually cease.
The prosthetic today has a myriad of complexities in its functionalities and application and with international research from the world’s most dynamic industries, this will only grow more advanced. The prosthetic is the cornerstone of the profound world of bioengineering and as we progress into the future, we progress towards a world where the artificial limb is perfected and the social challenges with wearing a prosthetic ceases.
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