Prosthetics, known formally as prostheses, are defined as artificial substitutes for missing parts of the body, but this downplays the significance that prosthesis has as part of amputee rehabilitation. Losing a limb can be a serious challenge for patients to overcome, both physically and mentally, but modern advances in prosthetic technology have made its integration into the human body ever more seamless. This article will discuss the evolution of prosthesis design, i.e. how an artificial limb can be made to adequately replace a biological one, and the various options available in the current medical scene will also be explained and evaluated in terms of their benefit to the patient.
Early ages
Casting an eye back at the history of synthetic replacement for limbs, it’s interesting to see that while we’ve had wooden toes and the like for thousands of years, it was only very recently that we’ve been able to produce both a highly functional and visually appealing solution. This is in part due to the greater availability of materials like plastics that are far superior to the heavy and cumbersome wood or iron that were used in the early days of prostheses (Fig.1). While not being capable of fine motor control, these early contraptions could be passively adjusted; for example, in Figure 1 the iron hand was cable of passive flexion and extension in the phalangeal joints, thereby allowing the user, a German knight by the name of Götz, to grasp reins and hold weapons.
Figure 1
Body-powered prosthesis
The next major innovations came in the early 20th and 21st centuries, starting with the “body-powered” prosthesis in 1916. Body-powered prostheses are still preferred by a certain percentage of amputees today, which might initially be surprising considering how far our electronics have progressed. By harnessing the force of joints and muscles more proximal to the amputation and transferring the excursion produced via a harness/cable system, it’s possible to operate terminal devices using the shoulders, chest and back. For example, a pincer-like terminal attachment can be controlled by scapular abduction, allowing objects to be picked up securely and moved around. Those that are involved in manual labour may choose this, due to it being more lightweight, robust and easier to maintain. That said, there are several disadvantages to this approach as well – the force needed to operate such a device is much larger than for a natural limb and this may cause pain or render some patients completely unable to use such a prosthesis.
Myoelectric prosthesis
Next came a more sophisticated approach - myoelectric prosthesis. This system is centred around an actuator of some sort, usually electric motors which can artificially manoeuvre the prosthesis. In this case, the supply signal is derived from EMG (Electromyographic) potentials generated by the residual muscles in the stump under voluntary control (left over after the amputation). For example, trans-radial amputees can use the preserved wrist flexor/extensor muscles to control a prosthetic hand, while trans-humeral amputees can use the biceps/triceps to control the prosthetic elbow. Non-invasive surface electrodes pick up the signals and transfer them to a processing unit within the prosthesis that determines what action is to be performed. For simpler devices, EMG amplitudes can be analysed to determine the desired strength of “muscular” contraction, whereas multi-function prostheses need a complex signal-processing algorithm to differentiate between signals from many muscle groups and then convert them into appropriate outputs in the prosthesis.
Obvious advantages to this lie on the cosmetic side of things– these units are sleek and can come with silicon skin-like overlays. In addition to this, the torques transmitted through the limb provide some somatosensory input for the user; some systems like the Hero ArmTM (a bionic limb produced by Open Bionics for trans-radial amputees), provide haptic vibrations that also contribute to this . However, there are also limitations to the functionality that a myoelectric prosthesis can provide.
Due to the requirement for translation between EMGs and instructions for the motor, there is a delay between the user thinking to move and the execution of this command. Shifting electrode positions or sweating can interfere with the EMGs and further delay the movement. Unlike body-powered devices, a battery is also needed, (often nickel-cadmium) and this inevitably brings the inconvenience of having to recharge the artificial limb. Moreover, it can be difficult for the patient to isolate muscle signals, requiring an extended learning phase.
Luckily, solutions have been developed to combat these limitations. TMR, (Targeted Motor Reinnervation), reroutes amputated peripheral nerves to intact spare muscles; the resultant EMGs of the target muscle become the input to the prosthesis. An example may be useful to illustrate this: the median nerve can be connected to the middle pectoralis major muscle. Then, when the user thinks “flex fingers”, the pectoral muscle contracts and the robust EMG produced makes the prosthetic hand move. This is more intuitive and allows the patient to control many joints at once, which isn’t really possible with a standard prosthesis.
Osseointegration (where titanium fixtures are inserted directly into living bone) ensures greater comfort and stability at the body/prosthesis interface, as well as removing socket-related problems like chafing. This intimate connection also increases vibratory sensation and allows the user more control over the limb.
Proprioception
While the most advanced myoelectric devices can provide sensory feedback, especially with the additional nerve treatments previously discussed, proprioception is difficult to simulate. Proprioception is roughly defined as the body’s sense of movement, balance and tension. Normally, muscle spindles and GTOs (Golgi tendon organs), operate in tandem to provide these sensations. The spindles follow the change in length of their parent muscle and the firing rate of their neurons is proportional to this change. To mimic this effect, incremental rotary encoders can be used with motors, such that the number of motor cycles is interpreted instead of the change in length in the muscle. Afferent nerves from the spindles also respond to vibration, so vibrations picked up in the prosthesis can be interpreted as a stretching sensation. GTOs lie in the junction between tendon and muscle body and interpret the sense of effort, (force needed to overcome an external resistance). Normally, the recruitment of additional motor units represents an increase in contractile force, but in the prosthesis the angular displacement of the effector can be compared to the current supplied to determine the effort.
Running blades
Aside from prostheses for everyday life, something interesting to consider is the design of the running blade. While normal prostheses try to match the shape and form of human limbs as best they can, the distinct sickle-shape of sports prostheses seems rather unnatural. In fact, the designer of the original running blade was inspired by the hind legs of the cheetah, (see Fig. 3), as well as the mechanics of a diving board! The way the leg works is as follows: in a natural human leg, the ankle works as a store of energy when flexed, which is released in each step – the curvature in the running blade mimics this and allows the runner to get a return on energy they put into the ground. This is simple in theory, but has allowed countless people who have lost a limb to continue running.
Figure 3
Conclusion
While this article has focused more on the scientific aspect of prostheses by looking at the design of various types of artificial limb, it is important to note that there is a significant psychological aspect to amputation. Patients still require significant support to get used to the new limb and even then, they may still experience issues. Phantom limb pain, a phenomenon where an amputee experiences pain from their missing limb, is one of the reasons why amputees sometimes refuse to wear their prosthetic limb. As far as technology has come, with 3D modelling and printing to provide a custom-fit for patients or advanced neural connections, there is still room for progress.