Although prosthetic arms have a wide range of applications, one of the major limitations of this technology is the inability to interact with the real world. For instance, the contact between a stiff robot arm and a hard surface will lead to contact instability which occurs due to indefinite modeling of the robot, environment and their interaction.
While the current electric prosthetic elbows are capable of achieving 12.2 Nm of elbow lifts at 2 rad/s, the lifting of the body-powered elbows are restricted by the capability of a connector connecting the component and the user, speed and the user’s strength. The elbow components achieve humeral rotation with the help of friction joints/turntables fixed manually. Hence, it is obvious that prosthetic arm designs still require improvements for a sustainable future.
The limitations of the prosthetic arm can be resolved by altering its design characteristics. One of the major design considerations required while fitting prosthetic arms is that the weight of the prosthetic arm should not exceed that of the original due to the discomfort caused by suspending heavy weight. Another critical factor is the selection of power source (i.e., body power or external power). In case of body power, excess energy is not needed as the individual’s musculature acts as an actuator. However, while using external energy, the power source must be capable of operating the arm without the need to recharge.
Research and Biomedical Application
Buffart LM et al (2007) evaluated 20 subjects including nine prosthetic users and 11 non-prosthetic users of age between 4 and 12 years having unilateral transverse upper limb reduction deficiency to check upper extremity functioning with the help of standard equipments. The experimental results demonstrated that the children are capable of achieving 68% of daily activities efficiently using the prosthesis. In addition, the children mostly used the prosthesis for specific activities than daily activities. The validity of the prosthetic upper extremity functional index and assisting hand assessment was best and reliability varied from good to excellent. It was concluded that appropriate information related to the functioning of children suffering from upper limb reduction deficiency can be obtained using standard equipments.
Research by Zhou P et al (2007) focusses on the control of artificial limbs by decoding a neural-machine interface (NMI). The team developed a new NMI named as targeted muscle reinnervation (TMR) for enhancing the artificial arms function in amputees. The residual amputated nerves were transferred to the amputee’s non-functional muscles using TMR, where the reinnervated muscles amplify the signals from the nerves.
High-density surface electromyogram (EMG) electrode arrays were used to examine the amount of control information obtained from the reinnervated muscles by recording the surface EMG signals. Following this, the signals were subjected to various pattern classification techniques. As a result, the classification of movements of 16 desirable hands, arms and fingers were highly accurate. Moreover, the results revealed that complex commands can be transmitted by the central motor control system without the need for a peripheral feedback. The study concluded that the prosthetic limb function can be enhanced using TMR and pattern-recognition techniques.
In support of this research on limb control using a neural interface system, the University of Pittsburgh Medical Center Rehabilitation Institute have recently worked on the biomedical application of such technology by presenting a patient case involving a paralyzed man being able to move a robotic arm with his thoughts:
In 2006, DARPA launched the Revolutionizing Prosthetics program that offers prosthetic arm solutions for warriors injured in the battlefield. A group of researchers helped develop two advanced anthropomorphic prosthetic arms, technology that has the benefit of improved motion range, control options and dexterity. DEKA Integrated Solutions Corporation, one of the participating teams, developed a Gen-3 Arm System for which 35 volunteer amputees provided design feedback. Following testing and refinement, a notification was submitted to FDA by DEKA for making the system commercially available.
The other team (also part of the research group investing in this initiative), Johns Hopkins University Applied Physics Lab developed an advanced arm system that yielded promising results on brain control. This work experimented on tetraplegic volunteers proved the efficiency of advanced prostheses on paralyzed victims. In addition, the program has also developed dexterous hand capabilities that are used in small robotic systems for handling unexploded ordnance.
Success of Upper-limb Prosthetics - Expert Opinion
A survey conducted by Schultz AE et al (2007) reviewed the experts’ opinions on success factors for upper-limb prostheses. It was observed that the opinions associated with the function and comfort of the unilateral amputees vary from that of the bilateral amputees. The average scores of function were higher than that of comfort and cosmesis for both the groups. Hence these opinions may reveal the assumptions on the needs of both unilateral and bilateral amputees.
However, the subcategories were analyzed for more specific results. Most of the respondents opted socket-interface comfort to bear the weight of the prosthetic arm. The results also suggested focusing on dexterity as a part of mechanical enhancement of the prosthetic arms instead of improving the power of the components. On the other hand, the results emphasized that it is necessary to focus on the shape of the prostheses than on the color.
Although the study examines cosmetics and its subcategories, function and comfort of the prostheses arms, it failed to discuss about other important factors such as durability, reliability, cost and maintenance. Hence, a more comprehensive study dealing with all these factors would reveal the differences in opinions of experts in various fields. Moreover, a number of results and responses were required to derive a detailed conclusion.
Sources and Further Reading
- Buffart LM, Roebroeck ME, Heijningen VG, Pesch JM. Evaluation of arm and prosthetic functioning in children with a congenital transverse reduction deficiency of the upper limb. Journal of Rehabilitation Medicine. 2007;39:379–386.
- Zhou P, Lowery M, Englehart KB, Huang H, Li G, Hargrove L, Dewald JPA, Kuiken TA. Decoding a New Neural–Machine Interface for Control of Artificial Limbs. Journal of Neurophysiology. 2007;98:2974–2982.
- Shultz AE, Baade SP, Kuiken TA. Expert opinions on success factors for upper-limb prostheses. Journal of Rehabilitation Research and Development. 2007;44:483–490.
- Revolutionizing Prosthetics - DARPA