Head of Medical Engineering, University of Applied Sciences Upper Austria
Press Conference “Application of first feeling prosthesis” Concordia Center, Vienna,
8th of June 2015
Arm and leg prostheses replace one or more functions of the biologic human limbs. Due to the fact that materials prostheses consist of are not biologic and their functions are limited, artificial limbs can only be a surrogate. Even in the best case they represent just a compromise. The gap, however, between healthy natural and artificial limbs is steadily decreasing so that modern high-tech-prostheses improve the individual’s quality of life which was never as good as today. Science in the field of prosthetics is relatively young. Further research is needed to gain a better understanding of the overall function of prostheses and the interaction with its user in everyday life. Many details determine the individual’s most appropriate prosthesis. A lot of them are yet to be discovered and so they are the main focus of the projects described below. At the end improved knowledge gained from research will hopefully lead to improved fitting of prostheses to persons who need them.
As the physiologic ankle-foot system is amazingly complex and the foot’s bottom represents the junction of body and floor, foot prostheses play an important role in the fitting process. Unfortunately their significance is frequently underestimated. Prosthetic feet have to be light in weight, but yet of high strength. They should offer a variety of functions such as automatically changing the alignment of the ankle to adapt to different terrains in walking and standing. Furthermore shock absorption in the early stance phase, energy return in the late stance phase and cosmetic are other important design considerations. The alignment of the foot in relation to the socket largely contributes to the overall function.
A prosthetic knee has to mimic the function of the physiologic knee while providing stability and safety. As the normal gait cycle is divided into two major phases, stance phase and swing phase, prosthetic knees can be classified into two distinct types: those that use mechanical control of the knee joint and those that use microcontroller to control the swing and/or stance phases of gait. While in mechanical knee prostheses the joint’s friction is typically controlled manually, by weight or joint-angle, microprocessor controlled prostheses use sensors (to measure, e.g. force, torque or tilt) that provide a variety of information so that the knee “knows” which gait phase it’s actually in. This allows adapting to different walking speeds, terrain and environmental conditions. Research in this field focuses on control strategies for the wearer’s needs, less weight, high strength, less maintenance and, last but not least, low cost.
Usually the prosthetic socket comprises a sort of silicon liner, a matching closure system and a load-bearing outer layer. These three components serve as the connection to the knee joint or prosthetic foot. The silicon liner, typically a flexible material, acts as thin protective layer rolled over the residual limb connecting it to the prosthesis. Many manufacturers point out that the sensitive skin is protected in this way because the airtight silicon liner reduces friction and pressure spots. That may be true, but we certainly must not conclude that there are no problems left. A lot of people wearing prostheses complain about sweating and skin irritation due to the airtight silicon liner. Research in this field could focus, e.g. on air permeable textile material reducing skin irritation. Wearer of prostheses would be more than grateful, if an alternative solution was found.
Upper extremity amputations include finger and partial hand amputations, wrist disarticulation, below elbow, elbow disarticulation, above elbow, and shoulder disarticulation. Three types of socket design determine the suspension: suction, bony lock, and harness, each type having advantages and disadvantages. Hand/Arm prostheses can be passive or active. Often complete passive hand/arm prostheses fulfil only cosmetic purposes not offering any functional benefit. Hooks can be light and efficient; however, they do not look natural. Active prostheses are either body-powered or battery-powered. Typically battery-powered prostheses use myoelectric signals (EMG) for their control. Research in this field focuses on both: to mimic the natural limb as closely as possible and to control it as intuitively as possible.
Prostheses Control Strategy
Myoelectric prostheses are controlled by voluntary muscle contraction. Two battery powered electrode units built into the socket pick up the electromyogram (EMG) created when muscles in the residual limb are contracting; the output of the two electrode units placed on the flexor and extensor muscles deliver control signals SCF(t) and SCE(t) proportional to muscle contraction. These signals turn a single joint’s motor into right and left rotation. A wide-spread method to control more motors consecutively, i.e. more joints with the same signal-pair SCF(t) and SCE(t) is the so-called “co-contraction”: when flexor and extensor muscle are contracting jerky and simultaneously a microcontroller routs the signal-pair SCF(t) and SCE(t) to a second joint’s motor disconnecting the first one. In theory the method can be extended to several joint’s motors, in practice it’s limited by an inacceptable expenditure of time. Research in this field focuses on sophisticated control strategies involving more than two electrode units to control several joints simultaneously.
Targeted Muscle Reinnervation (TMR)
Control based on co-contraction doesn’t allow controlling multiple degrees-of-freedom prostheses simultaneously. By transferring the residual nerves of amputated limbs to muscles out of use more than one signal-pair SCF(t) and SCE(t) can be obtained. Residual nerves delivering natural movement commands control multiple-degrees-of-freedom prostheses in a similar way as they controlled the former natural limbs. Research in this field focuses on sophisticated control strategies like pattern recognition methods using array electrode units to control a multiple degrees-of-freedom prostheses both intuitively and simultaneously.