Aimee E. Schultz

Neural Interfaces for Control of Upper Limb Prostheses: The State of the Art and Future Possibilities

Current treatment of upper limb amputation restores some degree of functional ability, but this ability falls far below the standard set by the natural arm. Although acceptance rates can be high when patients are highly motivated and receive proper training and care, current prostheses often fail to meet the daily needs of amputees and frequently are abandoned. Recent advancements in science and technology have led to promising methods of accessing neural information for communication or control. Researchers have explored invasive and noninvasive methods of connecting with muscles, nerves, or the brain to provide increased functionality for patients experiencing disease or injury, including amputation. These techniques offer hope of more natural and intuitive prosthesis control, and therefore increased quality of life for amputees. In this review, we discuss the current state of the art of neural interfaces, particularly those that may find application within the prosthetics field.

Sensory capacity of reinnervated skin after redirection of amputated upper limb nerves to the chest

Targeted reinnervation is a new neural-machine interface that has been developed to help improve the function of new-generation prosthetic limbs. Targeted reinnervation is a surgical procedure that takes the nerves that once innervated a severed limb and redirects them to proximal muscle and skin sites. The sensory afferents of the redirected nerves reinnervate the skin overlying the transfer site. This creates a sensory expression of the missing limb in the amputees reinnervated skin. When these individuals are touched on this reinnervated skin they feel as though they are being touched on their missing limb. Targeted reinnervation takes nerves that once served the hand, a skin region of high functional importance, and redirects them to less functionally relevant skin areas adjacent to the amputation site. In an effort to better understand the sensory capacity of the reinnervated target skin following this procedure, we examined grating orientation thresholds and point localization thresholds on two amputees who had undergone the targeted reinnervation surgery. Grating orientation thresholds and point localization thresholds were also measured on the contralateral normal skin of the targeted reinnervation amputees and on analogous sites in able-bodied controls. Grating orientation thresholds for the reinnervated skin of the targeted reinnervation amputees were found to be similar to normal ranges for both the amputees’ contralateral skin and also for the control population. Point localization thresholds for these amputees were found to be lower for their reinnervated skin than for their contralateral skin. Reinnervated point localization thresholds values were also lower in comparison to homologous chest sites on the control population. Mechanisms appear to be in place to maximize re-established touch input in targeted reinnervation amputees. It seems that sound sensory function is provided to the denervated skin of the residual limb when connected to afferent pathways once serving highly functionally relevant regions of the brain. This suggests that tactile interface devices could be used to give a physiologically appropriate sense of touch to a prosthetic limb, which would likely help with better functional utilization of the prosthetic device and possibly help to more effectively integrate the device with the users self-image.

Vibrotactile detection thresholds for chest skin of amputees following targeted reinnervation surgery

Recent advances in the design of prosthetic arms have helped upper limb amputees achieve greater levels of function. However, control of upper limb prostheses is limited by the lack of sensory feedback to the user. Targeted reinnervation, a novel surgical technique for amputees, offers the potential for returning this lost sensation. During targeted reinnervation surgery, truncated nerves are directed to reinnervate new muscle and skin sites. Contractions of reinnervated muscles generate electrical signals that are used to control prosthetic arms. In addition, stimulation of reinnervated skin is perceived on the missing limb. Vibration detection thresholds were measured at four frequencies on the reinnervated chest skin of three shoulder-level amputees following targeted reinnervation surgery. Thresholds were also measured on the contralateral chest and arm skin of these amputees, as well as on the chest and arm skin of a control population. Vibrations applied to reinnervated skin were perceived at various locations on the missing arm and hand. Thresholds for the reinnervated chest skin were generally within the range of values measured on the chests of the control population. For the two unilateral amputees, these thresholds were similar to measures on their contralateral chests, but greater than measures on their contralateral hands. Targeted reinnervation appears to result in near-normal vibration-detection ability with respect to the target tissue, suggesting the functional reinnervation of mechanoreceptors by the reinnervating afferents. The functional limb sensation following targeted reinnervation could be used to provide prosthesis users with a sense of touch.

Multifunctional Whisker Arrays for Distance Detection, Terrain Mapping, and Object Feature Extraction

Several species of animals use whiskers to accurately navigate and explore objects in the absence of vision. We have developed inexpensive arrays of artificial whiskers based on strain-gage and Flex Sensor technologies that can be used either in passive (“dragging”) mode, or in active (“whisking”) mode. In the present work we explore the range of functions that whisker arrays can serve on a rover. We demonstrate that when mounted on a rover, whisker arrays can (1) Detect obstacles and determine obstacle distance (2) Map terrain features (3) Determine ground and surface texture (4) Provide an estimate of rover speed (5) Identify “slip” of the rover wheels, and (6) Perform 3-dimensional extraction of object shape. We discuss the potential use of whisker arrays on planetary rovers and as an investigative tool for exploring the encoding of sensory information in the nervous system of animals.

Expert opinions on success factors for upper-limb prostheses

The goal of this study was to gather the opinions of prosthetics experts on the most important factors for the successful use of upper-limb (UL) prostheses, compare them with those of prosthesis users, and ultimately direct research efforts in this field. UL prosthetics experts were asked to compare the importance of the comfort, function, and cosmesis of a prosthetic device for a transhumeral amputee. Categories were subdivided into weight, socket-interface comfort, power, agility, color, and shape. The majority of those who responded viewed comfort as the most important factor for a unilateral amputee and considered socket-interface comfort to be more important than weight. Function was considered to be the most important factor for a bilateral amputee, with agility considered more important than power. Cosmesis was consistently reported as being less important than comfort and function, and shape was considered more important than color.

Toward the development of a neural interface for lower limb prosthesis control

Lower limb amputees form a large portion of the amputee population; however, current lower limb prostheses do not meet the needs of patients with high-level amputations who need to perform multi-joint coordinated movements. A critical missing element is an intuitive neural interface from which user intent can be determined. Surface EMG has been used as control source for upper limb prostheses for many years; for lower limb activities, however, the EMG is non-stationary and a new control strategy is required. This paper describes the work completed to date in developing a novel lower limb neural interface.

Targeted reinnervation in the transfemoral amputee: A preliminary study of surgical technique

Background: Lower limb amputation is a common and growing problem in the United States. Current prosthetic technology is insufficient for transfemoral amputees to safely control their prostheses for demanding exercise such as stair climbing. Using a technique called targeted reinnervation, intuitive control of prosthetic devices has been achieved for upper limb amputees. To bring this technique to transfemoral amputees, a comprehensive understanding of the location of motor and sensory nerves is required. Methods: Five lower limbs were dissected and the locations of motor points for 13 muscles of the thigh were documented, as was the location of the posterior femoral cutaneous nerve of the thigh. A transfemoral amputation was performed on one limb to demonstrate the targeted reinnervation procedure. The tibial and common peroneal divisions of the sciatic nerve were coapted to the motor points of the semimembranosus and biceps femoris, respectively. The posterior femoral cutaneous nerve was coapted in end-to-side fashion to the tibial nerve. Results: The average number of motor points per muscle were as follows: sartorius, 4.75; rectus femoris, 3.25; vastus lateralis, 4.5; vastus intermedius, 4.5; vastus medialis, 4; adductor brevis, 2.3; adductor longus, 3; adductor magnus, 2.7; gracilis, 3; semitendinosus, 1.5; semimembranosus, 2.5; biceps femoris long head, 2.75; and biceps femoris short head, 1. Conclusion: The results of this study indicate that targeted reinnervation is technically feasible in a transfemoral amputee.

Use of two-axis joystick for control of externally powered shoulder disarticulation prostheses

We explored a new method for simple and accurate control of shoulder movement for externally powered shoulder disarticulation prostheses with a two-axis joystick. We tested 10 subjects with intact shoulders and arms to determine the average amount of shoulder motion and force available to control an electronic input device. We then applied this information to two different input strategies to examine their effectiveness: (1) a traditional rocker potentiometer and a pair of force-sensing resistors and (2) a two-axis joystick. Three nondisabled subjects and two subjects with shoulder disarticulation amputations attempted to control an experimental externally powered shoulder using both control strategies. Two powered arms were tested, one with powered flexion/extension and humeral rotation and one with powered flexion/extension and adduction/abduction. Overwhelmingly, the subjects preferred the joystick control, because it was more intuitively linked with their shoulder movement. Additionally, two motions (one in each axis) could be controlled simultaneously. This pilot study provides valuable insight into an effective means of controlling high-level, externally powered prostheses with a two-axis joystick.

Pattern recognition control of multifunction myoelectric prostheses by patients with congenital transradial limb defects: a preliminary study.

Background: Electromyography (EMG) pattern recognition offers the potential for improved control of multifunction myoelectric prostheses. However, it is unclear whether this technology can be successfully used by congenital amputees. Objective: The purpose of this investigation was to assess the ability of congenital transradial amputees to control a virtual multifunction prosthesis using EMG pattern recognition and compare their performance to that of acquired amputees from a previous study. Study Design: Preliminary cross-sectional study. Methods: Four congenital transradial amputees trained and tested a linear discriminant analysis (LDA) classifier with four wrist movements, five hand movements, and a no-movement class. Subjects then tested the classifier in real time using a virtual arm. Results: Performance metrics for the residual limb were poorer than those with the intact limb (classification accuracy: 52.1%±15.0% vs. 93.2%±15.8%; motion-completion rate: 49.0%±23.0% vs. 84.0%±9.4%; motion-completion time: 2.05±0.75 s vs. 1.13±0.05 s, respectively). On average, performance with the residual limb by congenital amputees was reduced compared to that reported for acquired transradial amputees. However, one subject performed similarly to acquired amputees. Conclusions: Pattern recognition control may be a viable option for some congenital amputees. Further study is warranted to determine success factors.