Cognitive Engagement of the Prosthetic Hand
February 2020 Issue
CHALLENGES OF UPPER-LIMB PROSTHETIC ACCEPTANCE
Unlike the lower limb, upper-limb prosthetic management involves the additional challenges of active function and control of the components, operational comfort of the interface, maximizing sensory surface area, and movement of the residual limb. These needs are further complicated by the greater visibility of the prosthesis, which affects interpersonal societal acceptance and may require additional attention to the cosmetic appearance of the device. The prosthetist must prioritize and balance these factors during an extensive evaluation and discussion of the patient's individual needs.
In a previous survey, the five factors that influence 84 percent of acceptance are amputation level; relative advantage, or to what degree the prosthesis meets the functional expectation of the patient; relative comfort of the interface and harnessing; and the confidence of the prosthetist and availability of occupational therapy (OT), which, counterintuitively, are ranked ahead of cosmetic concerns (Figure 1).1
Most prosthetists see on average 2.3 patients with upper-limb loss per year so are uncomfortable with the fitting and adjustment of the upper-limb prosthesis.1 Unlike lower-limb prosthetics, clinicians have not developed confidence in an experiential model for the upper limb due to their limited exposure. The need for knowledgeable occupational therapists for upper-limb patients is widely accepted in European countries, but OT may be difficult in the United States due to reimbursement limitations. Finally, cosmetic concerns are listed as a factor, which does not necessarily indicate that the terminal device must be a prosthetic hand, but it must be acceptable to the prosthetic user within his or her social context and need for function.
These functional and personal factors conspire to keep estimated prosthetic acceptance rates to 79.6 percent for transradial, 57.8 percent for transhumeral, and 32.8 percent for shoulder disarticulation.1 Even with technological advantages in materials, microprocessor controls, control sensors, power sources, and mechanical design, acceptance rates remain fairly similar to the acceptance rates described in 1958, which were 75 percent for transradial, 61 percent for transhumeral, and 35 percent for shoulder disarticulation.2
As a result, one may question if technologic improvements have had much of an effect on the acceptance of prosthetic devices. Undoubtedly the individual users of the various prosthetic components would indicate that their functional and social needs are being met by their prostheses. It may be that these needs vary greatly from patient to patient and dynamically shift over the patient's lifetime. The overarching principle of acceptance may not just be the use of technology, but the degree to which it interfaces with the cognitive needs of the prosthetic user.
THE HAND IN HUMAN EVOLUTIONARY COGNITIVE DIFFERENTIATION
The hand's 26 bones, 29 joints, 123 ligaments, 34 muscles, 48 nerves, 30 arteries, and 23 degrees of freedom attest to the evolutionary importance of the hand in our development as humans.3
Not only does the hand play a primary functional role, but it also serves as an instrument of human communication to convey nuances of emotion, sensing and exploring objects, and driving cognitive learning. Current belief, based on anthropologic evidence, has shown that the modern hand derived from similar long-thumbed ancestors rather than present-day, short-thumb, hand-walking apes. It is thought that the hand actually played a significant role in human cognitive development and hyperplastic remodeling of the brain.3,4
An example of the brain-hand connection is evidenced in people who use their hands for repetitive or intricate movements in knitting, music, or art. They often exhibit different patterns of thinking,4 and may have organic and physiologic differences in the brain architecture as well.5 The hand's importance can be observed in the sensory and motor homunculus where one quarter of our neural cortex is dedicated to the hand, with the largest amount of neurons concentrated to the thumb.4
Our cognitive development has been further refined so that the hand developed direct monosynaptic neural connections with our facial expressions. This is why so many people make facial expressions and speak with their hands involuntarily—our face and hands are hardwired to the behavior of our brain. As social animals, we are equally sensitive to these gestures, which often serve subconsciously to convey our instant emotions and group integration. 6
THE HAND IS MORE THAN A TOOL
Chimpanzees and apes can grasp objects with their thumb and index finger. But what makes the human hand unique is the ability to touch all the fingertips with our thumb, made possible by the transverse arch of the hand and greater mobility of the carpometacarpal joint of the thumb in ulnar opposition. This enables the cupping and increased grip of irregular objects (such as opening a large pickle jar) with multiple prehension patterns.7 Some have speculated that the increased intrinsic strength of the hand also allowed it to be formed into a fist for evolutionary advantages.4,8
A study by Cusack, et al. showed that prosthesis users may cognitively regard the prosthesis as a tool rather than an integrated part of their body image as a hand. The use of a prosthesis may disrupt cognitive somatotopic processes of the mirror neuron system, or the "mechanism by which we understand, learn, and imitate the actions of others from our own perspective."10 Perhaps this is a major reason 28-72 percent of potential prosthetisis wearers reject their prostheses depending on amputation level and control method.9-12
The somatosensory mirror neuron can also be stimulated by mirror therapy wherein the prosthesis user mimics the movement of their intact hand with the movement of the prosthetic hand. The visual feedback of the mirror, even though it is an illusion, provides confirmation and imprinting on the cognitive aspects of the brain to increase unconscious tacit motor learning. This not only has benefits to control of the prosthesis but may increase the awareness of the phantom limb necessary for more advanced control systems.
Emulating this physiologic function may be directly related to the amount of prosthetic training the patient receives. Andrew Carter, a lawyer, writer, and transradial amputee developed his own training regimen: "There was a learning curve with my multiarticulating hand design because of its multiple thumb settings and the sudden increase in speed and accuracy from my previous prosthetic hand. I come from the school of thought that something needs to be practiced 1,000 times before you get it right, so I learned to use the new hand by using a ring stack toy, a set of stackable multicolored cups, and a foam rubber bowling set. At least once an hour, I would stop and play with my toys for five or ten minutes, and after about a month using my prosthesis was second nature."
Carter describes a process by which the somatosensory cortex imprints on the motor cortex, which lie next to each other in the brain (Figure 2). The OT and prosthetist provide instruction for this process through repetition to create motor learning that enables tacit or unconscious function of the prosthesis. When done successfully, the patient begins to exhibit what is referred to as kinesthetic projection or phenomenological osmosis where the prosthesis is incorporated into the patient's body image. This is enhanced with a functional visual feedback loop in which when the operation of the device renders a successful outcome the patient develops stronger ties to the prosthesis.
Have you ever stepped on an escalator or people mover that you did not notice was inoperable, but experienced the odd perception of movement? Your brain expected it to happen from the experience of your somatosensory cortex that had imprinted on your motor cortex, so when you stepped on the people mover it was supposed to move. When it didn't, you found you had cognitive paradox. Similarly, prosthetists and OTs work to create the illusion that the prosthesis is integrated as part of the patient's body image. If other feedback loops are provided, such as haptic (touch), proprioception (body position), and kinesthesia (body movement), then the goal of prosthetic embodiment may be realized. One test of proprioception and kinesthesia is to sit and close your eyes, then with the index finger of the right hand reach to touch the ring finger of the left hand held stationary. In many cases you will slightly miss the target especially if brain function is impaired by alcohol or another substance. However if you move both the index finger of the left hand to touch the ring finger of the right hand with both arms moving, you should succeed because the somatosensors in the shoulder and elbow are engaged in both arms.
The brain architecture varies from patient to patient depending on when the amputation occurred or if the limb absence is congenital. The homunculus, literally "little man," is the image the brain has of itself, and it "sees itself" as having an inordinately large face, lips, and hands. The amount of brain surface dedicated to each part of the body indicates how significant those areas are to human function.
Notably, in the sensory and motor homunculus, one quarter of our neural cortex is dedicated to the hand with the largest number of neurons concentrated to the thumb.4 It is easy to see how after amputation the neural connections remain and the phantom limb is present.
In many patients, within weeks or months of an amputation, the brain will reorganize the available space of the limb and may rededicate it to the face (Figure 3). Neuroscientists are finding that the physical architecture of the brain is far more flexible and plastic than previously believed. Experiments have shown activating areas of the face caused sympathetic feeling of the phantom limb in 42 percent of patients.10 With congenital limb deficiency, the brain's mapping may be different since no previous sensation of the physiologic limb remains. In people with congenital limb deficiencies, the area of the brain's sensory map appears to be smaller. However some of these patients report vestigial phantoms, which may indicate that the amputation occurred in utero due to amniotic band syndrome.
FUNCTIONALITY OF THE HUMAN HAND AS THE PROSTHETIC GOAL
The hand can move quickly to grasp objects at up to 4,000 mm/s, but often moves at 310 mm/s during normal use. The hand must be able to move quickly and to provide firm grasping.11 The hand can grasp objects at 21.5 lb.-f in palmar prehension and 23.2 lb.-f in lateral prehension.13,14 Generally we grasp objects at 5 lb.-f about 1,500 times on a daily bias.4,5 These two grasping patterns that require moving the thumb from palmar to lateral prehension represent a frequency of 50 and 33 percent respectively when picking up an object and 88 and 10 percent when holding an object.5
With multiple feedback loops, the hand also provides the role of one of our main external sensors for touch, pressure, thermal sensing, kinesthesia, and proprioception. This complex feedback network of kinesthesia and proprioception places an increased demand on our motor ability to process where the hand is spatially, as evidenced by the attempt to touch one's nose when the eyes are closed.
Because of the ability to move between six main types of grip patterns and various prehensile sub-classifications, the thumb is said to contribute to more than half of all functions of the hand.5 As a result, surgeons will often utilize a number of surgical alternatives to restore thumb function, such as webspace deepening, finger transplantation, or hallux transfer to maximize the number of prehension patterns.
Prosthetic design engineers have informally determined the main prosthetic hand specifications to be: 1) physiologic speed, 2) functional grip strength, 3) position and control, 4) sufficient grip rigidity, and 5) maximum number of grip options. One of the limiting factors of any hand design is that the patient uses visual feedback to provide positioning and control. Body-powered systems often provide a greater amount of proprioceptive kinesthetic feedback since a gross motor movement is utilized to measure movement. However, externally powered designs provide a greater amount of harness-free operation with greater rigidity and near physiologic grip strength.
The original myoelectric systems achieved a significant number of these functions, including speed, control, durability, and grip strength. With further development of multiarticular hand designs, function has evolved to include intuitive control, multiple degrees of freedom, articulated finger modes, near physiologic speed, sense of feeling, pressure feedback, and more. In many cases the mechanism has outpaced the ability and technology of the patient to control them.
FOSTERING THE PATIENT-PROSTHESIS SOCIAL CONNECTION
Bambi Lombardi, OTR/L, an occupational therapy specialist in upper limb at Hanger Clinic in Fort Smith, Arkansas, says, "My objective is to see a patient that can hold the hand any number of new ways such as crossed arms, clapping, or touching the shoulder of a friend." These psychosocial gestures are crucial, Lombardi adds: "When you watch the end user, you should be able to tell they no longer think of the prosthesis as a tool or a prosthetic device. It seems to be more truly incorporated into their body image."
Dale Feste, a transradial prosthesis user, agrees. "I did not realize how much my prosthetic devices were changing my interactions with people. The unexpected added benefit with the newer multiarticular designs was that it became an extension of me and not just a tool."
Carter says, "When there are quantum leaps in technology and the way I hold things, it enables me to take my integration skills to the next level. It's actually caused people who have known me for years to do double takes and forget which one of my hands is prosthetic."
DESIGNED FOR MORE THAN FUNCTION
Pattern recognition controls may help integrate prosthesis control by utilizing natural intuitive models. The myoelectric activity of the entire residual limb can be mapped and identified for various functional needs with eight pairs of electrodes. The resulting pattern of myoelectic activity emulates certain movements by tracking 80 different factors simultaneously that correspond with desired movements. This places the patient at the center of the control again, by using his or her own intuitive image of what the prosthetic control should be.
The implication is that the patient is no longer using a cognitive abstraction of opening and closing the hand by flexing and extending the wrist, but rather imagining opening and closing the image of the phantom hand itself. This use of the physiologic image applies to wrist and elbow function as well as desired grip patterns. Since there is less disruption of the intuitive operation of the hand, the patient adopts the controls more fully as a kinesthetic projection of his or her own movements.
The prosthesis no longer requires intermediary operations, but simply uses the muscles a physiologic hand and wrist would. The homunculus can embrace the imagery of the hand more readily and creates the cognitive engagement necessary for more complex hand and finger presentations. The patient does not need to isolate individual muscles but utilizes a matrix of the subtle signals that are presented by the individual residual limb. The patient really defines the configuration of contractions as the system learns of his or her movements.
When the physiologic functionality and relevance of the hand and its control is compared to prosthetic design, it is clear that one of the main goals is not only to restore functionality, but also to provide a device that can be fully integrated with the person. With increased cognitive engagement the prosthesis may provide greater social interaction in terms of language, expression, and communication and form the basis for greater integration and acceptance.
Gerald Stark, PhD, MSEM, CPO/L, FAAOP, is a senior clinical specialist at Ottobock Healthcare, Austin, Texas.
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