Patient Population
In 2005, approximately 541,000 Americans were living with upper limb loss (Behrend et al. 2011). With 6,000-10,000 upper limb amputations each year (Malone et al. 1981), and slightly more than 15 congenital upper limb anomalies per 100,000 live births (Behrend et al. 2011), individuals will continue to be affected. Upper limb loss can be a life-altering condition. Without one, and especially two, functional upper limbs, routine tasks like buttoning shirts, tying shoe laces, and bathing can become exceedingly challenging and cumbersome. Furthermore, the cultural significance and constant visibility of the hand in daily life can unduly remind individuals of their deficiency (Morris 2008). This can lead to self-esteem issues, body-image concerns (Morris 2008), and general dissatisfaction with life (Østlie et al. 2011).
Upper extremity deficiencies have been categorized into different groups based on the location of limb loss. The commonly defined groups are the following: wrist disarticulation, transradial, elbow disarticulation, transhumeral, shoulder disarticulation, and forequarter. Among these groups, transradial limb loss is the most common congenital deficiency (Nelson 2011) and amputation level (Atkins et al. 1996). Upper limb loss is also categorized as unilateral and bilateral. Unilateral deficiencies affect only one limb, while bilateral deficiencies affect both.
Existing Solutions and Technology
Upper extremity prostheses can be classified into one of three categories: body-powered, externally-powered, and passive. Body-powered devices use the mechanical motion of residual limbs or other functional muscle groups as the energy input. Typically a cable or harness system that is strapped to the patient transmits the force to the region of interest. Externally-powered devices generally rely on an electrical system to stimulate movement of the device. Myoelectric prostheses are a subset of this category, using the electromyography (EMG) signal produced by residual or neighboring muscles, as the electrical input. Unlike the body-powered and externally-powered devices, passive devices do not provide much functional benefit to the user. Instead, they simply serve as sturdy attachments to the residual limb and often provide a cosmetic resemblance to a real hand. In all three cases the device securely attaches to the residual limb in a socket. Sockets are custom made for each patient and are made of carbon fiber or epoxy to limit weight. The prosthetic device fits tightly to the residual limb and often suspends off of the elbow condyles for transradial devices (Fryer & Michael 1992).
Passive Hand Design
The passive hand, a type of terminal device, is designed with a rubber covering called a cosmetic glove, which resembles the external features of a human hand. Cosmetic gloves can be manufactured as stock gloves, custom-production gloves, and custom-sculpted gloves. Stock gloves are produced from a common mold specialized for each patient based on hand size and skin tone; custom-production gloves match a closely similar existing mold to the shape of the patient's remaining hand; and a custom-sculpted glove is designed from the exact mold of the patient's remaining hand. The gloves are generally made of polyvinyl chloride (PVC), or silicone rubber (Fryer & Michael 1992).
Body-Powered Prosthesis Design
Terminal devices are the most distal elements in an upper extremity prosthesis and usually attempt to provide functionalities similar to those of a hand. Body-powered prostheses use active terminal devices, which primarily enable prehension, or grasping. The most common terminal device is the split hook. The design for the hook was first patented by David Dorrance in 1912 (Dorrance 1912) and continues to be one of the most functional terminal devices. Split hooks are generally made of stainless steel or aluminum alloy, and manufactured in a variety of shapes suitable for different grasping patterns, and special functions like mounting and locking instruments to the device. Terminal devices with grasping capability can also be designed to fit a cosmetic glove, thus combining the cosmetic appeal of passive devices and functional benefits of active devices. The Robin-Aids Soft Mechanical Hand and Sierra Voluntary Opening Hand are examples of such devices. Prosthetic users often have multiple terminal devices that they switch between depending on the specific task they are performing. Specialty terminal devices are available for tasks such as golfing, bike riding, and baseball. For body powered prostheses, tension applied to a control cable will either open or close the terminal device, depending on the rest state of the device. Generally, a harness is placed around the patient's opposite shoulder and shoulder movement causes tension in the cable. A patent for a cable-controlled prosthetic device was first filed in the US in 1857 by William Selpho (Selpho 1857). Since then the design has seen little change, with improvements mainly focusing on limiting device weight.
Terminal devices attach to the patient's limb via a wrist unit. An important function of the wrist unit is providing patients who have limited supination and pronation capabilities a means of adjusting the position of the terminal device with respect to the forearm. In addition, the wrist unit must provide friction to stabilize the position of the terminal device. While basic wrist units restrict patients to the supination angle the terminal device is initially locked into, more advanced wrist units permit a range of rotational movement through a source of constant friction. This locking mechanism is also used in wrist units that provide flexion. Supination/pronation and flexion movements can additionally be combined into one wrist unit with a ball-and-socket joint. For patients with proximal transradial limb or transhumeral limb differences, a hinge, which operates similar to the wrist unit, must be used to either enhance or enable flexion at the elbow joint (Fryer & Michael 1992).
Externally-Powered Prosthesis Design
In addition to sockets, terminal devices, wrist units, and other mechanical connections within the prosthetic limb, externally-powered devices also have actuators. Actuators are in essence motors, converting the energy from the external source into movement in the prosthetic device. Over the last few decades, the utility of different actuators in prosthetic devices has been evaluated. These include DC micromotors, servomotors, hydraulic actuators, piezoelectric actuators, rotaries, ultrasonic motors, metal alloys, and shape memory alloys. Although not actuators in the traditional sense, contractile polymer gel and rubber have been proposed to serve as "artificial muscles" that can stimulate movement. To produce the desired response in the prosthetic device, however, the actuator must be coupled to a mechanism that appropriately transfers the mechanical energy. Some of the mechanisms used are the multiple transmission mechanism, CT Arm 1, slider crank, shape metal alloy finger, and accommodation mechanism (Del Cura et al. 2003).
Myoelectric prostheses include sensors that detect nerve and EMG signals, and a microcontroller computer that translates the EMG signal into an electrical input for the actuator. Myoelectric prostheses often utilize EMG signals from muscle contractions in the residual limb to control opening and closing of the terminal device. The myoelectric prosthesis was first invented by Reihold Reiner in 1948 and was first used clinically in Moscow from 1957-60 (Scott 1992) (Singh et al. n.d.). Advancements in technology have optimized circuit design, reduced power usage and increased battery life. Additionally, changes in methods for receiving, amplifying, and transmitting the EMG signals from the residual limb have allowed for reduced weight, increased EMG electrode inputs, and better device control. A major disadvantage of myoelectric devices is greater weight compared to body powered devices. Myoelectric devices are also less durable and have higher associated repair costs. Recent advancements in terminal devices for myoelectric prostheses allow for individual finger movement, providing improved functionality. The Michelangelo hand from Otto Bock and the i-LIMB from Touch Bionics are two of these devices (Behrend et al. 2011). The i-LIMB Ultra, invented by David Gow, has individual finger and joint movement (Gow 1999). A major shortcoming of all the aforementioned prosthetic devices is that they do not provide sensory feedback to the user. Research in targeted re-innervation performed at the Rehabilitation Institute of Chicago has allowed for sensory feedback and more intuitive control of prostheses (Kuiken et al. 2007). Although there have been major advancements in myoelectric prostheses, due to cost, the majority of transradial myoelectric prostheses that are regularly used only have opening and closing functions. Developments in 3D printing technology have the potential to change the prosthetic industry, offering low cost and rapidly developed custom prosthetic solutions. Beginning in 2011, Richard Van As began developing a prosthesis with 3D printed parts called the Robohand (Van As 2013). The original Robohand, an open-source design, utilizes wrist motion to open and close mechanical fingers. The estimated parts cost for the Robohand is $500. Since then, Van As has continued development to extend the design to those who do not have wrist motion.
Rejection Rates and Design Challenges
The development of suitable upper extremity prostheses has been a challenging endeavor. Conferring 27 degrees of freedom for movement, the human hand is challenging to emulate (Elkoura & Singh 2003). Although there are a variety of devices available on the market today, many of them are unable to provide the same level of dexterity as the human hand, and users are generally dissatisfied with the available options (Atkins et al. 1996; Resnik et al. 2012; E. Biddiss & Chau 2007). In fact, studies over the last couple of decades have indicated that a large number of prosthesis users eventually abandon their devices (E. Biddiss & Chau 2007; E. A. Biddiss & Chau 2007a). It was found that mean rejection rates for body-powered and electric prostheses were 45% and 32%, respectively (E. A. Biddiss & Chau 2007a).
A variety of factors have been attributed to upper extremity prosthesis rejection. These include poor training (Carter et al. 1969; Herberts et al. 1980), adjustments, and fittings, inappropriate device selection, and poor understanding of device use and limitations (Dakpa & Heger 1997). Rejection rates were found to vary by amputation level: transradial (6%), wrist disarticulation (46%), transhumeral (57%), and shoulder disarticulation and forequarter (60%) (Wright et al. 1995; Sturup et al. 1988). Variation also exists among prosthesis type, with myoelectric devices, body-powered hooks, and passive devices having rejection rates of 39%, 53%, and 50%, respectively (E. A. Biddiss & Chau 2007b). Based on a multivariate analysis, the largest predictor of individuals accepting a prosthesis was found to be the fitting timeframe. Individuals who were fitted to a prosthetic device within 2 years of birth for a congenital limb absence and within 6 months for an amputation were 16 times more likely to accept a prosthesis than individuals fitted outside of that timeframe (Biddiss & Chau 2008). Additionally, cost was cited as an influential factor in prosthesis acceptance. One study reported that, among participants who did not use prostheses, 48% considered cost an influential factor in their decision (Biddiss et al. 2011). Body-powered devices may cost up to $10,000 and externally-powered devices may cost up to $75,000 (Resnik et al. 2012). Even more costly, terminal devices such as the i-LIMB can be up to $100,000 (Webster 2013). It is important to note that prostheses must be replaced every time the user outgrows the device. This significantly adds to the financial burden as prostheses are usually replaced at least every two years (Resnik et al. 2012). One study reported that 68% of amputees who rejected their prostheses would be willing to reconsider usage of another if improvements in technology were made at a reasonable cost (Biddiss et al. 2007). Another source of dissatisfaction, particularly among body-powered prostheses users, was discomfort from rubbing of the prosthesis against the skin (Biddiss et al. 2007). In light of levels of prosthesis rejection, consumer feedback was elicited to rank priorities for future designs. Reducing weight and cost were ranked high for both electric hands and body-powered hooks, and decreasing discomfort with the harness/straps was suggested specifically for body-powered hooks.
Need
Abandonment of prostheses for reasons other than lack of functional need is a source of concern. Potential issues include development of one-handedness, presence of residual limb and phantom pain, and limitations in strength, flexibility, endurance, and mobility (Resnik et al. 2012). In addition, compensation of the missing limb's function with the intact limb can lead to injuries and secondary health concerns (E. Biddiss & Chau 2007). Given the high levels of prostheses rejection and consumer dissatisfaction with current devices today, it is clear there is a compelling need for improved designs.
In 2005, approximately 541,000 Americans were living with upper limb loss (Behrend et al. 2011). With 6,000-10,000 upper limb amputations each year (Malone et al. 1981), and slightly more than 15 congenital upper limb anomalies per 100,000 live births (Behrend et al. 2011), individuals will continue to be affected. Upper limb loss can be a life-altering condition. Without one, and especially two, functional upper limbs, routine tasks like buttoning shirts, tying shoe laces, and bathing can become exceedingly challenging and cumbersome. Furthermore, the cultural significance and constant visibility of the hand in daily life can unduly remind individuals of their deficiency (Morris 2008). This can lead to self-esteem issues, body-image concerns (Morris 2008), and general dissatisfaction with life (Østlie et al. 2011).
Upper extremity deficiencies have been categorized into different groups based on the location of limb loss. The commonly defined groups are the following: wrist disarticulation, transradial, elbow disarticulation, transhumeral, shoulder disarticulation, and forequarter. Among these groups, transradial limb loss is the most common congenital deficiency (Nelson 2011) and amputation level (Atkins et al. 1996). Upper limb loss is also categorized as unilateral and bilateral. Unilateral deficiencies affect only one limb, while bilateral deficiencies affect both.
Existing Solutions and Technology
Upper extremity prostheses can be classified into one of three categories: body-powered, externally-powered, and passive. Body-powered devices use the mechanical motion of residual limbs or other functional muscle groups as the energy input. Typically a cable or harness system that is strapped to the patient transmits the force to the region of interest. Externally-powered devices generally rely on an electrical system to stimulate movement of the device. Myoelectric prostheses are a subset of this category, using the electromyography (EMG) signal produced by residual or neighboring muscles, as the electrical input. Unlike the body-powered and externally-powered devices, passive devices do not provide much functional benefit to the user. Instead, they simply serve as sturdy attachments to the residual limb and often provide a cosmetic resemblance to a real hand. In all three cases the device securely attaches to the residual limb in a socket. Sockets are custom made for each patient and are made of carbon fiber or epoxy to limit weight. The prosthetic device fits tightly to the residual limb and often suspends off of the elbow condyles for transradial devices (Fryer & Michael 1992).
Passive Hand Design
The passive hand, a type of terminal device, is designed with a rubber covering called a cosmetic glove, which resembles the external features of a human hand. Cosmetic gloves can be manufactured as stock gloves, custom-production gloves, and custom-sculpted gloves. Stock gloves are produced from a common mold specialized for each patient based on hand size and skin tone; custom-production gloves match a closely similar existing mold to the shape of the patient's remaining hand; and a custom-sculpted glove is designed from the exact mold of the patient's remaining hand. The gloves are generally made of polyvinyl chloride (PVC), or silicone rubber (Fryer & Michael 1992).
Body-Powered Prosthesis Design
Terminal devices are the most distal elements in an upper extremity prosthesis and usually attempt to provide functionalities similar to those of a hand. Body-powered prostheses use active terminal devices, which primarily enable prehension, or grasping. The most common terminal device is the split hook. The design for the hook was first patented by David Dorrance in 1912 (Dorrance 1912) and continues to be one of the most functional terminal devices. Split hooks are generally made of stainless steel or aluminum alloy, and manufactured in a variety of shapes suitable for different grasping patterns, and special functions like mounting and locking instruments to the device. Terminal devices with grasping capability can also be designed to fit a cosmetic glove, thus combining the cosmetic appeal of passive devices and functional benefits of active devices. The Robin-Aids Soft Mechanical Hand and Sierra Voluntary Opening Hand are examples of such devices. Prosthetic users often have multiple terminal devices that they switch between depending on the specific task they are performing. Specialty terminal devices are available for tasks such as golfing, bike riding, and baseball. For body powered prostheses, tension applied to a control cable will either open or close the terminal device, depending on the rest state of the device. Generally, a harness is placed around the patient's opposite shoulder and shoulder movement causes tension in the cable. A patent for a cable-controlled prosthetic device was first filed in the US in 1857 by William Selpho (Selpho 1857). Since then the design has seen little change, with improvements mainly focusing on limiting device weight.
Terminal devices attach to the patient's limb via a wrist unit. An important function of the wrist unit is providing patients who have limited supination and pronation capabilities a means of adjusting the position of the terminal device with respect to the forearm. In addition, the wrist unit must provide friction to stabilize the position of the terminal device. While basic wrist units restrict patients to the supination angle the terminal device is initially locked into, more advanced wrist units permit a range of rotational movement through a source of constant friction. This locking mechanism is also used in wrist units that provide flexion. Supination/pronation and flexion movements can additionally be combined into one wrist unit with a ball-and-socket joint. For patients with proximal transradial limb or transhumeral limb differences, a hinge, which operates similar to the wrist unit, must be used to either enhance or enable flexion at the elbow joint (Fryer & Michael 1992).
Externally-Powered Prosthesis Design
In addition to sockets, terminal devices, wrist units, and other mechanical connections within the prosthetic limb, externally-powered devices also have actuators. Actuators are in essence motors, converting the energy from the external source into movement in the prosthetic device. Over the last few decades, the utility of different actuators in prosthetic devices has been evaluated. These include DC micromotors, servomotors, hydraulic actuators, piezoelectric actuators, rotaries, ultrasonic motors, metal alloys, and shape memory alloys. Although not actuators in the traditional sense, contractile polymer gel and rubber have been proposed to serve as "artificial muscles" that can stimulate movement. To produce the desired response in the prosthetic device, however, the actuator must be coupled to a mechanism that appropriately transfers the mechanical energy. Some of the mechanisms used are the multiple transmission mechanism, CT Arm 1, slider crank, shape metal alloy finger, and accommodation mechanism (Del Cura et al. 2003).
Myoelectric prostheses include sensors that detect nerve and EMG signals, and a microcontroller computer that translates the EMG signal into an electrical input for the actuator. Myoelectric prostheses often utilize EMG signals from muscle contractions in the residual limb to control opening and closing of the terminal device. The myoelectric prosthesis was first invented by Reihold Reiner in 1948 and was first used clinically in Moscow from 1957-60 (Scott 1992) (Singh et al. n.d.). Advancements in technology have optimized circuit design, reduced power usage and increased battery life. Additionally, changes in methods for receiving, amplifying, and transmitting the EMG signals from the residual limb have allowed for reduced weight, increased EMG electrode inputs, and better device control. A major disadvantage of myoelectric devices is greater weight compared to body powered devices. Myoelectric devices are also less durable and have higher associated repair costs. Recent advancements in terminal devices for myoelectric prostheses allow for individual finger movement, providing improved functionality. The Michelangelo hand from Otto Bock and the i-LIMB from Touch Bionics are two of these devices (Behrend et al. 2011). The i-LIMB Ultra, invented by David Gow, has individual finger and joint movement (Gow 1999). A major shortcoming of all the aforementioned prosthetic devices is that they do not provide sensory feedback to the user. Research in targeted re-innervation performed at the Rehabilitation Institute of Chicago has allowed for sensory feedback and more intuitive control of prostheses (Kuiken et al. 2007). Although there have been major advancements in myoelectric prostheses, due to cost, the majority of transradial myoelectric prostheses that are regularly used only have opening and closing functions. Developments in 3D printing technology have the potential to change the prosthetic industry, offering low cost and rapidly developed custom prosthetic solutions. Beginning in 2011, Richard Van As began developing a prosthesis with 3D printed parts called the Robohand (Van As 2013). The original Robohand, an open-source design, utilizes wrist motion to open and close mechanical fingers. The estimated parts cost for the Robohand is $500. Since then, Van As has continued development to extend the design to those who do not have wrist motion.
Rejection Rates and Design Challenges
The development of suitable upper extremity prostheses has been a challenging endeavor. Conferring 27 degrees of freedom for movement, the human hand is challenging to emulate (Elkoura & Singh 2003). Although there are a variety of devices available on the market today, many of them are unable to provide the same level of dexterity as the human hand, and users are generally dissatisfied with the available options (Atkins et al. 1996; Resnik et al. 2012; E. Biddiss & Chau 2007). In fact, studies over the last couple of decades have indicated that a large number of prosthesis users eventually abandon their devices (E. Biddiss & Chau 2007; E. A. Biddiss & Chau 2007a). It was found that mean rejection rates for body-powered and electric prostheses were 45% and 32%, respectively (E. A. Biddiss & Chau 2007a).
A variety of factors have been attributed to upper extremity prosthesis rejection. These include poor training (Carter et al. 1969; Herberts et al. 1980), adjustments, and fittings, inappropriate device selection, and poor understanding of device use and limitations (Dakpa & Heger 1997). Rejection rates were found to vary by amputation level: transradial (6%), wrist disarticulation (46%), transhumeral (57%), and shoulder disarticulation and forequarter (60%) (Wright et al. 1995; Sturup et al. 1988). Variation also exists among prosthesis type, with myoelectric devices, body-powered hooks, and passive devices having rejection rates of 39%, 53%, and 50%, respectively (E. A. Biddiss & Chau 2007b). Based on a multivariate analysis, the largest predictor of individuals accepting a prosthesis was found to be the fitting timeframe. Individuals who were fitted to a prosthetic device within 2 years of birth for a congenital limb absence and within 6 months for an amputation were 16 times more likely to accept a prosthesis than individuals fitted outside of that timeframe (Biddiss & Chau 2008). Additionally, cost was cited as an influential factor in prosthesis acceptance. One study reported that, among participants who did not use prostheses, 48% considered cost an influential factor in their decision (Biddiss et al. 2011). Body-powered devices may cost up to $10,000 and externally-powered devices may cost up to $75,000 (Resnik et al. 2012). Even more costly, terminal devices such as the i-LIMB can be up to $100,000 (Webster 2013). It is important to note that prostheses must be replaced every time the user outgrows the device. This significantly adds to the financial burden as prostheses are usually replaced at least every two years (Resnik et al. 2012). One study reported that 68% of amputees who rejected their prostheses would be willing to reconsider usage of another if improvements in technology were made at a reasonable cost (Biddiss et al. 2007). Another source of dissatisfaction, particularly among body-powered prostheses users, was discomfort from rubbing of the prosthesis against the skin (Biddiss et al. 2007). In light of levels of prosthesis rejection, consumer feedback was elicited to rank priorities for future designs. Reducing weight and cost were ranked high for both electric hands and body-powered hooks, and decreasing discomfort with the harness/straps was suggested specifically for body-powered hooks.
Need
Abandonment of prostheses for reasons other than lack of functional need is a source of concern. Potential issues include development of one-handedness, presence of residual limb and phantom pain, and limitations in strength, flexibility, endurance, and mobility (Resnik et al. 2012). In addition, compensation of the missing limb's function with the intact limb can lead to injuries and secondary health concerns (E. Biddiss & Chau 2007). Given the high levels of prostheses rejection and consumer dissatisfaction with current devices today, it is clear there is a compelling need for improved designs.