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Electric Wrist Rotation in Proportional-Controlled Systems

Harold H. Sears, PhD
Julie Shaperman, MSPH, OTR

ABSTRACT

The purpose of this study was to define specifically the functional usage of electric wrist rotation (EWR), and assess its value to persons with an amputation wearing proportional-controlled prostheses. Key Points: Survey results of 32 persons with an amputation who were fitted with EWR included data on extent of usage, battery usage, specific tasks performed, and advantages and disadvantages. Conclusions: A strong majority of the wearers surveyed used the EWR effectively, and preferred the component to its absence. Surprisingly, both persons with unilateral and bilateral amputation used the EWR for active rotation and prepositioning. Use of the EWR correlated highly with the extent of usage of the prosthesis that is, those who use their prosthesis extensively also will tend to use electric wrist rotation.

Key Words: Wrist rotation; myoelectric; electric-hand prostheses; upper-limb amputees

Introduction

The natural functions of the wrist, i.e., pronation/supination and flexion, are among the most useful that have been lost by persons with an upper-extremity amputation, second only to prehension. Historically, designs for electric prostheses reported in the 1970s and 1980s incorporated wrist rotation in attempts to replace the multiple functions lost by persons with arm amputation.^1-6 By 1972, a commercially available, adult-sized component was available and could be integrated with an electric hand, at least in persons with mid-length transradial or higher level amputations.^7,8 A child-sized electric rotator has also been available since 1993.⁹

Although available with electric hands for over 25 years, use of EWR has been rarely reported in the literature. Notably, the evaluation of body-powered and electric prostheses published by Stein and Walley in 1983,¹⁰ reported positively on the increase in range of motion for users of electric wrist rotators.

The 1996 publication by Atkins, Heard, and Donovan,¹¹ summarizing a survey of 1,575 persons with upper-limb amputations, very strongly showed the need for wrist function, particularly rotation by both body-powered and electric prosthesis wearers. Although the actual number of electric wrist users among the 438 electric prosthesis wearers in the survey was not reported, it is significant that wrist rotation and flexion were both among the top ten "preferences for improvements" listed, and for body-powered transradial prosthesis wearers, wrist rotation was the most desired improvement listed. In other words, whether body-powered or electric components are used, the need for wrist rotation function is strongly felt by the amputee population.

We speculate that prescription and utilization of the available electric wrist components has been limited because of four perceived problems:

1. Additional length (6.7 cm., or 2.6 in., for the present component) is required in the forearm, which eliminates people with mid-length to long transradial amputation as candidates.
2. The electric wrist rotator adds weight (about 96 gms, or 3.4 oz, for the present component⁸).
3. Many prescribing physicians and prosthetists have considered the function of EWR unnecessary and unused by people with amputation, especially those with unilateral limb loss. The "conventional wisdom" could be summarized as, "the wearer can just as easily rotate the wrist with the sound hand or move his entire arm." This opinion is clearly based upon the assumptions that unilateral prosthesis wearers require wrist rotation only for prepositioning and that prosthesis wearers require wrist rotation only when the sound hand is free.
4. Myoelectric control of wrist rotation has not always been easily implemented because easily used control methods have not been available heretofore.

The introduction of a proportional controller in 1989 (Utah ProControl System from Motion Control, Inc.), which allows a simple switching method using muscle contraction, was designed to impact the last two above-listed problems. The control method is easier to learn and perform than previous control options, and allows proportional control of wrist rotation. When powered by a 9-V battery, greater speed of wrist rotation also is provided, compared to earlier 6-V batteries (optionally, the system can be configured with a 6-V, 9-V, or 12-V battery).

The survey sought to evaluate the actual usage of electric wrist rotator, including the specific functions, as reported by the wearers of proportional electric systems. Ratings of comfort, convenience, appearance, and function were gathered from each wearer in the survey. Functional areas included eating, kitchen tasks, communication tasks, hobbies, driving, etc. Activities using active rotation (i.e., turning of the object while performing a task), as distinct from passive rotation (i.e., prepositioning) were differentiated. The wearers also defined the advantages and disadvantages of the electric wrist specific to their own use.

Method

Prosthetists contacted persons with amputations fitted with a ProControl System, which included an Otto Bock electric wrist rotatorb, and obtained permission to submit their name and telephone number for the survey. An occupational therapist familiar with prosthesis usage, the second author, interviewed each of them by telephone using a structured interview format, designed to thoroughly record all of the uses of the electric wrist, as well as the wearer's qualitative rating of the prosthesis function, comfort, convenience, and appearance. The interviewer was not a member of the staff of Motion Control, Inc. or Otto Bock Orthopedic Industry, and encouraged participants to report freely their likes and dislikes about their prostheses.

Results

Survey Participants: Thirty-six individuals were contacted, and 32 responded to the survey. Among the four not included, two had recently been fitted with the prosthesis and had not worn the limb beyond the fitting process at the time of the survey. Two others declined to participate when called. The participants included 30 males and two females, aged 20 to 75 years (mean age = 41 years). Amputation level and side are shown in Table 1 .

Occupational information also was collected. Six were unemployed and four were students. The average age of the 22 employed respondents was 46 years (24 to 62), and the jobs they held included farmer, restaurant manager, administrator, banker, computer technician, bartender, paramedic, printer, heavy equipment operator, and construction supervisor. The average age of the students was 28 years (20 to 31), and all four had unilateral transradial amputations. The average age among those not employed was 43 years (29 to 75), and included two men of retirement age and three who had bilateral amputations.

Description of the Present Prosthesis, Wearing and Use: The respondents described their present and previous prostheses, time worn, and use patterns. Use information included estimated percentage of waking hours worn, estimated percent of time using the prosthesis, primary purpose (i.e., for grasp or other purpose), and whether it was primarily for specific tasks or for cosmetic purposes. The primary prehensor used by 25 respondents was the myoelectric hand. Six used the Greifer predominately and one used the hand and Greifer equally. Wearers estimated the percentage of waking hours they wore the prosthesis (Table 2) . On average, the respondents reported wearing their prosthesis 65% of their waking hours. The average was slightly higher (71%) for participants with bilateral amputation. Respondents were asked to estimate the percentage of the time they actually used the prosthesis for task performance (Table 3) . Their estimates averaged 44% of the time for the group as a whole, with slightly lower frequency (39%) for participants with unilateral amputations and higher (57%) for people with bilateral amputation. Respondents were also asked whether their prosthesis was used primarily to grasp and hold objects, to push and pull (without prehension), or for cosmesis. Responses are grouped by amputee type in Table 4 .

Comparisons with Previous and Concurrent Prostheses

Respondents based their opinions of the present prosthesis with wrist rotator on their experiences with their previous and/or concurrent prosthesis. Eighteen of the 32 respondents (56%) no longer wore the previous prosthesis; ten (31%) had worn digitally controlled myoelectric hands; and eight (25%) had worn body-powered prostheses. Nine of the 32 respondents still wore the previous prosthesis part time, including one who wore a digitally controlled hand to work, six who still wore a body-powered prosthesis part of the time, one who wore an electric hook on one side of bilateral prostheses, and one who had a passive, cosmetic hand. Nine of the 32 respondents who wore another prosthesis concurrently include six who wore it more than 50% of the time and three who wore a body-powered hook occasionally. The overall average wearing time for a concurrent prosthesis worn by the nine participants was 15% of the time. Twenty-six respondents were able to compare the ProControl prosthesis and electric wrist rotator with their previous or concurrent prosthesis. The other six had no basis for comparison since one had a passive, cosmetic hand and five had no previous or concurrent prosthesis. Comparison prostheses are shown in Table 5 .

When comparing the ProControl prosthesis with a previous prosthesis, the 26 respondents rated five features: (1) convenience (ease of use, donning and doffing, maintenance), (2) weight (the key comfort criteria), (3) appearance, (4) function, and (5) frequency of use of the electric wrist rotator. Ratings were on a Likert Scale of "much better, better, same, worse, much worse." Ratings of the 26 respondents are in Table 6 .

Respondents indicated which of the features were most advantageous and least advantageous to them. All 32 participants were asked to respond to this question, since no comparison was needed. Twenty-seven responded to this question and 22 identified disadvantages. Some selected more than one feature, ranking them "first" and "second." Responses are in Table 7 .

Changes in Prosthesis Use

Respondents were asked, "Considering the tasks you perform using your prosthesis, how has the way you use your prosthesis changed since receiving the ProControl with electric wrist?" Responses can be summarized as follows (number of responses = n):

1. "I have greater comfort than with a harness and can reach an increased work space." (n = 5)
2. "I look more normal when using this limb." (n = 4)
3. "I have better grip on objects." (n = 3)
4. "It's easier to operate, therefore I use less energy." (n = 2)
5. "I have better control over speed of operation and size of opening of the hand." (n = 2)
6. "The wrist rotator gives a more normal appearing function and I need less ‘body English' to perform tasks." (n = 3)

Maintenance and Repairs

Respondents used one of three battery types: a 6-V system, a 9-V nickel cadmium, or 9-V disposable battery systemc. Some used more than one type of battery. Table 8 is a summary of their reports on battery life for the battery used most of the time (primary battery) and the alternate type (secondary battery). The responses to the questions on battery life were estimates and, as such, varied quite widely. The respondents were especially vague about battery life for their secondary battery; as reflected in the wide range of reported battery usage. The 9-V alkaline disposable batteries were the most popular batteries to carry as spares and for travel when carrying a charger was inconvenient. Two respondents reported that they recharge the disposable batteries (although this is not advised by manufacturers since it is a fire hazard). To emphasize their convenience, one respondent carries one, literally, "in his hat." Those who used a 9-V system described greatly improved speed of operation and voiced strong preferences for it.

Twelve of the respondents to the survey reported that they had never had any malfunctions or breakdowns of the prosthesis. The remaining 20 respondents reported an average of 1.7 breakdowns per person (range 1 to 5) over the total wear period (average 25 months, range 2 to 85 months). These data do not include socket problems or structural breakage of the hand or Greifer. Electronic problems comprised the majority of prosthesis malfunctions. Only two respondents reported the need for repair of the electric wrist rotator.

When asked about the cost of the prosthesis, six respondents said cost might be a problem in the future. Only one respondent said that cost of the prosthesis would prevent getting another similar limb, although most of the respondents volunteered that their prosthesis was funded through worker's compensation and other employer insurance.

Use of the Electric Wrist Rotator

Respondents rated frequency of wrist rotator use on a four-level scale. Fifteen (47%) used it constantly, nine (28%) used it occasionally, five (16%) used it rarely, and three (9%) did not use it at the time of the survey. Respondents were asked to identify tasks they performed using the electric wrist rotator. The interviewer also mentioned specific tasks often requiring wrist rotation and asked if the respondent used the electric wrist rotator for these tasks. These tasks included specific functions in eating, dressing, food preparation, hobbies, desk/office use, driving, and shopping. The interviewer prompted the subject to describe how he or she performed the task and to clarify whether the wrist was prepositioned or used actively to turn an object while performing the task. On average, respondents identified seven tasks involving prepositioning (0 to 18 tasks) and an average of two tasks involving active rotation during task performance (0 to 10 tasks).

Table 9 lists the most frequently identified tasks and the number of respondents who used the electric wrist rotator for prepositioning or active rotation while performing the task. These included cutting food with knife and fork, opening a bag, pushing a cart at the market, holding a wallet to remove bills, passing a plate or tray, holding a menu, and holding the steering wheel of a car. People with bilateral amputation mentioned additional tasks such as use of the button hook, keeping spoon or fork level while eating, writing, managing coins, and using the telephone. A very frequently mentioned use of the electric wrist rotator for people with both unilateral and bilateral amputation was making fine adjustments of the wrist position after they grasp the object so that the object is in the optimum position for performing the task. Clearly, those who used the electric wrist rotator for making fine adjustments in wrist position had learned to quickly and smoothly switch modes between use of the terminal device and the wrist.

The surveys were rich in detail, encouraged by the open-ended format of the survey. Two respondents described ways they amuse others by using the wrist rotator to make the terminal device twirl and thus perform tricks! One respondent with a shop makes models and uses the rotator to turn small parts to examine them while in preparation.

Comparing results of participants with bilateral amputation with those with unilateral amputation is also very revealing and contradicts the "conventional wisdom" cited in the introduction, which presumes that people with unilateral amputation would not use EWR. Noticing "percentage of waking hours worn" and "functional usage as a percent of prosthesis wearing time," the usage by some people with unilateral amputation was clearly of a very high level that is, six of the nine wearers surveyed who used their prosthesis over 50% of their wearing time are unilateral (Table 3) . Also, 10 of 14 wearers who reported wearing time over 75% of waking hours had a unilateral amputation (Table 2) . Considering also the tasks mentioned, those with unilateral amputation revealed frequent usage, especially when the sound hand was occupied, and could not preposition the terminal device. Active wrist rotation was well represented also, including eating, dressing tasks, handling paper money, paper and envelopes, etc., as pictured in Figure 1 , Figure 2 , Figure 3 , and Figure 4 . Other tasks that illustrate the function of the wrist rotator include dressing (Figure 5 and Figure 6 ) and opening doors (Figure 7) .

Driving tasks were mentioned by unilateral wearers, who frequently used active wrist rotation to change radio or heater controls, while steering with the sound hand. Table/Kitchen tasks were also performed with the use of active rotation. Prepositioning without requiring the sound hand (for those with unilateral amputations), or other methods (for bilaterals) seemed to be very useful in tasks such as reaching for doors. Figure 2 shows a person with a bilateral amputation drinking from a glass. Those with a unilateral amputation did not report drinking, but would use active rotation for pouring from a bottle to a glass or to reposition a utensil. Since eating is often a public task, the motive mentioned, "a more normal appearing function," was also important to some.

Preferences for Future Prostheses

The 32 respondents were asked, "If you needed a new prosthesis now, would you want to continue to use the ProControl?" and "If you needed a new prosthesis now, would you want another electric wrist rotator?" A large majority chose to stay with their present equipment. As one would expect, a significant relationship was found between the number of tasks performed using the electric wrist rotator and those who want their next prosthesis to include the electric wrist rotator (p less than or equal to 0.01). Similarly, reported use of the prosthesis 50% of the time or more strongly correlated with those who would want the ProControl on their next prosthesis (p less than or equal to 0.05) (Table 10) . When asked why they chose to stay with their present prescription, 14 of the 32 respondents mentioned the functional advantages of the electric wrist rotator as an important factor in their decision. The same number cited the control over the amount of grip and speed of opening/closing, and proportional control for rotating the wrist. Specific factors important in choosing to continue with the same type of prosthesis included:

• Greater speed with a 9-V battery
• Convenience of the disposable battery
• Natural appearance during function and knowing that they have the best limb available (each factor was mentioned by three or four respondents).

Negative features mentioned by current wearers included inability to determine whether the hand or wrist would operate when they next activated the system (mentioned by 10 respondents). Another way they described the same problem was to ask for an on/off switch for the wrist rotator so they could prevent its operation during hand function (six respondents). Several respondents related humorous and/or embarrassing experiences that occurred when they intended the hand to open, but the wrist turned instead. The other problem frequently mentioned was weight of the prosthesis (seven respondents). Less frequently mentioned negative features included fitting problems, bulkiness, inconvenience, and dissatisfaction with glove color. The problems mentioned sometimes related to the prosthesis as a whole rather than specifically the ProControl system or the electric wrist rotator, again reflecting the open-ended nature of the survey format.

At the end of the interview, respondents were asked what they wished they could have in a future prosthesis. Their wish list included lighter weight, wrist flexion, finger dexterity, greater versatility in grasp patterns, touch feedback, less bulk, improved appearance, and longer-lived batteries.

Discussion and Conclusion

In this study group respondents clearly preferred the electric wrist rotator, and proportional control. A previous survey indicated the preference for proportional control.¹² Examination of the usage patterns of those preferring wrist rotation clearly demonstrated a correlation between active use of the prosthesis and active use of the electric wrist rotator. Thus, a conclusion that can be drawn from this survey is that electric wrist rotation will be used by many people with unilateral and bilateral amputation if made available to them with a control mode they can conveniently use. Not surprisingly, those using the function of the electric wrist rotator are those who also use the other functions of the electric prosthesis, i.e., those who desire and need maximum function use all the functions available to them. Significantly, desire for function for these wearers overrides the relative inconvenience of additional weight, battery charging and replacement, higher cost, and training.

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