Wrist Rotation Activation in Myoelectric Prosthetics -- An Innovative Approach: A Case Study

Eugene Banziger, CPO


Historically, external powered wristrotation control was achieved with the means of muscle contraction. This was achieved by utilising particular muscle contractions, e.g. strong or fast contractures or a combination of it with a muscle or musclegroups. There are other, less often utilised means of wristrotation activation, e.g. mechanical switch or stump activated. This paper will describe a new and innovative approach in the control of external powered wristrotator which was utilised and tested on a field trial basis. The control and activation of it is simply achieved by slight shoulder abduction or adduction and/or slight residual supination or pronation. It is a simple yet ever so effective alternative to provide the amputee with the needed function of wristrotation. This new approach may force us to revisit our thinking about the utilisation of external powered (electric) wrist rotators.

INTRODUCTION

RMG is a 45 year old male who sustained a left below elbow amputation 15 years ago due to crush injury. He, at that time, was employed in the construction industry. After amputation he presented a left below elbow residual limb of good length, (just over 1/2 of his forearm length), there is excellent soft tissue coverage and he is pain free. He is married and has two children.

Subsequently after his stump healed he was fitted with a conventional BE prosthesis consisting of double wall construction, WD400 wrist unit, 5XA and #7 hooks, NW figure 8 harness system and cable assembly. After the rehabilitation and training he returned to his hometown in the Interior of British Columbia where he continued working in the construction business. One year after his discharge he returned to the Rehab Center for a myoelectric assessment. Subsequently he was fitted with a myoelectric prosthesis with the primary purpose to enhance his casual and social life. That prosthesis consisted of a munster type socket design, Otto Bock componentry, such as hand, electrodes, quick disconnect wrist unit and a electric wrist rotator. The fittings went uneventful, as RMG is a well co-ordinated individual of good intelligence. The request for the myoelectric prosthesis was patient initiated, the choice of componentry was assessed and decided by the clinic team. About 6 months later the patient returned with the request to have the wristrotator removed, "as he was not really using it, and this may reduce the weight of the prosthesis". This request was agreed to and the device was taken out. He, after that, only returned for the occasional repairs and maintenance needs like new gloves, batteries for the myoelectric, and cable harness and hook needs for his conventional prosthesis. Over the years his professional life has taken him from construction worker to contracting, and 2 years ago he became the owner of a 250 bed hotel in his resort town. His activities therefore have changed, and in his managerial role he now exclusively wears his myoelectric prosthesis. He now only utilises his conventional prosthesis for special activities around the home and for certain sports. He, after all those years, is still wearing his original prosthetics and, as they may need to be replaced soon, he feels they are a part of him and he dislikes the thought of getting used to new devices.

DESIGN

In late summer of 1995 at the Myoelectric Symposium in Fredericton, New Brunswick a scientific paper was presented by Steve Dillon of Maryland Electro Limb on a new component which he recently designed to activate electric wrist rotation componentry, especially Otto Bock (I OS 12-6) types. The assembly (Picture 1. ) consists of two mercury-switches, similar in function to the ones in the thermostat in your home, a housing, a manual on /off switch and all necessary cables. It will replace the 13E165 and the older 13E103 Otto Bock electronic control unit which is positioned behind the wristrotation unit. It is 3/8 inch or 10 mm less in thickness than the Bock module, thus allowing for a slightly longer stump (Picture 2. ). It slides on to the wristrotator like the Otto Bock module. Wrist rotation will be activated by slight pro and supination. By this slight motion the mercury-switch will go from the off or neutral position, to one of the two on positions, In other terms, the result could be described as a "pronation-supination- amplifier". Slight abduction and adduction at the shoulder joint will also activate wrist rotation.

Fitting

As I expressed interest in this new product, Steve Dillon offered to me one of his wrist rotation activation modules for field testing. I felt that in order to objectively evaluate this component it should be used by a previous wristrotator user. I, in the past, experienced several rejections of wrist rotators and I wanted to select a good prosthetic user who would hopefully agree for a trial and provide me with good feedback. RMG was contacted and he agreed to participate in this endeavour. At his subsequent visit to us we installed an old wrist rotator we still had hanging around and the new wrist rotation activation unit. The installation took about 30 minutes, and after initial calibration (Picture 3. ) of the unit the patient was asked to try the prosthesis on to do some functional testing. He was well prepared as I explained the function and design of the unit prior to the fitting. After donning of the prosthesis the device is calibrated to the neutral position, (neutral shoulder and neutral forearm position).This position varies from patient to patient. RMG was able to activate the supination and pronation of his prosthesis instantly without difficulty (Picture 4. and Picture 5. ). He was able to do hand and wristrota-tion motion simultaneously, which he was never able to do in the past. All his motions were instantaneous and natural. He appeared very exited about the whole thing and he even mentioned that the additional weight of the wristrotator is not really noticeable. He left with the understanding that he would provide us with regular feed-back.

Conclusions

In this single case study it became apparent that wristrota-tors were possibly rejected for other reasons than simply weight. If a device cannot be utilised in a simple, efficient and natural way other negative characteristic are amplified. On the other hand, if a device can be activated in a simple and natural way, the patient will not notice the negative characteristics, and those are dampened. This may well compare as an ill fitting prosthesis appears to be heavier then a good fitting one. In general, prosthetic users who do not have access to a electric wrist rotator may choose to perform certain activities by prepositioning the terminal device with the sound hand. This may be necessary at times, but most amputees will prefer to utilise an unnatural motion, e.g. excessive shoulder abduction and adduction to achieve the same results. The reason for this is simply that it is instantaneous and "natural". I have been informed that concurrently 6 additional wrist rotation activators are being tested and with similar results. (Data as of April 1996) This has forced me to change my opinion on the utilisation of the application of wrist rotators for the uni-lateral BE amputee. It is my belief that certain standards are difficult to change, particulary if it adds additional expenses to a prosthesis, but I'd like to urge all practitioners to have a second look at wrist rotators activation with this new control unit. I also think that this device will find rather quick acceptance in bilateral amputees who are more likly to rely on powered wrist rotation. Above elbow amputees are another category of amputees to benefit from this device due to it's simplicity, as it does not require another electrode site or harness switch etc. Even more, this activation-tool can be extended to the adolescent upper extremity amputee or even child size prostheses users with Variety Wristrotation Systems.

This unique method of activation can also be utilised for the control of externally powered elbows and hands in special applications where other means of control sites are difficult or impossible to find. Let your imagination run wild!

At present time, the wristrotation controller is available in the United States through Liberty Technology; in Canada through Variety Ability Systems.

This paper was previous published in O&P Bussiness News, July l5. 1996.