The Evolution of Otto Bock Myoelectric Systems for the Pediatric Patient

Terry Sanderson, R.T.P. (c)


History

The first work done with myoelectrics was in the early 1940's by Reinhold Reiter, a physicist. His device was shown at an International Industrial Exhibition in 1948 [6]. The system operated by utilising a vacuum tube amplifier. This made the device rather large and non-portable, thus restricting the user to a fixed place [1]. A Russian development in i960, the first to use transistors, was portable and quite functional. The power source however was rather large and was required to be worn around the waist [1].. Research and development of myoelectric systems was carried out at various centres around the globe. In 1965, Robert Scott and Bill Sauter fit the first U.N.B. three state system at the previously known as the Ontario Crippled Children's Centre and now called the Bloorview MacMillan Rehab. Center. By the late 1960's, myoelectric components and prostheses were becoming routinely prescribed for adolescents or adults.

In the early 1970's, Dr. Rolf Sorbye from Sweden began fitting children, some as early as 18 months of age. This changed the early thinking that myoelectric prostheses should be restricted to adolescents and adults [6]. A study conducted in 1987 of Dr. Sorbye's work concluded that 83% of the patients fit in the early 1970's still used their prostheses. Recommendations from the study showed the best age for initial myoelectric fitting to be 2 1/2 to 4 years of age. Dr. Sorbye concluded by stating "it had been proven in their patient population that early myoelectric fittings strongly correlated with positive functional and psychological outcome" [5].

A 1985 report from the U.K. showed that 60% of the children fit with a myo-prosthesis wore them continuously and benefited from their use, 20% used them part-time and the remaining 20% did not wear their prostheses. From these results, the National Health Limb Fitting Service would continue support for child amputees [5].

A 1989 study conducted at the Hugh MacMillan Rehabilitation Centre surveyed 89 children ages 6 and under that had been fit with externally powered prostheses. Results showed 68.6% of the children polled used their devices full-time, 12.3% used their devices occasionally, and 19.1% rejected their prosthesis. Evidence showed that children fit under the age of 2 years of age were more likely to be full-time wearers of their prosthesis. Also, a myoelectric prosthesis was a standard option at this clinic for children [5].

The Institute for Rehabilitation and Research (T.l.R.R.) in Houston, Texas conducted a postal survey study in 1993. Initially, over 6,200 single page surveys were sent out to upper extremity patients via 104 various clinics, hospitals, prosthetists, support groups, and manufacturers. Over 2,400 surveys were completed and returned, and the results were entered into a database. Expanded 7 page surveys were mailed out, and 1,505 were completed and returned. Of this

number, 399 surveys were specific to myoelectric prosthesis users. 212 patients were children with 186 being adults [2].

Table 1. shows some key results from the study [4].

A notable conclusion drawn from the survey; "There is an emerging trend, from the single page survey, which suggests that children are being fit with myoelectrics more often than before" [3]. Also, "overall, children are occasional wearers and users of prostheses as opposed to all the time users, which is more common among adults" [4].

From the various studies conducted, one could describe the paediatric amputee population as having a high percentage of congenital cases with most of those children

being below elbow or wrist disarticulation amputees. With current clinical practice, paediatric amputees will be fit with some form of powered upper extremity device at a young age and will most likely become a full-time user or at least an occasional user of their device.

Past

The earliest myoelectric componentry offered by Otto Bock was a 12 volt system. This system offered modular type componentry for ease of assembly. Hand, cable, electrode, and battery connections were made by plugging the components into each other.

Being a 12 volt system, the energy source or battery pack was quite large. The battery was worn or attached to the patient's waist, chest pouch, or on a band around the humeral section. Wire breakage and electrical interference were common problems with early systems.

Structurally, the hand was made of aluminum, making it light in weight. The original finger and thumb segments had a flat profile which was easily bent or broken. An inner hand covered the mechanical components of the hand as well as providing the 4th and 5th fingers. The inner hand also provided shape for the cosmetic glove.

The 12 volt system originally provided 3 sizes; 7 3/4, 7 1/4, 6 3/4. The 7 3/4size , as a general rule, was used for men, the 7 1/4for women, and the 6 3/4 size could fit children from as early as the age of 8.

By the early 1970's, Otto Bock developed a 6 volt myoelectric system. The difference from the 12 volt was that the battery size was smaller and could be incorporated into the forearm of the prosthesis, providing better cosmesis. This also eliminated the problem of exterior battery placement.

The 6 3/4 size was made available in various configurations, a quick disconnect, a wrist disarticulation design, and a threaded stud design.

This size offered a small size and a good cosmesis with 18 cosmetic glove colours to choose from. This particular size however had a slower opening and closing speed compared to the adult size hand. The slower speed was due to a smaller drive unit and a different gearing system. Grip force was also reduced as the transmission assembly found in larger hands, did not fit this size of hand.

In the wrist disarticulation, (WD.) style the original 12 volt system had 4 screws connecting the hand chassis to the lamination ring. The hand was pre-positioned and the screws, when tightened, maintained this position. In the later version, the hand chassis and wrist unit were threaded and screwed together. A spacer ring allowed for pronation and supination. Also, in later designs of the W.D., the connector cable travelled through the centre of the wrist. This protects the cable from damage. In earlier designs, the cable travelled outside the wrist area.

In the late 1980's, after many years of research and development, Otto Bock introduced the 8E50=5 1/2 Electrohand 2000. This system was developed specifically for children. The size, 5 1/2 would fit patients between the ages of 3 and 6.

The Electro 2000 design featured a new finger/thumb relationship. This new concept allowed the fingers and thumb to rotate around the same axis, therefore simulating a more natural grasping motion. Small objects are easier to grasp, requiring less compensatory movement of the elbow and upper arm. The shape and positioning of the 4th and 5th finger do not obstruct objects when grasped.

Assembly of the Electrohand 2000 was done by connecting the threaded portion of the hand to the threaded lamination ring. The electrical connection was made by connecting the electrodes to cables then into a switching circuit. The development of new rectangular electrodes (13E125) are ideal for children's socket designs as they are 40% smaller than the original round electrode design. Smaller rubber pins secure the electrode into the socket, but also allowed for up to 2.5 mm of movement if needed due to volume change of the patient's residual limb. The battery was connected to the switching circuit by means of a cable. The battery was reduced from a 6 volt battery to a 4.8 volt battery. The battery, because of its smaller size, fits cosmetically into most forearm sizes for children. The switching circuit had a coaxial plug that was inserted into the hand through a hole in the lamination ring. The circuit, secured to the lamination ring by a plastic retaining ring, completed the electrical system.

Present

Advancement in materials, engineering and manufacturing techniques has allowed the System 2000 to evolve to its present state. Currently, there are four sizes available (8E51=5, 5 1/2 ,6, 6 1/2), fitting patients in age from 1 1/2 to 13 years. The newer design still maintains its unique prehensile characteristics, as well as some mechanical and electrical improvements from the 8E50 design.

The manufacturing process of the hand mechanism is different from other myoelectric hands currently available. Cylindrical lengths of special grade aluminium are used for the finger and thumb segments. The pieces are heated, then pressed in a mould to initially form their unique shape They are then machined and milled to further define their shape and parts for the mechanical function. A special plastic coating is then applied to the finger and thumb. This coating serves as a protector for the hand mechanism acting similar to the inner hand of the adult sizes.

The drive mechanism for sizes 5 1/2, 6, and 6 1/2 operates on a two motor system. These motors are at either end of the housing assembly. One motor provides movement (speed) in opening and closing. The second motor, once current draw is increased due to an object being grasped, is activated and provides the grip force. A combination of reduction units, planetary gears, and a break away clutch completes the drive assembly.

The electrical signal transfer has been changed as well. A series of contact rings on the switching circuit make contact with wires that are integrated into the wrist of the hand. This feature eliminates the coaxial plug of the earlier design and also reduces the overall length.

The Electrohand 2000 is secured to the prosthesis by screwing together a ring on the hand, ortto the threaded portion of the lamination ring. Passive pronation and supination is unrestricted because of the unique assembly of the threaded ring on the hand. Also, rotational friction between the hand and wrist can be varied by changing the number of rubber rings in the lamination ring.

The size 5 hand maintains the same finger/ thumb relationship of the larger sizes, however, the drive mechanism is uniquely different. This size operates with a single motor only. Two switching circuits are available for this size, giving the prosthetist an option of single or dual electrode sites. If a single site is selected, the hand is myoelectrically opened, and a spring mechanism closes the hand. The hand may also be manually opened by the patient or care giver. If dual electrode sites are prescribed, then the hand is myoelectrically opened and closed. If electrode sites are not found on the patient's residual limb, or the residual limb length does not permit electrode placement, it is possible to use pull or push switches with the System 2000. However, modification to the switch cable is required.

The switching circuits have been modified as well. The electronics within the circuit use a microprocessor that has improved function in the following ways:

  • reduced power consumption
  • the shift point between speed and grip is automatically adjusted with respect to the condition of the glove
  • wear of the gears is reduced by a lower cut-off level in the opening direction

Switch circuits have been designed for single or dual electrode sites, as well as for right and left hands. Table 2. shows switch circuit selection.

The power source is still maintained by the same 4.8 volt battery. An on/off switch has been

integrated into the battery.

The batteries are recharged by a unique new charger that operates on a pulse charging current. This type of charging improves battery efficiency by the use of LED. lights indicating to the patient that the battery is fully charged. When the battery is first inserted into the charger, the lights flash slow. When the battery increases in charge, the lights increase their flash rate. Thus, cycling of the battery is improved and longer battery life is achieved.

The changes, both mechanical and electronic previously described, are quite noticeable to the overall system. However, smaller upgrades within the hand are continuing, providing smoother function and less current consumption. For example, a new rubber gasket, sealing the connection point between the fingers and thumb, greatly assists in preventing moisture and dirt from entering the mechanics of the hand.

The contact springs now have a flexible housing that decreases fracture of this electrical contact point The contact springs are secured to the thumb with small plastic nails. Previously this piece was bonded and the adhesive did not always secure the plug, causing a loss in contact. Continued testing and self-evaluation of componentry and design is ongoing.

Future

What does the future hold for the System 2000? Is electric wrist rotation or proportional speed and grip force something to look forward to? Is the unique finger/thumb relationship going to expand into larger adult sizes? These are all possibilities. What would practitioners like to see? More importantly, what would the patient like to see from Otto Bock? Let us know.

This paper was previously presented at the Myoelectric Controls / Powered Prosthetics

References:
1. Childress D.S. "Historical Aspects of Powered Limb Prostheses", Clinical Prosthetics & Orthotics, Volume 9, Number 1,2-3, 1995.
2. Heard D.C.Y. et al., "Priorities For Improving Myoelectric Prostheses As Defined By The Child Users And Their Parents", UNB's Myoelectric Controls / Powered Prosthetics Course and Symposium, 114-120, 1994.
3. Heard D.C.Y. et al, "Year One Summary - The Next Generation Myoelectric Prosthesis". The Institute for Rehabilitation and Research, 1333 Moursund, Houston, Texas 77030., Sept. 1,1992 - Aug. 31,1993.
4. Heard D.C.Y. et al, "Priorities For Improving Myoelectric Prostheses As Defined By The Child And Their Parents", U.N.B.'s Myoelectric Controls/Powered Prosthetics Course and Symposium, Slide Presentation. 1994.
5. Hubbard S.A., Bush G. et al, "Myoelectric Prostheses For The Limb - Deficient Child". Physical Medicine and Rehabilitation Clinics of North America , 2 (4), 847-866, 1991.
6. Hugh MacMillan Rehabilitation Centre, Powered Upper Extremity Prosthetics Program. Forefront of Technology, Annual Review April 1994 - March 1995.
7. Otto Bock Orthopadische Industrie GmbH & Co, Post-fach 1260, D-37105. Duderstadt, Germany.
8. Otto Bock Austria. Ges. m.b.H.. Kaiserstrasse 39 A-1070 Vienna, Austria.