Sockets for transradial myoelectric prostheses have not changed significantly during the past 30 years. The Otto Bock Muenster style socket (MyoBock, Otto Bock Healthcare US, Minneapolis, MN) was developed in the late 1960s by Fruzinski based on the original Muenster design of Hepp and Kuhn.1 This socket was designed to complement the newly available components of the time that allowed for self-contained, self-suspending myoelectric transradial prostheses. The Northwestern University socket was first introduced by Billock2 in 1972. Elements of these two socket designs represent the critical design elements of state-of-the-art transradial interface designs even today. Flexible thermoplastics have helped to improve the dynamics of these socket systems, but the fundamental socket design did not change significantly as a result of the more flexible materials. It should be noted that these flexible materials are nonelastic.
INFLUENCE OF ROLL-ON SILICONE SOCKETS AND SILICONE HAND PROSTHESES
During the past 10 years, roll-on silicone sockets gained favor in fitting some patients with myoelectric sockets.3,4 One benefit of silicone suction socket technology is that a true suction suspension can easily be achieved. Suction suspension is particularly advantageous for fitting the long transradial limb because forearm rotation can be preserved due to the low and flexible trimlines, obviating the need for supracondylar suspension, which blocks all physiological forearm rotation. With the introduction of snap electrodes by Motion Control Incorporated (Salt Lake City, UT), electrode contact with the skin was ensured because of the elastic properties of the liner to which the snaps are attached. Two drawbacks of myoelectric fittings using roll-on designs are the need for some type of locking mechanism and the nuisance of having to attach each of the electrodes separately as part of the donning process when snaps are used.
Concurrently, there has been a proliferation of manufacturers of high-definition silicone hands that offer excellent appearance and are also suspended by suction. These high-definition silicone hands, when fitted to long transradial and partial hand residual limbs, provide a suction silicone socket as a structural component of the hand prosthesis. Because the silicone socket is a structural component of the prosthesis, it does not require a means of locking it to the prosthesis as does the roll-on silicone socket design. These silicone hand prostheses are donned by lubricating the skin and then sliding into the prosthesis while working any air out by pressing the flexible walls to direct the air to the proximal trimline.
CUSTOM SILICONE TECHNOLOGY
With the introduction and commercial availability of custom silicone sockets (available through Otto Bock Custom Silicone Services, Toronto), the materials and fabrication methods are now available to produce sockets that combine many of the desirable features of the outlined silicone designs. Material thickness, stiffness, and color can be precisely controlled. It is also possible to incorporate hardware such as electrode mounts, screw attachments, zippers, and wrist mounts into the silicone during fabrication. To date, patients with partial hand, transcarpal, wrist disarticulation, long transradial, elbow disarticulation, and long transhumeral amputations have been fitted by the authors; however, this article focuses on applications below the elbow.
Two basic construction methods have been used for fitting long transradial, wrist disarticulation, and partial hand prostheses. The first method uses an all-silicone construction with the components molded into the silicone. The second method uses a hybrid construction with silicone for the interface attached to a plastic laminated frame. In either case, a silicone test socket is constructed to evaluate the fit, myoelectric sites, and component locations similar to conventional methods (Figure 1).
The all-silicone prosthesis provides a very flexible prosthesis with minimal bulk. Tear strength is increased in critical areas by incorporating fabric into the silicone during fabrication. Stiffness can be varied through the use of different shore ratings (a measure of material stiffness) of silicone, creating an intrinsic silicone frame based on the biomechanical requirements of the particular prosthesis.
The battery placement is determined by the length of the limb, the need for passive wrist rotation and the perceived benefit of an “internal” battery versus a removable battery. For wrist disarticulation amputees, when using the all-silicone construction, the Otto Bock transcarpal hand has been used. If an internal battery is desired, then two mounting plates, spaced apart, are used to create a cavity for the battery and charge plug assembly (Figure 2). This method does not provide passive prosthetic wrist rotation. Therefore, this method is best used for individuals who have good physiological forearm rotation, such as in most wrist disarticulation cases. Another method has been used when space is limited either by the presence of carpal bones or when fitting an amputee of short stature, where even a wrist disarticulation amputation requires the use of the transcarpal hand in its shortest configuration to achieve anatomical forearm length. This method attaches the wire basket of the hand mounting plate into the silicone. The Otto Bock battery box is also molded directly into the silicone. This method does not provide passive prosthetic wrist rotation, and it is more bulky at the mid-forearm because of the battery, but the overall length matches the sound side. When passive wrist rotation is desired, the transcarpal hand can be fitted with the Otto Bock quick disconnect mechanism, and the lamination collar can be molded into the silicone or laminate. The battery is again housed in a battery box molded into the silicone at the mid-forearm level.
Laminated rigid plastic has been used in combination with the custom silicone sockets when a more heavy-duty prosthesis is desired. The fabrication of the wrist connection is performed in the conventional manner. Attachment of the silicone to the laminated frame has been accomplished either by direct bonding to the laminate or by screw anchors that are formed into the silicone. The laminated struts can be terminated short or long, depending on the anticipated load the amputee will apply during use (Figure 3).
Batteries have been mounted in similar fashion to the all-silicone methods discussed above. One unique method has been used where a pocket was created within the silicone and a zipper was installed to provide access to the battery (Figure 4). A zipper has also been installed in the silicone to allow the very bulbous end of a severely scarred limb to be easily inserted and removed from the prosthesis without subjecting the skin to the shear forces associated with forcing the bulbous end through the narrow wrist section (Figure 5).
METHODS OF INTEGRATING ELECTRODES IN SILICONE
For externally powered devices, suction sockets have become the preferred suspension method in many cases. Early attempts at combining silicone with myoelectric control used simple cut-outs in the silicone where the electrode was located within the rigid frame or electrodes mounted in the silicone attached by wires to preamplifiers. These systems worked but were difficult for many amputees to manage, and some patients experienced window edema, where holes were cut in the liner for electrode contact. The relatively unprotected wires were also prone to electronic problems associated with remote preamplification yielding inconsistent results. As mentioned, snap electrodes offered some improvement but were not considered an ideal solution. Custom silicone construction allows electrodes, such as the Otto Bock design, to be formed directly into the silicone. However, this design does not seal the electrode to the silicone, allowing undesirable flow of air and fluids around the edges of the electrode. Recently, a new prototype electrode mount developed by Otto Bock has made full suction suspension possible using a standard Otto Bock electrode (Figure 6). These electrodes are identical to the 200 series electrodes but do not have any mounting extensions. They simply snap into the rectangular cutouts of laminated, thermoplastic, or custom silicone sockets. This new mounting system allows for suction to be maintained around the electrode. With the edges around the electrode sealed, previous problems of donning gels and sweat flowing behind the socket and causing corrosion and electrical failures have been resolved. The new mount allows for easy installation and removal and yields a low profile in the finished socket.
DONNING AND DOFFING
To provide a durable and simple suction system, valves are not installed in the distal sockets. A simple and effective method of releasing air upon insertion of the limb has been developed in which a thick nylon cord is draped down the socket wall and allowed to curl around the distal end of the socket. This creates an air channel allowing for evacuation of air from the socket as the limb displaces the air. Once the limb is fully inserted, the cord is easily pulled out and an air-tight suction suspension achieved. To remove the prosthesis, many of the patients have been able to slide a finger into the socket and then force the air pocket created by the finger to the distal end, thereby breaking the suction seal. Some newer amputees who have more sensitive limbs are unable to tolerate the pulling associated with this first method and have preferred to slide a thin corset stay into the arm, creating an air channel to the end of the socket, allowing easy removal.
PARTIAL HAND APPLICATIONS
Recent improvements in prosthetic components and materials have allowed marked improvement in the functional rehabilitation of partial hand amputees when using electric components. Manufacturers have produced smaller and lighter components, such as electrodes, switches, batteries, and programmable microprocessors, that better meet the requirements of prostheses for partial hand amputees where there is limited space for such components. In many partial hand cases, the sockets do not pass the wrist. This allows full range of motion at the wrist and produces lighter weight prostheses, leading to greater functionality and acceptance by the users.
The flexible and elastic qualities of silicone allow the socket to maintain suction and thereby provide secure suspension without crossing the wrist. Bony prominences, as often encountered when fitting partial hands, can be relieved through precise positioning of softer silicone.
Size reduction of electrical components has allowed for more natural appearing devices without the large bumps and bulges of previous systems. Some batteries are small enough to be placed inside the hand shell, completely eliminating any resulting bulge in the frame (Figure 7). Although not for an active adult user, Li-Polymer batteries by Liberating Technologies, Inc. (Holliston, MA) provide the smallest dimensions and lightest option presently available, but battery life may be sufficient only for light duty users.
Smaller size electrodes and preamplifiers have also allowed for a much smaller device at the residual hand. If there is movement of the skin in the socket, as there is sometimes in transcarpal levels depending on the weight of the object being lifted, a force sensitive resistor (FSR) is recommended for the control input. FSRs afford a good means of control at the partial hand level and add minimal thickness.5
Hands for transcarpal amputees are available from Otto Bock and are much shorter and lighter than previously available electric hands. These hands weigh about 1/3 as much as the larger version and save approximately 1.25" in length.
These new component options in combination with custom silicone socket technology allow the transcarpal amputee to be successfully fit with a more cosmetic and functional electric hand prosthesis than was possible previously. Continued development of smaller hand and finger mechanisms and improved fitting techniques hold promise to improve the functional prosthetic options for this previously underserved segment of the amputee population.
To date, all of the fittings using custom silicone socket technology in our series (n = 11) have been successful. There have been no rejections, no skin irritations, and only minor adjustments required. The socket adjustments were required because of shrinkage of the residual limb or volume increase because of weight gain. Three sockets required modification, with two of those refabricated due to shrinkage of the residual limb. Both patients requiring socket replacements were new amputees. The replacements were done 7 months and 4 months, respectively, after the initial delivery of the custom silicone socket. In the cases where the residual limb reduced in size, the patients reported discomfort or pain, red to purple skin upon removal of the prosthesis, and swelling. These problems would be expected in a total suction socket whenever residual limb volume reduces. These problems have been corrected by adding a urethane disc in the area of swelling. Volume increase caused by weight gain occurred in one patient and required refabrication of the prosthesis.
The longest follow-up using custom silicone construction in our series is 2 years. Material durability has proven to be acceptable for all patients fitted, with no significant damage reported.
Custom silicone socket technology has expanded and improved the options for long below-elbow amputees. It is the authors’ opinion that comfort and cosmetic appearance can be improved over previous constructions techniques by providing soft, flexible sockets with suction suspension. Function is improved when physiological motions can be preserved, such as forearm rotation and wrist motions, as is possible with the designs discussed. With continued improvement in function, natural appearance, control strategies, and miniaturization of all the component parts of myoelectric hands, key shortcomings of previous systems are being addressed. Continued improvements in these areas are needed, but clearly progress is being made.
The authors thank the technical staff of Custom Silicone Services at Otto Bock Canada. This dedicated team is constantly making improvements that benefit the authors’ patients. Special thanks to Sara Creighton, Wilson Cisneros, and Kyla Lau.
1.Hepp O, Kuhn GG.Upper Extremity Prostheses Prosthetics International, Proceedings of the Second International Prosthetics Course, Copenhagen, Denmark, July 30 to August 8, 1959. Copenhagen: Committee on Prostheses, Braces and Technical Aids, International Society for the Welfare of Cripples, 1960:133–181.
2.Billock JN. The Northwestern University supracondylar suspension technique for below elbow amputations. Orthot Prosthet
3.Daly W. Clinical application of roll-on sleeves for myoelectrically controlled transradial and transhumeral prostheses. J Prosthet Orthot
4.Salam Y. The use of silicone suspension sleeves with myoelectric fittings. J Prosthet Orthot
5.Rotter D, Ackman J.External powered fitting using touch pad sensors for the transcarpal level amputee. Paper presented at Association of Children’s Prosthetic-Orthotic Clinics Annual Meeting, St. Petersburg Beach, Florida, May 19–22, 1999.