December 14, 2014

HCR Silicone

What do you think about when you consider the use of silicone in prosthetics fabrication? Do you think about the aesthetics of custom restoration, such as silicone gloves? Do you think about gel liners? Those are both correct, of course, but if you don’t also think about the use of silicone for fabricating custom interfaces and embedding component and structural elements, then you are missing new and exciting possibilities.

This article addresses high consistency rubber (HCR) silicone as a fabrication material in upper-limb prostheses and gives examples of creative uses, from partial-finger designs through shoulder disarticulation devices. Silicone can be used to customize fit, optimize function and range of motion (ROM), as well as provide increased comfort.

Properties of Silicone

Silicone has many advantages for patients. High surface adhesion is a natural property of silicone, which improves suction suspension and provides a secure linkage to the prosthesis. Silicone is extremely comfortable, which is especially advantageous for people who have scar management issues or sensitive skin, according to clinical reports. Even for people without such issues, the flexibility and elasticity of silicone can provide greater ROM and increase the number of hours they can wear a prosthesis during the day.

Can Anyone Fabricate Using HCR?

There is a learning curve when working with silicone. However, for a skilled prosthetist who frequently sees patients with upper-limb amputations, silicone can offer features that are not possible with other materials. It allows for creative solutions to complicated problems, such as designing task-specific oppositional devices, adding zippers to ease donning and doffing, embedding electronics and channels for wires, and custom coloring. Prosthetists who routinely work with silicone find techniques and designs that work for them and can repeat those techniques for different patients. As their arsenal of knowledge grows, they can also find new applications.

What Are the Disadvantages of Silicone?

Although silicone offers many unique properties, there are limitations to its use in prosthetics. It should only be used for mature and volumetrically stable residual limbs since it is very difficult to adjust for volume with silicone. Additionally, it is to be considered for lightand medium-level prosthetic use. If it is used for a patient who tends to put more strenuous wear on his or her prosthesis, durability will be an issue, and the prosthesis will require more frequent replacement. In this case, the area most commonly prone to excessive wear is the connection between the silicone and the frame. Designing the prosthesis so that the force is not applied through the attachment, adding carbon surrounding the silicone, or maximizing the surface area in contact can all help to enhance durability.


A custom silicone dorsal cable anchor system. Partial M-Fingers are a good body-powered solution for amputations at the proximal interphalangeal joint (PIP) level. In this photograph, silicone sockets surround the residual limb. The Partial M-Fingers are held on with suction between the silicone interface and the residual finger. There is a carbon fiber support system that stabilizes the interface against the pull between the joint and hand anchor system. PIP flexion causes the fingers to flex and grasp.

Regal thumb with custom HCR silicone socket. (Oppositional/passive/activity specific) For patients who want opposition and desire an aesthetic prosthesis, using a combination of parts from a Regal glove and integrating it with a silicone interface can provide a reasonable solution.

ABOVE RIGHT: Designs combining HCR silicone with a multiposition passive thumb can be used with either the Vincent Systems thumb or the Partial Hand Solutions M-Thumb. Although the pictured prosthesis has a zipper for donning, most hand configurations do not require it.

AT LEFT: This prosthesis uses a combination of Partial M-Fingers for digits two and three, and passive silicone extensions for digits four and five.
BELOW: Body-powered M-Fingers (carbon shown, also available in pigmented resin) consist of wrist-driven, cable-actuated mechanical fingers with or without a multiposition thumb. Displayed is a custom-made HCR silicone socket, a lightweight composite frame, and wrist joint. The cable controls are adjusted with a rapid-adjust cable tensioning system (BOA Technology, Denver, Colorado).

This powered finger prosthesis has a carbon fiber prepreg finger stabilization system and a custom HCR silicone interface with incorporated control system, power supply, and electronic wiring. Zippers incorporated in silicone allow easy donning/doffing and access to electronic components.A: A custom silicone socket with air expulsion valve fixed to a carbon fiber prepreg internal frame. The valve allows evacuation of air for donning and release of suction for doffing. The prepreg carbon fiber internal frame prevents silicone from buckling inward under loading conditions for secure linkage between the prosthesis and user, improving prosthetic function and comfort.
B: This infracondylar socket design for mid-forearm or longer residual limbs provides suction suspension and full elbow ROM.

This partial hand prosthesis uses a Vincent System thumb and second digit. It has a zipper for ease of donning and incorporates a carbon fiber prepreg frame. Electrodes are embedded in the silicone, and a battery box is concealed beneath the silicone, as are the on/off switch and wires. Additionally this example demonstrates that silicone can be pigmented in a variety of colors, including kelly green, if the patient desires.
A custom silicone socket is highly flexible and elastic. This provides secure suction suspension and unrestricted forearm rotation, improving patient function and comfort. This is a way to manage the wrist disarticulation patient who uses a custom silicone liner with a carbon fiber frame. There is excellent suspension provided through suction, and depending on the trim lines of the frame, pronation and supination can be retained due to the elasticity and flexibility of the silicone.

Using silicone as a liner for the frame of a shoulder disarticulation prosthesis softens the edges as the person goes through ROM and activities of daily living. Patients have commented on how much more comfortable it is to move around and bend over because the silicone acts as a soft liner to ease the rigid edges of the frame. That silicone is easy to clean and hygienic is an additional benefit.

AT LEFT: For a patient with a shorter transradial prosthesis, retaining ROM while achieving suspension can be especially challenging. With a silicone interface, the prosthetist can use the elasticity of the material to allow for muscle expansion anteriorly as well as room for the olecranon posteriorly if trim lines of the frame are strategically adjusted to allow for this. Supracondylar suspension can be used, and many practitioners are finding that a humeral sleeve can provide auxillary suspension, which is beneficial.

For individuals with transhumeral amputations, silicone provides the advantage of increased comfort and ROM over the shoulder. It allows the prosthetist to use the Sauter modification and expose the lateral deltoid with the prosthesis. Traditional laminated sockets gap over the superior aspect of the shoulder when the patient abducts, flexes, or extends his or her limb. However, a silicone interface moves with the patient and follows the natural contours of the shoulder, which also provides a more natural appearance under clothing.

Are There Special Casting Instructions?

The same principles apply to casting with silicone as they do with other casting materials. Joint position to maximize functionality is important but often overlooked. The impression should provide a good foundation from which to fabricate the prosthesis, and it is therefore important to consider how the end product will be used and which components will be needed before starting. For example, with a partial hand amputation where the thumb is intact and will be used for opposition, a relaxed, functionally opposed thumb is desirable. Conversely, with a partial hand amputation where the thumb has also been amputated, it is desirable to place the prosthetic thumb in a “thumbs-up” position so that it is out of the way of components. For impressions of amputations distal to the elbow (such as transradial amputations, wrist disarticulations, or partial hand amputations), I prefer Reynold’s Body Double Fast Set®, which is a life-casting silicone rubber that can be applied safely directly to the skin, according to the manufacturer. (Author’s note: For more information, The working time is 90 seconds, so it requires rapid application. This can seem overwhelming the first time a practitioner uses the product, but once he or she is accustomed to it, the process goes smoothly. Clipping the patient’s hair before applying the molding material makes the casting procedure more comfortable. Alginate can sometimes be used, but the residual limb cannot be seen while dipped into the alginate and usually the position of the joints is undesirable. For transhumeral amputations, a traditional plaster impression works well.

In conclusion, there are many benefits to using silicone as an interface material in prosthetics. A creative and skilled prosthetist can address many challenges routinely faced by patients. Advantages include increased comfort and wear time, better ROM, and better suspension compared to traditional materials.

Elaine N. Uellendahl, CP, is the owner of New Touch Prosthetics, Cave Creek, Arizona. She is a graduate of Northwestern University Prosthetics-Orthotics Center (NUPOC), Chicago, Illinois, where she later became a prosthetics instructor for 12 years. She has also done international work in prosthetics, written training manuals, and taught in Latin America.

Academy Society Spotlight is a presentation of clinical content by the Societies of the American Academy of Orthotists and Prosthetists in partnership with The O&P EDGE.


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