Case Study: Fabricating a Custom Silicone Partial Hand Prosthesis

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By Ahmahn Peeples, CPO/L, ACSM-CPT, EIM

Fabricating a passive partial hand or finger prosthesis from silicone requires specialized equipment and techniques applied to a unique patient presentation, making it a challenge for O&P professionals to perform effectively. The goal of this project was to fit a congenital partial hand/finger patient with a low-cost device that would restore her lost lever arms to allow for simple opposition and a more natural grasp pattern.

 

One of the most important factors in achieving successful outcomes is getting a clear understanding of the patient's expectations. This case study describes how the functional needs of a patient with a partial hand amputation helped influence the design of her prosthesis. We will also show how innovative fabrication techniques are utilized to demonstrate possibilities for restoring function to this patient population.

 

Study

The patient is a 24-year-old woman who reported frustration with impairment caused by a congenital limb loss. She is an otherwise healthy individual with minor health concerns. She was referred to the clinic for a consultation and possible device options that could be fabricated for her unique presentation of the left, nondominant hand (Figures 1 and 2).

 

The patient presentation involves the left middle, ring, and pinky fingers at different levels. In addition, the patient has small tremors and limited range of motion (ROM) of the remaining digits. This was her first attempt at prosthetic intervention for her limb loss, and outcomes measurements were administered to assess her functional dexterity index deficit. The measurements used were Quick DASH and the Trinity Amputation and Prosthesis Experience Scales-Revised. It is important to note that the patient elected to have a revision surgery due to compromised bone integrity in the residual limb, which was essentially disintegrating. An unsuccessful attempt was made to stabilize the residual bone with hardware, which had to be removed, leaving the patient with even less function in the pinky finger. 

 

As she entered college and got a job as a stocker in a pet store, she found she had more public interaction and more activities of daily living, so she wanted to consider a prosthesis. Some of the difficulties she faced included holding objects such as her phone or tablet in one hand, typing, and an increase in social anxiety from the visibility of her limb loss. There is very little room between the digits, which made it a challenge to fit her with the mechanical devices that fit with a ring-like posting. The patient was given the option of a revision surgery to completely remove the residual limb to make a metacarpal disarticulation. This would create an opportunity to fit her more as we would someone with a partial hand absence than partial finger absence, with potentially more socket options and terminal device applications. This would also create more ROM of the terminal device to articulate, thus providing more functional grasp patterns. Due to the trauma from the previously failed revision and possibilities of complications without a guarantee of a successful outcome, the patient rejected further surgical intervention. Her father was also involved with her decision-making and wanted us to try to work with the current presentation. After a lengthy discussion and managing expectations for limitations of the available prosthetic interventions, it was determined that a silicone restoration would be a baseline and we would grow from there to improve overall function.

 

Design

Some important elements to consider for any prosthesis are to create a comfortable socket that suspends well, transfers anatomical motion into mechanical motion, and minimizes limitations to physiological ROM. Considering the patient's presentation, the obstacles to overcome included:

  • Preventing the prosthesis from detaching with either flexion or extension of the digits
  • Preventing the prosthesis from detaching when holding or manipulating an object
  • Good suspension
  • Ease of donning/doffing while also maintaining a secure fit
  • Flexibility of the material over the metacarpal without causing the material to tear or rip
  • Durability of the material
  • Compliance

One of the more difficult problems to address was having adequate surface area for proper suspension of the digits. The middle and ring finger are essentially just soft tissue, therefore unable to anchor to bone. Because of this patient's presentation, both a partial hand design and partial finger design were considered, one utilizing an open-palm design, the other a closed-palm design. With an open-palm design, there was a concern of stabilization when an extension force would be applied to the digit. A closed-palm design would offer more stabilization and a more secure fit but would lead to heat buildup and loss of tactile sensation.

A hybrid solution involved the design of a palmar strap connecting the middle, ring, and pinky splitting the thenar and hypothenar eminence. This palmar strap was constructed using one layer of heat cured rubber (HCR), which was a compromise between open- and closed-palm designs (Figures 3 and 4). This ensured that adequate suspension was maintained through extension forces and allowed for heat dissipation and tactile sensation when grasping objects.

Initially, the socket included the pinky to try to achieve a symmetrical grasp as seen in the test socket and first socket design (Figures 5 and 6). But it was determined that this design was too difficult to don and limited her pinky ROM. Although the pinky does have less strength and limited ROM, it still has more than the middle and ring finger, so we separated the pinky in the second socket design (Figures 7 and 8).

In regard to the material selection, an interface made with HCR silicone has less tackiness when it comes into contact with the patient's skin. Due to the patient's short residual limb, any perspiration would lead to slippage, so suction alone would not maintain proper suspension. Increasing the tackiness of the interface enhanced the suspension of the prosthesis in conjunction with the anchor around the wrist.

After a silicone impression was taken using Reynold's Body Double, the fabrication technique was as follows: Dental plaster was used to create a positive model for which the 35- and 70-durometer shore and HCR was applied over the mold. Pre-preg carbon was used on the first socket design to help stabilize the pinky and prevent contact pressure. This pre-preg add-on added to the limitation of her ROM and the silicone digit was later separated from the socket. The silicone itself was electric milled to blend the different colors and get out any air bubbles, then rolled onto the mold by Richard Shapcott, Cosmetic Restoration Specialist with the Hanger Upper Limb Program. The mold with silicone was kept in an oven at 60 degrees Celsius overnight.

The next objective was to make sure current ROM was maintained or enhanced by adding a device onto the patient's residuum. The middle and ring fingers needed to be pre-flexed to allow for additional grasp patterns to be archived. The silicone over the knuckles also had to be of a soft yet durable shore material. Sometimes openings are used over the knuckles, but this can lead to potential tears in the material so this was avoided at the time. In this case, 35 shore was used on the socket and 70 shore for the digits. The overall structure was approximately 1mm thick graduated to 2mm as it blended into the digits. The patient reported this was a comfortable design. She was able to don with the use of a moisturizing hand sanitizer that allowed her to push into the device or pull the device on like a glove. This will be monitored over the next one to two years to make sure the material does not lose its memory from repeatedly stretching open for donning. As you can see in Figures 7-9, small modifications were done to the second socket design to ensure a better fit.

The pinky digit was completely separated from the socket and maintained suspension with a suction fit. The overall material at the residual limb was thinned to allow for flexion at the knuckles and a proximal strap was used for a more secure fit and to prevent slippage.

An alternative to the push-in design used in this case includes a zipper or strap design. Both of these were considered, but we realized either would add more bulk, weight, and rigidity of the silicone material needed to fabricate the device. Also, this is not as aesthetic or low profile, which might add to the anxiety the patient has with wearing a device. The current wrist strap does help prevent the material from stretching out too much after repeated donning and doffing.

Another consideration for a successful prosthetic outcome was the position and design of the fingers themselves. Because of the patient's presentation and funding restrictions, there were limited options for finger design. The options for this patient were between using low- or high-durometer silicone. High durometer can be more durable but does not flex as easily, which might make it harder to gasp objects. Conversely, a material that is too soft would make it hard for the user to hold the device because there is not enough oppositional rigidity and the device might not be as durable. So a progressive design was utilized with the silicone blended from the socket, which was a softer durometer, to the fingers, which were a firmer durometer.

The digits were hand molded over the socket to get the correct position (hanging angle), length, and shape (Figures 3 and 4). The initial test socket fit was good, but the finger position and shape got distorted in shipping to our fabrication lab. Figure 10 shows our second attempt with a different shore silicone to get a better position and shape. Armature wire was considered to allow the patient to be able to flex the digits in different positions to be able to grasp different objects, but due to the length of her residual limb and length of the material, there was a concern that the wire might tear through the silicone and either destroy the device or harm the patient.

Conclusion

We were able to get the correct hanging angle, which looked more natural, a low profile, and provide adequate opposition for holding devices in the final silicone prosthesis (Figure 11). The patient can use the Velcro strap as needed, but the memory of the material will still be monitored over time to see if the elastic nature of the material holds its shape. The bigger goals of preventing overuse of the sound side and the intact fingers should decrease over time. Bimanual tasks became easier as the patient could wear the device in her new job at a veterinarian's office, managing animals of all kinds. Because she had a congenital limb absence, the patient reported the digits "felt weird, almost like they are in the way. It will take time to get used to them." Routine follow-ups will be done to monitor her progress and acceptance.

The use of custom silicone to design functional partial hand and finger prostheses can be challenging. It is important to make sure you can partner with a good, knowledgeable technician who can work with this material to ensure great outcomes. A key element to our successful outcome was the patient's willingness to try different designs and ask questions throughout the process, ensuring we heard her input every step of the way.

 

Ahmahn Peeples, LCPO, ACSM-CPT, EIM, is a clinician with more than 15 years of experience in patient care and education. He is a staff clinician and Outcomes Southwest Regional Champion with Hanger Clinic. He can be contacted at apeeples@hanger.com.

Author's note: Melissa Turncliff, CPO, was a contributing practitioner on this case study. She is currently a clinician in the New Mexico VA Health Care System in Albuquerque. She can be contacted at melissa.turncliff@gmail.com.

Resources

Uellendahl, E. 2014. There's a New Touch in Prosthetics: Have You Considered Silicone Lately? The O&P EDGE  13(12):30-37.

Dawson, G. and C. Lake. 2016. Fabrication of a Silicone Partial Hand Prosthesis. The American Academy of Orthotists & Prosthetists.

Stevens, P. 2018. "Updates on Prosthetic Rehabilitation Following Partial Hand Amputations." The O&P EDGE 17(9):44-52.

Imbinto, I., C. Peccia, and M. Controzzi, et al. 2016. Treatment of the partial hand amputation: an engineering perspective. IEEE Reviews in Biomedical Engineering 9:32-48.

Kuret, Z., H. Burger, G. Vidmar, and T. Maver. 2016. Impact of silicone prosthesis on hand function, grip power and grip-force tracking ability after finger amputation. Prosthetics and Orthotics International 40(6):744-750.

Kuret, Z., H. Burger, G. Vidmar, and T. Maver. 2019. Adjustment to finger amputation and silicone prosthetic use. Disability and Rehabilitation  41(11):1307-1312.