All Hands, Hooks, and Fiberglass on Deck: The Many Materials of Upper-limb Prosthetics

By Phil Stevens, MEd, CPO, FAAOP

In contemplating writing an article about the various materials that can be used in our profession, my thoughts turned to a recent international case with an atypical bilateral partial hand amputation presentation. Because this would be the patient's only chance to receive modern prosthetic intervention and management for years to come, my colleagues and I wanted to provide him with a number of different devices, each designed with different activities and usage environments in mind. In doing so, we drew from a range of components, techniques, and materials that included plaster, thermoplastics, laminations, fiberglass, 3D printing, liquid and high-definition silicones, wax, and leather. This experience underscores the variety of options available in the world of prosthetic rehabilitation.

Figure 1

Figure 1. (a, left) Ulnar view of the patient's right partial hand amputation during a Skype call. (b, right) Ulnar views of both extremities in maximum wrist flexion.

PREPARATION

While the balance of this article focuses on components and fabrication techniques, the details of the case warrant some mention. The patient lives in a remote region of Africa, over 8,500 miles from our office. Before any treatment, medical visa application, or fundraising for transportation could begin, we needed to determine his general prosthetic candidacy and start to develop several parallel treatment plans. Fortunately, modern technology enabled a remote evaluation. In this case, a Skype videoconference was held several months prior to the patient's arrival in the United States, which allowed a real-time assessment of his presentation.

Figure 2a

Figure 2a. The left digit in a closed position against the prosthetic thumb through wrist flexion.

The patient presented with unusual, bilateral partial hand amputations. The absence of any x-rays precluded an exact awareness of what bony elements were spared, but the patient was left with a single metacarpal on each hand, presumably from his thumbs (Figure 1a). Importantly, the remote evaluation allowed an appreciation of the patient's active range of motion at the wrist. Though he lacked wrist extension, he presented with normal mobility into wrist flexion (Figure 1b). Further dialog determined that the residual carpal bones (hereafter referred to as digits) were largely sensate and he could feel temperature and pressure through them. The patient was also able to demonstrate functional wrist flexor strength, suggesting that the residual digits could actively contribute to a prehensile grip. The insights gained during the remote evaluation enabled the development of preliminary treatment plans.

TWO-POSITION THUMB POSTS

Based on the presence of mobile, sensate residual digits, we determined that the patient's primary devices would be simple, durable opposition posts. Our plan to use the two-position APRL thumb to obtain this opposition was verified at the first face-to-face encounter. However, the unique nature of the residual limbs, with the digits articulating at the wrists rather than the metacarpals, required an atypical placement of the standard component. Manual positioning of the prosthetic thumbs proximal to the wrist joints verified the patient's ability to use wrist flexion to bring the residual digits against the prostheses to obtain a crude prehension characterized by both sensory feedback and active movement of the residual digits (Figure 2). The ability of the APRL thumb component to obtain two grip positions would permit the patient to grasp small and large objects.

Figure 2b

Figure 2b. The left digit in an open position, obtained through wrist extension.

PLASTER, DURAFLEX, AND FIBERGLASS

With this plan in place, the concept had to be tested in the form of dynamic evaluation prostheses. Simple plaster casts were used to produce positive plaster models. These were used to pull inner sockets of flexible plastic using 3mm Duraflex. Prior to committing to a laminated design, test frames were quickly fabricated by wrapping the flexible plastic with fiberglass and trimming the frames to the desired trim lines. This done, the base of the prosthetic thumbs could be epoxied to the fiberglass frames to verify functional, comfortable socket fit as well as optimal thumb placements for tip prehension (Figure 3).

Figure 3

Figure 3. (a, left)The patient demonstrates functional grip with the opposition prosthesis while writing. (b, right) Proper alignment of the thumb against the residual digit is verified.

CARBON FIBER, LAMINATE, AND LEATHER

Once the shape of the frames and the alignment of the thumbs were verified, the frames were replaced with definitive laminated frames. Given the patient's access to leatherwork after he returned home, leather covers that could be replaced as required were sewn for the opposition thumb posts (Figure 4).

Figure 4

Figure 4. Definitive two-position opposition post prostheses.

3D-PRINTED AESTHETIC SHEATH

The patient recognized the functional benefits associated with these devices, but was interested in a cosmetic element that could be worn situationally. Prototype covers, designed by Jeff Erenstone, CPO, were fabricated using 3D printing. The devices were designed to be worn over the dorsal surface of the prostheses, providing a more anthropomorphic shape that, once color matched, would draw less attention than the uncovered devices. Because these covers would come with functional compromises, they were intended to be removable to allow maximum function when cosmetic concerns were not present, such as in the home or with friends and family. While prototypes were printed and definitive forms established in CAD (Figure 5), technical challenges precluded final fabrication of the sheaths in the available time window.

Figure 5

Figure 5. (a, left) The 3D-printed prototype of a cosmetic sheath that could be worn over the dorsal surface of the opposition post prosthesis. (b, right) The CAD design of the definitive shape.

MOLDING SILICONE, WAX, AND CUSTOM SILICONE

In addition to the more traditional opposition posts described above, the patient also desired custom silicone restorations. These were fabricated in collaboration with anaplastologist Paul Tanner, CCA, MBA. A two-part liquid silicone was used to obtain negative impressions of the patient's residual hands. In the absence of a sound hand to duplicate, the same material was poured over the patient's brother's hands to provide a starting point for the desired shape of the silicone restorations.

In considering the available wrist flexion that would be used for the opposition posts described earlier, the decision was made to duplicate elements of this prehensile strategy in the silicone restorations. Observing that wrist flexion in the silicone test sockets was limited by the bunching of the silicone at the wrist crease, the decision was made to cut away the palmar aspect of the silicone at the wrist to allow full wrist motion (Figure 6).

Figure 6

Figure 6. Cutting away the palmar surface of the silicone at the wrist increases available wrist flexion.

Wax models of the patient's brother's hands were then modified to accommodate the anatomy of the patient's hands. More specifically, the thumbs had to be moved proximal to the wrist joints and into positions of opposition. In addition, the proximal phalanx of the second and third digits on each prosthesis were merged to allow enough space to contain the patient's residual digits. The result was a custom silicone restoration that allowed for functional prehension as wrist flexion brought the silicone fingers into opposition against the relocated silicone thumbs (Figure 7).

wrists in relative extension

Figure 7. Silicone restorations with the wrists in relative extension (a, above) and flexion (b, below), creating functional prehension (c, below bottom image). The patient's digits are housed in the proximal phalanges of the second and third digits in the restoration. Slits at the palmar surface of the wrist increase the available range of motion.

wrists in relative extension
wrists in relative extension

HOOKS, URETHANE ANKLE JOINTS, AND LAMINATIONS

In addition to the devices described to this point, the patient required something that would allow him to lift heavy objects and engage in heavy manual labor. This was accomplished in the form of bilateral handi-hook prostheses with a voluntary opening terminal device on the right and a voluntary closing terminal device on the left.

In contrast to the opposition posts, where the flexible inner sockets were trimmed distally to allow wrist flexion distal to the sockets themselves, these more robust devices encased the residual digits so their movements could be used to preposition the terminal devices in space. The flexible inner sockets were slit at the dorsal and palmar surfaces at the level of the wrist, and open along the ulnar aspect proximal to the wrists. As with the oppositional devices, evaluatory rigid frames were fabricated over the flexible plastic. Unlike the oppositional devices, the fiberglass was wrapped over the molding dummies for urethane Tamarack Flexure Joints at the radial aspect of the wrist joints, and was cured under vacuum to ensure adequately tight tolerances against the urethane joints.

Figure 8

Figure 8. Right handi-hook prosthesis in relative pronation (a, left) and supination (b, middle). The Tamarack joint at the wrist permitted active wrist flexion and extension as well as radial and ulnar deviation (c, right).

Figure 9

Figure 9. Laminated right, voluntary opening handihook with a Tamarack Flexure Joint at the wrist. A two-position opposition post on the left hand.

The addition of urethane joints at the wrists allowed the patient to use his residual wrist motion into flexion, extension, and both radial and ulnar deviation to preposition the prosthetic hooks in space. Coupled with his residual pronation and supination, the patient was able to position his hooks through a large functional workspace. With the fit and function of the socket shapes, control cables, and harnesses verified, the fiberglass frames could be replaced with definitive carbon fiber laminations (Figure 9).

SUMMARY

The range of devices provided to this case subject highlights the wealth of materials and fabrication techniques employed within the prosthetic field to obtain the desired objectives of practitioners and patients. Positive impressions can be obtained through traditional plaster casts or more detailed two-part liquid molding silicones. Flexible, adaptable sockets are routinely obtained with flexible plastics and quick-setting fiberglass tapes. Rigid, durable devices are often the product of metal components laminated into rigid carbon frames. Aesthetic solutions can be fabricated out of high-definition silicones when time and finances allow, with the developing potential of 3D-printed solutions when time and funding are in shorter supply. Collectively, practitioners have a broad range of material solutions and fabricating methods at their disposal to address the various needs of their patients.

Phil Stevens, MEd, CPO, FAAOP, is in clinical practice with Hanger Clinic, Salt Lake City. He can be reached at

Collaborators on this project include Jeff Erenstone, CPO, who can be reached at , and Paul Tanner, CCA, MBA, who can be reached at