Shaping a Transradial Forearm Prosthesis

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By Justin Eitel

New technology usually involves a new production process that requires training for technicians. Fortunately, that's not the case with shaping a transradial forearm prosthesis. At Ottobock, we have been using the same process for about 40 years. The industry-wide process for shaping a transradial forearm has gone virtually unchanged because it provides excellent results whether the prosthesis is passive, body-powered, or myoelectric. That's pretty rare in an industry that is quick to adapt to any improvement in quality or efficiency, so most technicians appreciate a reliable process that stays within their skillset and comfort zone for an infrequent project like an upper-limb prosthesis.

Figure 1

Figure 1

This time-tested process allows us to create an anatomical outer shell that is capable of containing and concealing any mechanical or electronic components (Figure 1). Creating space for the components along with a natural-looking shape can be a challenge, but these steps will help you clear those hurdles. Better yet, it works whether the inner socket is thermoplastic or laminated.

Prepare the inner socket. Start by securing the thermoplastic inner socket in a laminating stand. Fill any voids with molding clay to ensure that all attachment plates and any electrodes are covered for smooth transitions (Figure 2).

Figure 2

Figure 2

Figure 3

Figure 3

Add a layer of separation. Pull a PVA bag or casting balloon over the inner socket and secure it to the suction pipe to prevent the foaming mixture from bonding to the inner socket.

Attach a ring of foam padding. Cut a 1/2-in. strip of 1-in. thick foam padding that is long enough to wrap around the bottom of the inner socket. Use double-sided tape to attach the foam strip about 1 in. from the trim line to ensure that the transition of the forearm blends into the inner socket; later it will also assist in removing the laminated forearm (Figure 3). Use high-density foam to minimize compression and to allow you to cut away the outer shell safely.

Figure 4

Figure 4

Figure 5

Figure 5

Attach a forearm-shaped cone. Using a 1mm or 2mm sheet of polyethylene, form a cone that fits snuggly around the foam ring. Make sure the cone extends at least the length prescribed by the practitioner and provides the desired circumference at the wrist. Adjust the angle of the cone to align with the natural progression of the inner socket, and then tape the cone in position to the mold. The foam strip will seal the bottom of the cavity formed by the cone (Figure 4).

Pour the foam mixture. You will need to use a medium-density foam to allow you to grind and shape the forearm later. To produce consistent results in the foam, Ottobock uses a one-to-one ratio mixture of Ottobock Pedilen® Rigid Foam 200 (617H12) and Hardener (617P21) to achieve the right density to begin shaping. Follow the instructions for blending the two-part foaming agent, and pour it into the cone. Be sure to have good ventilation while you do this.

Allow several minutes for the foam to cure. The lightweight foam will harden to a density similar to wood. Once the exothermic reaction cools, remove the polyethylene mold (Figure 5). Detach the PVA bag from the suction pipe, then carefully pull the Pedilen foam off the inner socket and discard the PVA bag.

Shape the Pedilen foam. When forming a lower-limb prosthesis, we tend to focus on making a lightweight, structurally sound socket that captures the correct alignment. When fabricating an upper-limb prosthesis, we shift our focus to cosmetics. Our goal is to create a prosthesis that reflects the anatomical shape of the limb. This step advances the project toward a natural-fitting device by creating space for components and a smooth transition from the inner socket to the outer shell, and to the attachment point for the hand and wrist unit.

Now you can unleash your artistic side. Use your aesthetically tuned eye to sculpt a forearm that replicates the size and shape of the patient's sound limb. When grinding the foam, let the circumferential measurements provided by the clinician be your guide. Keep in mind the minimum space required for electronic components.

With a rough-coarse sanding cone, start to create the contours of the forearm. Take measurements repeatedly as you approach the desired dimensions. Did the clinician order a round or oval wrist unit? Shape the distal forearm to transition to that standardized shape and size.

When you achieve the rough shape, switch to a medium-coarse sanding cone, and begin shaping the transitions from the battery and electrode placements. Apply light putty to create the transitions and fill in places where grinding removed too much foam. When you achieve the desired shape, switch to a fine sanding cone and smooth all the transitions.

Position electronic components, if needed. If a battery or wiring dummy is to be added, place it near the proximal trim lines of the forearm and the distal end of the inner socket. Following the natural contours of the inner socket will assist in blending these components.

The final location often depends on customer preference. For example, a patient using an older prosthetic hand might want an external placement for easy exchange of the battery, while a patient with a newer terminal device might want internal placement of a battery that uses an external charging port.

Figure 6

Figure 6

Either way, use a razor or rotary tool to carve out a space for the battery, typically on the medial aspect of the forearm. For internal placement, work on the inner surface of the foam. For external placement, carve into the outer surface and build up the surrounding area with light putty. By blending an external placement with the contour of the mold, it will be easier to conceal the battery within the forearm.

Identify placement of the wrist unit. This can be done two different ways. If you are working from a rough trial, transfer the alignment through a rod-and-dummy system to capture the alignment and length of the wrist unit. If you are working off measurements, follow the natural progression of the forearm, and mount the fabrication dummy for the wrist unit perpendicular to the line of progression. To achieve a perpendicular surface, plane the distal surface of the foam (Figure 6).

Laminate the outer shell. Once you achieve the desired length and shape, apply a PVA bag, lay up stockinette and any carbon fiber, and follow your usual lamination process. When the lamination cures, cut out and finish the outer shell and inner socket.

Figure 7

Figure 7

Install components. Expose the attachment holes, and remove any dummies from the inner socket and outer shell. Installation may include the following (Figure 7):

  • Mounting control units on the inner socket.
  • Bonding the wrist unit to the outer shell.
  • Completing placement of the electrode, battery housing, and wiring.
  • Connecting cables to the control units.
  • Connecting the terminal device and the battery.

Apply a cosmetic skin over the hand and forearm to complete a natural-looking prosthesis with an outer shell that provides connection between the inner socket and prosthetic hand.

Make this your standard process, and you'll get a consistent, quality outcome when fabricating transradial forearm prostheses.

Justin Eitel is the technical orthopedics lead for Ottobock US. He oversees all orthotic and prosthetic fabrication at the Ottobock technical center, Minneapolis, Minnesota.