Modifying the Plaster Model of an AFO
April 2016 Issue
Modifying the plaster model of an AFO is not difficult, and since AFOs are the most common device fabricated in most O&P fabrication facilities, it is vital to do it right. I recommend a technique that works well with the following types of thermoplastic AFOs:
- Flexible AFOs or leaf spring AFOs for dorsiflexion assistance
- Rigid-ankle AFOs to block ankle movement for medial-lateral stabilization of the subtalar joint or control of forefoot abduction or adduction
- AFOs with simple ankle joints to allow or restrict ankle dorsiflexion or plantarflexion
PREP FOR SUCCESS
To make work easier, I follow these standard parameters before starting a project.
- Make sure you have a clean work area and your hand tools are clean and easy to access.
- Make time to familiarize yourself with the negative wrap, order form, and device you will be fabricating. This is especially helpful if you are working on several projects that day.
- Make sure you understand the practitioner's instructions and that all your questions have been answered.
Follow these steps to consistently produce AFOs that improve function and ensure comfort for the patient.
1. Prepare the negative wrap.
Even though it is recommended to ensure that the patient's limb is aligned to its corrected position during the casting procedure, the negative wrap can be altered before pouring the plaster model, if needed. Be aware that any cutting of and alteration to the negative wrap changes its shape and may create unforeseen complications during the fitting of the finished device.
A common adjustment to an AFO wrap is to allow for increased dorsiflexion or plantarflexion. To increase dorsiflexion, make a wedge-shaped cut on the anterior portion at ankle height and a straight cut on the posterior aspect. Flex the wrap into the wedge and fix it in the desired position with staples or plaster bandage. Do the opposite to increase plantarflexion.
2. Pour the plaster.
Before you fill the cast, keep in mind that a traditional plaster wrap needs some sort of parting agent. The most common parting agent is a soap-water mixture. Liberally coat the inside of the cast with the soapy mixture, making sure the entire surface is covered. Remove and discard any excess soap mixture.
Pour the model in a standard manner, using your favorite type of plaster. Don't forget to secure a metal pipe in the cast. This will make it easier for further processing.
3. Prepare the plaster model.
After the plaster has cured, carefully remove the negative wrap, using a flathead screwdriver to pull out all the staples. Use an indelible ink pencil to retrace bony landmarks and other areas that were marked on the negative wrap. These will be reference points during the modification process. Keep the indelible pencil close to retrace these areas as necessary.
Ridges and bulges in the plaster indicate that the plaster bandage altered the normal shape of the soft tissue. A good rule of thumb is to remove about half of the convexities surrounding a constriction and to fill the constricted concavities. Use a half-round Surform rasp to remove bulges and smooth the surface. Then use plaster to fill indentations and small surface imperfections caused by air entrapment.
4. Add heel height and extend
The plantar surface of the orthosis should fit the patient's foot and shoe, accommodating the heel height of the shoe without distorting the relative position of the AFO's plantarflexion or dorsiflexion. Ideally, the negative wrap was created with the patient bearing weight. In that case, the forefoot and heel should appear flat on the plantar surface of the model and should be in parallel planes with the difference between the forefoot and heel height. To achieve this, set up your positive model on a flat surface.
I prefer a smooth plastic surface to aid in the removal of the model after the plaster has cured. If you have the patient's approximate shoe size, add about one inch of freshly mixed plaster to the forefoot section. The plaster should be added from the apex of the first metatarsal head and the apex of the fifth metatarsal head and extend about one inch past the toes. I also add plaster to fill in and square the sides of the heel. This will make it easier to create a flat spot on the heel.
After the plaster cures, move the model with the extended foot to a horizontal modifying fixture or vise. Use the half-round Surform to create the desired foot shape and restore the original roundness of the heel, blending the new plaster to the original model. Most likely, you will see imperfections in the added plaster on the plantar surface of the original model. Mix up additional plaster and fill any voids, taking care to not distort the original arch area (Figures A and B).
5. Buildup at bony landmarks.
Next, drive tack nails into any marked bony prominences and pressure-sensitive areas. The most common areas are:
- Medial and lateral malleolus
- First and fifth metatarsal heads
- Base of the fifth metatarsal
- Posterior aspect of the calcaneus
Depending on leg deformities or practitioner preference, buildups can be between one-eighth inch and one-quarter inch. Measure to ensure the correct height.
For this example, I measured one-eighth inch on all areas that need buildups. Using a mixing bowl and spatula, mix up enough plaster to cover all the tacks on the model. Wait for the plaster to partially cure, since it is easier to apply thickened plaster to the model. Apply the plaster with the spatula. Using the half-round Surform, blend and feather the added plaster from the apex of the nail to the original model. You may have to repeat this step several times to get all the bony landmarks covered (Figure C).
6. Extend the medial and lateral
Because the leg narrows at the ankle, it would be hard to don and doff a finished AFO contoured exactly like the patient's leg. To aid in donning and doffing the brace, especially for patients with physical limitations, I straighten and extend the medial and lateral walls from the ankle to the toes and from the ankle to the top of the cast. This also creates a flat plane if you have to attach joint dummies to the model.
Position the plaster mold in the modifying fixture with the toes pointing up. Mix some plaster and use a spatula to apply a generous amount along the anterior edge of the dorsum of the foot from the apex of the medial malleolus to the metatarsal heads. Do the same on the lateral side. When the plaster has partially cured, shave off excess plaster from the apex of the malleolus to the anterior edge of the plaster model. Hold your Surform angled slightly inward to ensure that the added plaster does not flare out larger than the widest medial-lateral measurement of the cast.
Repeat this technique to create an easy-entry modification on the calf section of the AFO. Add more plaster from the narrowest part just proximal of the malleoli to the proximal edge of the plaster model (Figure D).
7. Smooth the surface.
Use a sanding screen and water to smooth out your model. Depending on the imperfections and blended areas, it might help to mix additional plaster and apply a thin layer by hand to fill any holes and dimples, followed by the wet sanding screen (Figure E).
Your model should now be ready for the thermoplastic application.
Alex Stein is a founding partner of Motion Unlimited, Minneapolis. Since studying material science and fabrication at Carl-Bosch-Schule in Germany, he has spent 20 years refining production of high-quality, custom orthotics and prosthetics.