The Art of Mold Modification

Content provided by The O&P EDGE
Current Issue - Free Subscription - Free eNewsletter - Advertise
Tony Wickman

For five weeks this summer, we were lucky enough to have an intern from one of the O&P technical education programs working with us. We like working with interns because it gives us a chance to see the relative effectiveness of the various technical education program curricula and because we get some free labor! Another benefit of having an intern, which is often overlooked, is the learning experience for the teacher. Every time I teach somebody something, it helps me as well. Transferring knowledge and skill requires me to organize my thoughts in a way that is specific and digestible. More importantly, it makes me think about why I do what I do. Every time I try to teach someone something, I end up with a better understanding of my own job.

One of the areas we isolated for our summer intern as a “task of focus” was mold modification. Some technical education programs don’t put much focus on this subject because historically it has been the domain of the practitioner (more so in prosthetics than orthotics). This is changing rapidly, however. As practitioners become more involved in patient management and central fabrication becomes more prevalent, that old dividing line is shifting more toward the technician. Some have argued that this is a bad thing, using the rationale that since the practitioner consulted directly with the patient, the practitioner will know best what needs to be done to the mold. I can certainly see that point, but it is hard to expect a practitioner who has never been trained to work with his or her hands to make an artful mold. So I don’t think the answer is to have the practitioner modify the mold, but instead for the practitioner and technician to better communicate the desired results.

Of course it isn’t much more reasonable to expect technicians to make an artful mold when they haven’t been trained to do it. When I wanted to learn how to modify a mold, I sought out the best guy I could find and did everything I could to emulate the way he did it. He was an artist, and he taught me some fundamental rules about how to work with plaster. He made the nicest, strongest, and most attractive molds I had ever seen, and the technique is easy.

Rule #1: Smoothing the Mold

The first rule I learned was that the mold should be filed smooth before any plaster is added. Virtually all of the material you remove should be removed in the beginning. When you file a cast, look at the horizon, not the part of the mold that is facing up. By looking at the horizon, you can easily see the line you are filing. The contrast of the cast with the floor allows you to see a straight line much better than trying to look at the part of the cast that is facing up.

Also, always hold your file in a way that allows it to span across the high spots. Holding the file parallel to the cast (proximal to distal) allows the file to remove only the high spots. If you see file marks in the “valleys,” you are exacerbating the roping! Move the file in a spiral pattern and remember, pull a file for precision and push it for power.

Rule #2: Building Relief

Another fundamental rule is that half of the “relief ” you make on a mold comes from compressing the tissue surrounding a bony prominence; the other half comes from building up on the prominence itself. If you try to relieve a bony prominence just by building it up, you won’t effectively control motion and the prominence will still contact the plastic whenever the patient shifts in the device.

Rule #3: Saturate the Mold

Here is where the controversy starts.

Once the mold has been smoothed with a file and all the tissue compression is complete, it should be saturated. Most people work their molds with some moisture still present, and some people actually go out of their way to dry a mold before adding plaster. My way might sound crazy, but hear me out.

Have you ever noticed when you are trying to wet sand plaster, sometimes your buildups are much harder than the original material? This makes getting a smooth transition between the buildups and the original material difficult because the original plaster is softer than the buildups. This means the original material will sand away much faster than your buildups. The reason this happens is because when the buildup material is applied to a dry cast, the dry cast wicks the moisture out of the buildup, thereby increasing the plaster-to-water ratio. The higher the ratio of plaster to water, the harder the plaster will be. Subsequently, your buildups will be harder than the original material.

The method I use to bring these two materials into balance is to saturate the original mold in water. If the original mold is saturated, it won’t wick moisture from the buildup material. If you add material to a mold and you notice the buildup instantly becomes more dense, the mold isn’t saturated enough. If you use this technique, you don’t have to put vermiculite in your mold and your transitions will be as smooth as you want. This has the added benefit of keeping your buildups from falling off. Over the years I have seen some really crazy techniques for making buildups stick—adding Elmer’s glue, painting the mold with resin, and using plaster bandages for buildups. All of these may work on some level, but none of them will allow for a smooth transition and they all add a layer of avoidable effort.

With this process, you should allow the buildups to harden completely before final filing and sanding begins. You could argue that this takes more time, but it’s linear time, not “man hours.” It’s a bit like arguing that that it takes too long for a CAD carver to cut a mold. As long as it doesn’t increase the amount of work you have to do, it is irrelevant.

Exceptions Are the Rule

Over the years, we have spent a lot of time analyzing the process of modification, developing “algorithms” to help us understand what to do and when. Still, there are more exceptions than rules. Gone are the days of simply applying our standard matrix of modifications to every mold that comes in the door. You may use 1/4 inch on the medial malleoli, and 3/16 inch on the lateral for a starting point, but it is a guideline, not a rule. You must have an understanding of anatomy and pathology. You have to be able to imagine how the device is going to react to the patient’s inputs, and you have to be able to understand how the device will react to contact with the floor.

The next critical lesson is to keep it anatomical. The brace should be shaped like the patient. If the cast was taken properly, there really shouldn’t be much reason to change the overall shape. Humans, as a rule, are smooth. They are round, and they rarely have sharp edges. Our molds should reflect this. Sometimes we have to make mechanical changes to accommodate joints or flair the edges of the plastic, but the bulk of the brace should look human. The plantar surface of the foot, the areas around the malleoli, the shape of the thigh and around the knee—all of these shapes are dictated by the shape of the patient’s anatomy.

Accurately and comfortably translating the desired forces onto a patient’s anatomy is the foundation of modern O&P. How many times have you heard a patient say he or she had a thermoplastic brace once and it just didn’t feel right, and, subsequently, he or she never wants another one? It always makes me wonder what that first one looked like and whether the results could have been different if the mold had been more artfully modified.

Tony Wickman, CTPO, is the CEO of Freedom Fabrication, Havana, Florida. He can be reached at

Bookmark and Share