Modification Techniques

Cast Rectification Options



Every one knows that the fit of any prosthesis or orthosis is wholly dependant on two things. The mold and the mold. If the cast isn’t taken well there’s little chance of the technician being able to fabricate a device that fits. It’s a perfect example of “Garbage In, Garbage Out”. There’s just so much you can do to fix things that are broken.


That said, there are some things you can do to help every cast along. The decisions you make regarding the cast now will have a direct and irreversible impact on how the device fits the patient. This is your big chance to work your magic! Let’s take a look at a few simple techniques that you can employ at your lab that you may find useful.


Cast Preparation


There are a lot of things that can go wrong during the casting process, not all of which are under the control of the clinician (that’s my story and I’m sticking to it!). As technicians it’s our job to rectify these mishaps, because ultimately a bad fit will be blamed on us.


Correcting the angle of the cast at the ankle is a common cast correction, but it can be made easier and more accurate if done properly. Many of us older guys were first taught to cut the cast across the ankles to the heel from the dorsal crease. You’d remove a wedge of cast to correct plantar flexion or just open it up to correct dorsi flexion. The biggest problem with this method is that a large section of the most important part of the cast is removed. The shape and height of the ankle is important information.


To get by this unnecessary destruction of the cast, all we have to do is draw the ankle centers on the outside, cut across the Achilles tendon and across the ankle crease. Leave a half inch section of cast at each ankle center that will act as a hinge. Use this as a pivot point and set the cast to the desired position. Seal it back up with some more plaster bandage.


This method can be used in the transverse plane too, and depending on where you put the pivot points, you can also correct varus/valgus, supination/pronation and clubfoot deformities.


Mold Rectification


Once the cast is filled and the angles are correct, it’s time to sling some plaster. We use plaster to build up boney prominences and fill in areas that might impinge on the patient. This is where the creative, artistic nature of a good technician will come in to play. All of the best modifiers are closet sculptors.


Of course “fit” is the most important criteria when it comes to cast modifications, but we shouldn’t have to sacrifice form for function. When we make smooth modifications and build-ups, the device not only looks better, it’s bound to fit easier and be more pleasing to the patent. If the patient likes the device they’ll be more inclined to wear it. We all know that compliance is everything. You know the orthosis or prosthesis isn’t going to help if it spends all it’s time on a shelf, so making the device look nice is also very important.


Many people use what we call “Brads” nails (small, headless finishing nails) to assure proper build-up height. These small nails are usually hammered into the build-up area one at a time, spaced apart to maximize coverage and tapped down to the desired height. This method is a little time consuming, especially when modifying the tibia and tibial crest on a something like a PTB. That’s an awful lot of Brads nails.


A better and faster method of guaranteeing your build-up height is to use staples. Staples have several advantages. First and foremost is the fact that you can easily and quickly put in enough staples with the staple gun to modify and PTB in less than a minute. Pneumatic guns are the fastest and easiest, but the regular Arrow type works well too. Just tap the staples to the desired height and modify away.


Another advantage is that the staples stick up like little crochet wickets, holding the build-up in place. You may not see any advantage on the first pull, but if you have to vacuum form a replacement in a few weeks time when the cast has dried out, you’ll be glad those staples are there. Even though the plaster may be starting to flake off, the build-ups will stay in place, held there by the staples.


Spreading Plaster


When it comes to applying plaster to the cast, most labs teach new technicians to use either their fingers or the tongue depressor they used to mix the plaster up with. These methods offer a degree of control, but they lack precision and accuracy. Fingers are just too bumpy to offer a smooth flow and tongue depressors are too narrow and tend to warp when wet.


I’ve seen some ingenious ways to spread plaster before, but none better than what the auto body industry uses. They do almost as much “body work” as we do. They have a simple tool called the “bondo spreader” and it’s basically just a tapered piece of plastic, available in different sizes. Best part is that you can pick them up at any auto parts store, cheap.


Once you get used to using something different to spread the plaster, you’ll find numerous advantages to the bondo spreader. They’re larger than tongue depressors so you can put down a bunch of plaster, quickly. Because of the lager surface area, dips from cast roping can be filled in smoothly the first time, often eliminating a second carving with the sureform.


They’re also slightly flexible; making smooth compound shapes without dificulty. Use either the sharp edge to get into tight spots or the long flat area for long flowing areas. Easy entry modifications and lordosis spanning were never easier. Try it, you’ll like it.


Once the plaster has been applied and shaped with the bondo spreader, I use a piece of moistened ¼” felt, around 1 ½” x 2 ½” rectangle, and work the hardening plaster into the cast. This way I can fine tune the build-up shape as well as encourage the build-up to soak into the cast better, making it stronger. Try various sized felt pieces to determine the right size for you. The build-up can then be either carved and/or sanded to the final finish.


Carving the cast


The other part of cast rectification is plaster removal. Each cast is carved, usually with a draw knife and/or sureform, prior to adding build-ups and filling in the low spots. The sureform is the primary tool for this job, but it’s surprising how many different ways people use them.


The main ways a sureform is used is by either pulling it toward you or pushing it away from you, cutting the plaster beneath with the blades. It sounds like the same thing, but the two actions couldn’t be more different.


When I was in art school, we were taught a story about Michelangelo. When Pope Julius II was looking for someone to paint the Sistine Chapel, he sent out an emissary to recruit the best artist that could be found. When asked for a submission, the great artist simply drew a perfect circle with only two strokes of his brush, each stroke starting at the top of the canvas and drawn downward, toward the body. This story is supposed to demonstrate that a person has more biomechanical control over their movements pulling toward them rather than away.


That said, although there is some lose of control, you can remove more material from a cast while pushing the sureform away from you. The weight of your body contributes to this fact, mostly. The control over how deep the cut and the shape of the cut aren’t as good, but you can remove large amounts of plaster more quickly this way.


I know some of you techs are going to say “But that’s the way I always cut with a sureform and it’s always worked for me”. That may be true, but was the result as good as it could have been? Hard to gauge, I know, but “The Master” knew what he was talking about and I tend to agree. Try using both methods and see how well you do.


There is a third way to use a sureform that is really more my speed though. It’s a rather new invention called the “Freedom File”, from Freedom Fabrication. This sureform adaptation uses compressed air to power the sureform back and forth, cutting plaster with nearly no movement from the user other than to position the file. In the words of Tim “The Toolman” Taylor, it has “MORE POWER!” (grunt, grunt, grunt)…


While this tool does some serious cutting quickly, it’s not really intended for the close-in work but it makes short work of body jackets with lots of “redundant tissue”. The Freedom File is a quantum leap in sureforms, but I wouldn’t take one to a pediatric footplate. In my opinion, there’s a time for every tool and a tool for every situation. Okay, so it’s not a very catchy slogan, but what did you expect from a guy like me. Poetry?


Sanding Plaster


The final step in modification is when we sand it smooth. There are those of us who use sandscreen wet and those of us who only use it dry. In my opinion, it cuts too quickly when wet and tends to leave the cast soaking wet too. Then we simply throw our used sandscreen into a bucket of mildly soapy water and let it stay wet until it’s time to clean them. It eliminates having to clean dried plaster off of the sandscreen. That’s just my opinion, however. Use whatever works for you in your situation. On this point, I don’t think it’s all that important.


One thing you can count on when using sandscreen is that, wet or dry, the smoothest results are had when using the “shoeshine method”. That is to say you grasp each end of a strip of sandscreen and sand the mold in a back and forth motion. This keeps direct the pressure of the hands off of the cast and preventing accidental gouging. Sanding with your fingers behind the sandscreen can cause channels to be sanded into the cast unintentionally.


Other than that, the idea is to simply make the outside of the cast as smooth as possible. If you can do that with wet paper towels, more power to you.


These tips and techniques are just one more way to do things and may not be suitable to your situation. In fact, you may have some tips of your own you’d like to share. If so, write an article and send it in and you may see your writings in this magazine. Send Technicians Corner articles to: Steve Hill c/o OTS Corp., 220 Merrimon Ave, Weaverville, NC 28787 or email me at


Captions for the photos are as follows;


File name                      Caption

Cast correction             You can see how the cast was cut to form a hinge allowing the cast to be positioned as desired.


Staple height                 Using a staple gun, staples are driven into the cast and tapped to the desired build-up height. Staples also hold the build-up on the cast.


Bondo spreader            Using a bondo spreader can make applying plaster much easier.


Felt build up                 A moistened piece of ¼” felt helps smooth and shape the plaster build-up.


Freedom file                 A pneumatic sureform, the Freedom File can quickly and easily remove large amounts of plaster.