Tony Wickman, CTPO

It’s been said that America’s favorite color is shiny, and I agree.

Pre-preg is great. Really, it’s a wonderful material—high strength-to-weight ratio, easy to engineer, highly repeatable—but there’s just one problem…it’s ugly! The usual method of using a cloth wick and a breather layer to remove excess resin during the curing process leaves a dull, abrasive finish that just isn’t what we are used to in this industry. Usually, your only options are either to be okay with ugly or do a lot of post-production work to make it pretty. Some people sand the material smooth and spray it with a coating, and some actually laminate over it to achieve a nicer finish. Both of these methods work but require investing a lot of extra time. I was never really satisfied with either of these options, so my team kept working to figure out some way to get a really nice finish on our pre-preg components without having to do any of the crazy, time-consuming things I’ve seen people try to make this stuff acceptable to the consumer. I mean, let’s face it—it can be an amazing material, but if it’s ugly, we can’t sell it. After a lot of trying, we finally got the solution down.

An outer layer of polyethylene is applied over the layup.

The fix actually came from some early problem solving. When we first started to work with this material, everyone said that the water in the plaster molds was a major problem for the pre-preg, so a lot of people suggested using dental plaster, extra hardeners, or all kinds of voodoo fixes that would help maintain the strength of the mold while it was being subjected to the extensive drying cycles. Most of this came from the idea that the experts were using PVA bags as their inner surface! PVA is smooth and easy to apply, but it’s water permeable! If water was the problem, why on earth would we use a permeable barrier? We fixed the problem by using a very thin polyethylene inner layer. It’s cheap, easy to apply, allows us to use the same plaster mold material we use for all our other molds, and no matter how wet the mold is, it never interferes with the pre-preg! One byproduct of this technique was that the inner surfaces of the devices we fabricated were beautiful! Their outsides, however, were not that nice looking. We tried PVA, we tried silicone, and we did have some success, but we just couldn’t get the outside to look like the inside.

The obvious answer was to use a polyethylene layer on the outside as well, but it wasn’t that simple. Pulling an inner layer of polyethylene is easy—you just use a separator (a fabricating hose) to wick the air out—but if we used a fabricating hose between the inner and outer layers, it left a texture on the inside of the outer piece of plastic, which then transferred to the outside of our pre-preg. If we didn’t use a separator, the material cooled in random patterns and wasn’t smooth.

So the trick became pulling a very thin layer of polyethylene over the polyethylene inner layer without using any kind of wicking layer that would leave a pattern on the inside of the outer layer. We tried a few different parting agents until we finally settled on Liquid Wrench® Dry Lubricant with Cerflon. It is designed to go on wet and then dry out, leaving a layer of Cerflon (a combination of PTFE and boron nitride) behind. This effectively stops the layers of plastic from sticking together and still gives a very smooth surface.

The finished layup is shown under vacuum.

At this point, you might be wondering how we made up for the volume of the pre-preg material between the two layers of plastic since we didn’t allow for that in forming the outer layer of plastic. The answer is simple: we didn’t. We just cut the outer layer a few centimeters beyond our intended trim lines, layed up our pre-preg, and then applied the outer layer of plastic over the pre-preg material. By adding a few layers of fabricating hose over the outer layer of plastic and covering that with a PVA bag to act as our vacuum vessel, we could pull an effective amount of vacuum, and as the material ramped up to temperature, it “reformed” to fit the mold.

The process is simple: Modify the mold as standard. Add one layer of fabricating hose and pull a uniform layer of polyethylene over the entire mold. If no liner is to be used, then 1/16 in. is fine; if you plan to use a liner, you can pull a layer consistent with the thickness of liner you will use (1/8 in. liner = 1/8 in. polyethylene, for example). Sand down any seams that may get in the way, spray with the Liquid Wrench dry lubricant, and let it dry. A quick buff of the lubricant, and you’re ready for your next layer of polyethylene. For this layer, 1/16 in. is fine, and feel free to stretch it as thin as you can. Remove the entire outer layer. Trim this outer layer to extend just a few centimeters beyond your desired trim lines and discard the remainder of the second layer.

The finished pre-preg is smooth and shiny, inside and out.

Now, just lay up your pre-preg in the predetermined pattern directly on top of the first layer of plastic and then apply the second layer over that. You still need to be concerned with removing as much excess resin as possible, so apply a few layers of smooth wicking agent to the outside of the entire mold and then apply a PVA bag to the outside of that. Apply vacuum as usual, and the layup will be de-bulked. Use a slightly longer ramping time to ensure that you evacuate as much resin as possible, then give it a short heat cycle to ensure the best curing of the final piece. The result is a component that is strong and attractive, with no postproduction effort required.

Also published in the May 2010 edition of the O&P Edge. © 2010 O&P Edge

Update – August 17, 2010

I created a pre-preg PTB brace, utilizing the techniques outlined above. The orthosis was super lightweight with really good axial loading capabilities.

PTB brace utilizing our pre-preg 'should be patented' process.

Tony Wickman, CTPO

Metal orthoses are simple to fabricate. I once visited a brace shop in Port Au Prince, Haiti, which was staffed entirely by people who were deaf mute. They made crude but very effective orthoses with little more than an anvil, a drill press, and a lot of files. They made their own rivets out of discarded nails, used cold-rolled steel for the bands and bars, and just filed the ends of the bars to create joints. It’s becoming less and less common to see even basic metalworking tools in most facilities these days, so a lot of technicians become frustrated by metal work. Though metal orthoses are simple to fabricate, some specialized tooling is needed to make the job easy and efficient. They also require practice. It’s hard to get really good at something you do twice a year with half the tools you need.

One of the primary reasons metal orthoses are used is that they’re really rugged. Even today, with modern thermoplastics and composites at our disposal, just the phrase “rugged orthosis” conjures images of Mel Gibson in the Mad Max movies wearing his old Pope Klenzack AFO with the joints on backward. This is one Hollywood image that is rooted in reality. These designs have long been favored by hard-working farmers, factory workers, and heavy-equipment operators. However, they can also be made to be quite light. Using lightweight aluminum and thin-gauge bands, you can produce devices whose weight, with the shoe factored in, compares to some thermoplastic designs.

Metal braces also offer a level of torsional rigidity that can’t be matched. When you’re trying to reduce genu varum or genu valgum, that rigidity allows the device to resist medial or lateral loads very well, even when there is a rotational element. If your primary pathological force is in the sagittal plane, such as a knee-flexion contracture or hyperextension, a conventional metal device is a great way to manage those forces. The inherent shape of the mechanical structure is normally a dual parallelogram. This shape allows the device to resist forces in every plane with a very small cross section. That means we get a lot of strength from a fairly small mass, and the patients get a lot of control without a lot of weight.

Metal orthoses usually offer a very small contact surface. This can be a disadvantage in cases with tissue damage or where tissue requires a broad, low-pressure contact force. A lot of people don’t need that, especially in warmer climates! If the patient has good tissue and the forces required for correction can be limited to a small area, then a metal orthosis can work well. Even in cases in which the contact patch has to be increased, this can often be done with a leather lacer. Less contact means less heat build up, less opportunity for impingement, and more comfort.

Metal orthoses are normally attached to a shoe, which presents its own set of pluses and minuses. Frequently, (especially with KAFO patients) the foot may not actually be involved in the pathology. If, for instance, the device is intended simply to correct genu varum, involving the foot could be counter-productive. When a thermoplastic foot cup is used, the patient will lose some ankle motion, inversion/eversion, and proprioception. These are all important mechanisms for normal locomotion. The attachment of the device to a shoe will maintain a lot of these factors as “normal.” Attaching the device to the shoe might limit footwear choices, even when a split stirrup is used, but the use of a plastic foot cup does not imply an unlimited choice of shoe options. Lastly, having a visible stirrup can be perceived as less cosmetically acceptable for some wearers, but for many the increased comfort and function are more than enough to offset this.

Of course, we get to use a lot of different materials in this industry, and just about all of them have a demographic to which they can be appropriately applied. Whether it is the comfort and control of thermoplastics, the low mass and high strength of composites, or the open, rugged designs of conventional metal, there is something for everyone.

Also published in the November 2009 issue of the O&P Edge. © 2009 O&P Edge

Tony Wickman, CTPO

When I started out as an orthotic technician over 25 years ago, I believed you should always flatten the plantar surface of an AFO mold. That’s what I was taught because it was the conventional wisdom of the day. Then I had the opportunity to work with a group of physical therapy students. I felt secure in talking with the students about my techniques, because I thought I was applying the best we had to offer to the science of rehabilitation.

The students toured the lab and remarked that we were doing some amazing stuff, until they saw how we were modifying our molds. One of the students posed a question, “Why do you flatten the bottom of the foot when you modify the mold?” I answered that we were building mechanical devices, and that flat surface was our foundation. Wasn’t it?

After that, I started to question my fundamental beliefs on how to build a quality device. I was fortunate to have spent my first few years as a technician with some visionary orthotists, and what they taught me made sense, but at this point, I started to think we might be overlooking a very important part in the process, the part where modifications are made so that a device will become an extension of the patient. I noticed when we make arch supports, we never flatten the plantar surface. On the contrary, arch supports mirror the plantar surface. So what’s the difference between making an arch support and an AFO? Why would we think an arch support should take advantage of the curvatures of the plantar surface, but the bottom of an AFO or other orthoses should be flat?

Not long after that encounter with the physical therapy student, I had the opportunity to work with another visionary, a physical therapist and author, who was teaching a class on Neuro-Developmental Treatment (NDT) at that same physical therapy school. She had a test subject for the NDT class and needed a volunteer to fabricate an AFO. Naturally, our lab was offered and I worked one on one with her, and she helped me understand the intricacies of the foot: its bones, nerves, tendons and vascular structures, and how each of these components have their own set of needs. After working with her, I was convinced that technicians must see the foot as the foundation of an orthoses, not the floor! I knew I had to learn more about pedorthics, tone inhibition and neuro-developmental techniques if I wanted to create bio-mechanical devices, devices that fit properly, as well as focus on the specific pathology without causing any collateral damage.

This level of knowledge is beyond the standard education for technicians, but if I am modifying a mold, it’s my responsibility to know about physiology, neurology, and pressure mapping. Every patient who needs a brace has a pathology. Seldom are those pathologies purely orthopedic, there are typically underlying vascular or neuropathic disorders as well. There is no orthopedic solution for a neurological problem.

Technicians need to be aware of this when building a device, and practitioners need to be aware of this when taking a cast. If the cast is taken weight bearing, on a flat surface, much of the plantar data disappears. When a cast can be taken on a foam block, semi weight bearing, or even hand manipulated, that cast takes on a whole new level of function. The surfaces of the foot become much more natural and the load pressures can be more accurately distributed. Neurological inputs can be reduced and deep-tissue weight-bearing techniques can be utilized, and the brace can better fit the patient. One of the issues with casting a patient on a flat surface or flattening the bottom of a mold is that the device created from that mold can cause collateral damage, by creating excessive pressure on at risk tissue, or exacerbating existing problems and putting the surrounding tissues at risk.

No matter the casting technique, I can make a device, but it may not function properly, and it will certainly be less comfortable for the patient. I see these issues most frequently with neuropathic (CROW) walkers and pediatric devices. I’ve worked with many practitioners to help them change their casting technique for these devices, and by doing so, they are seeing better results with their patients.

I believe a technician’s role is to understand and interpret the orthotist’s vision for the patient and make that vision a reality. I also believe the practitioner and technician should work together as a team to make a device that will assist the patient in the best manner possible. If I don’t have an understanding of the nerve structures on the foot, then it’s probable that I won’t make a properly functioning device. Orthoses are not “widgets” that come off an assembly line, they are custom-made bio-mechanical devices that help improve the quality of life for the people who use them.

In order to increase the overall caliber of the rehab team, I suggest that you learn as much as you can about pedorthics, tone inhibition, and neuro-developmental technique. The manipulation of the surfaces of the foot is the foundation of a good lower limb orthosis and these specialized disciplines will give you a good foundation upon which to build your skills.