Custom ORTHOSES
Introduction
Building Orthoses - Why and How
To enable corrective surgical procedures for hand positioning in cases of radial aplasia, the «too short» soft tissues causing the characteristic bent hand position must be stretched or during the surgical intervention, the surgeon must shorten the ulna. Otherwise, straightening the hand is not feasible.
Irrespective of the planned intervention type, it’s advantageous to carefully maximize the expansion of soft tissues to allow passive movement of the hand toward the neutral position.
Currently, many surgeons recommend using night positioning splints to perform initial stretching before transitioning to an external fixator for the required extension prior to the corrective procedure.
Typical splints are crafted from low-temperature thermoplastic material that is heated to around 60-70°C in water and then shaped around the extended arm, maintaining the position upon cooling.
However, the effectiveness of these splints is minimal, as they primarily fix an already attainable hand position. With CAD-modeled orthoses based on a 3D scan, a significantly more substantial effect can be achieved, reducing the later dependence on an external fixator or the extent of necessary shortening of the ulna.
If you haven’t already, start by reading our page on soft tissue distraction, where I explained why we decided to build the orthoses for our child ourselves, more about what we wanted to achieve and our conclusions.
And don’t forget to read the disclaimer in the «legal» section!
Before you begin…
The starting point should always be a thorough familiarization with the topic:
– Understand the anatomy of the forearms and wrists, bones, muscles, and tendons. You will find some basics covered in the glossary.
– Learn the technical terminology related to possible movements (e.g., pronation/supination/extension/flexion), each of which unfortunately has various designations, to communicate effectively with doctors and orthopedic specialists on a reasonable level. Different procedures result in different (extents of) possible movements.
– Familiarize yourself with standard procedures and interventions, their advantages and disadvantages.
Step 1: 3D Scan
Obtaining a CAD Model of the Status Quo
We conducted the 3D scan of the arms using a «Structure Sensor» (Mark I) by Structure – a very cost-effective 3D scanner. However, using this scanner requires an iPad Pro and a Mac(Book): the iPad acts as a holder and screen for scanning and is needed for data transfer to the Mac, which processes the data into a model. The manufacturer offers the «Skanect» program for this purpose, available in a free version that suffices for many cases.
Nevertheless, this was in 2020 – nowadays, there are likely alternative good options, especially for those who do not happen to own both an iPad Pro and a Mac(Book). Ultimately, obtaining an accurate forearm scan up to the elbow and the hand up to the base of the fingers is essential.
We conducted the scan while the baby was asleep – even so, we required several attempts due to minor tremors that rendered the scan unusable. With the baby lying covered by a blanket on a mat on the floor, the arm was extended upwards – delicately held and stretched by the fingers – and then a helper performed a 370° scan around the extended arm. Be ready to have to repeat several times!
Tip: An extended arm doesn’t provide many reference points for a scanner, so ideally, additional markers should be added, e.g., placing building blocks around the upper arm on the blanket.
Step 2: Modeling
CAD Modeling for a Slightly Corrected Hand Position
The modeling phase proved to be the most time-intensive aspect of orthosis creation. For this purpose, the free software «Blender» is a good choice, with numerous helpful tutorials available on YouTube.
Another handy free tool for editing mesh bodies, such as cropping the 3D scans, separating and hollowing out the body to create a shell for 3D printing for fitting, etc., is Meshmixer.
In a nutshell, the modeling process entailed embedding a spline «within» the arm and subsequently fine-tuning it by a few degrees – guided by intuition, oriented towards the alignment of a typical arm. The extent to which the result perfectly fits the arm can only be determined through trial and error (actually, it should not «fit,» but gently push the arm into the desired shape. Once it fits perfectly, it needs to be replaced!).
Potentially, the effort of modeling can be reduced by mirroring the edited hand for the orthosis of the other hand (if necessary).
Ultimately, all iterations of the orthoses we crafted – 8 sets (of 2, that is, for RH/LH!) over about two years – were based on the same initial scans. However, each iteration involved one to two attempts that didn’t fit perfectly, resulting in over 30 completed orthoses!
Step 3: 3D Printing
Producing the Positive Molds for Vacuum Molding
The digital positive model, previously created during the modeling stage, must be physically available to manufacture an orthosis using the vacuum-forming process.
For this purpose, 3D printing is a suitable approach. Investing in a personal 3D printer for this task is advisable, as ordering through online portals does not match the necessary speed of iterations (sometimes involving three attempts over a weekend until an exact fit is achieved).
An affordable entry-level device is sufficient. It should be capable of printing PLA and have a sufficiently large build volume – ideally, the positive models can be printed standing up. Otherwise, the model would need to rely on support material, leading to post-processing and potentially a less smooth surface. I used a MakerBot Z18, which was already available beforehand.
Print sturdy models – with a minimum of 4 shells and 25% infill. Otherwise, there is a risk of them collapsing under the vacuum and the heat from the thermoplastic orthotic material.
Determine the scaling of the prints iteratively – and always meticulously document everything / save screenshots!
Step 4: Vacuum Molding
Vacuum Molding Orthoses
Since our orthoses are vacuum-formed around a 3D-printed model and not directly shaped on the patient’s arm, we can use materials molded at higher temperatures (e.g., 120-135°C) and boast significantly improved qualities, notably a softer texture. I used Streifeneder’s EVA (ethylene vinyl acetate) material, Streifyflex. While various alternative options exist, using a product approved for orthotic construction and skin contact from a reputable manufacturer is essential. A video from Streifeneder on this subject can be found on YouTube.
Firstly, the form must be prepared. This involves initially drilling a hole in the head end of the borehole provided in the model – around 5mm in diameter – passing only through the shells and into the infill. This ensures the infill is adequately permeable to the vacuum in all directions. Following this, several small holes (approximately 1mm diameter) are drilled externally into the concave areas of the model to ensure the contact of the EVA material with the model at these points.
Next, the model is coated with a release layer to prevent the EVA material from adhering to it. This is the material we used (in the 3cm-variant). This layer is coated with silicone spray – unless it’s desired for the release layer to adhere to the EVA material, thus later padding the orthosis internally. This approach has advantages – soft yet thin cushioning – and disadvantages: challenging to keep clean, poor adhesion of edge padding (see «post-processing «) will likely occur, and it may detach after some time. We also tried this material to achieve thicker padding. If adherence is desired, the EVA material should be processed at the upper end of its processing temperature range. However, this is a bit more challenging as the material becomes quite stretchy. In any case, the release layer must lie flat against the model everywhere. If no padding is applied, the child should wear a thin, close-fitting long sleeve while wearing the orthosis. A light wool-silk combination has proven effective.
Now, the prepared model can be placed onto the tube, which is then secured in a holder – such as a vise. At the rear end of the tube, a vacuum hose is attached to a vacuum pump. An adapter may need to be 3D-printed here (see images). Since I used a vacuum pump that must not anspirate small debris from drilling into the infill, I placed a «filter» – a piece of nylon stocking – at the end of the tube before attaching the hose adapter. A vacuum cleaner might also serve as a vacuum source, although I did not attempt this.
Subsequently, the EVA material is heated to its processing temperature. For this purpose, I used a conventional oven, although most oven thermostats are very imprecise. Hence, I purchased a 2-channel thermometer with two K-type probes (UNI-T UT320D for about €30), attaching one probe to the baking sheet and positioning the other freely inside the oven. Using a brand-new, Teflon-coated baking sheet exclusively for this purpose is essential. Beforehand, the baking sheet should be lightly coated with silicone spray to prevent material adhesion. Importantly, the side of the EVA that lies on the baking sheet (and therefore comes into contact with the silicone spray) must end up being the outer side of the orthosis. Otherwise, the areas beneath the model won’t bond, and the vacuum won’t pull the material towards the model.
Then, the oven is heated to the correct temperature – in my case, 135°C. The baking sheet heats up more slowly than the air in the oven, so I waited until the baking sheet reached the right temperature and then adjusted the oven’s air temperature by opening the door. Now, the generously cut EVA material is placed on the baking sheet, and we wait until it becomes shiny. The exact time can be taken from the manufacturer’s data sheet – or determined through experimentation. In my case, it was approximately 90 seconds. Make sure the distance between oven and prepared mold is very short!
When the material is ready for processing, it’s placed on the model (with the release layer!) and sealed all around, as seen in the video. Don’t forget to activate the vacuum beforehand.
Once the material has cooled, the vacuum can be turned off, and the orthosis can be gently removed from the model (by cutting away the area through which the tube enters the model using a scalpel).
The making of an orthosis. Unfortunately, the vacuum pump caused vibrations in the video. The white stripe on the orthosis is a reinforcement made of the same material, pressed onto the pink material, fusing with it during heating.
Step 5: Post Processing
A Few Post Processing Steps... And You're Done!
After removing it from the mold, the blank must be washed with soap to remove any traces of silicone spray. Afterward, all edges should be deburred, for which a rotary tool with sanding cylinder, like the Dremel shown in the pictures, is suitable. Very fine burrs can be gently heated with a lighter and rounded off with fingers. If the orthosis shape slightly deviates from the desired state, it can be carefully adjusted with a heat gun.
I then padded the edges with Cellona cast edge padding. An orthosis should never have hard or sharp edges and should not cause pressure sores on the skin.
Next is the closure: Stick small pads on the orthosis using self-adhesive hook tape (e.g., Velcro, 2 or 3 pads per side depending on the length of the orthosis) and cut both-sided, soft loop tape to the right length. Before sticking, degrease the corresponding areas thoroughly with isopropanol and press the pads on very firmly. Occasionally, some pads may need replacement over time.
Before wearing the orthosis beyond fitting / for an extended period, it is essential to consult a qualified healthcare provider (e.g., orthopedist). Wearing an improperly shaped orthosis can cause significant harm. When using it, be sure never to tighten the orthosis so much that it compromises blood circulation (causing cold fingers).
The Result
A CAD Modeled Orthosis made of EVA
EVEN MORE ORTHOSES
3D Printing Splints
An Option for Regular Splints
3D printing orthoses directly is an option, but achieving a consistent and gentle straightening force without causing pressure sores is challenging. This requires the integration of a spring mechanism, as described in the section below, which I haven’t quite managed to implement without causing any pressure sores. If you only need regular, stiff splints, 3D printing is a good alternative and much quicker to implement than vacuum molding. There are also excellent tutorials on YouTube for modeling in Blender and specific add-ons for Blender to that end.
When 3D printing orthoses, using a material suitable for prolonged skin contact is essential. I conducted my experiments with antibacterial PLA from Copper3D, which even has FDA approval and is considered suitable for splints according to Copper3D. However, I never came to use these orthoses permanently.
A Dynamic Approach
Lots of Ideas..
From the beginning, I always had the idea to build an orthosis with a spring mechanism. On the one hand, this mechanism should maintain the wrist’s mobility while gently pushing the hand into the desired position.
However, all the various designs and prototypes ultimately failed because it was impossible to introduce the force into the hand without causing disturbing side effects. When the hand was secured too tightly, it led to increased pressure and reduced blood circulation in the hand (cold fingers). If the fixation of the hand was too loose, it would slip within the orthosis or deform inside it (with the thumb pressing into the palm).
I mainly experimented with leaves from a high-quality stainless spring steel feeler gauge as a spring mechanism. The advantage here is that the various thicknesses of the leaves exert different forces. A spring package made of different thicknesses also has its distinct characteristics.
I developed multiple supports for this purpose: unpadded and padded, precisely fitting 3D prints made of Copper3D PLA and variations made of EVA material. The PLA variants have the advantage that the point of force application on the hand can be placed flexibly, and wrist extension/flexion can be achieved while simultaneously applying pressure in the direction of the ulna (supination).
Another possibility is selectively reinforced or cut EVA orthosis variants, but I could not achieve a satisfactory result with them.
If you find a working solution, have ideas or questions, please get in touch!
