Article Engineeringnet - Surgical customization for prosthetic skulls

Article Engineeringnet - Surgical customization for prosthetic skulls

CADskills patented its innovative temporomandibular joint replacement. Usually, a "standard prosthesis" or a prosthesis adapted to the patient is used.

ENGINEERINGNET.BE - R&D engineer Stijn Huys: “We go further. Our 3D-printed prosthesis is fully adapted to the patient's anatomy.”

The customization starts with the patient's CT scan, on a scale of 1/1. On the segmented 3D model, the CADskills engineer builds the component, using Freeform design software and a haptic pen. On top of the patient's skull / jaw profile. Customization. "Our showpiece." A more precise fit than the direct fit is hardly possible. The designers also take into account the fact that the temporomandibular joint not only moves up and down, but also rotates and translates.

“It is the only joint in the body that hangs on both sides. For example, in our design we take into account that if one joint moves, the other must move, the so-called Bennett shift. This is not the case by default.” Similarly, when the jaw opens, an angle is created forward by the combination of rotation and translation. The so-called condylar slope. The biggest innovation is probably in the scaffold that can be made thanks to 3D printing.

In a common temporomandibular joint prosthesis, the pterygoid lateralis muscle, which provides lateral movement of the jaw and grinding food, is cut loose. “We cut the piece of jawbone, to which the muscle is attached, from the jaw and pull it against the titanium prosthesis. It has a field with micro roughness exactly on that spot. A few weeks later, there is bone ingrowth in the prosthesis, so we also have muscle adhesion to the prosthesis at the same time.”

This new implant technique has been tested on a dozen sheep / ruminants for nine months. "That equates to about twenty years in humans." The 3D-printed TMJ component is given a wear-resistant HadSat coating on the joint itself after polishing. Hard As Diamond, Slick As Teflon. The bowl itself is in UHMW polyethylene. "The life expectancy of the whole is 15 years, at least."

The latter component cannot be steam sterilized like other 3D-printed pieces in the hospital autoclave, just before surgery. He could deform. The alternative is gamma sterilization. This has the advantage that it causes cross-linking (compaction) of the PE, which makes it even more wearresistant. The gamma sterilization is outsourced in Italy. In the same Italy, CADskills won a tender to provide the hospital with the most TMJ patients with its prostheses. It concerns about 30 patients per year. Is that much? Little? “This intervention is the last chance. When nothing else helps. ”

3D printing has several steps

The necessary CT scans are done in the hospital but are verified by CADskills. Material segmentation software converts the CT scans into 3D files. Production manager Ruben Van de Sande, uses the haptic pen and models the pieces with Freeform (Geomagic) on the model of the skull or jaw. ANSYS-Discovery uses finite elements to check whether the designed implants are also strong enough. Magics prepares the STL files for the 3D printer. The 3D printing is done in house on the SLM Solutions metal printer with a chamber of 12.5 x 12.5 x 12.5 cm.

In addition, there is an oven to remove the tension from the printed parts afterwards. Then the optical scan of the print follows with the GOM Inspect. Is there no distortion compared to the model? And cleaning. Cleaning with acid, for example, will remove semi-sintered grains. This is followed by ultrasonic cleaning and packaging.

Some implants, such as AMSJI implants (see box), are anodized. The anodization of such a pinkish print prevents the otherwise greyish metal from shining through the gums. Anodizing is done outdoors. All subcontractors are medically certified. In the hospital, the component is sterilized with steam before the procedure.

The company was certified ISO13485: 2016 in 2019, a Medical Devices standard. “We mainly work with class 2B and 3.” Jaw prostheses, like pacemakers, are, for example, class 3. There is no FDA Approval (yet). "There will soon be special legislation."

Movement

Stijn Huys is an industrial engineer in electromechanics (Antwerp). Subsequently, he became a civil engineer (biomedical) at KULeuven. Today he is working on his PhD on TMJReplacement and further refining the prosthesis. “We looked at 4D CT scans. These are ordinary scans, the images of which follow in rapid succession. That creates a moving image of about 1.5 seconds.” So even more information to work with. There is collaboration with prof. Vander Sloten (KULeuven), prof. Nico Buls (UZ Brussel) and prof. Maurice Mommaerts, the founder of CADskills.

Can it be even more patient-specific? There can still progress be made, Huys believes. The occlusion? Are the teeth well aligned?” If we lower the axis of the prosthesis, what is the effect on the jaw muscles and teeth? The patient is not obliged to be operated on both sides of the jaw. What then? At this time, no one removes the low frictional forces from the common joint with a prosthesis. More can be planned in advance. Reverse engineering? ”

Skull plates

CADskills also makes cranial plates (cranioplasty). These are / were often made from PEEK, with which however very often infections occur. Then those plates have to come out again. It is possible that the patient had already undergone five surgeries at that time to lower the pressure on the brain, to place the own bone flap, to remove that bone flap again, to place the PEEK plate and to remove it after infection. Tricky and a huge risk for the patient. The CADskills solution: the CeTi (to Ceramic Titanium).

It is a sturdy but thin Ti6Al4V plate with a grid structure on the side that contains hydroxyapatite. This stimulates bone ingrowth so that the plate will form a whole with the rest of the skull. This is never the case with PEEK. However, the ceramic provides a primary fixation (which is later taken over by bone ingrowth). The surgeon can provide an additional fixation with screws. Other features of the CeTi range: mirroring of the skull (aesthetics), small hooks for the suspension of the temporalis muscle and the dura, a polished inner surface reduces friction and special material recesses if, for example, there is not enough skin to close the wound again. The antibacterial properties of the CeTi combination are pronounced.

Growing fast

CADskills was launched to address the lack of innovation in cranio-maxillo-facial and facial surgery. Its asset is "the perfect fit" of prostheses. The company employs five people. Manager / founder, Prof. Maurice Mommaerts, is a CMF surgeon. There is a CAD / CAM dental technician, a finisher for the 3D prints and two engineers.

All team members are under 30 (except the founder). “Everything is there to grow quickly. That is also the intention. ”Products are shipped within Belgium, to the Netherlands, France, Italy, Switzerland, Spain, Canada ... “Our approach is safer and faster. Our customization in TMJR is no more expensive than existing stock products. We are now breaking into a niche market with an even better solution. However, it requires knowledge… The challenge is therefore to get experienced surgeons and assistants to cross the bridge.” CADskills provides workshops and training. "We offer surgeons, among other things, to observe surgery with another surgeon."

By Luc De Smet, Source: Engineeringnet.be

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