Cases
Materialise Assists in True Rapid Design and Manufacturing of a Custom Skull Implant
“Using Materialise software to design a custom-made implant decreased the design time by 68% in comparison to other CAD software. At the same time, the new EBM production process reduced the production time by 53% in comparison to conventional production processes. The option of designing a custom implant makes performing an operation easier on the surgeon and results in a 50% reduction of the time needed in the operating theatre. Materialise software is and will be our software for designing custom implants”
A few years ago, a young woman was injured in a traffic accident. During the first critical days after the accident, her neurosurgeon had to perform an extensive craniotomy on the patient’s skull to release the pressure on her brain. Three years later, the surgeon turned to Materialise' SurgiCase CMF software to examine the patient. Integrated in SurgiCase CMF is a list of CMF partners, which are part of the CMF network, offering a central simulation, design and ordering system for cranio-maxillofacial guides, aids and instruments. The surgeon relied on the CMF network to have a customized cranial implant designed and manufactured. This case study describes how rapid design and rapid manufacturing helped the surgeon three years later to reconstruct the large cranial defect easily and quickly. Where other CAD-software failed, 3-matic proved to be the best solution to design the implant, because it enables the user to incorporate scanned anatomical data into the design easily and accurately.
Due to the complexity of the case, the input of the cranio-maxillofacial surgeon was indispensable during the design process. The case study below takes you through the journey Engineer Maikel Beerens* and Dr. Jules Poukens** took in producing the ideal prosthesis for this patient. The team brainstormed over different designs. Each step they took caused them to consider additional clinical aspects and constraints that would achieve the best design.
Pathology
Because the patient had lived with the cranial defect for over three years, her scalp had tightened over it.
Since there was not enough skin to cover the implant completely, it was impossible simply to design the implant as a symmetrical copy of the cranium’s left side. One solution was to inflate balloons beneath the patient’s skin in order to stretch it. However, weary of the cycle of visits to specialists and hospitals, the patient declined the procedure. This meant that the implant’s purpose would be to cover the defect and protect the patient, not to effect cosmetic (symmetric) reconstruction.
Fast processing of the patient’s CT data to an accurate 3D model
Using Mimics software, the team processed the CT scan data of the patient’s skull. By varying the threshold values, the cranium and loose bone parts were calculated from the image data. To ensure that the inside surface of the implant did not interfere with the meninges, a Mimics segmentation of the same was also made. (Fig. 1)
When designing an implant that covers a large defect, it is important to use reliable design references to ensure the implant fits properly. The surgeon decided to use the loose bone parts lying on the meninges and the meninges itself as a reference for this implant design.
Testing various designs to obtain the optimal implant
Maikel Beerens produced several designs using 3-matic, which contributed to the optimization process. It gave Beerens the opportunity to try out several approaches, which he discussed with the surgeon in order to arrive at an optimal implant. Maikel Beerens says: “3-matic is user-friendly and fast. It simplifies working with complex STL-files, such as the 3D files obtained from CT, MRI and other scanner data.”
Design 1: Exploring 3-matic
Before deliberating with the surgeon, Maikel Beerens learned to use the 3-matics software by producing a preliminary design. He found 3-matics a handy tool for quickly mirroring the intact left side of the meninges, thus restoring the skull’s symmetry (Fig. 2). However, because there was not enough skin to cover the implant completely, he had to throw out the design.
Design 2: Concave path towards the meninges
A closer look at the patient’s CT images revealed that the pillow used to position the patient’s head during the CT-scan had caused an indentation of the meninges. As a result, part of the meninges and several loose bone parts could not be used as a reference for the implant design.
3-matic allowed the designers to import the contours of the cranium, meninges and loose bone parts easily into a sketch. It was easy to draw guiding curves on the usable references (part of the meninges [blue] and bone particles [yellow] in Fig. 3) to indicate the necessary curvature of the implant. These guide lines (turquoise in Fig. 3) followed a convex path over the indented part of the meninges to prevent the implant from pushing onto the meninges. These guiding curves ensured the implant is not designed by sight, but is calculated ingeniously, based on the patient’s anatomy and other clinical features. This is an important advantage 3-matic software offers over other CAD packages. Furthermore, 3-matic’s section view capability ensured that the implant did not touch the meninges (Fig. 4).
Design 3: Mirroring meninges
Because the indentation from the positioning pillow could have caused side effects like “brain shifting” (movement of the brain) and movement of the meninges, the surgeon proposed an alternate third design. Movement of the meninges would cause bad design references and a bad implant design. 3-matic was used for the third implant design to mirror the patient’s meninges (Fig. 5) and to design the implant with this mirrored meninges as a reference (Fig. 6). Because 3-matic assigns a contrasting color to each part, it is easy to compare them.
Design 4: Convex concave
The patient had already undergone reconstructive surgery for the left part of the skull a month after the accident. However, reconstruction failed to restore the cranium’s original shape. That meant the mirrored meninges could not serve as a good reference for designing the implant. Design 2 had turned out to be too concave, while design 3 was too convex to close the defect successfully.
In order to arrive at the perfect implant, the surgeon and the engineer decided to combine both designs in a fourth implant design (Fig. 7). This new implant had the potential not only to protect the patient, but also to reconstruct the cosmetic facial forehead contour without stretching the patient’s skin.
Design 5: Final convex concave
Another clinical aspect that had to be taken into account was the attachment of the chewing muscle which had fused with the meninges and the attendant scar tissue. The surgeon’s task was to try to separate the muscle from the meninges and attach it onto the implant. At first, the team decided to add a lattice structure, to which the muscle could be sutured during the operation. The team theorized that this step would maximize the muscle’s attachment. Therefore, they added a 1-millimeter-deep groove in the implant where two lattice structures were positioned at 0.2 millimeter from each other. 3-matic made it easy to give this lattice structure the same outer shape as the original implant surface (Fig. 8).
However, research revealed that such a lattice structure, which included large interstitials where infections could occur, might pose a severe risk for the patient. These interstitials prevent cleansing bodily fluids and antibodies from reaching the core of an infection. A final design based upon this information and the complete clinical information, in which a row of holes replaced the lattice structure, was produced (Fig. 9).
The final design steps, made in 3-matic software, were:
- Five fixation lips were designed on the implant.
- The 93/42/EEG directive for the registration of custom-made medical device mandates that a product number be added to an implant. Using the “surface move tool” in 3-matic, this number was easily deepened into the implant surface.
- 12 holes - in pairs - which pull the meninges towards the inner surface of the implant. This reduces the chance of infection and gives the meninges the opportunity to grow into the implant surface.
- Since a lattice structure would increase the risk of infection, alternatively a pattern of holes was designed, through which the muscle can be attached during surgery.
Rapid manufacturing of the implant using electron beam melting (EBM)
EBM is a 3D printing technology developed by the Swedish company Arcam. The implant was manufactured by melting thin layers of titanium powder TI6Al4V (Titanium 6 Aluminum 4 Vanadium). This technology allows the construction of a thoroughly dense implant. The resulting implant promotes osseo-integration of bone and soft tissue.
Successful implantation
During surgery the implant was screwed onto the cranium using titanium medical grade screws and the fixation lips. The implant turned out to fit perfectly and healed without complications. The “convex concave” implant design enabled the surgeons to reconstruct the shape of the forehead aesthetically. The back part of the implant only protects the patient’s brain tissue. Since it is covered up by the patient’s hair its concavity is not visible.
Conclusion
Using 3-matic software to design a custom implant decreased the design time by 68% in comparison to other CAD-software. This excludes redesigns/adaptations to the implant. The new EBM production process reduced the production time by 53% in comparison with conventional production processes. The option of designing a custom implant increases the surgeon’s confidence in the implantation. This eventually can lead to a reduction of 50% in the operation time. For IDEE, the Materialise software is and will be the software for designing custom made implants.
Why choose 3-matic for the design of cranial plates?
3-matic has integrated several automated functions in order to design the shape of a cranial plate quickly:
The design of the base shape of the cranial plate
First, indicate the outline of the defect on the 3D model of the scanned skull. If necessary, you can design arches or guiding lines in the defect. These arches define the height and shape of the resulting prosthesis. Based on this information and the parameter settings like thickness and clea rance, the base shape of the cranial plate is created automatically. As illustrated in the above case study, the arches are crucial to obtaining the required design. Each modification of the arches produces a new design.
The removal of undercuts
For thicker plates (+/- 5mm), e.g. plates that will be made in a ceramic material, the 3-matic software provides an automatic undercut removal function. This function is driven by the indication of the insert direction and will remove all material from the plate that would prevent insertion in this direction.
Creating varying thicknesses
With the integrated morphology functions in 3-matic, it is easy to create a prosthesis that is thicker at the borders and thinner in the center.
