Abstract
Three series of cone-beam computed tomography (CBCT) patient data sets were obtained. These data were exported into DICOM and MIMICS (Materialise’s Interactive Medical Image Control System; Materialise, Leuven, Belgium) and were imported for differentiation of various tissues (bone, teeth, and nerve). After transferring the data to an additive manufacturing machine, three-dimensional (3D) haptic models were fabricated using clear and opaque materials. These models were integrated into phantom heads normally used for education in undergraduate dental education. 3D prototype CBCT-based haptic patient models can be used in undergraduate and postgraduate education. Students can simulate routine standard oral surgical procedures with supervision under ‘dry conditions’. Residents can simulate advanced and complex cases before performing the real operation.
Obtaining sufficient information about the size and shape of an object is the key to being able to reproduce it successfully. This information can be obtained from computed tomography (CT) by means of three-dimensional (3D) reconstruction, which reformats the transformed CT data so that it can be visualized on a TV monitor as a two-dimensional (2D) image with the third dimension shown in perspective .
The Styrodur cutting and milling machine (HEC, Lübeck, Germany) was first introduced for radiotherapeutic purposes. The next step was combining 3D reformatting with this type of milling machine for the fabrication of 3D models of the skull and using them to simulate maxillofacial surgical procedures . Model fabrication based on laser technology was developed in the early 1980s for rapid industrial prototyping. Its medical application was delayed because of problems with digital recognition and assessing the contours of anatomical structures as well as with the transformation of the CT and MR (Magnetic Resonance) data.
The first milled models were used to plan and simulate maxillo-craniofacial surgery in three-dimensions . Laser-hardened acrylic resin had been shown to be a useful alternative . Today, individual medical models are routinely generated by rapid prototyping. Physical models are produced by selectively solidifying UV-sensitive liquid resin using a laser beam. The technology was first introduced in mechanical engineering and has attracted attention in clinical dentistry .
Later, cone-beam CT (CBCT) was introduced as a high-resolution imaging procedure in oral and maxillofacial radiology. CBCT technology has a 2D sensor and uses a cone-shaped X-ray beam. The characteristics of CBCT are lower entrance doses and a higher resolution compared with conventional CT . This allows a more detailed and exact radiological diagnosis of complex anatomical situations in the dento-maxillofacial region. The method of generating 3D prototype models based on CBCT has been described . The conclusion of that work was that automated routine fabrication was too time consuming and therefore too expensive.
This technical note describes a further development, with different infrastructures, leading to the option of mass producing CBCT data-based, haptic models for educational purposes.
Different datasets for three patients were established by 3D CBCT (Accuitomo, J. Morita, Kyoto, Japan). One dataset was used as an example ( Fig. 1 ). The imaging volume was a cylinder measuring 40 mm (diameter) × 40 mm (height) at the X-ray rotational center. Images were taken under the exposure conditions of 75–80 kV and 4–6 mA, which are the standard parameters, and were changed for different patients, if necessary. The cone-beam technique involves a single 360° scan in which the X-ray source and a reciprocating area detector synchronously move around the patient’s head. The top of the cone is the source of radiation and the bottom of the cone is the sensor, an image intensifier with a CCD (charge coupled device) camera. Within 17 s the region of interest is scanned to obtain 512 frames of 2D images; these are recorded as avi files.
From this dataset, single slices are calculated from a volume of 3× 4 cm, by selecting them in Dicom with a voxel volume of 0.125 mm × 0.125 mm × 0.125 mm. The dataset is imported with a slice thickness of 0.5 mm into the planning software MIMICS (Materialise’s Interactive Medical Image Control System, Materialise NV, Belgium). MIMICS is a link between the scanner data (CT, MR, CBCT) and a simple virtual 3D representation. The MIMICS software uses segmentation algorithms to obtain a 3D visualization. The data, which were acquired by i-Dixel, can be read into MIMICS and are ready to be viewed in axial, sagittal and coronal dimensions.
The ‘reslice project’ function allows the user to change the direction of the slice axes. A correct direction of axis simplifies later use of the data. The ‘threshold’ function enables the user to tell the computer the gray value within the grayscale from which the program should detect and color the voxels.
To work with the data, a ‘mask’ is generated. The threshold is necessary to create a first separation of the single anatomical structures (tooth, bone, nerve). A mask is generated for each structure and makes it possible to proceed within the work flow. The masks can be edited, slice by slice, by simply adding or removing voxels manually. Bone, teeth and soft tissue were separated using the above-mentioned functions. The masks can be exported as an STL (Surface Tessellation Language) data representation, which is a simple triangulated 3D surface dataset.
The acquired 3D data can be imported as an STL dataset. Since the major part in the previous process was edited manually, the resulting surface of the anatomical structures was relatively rough. For this reason, a remesh was performed. The triangulated surface was smoothened and the edges of the triangles were assimilated. Using Boolean operations, figures without overlaps can be generated. These smoothed data were reimported into MIMICS as an STL dataset.
The 3D models are produced by the PolyJet Matrix method on an Objet Connex 500 (Objet Technologies, Rehovot, Israel). Prototypes with high resolution and excellent surface quality can be produced layer by layer.
High resolution and good surfaces (without ‘steps’) are obtained because the layers are 0.016 mm and 0.030 mm thick. This allows the production of high quality parts containing thin walls down to 0.6 mm with an accuracy of 0.1–0.3 mm.
The PolyJet Matrix technology uses 96 nozzles in each of the eight print heads to jet photopolymer material onto a build tray, layer by layer, until the part is completed. Each photopolymer layer is cured by UV light immediately after it is jetted, producing full cured models that can be handled and used immediately, without post-curing.
For a single model the printing process takes 5–6 h depending on the layer thickness (i.e. vertical resolution) used. The benefit of this process is that one batch of 20 mandibles can be finished in 15–20 h. The support material is a non-toxic photopolymer that is completely removed by water jetting.
This rapid prototyping technique is the only one capable of printing models with two discretely different materials. This is based on the principle that each head can print a separate material. The material used to create teeth and nerve structures is very similar to the material used for the bony anatomy. The main difference is that it is an opaque version of the acrylic photopolymer instead of a semi-transparent version. The unique properties of the PolyJet production technique allow it to jet material through small nozzles in an ink-jet fashion. The particular machine used (Connex 500) can print two different model materials at the same time, thus jetting some white and some amber on the same layer. The polymerization method is exactly the same for both materials, they both use UV light for curing on a layer by layer basis.
In the future, it may be possible to print many more materials at one time, but now the nervus alveolaris inferior and the teeth are printed in separate materials ( Fig. 2 ). The region of interest is integrated in a standardized model of the lower jaw which is incorporated in a phantom head used for education purposes ( Fig. 3 ). The macroscopic in vitro view ( Fig. 4 ) for undergraduate students resembles the in vivo situation but the soft tissues are missing. This problem will be solved in the future.
