Digital Dentistry. Группа авторов

Figure 1.1 Three‐dimensional objects imported in different coordinates of the 3D space (screen capture of MeshMixer software, Autodesk). Note that the fixed bridge is closer to the screen than the molar crown. The dynamic grid is used to orientate the spatial disposition of the 3D objects.

      Three‐dimensional images can be manipulated in various ways, depending on the characteristics of the software. For example, with DICOM and STL files, using the CAD software one can plan and perform digital surgery of dental implants and wax‐up of future prostheses. After digital planning, the implant surgery guide, temporary crowns, and definitive crowns can be printed with additive manufacturing devices or milled by subtractive manufacturing devices [5, 6].

       1.1.2 Coordinates and Planes

All 3D images are created or rendered in a virtual space of coordinates and planes. Any objects that are digitally designed within the 3D coordinates can be fully edited in the virtual space, before being manufactured. The coordinate system is a method of assigning numbers to points. In three dimensions, three numbers are required to specify a point. Plain 2D images have numbers related to only two coordinates (x and y). The coordinate that represents the third dimension is usually an axis called z. The z‐axis is perpendicular to both the x‐axis and the y‐axis (Figure 1.2).

      The coordinates and the respective planes provide references for the location, size, and volume of the 3D images. All 3D objects have their coordinates fixed in a virtual plane of the imaging software. It is important to make sure that multiple 3D objects to be manipulated or aligned are positioned in the same spatial coordinates, which can be used as spatial references. Therefore, 3D files from different imaging methods should be in the same 3D coordinates in order to be superimposed or combined with the aim of creating a virtual patient, as explained further in this chapter.

Figure 1.2 A 3D object (reconstructed model of a maxillary CBCT scan) is positioned in the 3D space of a software (Ultimaker Cura) to be 3D printed. Note the three axes depicted by the software in different colors (x‐axis in red, y‐axis in green, z‐axis in blue).

       1.1.3 Computer‐Aided Design and Computer‐Aided Manufacturing (CAD‐CAM)

      The term computer‐aided/assisted design is usually abbreviated as CAD. The methods used for image acquisition (CBCT, scanning imaging, photographs) and manipulation (software programs) can be included in CAD. On the other hand, computer‐aided/assisted manufacturing (CAM) includes processes such as 3D printers (additive manufacturing) and milling devices (subtractive manufacturing). CAD‐CAM technologies are currently used in biomedical engineering, clinical medicine, customized medical implants, tissue engineering, dentistry, artificial joint manufacture, and robotic surgery. Furthermore, the use of CAD‐CAM technologies has been increasing in various fields of study of medicine and dentistry [5, 6]. Among the main devices that can be digitally designed and manufactured are different types of dental restorations and prostheses, surgical guides, occlusal splints, dental casts, and orthodontic aligners [5, 7]. Details of the main clinical applications of CAD‐CAM in dentistry are further addressed in the next chapters.

       1.1.4 Mesh

      The term mesh is used to describe the surface of a 3D object composed of triangular or polygon faces. A mesh object does not have any actual curvature. Instead, the appearance of curvatures in a 3D image composed of meshes is obtained by increasing the number of surfaces. The most common file format of these 3D images is the STL file [5], which will be discussed in detail in the next chapter.

       1.1.5 Image‐Guided Treatment

      Since 3D patient scans are taken prior to dental treatment, CAD‐CAM technology can be used for the fabrication of surgical guides, preparation guides, and maxillofacial surgical templates. Most of these applications require 3D hard and soft tissue images generated by CBCT and optical scanning image modalities, respectively. Based on such images, CAD‐CAM guides can be designed and manufactured to orientate directions of drilling procedures and incisions [5].

       1.1.6 Image Superimposition/Alignment

      Distinct 3D image files like DICOM and STL can be overlaid or aligned using CAD software. In the field of digital dentistry, aligning DICOM and STL is useful to plan implant placement. Details of image alignment will be addressed in the next chapter.

       1.1.7 Resolution

      In 2D images, the resolution depends on the number of pixels. A pixel is the smallest unit of a digital image that can be displayed and represented on a digital display device, also known as a picture element (pix = picture, el = element). A pixel is represented by a dot or square on a computer display screen. Pixels are the basic building blocks of a digital image or display and are created using geometric coordinates. Depending on the graphics card and display monitor, the quantity, color combination, and size of pixels vary and are measured in terms of the display resolution. A full high‐definition (full HD) image is 1920 pixels in width and 1080 pixels in height, totaling 2.07 megapixels. Ultra HD (also known as 4 K) resolution has 3840 × 2160 pixels, totaling 8.3 megapixels.

      The 3D version of a pixel is called a voxel. In general, the smaller the voxel size is, the better quality a 3D reconstructed model will have.

      The quality of radiographic images depends on contrast resolution and spatial resolution. Contrast resolution is proportional to the size of the contrast scale available to produce the image. As a result, the higher the contrast resolution of an image, the easier it will be to distinguish between multiple densities. In digital imaging, contrast resolution depends on the bit‐depth of the imaging method, following a logarithmic scale. Therefore, a panoramic radiograph produced with an 8‐bit system can show 2⁸ = 256 different gray‐scale levels distributed from black to white. A CBCT device with a 12‐bit system will offer 2¹² = 4096 gray‐scale values. Spatial resolution is the ability of an imaging method to identify the actual limits and differentiate two adjacent structures [2–4].

      Resolutions in 3D CAD files basically depends on the size and densities of the meshes. The quality of the respective manufactured device, however, is also dependent on factors related to CAM (e.g., resolution of 3D printers or milling devices). For 3D printers, there will be factors related to the resolution such as the number of layers and layer thicknesses. For milling machines, the resolution will be dependent on the number of axes and size of burs (see Chapter 3).

1.2 History of Digital Dentistry

      Science and technology are the foundations of human development. From the rudimentary creation and improvement of stone tools, accompanied by the breakthrough in learning to control fire and the Neolithic revolution, which multiplied the sustenance availability, to the significant invention of the wheel which allowed humans to travel and produce machinery, or the overcoming of physical barriers with advancements in communications, technology is what sets humanity apart.

      Alongside technology, a lexicon development has always been necessary to provide a common understanding of innovations in the meaning and usage of new or existing words. The technological lexicon expansion will often plainly exhibit a