Description of STL

The main purpose of the STL file format is to encode the surface geometry of a 3D object. It encodes this information using a simple concept called “tessellation.”

Tessellation is the process of tiling a surface with one or more geometric shapes eliminating overlaps or gaps, such as a tiled floor or wall. Tessellation can involve simple geometric shapes or very complicated (and imaginative) shapes.

The basic idea was to tessellate the two‐dimensional (2D) outer surface of 3D models using tiny triangles also called “facets” and store information about the facets in a file.

For example, if you have a simple 3D cube, this can be covered by 12 triangles. In a cube, there are two triangles per face. Since the cube has six faces, it adds up to 12 triangles.

If you have a 3D model of a sphere, then it can be covered by many small triangles.

The Albert Consulting Group for 3D Systems realized that if they could store the information about these tiny triangles in a file, then this file could completely describe the surface of an arbitrary 3D model. This formed the basic idea behind the STL file format.

In a nutshell, an STL file stores information about 3D models. This format describes only the surface geometry of a three‐dimensional object without any representation of color, texture, or other common model attributes. This format allows the image to be 3D printed, models to be generated and crowns to be milled.

Description of 3D Volume

Today, the CBCT is a very common tool available in most of the dental practices. It provides lower radiations in order of 92–118 μSv. As discussed in the previous chapters, it provides an abundant information on anatomical landmarks and different images, sagittal, axial, and coronal. In relation to our needs, it provides the bone and the teeth known as high density tissue. One of the setbacks in the images collected, we can get artifacts in the shape of streaks originating from restorations and crowns. This can be an impediment since the guides and prosthetic work‐up will depend on the clarity of the teeth image. To limit artifacts: reduction of the field of view (FOV), Placing cotton rolls between the arches and the mucobuccal fold during the acquisition may decrease the beam hardening artifact. Patient‐related artifacts are typically due to motion during the acquisition of the data volume, such as pronounced respiration, eye motion, or tremors. Movement artifact ranges from blurring to double contours of bony outlines.

2D images are made of several pixels represented as squares, with height and width. The smaller the pixel, the better the quality of the picture. The same concept applies to a 3D data volume. A voxel is the smallest 3D element of the volume, and is typically represented as a cube or a box, with height, width, and depth. Each 3D voxel represents a specific X‐ray absorption. The voxel size on CBCT images is isotropic, which means that all the sides are the same dimension with uniform resolution in all directions. This is considered an advantage of the CBCT because if a certain structure needs to be measured, the measurement will be exact in all the three orthogonal planes. There are different voxel sizes depending on the capabilities of each unit. Voxel size needs to be smaller than the desired anatomical structure for adequate representation. For example, the first sign of periapical inflammatory lesion is discontinuity of the lamina dura; thus, if visualization changes to the periapical area (lamina dura and PDL space) is desired, a CBCT less than 0.2 mm needs to be acquired. Structures smaller than the voxel size will not be visualized in the scan (example, small cracks in the enamel). As previously mentioned, soft tissue structures (mucosa, gingiva, cartilage, nerves, blood vessels) cannot be evaluated in a CBCT study.

During the reconstruction phase, the set of 150–600 basis images will be sent to a dedicated software that will then create a 3D volume reconstruction of the data volume which is called rendering. To render a 2D projection of the 3D data set, one needs to define the opacity and color of every voxel. This is usually defined using an RGBA (for red, green, blue, alpha) transfer function that defines the RGBA value for every possible voxel value. We are focused on teeth and the bone which each will have a value. This will allow the clinicians to do a segmentation of the rendering in certain planning software’s making the image clearer from the artifacts or un‐needed structures.

Intra‐Oral Scanners (IOS)

There are different IOS in the market. The older version required powder in order to capture the image not anymore in the new models. The principle is to stitch multiple images to make them continuous. Based on the literature, not much of difference between all of them other than the rapidity in capturing the images based on the stitching and the software that allows it to happen but yet a recent publication showed there is a difference between some (Nedelcu et al. 2018). The IOS have shown similar accuracy to conventional impressions (Ender et al. 2016) and some suggested can be used as a replacement for conventional impressions when restoring up to ten units without extended edentulous spans. In a Prosthetic case, a body scan is needed to capture the position of the implant and with the IOS you can even capture the emergence profile that was created without resorting to multiple steps done during the conventional way (Elian et al. 2007