Is STL enough? When are PLY and OBJ necessary?

    Conventional impression methods are increasingly being replaced by digital scanning. Data acquired through intraoral scanners can be quickly transformed into restorations using milling machines or 3D printers. However, a critical question arises: in which file format should the scan be saved? There are different file types such as STL, PLY, and OBJ, each containing different types of information. Choosing the correct format is essential for seamless communication with the laboratory and for achieving accurate results.

 

STL (Standard Tessellation Language)

-      STL is the most commonly used file format in digital dentistry and offers universal compatibility across software systems.

-      The surface is represented as a mesh of triangles. Each triangle is defined by three vertices and a normal vector. Depending on the complexity of the surface, the number of triangles varies; simpler and flatter surfaces require fewer and larger triangles, whereas more complex anatomies are represented with smaller and more numerous triangles.

    The main disadvantage of STL is that it does not contain color or texture information. This limitation can make it more difficult to distinguish between tooth structure and gingiva, especially when defining restoration margins during the design phase.

 

PLY (Polygon File Format)

-      PLY is a 3D model format that includes color and texture information, while still using a mesh-based structure.

-      Additional data (such as color) can be stored for each vertex. This allows for a more realistic representation of tooth color, soft tissues, and surface details compared to STL.

This feature provides significant advantages during restoration design. However, not all CAD software supports PLY, and some intraoral scanners may not offer PLY export.

 

OBJ (Object File Format)

-      OBJ is another 3D file format, but with a more advanced data storage capacity.

-      It can include not only geometry but also color, texture, and surface normal information.

-      OBJ files are typically larger than STL and PLY files. When exporting as OBJ, additional files such as MTL (material file) and JPG/PNG (texture maps) are generated. Unlike PLY, texture information is stored as a separate file, which can be thought of as a layer wrapping around the 3D surface.

    The use of OBJ in dentistry is still less common compared to STLand PLY, and not all systems support it.

    The choice of file format depends on the compatibility of the scanner and the software being used. While all three formats provide similar geometric accuracy, the key difference lies in the type and amount of data they store.