  Objectives: An increasing number of CAD/CAM (computer-aided
design/computer-aided manufacturing) hybrid materials have been introduced to
the dental market in recent years. In addition, CAD/CAM hybrid materials for
additive manufacturing (AM) are becoming more attractive in digital dentistry.
Studies on material microstructures using micro-computed tomography ($\mu$-CT)
combined with scanning electron microscopy (SEM) have only been available to a
limited extent so far.
  Methods: One CAD/CAM three-dimensional- (3D-) printable hybrid material
(VarseoSmile Crown plus) and two CAD/CAM millable hybrid materials (Vita
Enamic; Voco Grandio), as well as one direct composite material (Ceram.x duo),
were included in the present study. Cylindrical samples with a diameter of 2 mm
were produced from each material and investigated by means of synchrotron
radiation $\mu$-CT at a voxel size of 0.65 $\mu$m. Different samples from the
same materials, obtained by cutting and polishing, were investigated by SEM.
  Results: The 3D-printed hybrid material showed some agglomerations and a more
irregular distribution of fillers, as well as a visible layered macrostructure
and a few spherical pores due to the printing process. The CAD/CAM millable
hybrid materials revealed a more homogenous distribution of ceramic particles.
The direct composite material showed multiple air bubbles and microstructural
irregularities based on manual processing.
  Significance: The $\mu$-CT and SEM analysis of the materials revealed
different microstructures even though they belong to the same class of
materials. It could be shown that $\mu$-CT and SEM imaging are valuable tools
to understand microstructure and related mechanical properties of materials.
