**4. Challenges**

Despites their benefits, all ceramic dental materials and their applications shows challenges which still need to be tacked.

The challenges in dentistry remain in understanding and improving the clinical performance of the biocompatible restorative materials by improving definition of failures, laboratory testing, and clinical studies. In fact, material factors, including differences in thermal conductivity and coefficient of thermal expansion between core and veneer, likely create residual stresses that redispose a restoration to chipping. Only requirements of patients further complicate the challenge of understanding factors that contribute to long term success of restoration. In this context, some works include report patient or provider factors or patient control groups.

Few recent works [119, 120] have been reported on clinical trials. Several improvements have been recently made in structural reliability via damage tolerance and flaw control [121, 122]. Predictive laboratory tests can reduce the need for expensive and time-consuming clinical tests, which sometimes exceed the commercial lifetime of the materials being evaluated. In addition, laboratory tests, likely over estimate clinical lifetimes, can replicate clinical failure modes.

Several parameters like dimensional accuracy, surface, and mechanical properties of ceramic dental materials should be improved to obtain high quality final products [123]. Another challenge is bacteriological safety of the final products which are in contact with human organs and tissues. However, it is necessary to make sterilized protocols while keeping intrinsic properties [123]. In addition, Lee et al. [124] showed that the accuracy in Z-direction is harder to enhance than in the directions X and Y because the presence of uncontrolled parameters like evaporation of material during machining, shrinkage, and spreading densification of the powder. Hence, the porosity of the ceramics is another challenge in dentistry. It has been showed that the porosity was reduced by adding dopants or viscous liquid forming phase, choosing the corresponding powder granulometry, and applying HIP to the green body [125]. Several studies revealed that the surface quality of ceramic materials depends strongly on the technique, raw material characteristics, and processing conditions [126, 127].

Moreover, there maining challenges for future advances are present abundant arenas for future innovations. Moreover, it will be important to determine where and how informed simplifications in testing conditions can be made.

Machining techniques and design methods should to be improved and innovated to achieve good ceramic restorations with subsurface damage and little surface.

CAD and CAM and fabrication processes creating veneers and cores separately will further evolve [128]. Thus, these approaches will be complemented by additive approaches, laying down materials only in places where it is needed to create a restoration [129]. However, these approaches have shown significant substantial hurdles.
