**4. Discussion**

This study evaluated the differences in the retention of different types of custom post and cores. Significant differences in retention were observed when the groups were compared. Therefore, the tested null hypothesis could be rejected; however, retention varied according to the fabrication technique and material used for manufacturing custom post and cores.

Several studies have reported acceptable clinical outcomes with CCNPPCs, which have been used for many years; hence, they were used as the control group in this study [2,4–8,18]. Advancements in materials science and the use of CAD/CAM have transformed the approach to fabricating indirect restorations in the field of prosthodontics, including the fabrication of custom post and cores. These new materials are easier to fabricate when compared with non-precious alloys. The current literature lacks evidence to support that the newly introduced custom-printed and -milled titanium post and cores offer comparable retention and effectiveness.

Various studies have reported that the success of teeth restored with custom post and cores correlated with the design, technique, and material used for the fabrication of the post and cores [6,7,42,44,45]. Recently, Wei Liu et al. found that post and cores fabricated with the CAD-CAM milled technique using a cobalt–chromium alloy could be an alternative to conventional casting for metal-post-and-core fabrication [46]. However, the retention of CAD/CAM milled or printed post and cores was neither evaluated nor tested.

The results of the present study showed that there was a significant difference (*p* < 0.05) between the retention of teeth restored with custom-printed and -milled titanium post and cores and that of teeth restored with CCNPPCs and CMZPCs. The retention difference between the two groups of custom-printed and -milled titanium post and cores was not significant (*p* > 0.05), and the CMZPC group revealed significantly lower retention (*p* < 0.05). The possible reason for the difference between custom post and cores and the retention of the zirconia group could lie in the post-surface configuration and roughness of the printed titanium post, which allows the post-and-core material to form micromechanical retention locks, whereas the smooth surface of zirconia reduces mechanical retention. Maya et al. reported similar results, with metal posts revealing significantly greater retention (495.5 N ± 75.9 N) than zirconia posts (241 ± 89.3 N) [47]. In addition, the study by Cohen et al. reported that zirconia posts had extremely low retention values (104.5 ± 34.8) [48]. These findings are consistent with the results of our study. Previous research has already demonstrated that resin cement can provide greater retention than

non-resin cement [49,50]. However, this observation was not confirmed in this study, as we did not evaluate post and cores cemented with non-resin cements.

The finishing and polishing of post-and-core surfaces may improve the fit; however, this could affect the retention. It was noted that among the custom post and cores, those that were made from printed titanium required less adjustment and showed a better fit than the other groups. Studies have reported that differences in retention may also be due to differences in the adaptation of the posts [51–53]. Poor adaptation can lead to an increase in resin-matrix cement layer thickness which can increase in cracks, pores, and micro-spaces. These structural defects can cause stress concentration leading to a reduction in interface strength [51–53]. Amin et al. [54] reported that a higher volume of resin cement can also cause higher polymerization shrinkage leading to poor bond strength.

To date, due to the complexity of casting post and cores and the wide range of available materials, few studies have quantitatively evaluated the fit and accuracy of custom post and cores using micro-CT scanning, nor have they evaluated the fit and accuracy of custom post and cores via a visual inspection or direct measurements of the gap filled with cement material, as the latter has only been used to evaluate the internal fit and adaption of dental restorations [55,56].

The limitations of this study and recommendations for future ones are listed below:

