**1. Introduction**

Post and cores are commonly used to restore endodontically treated teeth with the extensive coronal structural loss [1]. The retention of the post is a fundamental factor influencing definitive restoration longevity and success. Post length, shape, diameter and

**Citation:** Alqarni, H.; AlHelal, A.A.; Jekki, R.; Kattadiyil, M.T.; Sayed, M.E.; Jain, S.; Vahdati, S.A.; Dehom, S. In Vitro Study Comparing Retention of Custom Post and Cores Fabricated Using Conventional, CAD/CAM Milling and 3D-Printing Techniques. *Appl. Sci.* **2022**, *12*, 11896. https:// doi.org/10.3390/app122311896

Academic Editor: Mary Anne Melo

Received: 17 October 2022 Accepted: 19 November 2022 Published: 22 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

surface texture, and the type of cement used are associated factors that may affect the retention and stability of the post [2].

Custom-cast post and cores (CCPCs) have been reported to have superior adaptation and fit the radicular post-space walls when compared with prefabricated posts [3,4]. Compared with other post-and-core systems, CCPCs are advantageous, as they exhibit higher resistance to rotational movement forces [5], have a superior success rate [6,7], and have better retrievability for endodontic re-treatment [6,8]. Various materials can be used for fabricating custom post and cores. These include gold alloys [9–11], titanium alloy [9,11], base-metal alloys [9,12–14], zirconium oxide [11,15,16], glass fiber-reinforced composites [11,17], etc., which can be fabricated by different techniques such as casting [9–13], CAD/CAM milling [11,12,15–17], and 3D printing [11,12,14]. Various alloys are used for the fabrication of CCPCs, and most of them exhibit good retention and fracture strength [18,19]. However, CCPCs have been associated with catastrophic abutment fractures due to their high stiffness [4,5].

Custom post and cores have also been reported to be fabricated out of zirconia to overcome the esthetic limitations associated with metal CCPCs. Zirconia has been reported to have high flexural strength, high fracture toughness, chemical stability, biocompatibility, favorable optical properties, greater toughness, and maximum adaptability to the canal, as well as good esthetic characteristics [20,21]. As discussed by Baba et al. [22] and Ozkurt et al. [23], zirconia as a post-and-core material has a few disadvantages. These include more frequent root fractures (due to high rigidity) than fiber posts, decreased retention of the post (due to poor bonding between zirconia and resin cement), and poor retrievability in cases that need endodontic re-treatment.

Titanium has also gained wide acceptance in dentistry due to its biocompatibility, excellent corrosion resistance, reduced cost, ease of fabrication, and superior mechanical properties, which make it suitable as a post-and-core material [24–27]. Computeraided design/computer-aided manufacturing (CAD/CAM) additive (3D printing) and CAD/CAM subtractive manufacturing techniques (milling) can be used to manufacture custom post and cores.

To the best of our knowledge, the current literature lacks the relevant information assessing the relationship between the retention of custom-cast non-precious post and cores (CCNPPCs), custom-milled titanium post and cores (CMTPCs), custom-printed titanium post and cores (CPTPCs) and custom-milled zirconia post and cores (CMZPCs). Therefore, this study aims to evaluate the differences in the retention of CCNPPCs (control group), CPTPCs, CMTPCs, and CMZPCs. The tested null hypothesis was that there were no differences in the retention values of the various tested custom post and cores.
