This in vitro study examined the fracture resistance of three aesthetic materials utilized as post and core restorations manufactured using CAD-CAM technology. Among the three tested groups, the custom PEEK post and core restorations showed the least values in fracture resistance. The fracture resistance of the custom FRC and PICN post and core restorations showed notably superior results when compared to the customized PEEK post and core restorations. The null hypothesis, suggesting no difference in fracture resistance among these three materials, was rejected.

In the present study, the selection of the maxillary central incisors was based on their pronounced need for aesthetic restoration. In addition to their characteristic feature of possessing a straight, single, and flared root canal. This choice was made to facilitate standardization and enhance validity of the study in comparison to utilizing artificial teeth. A standardized post space of 9 mm in length was created to align with the protocols followed in previous studies [43, 53].

An intraoral scanner was utilized to scan the teeth, including post spaces, enabling the fabrication of post and core restorations through a fully digital workflow. This method aimed to enhance precision, reduce time consumption, and confront the limitations associated with traditional methods. Alhasher et al. [8] proposed that employing intraoral scanners for constructing single-piece post and core restoration could offer a reliable alternative to conventional methods. Elter et al. [35] suggested that intraoral scanners are viable for capturing impressions of the post space when the post depth measures less than 14 mm, thus facilitating a streamlined impression procedure. Additionally, Vogler et al. [36] concluded that the fully digital chairside workflow, coupled with CAD-CAM-fabricated post and core, displayed superior accuracy of fit and practicality in impression-taking compared to conventionally fabricated cast post and core. Consequently, CAD-CAM technology holds the potential for enabling single-session restoration of endodontically treated teeth with customized post and core solutions, offering efficiency and precision in dental practice.

Previous studies employed ceramic materials with high elastic modulus such as lithium disilicate and zirconia to construct custom post and core restorations. It was found that there was stress concentration on root dentin resulting in catastrophic failure [44, 47, 54,55,56,57,58]. The investigated materials in the current study exhibit a modulus of elasticity close to dentin, approximately 18 GPa, [59] leading to a more uniform distribution of stress along the root. Consequently, this even distribution mitigates the occurrence of vertical root fractures.

In the present study, the exclusion of crown fabrication aimed to amplify the effect of post and core materials in the assessment of fracture resistance of endodontically treated teeth [41,42,43,44]. On the other hand, studies [40, 60, 61] that used a crown or coping to cover post and core restorations had fracture resistance higher than the current study, however, this might conceal the actual differences between the used custom made post and core materials. Thermal and mechanical aging have significant impacts on the bonding and fracture resistance of endodontically treated teeth restored with post and core restorations. Despite the relatively brief duration of thermomechanical aging in our study, the observed outcomes align with findings from comparable studies, [45, 46] which also reported similar statistical differences and failure modes. Notably, mechanical loading exerted direct effect on the integrity of the post and core restoration, while thermal cycling notably affected the interface of cemented surfaces, exacerbating the degradation of bond strength and mechanical properties of the post and core restoration [62, 63]. Moreover, several studies [9, 30, 42, 47, 55, 64] refrained from conducting artificial aging, emphasizing that their investigations were specifically focused on the materials and methodology rather than projecting outcomes under different conditions.

The applied force was oriented at a 135-degree angle in relation to the long axis of the tooth, targeting the palatal surface. This was achieved using a specially designed chisel head designed to emulate the lower central incisor, thereby replicating the clinical situation [47].

While PEEK exhibits comparable elastic modulus and flexural strength to dentin, excessive loading on PEEK post and core restorations tends to concentrate stress on the surrounding cement layer [19]. This often results in debonding, given that the sole bonding mechanism relies on mechanical interlocking attained through sandblasting [9]. Heightened stress can concentrate on the tooth, potentially causing catastrophic root fractures. The recorded values for PEEK posts and core restorations were 286.16 ± 67.09 N, aligning with findings by Özarslan et al. [65] who similarly assessed fracture resistance across PEEK, zirconia, and glass fiber. Their study revealed that the average failure loads of PEEK posts and core restorations were 306.7 ± 74.0 N.

Additionally, there is agreement with Teixeira et al. [20] who conducted a comparison of the fracture resistance between custom-made post-and-cores of PEEK and Nano-ceramic Composite. Their findings reported a mean fracture resistance of PEEK post and core restoration at 379.46 ± 119.8 N.

In contrast to our findings, Abdelmohsen et al. [39] investigated the fracture resistance of various post and core systems. They reported mean values of fracture resistance for CAD-CAM milled PEEK post and core restorations at 1055.25 ± 119.31 N. This discrepancy may be attributed to the study’s focus on premolars and the varied angulations of applied force.

The fracture resistance mean values for the PICN group in this investigation were measured at 426.76 ±77.99N. These findings were consistent with the observations of Spina et al. [43] where they reported mean fracture resistance values of 414.5 ±83.9N for PICN and 407.6 ±109N for FRC. Additionally, Elmaghraby et al. [45] also observed agreement in their research, noting fracture resistance mean values of 386.6 ±25.78N for one-piece post and core PICN restorations.

The findings in this study contrasted with those of Alkhatri et al. [30] who investigated the impact of materials on the apical extension of root fracture. Alkhatri et al. reported mean fracture resistance values of PICN as 271.06 ±69.57N. The variations in observed values could stem from employing diverse approaches in fabricating the post and core utilizing acrylic resin. These differences might arise due to distinct methodologies, such as variations in resin composition, application techniques, curing processes, or the specific protocols followed during fabrication.

The FRC group exhibited the highest mean failure load value of 452.60 ±105.90N. This finding is consistent with the results of Eid et al. [29] who investigated the fracture resistance and failure mode of endodontically treated teeth restored with custom-made FRC post and core restorations. They noted a mean fracture resistance of 367.06 ±72.34N for CAD-CAM glass fiber post and core restorations, further supporting the consistent performance of glass fiber reinforced composite materials in enhancing fracture resistance.

In the current study, the fracture resistance among the three tested groups fiber reinforced composite samples, PEEK, and polymer infiltrated ceramic network measured 452.60 ± 105.90 N, 286.16 ± 67.09 N, and 426.76 ± 77.99 N, respectively. These values exceed the recorded maximal occluding force generated by maxillary incisors, which stands at 146 ± 44 N [30]. This indicates that each of the three tested groups could be considered acceptable materials of choice for anterior tooth restoration requiring a customized post and core.

In terms of the failure mode, no significant difference was noted among the tested groups. PICN demonstrated the most favorable results, followed by PEEK, while the fewest number of restorable specimens were found in FRC, with counts of 7, 6, and 5, respectively. These findings align with the majority of previous studies [30, 43, 45, 61, 65].

Due to its relatively rigid molecular chain structure, PEEK demonstrates notable ductility, allowing for substantial deformation under unilateral stress during compression [20]. When subjected to stresses within its yield limit, the material undergoes elastic deformation. However, when these stresses surpass the yield limit, [19] PEEK experiences plastic deformation and bending without encountering fracture or chipping [21]. This behavior contributes to understanding the prevalent failure patterns observed in PEEK post and core restorations. Applied force tends to transfer to the intermediate cement, causing dislodgement without inducing fracture in any component, whether the post, core, or the tooth structure itself. Alternatively, this force concentration on the root can lead to a vertical root fracture as reported in the current study.

According to the study conducted by Özarslan et al. [65] which focused on comparing the fracture strength of endodontically treated teeth restored with different post-core systems, it was found that 40% of the specimens experienced decementation without fracture. This finding is consistent with our research results. On the other hand, Pourkhalili et al. [64] in their study on the fracture resistance of various post and core systems, reported that none of the specimens exhibited debonding. This disparity in results could be attributed to their use of premolars instead of anterior teeth and the application of force in a different direction in their study. Additionally, Kasem et al. [66] utilized customized PEEK post and core restorations for compromised teeth, documented a successful five-year follow-up. However, the favorable outcome observed can be explained by the ferrule effect of several millimeters used in the case report.

The failure of the FRC material stemmed from the rupture of its fibers. In instances where a fiber-reinforced material experiences failure, a crack initiates within the matrix and proceeds along the interface encircling a dispersed fiber, resulting in the rupture of the fiber itself. Subsequently, the load transfers to adjacent fibers, causing them to rupture in sequence. This mode of failure is commonly described as ‘brush-like’ cracking [37, 52]. The fractured samples in the PICN group exhibited a crack propagation failure mechanism, aligning with the observations made by Aboushelib et al. [24] They reported a diminished resistance in the polymeric matrix of resin-infiltrated ceramics, resulting in a linear crack and the formation of a slipstream path in the direction of crack propagation.

In the current study, the unfavorable outcomes were 61.5%, 53.8%, and 46.2% for glass fiber reinforced composite, PEEK, and polymer infiltrated ceramic network, respectively. However, these results still outperformed those reported by Hamdy et al. [55] where 80% of translucent zirconia post specimens were deemed non-restorable, as well as by Özarslan et al. [65] who found that 72.5% of zirconia post-core group samples were irreparable. Additionally, Alkhatri et al. [30] investigated the impact of post and core materials on the apical extension of root fracture in root canal treated teeth. It was observed that in the PICN group, the extension of root fracture tended to be more coronal compared to the metal and zirconia groups, likely due to variations in the modulus of elasticity among the tested materials. Additionally, Bittner et al. [57] conducted a study comparing a one-piece milled zirconia post and core with other post-and-core systems. Their findings indicated that all specimens in the one-piece milled zirconia post and core group experienced catastrophic root failure.

Similar to any other in vitro study, it’s important to note that the current research couldn’t entirely replicate in vivo conditions as most fractures occurring in vivo would likely be due to fatigue conditions rather than a compressive static load. This study doesn’t fully replicate the clinical situation as it doesn’t consider the use of crown restoration, which, when applied with adequate ferrule, significantly increases the fracture resistance of endodontically treated teeth (ETT).