Endodontically treated teeth are more susceptible to fracture compared to vital teeth due to structural loss from caries, trauma, and endodontic access cavity preparation.¹ The reduction in tooth stiffness and altered biomechanical behaviour necessitate restorations that can reinforce the remaining tooth structure while preserving as much sound tissue as possible. Premolars are at higher risk of fracture because of their anatomical characteristics and their exposure to both compressive and shear forces during mastication.²

Traditionally, full-coverage crowns have been recommended for restoring endodontically treated posterior teeth to protect them from fracture. However, full crowns require extensive tooth preparation, which may further weaken the remaining tooth structure.³ In recent years, advancements in adhesive dentistry and high-strength ceramic materials have encouraged the use of more conservative restorative options such as onlays and endo-crowns. Onlays restore cuspal coverage while preserving a significant portion of tooth structure.⁴ Endo-crowns, on the other hand, utilize the pulp chamber for macromechanical retention and adhesive bonding for micromechanical retention, eliminating the need for post placement. Although these restorations are considered less invasive, their ability to withstand functional occlusal forces compared to conventional full crowns remains a topic of debate.⁵

Therefore, this in vitro study aimed to evaluate and compare the fracture resistance and fracture patterns of endodontically treated premolars restored with onlays, endo-crowns, and full crowns. The null hypothesis was that there would be no significant difference in fracture resistance among the three restorative approaches.

Sixty freshly extracted human maxillary premolars free from caries, cracks, or restorations were selected. Teeth were cleaned and stored in distilled water at room temperature until use. Standardized root canal treatment was performed on all specimens using rotary nickel–titanium instruments, followed by obturation with gutta-percha and resin-based sealer. After endodontic treatment, specimens were randomly allocated into three groups (n = 20) according to the type of restoration. In Group I, teeth were prepared for ceramic onlays with cuspal reduction of 2 mm and rounded internal line angles. In Group II, endo-crown preparations were performed by removing coronal tooth structure to the level of the cemento-enamel junction and creating a standardized pulp chamber depth of 3 mm. In Group III, full crown preparations were carried out with a uniform axial reduction of 1.5 mm and occlusal reduction of 2 mm, with a circumferential chamfer finish line. All restorations were fabricated from lithium disilicate ceramic using a standardized laboratory protocol. The internal surfaces of the restorations were etched with hydrofluoric acid, silanated, and adhesively luted to the prepared teeth using a dual-cure resin cement. Specimens were stored in distilled water at 37°C for 24 hours before testing. Fracture resistance testing was conducted using a universal testing machine. Each specimen was mounted vertically and loaded axially with a stainless-steel indenter at a crosshead speed of 1 mm/min until fracture occurred. The maximum load at fracture was recorded in Newtons. Fracture patterns were examined visually and under magnification and classified as favourable (above the cemento-enamel junction) or unfavourable (below the cemento-enamel junction).

The mean fracture resistance values and standard deviations for each group are presented in Table 1. The full crown group demonstrated the highest mean fracture resistance, followed by the endo-crown group and the onlay group. One-way ANOVA revealed a statistically significant difference among the three groups (p < 0.05). Post hoc analysis showed that the full crown group had significantly higher fracture resistance than the onlay group, while the difference between endo-crowns and full crowns was not statistically significant.

Favourable fracture patterns were observed in 70% of specimens in the onlay group and 65% in the endo-crown group (Table 2). In contrast, the full crown group exhibited a higher incidence of unfavourable fractures extending below the cemento-enamel junction (Table 3).

Table 1. Mean fracture resistance values of the study groups

Table 2. Comparison of fracture patterns among the study groups

Table 3. Statistical comparison of fracture resistance between groups (Post hoc analysis)

The results of the present study demonstrated that full crowns provided the highest fracture resistance among the tested restorative options. This finding can be attributed to the circumferential coverage offered by full crowns, which helps distribute occlusal forces more evenly along the remaining tooth structure. However, despite superior fracture resistance values, the full crown group exhibited a higher frequency of unfavorable fractures extending below the cemento-enamel junction, which may render the tooth non-restorable in clinical situations.

Endo-crowns showed fracture resistance values comparable to full crowns, with no statistically significant difference between the two groups. This observation agrees with previous studies by Lamba et al (2024)⁶ and Abbas et al (2024)⁷ that have highlighted the biomechanical advantages of endo-crowns, particularly their ability to utilize the pulp chamber for retention while preserving radicular dentin. The monolithic structure and adhesive bonding of endo-crowns allow for more homogeneous stress distribution, thereby reducing stress concentration at the cervical region of the tooth.

Several authors Jin et al (2024)⁸, Abed et al (2022) ⁹ and Salah et al (2023) ¹⁰ have reported that endo-crowns perform favourably in premolars and molars when sufficient pulp chamber depth and enamel margins are available. Studies comparing endo-crowns with conventional post-core crowns have demonstrated similar or even superior fracture resistance for endo-crowns, while avoiding the risks associated with post placement such as root perforation and vertical root fracture.¹¹ The findings of the present study further support these observations, suggesting that endo-crowns can be considered a reliable alternative to full crowns in endodontically treated premolars.

Onlay restorations exhibited the lowest fracture resistance among the groups; however, the recorded values were still within the range of normal masticatory forces reported for posterior teeth. Importantly, onlays showed the highest percentage of favorable fracture patterns, indicating that failures associated with onlays are more likely to be confined above the cemento-enamel junction and therefore amenable to repair or retreatment. Previous studies have similarly reported that cuspal coverage onlays, when adhesively bonded, can effectively reinforce weakened tooth structure while maintaining a conservative approach.^12-14

The results of this study are consistent with existing literature emphasizing that fracture resistance alone should not be the sole determinant when selecting a restorative option for endodontically treated teeth. Preservation of tooth structure, mode of failure, and long-term prognosis are equally important considerations. Conservative adhesive restorations, such as onlays and endo-crowns, offer a favorable balance between mechanical performance and biological preservation

 Limitations

Limitation of this in vitro study include the absence of thermal cycling and dynamic loading, which may influence the clinical performance of restorations. Future studies incorporating fatigue loading and different restorative materials are recommended to better simulate intraoral conditions.

Within the limitations of this study, full crowns exhibited the highest fracture resistance, followed by endo-crowns and onlays. However, conservative restorations such as endo-crowns and onlays showed favorable fracture patterns and adequate strength to withstand functional forces. These restorations may be considered reliable alternatives to full crowns for endodontically treated premolars, particularly when tooth structure preservation is a priority

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2. Nasr M, Abd-Elhaliem NN. Effect of inlay, onlay, and endocrown restorations on the mode of failure and fracture resistance of endodontically treated maxillary premolar teeth. An in vitro study. Egyptian Dental Journal. 2024 Apr 1;70(2):2031-40.
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