Restoration of endodontically treated premolars: the challenge of mechanical resistance

The restoration of a maxillary premolar after endodontic treatment requires rigorous management of biomechanical constraints. The presence of a mesio-occluso-distal (MOD) cavity reduces the structural robustness of the dental organ by up to 54% compared to a healthy tooth, significantly increasing the risk of vertical fracture under the effect of occlusal forces. Faced with this risk, the practitioner must choose between direct techniques, prioritizing tissue preservation, and indirect monolithic approaches.

This in vitro experimental study evaluates the fracture resistance of 44 human premolars subjected to four standardized MOD restoration protocols: an innovative self-adhesive bulk-fill composite (Stela), a short-fiber reinforced composite (EverX Flow), a conventional nanohybrid composite (Filtek Z250 XT), and a CAD/CAM lithium disilicate endocrown (Emax).

The objective is to compare the mechanical performance of these solutions to determine whether new direct technologies — aimed at reducing polymerization stress or reinforcing the resin matrix — can constitute a reliable clinical alternative to the monolithic endocrown. The study tests the hypothesis of the superiority of indirect restoration regarding the resistance capacity to occlusal loads in this context of significant coronal destruction.

Experimental protocol and methodology

This in vitro study was conducted on a sample of 44 extracted human maxillary premolars. Following standardized endodontic treatment, mesio-occluso-distal (MOD) cavities were prepared by a single operator. The samples were divided into four experimental groups (n=11) according to the restorative material and technique used:

• Group F1: Direct restoration with Stela (SDI) self-adhesive bulk-fill composite, applied in a single 5 mm increment.
• Group F2: Restoration using EverX Flow short-fiber reinforced composite (GC), light-cured for 20 seconds, completed with a 2 mm occlusal layer of Filtek Z250 XT nanohybrid composite (3M ESPE).
• Group F3: Conventional direct restoration using Filtek Z250 XT nanohybrid composite exclusively.
• Group F4: Monolithic lithium disilicate endocrown (Emax) designed by CAD/CAM (CEREC MCXL system) after Omnicam optical impression and design via CEREC Premium 4.4 software.

For the direct technique groups, the proximal walls were reconstructed using a Tofflemire matrix. Fracture resistance tests were performed with a universal testing machine (Instron) equipped with a 5 kN load cell, recording the failure load in Newtons. Statistical analysis was conducted using SPSS 27 and GraphPad Prism, using Shapiro-Wilk and Kolmogorov-Smirnov tests which revealed a non-parametric distribution of the data.

Mechanical superiority of the lithium disilicate endocrown

Statistical analysis of fracture resistance (FR) tests reveals a highly significant difference between the tested restoration protocols (P < 0.0001). The results show a clear hierarchy, dominated by indirect endocrown-type restorations.

Inter-group comparisons and significance

The study highlights two distinct levels of performance:

• The advantage of indirect: The F4 group (Endocrown) statistically significantly outperforms all direct restoration techniques, with a strength more than twice that of the best composite tested.
• Similarity of direct techniques: Although variations in means were observed, no statistically significant difference was found between the three groups of direct composites:
  • Fiber reinforcement (F2) shows a higher average than the conventional nanohybrid composite (F3), but without reaching the significance threshold (P = 0.471).
  • The Stela self-adhesive bulk-fill composite (F1) shows a slightly superior performance compared to Z250 (F3), which is non-significant (P = 0.838).
  • The comparison between EverX Flow (F2) and Stela (F1) reveals no notable difference (P = 1.000).

In summary, while the lithium disilicate endocrown offers the most robust protection against occlusal forces for endodontically treated premolars with MOD cavities, direct materials show comparable performance to each other, regardless of their technology (fibers, bulk-fill, or nanohybrid).

Clinical analysis of mechanical performance

The results of this in vitro study are conclusive: the lithium disilicate endocrown (Group F4) clearly outperforms all direct restoration options for endodontically treated maxillary premolars. With a fracture resistance reaching 1427.73 N, it offers significantly superior mechanical protection (P<0.0001) compared to composites, whether fiber-reinforced or self-adhesive.

In the practice, this differential is explained by the monolithic design of the endocrown, which exploits the pulp chamber for its retention, creating a more rigid tooth-restoration block. Interestingly: although fiber-reinforced composite (EverX Flow, 619.87 N) and self-adhesive bulk-fill composite (Stela, 561.75 N) show slightly higher averages than conventional nanohybrid (Z250, 452.22 N), the difference is not statistically significant. The innovation brought by fibers or new polymerization initiators fails, in this MOD cavity configuration, to bridge the gap with synthetic ceramic.

Limits and perspective

The study highlights, however, an important clinical reality: although less resistant than the endocrown, direct materials exhibit fracture values that remain within a clinically acceptable range for common occlusal forces. The main limitation lies in the in vitro nature of the experiment (n=11 per group), which does not reproduce the complex thermomechanical cycles of the oral environment.

For the practitioner, these data confirm that if tissue preservation is a priority, the lithium disilicate endocrown remains the treatment of choice for restoring the structural integrity of a severely damaged premolar, thus minimizing the risk of vertical fracture feared on this type of tooth.

Synthesis of mechanical performance

The verdict is clear: the lithium disilicate endocrown massively outperforms direct restorations with a fracture resistance of 1427.73 ± 156.96 N (p < 0.0001). Among the direct solutions, no statistical difference emerges, despite a slight numerical advantage for the EverX Flow fiber-reinforced composite (619.87 N) and the Stela self-adhesive bulk-fill (561.75 N) compared to the conventional nanohybrid (452.22 N).

In concrete terms, for the practitioner:

• Aim for indirect for safety: on a maxillary premolar with an MOD cavity, the endocrown literally doubles the mechanical resistance compared to direct, thus securing fragile structures against occlusal forces.
• Put the choice of composite into perspective: if you opt for direct restoration, the choice between fiber reinforcement, self-adhesive bulk-fill, or classic composite does not significantly impact the final fracture resistance.
• Optimize your time: the use of Stela self-adhesive composite is clinically validated as a fast alternative to conventional composite, without measurable loss of mechanical performance in direct technique.

Technical lexicon of the study

Endocrown (CAD/CAM): Indirect restoration made of lithium disilicate via a computer-aided design and manufacturing system (Cerec). In this study, this device displayed record performance with a fracture resistance of 1427.73 ± 156.96 N, a value significantly higher than all direct restoration materials tested.

Short-Fiber-Reinforced Composite (SFRC): Low-viscosity composite (EverX Flow) reinforced with E-glass micro-fibers (25% by weight). Used as a dentin substitute under an occlusal layer of nanohybrid composite, this biomimetic material has shown an average strength of 619.87 N on endodontically treated premolars.

Stela (Self-adhesive bulk-fill): Direct restorative composite resin allowing insertion in a single 5 mm increment. This innovative material, designed to simplify clinical protocols in deep cavities, showed a fracture resistance of 561.75 N during the mechanical tests conducted in this study.

Fracture resistance (FR): Failure load measured in Newtons (N) using a universal testing machine (Instron®). This value quantifies the ability of a restored tooth to withstand occlusal forces; study results show a significant difference (P<0.0001) between indirect and direct restorations.

MOD cavity: Standardised cavity preparation involving the mesial, occlusal, and distal surfaces. This type of structural loss reduces dental toughness by up to 54% compared to a healthy tooth, making maxillary premolars particularly vulnerable to masticatory forces and micro-cracks.

Lithium Disilicate (Emax): Machinable ceramic material (CAD blocks) used for group F4. Selected for its adhesive properties and its ability to meet aesthetic requirements, it achieved the highest level of mechanical protection against occlusal forces within the framework of this experimental protocol.

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