Direct resin composite restoration of endodontically-treated permanent molars in adolescents: bite force and patient-specific finite element analysis

Abstract Objective To evaluate the influence of three levels of dental structure loss on stress distribution and bite load in root canal-treated young molar teeth that were filled with bulk-fill resin composite, using finite element analysis (FEA) to predict clinical failure. Methodology Three first mandibular molars with extensive caries lesions were selected in teenager patients. The habitual occlusion bite force was measured using gnathodynamometer before and after endodontic/restoration procedures. The recorded bite forces were used as input for patient-specific FEA models, generated from cone-beam computed tomographic (CT) scans of the teeth before and after treatment. Loads were simulated using the contact loading of the antagonist molars selected based on the CT scans and clinical evaluation. Pre and post treatment bite forces (N) in the 3 patients were 30.1/136.6, 34.3/133.4, and 47.9/124.1. Results Bite force increased 260% (from 36.7±11.6 to 131.9±17.8 N) after endodontic and direct restoration. Before endodontic intervention, the stress concentration was located in coronal tooth structure; after rehabilitation, the stresses were located in root dentin, regardless of the level of tooth structure loss. The bite force used on molar teeth after pulp removal during endodontic treatment resulted in high stress concentrations in weakened tooth areas and at the furcation. Conclusion Extensive caries negatively affected the bite force. After pulp removal and endodontic treatment, stress and strain concentrations were higher in the weakened dental structure. Root canal treatment associated with direct resin composite restorative procedure could restore the stress-strain conditions in permanent young molar teeth.


Introduction
First permanent molar teeth exert the highest force during chewing. 1 Masticatory performance is usually assessed by masticatory tests and bite force measurement. 2 Any structural alteration affecting tooth integrity may impact the masticatory process. 3 Permanent first molars are the first posterior teeth to erupt and the most affected by dental caries. 4 The deep sulcus and fissures on the occlusal surface facilitate accumulation of acid produced by bacteria, as well as the early eruption contributes further to caries progression. 4 Progression in dental caries can result in pulp injuries, periapical infection and tooth loss. 5 Damage caused by the caries process can be so extensive that the loss of structural integrity can affect masticatory functions. Dental caries can negatively impact adolescents, resulting in uncomfortableness, pain, functional impact, difficulty in chewing, and emotional problems. 6 Further progression of caries may result in premature tooth loss with serious consequences, including a collapse of the posterior dental arch space and, consequently, the extrusion of antagonist tooth, interfering in occlusal plane. 7 Therefore, management of carious permanent molars is important for the quality of life and growing process of children and adolescents. 8 Physiological bite forces have an important role on the progression of carious lesions to cavitation. 9 Bite loading applied over multiple cycles can cause and propagate stresses, contributing to the failures clinically observed. 10 Pulpal involvement caused by extensive caries may result in pain and reduce mastication force due to natural adaptation. When endodontic treatment is recommended, root canal access and pulp removal can eliminate the pain in the first section; however, without recovering the resistance of weakened tooth structure. Performing endodontic treatment followed by direct restoration with bulk-fill resin composite is a good alternative to maintain tooth function. 11 Resin composites are considered the first choice for restoration of severely damaged teeth in young patients due to their esthetic quality, conservative approach, and handling characteristics. 12,13 Bulk-fill represents a recent innovation to simplify resin composite insertion, which may also decrease polymerization shrinkage stresses and cuspal deflection. 14,15 Biomechanical analysis of endodontically-treated molar teeth with extensive loss of tooth structures caused by extensive caries in adolescents is scarce.
How restorative treatments improve the bite force and promote homogeneous stress distribution have not yet been determined. Moreover, large areas of unsupported enamel resulted from dentin destruction could increase the occurrence of tooth fracture under functional bite forces. 16 In addition, functional bite after pulp removal is lost, and the normal load applied on a weakened molar can result in tooth fracture. Our study sought to evaluate how different levels of tooth structure loss in endodontically-treated permanent molars restored with direct bulk-fill resin composite affect stress distribution and bite force in three adolescent patients. The stress was evaluated using patient-specific finite element models that represent three progressive structure losses of molar teeth, which models created from each individual patient's anatomy, allowing a loading that more truthfully mimics individual conditions. 17 The null hypothesis was that the root canal treatment followed by direct bulk-fill resin composite restorative process would not recover the bite force and would not improve the stress distribution of a permanent molar.

Patient selection
This study was approved by the ethics committee (protocol nº 1.685.725). Three patients were selected (9, 10 and 12 years old, hereinafter "adolescents") with a first lower permanent molar affected by caries with pulpal involvement resulting in dental pain requiring endodontic treatment, considered as biopulpectomy. The patients selected had teeth with various and progressive levels of tooth structure loss   to perform the structural analysis ( Figure 3C). A mesh was generated with a 0.5 mm element size subjected to convergence analysis before mechanical simulation.
All materials were considered linear-elastic, isotropic, and homogeneous. The applied material properties (elastic modulus, Poisson's ratio, tensile strength, and compressive strength) were obtained from the literature ( Table 1)

Stress and strain distribution
The modified von Mises stress distribution for the tooth structure at the initial unrestored condition and after resin composite restoration are shown for the three teeth in Figures 4, 5 and 6. For the M1 loading condition, in which the lower patient adaptive bite force was simulated, the stresses were significantly lower on the remaining coronal tooth structure ( Figures 4A, 5A and 6A). The M2 loading condition, in which the final (high) bite force was simulated on the unrestored tooth ( Figures 4B, 5B and 6B  structure strain. After restorative procedure, the tooth structure strain reduced significantly.

Discussion
Three different levels of the dental structure loss caused by caries and pulp involvement on first permanent mandibular molars were simulated to study the influence of the amount of remaining dental tissue on the bite force and stress distributions in mandibular molars. The amount of the dental structure loss had no effect on the bite force when comparing the initial conditions, demonstrating that the condition of the pulp may be the main factor for determining bite force adaptation. 22 Although this description can be interpreted as a speculative thesis for supporting bite force adaptation, it was very clear during the initial bite force measurement, when all patients reported that they could not apply more force because they are feeling pain and concerned about the tooth fracture.
However, a large difference was observed in bite force magnitude and stress distribution before and after endodontic treatment and resin composite restoration.
Therefore, the null hypothesis was rejected.
Bite force has been widely used as an acceptable test for masticatory system function. 17,23 The bite forces measured at initial condition for the three patients were substantially lower than after the endodontic and restorative procedures. The lower bite force could have been caused by chewing alteration due to pulp pain. 20 Dental pathologies, such as poor periodontal and dental conditions, can result in a reduced bite force. 24 A reflex withdrawal reaction to the pain evoked on biting and the induction of masseteric inhibitory periods 25 can explain the lower bite forces. 22 Loss of dental hard tissues due to extensive dental caries generated smaller occlusal contact areas. 24,25 Moreover, the weakened coronal tooth structure may determine natural adaptation by reducing the bite force to prevent tooth fracture. 28 Patients with extensive caries and/or pulpal involvement usually lose dental function and have difficulty in eating, which should be considered an indicator of oral problems and  to restore large cavities in endodontically-treated molar teeth. The option for bulk-fill composite resin for restoring endodontically-treated molar teeth is based on the good clinical performance described in recent systematic reviews and metanalysis when compared with incremental filling technique. 34,35 Other important aspect for adolescents is that restorations in molar teeth using regular paste bulk-fill resin composites require a shorter time to be performed than using the incremental filling technique. 36 This study showed, based on biomechanics, that bulk-fill resin composite restoration following endodontic treatment is a good strategy for maintaining molar teeth that have been severely affected by caries. Bulk-fill resin composite, besides presenting lower polymerization shrinkage stresses and cuspal deflection than conventional composite resin, 11,37 was shown to be able to provide relatively even distribution of stresses in the remaining tooth structure, as well as improved masticatory ability in young patients.

Conclusions
Extensive caries in molar teeth in the specific clinical conditions simulated in this study negatively affected bite force and caused higher stress concentrations in the weakened tooth structure. Early definitive resin composite restoration should be performed after endodontic intervention, since higher bite forces are achieved when dental pain is removed, thus increasing the possibility of tooth fracture. Endodontic treatment in molar teeth followed by direct resin composite restoration was an effective method to reestablish oral biomechanical performance in these three adolescents.