Three-dimensional (3D) computational models with different combinations of caries parameters (caries location, caries size and caries induced pulp shrinkage) and occlusal wear factors (enamel thickness, marginal ridge height and cuspal slope) were developed and analyzed using the extended finite element method (XFEM) to identify the stress distribution, crack initiation load and ultimate fracture load values. The effect of a non-drilling conservative treatment using resin infiltration on the recovery of mechanical properties of carious molar teeth was also investigated.
Presence of fissural caries, worn proximal marginal ridge and decreased enamel thickness due to occlusal wear, imparted a significant negative effect on the crack initiation load value of the lower molar models. Accordingly, models with intact and strong proximal marginal ridge, generally exhibited higher crack initiation loading, regardless of caries size and location. Presence of fissure caries drastically decreased (55%-70%) the crack initiation load compared to sound teeth. The depth of the fissural lesion and the presence of proximal caries did not have a major effect on crack initiation load values. However, increasing the caries size resulted in lower final fracture load values in most of the cases. Accordingly, the groups with combined and connected large fissural and proximal lesions experienced the largest drop in the fracture load values compared to sound tooth models. The worst condition consisted of two connected large proximal and fissural caries with no proximal marginal ridge, in which the fracture load dramatically decreased to only 25% of that for sound teeth with intact marginal ridge. On the other hand, decreased cuspal slope due to occlusal wear and shrinkage of the pulp due to caries appeared to have a protective role and a direct relation with the fracture resistance of the tooth. Following the application of resin infiltration on the carious models, the crack initiation load and the fracture load could recover up to 75% and 90% of the values for the corresponding sound tooth models, respectively.
Presence of fissural caries, if not treated (either with remineralization, resin infiltration or restoration), can be a major risk factor in the initiation of tooth fracture. When combined with decreased enamel thickness and loss of proximal marginal ridge due to mechanical or chemical wear, the weakening effect of the caries will be amplified specially in teeth with steep cuspal slopes. The application of a conservative treatment with resin infiltration can be an effective approach in prevention of further mechanical failure of demineralized enamel. The findings of this study emphasize the importance of early interventions in the management of caries for the prevention of future cuspal or tooth fracture especially in subjects with higher risk factors for tooth fracture such as caries, wear and bruxism.
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