This study evaluated the risk of failure for an endodontically treated premolar with MOD preparation and three CEREC ceramic restoration configurations.
Simulations were performed based on three 3D finite element (FE) models designed with CEREC ceramic inlay, endocrown and conventional crown restorations. Long-term failure probability in relation to varying load conditions was calculated by incorporating the Weibull function in FE analysis. Additionally, the final fracture strength and corresponding load value of the first acoustic emission (AE) activity in each specimen was recorded by performing in vitro AE analysis in CEREC restored teeth compressive testing.
Simulation results indicated that the stress values on the enamel, dentin and luting cement for endocrown restorations were the lowest ones among the corresponding values for inlay and conventional crown restorations. Weibull analysis indicated that failure probability was 95%, 2% and 2% for the inlay, endocrown and conventional crown restorations, respectively, for normal biting. AE analysis revealed that, although the significantly least load was required for the first AE activity for inlay configuration, the endocrown and conventional crowns did not significantly differ from each other.
This in vitro study, i.e. numerical and AE analyses, suggest that endocrown and conventional crown restorations for endodontically treated premolars with MOD preparation present a similar longevity.
Endodontically treated premolars present specific challenges for the restorative dentist and the prosthodontist because of their more brittle behavior when compared to vital teeth. This difference is not explained by their moisture content but rather the structural defect generated during tooth preparation . Preservation of tooth structure is important for its protection against fracture under occlusal loads and for its survival . Whereas structural strength depends on the quality and integrity of its anatomic form, the quantity of a sound tooth to retain and support the restoration is of priority concern . Therefore, the longevity of an endodontic treatment largely depends on the selection of an appropriate restoration that is concerned with efforts to save the tooth structure and the restorative materials used .
The preferred final restoration for endodontically treated posterior teeth remains contentious. The conventional means of restoring endodontically treated teeth is a build-up with a post and core, which utilizes adhesive procedures and placement of a full coverage crown with sufficient ferrule . However, preparing a post space also involves a certain degree of risk of accidental root perforation. Some studies claim that inlay/onlay restorations can provide the required protection and ensure the clinical success of the restoration based on the maximum preservation of tooth structure .
With advances in adhesive methods and ceramic materials, adhesive restorations are advantageous since macroretentive design is no longer a prerequisite if an adequate amount of tooth surface is available for bonding. Clinicians are particularly interested in restoring minimal or absent macroretentive preparations with extensive dentin- or enamel-bonded ceramic coverage . Increased demand in recent years for esthetic and metal-free restorations has led to the development of a computer design/manufacturing (CAD/CAM) system for fabricating ceramic inlays, onlays, and veneers. CAD/CAM system generated ceramics are currently available, which provide a novel means of restoring the large cavities in posterior teeth and achieving chair-side design and automated production of all-ceramic monolithic single-unit restorations . Therefore, all-ceramic crown restorations have become an alternative to metal crowns for tooth reconstruction. Moreover, the endocrown comprises a circumferential butt margin and a central retention cavity inside the pulp chamber and constructs both the crown and core as a single unit. This approach utilizes the surface available in the pulp chamber to ensure the stability and retention of a restoration through adhesive bonding. However, the biomechanical mechanisms involving the CAD/CAM ceramic inlay/onlay, endocrown and classic crown restoration effects on the stress transfer in restored teeth are unclear.
Capable of providing detailed mechanical responses, the finite element (FE) method is extensively adopted in dental biomechanical studies . Nevertheless, static analysis results normally fail to estimate the failure risk to dental restorations . Frequently used for estimating the probability of fracture in brittle materials, Weibull analysis also predicts cumulative failure probability at selected stress levels. Therefore, this study analyzes the stress distribution patterns and risks for failure using FE and Weibull analyses for a maxillary premolar with mesial-occluso-distal (MOD) preparation restored using CAD/CAM ceramic restorations with the inlay, endocrown and crown built up on a metal post–core unit. Additionally, simulation results are validated by performing parallel in vitro acoustic emission (AE) analysis in restored tooth fracture testing.
Materials and methods
Generation and verification of FE model
Micro-CT (Skyscan 1072, Skyscan, Aartselaar, Belgium) images with a voxel dimension of 38 μm were obtained from scanning (715 slices) a freshly extracted intact maxillary upper second premolar embedded in an epoxy resin block. All cross-sectional micro-CT image files were processed using commercially available software (Amira, v4.1, Mercury Computer Systems, Inc., Chelmsford, MA, USA) that identified the different hard tissue outer contours (enamel, dentin and pulp) and then entered that data into the FE program (ANSYS, v11.0, Swanson Analysis, Inc., Houston, PA, USA) to generate solid models. Based on the root-form geometry of the premolar, a simplified 0.3-mm periodontal ligament and alveolar bone (cortical and cancellous) were created. The cavity was designed as a typical MOD preparation. The pulpal floor was designed at two-thirds the distance between the buccal cusp tip and the cemento-enamel junction (CEJ). The isthmus width was half the size of the intercuspal width ( Fig. 1 a ). Preparation designs were then generated using a parametric cutting plane with adhesive CAD/CAM ceramic inlay, endocrown and crown built up on a metal post and core unit.
To verify the FE model, a convergence test was used to guarantee that our numerical model reached the converged results and that no further mesh refinement was necessary. To avoid quantitative differences in the stress/strain values in the models, all solid models for different restorations were derived from a single mesh pattern. Four mapping mesh patterns of different element sizes were derived based on the same mesh pattern using an eight node parametric brick element to perform the convergence test controlling the strain energy and displacement variations to less than 5%. Following the convergence test, the resulting 3D FE model was meshed with 39,728 linear eight-node structural solid elements and 41,827 nodes ( Fig. 1 b).
FE and Weibull analyses
To perform Weibull analysis, stress values on each node of the restored FE models under different loads were required to calculate the failure probability. Owing to the linear nature of the FE model, 100 N was applied and stresses of other loads (200–1500 N in 100-N increments) were calculated in proportion to the data of 100 N. The axial load with the resulting force crossing the central position at 100 N was then simulated by applying the load on the tooth with buccal and lingual cusp contact. Next, the exterior nodes at the mesial, distal and bottom surfaces of the bone block were fixed in all directions as the boundary conditions. Additionally, linear elastic, homogeneous and isotropic material properties of the tooth tissues, bone and restorative materials were assigned according to the volume definition from previous literature ( Table 1 ).
|Materials||Young’s modulus (MPa)||Poisson’s ratio ( ν )||Characteristic strength ( σ 0 , MPa)||Weibull modulus (m)||Reference||Simulated maximum principal stress (MPa)|
|Enamel||84,100||0.33||42.41||5.53||, Custom testing||44.35||44.94||6.73|
|Ceramic block (Vita Mark II)||68,900||0.3||71||19.1|
Failure was assumed to occur from the highest principal tensile stress on stress concentration areas. Weibull risk-of-rupture analysis was performed, in which the survival probability, P S , can be expressed as
where P S denotes the survival probability of node at stress σ (for load F ), σ represents the failure stress, σ 0 represents the characteristic strength, and m refers to the Weibull modulus, which is a material parameter. The values of σ 0 and m of different materials were adopted based on literature and custom testing data ( Table 1 ). When loaded, a restoration survives until the risk of rupture reaches a critical value at any multiple failure source. Hence, for a system of n = i sources, the overall survival probability, P S , is the product of individual survival probabilities:
where i = 1–4 in the case of the premolar MOD cavity with different restorations since the stress concentration regions of the enamel, dentin, cement and ceramic were observed at risk. Hence, the failure probability, P f , for the total system is