To evaluate the biomechanical influence of reconnecting the buccal and lingual walls of endodontically treated lower molar with a large MOD cavity.
3D models were created from a micro-CT scan of an extracted intact lower human molar. Finite element analysis was then used to compare the stress distribution and displacement of the structures between different models: Unrestored MOD on ETT (Unrest), MOD on ETT restored with composite resin (Norm), MOD on ETT reinforced with a non-invasive ring (Ring) and intact tooth as the control (Int).
This study showed that the Unrest model presented the highest stress values at the cervical level. The non-invasive Ring model showed the lowest stress values at the cervical level, and the lowest deflection of the cusps.
Mechanically reconnecting the remaining tooth structures has a direct positive effect on stress distribution and deformation of the remaining structures.
Reinforcing ETT in a non-invasive direct approach is efficient in terms of preserving sound tooth structures, reducing the time of treatment and eliminating the laboratory steps required to create the restorations.
After endodontic treatment, practitioners are facing several therapeutic possibilities for the restorative procedure. With the evolution of dental adhesive systems and techniques, everyday’s practice is heading towards maximal conservation of tooth structures , since this approach has demonstrated a better long term prognosis of the treatment . The endodontic procedure is known to weaken the tooth and to increase the risks of fracture. This is mostly due to the tissue removal required to access the root canals , and not due to dehydration since it was shown that this last factor was actually not very significant . In many studies, cusp fractures were very often associated with weakening of the tooth structure due to endodontic treatment but it was also shown that an endodontic treatment with the preservation of the proximal ridges did not have a severe effect on the fracture resistance of the tooth . Some authors suggested that endodontically treated teeth (ETT) with a conservative occlusal access cavity can be safely treated with a simple direct restoration. On the other hand, most of the studies agree that MOD cavities on ETT need to be treated with a cusp covering restoration . In amalgam restorations, the remaining buccal and palatal walls become completely separated after a MOD cavity. This means that there would be nothing left to retain the remaining walls from deflecting during the mastication. This is why a higher rate of cusp fracture was observed on teeth restored with non-bonded restorations such as amalgams . Adhesively placed resin composite restorations were thought to solve this problem as it was assumed that they reduced the deflection of the walls by adhering to them . The aim of this study was to evaluate the mechanical reinforcing effect of reconnecting the buccal and lingual walls of a large MOD cavity on an endodontically treated lower molar, through a novel non-invasive technique. Finite Element Analysis (FEA) was applied to compare stresses due to functional loading in the restored tooth. It was assumed that re-establishing the macromechanical link between the remaining walls of a cavity would increase the fracture resistance of the tooth.
Material and methods
Generation of the models (pre-processing)
An extracted intact mandibular human first molar was used to create the three-dimensional (3D) numerical models. A micro-CT scanner (SkyScan 1076 micro-CT, SkyScan, Aartselaar, Belgium) was used to scan the tooth at a spatial resolution of 15 μm. The software NRecon (NRecon 220.127.116.11, SkyScan, Aartselaar, Belgium) was used to reconstruct the images issued from the micro-CT scanner. The slices were then used to build the models with the image processing software OsiriX (OsiriX 3.9, Geneva, Switzerland). The different tissues of the tooth were separated by a process known as segmentation in order to better analyze them since they have different material properties. Segmentation is based on radiodensity, and it is performed by defining a range or thresholds of pixel density for every tissue. Pixels that have the same or very near values of density are grouped and selected together. Since the density of enamel is different from the density of dentine, at the end of this step, two 3D regions were obtained, one for enamel and one for dentine, and were saved as stereo lithography files (STL-files). In order to optimize the models for the FEA, the STL files were then remeshed, which is a process that reorganizes the geometry of the 3D object without changing its shape. After obtaining a valid mesh structure, two thirds of the roots were cut to reduce the time of the analysis, and a support cylinder was modeled. The parts were finally reassembled to obtain an intact tooth model (“Int”).
A 3D content-creation program (Blender 2.70, Amsterdam, the Netherlands) was then used to simulate the different cavities and restoration procedures, by performing multiple Boolean operations on the intact tooth model. Three other models were then created: Unrestored MOD cavity on an ETT (“Unrest”), MOD cavity on ETT restored with resin composite (“Norm”), MOD cavity on an ETT reinforced with a ring (“Ring”) ( Fig. 1 ).
To design the “Unrest” model, a conservative endodontic access cavity and a large and deep MOD cavity were simulated by performing a subtraction Boolean operation between the “Int” model ( Fig. 2 a) and a 3D object “Cutter” ( Fig. 2 b) that has the positive shape of the future cavity. Boolean operations permit to subtract, add or intersect different 3D objects. By positioning the “Cutter” in the wanted location of the future cavity ( Fig. 2 c), the common portion between the intact tooth and the “Cutter” can then be eliminated in order to obtain the unrestored model ( Fig. 2 d). The occlusal bucco-lingual width of the cavity was half the maximum bucco-lingual width of the tooth, and the cervical margin was 1 mm occlusal to the CEJ. All axial dentine was removed between the pulp chamber and the proximal boxes. The angle of divergence of the cavity walls was 5°. To design the “Norm” model, a series of Boolean operations was used between the “Int” and “Unrest” models in order to obtain the exact geometry for the composite restoration. To design the novel “Ring” model, a 2 mm wide unidirectional fiber-reinforced composite (FRC) band (everStick C&B, StickTech ltd., Turku, Finland) was modeled around the middle of the crown of the “Norm” model. The thickness of the ring was 1.5 mm.
In order to better understand the Ring model, an in vitro photograph is provided in Fig. 3 . Another configuration of the band could also be possible as seen in Fig. 4 : the proximal parts of the ring were crossed to the center of the cavity instead of using it as a ring shape. This configuration retains the same mechanical function between the buccal and lingual walls, and should permit the reconstruction of the proximal faces without any interference with the adjacent teeth.
Finite element analysis
The models were then introduced into a software for advanced engineering FEA (FEMAP 11.1, Siemens PLM software, Plano, Texas, USA). Different attributes were then created for each tissue to define the mesh size and the material properties. The Poisson ratio (n), modulus of elasticity (E) and shear modulus of the oral tissues and restoration materials were determined from the literature and are presented in Table 1 . All the materials were considered to be isotropic with the exception of the ring. The FRC ring was considered as an anisotropic material with its maximal young modulus and stiffness in the longitudinal direction of the fibers. After meshing the parts, loads were defined on the buccal and lingual distal cusps with a constant value of 600N and an angle of 45° as shown in Fig. 5 . The models were fixed by applying constraints in the x-, y- and z- axes on the support cylinder.
|Materials||Young modulus (gpa)||Poisson’s ratio||Shear modulus (mpa)||References|
A static analysis was then launched, and two types of results were mainly studied in the post-processor: The displacement and the maximum principal stress values. After running the analysis, the results were opened side by side in Nastran X. The loads and constraints were then hidden to have a better view on the model.
To study the deformation of the walls after loading, the distance change between the buccal and lingual walls had to be measured. To do that, two sets of paired reference nodes were chosen on both buccal and lingual surfaces of the enamel, on two different heights: the first set of pairs at the occlusal level near the cusp tips (OCC) and a second set of pairs at the cervical level 1 mm above the CEJ (CEJ). The coordinates of each node were recorded before and after applying the loads, and the distance between each two paired nodes was then calculated. The variation of the distance before and after applying the loads was represented in Fig. 6 . To study the stress distribution in each model, the maximum principal stress values were extracted and represented on the 3D models in four different views and lateral sections ( Figs. 7 and 8 ). The highest maximum principal stress values were also studied at four main zones: the interface between the cavity and the restoration, the cervical enamel, the cervical dentine and inside the restoration ( Fig. 9 ).