Stress distributions in adhesively cemented ceramic and resin-composite Class II inlay restorations: a 3D-FEA study

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1 Dental Materials (2004) 20, Stress distributions in adhesively cemented ceramic and resin-composite Class II inlay restorations: a 3D-FEA study Pietro Ausiello a, *, Sandro Rengo a, Carel L. Davidson b, David C. Watts c a Department of Cariology, School of Dentistry, University of Naples Federico II, Policlinico Edificio 14, Via Pansini 5, Naples 80131, Italy b University of Amsterdam, Amsterdam, The Netherlands c University of Manchester Dental School, Manchester, UK Received 4 November 2003; received in revised form 13 April 2004; accepted 11 May 2004 KEYWORDS Dental materials; 3D finite elements analysis; Occlusal loading; Stress-distribution simulation; Class II MOD inlay restorations; Resin cements Summary Objectives: The purpose of this study was to investigate the effect of differences in the resin-cement elastic modulus on stress-transmission to ceramic or resin-based composite inlay-restored Class II MOD cavities during vertical occlusal loading. Methods: Three finite-element (FE) models of Class II MOD cavity restorations in an upper premolar were produced. Model A represented a glass ceramic inlay in combination with an adhesive and a high Young s modulus resin-cement. Model B represented the same glass ceramic inlay in combination with the same adhesive and a low Young s modulus resin-cement. Model C represented a heat-cured resincomposite inlay in combination with the same adhesive and the same low Young s modulus resin cement. Occlusal vertical loading of 400 N was simulated on the FE models of the restored teeth. Ansyse FE software was used to compute the local von Mises stresses for each of the models and to compare the observed maximum intensities and distributions. Experimental validation of the FE models was conducted. Results: Complex biomechanical behavior of the restored teeth became apparent, arising from the effects of the axial and lateral components of the constant occlusal vertical loading. In the ceramic-inlay models, the greatest von Mises stress was observed on the lateral walls, vestibular and lingual, of the cavity. Indirect resin-composite inlays performed better in terms of stress dissipation. Glass ceramic inlays transferred stresses to the dental walls and, depending on its rigidity, to the resin-cement and the adhesive layers. For high cement layer modulus values, the ceramic restorations were not able to redistribute the stresses properly into the cavity. However, stress-redistribution did occur with the resincomposite inlays. * Corresponding author. Tel.: C ; fax: C address: pietausi@unina.it (P. Ausiello) /$ - see front matter Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

2 Ceramic and composite inlay Class II behavior 863 Significance: Application of low modulus luting and restorative materials do partially absorb deformations under loading and limit the stress intensity, transmitted to the remaining tooth structures. Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. Introduction Resin-composites are limited for direct restoration of the larger stress-bearing posterior Class II cavities, on account of polymerization shrinkage effects and some limitations in mechanical properties. Thermally post-cured resin-composite inlays, however, are recommended in preference [1]. Thermal post-curing does improve the mechanical qualities of composites [2,3]. Another advantage of resin-composite inlays, instead of direct placement, is that effects from the bulk curing-shrinkage can be evaded. As a further option for restoring large Class II MOD restorations, strong ceramics are now available that can function properly without metal support [4]. For both ceramic and composite materials, adhesive cementation is imperative to ensure reliable coherence of inlay and tooth structure. Moreover, the cementation with (dualcure) cements has to be accompanied by application of dentin bonding agents [5]. It is now customary to use resin-based luting cements in combination with dentin bonding agents for composite and all-ceramic inlay restorations to enhance (adhesive) retention and survival rates [6,7]. Unfortunately, luting also induces a significant adverse effect. Luting creates a long and narrow restoration, analogous to a Class I, made in materials that are principally inferior to the usual resin-composite restorative materials. This means that a weaker and higher shrinking material is inescapably present, that will set, moreover, in a most unfavorable configuration (high C-value) within the restored tooth [9]. Yet, with respect to wall-to-wall adaptation, adhesively luted resincomposite inlays score slightly better than direct composite restorations, whilst ceramic-inlays perform as well as cast-gold inlays [4]. Notwithstanding their widespread application, marginal integrity of tooth-colored direct or indirect restorations remains a major problem in today s dentistry [8]. The cement layer is not only subject to stresses that originate from curing shrinkage, but also from mastication. Therefore, marginal and internal adaptation of composite and ceramic inlays should also be studied after loading and fatiguing [10]. As a variety of materials with diverse mechanical properties are involved in inlay design and placement, analysis of the wall-to-wall integrity of inlay-restored teeth requires that attention be given to the elastic properties of the various materials at the interfaces. For instance, it has been demonstrated that incorporation of some elasticity (lining) to the restoration may decrease or even prevent interfacial separation [11 13]. However, particularly the fragile ceramic-inlays still require rigid support. In indirect adhesive Class II inlays, leakage often depends on the resincement properties and on its mechanical behavior [14]. Not only the shrinkage-strain, but the shrinkage-stress magnitude and kinetics [15], the Young s modulus and the thickness of the cement determine the totality of the developing stresses [16]. Ausiello et al. [17] showed, by means of a 3D finite element analysis (FEA) model analysis, the influence of occlusal loading and polymerization shrinkage-strain on the stress-distribution in an adhesive Class II MOD direct restoration for resincomposites of different elastic modulus. Also the influence of the adhesive-layer thickness on stressdistribution was illustrated by 3D FEA [18]. The aim of the present study was to analyze by 3D FEA the stress-distribution in all materials involved in adhesively luted Class II MOD inlay restorations, of both ceramic and resin-composite types. Materials and methods Finite element models A 3D model of a human upper premolar, as used in a previous study [18] was reused for this study. It was realized by digitizing a plaster human upperpremolar model on the scale of one to five with a laser scanner (Cyber-ware). Crown and roots were constructed in two different phases and subsequently assembled. For the crown, over 200 profiles were generated at 0.33 mm increments by vertical and horizontal scanning. Of these profiles, only 34 were selected, 17 vertical and 17 horizontal, at 2 mm increments, for use in the external shape definition of the solidtooth model. Literature data on the tooth

3 864 morphology for the definition of the dentine and enamel volumes [20] were used. The model data were assembled in a 3D wire-frame structure by means of a 3D CAD (Autocad 12, Autodesk, Inc., Neuchatel, Switzerland, 1992). The 3D curves were exported into Pro-Engineer 16.0 (Parametric Technology Co., Waltham, MA, USA, 1994), where a solid model was generated by fitting the horizontal and vertical profiles. The model was cut in the cervical area to obtain the final crown. The roots were modeled by their mesial-distal and buccal-lingual representations taken from literature. The two representations were scanned and eight vertical profiles were generated imitating the scanned images. The roots were constructed by fitting the vertical profiles. The pulp region was obtained in an analogous way and subtracted from the roots. The crown and the roots, with the pulp chamber, were assembled in the final model. A parametric cutting plane was chosen to generate different cavities and MOD preparations. In Fig. 1, the Class II MOD is shown (3.5 mm occlusal width). The cavity design was characterized by flat floor and sharp internal line angles. No bevel was considered at the proximal and occlusal margins. The preparation derived was flat from proximal to proximal surface. The solid model was transferred into a FEA program (ANSYS Rel. 6.0, ANSYS, Inc., Houston, TX, USA, 1994) where a 3D mesh was created. In the previous work [18], we explained the volumes that were redefined and meshed with 8-node-brick and 4-node-tetrahedral elements, resulting in 7282 elements (3376 hexahedral and 3906 tetrahedral shape elements) and 5236 nodal structures. P. Ausiello et al. Different material properties were now assigned to the elements, according to the volume definition. In particular, in the previous study, the adhesive layer was modeled in the FEM program using spring elements connecting the nodes from the cavity wall of the natural tooth with those of the composite restoration [18]. In the present study, technical enhancements in the finite element model generation were used to increase the structural relevance of the model itself. The modeling of the adhesive area in the Class II MOD preparation was differently realized. In this case, where an indirect Class II MOD restorationtype was simulated, one part of the adhesive area was modeled to be the adhesive layer, contacting the dental walls. The other part was modeled to be the resin luting cement, contacting from one side this adhesive layer and from the other side the filling material (Fig. 2). The new Class II MOD FE model used a different element mesh-size (the size of all the elements was reduced to obtain more detailed analysis results) and a different methodology to simulate the resin bonding and the luting cement layers. To investigate the strain-status of the total adhesive area under occlusal vertical loading simulation, shell elements (with membrane behavior) were employed both for the adhesive layer and for the luting cement layer (Fig. 1), instead of the spring elements used previously [18]. The volumes were redefined and meshed with 8-node brick and 4-node tetrahedral elements, resulting in a 27,140-element and 18,244-node structure, for a total solid elements number of 24,818. In particular, 1160 shell membrane elements were used (Fig. 2). In Ansys 6.0 software these elements are called shell 41 and they are 3D characterized for each of the 4 nodes of the single Figure 1 Finite element model of Class II MOD indirect restoration of an upper premolar with particulars relative to the shell modeling of the adhesive resin bonding and of the cement layers. Figure 2 Finite elements models of the cement and adhesive layers.

4 Ceramic and composite inlay Class II behavior 865 element. Because of their low-thickness, they do not show flexural deformation. In this way, it was possible to better simulate the mechanical behavior of these two different layers. Experimental model validation To validate this new Class II MOD FEM model, compression loading measurements were performed on sets of differently restored teeth until fracture of the samples. These were the Class II MOD designs, corresponding to Models A and C, described below. Ten caries-free human upper premolars were used for each test group. Class II MOD cavities were prepared with a diamond bur at high speed under water coolant. Axial and gingival walls were cut non-retentively, at approximately 1008 angles. No bevels were prepared at the cavosurface enamel angles. For the Class II MOD restoration, a heat-cured resin-composite with a Young s modulus of about 50 MPa (Gradia, GC, Japan) was used. Unifil Bond adhesive (GC, Japan) with a Young s modulus of 4.5 MPa was applied on the cavity walls. Unifil Flow (GC, Japan) with a Young s modulus of 9.6 MPa was used as resin luting material. The samples were inserted, as far as the cementum enamel junction, into steel cylindrical rings with the apical root area in contact with the steel-ring floor. Subsequently, spaces between roots and steel walls were filled with rigid resincomposite, so that only material deformation within the tooth would be measurable. The testrings were clamped to the universal testing machine and loading was applied vertically via a 6 mm diameter steel cylinder, with the axis normal to the tooth axis. To simulate one major occlusal force, a 1 mm/min compression rate was used. The vertical displacement and the axial load were recorded until each restored tooth fractured. This loading situation was also simulated using the FE analysis, generating closely matched results (Graph 1). An important parameter to be considered was the rigidity of the restored teeth, expressed under the loading conditions used in this analysis. Two components of rigidity were considered: axial and lateral. Axial rigidity directly measures the resistance to compressive forces. Lateral rigidity measures cuspal-displacement under flexural loading. Thus, the effect of the applied masticatory loading was to provide both a compressive load to the system and also displacement of the cusps. The FE analysis performed was linear and static. It used the over-position effect principle to Graph 1 Validation data: showing the theoretical plot, determined numerically, and the experimental plot of axial load versus displacement. determine the axial rigidity comparative parameter (%RCP A ) and lateral rigidity comparative parameter (%RCP L ). They represent, in percentage, the perceived rigidity variation of a system with respect to an other reference rigidity. If they are positive it means that the test system is more rigid than reference system. In formulas: %RCP A Z 1 K axial ÿ movement perceived axial ÿ movement reference!100 %RCP L Z 1K lateral ÿ movement perceived!10 lateral ÿ movement reference The sound tooth has been chosen as a reference model in numerical calculations [20]. Results are shown in Table 1. Numerical simulation The simplest approximation to the probable nature of occlusal loading is where forces to the teeth are applied statically and vertically. In all the FE models investigated here, the external roots nodes were constrained in all the spatial directions. Adhesive mechanical properties are listed in Table 2 for all the restored tooth models. The compression test with a 400 N occlusal load was conducted. The loading cylinder was modeled Table 1 Rigidity comparison of the systems. Model G-lC Model G-hC Model C Sound tooth %RCP A K %RCP L K

5 866 Table 2 Material Materials properties. Elastic modulus, E (GPa) Poisson ratio *Enamel Dentine Composite 50 Ceramic 90 Cement hm Cement lm 6 70 Adhesive *Verluis, 1996 [20]. Co., data. Thickness, T (mm) as a 3D elastic beam (Fig. 3). End rotations were not constrained. The common end was displaced in the central position of the loading cylinder section in the experimental test. The load was applied on the tooth at two points (Fig. 3) through the beams elements on the cusps (red areas). The resulting force F crossing the central position of the loading cylinder section was 400 N (Fig. 3, red arrow). The beam elastic properties were treated as being infinitely rigid compared to the tooth. Resincomposite support was not modeled and it was considered to be as rigid as the loading system. Moreover, the following assumptions were made: A static linear numerical analysis was performed. Thus, all materials were considered elastic throughout the entire deformation, which is a reasonable assumption for brittle materials in non-failure conditions. Dentin is an elastic and isotropic material. Enamel was treated as mechanically homogeneous and isotropic, as in Refs. [19,20]. In Fig. 2, the different thicknesses of the two interfacial layers is shown. The elements simulating Figure 3 The model structure is blocked avoiding movement in the three directions of the space. the luting cement and the adhesive are positioned between the tooth and the filling material. We hypothesize the perfect and absolute bonding between the two materials. In the FE analysis different conditions were simulated, modifying the thickness of the cement, not varying the adhesive resin bonding one, and including different filling materials properties (Fig. 2). Three different models of Class II MOD inlay restorations were considered in order to simulate three different clinical indirect restorations types. Model A (G-hC, Glass-core ceramic with high modulus Cement), in which an high modulus glass ceramic filling material was considered in combination with an high modulus cement; Model B (G-lC, Glass-core ceramic with low modulus Cement), in which an high modulus glass ceramic filling material was considered in combination with a low modulus cement; Model C (Composite restoration), in which a heatcured resin-composite inlay was considered in combination with a low modulus cement. In all the combinations, one resin bonding system was considered. Physical properties of the used materials are presented in Table 2. Results P. Ausiello et al. Inspection of the results revealed critical zones with particular stress behavior. The results are presented in terms of von Mises stress maps in MPa, which were computed within Ansys using the von Mises shear-strain-energy failure criterion, as an outcome of the 400 N occlusal loading. The figures utilize a false-color non-linear scale for stress. It should be understood that von Mises stress is essentially an aggregate stress, sometimes termed an octahedral stress. As such, it cannot be directly decoded into specific contributions from tensile, compressive or shear stress. However, other types of output from most FE programs can provide such information. Figs. 4 6 show the varying biomechanics arising from the differing rigidity of the three models: A, B and C. These figures show, respectively, stresses computed: at the surface of each model (Fig. 4a c), within each model cavity preparation (Fig. 5a c), within each MOD restoration (Fig. 5d f) and (in Fig. 6) from the interfacial areas (adhesive layerc resin-cement layer) between the inlay restoration

6 Ceramic and composite inlay Class II behavior 867 Figure 4 (a) von Mises stress-distribution of Model A (G-hC). (b) von Mises stress-distribution of Model B (G-lC). (c) von Mises stress-distribution of Model C. and the cavity preparation. In these interfacial areas, the biomechanical differences were especially critical. Experimental and theoretical validation curves are compared in Graph 1. The two similar stress strain behaviors gave good support to the validity of the model. Even the extreme (high strain) properties could be rather well approximated by the theoretical curve. The experimental curve shows a mild non-linear behavior near to the origin and a linear behavior thereafter. This apparent nonlinearity was not due to a real material or geometrical non-linearity but to the initial system assessment that included effects due to contact and sliding. These phenomena were unimportant in the FE model validation. The glass ceramic-restored teeth, respectively cemented with a high (10 GPa) and low (6 GPa) modulus cement material (Models A and B; Fig. 4a and b) may be compared with the behavior shown in Fig. 4c of the composite-restored tooth (Model C), luted with the low modulus cement. The highest stress values of about MPa for all three models were concentrated on the cuspal loading points. At the center of the occlusal surface, for Models A and B, a stress value of MPa was computed while for Model C it was generally much lower, ranging from 40 to 100 MPa. All the tooth models were low-stressed mesial-distally, with values of only MPa. On the vestibular and lingual sides, depending on the displacement of cusps, stresses appeared higher, about MPa. In Fig. 5a c, analysis was conducted within the cavity preparation, while the filling material was extracted from the cavity itself. This was possible

7 868 because of the CAD/FE model configuration. Stresses were particularly intense in Models A and B (10 40 MPa) compared with Model C (1 5 MPa), specifically localized on the vestibular and lingual cavity walls of Models A and B (Fig. 5a and b). In Model C, a lower modulus (50 GPa) heat-cured composite resin was simulated. In Fig. 5d f is displayed the stress behavior within the core of the restoration of Models A C. Slight differences are evident between the ceramic restorations of Models A and B, where von Mises stress values appear elevated but similar, irrespective of the modulus of the cement material used in the two simulations. Stress is concentrated in the core of the restoration, extending to the vestibular and lingual sides of the restoration itself and totally transmitted to the cavity walls, as already shown in Fig. 5a and b. P. Ausiello et al. In Fig. 5f, by contrast, where lower modulus (50 GPa) resin-composite was used for restoration, stress gradients from the internal area to the cavity walls were lower, matching the distribution seen in Fig. 5c. In Fig. 6 are shown for the three models the stressdistributions, as successive pairs, for the adhesive and the resin-cement. The change of stress scale (0 10 MPa) should be noted. For the adhesive layer, no differences are evident between ceramic Models A and B (Fig. 6a-1 and b-1). For the cement layer, higher stress was apparent with the higher modulus cement (Fig. 6a-2) compared with the lower modulus lute (Fig. 6b-2). Fig. 6c-1 and c-2 illustrates the interfacial stresses for the adhesive and cement layers within composite Model C. The lowest stress values were recorded for this condition. Figure 5 (a) von Mises stress-distribution within Model A (G-hC). (b) von Mises stress-distribution within Model B (G-lC). (c) von Mises stress-distribution within Model C. (d) von Mises stress-distribution within the restoration of the Model A (G-hC). (e) von Mises stress-distribution within the restoration of the Model B (G-lC). (f) von Mises stress-distribution within the restoration of the Model C.

8 Ceramic and composite inlay Class II behavior 869 Figure 5 (Continued) In Graph 1, a linear analysis on Model A and C is represented in which it can be seen that with increasing the loading from 400 to 800 N, the stresses proportionally increase, leading to a critical stress concentration in Model A, particularly on the lingual cusp. Discussion Teeth in posterior regions are subject to functional and para-functional forces of varying magnitudes and directions. In vitro mechanical tests on Class II adhesive posterior restorations revealed the different aspects related to the stress-distribution regarding the marginal and internal adaptation of adhesive Class II restoration [21]. The role of the filling material, of the adhesive resin and of the resin cement was clearly demonstrated and results indicated various important points to observe to obtain high performance of the restoration itself. The rigidity or elastic modulus of dental restorative materials was considered extremely important at the adhesive tooth-restoration interface. In the present work, the FEA method was used to investigate the stress-distribution resulting from occlusal loading within the restoration and in correspondence to that of the interfacial layers (adhesive and cement) between the cavity walls and the inlay materials. An arbitrary load of 400 N was applied in this test, which is probably lower than can be applied by the teeth in vivo. Different data are reported on this aspect. Tortopidis [22] found that 580 N was

9 870 P. Ausiello et al. Figure 6 (a-1) von Mises stress on Model G-hC, adhesive. (a-2) von Mises stress on Model G-hC, cement. (b-1) von Mises stress on Model G-lC, adhesive. (b-2) von Mises stress on Model G-lC, cement. (c-1) von Mises stress on Model C, adhesive. (c-2) von Mises stress on Model C, cement. the maximum bite force of healthy people in posterior areas. Other investigations [23] suggested that these values differ between males (522 N) and females (441 N). Under laboratory conditions, varied loading rates can be applied to the samples to investigate biomechanics of natural and restored teeth. In particular, fracture resistance of Class II

10 Ceramic and composite inlay Class II behavior 871 restorations in upper premolars submitted to vertical loading has been experimentally investigated [24]. It ranged for resin-composites in combination with dentin bonding systems between 700 and 800 N. These data were also confirmed recently [25], where the use of resin-composite and ceromers as restorative materials was considered. However, it was not the objective of this study to determine the absolute numerical stress levels created within the restoration but to examine their distribution and localization. The software used in this study was not programmed for evaluating the model to failure and therefore higher or lower loads would only change the magnitude of the stresses in the distribution pattern. In the ceramic Models A and B, where a ceramic inlay of high modulus (90 GPa) was used in combination with 70 mm thick resin-cements of two different Young s moduli, no major differences were found in terms of stress-distribution within the restoration. However, when the inlay modulus was reduced to 50 GPa, still in combination with the same parameters for the cement layer, the stressdistribution significantly changed. Comparing Fig. 5d and e with Fig. 5f (respectively, Model A and B, with Model C) the stress-distribution was more intensive where the 90 GPa modulus inlay was used and these stresses are almost totally transferred to the cavity walls, as shown in Fig. 5a and b; whilst for the 50 GPa inlay (Fig. 5f), the stresses are partially absorbed and partially transfered to the cavity walls. From the load-strain values as represented in Fig. 3, it can be derived that a 400 N loading in horizontal direction, when flexural deflections will take place in the prepared brittle tooth structure, destructive damage will occur earlier than with vertical, occlusal loading. Recently, Abu-Hassan et al. [26], used 3D-FEA to investigate stress-distributions associated with loaded ceramic onlay restorations with different designs of marginal preparation. It was possible to establish how vertical and horizontal forces act differently in correspondence with the total margins of the restoration. Hence interesting conclusions could be drawn regarding the optimum morphology of the butt-joint onlay preparation. In our investigation, the 400 N axial simulations showed that ceramic Models A and B transmitted higher stress to the cavity walls than composite Model C. Fig. 4a and b shows that Class II MOD preparations, restored by a 90 GPa ceramic inlay with the same 4.5 GPa adhesive but with either a 10 or 6 GPa resin-cement, do not show a substantially different stress-distribution after axial loading. By contrast, in Model C, a 50 GPa resin-composite inlay, with the same adhesive and the 6 GPa resin-cement, showed a lower stress with a more homogenous distribution. This indicates a greater stress-dissipating effect of the relatively compliant resin-composite than the more rigid glass ceramic inlay. Sorensen and Munksgaard [5] concluded from clinical trials that none of the dentin adhesives they tested were able to completely prevent interfacial gaps developing when inlays were cemented with a dual-cure resin-cement. But in absence of the adhesive, the failure rate was significantly higher. The investigation of the interfacial zone between the cavity and inlay margins has always represented an important tool in laboratory investigations as well as in clinical reports. A low-modulus poly-acid-modified glass-ionomer cement used with ceramic-inlays resulted in a high fracture rate and loss of marginal adaptation. The marginal adaptation of the lute was more durable at the enamel interface than that at the ceramic interface [27]. When comparing Fig. 6a-2 and b-2, the higher stresses in the contact area of the 10 GPa resincement is clear. Further studies on the role of cement layer thickness and minimal modulus to still sufficiently support the ceramic inlay are in progress. Conclusions From this FE analysis on stress-distribution in inlayrestored Class II MOD cavities under axial load, it is evident that both optimum stress magnitude and distribution are best served with low modulus restorative materials. FEA enabled investigation of optimal conditions, material selection and their interaction when adhesively restoring teeth. Class II MOD restorations by glass core inlay materials created higher stress levels at the cusp and at the internal sides. Thermally post-cured resin-composite Class II restorations presented elastic biomechanics similar to that of the sound tooth. References [1] Dietschi D, Spreafico R. Adhesive metal-free restorations: current concepts for the esthetic treatment of posterior teeth. Berlin: Quintessence Publishing Co., Inc; 1997 p [2] Asmussen E, Peutzfeldt A. Mechanical properties of heat treated restorative resins for use in the inlay/onlay technique. Scand J Dent Res 1990;98(6):564 7.

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