Influence of cavity preparation design on fracture resistance of ...

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THE JOURNAL OF PROSTHETIC DENTISTRY SOARES ET AL Fig. 7. Representative fracture modes observed after compressive loading test. A, Inlay. B, Onlay covering 1 cusp. C, Onlay covering all buccal cusps. D, Onlay covering all cusps. fracture <strong>of</strong> either the restorative material or dental structure, is an important method for improving restorative procedures. 1,2,10,12,16,17,28 There are a number <strong>of</strong> factors that may interfere with resistance to fracture, such as the tooth embedment method, type <strong>of</strong> load application device, and crosshead speed. 8 Thus, the experimental methods used for in vitro analyses do not faithfully represent real clinical conditions, in which failures occur primarily due to fatigue. 18 To minimize the discrepancy between experimental assessments and clinical failures, different methods have been used, such as the joint use <strong>of</strong> mechanical tests and fracture mode analysis according to predefined scales. 8,17,19 Mechanical fracture tests are performed to numerically quantify the influence <strong>of</strong> restorative material types, 2,10,16,17,19,20 luting procedures, 12,21 and <strong>preparation</strong> characteristics 14,22 for resistance to fracture when submitted to a concentrated and increasing load. These tests usually produce failure loads that exceed the load limit exerted by normal stomatognathic system movements. 33 In spite <strong>of</strong> this fact, higher loading situations can be compared to the situation in which the individual grinds a solid body <strong>of</strong> small dimensions and the force that would be distributed over the occlusal surfaces <strong>of</strong> posterior teeth is concentrated over a single tooth. If this tooth is structurally debilitated or prepared with an inadequate <strong>cavity</strong> <strong>design</strong>, the result may be fracture <strong>of</strong> the tooth, the restoration, or both. When performing mechanical tests, some factors are important to more closely approximate the clinical situation, such as the root embedment method to simulate the periodontal membrane, the loading apparatus, and the mode <strong>of</strong> load transmission. 8 The simulation <strong>of</strong> the periodontal ligament should be done with an elastomeric material that is able to undergo elastic deformation and reproduce the accommodation <strong>of</strong> the tooth in the alveolus, providing the nonconcentration <strong>of</strong> stresses in the cervical region <strong>of</strong> the tooth. Moreover, a simulated periodontal ligament is highly influential on the fracture pattern. 23 In this experiment, a polyether impression material was used in association with a polystyrene resin as an adequate method for fracture resistance tests. 17,24 Occlusal loading is another important factor. Burke et al 8 and Burke and Watts 9 concluded that the best method for measuring the resistance <strong>of</strong> premolars to fracture is the use <strong>of</strong> a cylinder <strong>of</strong> a defined diameter. 19 The use <strong>of</strong> a 6-mm steel sphere for resistance to fracture testing by Dietschi et al 25 and Soares et al 17 was shown to be ideal for molars because it contacts the functional and nonfunctional cusps in positions close to those found clinically. The results <strong>of</strong> the present study showed that the mean resistance to load <strong>of</strong> healthy teeth (IT:3143.1 6 635.5 N) was significantly higher than the resistance to load <strong>of</strong> teeth prepared with different types <strong>of</strong> <strong>preparation</strong>s and restored with a Leucite-reinforced ceramic; thus, the null hypothesis was rejected. This demonstrates that the restorative process, even when adhesive techniques are associated with cuspal coverage, is not able to restore the total resistance to load <strong>of</strong> healthy molars. This result is in agreement with the studies <strong>of</strong> Morin 426 VOLUME 95 NUMBER 6
SOARES ET AL et al, 1 St-Georges et al, 26 and particularly with that <strong>of</strong> Mondelli et al, 14 who showed a reduction in the resistance to fracture for teeth that had been prepared with greater removal <strong>of</strong> dental structure. Since ceramics have high elastic moduli and tend to concentrate stress inside the body <strong>of</strong> the restoration, 4,18,31 they have lower resistance to fracture than healthy teeth, even though the ceramic is reinforced by the inclusion <strong>of</strong> oxides. 27 This is because ceramics are not capable <strong>of</strong> undergoing elastic deformation at the same rate as tooth structure and resinous materials. Thus, stress concentrations are dependent on the geometry <strong>of</strong> the specimen material, loading conditions, and presence <strong>of</strong> intrinsic or extrinsic flaws. In addition, the resin luting agent under a ceramic restoration may act as a s<strong>of</strong>t layer and could reduce the effects <strong>of</strong> stress concentration. In spite <strong>of</strong> the adhesive process being fundamental for luting a leucite-reinforced ceramic restoration, 12,27 the cushioning effect <strong>of</strong> the resinous cement did not seem to be sufficient to absorb the stresses, which remain inside the ceramic restoration and demand deformation; if deformation does not exist, fracture may occur. Analyses <strong>of</strong> the different types <strong>of</strong> <strong>preparation</strong>s in this study showed that <strong>preparation</strong>s resulting in a greater loss <strong>of</strong> tooth structure appear to decrease the resistance to fracture <strong>of</strong> the tooth-restoration complex. Assif et al 22 analyzed the extent <strong>of</strong> <strong>preparation</strong> for amalgam restorations and found that endodontically treated teeth with a small amount <strong>of</strong> structure removed (conservative occlusal isthmus) and total cuspal coverage produced better resistance values. The discrepancies between the results <strong>of</strong> Assif et al 22 and the findings <strong>of</strong> the present study are likely due to the differences in the mechanical properties and adhesive characteristics <strong>of</strong> the respective restorative materials used. Amalgam, as a metal, can undergo elastic deformation; however, ceramics cannot due to their ionic and covalent bonds. Moreover, amalgam adhesion to the tooth structure is minimal; thus, extensive amalgam restorations demand cuspal coverage to protect teeth from fracture when the restorative material is undergoing deformation. Ceramics, in general, present a high elastic modulus and low strain capacity, so the stress tends to concentrate inside the material since stress can not be relieved by deformation the material fractures before stresses are transferred to the tooth. Leucite-reinforced ceramics have good adhesion to tooth structures, and according to St-Georges et al, 26 this enables an internal rather than an external splinting with cuspal coverage 10 to be created in some instances. Thus, the adhesive strength should be enough to support cusp deflection. With respect to the fracture resistance <strong>of</strong> all tested groups, the conservative inlay <strong>preparation</strong> showed the highest numerical value (2465.4 6 318.7 N), but was statistically similar to the extensive inlay group, in which a greater amount <strong>of</strong> tooth structure had been removed during <strong>preparation</strong> THE JOURNAL OF PROSTHETIC DENTISTRY (2278.1 6 586.4 N). The ceramic restoration presented less resistance to fracture when more tooth structure was removed during <strong>preparation</strong>, and the effect <strong>of</strong> cuspal coverage was not a benefit. In addition to discussing fracture resistance values, it may be important to analyze the fracture modes in each experimental group. For all groups, fracture was observed in the restoration itself. Similar findings were reported by Soares et al 17 when the authors tested feldspathic ceramic in extensive inlays, and also by Burke, 21 who affirmed that the ceramic fractures before the tooth. For the CsI group, it was found that the majority <strong>of</strong> the fractures occurred exclusively in the restoration, probably because <strong>of</strong> the larger volume <strong>of</strong> dental structure in the cusps, which resulted in a greater capacity for undergoing deformation rather than fracture. Cuspal coverage with an onlay apparently does not produce a clear benefit to the fracture resistance <strong>of</strong> the restored tooth, as has been shown for metal restorations. 11 This is related to the fact that the failure, as seen by the fracture modes, occurs almost exclusively in ceramics. If the restoration is expected to fail irrespective <strong>of</strong> the <strong>preparation</strong> <strong>design</strong>, then extending <strong>preparation</strong>s will not change this behavior, as seen in this study. The clinician may encounter a situation in which the tooth presents loss <strong>of</strong> a cusp or fracture <strong>of</strong> a portion <strong>of</strong> it. It is not advisable to establish an occlusal contact at the tooth-restoration interface, due to the difference in the mechanical behavior <strong>of</strong> the 2 structures. In this situation the need for cuspal coverage must be determined. Another concern is the need for covering nonfunctional cusps, which may be observed by comparing the groups <strong>of</strong> teeth with onlay <strong>preparation</strong>s covering just the functional cusps (CsO/b, 2158.4 6 321.7 N and ExO/b, 2204.5 6 353.0 N), and the onlay <strong>preparation</strong>s with total cusp coverage (CsO/t, 2062.3 6 488.4 N and ExO/t, 1551.4 6 443.3 N). According to the results <strong>of</strong> this study, it appears that there is no advantage in covering nonfunctional cusps. The comparison <strong>of</strong> the behavior <strong>of</strong> groups with great volumes <strong>of</strong> ceramic in occlusal isthmus (CsO/t and ExO/t) requires discussion. In spite <strong>of</strong> being statistically similar, ExO/t was almost 500 N less resistant than CsO/t, and it may be that the volume <strong>of</strong> ceramic material in the occlusal box and the thickness <strong>of</strong> remaining tooth structure in prepared cusps are responsible for this finding. Ceramic thickness, when either very thin or very thick, seems to be detrimental with regard to fracture. 34 In addition, sharp angles and knife-edge–prepared cusps tend to concentrate stress, resulting in greater susceptibility to ceramic restoration fracture 32 ; therefore, rounded internal angles are preferred for ceramic restorations. When tooth structure loss results in an extensive occlusal isthmus, this may necessitate coverage <strong>of</strong> all the cusps, a feasible alternative for restoration in the occlusal JUNE 2006 427
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