The aim of this study was to evaluate whether the compressive strength of lithium dissilicate ceramic inlays is influenced by the substrate (dentin or composite resin build-up) and to compare it to nonrestored teeth. Thirty freshly extracted human maxillary third molars were selected and randomly ascribed to three groups (n=10). Standardized Class II MOD preparations were made (bucco-palatal width = 2/3 of the intercuspal width and 2/3 of the width at the tooth equator for the proximal boxes), varying the extension of the preparations (Group 2: preparation limited to tooth structure; Group 3: pulpal floor of the preparation rebuilt with composite resin, IPS Empress Direct, restored with lithium dissilicate CAD/CAM ceramic inlays (e.max CAD) and cemented with a resin cement (Variolink II)). All groups were subjected to compressive strength test (1mm/min crosshead speed). The results showed that the fracture strength of G1 (control group) was significantly higher than G2 and G3, while within these groups there were no statistically significant differences. Both groups restored with lithium dissilicate restorations did not reach the fracture strength of the sound teeth but were statistically equivalent and sufficient to withstand physiologic masticatory forces.
Posterior teeth play a very important role in the masticatory system especially because they support the highest functional loads [
Cavities larger than one third of the intercuspal distance imply that an indirect restoration should be chosen. It should be taken into account that the polymerization shrinkage stress of composite resins is between 2 and 3% in volume [
The principles of tooth preparation such as resistance form, and 8° to 15° taper of the axial walls in the cervical-occlusal direction should be strictly followed to obtain long-term resistance of the restoration. Preparations for ceramic restorations must present rounded internal angles since the presence of sharp angles and edges generate stresses, leading to crack propagation and possible fracture of the restorative material [
The occlusal reduction should be between 1.5 mm and 2 mm and the axial reduction should at least be 2 mm. These depths are considered sufficient to obtain proper thickness of the ceramic restoration. This directly influences the restoration resistance to masticatory loads and thermal variations [
However, most of the time it is not possible to achieve an ideal preparation due to the loss of tooth structure. In these cases, it may be necessary to partially replace the lost portion to obtain a suitable preparation [
The reconstruction of the lost dental portion using composite resin is a conservative treatment alternative. It has been questioned whether teeth with a preparation build-up using composite resin behave similarly to teeth without composite build-up when both are restored with lithium disilicate ceramic inlays and submitted to compressive strength. Therefore, the aim of this study was to evaluate the influence of the preparation build-up using composite resin on the compressive strength of human third molars restored with lithium disilicate ceramic inlays. The hypothesis tested was that the preparation build-up using composite resin does not affect the compressive strength of human third molars restored with lithium disilicate ceramic inlays.
After authorization of the Research Ethics Committee of the Federal University of Santa Catarina (#1.025.321), thirty sound human maxillary third molars extracted for reasons not inherent to the present study were selected. All the subjects who donated the teeth signed a consent form, agreeing that their teeth would be used for this research. Only teeth that presented three cusps were used. The teeth were cleaned with pumice (Maquira, Maringá, PR, Brazil) and the remains of the periodontal ligament were removed with a #12 scalpel blade (Solidor, Barueri SP, Brazil) and were stored (0.1% thymol solution, pH=7) for a period up to 3 months before starting the research.
The teeth were inspected with a 3.5 X magnification glass (BIOART, São Carlos SP, Brazil, Batch #445) seeking for alterations such as cracks, gaps or anomalies that could compromise the study.
To facilitate the sealing of the teeth to carry out the preparation and cementation of restorations, the root portion of each tooth was fixed within a PVC ring (2 cm diameter, 2.5 cm length) with acrylic resin. The teeth were placed with the occlusal surface parallel to the base of the PVC tube, with a paralelometer (BIOART, São Carlos, SP, Brazil). Before placement on the base, the tooth was marked 2 mm below the cement-enamel junction with the aid of a periodontal probe and wet tipped pen, fixed with wax in a rod with the cylindrical end attached to the paralelometer. Then a metal base was used to keep the plastic ring static, isolated with petroleum jelly and the base was filled with acrylic resin, until 2 mm below the CEJ [
After fixing, all teeth were cleaned again using pumice and water and prepared according to the variables, varying the preparation depth of the pulp wall, except for the control group: Mesial-occlusal-distal (MOD) preparation; gingival walls 1 mm below the cementoenamel junction; Rounded internal angles; 10° to 12° wall taper.
Ceramic inlays (e.max CAD HT, shade A3,5, size C14, Ivoclar Vivadent Schaan, Liechtenstein, batch #R51252) were fabricated for each preparation and cemented using dual cure resin cement (Variolink II, Ivoclar Vivadent Schaan, Liechtenstein, batch # S46585) according to the manufacturer’s instructions.
Prior to preparation, impressions of the intact crowns were obtained using a PVS impression material (Virtual, Ivoclar Vivadent, Schaan, Liechtenstein, batch #SI4194) to fabricate a silicone index, which would guide the tooth preparation. Thus the preparation depth in the pulp wall was measured from the deepest portion of the sulcus bottom to the pulpal wall (Figure
Checking the preparation depth with a periodontal probe to assure that a 2 mm depth was achieved.
All preparations were performed with rounded-ended conical diamond burs (#3131, KG Sorensen, Cotia, SP, Brazil), replaced each five preparations. The preparations were standardized using a device [
Metal device used to adapt the high-speed hand piece. The goal is to keep the diamond bur parallel to the long axis of the tooth, without slant.
The intercuspal distance was measured with a digital caliper using and marking the occlusal and proximal surfaces with a wet tip pen. The proximal box was extended approximately 1 mm below the cementoenamel junction from the mesial marginal ridge. The preparation was extended to the buccal and lingual aspects resulting in a similar reduction; e.g., the volume of restorative material was proportional to the size of each tooth. Finishing of the preparation was accomplished using fine grain and extra-fine diamond burs (#3131F and #3131FF, respectively. KG Sorensen, Cotia, SP, Brazil), which were replaced each five teeth.
The teeth were divided according to the type of the preparation as follows: Group 1 (n = 10): control group, sound teeth; Group 2 (n = 10): mesio-occlusodistal (MOD) cavity with isthmus opening of 2/3 of the intercuspal width and 2 mm depth pulpal wall, measured from the sulcus to the pulpal wall. The axial reductions to prepare the proximal boxes were done as 2/3 of the distance from the dental equator in the buccal-lingual direction with 1 mm below the cementoenamel junction, and the distance from the axial-pulpal angle to the gingival wall of 1.5 mm. (Figures
(a) Occlusal view of the preparation of group 1. (b) Proximal view of the finished preparation of group 1. (c) Occlusal view of the preparation of group 2. (d) Proximal view of the finished preparation of group 2.
Group 3 (n = 10) is mesio-occlusodistal (MOD) cavity with deep pulpal wall, simulating a tooth with a large carious lesion, isthmus opening equal to 2/3 of the intercuspal distance and 1 mm below the cementoenamel junction. The axial reduction of the proximal boxes was equal to 2/3 of the distance of the dental equator and 1 mm below the cementoenamel junction. Composite resin was used to immediately rebuild the preparation (IPS Empress Direct, shade A2, Ivoclar Vivadent, Schaan, Liechtenstein). Isthmus opening is 2/3 of the intercuspal distance and has 2 mm depth. Proximal boxes are 2/3 of the distance of the dental equator, with 1 mm below the cementoenamel junction and 1.5mm distance from the axial-pulpal angle until the gingival wall (Figures
The depth of the pulpal wall was standardized (2 mm) from the main sulcus to the pulpal wall, with the aid of a previously prepared silicone guide.
To perform the composite resin build-ups, the teeth were etched with 37% phosphoric acid (BM4, Palhoça, SC, Brazil, batch number: 0115/1013) for 15 s in dentin and 30 s in enamel; then a light-cured adhesive system (Tetric N-Bond, Ivoclar Vivadent, Schaan, Liechtenstein, batch # R67742) was applied and light-cured for 20 seconds and they were restored with composite resin (Empress Direct, shade: A2 Dentin, Ivoclar Vivadent, Schaan, Liechtenstein, batch number: S38387). The composite resin was inserted in increments up to 2 mm thickness and light cured for 20 seconds with a LED light-curing unit (Bluephase G2 Ivoclar Vivadent, Schaan, Liechtenstein) (800 mW/cm2).
Then, for the cementation of the ceramic inlays restorations, the teeth were etched with 37% phosphoric acid (Power Etching, BM4, Palhoça, Brazil) for 30 seconds on enamel and 15 seconds in dentin and rinsed abundantly with water, and the dentin surface was protected with sterile cotton and the enamel air-dried. The adhesive system (Excite F DSC Ivoclar Vivadent, Schaan, Liechtenstein S36470) was applied to the etched surface and was not light cured until cementation.
Inlays were fabricated using a CAD/CAM technique (Cerec AC, v.4.0, Sirona Dental Systems, Bensheim, Germany). Each preparation was coated with a titanium dioxide spray (OptiSpray, Sirona Dental Systems, Bensheim, Germany, Batch: #2013140338), scanned (Bluecam, Cerec AC,
Scanning the preparation with Bluecam device.
Design stage in Cerec.
After the finishing and polishing steps, the ceramic restorations were embedded in a PVS impression material (Virtual, Ivoclar Vivadent, Schaan, Liechtenstein) leaving exposed only the inner surface, which were to be conditioned. The inner surfaces were then etched with a 9% hydrofluoric acid (Porcelain Etch, Ultradent Products, Brazil. Batch: #290414) for 20 seconds, rinsed with water for 20 seconds, cleaned in ultrasonic bath with distilled water for 3 minutes, air-dried, and silane-coated (Monobond Plus, Ivoclar Vivadent, Schaan, Liechtenstein, Batch number: S44734). All restorations were coated with a thin layer of an adhesive system (Excite F DSC, Ivoclar Vivadent, Schaan, Liechtenstein, Batch number: S36470).
Luting was carried out with a dual-cure resin cement (Variolink II, Ivoclar Vivadent, Schaan, Liechtenstein, Batch: #S411783) following the manufacturer’s recommendations. The cement was applied on the preparation and the inner surface of the restoration, followed by light finger pressure toK flow excess material. The specimens were then placed in a device applying a 1 kg force for 2 min [
Metal device used to exert constant pressure during the cementation step.
Once completed the restorative procedures, the specimens were stored in distilled water at 37°C for 24 hours, to allow water sorption of the resin cement, complete polymerization and achieve maximum compressive strength.
The compressive strength test was performed in a universal testing machine (EMIC DL200, São José dos Pinhais, PR, Brazil) (TRD 27 load cell, Tesc software, version 3.01, crosshead speed of 1 mm/min). A 6 mm diameter metallic ball, designed to touch the tooth/restoration interface of the three cusps simultaneously was used to accomplish the test (Figure
A 6 mm diameter metallic ball touching the tooth of the three cusps.
Data were submitted to one-way analysis of variance (ANOVA) and possible differences detected by the Tukey’s post hoc test, both at a significance level of 5% (IBM SPSS Statistics V.21 Armonk, NY, USA).
Average compressive strength values (N) obtained by the groups, along with their descriptive statistics (standard deviation, minimum and maximum coefficient of variation, and 95% confidence interval) are described in Table
Mean compressive strength (N) obtained by the control group, G2, and G3 and their descriptive statistics (standard deviation, minimum, maximum, coefficient of variation, and 95% confidence interval).
Groups | N | Average (Newtons) | Standard Deviation | Minimum | Maximum | CV | -IC(95%) | +IC(95%) |
---|---|---|---|---|---|---|---|---|
Control | 10 | 3276.14 | 623.52 | 2317.44 | 4092.60 | 197.17 | 2830.10 | 3722.19 |
| ||||||||
G2 | 10 | 1971.71 | 557.01 | 1322.57 | 2987.35 | 176.14 | 1573.25 | 2370.18 |
| ||||||||
G3 | 10 | 1707.82 | 436.30 | 1270.14 | 4092.60 | 137.97 | 1395.71 | 2019.94 |
The compressive strength values show to be normally distributed according to the Shapiro-Wilk normality test (Table
Test of normality.
Shapiro-Wilk’s | |||
---|---|---|---|
Groups | Statistic | Df | Sig. |
Control | ,880 | 10 | ,131 |
| |||
G2 | ,884 | 10 | ,144 |
| |||
G3 | ,892 | 10 | ,179 |
Levene’s homogeneity of variances test.
Levene’s statistics | df1 | df2 | Sig. |
---|---|---|---|
1,199 | 2 | 27 | 0,317 |
One-way analysis of variance test (ANOVA).
| Sum square | df | Mean square | F | Sig. |
---|---|---|---|---|---|
Between groups | 14102706.97 | 2 | 7051353.48 | 23.784 | 0.000001 |
| |||||
Within groups | 8004767.34 | 27 | 296472.86 | ||
| |||||
Total | 22107474.31 | 29 |
Average compressive strength values of each group and their respective standard deviations (SD).
Groups | Mean (±SD) |
---|---|
Control | 3276.14 (±623.52)a |
| |
G2 | 1971.71 (±557.01)b |
| |
G3 | 1707.82 (±436.30)b |
Mean followed by the same letters indicates no statistically significant differences between the groups according to Tukey’s post hoc at a significance level of 0.05.
Average resistance to compression values of each group and their respective standard deviations (SD). Averages followed by the same letter indicate no statistically significant differences between groups according to Tukey’s post hoc test at a significance level of 0.05.
Flaws were classified and evaluated using a magnifying lens and rated as described by Burke et al. [
Fracture Pattern classification.
The results of this study confirmed the hypothesis that the composite resin build-up of the preparation in human molars, prior to the fabrication of ceramic inlays, did not promote a decrease in the strength of the tooth/inlay assembly with respect to its compressive strength, when compared to cases where the preparations did not require composite resin build-ups. On the other hand, both tested groups (G2 and G3) showed to be significantly less resistant than the control group (healthy tooth). These results corroborate those found by a previous study [
Regarding the pattern of fracture, assessed on a percentage basis, it can be noted that most of the failures observed in Group 3 were type V, with severe coronal destructions, and more aggressive when compared to the group with support only on tooth structure (G2). However, even if there is this classification, it should be considered that these failures occurred in rather higher forces (MPa) than the ones that the tooth is clinically exposed to. Thus, it should be taken into account that even though Group 3 showed a higher concentration of severe failures, this approach is still a therapeutic alternative.
Unlike the results found in this study, Saridag et al. [
Using composite resin enables the adjustment of the walls and gives an appropriate and uniform depth of the preparation, along with providing an ideal thickness for the ceramic material, with a proper radiopacity without interfering with the final shade of the indirect restoration [
In this study, the occlusal isthmus of the preparation was standardized in two thirds of the intercuspal labiolingual distance while the opening of the proximal boxes was standardized in two thirds of the buccolingual distance measured from the prosthetic equator, unlike other studies, in which standardized preparations were carried out, ignoring the labial-lingual dimension of each tooth [
It has been found that industrially manufactured partial ceramic crowns should have at least 1.5 – 2.0 mm thickness in areas subjected to stress, in order to prevent crack formation and covery long-term fracture resistance of ceramic [
It is important to understand the composition and mechanical properties of dentin in order to understand how to dissipate the masticatory forces distributed throughout the tooth, along with allowing a prediction of how these forces may cause changes in the restorations. The compressive strength of dentin varies from 230 to 370 MPa, and its modulus of elasticity varies from 7 to 30 GPa [
Building-up the preparation with composite resin also facilitates image acquisition by the CAD/CAM system and reduces heat transfer to the pulp tissue. All-ceramic restorations became widespread in the dental market in the 1980s, when the ceramic reinforced by leucite and the possibility of them being conditioned with acids and cemented to the tooth structure was introduced. The material properties regarding strength and aesthetics were also improved, making a more reliable ceramic material [
In the same decade, CAD/CAM systems have been introduced and have been gradually expanding until today. Conventional impressions may now be replaced by image acquisition of the preparation using a scanner that allows designing and fabricating a restoration in the same appointment, saving time and avoiding clinical distortions, which are very common in the conventional protocols, producing restorations with good marginal adaptation, color stability and high success rates [
Holberg et al. [
In a 9-year-clinical follow-up study that evaluated the clinical outcomes of crowns made with lithium disilicate after 9 years of cementation the cumulative rate of 97.4% survival rate at 5 years and 94.8% after 8 years of clinical function was found [
There is no consensus in the literature regarding the location where the compressive forces should be applied, whether in the areas of cusp, at tooth/restoration interface, or merely on the restoration. In this study, the force was applied at the tooth/restoration interface, similar to previous studies, [
A previous study [
Considering the limitations of this study, it can be concluded that MOD cavity preparations with similar configuration do not present statistically significant differences after reconstruction with composite resin and dentin support, when subjected to compressive load. Also, the tested groups restored with lithium dissilicate did not reestablish the healthy tooth resistance yet provided sufficient fracture resistance to withstand physiological masticatory forces.
The data used to support the findings of this study are included within the article.
The authors certify that they have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company that is presented in this article.