INTRODUCTION
Ceramic onlays are a popular esthetic option,[1] however, literature about ceramic onlays is not readily available. Preparation guidelines for ceramic onlays are based on and modifications of their cast metal counterparts. Occlusal preparations follow occlusal anatomy like metal preparations, but with increased tooth structure removal and the rounding of internal angles.[2] Furthermore, ceramic onlay preparation designs take into account a combination of different aspects, such as marginal preparatory finish, and occlusal reduction.[1]
While marginal finish has been studied in depth, with strong evidence for butt joint finishes,[3,4] there is currently no literature that considers the effect of occlusal preparation on ceramic onlays. Studies investigating at the effects of a flat versus an anatomical cusp form have been lacking.[5] There are full crown studies that compared anatomical with flat preparations, such as that of Shahrbaf et al. who compared anatomical and flat cuspal preparations under a range of bonding conditions, finding that a flat occlusal preparation provided better fracture strength under certain cements.[6] In a finite element analysis study of full zirconia crowns, a flat occlusal preparation had a lower level of concentrated stress compared to anatomical preparations, further indicating that flat cuspal angles may reduce pinpoint stress on direct centered occlusal loading.[7]
Despite their esthetic advantage over cast metal, a major disadvantage of dental ceramics is their brittle nature as their atomic structure has no mechanism for yielding to stress without fracture.[6,8] Ceramics are unable to undergo deformation so during masticatory loading stresses will concentrate in micro-flaws within the ceramic. When tensile stresses exceed the nominal strength of the ceramic, it will fracture.[9] Stress raisers, areas, or interfaces that increase stress should be avoided and removed if possible. Modern ceramic restorative designs incorporate this, and aside from requiring sufficient bulk of ceramic for strength, stress raisers in the form of sharp angulated designs are rounded, and abrupt changes in preparation shape avoided.[1,8]
The ideal preparation, therefore, must take into consideration the physical properties of dental ceramics and tooth preparation geometry. Ceramic preparations require 2 mm preparation and the nature of the preparation results in the cement bond to both enamel and dentine, which have significantly different bonding strengths.[10] Surprisingly, the current ceramic onlay preparatory theory comes into conflict with the mechanical properties of ceramics.[1,11] According to Newton's third law of motion, all forces between two objects exist in equal magnitude in the opposite direction. An object, such as ceramic, can, therefore, be placed in a specific orientation, resulting in a neutralization of net forces acting on it.[12] The easiest method of achieving this net balance of force is on a flat plane. In such a configuration, the force across each point of the interface is equal.[13] However, the same ceramic on an inclined plane, such as ceramic on an angulated cusp, no longer has the same balancing forces present. The corresponding forces acting on the ceramic will act in different amounts and directions depending on the position of the interface. The center of the onlay at the central fossa will have both the vertical and horizontal bulk of the whole ceramic unit acting on it, whereas at the midpoint of the cuspal slope, the bulk component is now half the amount experienced at the base, resulting in half the net force acting on it.[12]
As ceramic acts as a single unit, if a ceramic onlay is to be placed on a premolar with two cusps, the introduction of a masticatory load in the center results in different directional forces acting on it. In this situation, the brittle ceramic undergoes varying amounts of force in multiple directions, including vertical and lateral shear forces. If the innate stresses in the ceramic are already present, then, it would require less force to surpass the nominal force of the ceramic, resulting in fracture. Another approach to this is to consider that a cuspal onlay preparation resembles a large wedge. As the angle of inclination increases, the net force on the object increases under a given load, and the shear forces acting on the unit are larger than that from a lesser inclination.[14]
A fundamental aspect of ceramic preparations is the importance of rounded internal angles.[1,15] This was pioneered in 1954 by Haskins et al.[4,16,17] carried forward by McLean in 1979, and a literature review further reinforced the optimal preparations of ceramics.[18] It is well known that sharp preparation angles act as stress raisers in the ceramic.[8] Modern ceramic preparations continue this concept of rounded internal angles with great success.[19] It is interesting to note that there is very little literature assessing the effect of cuspal angulations on ceramic onlays. Many studies have assessed the effect of preparation tapers on ceramic onlays restorations; however, few have analyzed the effect of occlusal preparation forms. In addition, there is no literature available that assesses the impact of rounding internal angles of preparations on the newer generation infiltrated ceramics. This could be partly due to the difficulty in sourcing extracted teeth and the inconsistent bonding that occurs when bonding to natural teeth. The bond with natural teeth can be influenced by the amount of enamel or dentine exposed as well as the orientation of enamel prisms and dentinal tubules.[24,25,26,27,28,29] The aim of this study, therefore, is to investigate the significance of geometric cuspal angulation and the effect of sharp internal preparation angles on the fracture strength of IPS e.max computer-aided designing (CAD) (e.max) onlays.
MATERIALS AND METHODS
For this in vitro study, two stylized geometric tooth shapes (33° and 22° preparation angles) were constructed using a three-dimensional printer. The dimensions of the specimens were based on the maxillary first premolar (10 mm × 10 mm). Postprinting, the master specimens were checked with digital callipers and protractor. A polyvinyl siloxane impression (light and regular body) (Exahiflex, GC America) was made of each specimen and poured in Exakto-Form (Bredent GmbH and Co. KG, Germany). Exakto-Form is an inert, dimensionally and structurally stable material that offers a hom*ogeneous and uniform bonding surface. Exakto-Form has a Shore hardness D 80, flexural strength of 50 N/mm2 and an e-modulus of 3900 N/mm2 (Bredent GmbH and Co. KG, Germany). Exakto-Form is a resin die-stone material and was used to enable bonding of the ceramic onlay to the specimen. For each of the 33° and 22° preparations, 10 specimens were tested.
For the rounded specimens, a correcting jig was made of methyl methacrylate (SR Ivolen, Ivoclar Vivadent, Liechtenstein) was constructed, fitting onto the Exakto-Form specimen with 1 mm of the cusp tip exposed. The exposed peak of the cusp was then removed with a tungsten carbide acrylic trimmer. The resulting modification was smoothed with pumice and 2000 grit sandpaper before being visually assessed under ×3 magnification loupes.
The onlays were made using e.max (Ivoclar Vivadent, Liechtenstein), and all blocks were milled at the School of Dentistry, University of Otago using the Amann-Girrbach Ceramill CAD/computer-aided manufacturing (CAM) system. The specimens were sprayed with IPS Contrast Spray Chairside (Ivoclar Vivadent, Liechtenstein) before being scanned through Ceramill Map 400. As the Ceramill system does not support nonanatomical onlays, a wax-up of the onlay was made directly onto the specimen and rescanned. They were then made to a uniform 2 mm thickness and assessed through the Ceramill Mind 2 system [Figure 1a]. The models were matched with the onlay wax-up in the Ceramill Mind 2 system [Figure 1b], and the recommended milling guidelines for e.max blocks were used. Each onlay was milled using e.max ceramic blocks through the Ceramill Motion 2 (5-axis) system using authorized components.
Postmilling, the ceramic was assessed for defects and overall fit using ×3 magnification. As per Ivoclar Vivadent guidelines, the fitting surface was not modified. The onlays were separated from the machined holder with a diamond disk and a fine diamond bur. The onlays were then sintered in a Programat P310, and the exterior surface of the onlays was glazed with E. max Crystal Glaze (Ivoclar Vivadent, Liechtenstein) following the Ivoclar protocol.
Bonding of the ceramic onlay was done using Variolink II (Ivoclar Vivadent, Liechtenstein), a dual-cure luting composite recommended for the e.max range. Bonding was done as per the manufacturer's instructions and standardized for all specimens. Both the internal ceramic surface and the bonding surface of the model were etched, the ceramic with IPS Ceramic etching gel (hydrofluoric acid) for 20 s (Ivoclar Vivadent, Liechtenstein) and the model with phosphoric acid gel (37% total etch, Ivoclar Vivadent, Liechtenstein) for 15 s. After etching, the onlay and model were thoroughly cleaned with water and air dried. Silanization of the ceramic was done using Monobond Plus (Ivoclar Vivadent, Liechtenstein) applied to the ceramic and air dried for 60 s. For the model, Syntac primer (Ivoclar Vivadent, Liechtenstein) was applied and air dried for 15 s before coating the model with Syntac adhesive (Ivoclar Vivadent, Liechtenstein) and air dried for 10 s. Heliobond (Ivoclar Vivadent, Liechtenstein) was then applied to both the ceramic and model. Followed this, the resin bonding agent, Variolink II, was applied by mixing the base and catalyst at a 1:1 ratio and coating the model. The ceramic onlay was then placed and cured. For uniform cement distribution, a custom-made penetrometer, giving 50 N of force, was applied to the center of the ceramic onlay during curing. After loading with the penetrometer, excess cement was removed, and Liquid Strip (an oxygen inhibitor by Ivoclar Vivadent, Liechtenstein) applied around the bonded interface. The interface was then light cured for 40 s on each side.
Fracture strength testing was performed using a universal testing machine (TAHD plus texture analyzer, stable microsystems, Godalming, United Kingdom). The machine was calibrated using a calibration weight before every test and recalibrated every 15 test cycles. The base of the models was retained in the base grip, and a compressive axial load through a 250 kg load cell was applied under a constant speed of 0.06 mm/min until initial fracture. The specimens were randomly allocated to the different loading strategies with 10 in each group. There were two different loading strategies (central fossa and cusp peak) and four different preparations (sharp 33° and 22° and round 33° and 22°). The preparations were loaded in the central fossa through a 4.65 mm diameter stainless steel ball bearing (two-point lateral loading) and single-point vertical loaded at the cusp peak through a stainless steel rod [Figure 2]. The 33° angle was chosen as this is a naturally occurring steep angulation found in human teeth.[30] The 22° angle was selected as it was the lowest inclination possible that the same sized ball bearing could be utilized and still have the contact area on the ceramic surface in a similar location (within 1 mm) and with the same contact area. Increasing the ball bearing size would also change the contact area of the ball bearing and in turn, would have skewed the results. In addition, any angulations < 22° or a decrease in the ball bearing size would result in a single point contact in the central fossa.
The ball bearings and rods were checked under ×5 magnification for visual defects before and after each test and a new ball used after 10 test cycles. For all tests, a dental dam sheet was placed between the ceramic onlay and the steel ball or rod to act as a stress breaker to dissipate local stresses.
The initial fracture strength was identified initially graphically from the time/load graph, and the load in Newtons was identified directly from the data spreadsheet value [Figure 3]. A biostatistician analyzed the data using Stata 14.1 statistical software (StataCorp LP, Texas, United States). Statistical significance was determined by two-sided P < 0.05 and all pairwise comparisons included estimated differences with 95% confidence intervals. A specimen size of 10 allowed detection of pairwise differences between the groups at 80% power in a two-sided test at the 0.05 level. Model diagnostics, including checks of normality and hom*oscedasticity of residuals, were carried out. Independent comparison between the groups for the central fossa loading and top loading was done through Kruskal–Wallis tests. Where the overall tests were statistically significant, an unadjusted post hoc Dunn's tests were then used to identify which groups differed.
RESULTS
The central fossa and cusp tip loading raw data showed evidence of skewing; the median is not typically in the 50th percentile range. In addition, the bottom and top for most groups have an uneven distribution from the 75th percentile to the maximum and the 25th percentile to the minimum value, respectively. Therefore, the use of the median as an indicative value is more suited than the mean. The outliers were determined using the standard rule of values 1.5 times the interquartile range away from the lower and upper quartiles [Table 1].
The central fossa loading results found a marked distinction in the fracture strength values between the two angulations. The round and sharp 22° preparations had comparable median fracture strengths of 1882 N and 1890 N, respectively. While the fracture strength of the round and sharp 33° preparations were lower, with the specimens having a median fracture strength of 1581 N and 1379 N, respectively [Table 1]. The Kruskal–Wallis test confirmed the significant differences between the different angulations when loaded to fracture. The post hoc testing through Dunn's showed that there was significant difference between the round 22° and round 33° preparations (P = 0.0011), the round 22° preparation and the sharp 33° preparation (P = 0.0001), and between the round 33° and sharp 22° preparations (P = 0.0056) and the sharp 22° and sharp 33° preparations (P = 0.0002) [Table 2].
Cusp peak vertical loading resulted in the round 22° preparation having the highest median fracture strength of 2087 N, followed by the round 33° preparation of 1850 N, the sharp preparations had the weakest fracture strengths with sharp 22° median of 1530 N and the sharp 33° median of 1571 N [Table 2]. Evidence of significance was seen from Kruskal–Wallis testing and post hoc testing which found statistically significant differences between the round 22° compared with the sharp 22° preparations (P = 0.0013) and the sharp 33° onlay (P = 0.0003). Furthermore, a significant difference was found between the round 33° specimens and the sharp 33° specimens (P = 0.0304) [Table 2].
DISCUSSION
The aim of this study was to investigate the impact of geometric shapes and angulation on the fracture strength of e.max ceramic restorations. This in vitro study used standard materials to provide the baseline for future comparisons. The results found that for each angulation, the rounded preparation was significantly stronger than the sharp counterpart, supporting the current paradigm regarding rounding preparations and is in-line with current literature, education, and the biomechanics of modern ceramics.[16,18,31] Interestingly, while photoelastic analysis has shown rounded preparation angles reduce stress in ceramics, there is no quantitative data on the difference between sharp and rounded angles. This study supports current literature, and the results indicate the influence rounding angles has on the fracture strength of e.max.
There were a number of limitations to this study; the main issue is the spread of fracture strength values recorded and the limited specimen size. The specimen number was limited due to cost and time restraints as manufacturing the onlays were both costly and time consuming. After milling, the machined attachment holder required manual trimming, and while trimming was done in accordance to manufacturers' guidelines, there exists the possibility of microcracks being formed in the onlay which can weaken the restoration. However, glass infiltrated lithium disilicate systems, such as the e.max system are known for strength, longevity, and quality.[32] While e.max Press offers a slightly stronger strength over its CAD/CAM counterpart, the errors in traditional laboratory processing (such as variability of slip-casting of ceramics) means that CAD/CAM milled ceramics offered advantages such as uniformity in the quality of ceramic as well as the reduced incidence of technique error affecting onlay fit and onlay size consistency.[1,8]
As the specimens prepared in this study were stylized specimens, it is possible to make limited correlation to a clinical situation. While natural teeth could offer a direct clinical correlation on the performance of e.max, a stylized specimen made of a uniform material. The use of Exakto-Form removed potential confounders and offered enhanced accuracy. Bonding to natural teeth is influenced by the amount of enamel or dentine exposed as well as the orientation of enamel prisms and dentinal tubules.[24,25,26] As ceramic preparations require at least 2 mm preparation, the nature of the preparation results in the cement bond to both enamel and dentine, which have significantly different bonding strengths.[10] Furthermore, the source of extracted teeth is a potential confounder that can compromise bonding consistency between specimens.[25,27,28,29]
The results of the study showed a marked difference in the fracture strength between the two preparation angles. For the central fossa loading, the sharp and round 22° preparation were found to have the highest median fracture strengths, being significantly higher than both sharp and round preparations with 33° preparations. This difference in strength between the 22° and 33° preparations can be attributed to the lowered lateral shear forces present on the cuspal inclinations. From classical mechanics, an object placed on a inclined plane will have a higher net force than if the same object was placed on a plane of lower inclination.[12] This correlates in less stress applied to the ceramic onlay at any given load force, explaining the higher fracture strength of the 22° preparation compared to the 33° preparation. Ceramics are hom*ogenous, and the ceramic unit deforms as an entire unit. When lateral forces are introduced, the induced force will push the onlay on two sides, forcing the inclined ceramic against the specimen. As the increased inclination results in a higher amount of stress, it requires less force applied to reach the point of fracture. The geometrical angulation of the cusp may therefore mitigate or facilitate such shear effects. With a cusp angulation of 33°,[33] a standard onlay with an anatomical preparation will therefore create a large wedge in the tooth compared to a preparation with a lower incline. Given these mechanical principles, a flat plane, without any inclinations, would theoretically result in a lower inherent stress in the ceramic body, correlating to a stronger ceramic as it would require a larger external force (e.g., masticatory force) to exceed its nominal force resulting in ceramic fracture.
For cusp peak loading, the rounded preparations were significantly stronger than the sharp preparations. The round 22° preparation had the highest median fracture strength, having a significant higher fracture strength compared to the sharp 22° (P = 0.0013). Analysis of the round 33° to the sharp 33° preparations also found the difference in fracture strength to be statistically significant. Comparison between the round 33° and round 22° preparations were not statistically significant (P = 0.0551), while the comparison between the round 33° specimens and sharp 22° specimens were statistically insignificant (P = 0.0799). This possibly could indicate that the influence of the preparation angle is negated when loading the cusp peak.
While this was an in vitro study, the authors feel that this research inherently supports current ceramic onlay preparation concepts and therefore fits well with clinical practice. During the occlusal preparation for e.max onlays, the results show that reducing cuspal angulation and rounding the preparation correlates to a stronger ceramic restoration. This study indicates further research into the field of ideal occlusal preparations for ceramic onlays would have merit.
CONCLUSION
Within the limits of the present study, the following can be concluded that:
- e.max geometric onlays with an occlusal preparation angle of 22° resulted in superior fracture strength compared to occlusal preparations with a 33° inclination when a central fossa load was applied
- e.max geometric onlays with a rounded occlusal internal line angles preparation resulted in a significantly (22° P = 0.0013 and 33° P = 0.0304) higher fracture strength for any given inclination when a cusp peak load was applied
- When loading the cusp peak, the occlusal preparation angle does not appear to influence the fracture strength of e.max.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
Acknowledgment
The authors would like to thank Mr. Andrew Grey for the statistical analysis. The authors also thank to Ms. Minshym Wong from the Faculty of Dentistry Technical Services Laboratory at the University of Otago for the use of the CAD/CAM facilities.
Additional thanks to Mr. Thomas Frerichs from Amann-Girrbach (Singapore) for instructions on modifying the CAD/CAM system, and to Ivoclar Vivadent, Liechtenstein for funding of materials.
REFERENCES
1. Rosenstiel SF, Land MF, Fujimoto J Contemporary Fixed Prosthodontics. 2006 St. Louis Elsevier Health Sciences
- Cited Here
2. Touati B, Miara P, Nathanson D Esthetic Dentistry and Ceramic Restorations. 1999 London, New York Martin Dunitz, Thieme
- Cited Here
3. da Costa DC, Coutinho M, de Sousa AS, Ennes JP. A meta-analysis of the most indicated preparation design for porcelain laminate veneers J Adhes Dent. 2013;15:215–20
- Cited Here |
- PubMed
4. Thompson MC, Thompson KM, Swain M. The all-ceramic, inlay supported fixed partial denture. Part 1. Ceramic inlay preparation design: A literature review Aust Dent J. 2010;55:120–7
5. Podhorsky A, Rehmann P, Wöstmann B. Tooth preparation for full-coverage restorations-a literature review Clin Oral Investig. 2015;19:959–68
6. Shahrbaf S, van Noort R, Mirzakouchaki B, Ghassemieh E, Martin N. Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement Dent Mater. 2014;30:234–41
7. Anami LC, Lima JM, Corazza PH, Yamamoto ET, Bottino MA, Borges AL, et al Finite element analysis of the influence of geometry and design of zirconia crowns on stress distribution J Prosthodont. 2015;24:146–51
8. Anusavice KJ, Shen C, Rawls HR Phillips' Science of Dental Materials. 2013 St. Louis Elsevier/Saunders
- Cited Here
9. McCabe JF, Walls A Applied Dental Materials. 2013 Oxford John Wiley & Sons
- Cited Here
10. Reis AF, Giannini M, Kavaguchi A, Soares CJ, Line SR. Comparison of microtensile bond strength to enamel and dentin of human, bovine, and porcine teeth J Adhes Dent. 2004;6:117–21
- Cited Here |
- PubMed
11. Shillingburg HT, Sather DA Fundamentals of Fixed Prosthodontics. 2012 Chicago, IL Quintessence Pub
- Cited Here
12. Shipman J, Wilson J, Higgins C An Introduction to Physical Science. 2012 Boston Cengage Learning
- Cited Here
13. Pelleg J Mechanical Properties of Ceramics. 2014 London Springer Science and Business
- Cited Here
14. Bansal RK A Textbook of Strength of Materials. 2010 New Delhi Laxmi Publications
- Cited Here
15. Shillingburg HT, Jacobi R, Brackett SE Fundamentals of Tooth Preparation: For Cast Metal and Porcelain Restorations. 1987 Illinois Quintessence Publishing Company
- Cited Here
16. Haskins RC, Haack DC, Ireland RL. A study of stress pattern variations in class II cavity restorations as a result of different cavity designs J Dent Res. 1954;33:757–66
17. McLean JW The Science and Art of Dental Ceramics. 1979 Chicago Quintessence Pub Co
- Cited Here
18. Banks RG. Conservative posterior ceramic restorations: A literature review J Prosthet Dent. 1990;63:619–26
19. Zimmer S, Göhlich O, Rüttermann S, Lang H, Raab WH, Barthel CR, et al Long-term survival of cerec restorations: A 10-year study Oper Dent. 2008;33:484–7
20. Al-Omari WM, Al-Wahadni AM. Convergence angle, occlusal reduction, and finish line depth of full-crown preparations made by dental students Quintessence Int. 2004;35:287–93
21. Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: An art form based on scientific principles J Prosthet Dent. 2001;85:363–76
22. Zidan O, Ferguson GC. The retention of complete crowns prepared with three different tapers and luted with four different cements J Prosthet Dent. 2003;89:565–71
23. El-Ebrashi MK, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part V. The concept of occlusal reduction and pins J Prosthet Dent. 1969;22:565–77
24. Sirisha K, Rambabu T, Shankar YR, Ravikumar P. Validity of bond strength tests: A critical review: Part I J Conserv Dent. 2014;17:305–11
25. Perdigão J. Dentin bonding-variables related to the clinical situation and the substrate treatment Dent Mater. 2010;26:e24–37
26. Watanabe LG, Marshall GW Jr, Marshall SJ. Dentin shear strength: Effects of tubule orientation and intratooth location Dent Mater. 1996;12:109–15
27. Harris EF, Hicks JD, Barcroft BD. Tissue contributions to sex and race: Differences in tooth crown size of deciduous molars Am J Phys Anthropol. 2001;115:223–37
28. DeWald JP. The use of extracted teeth for in vitro bonding studies: A review of infection control considerations Dent Mater. 1997;13:74–81
29. Humel MM, Oliveira MT, Cavalli V, Giannini M. Effect of storage and disinfection methods of extracted bovine teeth on bond strength to dentin Braz J Oral Sci. 2008;6:1402–6
- Cited Here
30. Misch CE Dental Implant Prosthetics. 2014 Missouri Elsevier Health Sciences
- Cited Here
31. Arnetzl GV, Arnetzl G. Design of preparations for all-ceramic inlay materials Int J Comput Dent. 2006;9:289–98
- Cited Here |
- PubMed
32. Kern M, Sasse M, Wolfart S. Ten-year outcome of three-unit fixed dental prostheses made from monolithic lithium disilicate ceramic J Am Dent Assoc. 2012;143:234–40
33. Misch CE Dental Implant Prosthetics. 2015 St. Louis, Missouri Elsevier Mosby
- Cited Here
Keywords:
Ceramic onlay; fracture strength; occlusal reduction
