journal of prosthodontic research 58 (2014) 107–114
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
Fracture strength of CAD/CAM composite and composite-ceramic occlusal veneers§ Andrew C. Johnson DDS, MDSa, Antheunis Versluis PhDb, Daranee Tantbirojn DDS, MS, PhDc, Swati Ahuja BDS, MDSd,* a
Advanced Prosthodontic Program, Department of Prosthodontics, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States b Department of Bioscience Research, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States c Department of Restorative Dentistry, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States d Department of Prosthodontics, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States
article info
abstract
Article history:
Purpose: To determine the effect of material type and restoration thickness on the fracture
Received 30 September 2013
strength of posterior occlusal veneers made from computer-milled composite (Paradigm
Received in revised form
MZ100) and composite-ceramic (Lava Ultimate) materials.
5 December 2013
Methods: 60 maxillary molars were prepared and restored with CAD/CAM occlusal veneer
Accepted 10 January 2014
restorations fabricated from either Paradigm MZ100 or Lava Ultimate blocks at minimal
Available online 11 March 2014
occlusal thicknesses of 0.3, 0.6, and 1.0 mm. Restorations were adhesively bonded and subjected to vertical compressive loading. The maximum force at fracture and mode of
Keywords:
failure were recorded. 2-Way ANOVA was used to identify any statistically significant
Occlusal veneer
relationships between fracture strength and material type or thickness. Spearman’s rank
Milled
correlation coefficient was used to analyze mode of failure with regard to fracture strength.
Conservative
Results: The average maximum loads (N) at fracture for the Paradigm MZ100 groups were
Posterior
1620 433, 1830 501, and 2027 704 for the material thicknesses of 0.3, 0.6, and 1.0 mm,
CAD/CAM
respectively. The Lava Ultimate groups fractured at slightly higher loads (N) of 2078 605, 2141 473, and 2115 462 at the respective 0.3, 0.6, and 1.0 mm thickness. Statistical analyses revealed that, while no significant difference existed among the various restoration thicknesses in terms of fracture strength (P > 0.05), the material type was found to be influential (P = 0.04). The maximum load at fracture (N) for Lava Ultimate averaged over all thicknesses (2111 500) was significantly higher than that of the Paradigm MZ100 (1826 564). No correlation between mode of failure and fracture strength was found. Conclusions: Under the conditions of this study, the maximal loads at fracture for these ‘‘non-ceramic’’ occlusal veneer restorations were found to be higher than human masticatory forces. Occlusal veneers made from the two materials tested are likely to survive
§ This study was presented as an oral defense for the degree designation: Master of Dental Science on 28th June 2013 at University of Tennessee Health Science Center. * Corresponding author at: 207, Giriraj, Neelkanth Valley, Ghatlopar East, Mumbai 400077, Maharashtra, India. Tel.: +91 9819365338; fax: +1 9014481294. E-mail addresses: [email protected], [email protected] (S. Ahuja). 1883-1958/$ – see front matter # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2014.01.001
108
journal of prosthodontic research 58 (2014) 107–114
occlusal forces regardless of restoration thickness, with those fabricated from the composite-ceramic hybrid material being more likely to survive heavier loads. # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
1.
Introduction
Pathologic loss of coronal tooth structure can be attributed to a multitude of individual or combined etiologic factors related to dietary and oral habits which ultimately cause abrasion and/ or erosion of enamel and dentin [1]. The deterioration of this tooth structure has been associated with unfavorable changes in esthetics [2], tooth sensitivity [3], occlusal stability [4], maxillomandibular relationships [5,6], and musculoskeletal harmony [7,8]. Given the pattern and extent of damage to severely worn teeth, the restoration of such defects becomes increasingly challenging and often requires multiple modifications to preparation design and possibly elective tooth devitalization and/or periodontal surgery [9]. In the interest of limiting the amount of adjunctive procedures required for dental reconstruction, a trend toward conservatism in preparation and restoration design is reemerging [10–12]. Considering the finite longevity of common fixed dental restorations [13–15] along with the increasing lifespan of both the patient and their natural dentition [16], maximizing the amount of tooth structure available for future re-treatment is an increasingly relevant consideration. The introduction of composite resin technology initiated a paradigm shift regarding the restoration of lost/damaged tooth structure [17]. However, while the advent of resinbonding has allowed for more conservative filling preparations and restorations [18], given the structural properties of commonly used resin-bonded all-ceramics, current tooth reduction parameters for indirect restorations have remained comparatively aggressive [19,20]. Yet, beyond the challenge of deciding upon the most appropriate treatment modality, along with the numerous technical hurdles associated with its stepwise accomplishment, lies the considerable financial burden the patient must assume upon initiating definitive and irreversible treatment on so many teeth at once. Often, patients simply cannot bear the considerable expense of a large-scale dental reconstruction as a whole. This limitation, all too frequently, is cause for postponing necessary care and perpetuating existing dental problems. Solutions for many of the technical challenges mentioned can be found in current Dental CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) systems. Furthermore, the concepts of CAD/CAM dentistry and conservative tooth preparation seem to be converging [21]. Given the technological advancements which now allow for better and finer reproduction of detail in machine-milled restorations, along with recent innovations of new restorative materials, computerized restorative dentistry may prove to be the best option for uniting tooth structure conservation with simple, expedient, and affordable restoration [22,23]. Considering the often regular pattern of structural loss often seen in cases of long-term bruxism and acid erosion [1,24], restoring such worn teeth with occlusal veneers can be relatively non-invasive, requiring little additional tooth
reduction. Using CAD/CAM technology, such minimalistic restorations can be machine-milled from various restorative materials to match the occlusal contours established in a diagnostic wax-up and adhesively bonded for extended, stable re-establishment of the posterior occlusion. Restorations of this type might not allow the same optimization of esthetics given the marginal visibility; however, their ease of fabrication and installation as a functional preview of a proposed posterior occlusion should be reason enough to pursue their development if not only as a means of effective, predictable temporary reconstruction of teeth. When provisionalized in this manner, subsequent definitive restoration of the posterior teeth can then be limited to sequential, segmental or even single-tooth treatments, allowing the patient a gradual transition from provisional to final restorations as he or she may accommodate. So far, little research has been done to investigate the viability of various materials utilized as minimal-thickness posterior occlusal veneers. Related experiments have tested the fracture strength and fatigue resistance of both ceramic and non-ceramic materials as conservative posterior occlusal restorations of various designs with results that appear to support the use of non-ceramic materials in these applications given their reported increased resistance to fracture [25,26]. Still, more evidence is necessary to support the clinical implementation of these minimally invasive, CAD/CAM posterior occlusal restorations as recommended alternatives to traditional means of restoring posterior occlusion. In the present study, the fracture strength of minimally thick (0.3, 0.6, and 1.0 mm) composite (Paradigm MZ100, 3M ESPE, St. Paul, MN) and composite-ceramic (Lava Ultimate, 3M ESPE, St. Paul, MN) occlusal veneers was tested by vertical compressive loading to fracture. The aim of this in vitro investigation was to determine any effect of restoration dimensions or material selection, and it operated on the null hypotheses that there would be no significant differences in the fracture strengths between material types or among thicknesses. Such findings would offer further support for the utilization of thin, non-ceramic occlusal veneers as functional and predictable means of posterior reconstruction.
2.
Materials and methods
One hundred extracted human maxillary molars of comparable dimensions and morphology were mounted in autopolymerizing acrylic resin using a cylindrical Telflon mold at a level 3 mm apical to their respective CEJs (Fig. 1A) and stored in distilled water.
2.1.
Preparation
Standardized tooth preparations replicating a worn occlusal table were accomplished using a diamond saw (Isomet Plus Precision Saw, Buehler, Lake Bluff, IL.). The mounted
journal of prosthodontic research 58 (2014) 107–114
109
Fig. 1 – (A) Extracted tooth specimen mounted in acrylic base. (B) Prepared specimen showing indexing notches and demonstrating intact enamel and dentin without pulp exposure. (C) Occlusal scanning form created from an anatomic denture tooth used in restoration design.
Fig. 2 – Flow chart of specimen selection and test group assignment.
teeth were sectioned axially, removing all coronal tooth structure 4 mm occlusal to the CEJs leaving exposed dentin centrally and peripheral enamel. Intra-enamel indexing notches were created on the mesial and distal finish lines (Fig. 1B) with a high speed round-ended tapered diamond rotary bur (#ZR 850 FG.01, Komet USA, Rock Hill, SC). Each mounted, prepared tooth was inspected for disqualifying characteristics such as pulpal exposure or cracks/fractures. Those with identifiable defects were eliminated and, from the eighty-four intact specimens remaining, sixty were selected. The specimens were randomly divided into 2 groups based on restorative materials: Paradigm MZ100 (3M ESPE, St. Paul, MN (MZ groups) and Lava Ultimate (3M ESPE, St. Paul, MN) (LU groups). Within each group, specimens were subdivided into 3 subgroups (consisting of 10 specimens each) based on assigned restoration thickness (0.3, 0.6, 1.0 mm) (Fig. 2).
2.2.
Preparation/occlusion scanning
A standardized occlusal form was created by trimming the cervical structure from an anatomic denture tooth (Phonares SR Typ NHC Mould NU3, Ivoclar Vivadent, Amherst, NY) to within 0.3 mm of the central fossa to be positioned on the prepared tooth specimens during the ‘‘occlusion’’ scans (Fig. 1C). Using a Cerec 3 acquisition unit operating software version 3.84 (Sirona Dental Systems GmbH, Bensheim,
Germany), scans of each prepared tooth surface were obtained as well as secondary scans of each specimen bearing the standardized occlusal form positioned in best agreement with its perimeter outline and axial rotation (Fig. 3A). During scanning, the specimens were positioned within a custom visual reference jig which provided the topographical reference indices necessary to correlate the ‘‘preparation’’ and ‘‘occlusion’’ images (Fig. 3B).
2.3.
Restoration fabrication
With the software set in ‘‘Master Mode’’, occlusal restorations were designed virtually using the ‘‘Correlation’’ feature which permitted the orientation of occlusal form contours with those of the prepared teeth. This resulted in restoration proposals with uniform occlusal morphology while allowing customized marginal adaptation. Using the virtual design tools within the software, the occlusal–cervical dimensions were adjusted to the prescribed thicknesses assigned to each test group. Groups MZ1 and LU1 were designed with a minimal thickness of 1.0 mm (measured from the depth of the central fossa to the preparation surface), groups MZ2 and LU2 were designed at 0.6 mm minimal thickness and groups MZ3 and LU3 at 0.3 mm. Milling sprue locations were designated on the distal surface of each final proposal, and restorations were computer-milled from either Paradigm MZ100 or from Lava Ultimate restorative materials.
110
journal of prosthodontic research 58 (2014) 107–114
Fig. 3 – (A) Scans of prepared specimen with and without occlusal form. Note the inclusion of peripheral reference jig topography allowing correlation of scans. (B) Scanning jig positioned on specimen coated in contrast media.
2.4.
Restoration bonding
After milling, sample restorations were inspected for defects and assessed for proper fit. Per manufacturer’s recommendations, all specimens were air-abraded with 40 mm aluminum oxide at 1.8 bar of pressure, cleaned with alcohol and dried with oil-free pressurized air. Prepared tooth surfaces were etched with 37.5% phosphoric acid for 15 s (Ultra-Etch; Ultradent Products, Inc, South Jordan, Utah), rinsed thoroughly and dried without dessicating dentin. Restorations were then bonded using self-adhesive, dual-cure resin cement (RelyX Unicem, 3M ESPE, St. Paul, Minnesota) per manufacturer’s instructions while a seating pressure of 6 N was applied by a customized device for 5 min (Fig. 4). Following initial light curing, excess marginal resin cement was cleaned and final light curing completed for 15 s per restoration surface at 1400 mW/cm2 (SmartLite Max LED curing light, Dentsply Caulk, Tulsa, OK, USA). After restoration bonding all specimens were returned to 37 8C distilled water storage for 1 week prior to fracture testing.
2.5.
Load testing
Each specimen was subjected to vertical load to fracture using a universal testing machine (Instron 5567, Norwood, MA). Those specimens mounted in the acrylic bases visibly in-line with direction of loading were placed directly on the testing platen. Those mounted slightly off-axis were re-oriented relative to the direction of loading by using an angleadjustment table. A custom 3.5 mm diameter spherical stainless steel tip was used to simulate an opposing cusp
and positioned to achieve equalized tripod contacts along the cuspal inclines surrounding the central fossae of the restorations. A 10 kN load cell was installed, the cross-head speed was set at 0.5 mm/min., and the automatic cut-off was set at 25% loss of peak load. The maximum vertical load at the point of fracture was recorded for each specimen. Homoscedasticity of variance and normal distribution of the data were tested using Bartlett and Anderson–Darling Normality test. Two-way ANOVA (Super Anova, Abacus Concepts, Berkeley, CA, USA) was performed to identify any difference in fracture strength as a result of material, thickness, or material–thickness interaction. Images of each specimen were recorded using a stereomicroscope with charge-coupled device (CCD) camera (SZX16 & UC30, Olympus, Tokyo, Japan) before and after fracture testing for failure mode comparison. The mode of failure for each specimen was categorized based on structures involved in the fracture (mode 1 = restoration only; mode 2 = restoration and enamel; mode 3 = restoration, enamel and dentin).
3.
Results
Mean maximum loads (N) standard deviation at the point of fracture for the MZ groups were 1620 433 (MZ3), 1830 501 (MZ2), and 2027 704 (MZ1) for the material thicknesses of 0.3, 0.6, and 1.0 mm, respectively. The LU groups fractured at slightly higher average loads (N) of 2078 605 (LU3), 2141 473 (LU2), and 2115 462 (LU1) at the respective 0.3, 0.6, and 1.0 mm thicknesses. The maximum, median, minimum, and upper and lower quartiles of the fracture loads are shown in a box plot
journal of prosthodontic research 58 (2014) 107–114
111
Fig. 5 – Box plot of failure load for each group. Dot is the median value, whiskers show maximum and minimum failure loads, box indicates the upper and lower quartiles.
Fig. 4 – Custom seating pressure device applying 6 N of vertical force during bonding procedures.
(Fig. 5). While there was a discernable trend suggesting a direct relationship between fracture strength and restoration thickness for the MZ material groups, no such relationship was observed among LU groups. Bartlett’s test revealed no significant difference between the variances (P = 0.668). The fracture strength values of the LU group had a high probability for a normal distribution (95.1%), while the MZ group had low probability (2.8%) (Anderson–Darling Normality test). Assuming a normal distribution, two-way ANOVA (Super Anova, Abacus Concepts, Berkeley, CA, USA) indicated that material type significantly affected fracture strengths averaged over restoration thicknesses (P = 0.04). However, no significant effect was attributable to restoration thickness or the material-thickness interaction (P = 0.43 and 0.55, respectively) (Table 1). Pairwise multiple comparisons showed that the LU1 and LU2 groups demonstrated higher fracture strengths than the MZ3 groups (P = 0.04 and 0.03, respectively) (Table 2). No notable difference was found between the fracture strengths of the 0.3 mm Lava Ultimate and any other group. 22 specimens displayed mode 2 fracture of the restoration along with some portion of peripheral enamel (Fig. 6A). 23 specimens showed mode 3 fracture of restoration, enamel and dentin (Fig. 6B), and 15 specimens fracturing only through the restorative material and tooth-restoration interface leaving the underlying enamel and dentin intact (mode 1) (Fig. 6C). 4 of
23 specimens displaying category 3 failure, while fractured through all structures, remained bodily intact (Fig. 6D, E) showing no delamination of restorative material. The rarest outcome appeared to be a complete adhesive failure (1 specimen) wherein all restorative material fractured away without damaging any underlying tooth structure (Fig. 6F). Fig. 7 (Fig. 7) shows the plot between failure mode and fracture load of each specimen. Spearman’s rank correlation coefficient did not identify a trend between mode of failure and maximum load at fracture. Additionally, no clear pattern was noted with respect to the number of specimens from each group and failure mode. For instance, the thicker restorations milled from the Lava Ultimate material displayed more fractures involving the underlying tooth while the thinner Paradigm MZ100 restorations more often fractured along with tooth structures.
4.
Discussion
The null hypothesis regarding restoration dimensions was accepted, no significant difference in fracture strengths was found between occlusal veneer thicknesses. However, the null hypothesis concerning material type was rejected as a statistically significant difference was found between the fracture strengths of Paradigm MZ100 and Lava Ultimate. In terms of ultimate restoration strength, these results do appear to demonstrate the initial feasibility of this minimally invasive treatment modality for restoring only the occlusal portions of severely worn teeth with rapidly prototyped, nonceramic materials. Considering that the mean fracture strengths for all sample groups were well above achievable human masticatory forces reportedly ranging from 585 to 880 N [27–29], it may be surmised that such restorations may be capable of clinical success in similarly ideal circumstances, and any complications/failures related to fracture strength would likely be due to factors well beyond normal maximum occlusal loading (i.e. cyclic fatigue, trauma, etc.). Similar studies have been done to investigate the strength of conventional ceramics and alternative materials as applied
112
journal of prosthodontic research 58 (2014) 107–114
Table 1 – Two-way ANOVA statistics. Source
df
Sum of squares
Mean square
F-value
P-value
Material Thickness Material thickness
1 2 2
1,220,370.817 501,886.233 348,519.633
1,220,370.817 250,943.117 174,259.817
4.215 0.867 0.602
0.0449 0.4261 0.5514
Fig. 6 – (A) Example of category 2 failure. (B) Type 3 fracture involving restoration, enamel and dentin. (C) Specimen demonstrating type 1 fracture of restoration material only. (D and E) Specimens demonstrating category 3 failure displaying fracture through all restoration and enamel–dentin but yet remaining bodily intact. (F) Specimen displaying restorationonly fracture and complete adhesive failure.
Table 2 – Pairwise multiple comparisons. Groups LavaUlt, 0.3 mm
LavaUlt, 0.6 mm
LavaUlt, 1.0 mm
MZ100, 0.3 mm MZ100, 0.6 mm
versus
P-value
LavaUlt, 0.6 mm LavaUlt, 1.0 mm MZ100, 0.3 mm MZ100, 0.6 mm MZ100, 1.0 mm LavaUlt, 1.0 mm MZ100, 0.3 mm MZ100, 0.6 mm MZ100, 1.0 mm MZ100, 0.3 mm MZ100, 0.6 mm MZ100, 1.0 mm MZ100, 0.6 mm MZ100, 1.0 mm MZ100, 1.0 mm
0.7954 0.8794 0.0623 0.3077 0.8346 0.9144 0.0349 0.2024 0.6400 0.0447 0.2423 0.7185 0.3863 0.0961 0.4159
in various restoration designs. When full-coverage, conventional all-ceramic crowns (1.5–2 mm occlusal thickness) were luted to extracted teeth and subjected to similar load testing, the range of forces causing restoration fracture (771–1183 N) were well below those found by the current study [30]. When all-ceramic restorations have been fabricated as conventional crowns or minimalistic, occlusal veneers, and subjected to artificial aging, the ‘‘ultra-minimal’’ restoration design did
display adequate strength to survive simulated masticatory function [31]. However, composite materials under similar comparisons of restoration strength seem to consistently demonstrate more favorable properties than their conventional ceramic counterparts. A study comparing the fatigue resistance of allceramic crowns to that of composite full-coverage restorations found that the non-ceramic specimens displayed significantly better (100%) survival when subjected to cyclic loading [32]. When composite crowns of variable convergence angles, margin design, cementation method and thickness (including a minimalistic 0.5 mm occlusal dimension) have been subjected to thermocycling and subsequent fracture strength testing, it has been shown that occlusal thickness significantly impacts restoration strength as fractures occurred at average loads of 1566 N and 1127 N for 1.3 mm and 0.5 mm thicknesses respectively [25]. In an experiment by Magne et al. involving vertical cyclic loading of 1.2 mm thick occlusal veneers CAD/CAM-fabricated from 2 ceramic materials as well as Paradigm MZ100 composite, the composite material was significantly stronger in such conservative applications given its 100% survival after all loading cycles (up to 1400 N) compared to the 30% and 0% survival rates reported for the ceramic groups [33]. A related study by Schlichting et al., compared the fatigue resistance of thinner (0.6 mm) occlusal veneer restorations fabricated from
journal of prosthodontic research 58 (2014) 107–114
Fig. 7 – Failure modes and fracture strength of all specimens.
tions under ideal circumstances was produced. While this information alone should not be the sole basis on which clinical decisions are made, it is complementary to that offered by studies with different testing protocols and results. The ‘‘vertical’’ or ‘‘compressive’’ nature of loading might be an oversimplification of the actual forces applied to the specimens. Given that the spherical load tip was prevented from striking the depth of the central fossa by tripodized contacts along the surrounding cuspal inclines, the true nature of the load applied during loading were necessarily not strictly vertical compression [34]. In fact, it is likely that most of the forces acting on those central areas were tensile in nature as the cusps were deflected around the advancing load tip. Similar mechanics have been identified through computer modeling and have been found to vary based on the type of material to which a simulated occlusal force is applied [34,35]. The fracture resistance demonstrated by these computermilled, non-ceramic materials (even at minimal dimensions) appears adequate to survive and function as lasting prosthetic replacements for lost occlusal tooth structure. The fact that they are able to be easily fabricated at a reduced cost and perhaps applied without the need for additional tooth reduction support the clinical implementation of such conservative restorations.
5. the same materials as well as an ‘‘experimental’’ fiberreinforced composite [26]. They reported that no ceramic specimen survived all loading cycles, but the survival rates demonstrated by the composite and reinforced-composite groups at these minimal dimensions was 60% and 100% respectively after final loading cycles at 1400 N. The fact that 2/3rds of the restoration fractures seen in the present study involved damage to natural tooth structure along with restorative material failure may suggest that these restorations possess strength at least similar to that of the worn teeth they are intended to restore. If these assumptions are ultimately proven, clinicians may attempt such simple and conservative procedures with confidence that no additional detriment is being brought to bear on the teeth as a result of this kind of restorative treatment. However, assumptions of clinical success might be premature due to limitations inherent in this study design. The large standard deviations are likely attributable to the sample size and variations between extracted tooth specimens. Although carefully selected, extracted teeth might contain subclinical flaws or irregularities and morphological variation. However, the ultimate clinical relevance of the experimental results was assumed to benefit from testing these restorations while bonded to natural enamel and dentin rather than utilizing a standardized preparation form and replicating artificial dies. Secondly, similar investigations often include thermocycling or cyclic loading to examine restoration strength as these afford a more clinically relevant representation of actual intraoral function. However, thermocycling and fatigue testing also introduce many new variables for which additional control or at least consideration must be made. In the present study, data concerning the maximum strength of such minimal restora-
113
Conclusions
Within the limitations of this in vitro experiment, fracture strengths of occlusal veneer restorations were above the forces producible by the human masticatory system. Material type affected the fracture strength of occlusal veneer restorations, and those fabricated from Lava Ultimate fractured at significantly higher loads than their Paradigm MZ100 counterparts. Restoration thickness had no statistically significant effect on fracture strength—implying that even for extremely thin occlusal veneers (0.3 mm), the restoration strength does not decrease substantially. No relationship between fracture strength and mode of failure was found.
Acknowledgements The authors thank Drs. David Cagna, Greg Paprocki, Ash Husein, Jim Simon and Mr. Brian Morrow for their help and guidance in conducting the study.
references
[1] Abrahamsen TC. The worn dentition—pathognomonic patterns of abrasion and erosion. Int Dent J 2005;55:268–76. [2] Mack MR. Facially generated occlusal vertical dimension. Compend Contin Educ Dent 1997;18:1183–6. [3] Erdemir U, Yildiz E, Kilic I, Yucel T, Ozel S. The efficacy of three desensitizing agents used to treat dentin hypersensitivity. J Am Dent Assoc 2010;141:285–96. [4] McHorris WH. Occlusion with particular emphasis on the functional and parafunctional role of anterior teeth. Part 2. J Clin Orthod 1979;13:684–701.
114
journal of prosthodontic research 58 (2014) 107–114
[5] Turner KA, Missirlian DM. Restoration of the extremely worn dentition. J Prosthet Dent 1984;52:467–74. [6] Fayz F, Eslami A. Determination of occlusal vertical dimension: a literature review. J Prosthet Dent 1988;59: 321–3. [7] Hellsing G. Functional adaptation to changes in vertical dimension. J Prosthet Dent 1984;52:867–70. [8] Ramfjord SP, Blankenship JR. Increased occlusal vertical dimension in adult monkeys. J Prosthet Dent 1981;45:74–83. [9] Shillingburg HTHS, Whitsett LD, Jacobi R, Brackett SE. Preparations for extensively damaged teeth. In: Fundamentals of fixed prosthodontics. Chicago: Quintessence; 1997: 119–38. [10] Cortellini D, Canale A. Bonding lithium disilicate ceramic to feather-edge tooth preparations: a minimally invasive treatment concept. J Adhes Dent 2012;14:7–10. [11] Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for posterior teeth. Int J Periodontics Restorative Dent 2002;22:241–9. [12] Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent 2002;87:503–9. [13] Kassem AS, Atta O, El-Mowafy O. Survival rates of porcelain molar crowns—an update. Int J Prosthodont 2010;23:60–2. [14] Pjetursson BE, Sailer I, Zwahlen M, Hammerle CH. A systematic review of the survival and complication rates of all-ceramic and metal–ceramic reconstructions after an observation period of at least 3 years. Part I: single crowns. Clin Oral Implants Res 2007;18:73–85. [15] Wang X, Fan D, Swain MV, Zhao K. A systematic review of all-ceramic crowns: clinical fracture rates in relation to restored tooth type. Int J Prosthodont 2012;25:441–50. [16] Prevention CfDCa. Public health and aging: retention of natural teeth among older adults—United States, 2002. Morb Mortal Wkly Rep 2003;52:1226–9. [17] Buonocore M, Wileman W, Brudevold F. A report on a resin composition capable of bonding to human dentin surfaces. J Dent Res 1956;35:846–51. [18] Ben-Amar A, Metzger Z, Gontar G. Cavity design for class II composite restorations. J Prosthet Dent 1987;58:5–8. [19] Lawn BR, Pajares A, Zhang Y, Deng Y, Polack MA, Lloyd IK, et al. Materials design in the performance of all-ceramic crowns. Biomaterials 2004;25:2885–92. [20] McLaren EA, Terry DA. CAD/CAM systems materials, and clinical guidelines for all-ceramic crowns and fixed partial dentures. Compend Contin Educ Dent 2002;23:637–41.
[21] Tsitrou EA, van Noort R. Minimal preparation designs for single posterior indirect prostheses with the use of the Cerec system. Int J Comput Dent 2008;11:227–40. [22] Tsitrou E, Helvatjoglou-Antoniades M, Pahinis K, van Noort R. Fracture strength of minimally prepared resin bonded CEREC inlays. Oper Dent 2009;34:537–43. [23] Tsitrou EA, Helvatjoglu-Antoniades M, van Noort R. A preliminary evaluation of the structural integrity and fracture mode of minimally prepared resin bonded CAD/ CAM crowns. J Dent 2010;38:16–22. [24] Moss SJ. Dental erosion. Int Dent J 1998;48:529–39. [25] Ohlmann B, Gruber R, Eickemeyer G, Rammelsberg P. Optimizing preparation design for metal-free composite resin crowns. J Prosthet Dent 2008;100:211–9. [26] Schlichting LH, Maia HP, Baratieri LN, Magne P. Noveldesign ultra-thin CAD/CAM composite resin and ceramic occlusal veneers for the treatment of severe dental erosion. J Prosthet Dent 2011;105:217–26. [27] Kikuchi M, Korioth T, Hannam A. The association among occlusal contacts, clenching effort, and bite force distribution in man. J Dent Res 1997;76:1316–25. [28] Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Occlusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981;46:443–9. [29] Bates J, Stafford G, Harrison A. Masticatory function—a review of the literature. J Oral Rehabil 1976;3:57–67. [30] Pallis K, Griggs JA, Woody RD, Guillen GE, Miller AW. Fracture resistance of three all-ceramic restorative systems for posterior applications. J Prosthet Dent 2004;91:561–9. [31] Skouridou N, Pollington S, Rosentritt M, Tsitrou E. Fracture strength of minimally prepared all-ceramic CEREC crowns after simulating 5 years of service. Dent Mater 2013;29: 70–7. [32] Kassem AS, Atta O, El-Mowafy O. Fatigue resistance and microleakage of CAD/CAM ceramic and composite molar crowns. J Prosthodont 2012;21:28–32. [33] Magne P, Schlichting LH, Maia HP, Baratieri LN. In vitro fatigue resistance of CAD/CAM composite resin and ceramic posterior occlusal veneers. J Prosthet Dent 2010;104:149–57. [34] Magne P, Stanley K, Schlichting LH. Modeling of ultrathin occlusal veneers. Dent Mater 2012;28:777–82. [35] Dejak B, Mlotkowski A, Romanowicz M. Finite element analysis of stresses in molars during clenching and mastication. J Prosthet Dent 2003;90:591–7.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
Fracture strength of CAD/CAM composite and composite-ceramic occlusal veneers§ Andrew C. Johnson DDS, MDSa, Antheunis Versluis PhDb, Daranee Tantbirojn DDS, MS, PhDc, Swati Ahuja BDS, MDSd,* a
Advanced Prosthodontic Program, Department of Prosthodontics, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States b Department of Bioscience Research, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States c Department of Restorative Dentistry, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States d Department of Prosthodontics, University of Tennessee Health Science Center, College of Dentistry, Memphis, TN, United States
article info
abstract
Article history:
Purpose: To determine the effect of material type and restoration thickness on the fracture
Received 30 September 2013
strength of posterior occlusal veneers made from computer-milled composite (Paradigm
Received in revised form
MZ100) and composite-ceramic (Lava Ultimate) materials.
5 December 2013
Methods: 60 maxillary molars were prepared and restored with CAD/CAM occlusal veneer
Accepted 10 January 2014
restorations fabricated from either Paradigm MZ100 or Lava Ultimate blocks at minimal
Available online 11 March 2014
occlusal thicknesses of 0.3, 0.6, and 1.0 mm. Restorations were adhesively bonded and subjected to vertical compressive loading. The maximum force at fracture and mode of
Keywords:
failure were recorded. 2-Way ANOVA was used to identify any statistically significant
Occlusal veneer
relationships between fracture strength and material type or thickness. Spearman’s rank
Milled
correlation coefficient was used to analyze mode of failure with regard to fracture strength.
Conservative
Results: The average maximum loads (N) at fracture for the Paradigm MZ100 groups were
Posterior
1620 433, 1830 501, and 2027 704 for the material thicknesses of 0.3, 0.6, and 1.0 mm,
CAD/CAM
respectively. The Lava Ultimate groups fractured at slightly higher loads (N) of 2078 605, 2141 473, and 2115 462 at the respective 0.3, 0.6, and 1.0 mm thickness. Statistical analyses revealed that, while no significant difference existed among the various restoration thicknesses in terms of fracture strength (P > 0.05), the material type was found to be influential (P = 0.04). The maximum load at fracture (N) for Lava Ultimate averaged over all thicknesses (2111 500) was significantly higher than that of the Paradigm MZ100 (1826 564). No correlation between mode of failure and fracture strength was found. Conclusions: Under the conditions of this study, the maximal loads at fracture for these ‘‘non-ceramic’’ occlusal veneer restorations were found to be higher than human masticatory forces. Occlusal veneers made from the two materials tested are likely to survive
§ This study was presented as an oral defense for the degree designation: Master of Dental Science on 28th June 2013 at University of Tennessee Health Science Center. * Corresponding author at: 207, Giriraj, Neelkanth Valley, Ghatlopar East, Mumbai 400077, Maharashtra, India. Tel.: +91 9819365338; fax: +1 9014481294. E-mail addresses: [email protected], [email protected] (S. Ahuja). 1883-1958/$ – see front matter # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2014.01.001
108
journal of prosthodontic research 58 (2014) 107–114
occlusal forces regardless of restoration thickness, with those fabricated from the composite-ceramic hybrid material being more likely to survive heavier loads. # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
1.
Introduction
Pathologic loss of coronal tooth structure can be attributed to a multitude of individual or combined etiologic factors related to dietary and oral habits which ultimately cause abrasion and/ or erosion of enamel and dentin [1]. The deterioration of this tooth structure has been associated with unfavorable changes in esthetics [2], tooth sensitivity [3], occlusal stability [4], maxillomandibular relationships [5,6], and musculoskeletal harmony [7,8]. Given the pattern and extent of damage to severely worn teeth, the restoration of such defects becomes increasingly challenging and often requires multiple modifications to preparation design and possibly elective tooth devitalization and/or periodontal surgery [9]. In the interest of limiting the amount of adjunctive procedures required for dental reconstruction, a trend toward conservatism in preparation and restoration design is reemerging [10–12]. Considering the finite longevity of common fixed dental restorations [13–15] along with the increasing lifespan of both the patient and their natural dentition [16], maximizing the amount of tooth structure available for future re-treatment is an increasingly relevant consideration. The introduction of composite resin technology initiated a paradigm shift regarding the restoration of lost/damaged tooth structure [17]. However, while the advent of resinbonding has allowed for more conservative filling preparations and restorations [18], given the structural properties of commonly used resin-bonded all-ceramics, current tooth reduction parameters for indirect restorations have remained comparatively aggressive [19,20]. Yet, beyond the challenge of deciding upon the most appropriate treatment modality, along with the numerous technical hurdles associated with its stepwise accomplishment, lies the considerable financial burden the patient must assume upon initiating definitive and irreversible treatment on so many teeth at once. Often, patients simply cannot bear the considerable expense of a large-scale dental reconstruction as a whole. This limitation, all too frequently, is cause for postponing necessary care and perpetuating existing dental problems. Solutions for many of the technical challenges mentioned can be found in current Dental CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) systems. Furthermore, the concepts of CAD/CAM dentistry and conservative tooth preparation seem to be converging [21]. Given the technological advancements which now allow for better and finer reproduction of detail in machine-milled restorations, along with recent innovations of new restorative materials, computerized restorative dentistry may prove to be the best option for uniting tooth structure conservation with simple, expedient, and affordable restoration [22,23]. Considering the often regular pattern of structural loss often seen in cases of long-term bruxism and acid erosion [1,24], restoring such worn teeth with occlusal veneers can be relatively non-invasive, requiring little additional tooth
reduction. Using CAD/CAM technology, such minimalistic restorations can be machine-milled from various restorative materials to match the occlusal contours established in a diagnostic wax-up and adhesively bonded for extended, stable re-establishment of the posterior occlusion. Restorations of this type might not allow the same optimization of esthetics given the marginal visibility; however, their ease of fabrication and installation as a functional preview of a proposed posterior occlusion should be reason enough to pursue their development if not only as a means of effective, predictable temporary reconstruction of teeth. When provisionalized in this manner, subsequent definitive restoration of the posterior teeth can then be limited to sequential, segmental or even single-tooth treatments, allowing the patient a gradual transition from provisional to final restorations as he or she may accommodate. So far, little research has been done to investigate the viability of various materials utilized as minimal-thickness posterior occlusal veneers. Related experiments have tested the fracture strength and fatigue resistance of both ceramic and non-ceramic materials as conservative posterior occlusal restorations of various designs with results that appear to support the use of non-ceramic materials in these applications given their reported increased resistance to fracture [25,26]. Still, more evidence is necessary to support the clinical implementation of these minimally invasive, CAD/CAM posterior occlusal restorations as recommended alternatives to traditional means of restoring posterior occlusion. In the present study, the fracture strength of minimally thick (0.3, 0.6, and 1.0 mm) composite (Paradigm MZ100, 3M ESPE, St. Paul, MN) and composite-ceramic (Lava Ultimate, 3M ESPE, St. Paul, MN) occlusal veneers was tested by vertical compressive loading to fracture. The aim of this in vitro investigation was to determine any effect of restoration dimensions or material selection, and it operated on the null hypotheses that there would be no significant differences in the fracture strengths between material types or among thicknesses. Such findings would offer further support for the utilization of thin, non-ceramic occlusal veneers as functional and predictable means of posterior reconstruction.
2.
Materials and methods
One hundred extracted human maxillary molars of comparable dimensions and morphology were mounted in autopolymerizing acrylic resin using a cylindrical Telflon mold at a level 3 mm apical to their respective CEJs (Fig. 1A) and stored in distilled water.
2.1.
Preparation
Standardized tooth preparations replicating a worn occlusal table were accomplished using a diamond saw (Isomet Plus Precision Saw, Buehler, Lake Bluff, IL.). The mounted
journal of prosthodontic research 58 (2014) 107–114
109
Fig. 1 – (A) Extracted tooth specimen mounted in acrylic base. (B) Prepared specimen showing indexing notches and demonstrating intact enamel and dentin without pulp exposure. (C) Occlusal scanning form created from an anatomic denture tooth used in restoration design.
Fig. 2 – Flow chart of specimen selection and test group assignment.
teeth were sectioned axially, removing all coronal tooth structure 4 mm occlusal to the CEJs leaving exposed dentin centrally and peripheral enamel. Intra-enamel indexing notches were created on the mesial and distal finish lines (Fig. 1B) with a high speed round-ended tapered diamond rotary bur (#ZR 850 FG.01, Komet USA, Rock Hill, SC). Each mounted, prepared tooth was inspected for disqualifying characteristics such as pulpal exposure or cracks/fractures. Those with identifiable defects were eliminated and, from the eighty-four intact specimens remaining, sixty were selected. The specimens were randomly divided into 2 groups based on restorative materials: Paradigm MZ100 (3M ESPE, St. Paul, MN (MZ groups) and Lava Ultimate (3M ESPE, St. Paul, MN) (LU groups). Within each group, specimens were subdivided into 3 subgroups (consisting of 10 specimens each) based on assigned restoration thickness (0.3, 0.6, 1.0 mm) (Fig. 2).
2.2.
Preparation/occlusion scanning
A standardized occlusal form was created by trimming the cervical structure from an anatomic denture tooth (Phonares SR Typ NHC Mould NU3, Ivoclar Vivadent, Amherst, NY) to within 0.3 mm of the central fossa to be positioned on the prepared tooth specimens during the ‘‘occlusion’’ scans (Fig. 1C). Using a Cerec 3 acquisition unit operating software version 3.84 (Sirona Dental Systems GmbH, Bensheim,
Germany), scans of each prepared tooth surface were obtained as well as secondary scans of each specimen bearing the standardized occlusal form positioned in best agreement with its perimeter outline and axial rotation (Fig. 3A). During scanning, the specimens were positioned within a custom visual reference jig which provided the topographical reference indices necessary to correlate the ‘‘preparation’’ and ‘‘occlusion’’ images (Fig. 3B).
2.3.
Restoration fabrication
With the software set in ‘‘Master Mode’’, occlusal restorations were designed virtually using the ‘‘Correlation’’ feature which permitted the orientation of occlusal form contours with those of the prepared teeth. This resulted in restoration proposals with uniform occlusal morphology while allowing customized marginal adaptation. Using the virtual design tools within the software, the occlusal–cervical dimensions were adjusted to the prescribed thicknesses assigned to each test group. Groups MZ1 and LU1 were designed with a minimal thickness of 1.0 mm (measured from the depth of the central fossa to the preparation surface), groups MZ2 and LU2 were designed at 0.6 mm minimal thickness and groups MZ3 and LU3 at 0.3 mm. Milling sprue locations were designated on the distal surface of each final proposal, and restorations were computer-milled from either Paradigm MZ100 or from Lava Ultimate restorative materials.
110
journal of prosthodontic research 58 (2014) 107–114
Fig. 3 – (A) Scans of prepared specimen with and without occlusal form. Note the inclusion of peripheral reference jig topography allowing correlation of scans. (B) Scanning jig positioned on specimen coated in contrast media.
2.4.
Restoration bonding
After milling, sample restorations were inspected for defects and assessed for proper fit. Per manufacturer’s recommendations, all specimens were air-abraded with 40 mm aluminum oxide at 1.8 bar of pressure, cleaned with alcohol and dried with oil-free pressurized air. Prepared tooth surfaces were etched with 37.5% phosphoric acid for 15 s (Ultra-Etch; Ultradent Products, Inc, South Jordan, Utah), rinsed thoroughly and dried without dessicating dentin. Restorations were then bonded using self-adhesive, dual-cure resin cement (RelyX Unicem, 3M ESPE, St. Paul, Minnesota) per manufacturer’s instructions while a seating pressure of 6 N was applied by a customized device for 5 min (Fig. 4). Following initial light curing, excess marginal resin cement was cleaned and final light curing completed for 15 s per restoration surface at 1400 mW/cm2 (SmartLite Max LED curing light, Dentsply Caulk, Tulsa, OK, USA). After restoration bonding all specimens were returned to 37 8C distilled water storage for 1 week prior to fracture testing.
2.5.
Load testing
Each specimen was subjected to vertical load to fracture using a universal testing machine (Instron 5567, Norwood, MA). Those specimens mounted in the acrylic bases visibly in-line with direction of loading were placed directly on the testing platen. Those mounted slightly off-axis were re-oriented relative to the direction of loading by using an angleadjustment table. A custom 3.5 mm diameter spherical stainless steel tip was used to simulate an opposing cusp
and positioned to achieve equalized tripod contacts along the cuspal inclines surrounding the central fossae of the restorations. A 10 kN load cell was installed, the cross-head speed was set at 0.5 mm/min., and the automatic cut-off was set at 25% loss of peak load. The maximum vertical load at the point of fracture was recorded for each specimen. Homoscedasticity of variance and normal distribution of the data were tested using Bartlett and Anderson–Darling Normality test. Two-way ANOVA (Super Anova, Abacus Concepts, Berkeley, CA, USA) was performed to identify any difference in fracture strength as a result of material, thickness, or material–thickness interaction. Images of each specimen were recorded using a stereomicroscope with charge-coupled device (CCD) camera (SZX16 & UC30, Olympus, Tokyo, Japan) before and after fracture testing for failure mode comparison. The mode of failure for each specimen was categorized based on structures involved in the fracture (mode 1 = restoration only; mode 2 = restoration and enamel; mode 3 = restoration, enamel and dentin).
3.
Results
Mean maximum loads (N) standard deviation at the point of fracture for the MZ groups were 1620 433 (MZ3), 1830 501 (MZ2), and 2027 704 (MZ1) for the material thicknesses of 0.3, 0.6, and 1.0 mm, respectively. The LU groups fractured at slightly higher average loads (N) of 2078 605 (LU3), 2141 473 (LU2), and 2115 462 (LU1) at the respective 0.3, 0.6, and 1.0 mm thicknesses. The maximum, median, minimum, and upper and lower quartiles of the fracture loads are shown in a box plot
journal of prosthodontic research 58 (2014) 107–114
111
Fig. 5 – Box plot of failure load for each group. Dot is the median value, whiskers show maximum and minimum failure loads, box indicates the upper and lower quartiles.
Fig. 4 – Custom seating pressure device applying 6 N of vertical force during bonding procedures.
(Fig. 5). While there was a discernable trend suggesting a direct relationship between fracture strength and restoration thickness for the MZ material groups, no such relationship was observed among LU groups. Bartlett’s test revealed no significant difference between the variances (P = 0.668). The fracture strength values of the LU group had a high probability for a normal distribution (95.1%), while the MZ group had low probability (2.8%) (Anderson–Darling Normality test). Assuming a normal distribution, two-way ANOVA (Super Anova, Abacus Concepts, Berkeley, CA, USA) indicated that material type significantly affected fracture strengths averaged over restoration thicknesses (P = 0.04). However, no significant effect was attributable to restoration thickness or the material-thickness interaction (P = 0.43 and 0.55, respectively) (Table 1). Pairwise multiple comparisons showed that the LU1 and LU2 groups demonstrated higher fracture strengths than the MZ3 groups (P = 0.04 and 0.03, respectively) (Table 2). No notable difference was found between the fracture strengths of the 0.3 mm Lava Ultimate and any other group. 22 specimens displayed mode 2 fracture of the restoration along with some portion of peripheral enamel (Fig. 6A). 23 specimens showed mode 3 fracture of restoration, enamel and dentin (Fig. 6B), and 15 specimens fracturing only through the restorative material and tooth-restoration interface leaving the underlying enamel and dentin intact (mode 1) (Fig. 6C). 4 of
23 specimens displaying category 3 failure, while fractured through all structures, remained bodily intact (Fig. 6D, E) showing no delamination of restorative material. The rarest outcome appeared to be a complete adhesive failure (1 specimen) wherein all restorative material fractured away without damaging any underlying tooth structure (Fig. 6F). Fig. 7 (Fig. 7) shows the plot between failure mode and fracture load of each specimen. Spearman’s rank correlation coefficient did not identify a trend between mode of failure and maximum load at fracture. Additionally, no clear pattern was noted with respect to the number of specimens from each group and failure mode. For instance, the thicker restorations milled from the Lava Ultimate material displayed more fractures involving the underlying tooth while the thinner Paradigm MZ100 restorations more often fractured along with tooth structures.
4.
Discussion
The null hypothesis regarding restoration dimensions was accepted, no significant difference in fracture strengths was found between occlusal veneer thicknesses. However, the null hypothesis concerning material type was rejected as a statistically significant difference was found between the fracture strengths of Paradigm MZ100 and Lava Ultimate. In terms of ultimate restoration strength, these results do appear to demonstrate the initial feasibility of this minimally invasive treatment modality for restoring only the occlusal portions of severely worn teeth with rapidly prototyped, nonceramic materials. Considering that the mean fracture strengths for all sample groups were well above achievable human masticatory forces reportedly ranging from 585 to 880 N [27–29], it may be surmised that such restorations may be capable of clinical success in similarly ideal circumstances, and any complications/failures related to fracture strength would likely be due to factors well beyond normal maximum occlusal loading (i.e. cyclic fatigue, trauma, etc.). Similar studies have been done to investigate the strength of conventional ceramics and alternative materials as applied
112
journal of prosthodontic research 58 (2014) 107–114
Table 1 – Two-way ANOVA statistics. Source
df
Sum of squares
Mean square
F-value
P-value
Material Thickness Material thickness
1 2 2
1,220,370.817 501,886.233 348,519.633
1,220,370.817 250,943.117 174,259.817
4.215 0.867 0.602
0.0449 0.4261 0.5514
Fig. 6 – (A) Example of category 2 failure. (B) Type 3 fracture involving restoration, enamel and dentin. (C) Specimen demonstrating type 1 fracture of restoration material only. (D and E) Specimens demonstrating category 3 failure displaying fracture through all restoration and enamel–dentin but yet remaining bodily intact. (F) Specimen displaying restorationonly fracture and complete adhesive failure.
Table 2 – Pairwise multiple comparisons. Groups LavaUlt, 0.3 mm
LavaUlt, 0.6 mm
LavaUlt, 1.0 mm
MZ100, 0.3 mm MZ100, 0.6 mm
versus
P-value
LavaUlt, 0.6 mm LavaUlt, 1.0 mm MZ100, 0.3 mm MZ100, 0.6 mm MZ100, 1.0 mm LavaUlt, 1.0 mm MZ100, 0.3 mm MZ100, 0.6 mm MZ100, 1.0 mm MZ100, 0.3 mm MZ100, 0.6 mm MZ100, 1.0 mm MZ100, 0.6 mm MZ100, 1.0 mm MZ100, 1.0 mm
0.7954 0.8794 0.0623 0.3077 0.8346 0.9144 0.0349 0.2024 0.6400 0.0447 0.2423 0.7185 0.3863 0.0961 0.4159
in various restoration designs. When full-coverage, conventional all-ceramic crowns (1.5–2 mm occlusal thickness) were luted to extracted teeth and subjected to similar load testing, the range of forces causing restoration fracture (771–1183 N) were well below those found by the current study [30]. When all-ceramic restorations have been fabricated as conventional crowns or minimalistic, occlusal veneers, and subjected to artificial aging, the ‘‘ultra-minimal’’ restoration design did
display adequate strength to survive simulated masticatory function [31]. However, composite materials under similar comparisons of restoration strength seem to consistently demonstrate more favorable properties than their conventional ceramic counterparts. A study comparing the fatigue resistance of allceramic crowns to that of composite full-coverage restorations found that the non-ceramic specimens displayed significantly better (100%) survival when subjected to cyclic loading [32]. When composite crowns of variable convergence angles, margin design, cementation method and thickness (including a minimalistic 0.5 mm occlusal dimension) have been subjected to thermocycling and subsequent fracture strength testing, it has been shown that occlusal thickness significantly impacts restoration strength as fractures occurred at average loads of 1566 N and 1127 N for 1.3 mm and 0.5 mm thicknesses respectively [25]. In an experiment by Magne et al. involving vertical cyclic loading of 1.2 mm thick occlusal veneers CAD/CAM-fabricated from 2 ceramic materials as well as Paradigm MZ100 composite, the composite material was significantly stronger in such conservative applications given its 100% survival after all loading cycles (up to 1400 N) compared to the 30% and 0% survival rates reported for the ceramic groups [33]. A related study by Schlichting et al., compared the fatigue resistance of thinner (0.6 mm) occlusal veneer restorations fabricated from
journal of prosthodontic research 58 (2014) 107–114
Fig. 7 – Failure modes and fracture strength of all specimens.
tions under ideal circumstances was produced. While this information alone should not be the sole basis on which clinical decisions are made, it is complementary to that offered by studies with different testing protocols and results. The ‘‘vertical’’ or ‘‘compressive’’ nature of loading might be an oversimplification of the actual forces applied to the specimens. Given that the spherical load tip was prevented from striking the depth of the central fossa by tripodized contacts along the surrounding cuspal inclines, the true nature of the load applied during loading were necessarily not strictly vertical compression [34]. In fact, it is likely that most of the forces acting on those central areas were tensile in nature as the cusps were deflected around the advancing load tip. Similar mechanics have been identified through computer modeling and have been found to vary based on the type of material to which a simulated occlusal force is applied [34,35]. The fracture resistance demonstrated by these computermilled, non-ceramic materials (even at minimal dimensions) appears adequate to survive and function as lasting prosthetic replacements for lost occlusal tooth structure. The fact that they are able to be easily fabricated at a reduced cost and perhaps applied without the need for additional tooth reduction support the clinical implementation of such conservative restorations.
5. the same materials as well as an ‘‘experimental’’ fiberreinforced composite [26]. They reported that no ceramic specimen survived all loading cycles, but the survival rates demonstrated by the composite and reinforced-composite groups at these minimal dimensions was 60% and 100% respectively after final loading cycles at 1400 N. The fact that 2/3rds of the restoration fractures seen in the present study involved damage to natural tooth structure along with restorative material failure may suggest that these restorations possess strength at least similar to that of the worn teeth they are intended to restore. If these assumptions are ultimately proven, clinicians may attempt such simple and conservative procedures with confidence that no additional detriment is being brought to bear on the teeth as a result of this kind of restorative treatment. However, assumptions of clinical success might be premature due to limitations inherent in this study design. The large standard deviations are likely attributable to the sample size and variations between extracted tooth specimens. Although carefully selected, extracted teeth might contain subclinical flaws or irregularities and morphological variation. However, the ultimate clinical relevance of the experimental results was assumed to benefit from testing these restorations while bonded to natural enamel and dentin rather than utilizing a standardized preparation form and replicating artificial dies. Secondly, similar investigations often include thermocycling or cyclic loading to examine restoration strength as these afford a more clinically relevant representation of actual intraoral function. However, thermocycling and fatigue testing also introduce many new variables for which additional control or at least consideration must be made. In the present study, data concerning the maximum strength of such minimal restora-
113
Conclusions
Within the limitations of this in vitro experiment, fracture strengths of occlusal veneer restorations were above the forces producible by the human masticatory system. Material type affected the fracture strength of occlusal veneer restorations, and those fabricated from Lava Ultimate fractured at significantly higher loads than their Paradigm MZ100 counterparts. Restoration thickness had no statistically significant effect on fracture strength—implying that even for extremely thin occlusal veneers (0.3 mm), the restoration strength does not decrease substantially. No relationship between fracture strength and mode of failure was found.
Acknowledgements The authors thank Drs. David Cagna, Greg Paprocki, Ash Husein, Jim Simon and Mr. Brian Morrow for their help and guidance in conducting the study.
references
[1] Abrahamsen TC. The worn dentition—pathognomonic patterns of abrasion and erosion. Int Dent J 2005;55:268–76. [2] Mack MR. Facially generated occlusal vertical dimension. Compend Contin Educ Dent 1997;18:1183–6. [3] Erdemir U, Yildiz E, Kilic I, Yucel T, Ozel S. The efficacy of three desensitizing agents used to treat dentin hypersensitivity. J Am Dent Assoc 2010;141:285–96. [4] McHorris WH. Occlusion with particular emphasis on the functional and parafunctional role of anterior teeth. Part 2. J Clin Orthod 1979;13:684–701.
114
journal of prosthodontic research 58 (2014) 107–114
[5] Turner KA, Missirlian DM. Restoration of the extremely worn dentition. J Prosthet Dent 1984;52:467–74. [6] Fayz F, Eslami A. Determination of occlusal vertical dimension: a literature review. J Prosthet Dent 1988;59: 321–3. [7] Hellsing G. Functional adaptation to changes in vertical dimension. J Prosthet Dent 1984;52:867–70. [8] Ramfjord SP, Blankenship JR. Increased occlusal vertical dimension in adult monkeys. J Prosthet Dent 1981;45:74–83. [9] Shillingburg HTHS, Whitsett LD, Jacobi R, Brackett SE. Preparations for extensively damaged teeth. In: Fundamentals of fixed prosthodontics. Chicago: Quintessence; 1997: 119–38. [10] Cortellini D, Canale A. Bonding lithium disilicate ceramic to feather-edge tooth preparations: a minimally invasive treatment concept. J Adhes Dent 2012;14:7–10. [11] Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for posterior teeth. Int J Periodontics Restorative Dent 2002;22:241–9. [12] Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent 2002;87:503–9. [13] Kassem AS, Atta O, El-Mowafy O. Survival rates of porcelain molar crowns—an update. Int J Prosthodont 2010;23:60–2. [14] Pjetursson BE, Sailer I, Zwahlen M, Hammerle CH. A systematic review of the survival and complication rates of all-ceramic and metal–ceramic reconstructions after an observation period of at least 3 years. Part I: single crowns. Clin Oral Implants Res 2007;18:73–85. [15] Wang X, Fan D, Swain MV, Zhao K. A systematic review of all-ceramic crowns: clinical fracture rates in relation to restored tooth type. Int J Prosthodont 2012;25:441–50. [16] Prevention CfDCa. Public health and aging: retention of natural teeth among older adults—United States, 2002. Morb Mortal Wkly Rep 2003;52:1226–9. [17] Buonocore M, Wileman W, Brudevold F. A report on a resin composition capable of bonding to human dentin surfaces. J Dent Res 1956;35:846–51. [18] Ben-Amar A, Metzger Z, Gontar G. Cavity design for class II composite restorations. J Prosthet Dent 1987;58:5–8. [19] Lawn BR, Pajares A, Zhang Y, Deng Y, Polack MA, Lloyd IK, et al. Materials design in the performance of all-ceramic crowns. Biomaterials 2004;25:2885–92. [20] McLaren EA, Terry DA. CAD/CAM systems materials, and clinical guidelines for all-ceramic crowns and fixed partial dentures. Compend Contin Educ Dent 2002;23:637–41.
[21] Tsitrou EA, van Noort R. Minimal preparation designs for single posterior indirect prostheses with the use of the Cerec system. Int J Comput Dent 2008;11:227–40. [22] Tsitrou E, Helvatjoglou-Antoniades M, Pahinis K, van Noort R. Fracture strength of minimally prepared resin bonded CEREC inlays. Oper Dent 2009;34:537–43. [23] Tsitrou EA, Helvatjoglu-Antoniades M, van Noort R. A preliminary evaluation of the structural integrity and fracture mode of minimally prepared resin bonded CAD/ CAM crowns. J Dent 2010;38:16–22. [24] Moss SJ. Dental erosion. Int Dent J 1998;48:529–39. [25] Ohlmann B, Gruber R, Eickemeyer G, Rammelsberg P. Optimizing preparation design for metal-free composite resin crowns. J Prosthet Dent 2008;100:211–9. [26] Schlichting LH, Maia HP, Baratieri LN, Magne P. Noveldesign ultra-thin CAD/CAM composite resin and ceramic occlusal veneers for the treatment of severe dental erosion. J Prosthet Dent 2011;105:217–26. [27] Kikuchi M, Korioth T, Hannam A. The association among occlusal contacts, clenching effort, and bite force distribution in man. J Dent Res 1997;76:1316–25. [28] Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Occlusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981;46:443–9. [29] Bates J, Stafford G, Harrison A. Masticatory function—a review of the literature. J Oral Rehabil 1976;3:57–67. [30] Pallis K, Griggs JA, Woody RD, Guillen GE, Miller AW. Fracture resistance of three all-ceramic restorative systems for posterior applications. J Prosthet Dent 2004;91:561–9. [31] Skouridou N, Pollington S, Rosentritt M, Tsitrou E. Fracture strength of minimally prepared all-ceramic CEREC crowns after simulating 5 years of service. Dent Mater 2013;29: 70–7. [32] Kassem AS, Atta O, El-Mowafy O. Fatigue resistance and microleakage of CAD/CAM ceramic and composite molar crowns. J Prosthodont 2012;21:28–32. [33] Magne P, Schlichting LH, Maia HP, Baratieri LN. In vitro fatigue resistance of CAD/CAM composite resin and ceramic posterior occlusal veneers. J Prosthet Dent 2010;104:149–57. [34] Magne P, Stanley K, Schlichting LH. Modeling of ultrathin occlusal veneers. Dent Mater 2012;28:777–82. [35] Dejak B, Mlotkowski A, Romanowicz M. Finite element analysis of stresses in molars during clenching and mastication. J Prosthet Dent 2003;90:591–7.
