Qu
by ht
opyrig
C All eR ech te
vo rbe ha lte n
i
nt Masticatory Fatigue, Fracture Resistance, and e ss e n z Marginal Discrepancy of Ceramic Partial Crowns With and Without Coverage of Compromised Cusps Christian F.J. Stapperta/Pia Abeb/Volker Kurthsc/Thomas Gerdsd/Jörg R. Strube
Purpose: To evaluate preparation designs of compromised cusps and whether or not they influence masticatory fatigue, fracture resistance, and marginal discrepancy of ceramic partial-coverage restorations (PCRs) luted on mandibular molars. Materials and Methods: Sixty-four caries-free molars were equally divided into four groups. Control group NP received no preparation (NP). Group B-IN received a basic inlay (IN) preparation with buccal (B) cusp conservation and occlusal reduction of both lingual cusps. Group B-ON was prepared in the same way, except buccal cusps were prepared with an angle of 45 degrees to the occlusal plane (buccal onlay). Group B-OV preparation was similar to group B-ON, but buccal cusps received a further shoulder preparation on the buccal aspect (buccal overlap). Forty-eight all-ceramic IPS e.max Press PCRs were fabricated and luted adhesively. Specimens underwent mouth-motion fatigue (1.2 million cycles, 1.6 Hz, 49 N) and 5500 thermal cycles (5°C/55°C). Fracture patterns were observed. Surviving specimens were loaded until fracture. Marginal discrepancies were examined. Results: Only one specimen of group B-ON fractured during fatigue. Median fracture loads (N) [IQR=x.25-x.75]: group NP = 1604 N [1182-1851 N], group B-IN = 1307 N [1262-1587N], group B-ON = 1396 N [817-1750N], group B-OV = 1205 N [1096-1542N]. No significant differences in fracture resistance were found between restored molars and unprepared teeth (p ≥ 0.18). Different preparation designs showed no significant influence on PCR fracture resistance. Mouth-motion fatigue caused a significanty decrease of marginal accuracy in groups B-IN (p = 0.009) and B-ON (p = 0.008). Marginal discrepancy values of groups B-IN and B-OV were significantly different after fatigue (p = 0.045). Conclusion: Ceramic coverage of compromised cusps did not demonstrate an increase of fracture resistance after fatigue when compared to less invasive partial-coverage restorations. However, enhanced exposure of restoration margins to occlusal wear could result in more extensive marginal discrepancies. Keywords: fracture resistance, marginal discrepancy, partial coverage restoration, ceramic, fatigue. J Adhes Dent 2008; 10: 41-48.
Submitted for publication: 12.10.06; accepted for publication: 01.05.07.
D
a Assistant Professor, Department of Periodontology and Implant Dentistry, De-
b c d e
partment of Biomaterials and Biomimetics, New York University College of Dentistry, New York, NY, USA; Associate Professor, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany. Research Associate, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany. Research Associate, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany. Statistician, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany. Professor and Chairman, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany.
Correspondence: Dr. Christian Stappert, DDS, Department of Biomaterials and Biomimetics, New York University College of Dentistry, Arnold and Marie Schwartz Hall of Dental Sciences, 345 East 24th Street, New York, 10010 NY, USA. Tel: +1212-998-9939, Fax: +1-212-995-4244. e-mail:
[email protected]
Vol 10, No 1, 2008
ental ceramics present an interesting option in terms of biocompatibility and esthetics for the restoration of lost tooth structure.47 With improvements in the physical and biomechanical properties of ceramic materials along with the use of adhesive cementation, the application of all-ceramic restorations in the anterior and posterior dentition is justified.3 Fabrication techniques such as heat pressing or computer-assisted machining of prefabricated ceramic ingots or blocks are used to produce even small-sized all-ceramic restorations, such as inlays and onlays, with high fracture resistance.18 Long-term clinical studies have well documented the good performance of all-ceramic inlays, and they are widely accepted as treatment procedures.16,18,19,31,37,44,45 Mean annual failure rates in posterior stress-bearing preparations of 1.9% for ceramic inlays and 1.7% for CAD/CAM ceramic 41
Stappert et al
Qu
i
nt
Fig 1 Control group NP (no preparation).
Fig 2 Preparation of group B-IN.
restorations were found.38 Survival rates of ceramic inlays are described as ranging from 74% to 100% with observation periods of up to 12 years.3,22 Ceramic onlays also occur in these investigations, but their numbers are small and failure rates are not strictly separated from those of inlay restorations. Although preparation design is considered one of the important factors responsible for the success of ceramic restorations,18 evidence-based preparation guidelines for posterior ceramic partial-coverage restorations (PCRs) are hard to find in the literature. Many preparation designs are based on experience with cast metal partial-coverage restorations and have been modified to optimize the performance of ceramic PCRs.2,39 Guidelines for conventionally cemented cast metal restorations53 include almost parallel preparation surfaces or box designs to gain stability by retention against mainly horizontal mastication forces. Due to strong adhesive bonding,30 preparation design of all-ceramic PCRs is less dependent on retention form, which allows a higher variation in preparation when compared to conventionally cemented restorations. A general consensus exists on ceramic preparation standards, such as rounded line angles17,31,39,58 and passive seating of PCRs by diverging preparation angles.39,58 Adequate material thicknesses and ideal extensions of a ceramic restoration, however, are less well defined. In the case of a large tooth defect, reduction and overlapping of the weakened cusps by a PCR has been recommended to reinforce the compromised tooth structure.5,9 Invitro studies demonstrated an improvement of fracture resistance for endodontically treated premolars with amalgam41 or conventionally cemented cast-metal cusp coverage.59 When restorations were adhesively luted, cusp coverage by cast-metal59 and ceramic restorations55 did not show a clinically relevant increase of fracture resistance compared to an inlay restoration. The aim of the present study was to evaluate different preparation designs of compromised cusps and whether or not they influence masticatory fatigue, fracture resistance, and marginal fit of all-ceramic PCRs luted on mandibular molars. A lithium disilicate press ceramic (IPS e.max Press; Ivoclar Vivadent; Schaan, Liechtenstein) was used to fabricate the ceramic restorations. 42
by ht
opyrig
C All eR ech te
vo rbe ha lte n
e ss e n z
Fig 3 Preparation of group B-ON.
MATERIALS AND METHODS Extracted caries-free human mandibular molars were examined under a reflected light microscope at 10X to 50X magnification (Stemi 2000 CS; Carl Zeiss; Jena, Germany). Sixty-four teeth, equal in size (deviation of < 5%) (Digital Micrometer, Protective System IP 65, Mitutoyo; Kyoto, Japan) and free of hypoplastic defects and cracks, were selected. Teeth were cleaned by scaling and stored in 0.1% thymol solution at room temperature, for three months maximum. The molars were divided into 4 groups of 16 specimens each. Group NP (no preparation) served as a control of natural, unprepared teeth (Fig 1). The remaining teeth were prepared. Tooth Preparation Forty-eight teeth were prepared according to specific guidelines (groups B-IN, B-ON and B-OV). Teeth were prepared freehand by one clinician using a high-speed handpiece and diamond burs under water cooling. Primary preparation was conducted with 80-μm-grit preparation diamonds (837KR.314.012, 847KR.314.016), while finishing was carried out with finer diamonds (30- to 40-μm grain size, 8837KR.314.012, 8847KR.314.016, 8390.204.016, Komet, Brasseler; Lemgo, Germany). Group B-IN (buccal-inlay preparation) was prepared with a mesio-occlusal-distal (MOD) inlay cavity with a 3-mm-deep occlusal box and an isthmus width of 3 mm as well as an overall preparation angle of 6 degrees towards the occlusal aspect. Proximal finishing lines were 1 mm above the cementoenamel junction (Fig 2). Both lingual cusps were reduced by 2 mm with an angle of 45 degrees to the occlusal plane. Teeth of group B-ON (buccal-onlay preparation) were prepared in the same manner as in group B-IN, except for an additional reduction of both buccal cusps by 2 mm with an angle of 45 degrees to the occlusal plane (Fig 3). In group BOV (buccal-overlap preparation), the teeth were prepared as in group B-ON, but with an additional overlapping shoulder preparation of the buccal cusps by 0.8 mm with rounded inner edges (Fig 4). Prior to preparation, two silicone impressions were taken of each tooth (Formasil Xact, Heraeus Kulzer; Wehrheim, Germany). For impression taking and preparation, pairs of teeth were provisionally secured in a prefabricated plaster mold (GC FujiRock EP, type 4 dental stone, GC The Journal of Adhesive Dentistry
Qu
Europe; Leuven, Belgium). One impression was used as an orientation aid for the subsequent wax-up of the ceramic restorations, while the other one was cut in a buccal-oral direction to serve as an aid in the removal of tooth structure according to the preparation guidelines. Accuracy of preparation dimensions was re-evaluated (Digital Micrometer, Protective System IP 65, Mitutoyo) under light microscopy (5X) (Stemi 2000 CS; Carl Zeiss). Fabrication of the Ceramic Restorations Following double-mixing technique impressions with polyvinyl siloxane material (Dimension Garant L and Permagum Putty Soft, 3M-ESPE; Seefeld, Germany), the master models were poured using dental stone (GC Fujirock type 4, GC; Tokyo, Japan). Critical line angles along the preparation margin of the master models were strengthened and enhanced by die hardener (Margidur, DUS Dental-U; Richmond, Canada). Die spacer (Purargent 20ml, DUS Dental-U) was applied to the cavity surfaces (approximately 10 μm) 1.5 mm from the marginal areas. IPS e.max Press (VP 1989/4) PCRs were fabricated by Ivoclar Vivadent (Schaan, Liechtenstein). Full PCR wax-ups were made according to the silicone guides of the unprepared teeth. After investment of the wax-up, the investment cylinders were preheated in a furnace (Type 5636; KaVo Dental; Biberach, Germany) at 850°C. For the pressing procedure of the ceramic PCRs, a hot-press furnace (EP 500; Ivoclar Vivadent) was used. The investment cylinders along with the glass-ceramic ingots (VP 1989/4, Ivoclar Vivadent) were placed at the center of the EP 500 press furnace and pressed at a temperature of 915°C. After divestment, the pressed PCRs were cut from the sprues with a water-cooled diamond-coated disk (Diaflex H347, Horico Dental; Berlin, Germany) and cleaned with a jet steam machine (EV1 SJ, Silfradent Sync; Sofia, Italy). PCR fit was verified on the master model. Two glazing procedures were performed in a firing oven (Programat P100; Ivoclar Vivadent) with C27688 Empress 2 glazing material (Ivoclar Vivadent). The inner surfaces of the PCRs were airborne-particle abraded with a high-grade alumina (Type 100, KaVo EWL blasting medium, white, KaVo Dental) at 2 bar pressure for 10 s and cleaned with a jet steam machine (EV1 SJ, Silfradent Sync).
Vol 10, No 1, 2008
vo rbe ha lte n
Adhesive Luting n Prior to placement of the PCRs, the prepared teeth twere es z cleaned with synthetic rotary brushes and fluoride-free pol-s e n ishing paste (Pell-ex, Hawe Neos Dental; Bioggio, Switzerland). Subsequently, the teeth were etched (enamel for 60 s, dentin for 15 s) with 37% phosphoric acid (Total Etch, Ivoclar Vivadent) and conditioned with Syntac Primer for 15 s and Adhesive for 10 s (Ivoclar Vivadent). Heliobond (Ivoclar Vivadent) was applied onto the preparation surface and dispersed as a thin layer using clean, dry air. Pooling or insufficient coverage of the adhesive was avoided. The layer of Heliobond (Ivoclar Vivadent) was polymerized with a light-curing unit (Elipar Free Light 2; 3M ESPE, Seefeld, Germany) for 20 s. IPS e.max Press PCRs were cleaned with 99% isopropanol, inner surfaces were etched with 4.9% hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar Vivadent) for 20 s, thoroughly rinsed with water for 60 s, and dried with oil-free compressed air. Afterwards, a single-component bonding silane (Monobond-S, Ivoclar Vivadent) was applied to the etched surfaces. Forty-eight ceramic PCRs were luted with a dual polymerizing resin composite (Variolink II, Ivoclar Vivadent). Before light polymerizing, excess composite was removed using synthetic pellets. To avoid oxygen inhibition during polymerization, glycerine gel (Liquid Strip, Ivoclar Vivadent) was applied to the marginal area. The PCRs were light polymerized with a minimum of 650 mW/cm2 light intensity (Elipar Free Light 2; 3M ESPE) in increments from the occlusal to the buccal, oral, and proximal aspects for at least 40 s each. Finishing was performed using hand instruments (#15c scalpel, #371716, Bard-Parker; Becton, Dickinson and Co; Franklin Lakes, NJ, USA), finishing disks (Sof-Lex Pop-On; 3M; St Paul, MN, USA), and fine silicone ceramic polishers (#9418.204.030, #9419.204.030, #9547.204.030; Komet Dental, Brasseler; Lemgo, Germany). Specimens were stored in 0.1 thymol solution prior to fatigue testing for at least 30 days.
i
Fig 4 Preparation of group B-OV.
by ht
opyrig
C All eR ech e Stappert et tal
Mastication Simulator To simulate the periodontal ligament, roots were coated with a gum resin (Anti-Rutsch-Lack, Wenko-Wenselaar; Hilden, Germany) of 0.25 mm thickness, 2 mm below the cementoenamel junction. Afterwards, the molars were embedded in a self-polymerizing resin at an angle of 90 degrees to the horizontal plane (Technovit 4000, Heraeus-Kulzer). All specimens were subjected to a dynamic load of 49 N at 1.6 Hz with synchronized thermal cycling in a dual-axis masticatory simulator (Willytech; Munich, Germany). A total of 1,200,000 loading cycles was performed, simulating almost 5 years of clinical service.12,32 The specimens were thermocycled between 5°C and 55°C 5500 times simultaneously with the mechanical cycling, with a 60-s dwell time at each temperature. The dynamic load was applied on the occlusal-buccal edge between the two buccal cusps, using a ceramic antagonist (steatite; Hoechst Ceram Tec; Wunsiedel, Germany) 6 mm in diameter. The invariable load was applied with an ellipsoidal force profile, with a vertical movement of 6 mm and a horizontal movement of 0.5 mm. The rising speed was 60 mm/s, the forward speed was 60 mm/s, the descending speed was 55 mm/s, and the backward speed was 55 43
Stappert et al
vo rbe ha lte ter taking the replicas out of the molds, they were fixed n along n
Qu
t esAd- z the center axis on aluminum sample holders with Epoxy se n hesive (Epoxy Kit 8778-00; Cole-Parmer Instrument Company; Vernon Hills, IL, USA). For image acquisition and evaluation of marginal regions with stereo light microscopy (Stemi 2000 CS; Carl Zeiss), replicas were coated with a fine gold layer (200 Å) using a high-vacuum sputter device (SCD 050, Bal-Tec; Witten, Germany). This study adopted the definition of marginal width ("absolute marginal discrepancy") by Diedrich and Erpenstein14 as the space between the restoration margin and the external preparation margin. To investigate the restorations’ marginal fits, a stereo light microscope (Stemi 2000 CS; Carl Zeiss) at 200X magnification, a 3 CCD color video camera (Sony; Cologne, Germany), and an IBM compatible PC equipped with a Microsoft NT 4.0 operating system were used. The program Analysis 3.0 (Soft-Imaging Software; Munich, Germany) was applied for image investigation. Measurements of marginal discrepancies were taken at 100-μm intervals around the circumference of the restoration margins. Due to discrepancies in shape and size of the human mandibular molars and preparation design, differing numbers of measurements for specimens were taken. To establish a number range for statistical analysis, marginal discrepancy measurements were averaged from between 400 and 500 points for each tooth. The averaged marginal widths values were used for computing means and confidence intervals for marginal accuracy assessment for each group after adhesive cementation and aging process (fatigue and thermocycling). Estimation of confidence intervals and t-test were based on logarithmically transformed values. To achieve a global level of significance (α = 0.05), the p-values resulting from comparisons of unpaired t-tests (between groups) and paired t-tests (between stages) were corrected by the Bonferroni-Holm method. i
Fig 5 PCR of group B-OV loaded until fracturein a universal testing machine. To prevent local force concentrations, a 1-mm-thick piece of tin foil was placed between the tip of the punch and the specimen.
mm/s. During dynamic loading, all specimens were examined twice a day. Failure was defined by bulk fracture of a specimen, not subcritical crack growth. The real time of mouth-motion loading and thermal changes per test group was 10 to 12 days. After the masticatory simulation test, all specimens were photographed from occlusal and palatal aspects with a 4X magnification lens (4T; Nikon F5; Nikon camera systems; Tokyo, Japan), examined for fractures under an optical microscope (Stemi 2000 CS; Carl Zeiss) at low power (50X) stereomagnification using incident light. All specimens which survived the masticatory simulation were loaded until fracture using a universal testing machine (Zwick Z010/TN2S; Ulm, Germany). The force was applied axially to the buccal cusps through a steel wedge, using a consistent crosshead speed of 1.5 mm/min. A 1-mm-thick tin foil was placed over the occlusal surface of the tooth to achieve homogenous stress distribution (Fig 5). After failure, all specimens were re-examined for fracture modes under optical light microscopy (Stemi 2000 CS; Carl Zeiss) (50X to 200X) by two independent observers. Load-to-fracture values were recorded by Zwick testXpert V 7.1 software. Statistical analysis of data was performed using the Kruskal-Wallis test (ANOVA) (α = 0.05). For the comparison of independent groups, the unpaired Wilcoxon rank sum test (α = 0.05) (S-PLUS statistics program, version 3.4 release 1 for Sun SPARC, Insightful; Seattle, WA, USA) was used. A Bonferroni-Holm adjustment of the p-values was performed. Marginal Accuracy Individual impressions (Permagum Putty Soft, Dimension Garant L, 3M-ESPE; Seefeld, Germany) were made for every adhesively luted specimen to examine the marginal fit of each restoration. Further impressions were taken after mouth-motion fatigue, provided that the specimen did not fail during masticatory simulation. Impressions were poured with epoxy resin (Epon 812, Sigma Chemie; Munich, Germany). To produce replicas without bubbles, the impressions were placed in a furnace at 60°C for 24 h (Type 5636; KaVo Dental). The epoxy material was mixed according to manufacturer’s instructions. For thorough polymerization, impressions were incubated for 24 h at 60°C in the furnace. Af44
by ht
opyrig
C All eR ech te
RESULTS Only one specimen of group B-ON fractured during masticatory fatigue. The following median fracture resistance values (N) [IQR=x.25-x.75] were recorded: group NP 1604 N [11821851N], group B-IN 1307 N [1262-1587 N], group B-ON 1396 N [817-1750 N], group B-OV 1205 N [1096-1542 N]. Irrespective of the fractured specimen of group B-ON, all groups demonstrated a minimum fracture resistance above 690 N after fatigue. The highest fracture resistance value was observed in group B-ON (3048 N) (Table 1). For each group, statistics were computed for fracture resistance values after fatigue loading. These results are represented in box plots (Fig 6). No significant differences in fracture loads were found between restored molars when compared to natural teeth (p ≥ 0.18). The different preparation designs demonstrated no significant influence on the fracture resistance of PCRs (B-IN/B-ON p = 0.61; B-IN/B-OV p = 0.30; BON/B-OV p = 0.89). In group NP, 10 of the natural mandibular molars demonstrated fractures exclusively in enamel. Six teeth showed a participation of dentin. Most of these fractures were coronal; only one longitudinal fracture occurred. In contrast, most of The Journal of Adhesive Dentistry
Qu
Table 1 Load-to-fracture test results [N]
nt
i
Group
Median
IQR [Q1(x.25)-Q3(x.75)]
Minimum
Mean
Maximum
SD
NP (n = 16) no preparation
1604
[1181.9-1851.3]
769
1502
2322
477
B-IN (n = 16) buccal inlay preparation
1307
[1261.7-1586.9]
699
1440
2138
405
B-ON (n = 15) buccal onlay preparation
1396
[817.1-1750.2]
49
1326
3048
773
B-OV (n = 16) buccal overlay preparation
1205
[1095.9-1541.7]
735
1295
2145
372
Fig 7 Diagram of geometrical means [95% confidence limits] of marginal discrepancy analysis; cemented adhesively (luted) and after mouth-motion simulation (aged). No significant differences between the groups were found after luting (p ≥ 0.5). Fatigue led to significantly different marginal width values of groups B-IN and B-OV (p = 0.045).
Table 2 Absolute marginal discrepancies in μm of PCRs cemented adhesively (luted) and after mouth-motion simulation (aged) Group
Min
Mean
Max
Median
IQR [Q1(x.25)Q3(x.75)]
Geomean
SE
CI-95%
B-IN luted B-IN aged B-ON-luted B-ON-aged B-OV-luted B-OV-aged
54 73 42 63 63 68
81 95 78 93 81 86
119 124 105 122 109 104
82 93 76 92 82 81
67– 92 89–101 67– 92 81–100 69– 91 79– 97
79 94 76 91 80 86
1.1 1.1 1.1 1.1 1.1 1.1
70– 90 80– 92 67– 87 83–101 73– 88 80– 92
Vol 10, No 1, 2008
vo rbe ha lte n
e ss e n z
Interquartile range (IQR) = difference between lower quartile, Q1 (x.25), and upper quartile, Q3 (x.75).
Fig 6 Box plot of fracture resistance results in N. NP = natural teeth with no preparation; B-IN = PCRs with inlay preparation of the buccal cusps; B-ON = PCRs with onlay preparation of the buccal cusps; B-OV = PCRs with overlapping preparation of the buccal cusps.
by ht
opyrig
C All eR ech e Stappert et tal
45
Stappert et al
Extracted human teeth were chosen as substrates because their modulus of elasticity, thermal conductivity, bonding characteristics, and strength mimic the clinical situation better than plastic, metal, or animal teeth would.48,57 Natural teeth have been used in various investigations,7,54,56 since substrate material properties demonstrated a significant influence on fracture resistance of ceramic restorations.48 To minimize variation in preparation and restoration dimensions, all teeth were prepared by only one clinician using silicone guides. Accuracy of preparation dimensions was reevaluated under light microscopy using a digital micrometer. Tooth mobility has been reported to be a relevant factor in the evaluation of fracture strength.25,26,49 When a small abutment rotation is allowed during testing, failure is more likely to occur.26 To mimic physiological tooth mobility,42 a layer of gum resin was applied to the roots.28 The necessity of applying an artificial periodontium during masticatory simulation, thermocycling, and fracture testing has recently been demonstrated by Rosentritt et al49 on all-ceramic fixed partial dentures. Omitting the artificial periodontium during mouth-motion fatigue caused fracture results almost twice the fracture force compared to tests with periodontium. The influence of the artificial periodontium during fracture testing was more distinct: rigidly embedded teeth withstood three times the loading force than those with a polyether periodontium layer did. The main objective of mouth-motion simulation tests is to introduce fatigue damage to dental restorations under conditions that are as close as possible to the clinical situation.27 The parameters used for masticatory simulation were adjusted to physiological values found in the literature.7,29,33 These in vitro studies have considered a functional loading force of 49 N, which arises during mastication or swallowing. Despite higher forces measured in the posterior dentition,11,20 a value of 49 N was considered to be more applicable as an average constant force in this investigation. Julien 46
mm (female) and 64 mm (male) on posterior teetht during e ss e n z maximum intercuspation. In vivo, mastication forces are distributed to the total contact area, which reduces the simple contact pressure significantly.27 In consideration of the given material properties (ESteatite=138 GPa, EIPS e.max Press=69 GPa), the radius of the Hertzian indenter, and test conditions (P = 49 N), a stress level of approximately 800 MPa at or near the ceramic contact surface was calculated. A simple contact pressure of 5 to 890 MPa was reported to encompass the range of realistic average bite forces.27 Dependent on the diameter of the wear facet,27 the applied load of 49 N was able to induce a contact stress level at the upper end of this range. Variables such as surface structure, inclination of the cusps, and wear behavior of the ceramic and indenter materials35 cause variations in the contact stress level. An average of 250,000 masticatory cycles per year was demonstrated by participants in clinical investigations.12,50 Based on this, an in vivo service time of almost 5 years was performed.21,32 If ceramic restorations are subjected to preliminary masticatory fatigue and thermocycling, the fracture resistance may decrease up to 50% compared with the baseline values.13 Superficial and deep microcracks are responsible for this decline.35 In this study, only one specimen did not survive the exposure to the mastication simulator. It would be ideal to subject a high number of specimens to mouth-motion fatigue under different loads and time periods until failure (step-stress profiles). In this way, the reliability of a specific restoration system would be more predictable.27,35 However, high costs of dental restoration specimens and machine running time restrict this research approach. Since most of the specimens survived fatigue, loadto-fracture tests7,28,55,56 were performed to examine the influence of overlapping the compromised cusps on the fracture resistance of ceramic PCRs. Load-to-fracture tests require high uni-axial loads which might cause sharp or blunt indentation stresses on the surface of a ceramic specimen.27 When a pure steel wedge was applied, contact damage (eg, cone cracks) was observed. To avoid superficial damage, tin foil was used to distribute the load. When surface contact stresses are kept low enough, radial cracks can develop at the bonded ceramic undersurface.34 This is reported when a ceramic layer is uniformly supported by and bonded to a material of lesser stiffness, since high tensile stresses develop in the ceramic at its interface with the cement directly below the loaded area.27,35,36 These interfacial stresses arise from strain differences in the ceramics, cement, and dentin, because the ceramic material possesses a higher modulus of elasticity. This is why cracks usually initiate at the interface level, leading to a subsequent total failure of the restoration.24,27 Lightmicroscopy observation identified bottom radial cracks as being primarily responsible for the fatal damage of the restorations under load to failure. Unknown stress patterns under extreme single loading and collateral damage caused by overshooting loads after sudden stress release27 limit the ability to identify the fracture initiation. Failure modes at the interface and surface are often combined. The intention of the load to failure test was to compare the fracture resis-
i
DISCUSSION
vo rbe ha 23 lte et al described an average clinical total contact area of 52 n n 2 2
Qu
the restored specimens of groups B-IN to B-OV fractured in a different manner. The majority of specimens showed a combination of coronal and cervical fractures, inducing damage to tooth structure and ceramic. In groups B-IN, BON, and B-OV, 13 (81%), 13 (81%), and 10 (62.5%) fractures exceeded the ceramic restorations and propagated into the natural tooth structure, respectively. No debonding of a ceramic restoration occurred. Marginal discrepancies (geometrical mean) [95% confidence limits] of luted PCRs were recorded as followed: B-IN 79 μm [70-90], B-ON 76 μm [67-87], and B-OV 80 μm [7388]. No significant differences were found between the groups (p ≥ 0.5). Mouth-motion fatigue and thermocycling caused a decrease of marginal accuracy (B-IN 94 μm [8092], B-ON 91 μm [83-101], and B-OV 86 μm [80-92]; Fig 7); in groups B-IN (p = 0.009) and B-ON (p = 0.008) the decrease was significant. Marginal width values of groups B-IN and B-OV were significantly different after fatigue (p = 0.045). All values of marginal discrepancy analyses are summarized in Table 2.
by ht
opyrig
C All eR ech te
The Journal of Adhesive Dentistry
t es plete recovery of fracture strength irrespective of the prepaz ration design. Hence, it may be necessary to reconsider thes e n imminent need of cusp coverage when a margin of an inlay or onlay approaches within 1.5 mm of a functional cusp.2 An FEA study examined the loading characteristics of ceramic onlay restorations and compared their associated stress levels with different designs of marginal preparation.1 It was concluded in that study that the buccal aspect of a restoration is subjected to high stresses when horizontal forces are applied. The authors suggested that the restorative material in this area should be sufficiently thick to withstand these stresses. In the present study, vertical and horizontal forces on the buccal aspect did not reveal insufficient material or tooth structure behavior. i
Vol 10, No 1, 2008
vo rbe ha lte ceramic partial-coverage restorations demonstrated n a comn
Qu
tance of PCRs with different preparation designs after fatigue under a standardized protocol. The authors did not expect to mimic clinical failure modes with this method. Further investigations could use fractography51 to verify the crack initiation under single loading. Adhesive bonding proved to be effective in stabilizing the all-ceramic material, since recorded fracture patterns of groups B-IN to B-OV showed no ceramic delamination and involved ceramic and natural tooth structure. In general, it might be preferable if only the restoration itself fails, and does not involve the remaining tooth structure.46 Mean marginal discrepancies of 78 to 95 μm were observed on current glass-ceramic PCRs before and after mouth-motion fatigue. These values met the requirements of maximally acceptable marginal discrepancies between 25 μm and 120 μm as proposed in the literature.8,30,40,52 Nevertheless, a decrease of marginal accuracy of all-ceramic PCRs due to off-axis mouth-motion fatigue and thermal changes was recorded. Water sorption phenomena of composite resin cement,43 microleakage,6 degradation of luting cement as a result of the moist oral environment, and wear should be considered as decisive factors in these findings. An increase of marginal discrepancies by masticatory simulation was also demonstrated in a previous investigation on glass-ceramic PCRs luted to maxillary molars.54 In contrast to the present study, positioning of the restoration margins at the occlusal surface did not lead to significantly higher marginal gap values. In the current investigation, the steatite antagonist was positioned between the buccal cusps and crossed the restoration margins of group B-IN during occlusal horizontal movement. Therefore, marginal wear of this group was increased. This was not the case in the previous study54 when the load was applied vertically onto the center of the occlusal surface of the partial coverage restorations. According to these findings, occlusal contact points sliding over all-ceramic restoration margins could compromise the integrity of the marginal seal of a ceramic PCR under clinical conditions. An in vitro fracture resistance study about restored natural posterior teeth showed maximum fracture strengths of 2680 N for Vitadur N inlays, 1662 N for Ceramco II inlays, and 3547 N for unprepared molars.15 Bremer and Geurtsen4 also reported an average fracture resistance value of 2102 N for human molars, which was higher than in the present study. Under similar load-to-fracture conditions in an earlier investigation, natural maxillary molars achieved higher mean fracture loads (2041 N) than the mandibular molars in the current study (1502 N).57 Maxillary molars restored with IPS e.max Press PCRs demonstrated higher median fracture resistance values of 1567 N to 1870 N than the restored mandibular molars (1205 N to 1396 N).57 IPS e.max Press PCRs on natural maxillary premolars demonstrated median fracture resistance values of 776 N to 1300 N after fatigue, depending on the preparation design.55 Despite different preparation designs, the statistical analysis demonstrated neither a significant difference in fracture resistance between restored teeth nor in unprepared mandibular molars in the current study. Fracture resistance of a natural tooth is described to be compromised after tooth preparation or caries.10 Adhesive
by ht
opyrig
C All eR ech e Stappert et tal
CONCLUSION Ceramic coverage of compromised cusps did not demonstrate an increase in fracture resistance after fatigue when compared to less invasive partial coverage restorations. However, enhanced exposure of restoration margins to occlusal wear could result in more extensive marginal discrepancies.
ACKNOWLEDGMENTS We express our appreciation to Wael Att, Visiting Professor, UCLA School of Dentistry, Weintraub Center for Reconstructive Biotechnology, Los Angeles, CA, USA for his consistent support, and to Hans-Peter Foser, Master Dental Technician, Ivoclar Vivadent, Schaan, Liechtenstein, for his efforts in the fabrication of the ceramic restorations.
REFERENCES 1. Abu-Hassan MI, Abu-Hammad OA, Harrison A. Stress distribution associated with loaded ceramic onlay restorations with different designs of marginal preparation. An FEA study. J Oral Rehabil 2000;27:294-298. 2. Banks RG. Conservative posterior ceramic restorations: a literature review. J Prosthet Dent 1990;63:619-626. 3. Blatz MB. Long-term clinical success of all-ceramic posterior restorations. Quintessence Int 2002;33:415-426. 4. Bremer BD, Geurtsen W. Molar fracture resistance after adhesive restoration with ceramic inlays or resin-based composites. Am J Dent 2001;14:216-220. 5. Broderson SP. Complete-crown and partial-coverage tooth preparation designs for bonded cast ceramic restorations. Quintessence Int 1994;25:535-539. 6. Browning WD, Safirstein J. Effect of gap size and cement type on gingival microleakage in Class V resin composite inlays. Quintessence Int 1997;28:541-544. 7. Chitmongkolsuk S, Heydecke G, Stappert C, Strub JR. Fracture strength of all-ceramic lithium disilicate and porcelain-fused-to-metal bridges for molar replacement after dynamic loading. Eur J Prosthodont Restor Dent 2002;10:15-22. 8. Christensen GJ. Marginal fit of gold inlay castings. J Prosthet Dent 1966;16:297-305. 9. Christensen GJ. Alternatives for the restoration of posterior teeth. Int Dent J 1989;39:155-161. 10. Cotert HS, Sen BH, Balkan M. In vitro comparison of cuspal fracture resistances of posterior teeth restored with various adhesive restorations. Int J Prosthodont 2001;14:374-378. 11. De Boever JA, Mc Call WD, Holden S, Ash MM. Functional occlusal forces: an investigation by telemetry. J Prosthet Dent 1978;40:326-333.
47
Stappert et al
t es
i
48
39.
vo rbe ha McDonald A. Preparation guidelines for full and partial coverage ceramic lte n restorations. Dent Update 2001;28:84-90. n
Qu
12. DeLong R, Douglas WH. Development of an artificial oral environment for the testing of dental restoratives: bi-axial force and movement control. J Dent Res 1983;62:32-36. 13. Deng Y, Lawn BR, Lloyd IK. Characterization of damage modes in dental ceramic bilayer structures. J Biomed Mater Res 2002;63:137-145. 14. Diedrich P, Erpenstein H. Scanning electron optical analysis of the marginal space of in vivo incorporated shoulder crowns and inlays [in German]. Schweiz Monatsschr Zahnmed 1985;95:575-586. 15. Dietschi D, Maeder M, Meyer JM, Holz J. In vitro resistance to fracture of porcelain inlays bonded to tooth. Quintessence Int 1990;21:823-831. 16. El-Mowafy O, Brochu JF. Longevity and clinical performance of IPS-Empress ceramic restorations--a literature review. J Can Dent Assoc 2002;68:233237. 17. Federlin M, Schmidt S, Hiller KA, Thonemann B, Schmalz G. Partial ceramic crowns: influence of preparation design and luting material on internal adaptation. Oper Dent 2004;29:560-570. 18. Felden A, Schmalz G, Federlin M, Hiller KA. Retrospective clinical investigation and survival analysis on ceramic inlays and partial ceramic crowns: results up to 7 years. Clin Oral Investig 1998;2:161-167. 19. Frankenberger R, Petschelt A, Krämer N. Leucite-reinforced glass ceramic inlays and onlays after six years: clinical behavior. Oper Dent 2000;25: 459-465. 20. Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Oc clusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981;46:443-449. 21. Gibbs CH, Mahan PE, Mauderli A, Lundeen HC, Walsh EK. Limits of human bite strength. J Prosthet Dent 1986;56:226-229. 22. Hickel R, Manhart J. Longevity of restorations in posterior teeth and reasons for failure. J Adhes Dent 2001;3:45-64. 23. Julien KC, Buschang PH, Throckmorton GS, Dechow PC. Normal masticatory performance in young adults and children. Arch Oral Biol 1996;41:6975. 24. Kelly JR, Giordano R, Pober R, Cima MJ. Fracture surface analysis of dental ceramics: clinically failed restorations. Int J Prosthodont 1990;3:430-440. 25. Kelly JR. Perspectives on strength. Dent Mater 1995;11:103-110. 26. Kelly JR, Tesk JA, Sorensen JA. Failure of all-ceramic fixed partial dentures in vitro and in vivo: analysis and modeling. J Dent Res 1995;74:12531258. 27. Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999;81:652-661. 28. Kern M, Douglas WH, Fechtig T, Strub JR, DeLong R. Fracture strength of all-porcelain, resin-bonded bridges after testing in an artificial oral environment. J Dent 1993;21:117-121. 29. Kern M, Strub JR, Lu XY. Wear of composite resin veneering materials in a dual-axis chewing simulator. J Oral Rehabil 1999;26:372-378. 30. Krämer N, Lohbauer U, Frankenberger R. Adhesive luting of indirect restorations. Am J Dent 2000;13:60D-76D. 31. Krämer N, Frankenberger R. Clinical performance of bonded leucite-reinforced glass ceramic inlays and onlays after eight years. Dent Mater 2005;21:262-271. 32. Krejci I, Lutz F. In-vitro test results of the evaluation of dental restoration systems. Correlation with in-vivo results [in German]. Schweiz Monatsschr Zahnmed 1990;100:1445-1449. 33. Krejci I, Reich T, Lutz F, Albertoni M. In-vitro test procedure to evaluate dental restoration systems. 1. Computer-controlled masticatory simulation [in German]. Schweiz Monatsschr Zahnmed 1990;100:953-960. 34. Lawn B, Deng Y, Miranda P, Pajares A, Chai H, Kim D. Overview: Damage in brittle layer structures from concentrated loads J Mater Res 2002; 17:3019-3036. 35. Lawn BR, Deng Y, Thompson VP. Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review. J Prosthet Dent 2001;86:495-510. 36. Lawn BR, Deng Y, Lloyd IK, Janal MN, Rekow ED, Thompson VP. Materials design of ceramic-based layer structures for crowns. J Dent Res 2002; 81:433-438. 37. Lehener C, Studer S, Brodbeck U, Schärer P. Six-year clinical results of leucite-reinforced glass ceramic inlays and onlays. Acta Med Dent Helv 1998;3:137-146. 38. Manhart J, Chen H, Hamm G, Hickel R. Buonocore Memorial Lecture. Review of the clinical survival of direct and indirect restorations in posterior teeth of the permanent dentition. Oper Dent 2004;29:481-508.
by ht
opyrig
C All eR ech te
se nz
40. McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-111. 41. Mondelli RF, Barbosa WF, Mondelli J, Franco EB, Carvalho RM. Fracture strength of weakened human premolars restored with amalgam with and without cusp coverage. Am J Dent 1998;11:181-184. 42. Mühlemann HR. Physiologic and pathologic dental mobility [in German]. SSO Schweiz Monatsschr Zahnheilkd 1951;61:1-71. 43. Ortengren U, Elgh U, Spasenoska V, Milleding P, Haasum J, Karlsson S. Water sorption and flexural properties of a composite resin cement. Int J Prosthodont 2000;13:141-147. 44. Otto T, De Nisco S. Computer-aided direct ceramic restorations: a 10-year prospective clinical study of Cerec CAD/CAM inlays and onlays. Int J Prosthodont 2002;15:122-128. 45. Pallesen U, van Dijken JW. An 8-year evaluation of sintered ceramic and glass ceramic inlays processed by the Cerec CAD/CAM system. Eur J Oral Sci 2000;108:239-246. 46. Peters MC, de Vree JH, Brekelmans WA. Distributed crack analysis of ceramic inlays. J Dent Res 1993;72:1537-1542. 47. Rosenblum MA, Schulman A. A review of all-ceramic restorations. J Am Dent Assoc 1997;128:297-307. 48. Rosentritt M, Plein T, Kolbeck C, Behr M, Handel G. In vitro fracture force and marginal adaptation of ceramic crowns fixed on natural and artificial teeth. Int J Prosthodont 2000;13:387-391. 49. Rosentritt M, Behr M, Gebhard R, Handel G. Influence of stress simulation parameters on the fracture strength of all-ceramic fixed-partial dentures. Dent Mater 2006;22:176-182. 50. Sakaguchi RL, Douglas WH, DeLong R, Pintado MR. The wear of a posterior composite in an artificial mouth: a clinical correlation. Dent Mater 1986;2:235-240. 51. Scherrer SS, Quinn JB, Quinn GD, Kelly JR. Failure analysis of ceramic clinical cases using qualitative fractography. Int J Prosthodont 2006;19:185192. 52. Schmalz G, Federlin M, Reich E. Effect of dimension of luting space and luting composite on marginal adaptation of a class II ceramic inlay. J Prosthet Dent 1995;73:392-399. 53. Shillingburg H, Jacobi R, Brackett S. Fundamentals of tooth preparations. Chicago: Quintessence, 1987:13. 54. Stappert CF, Denner N, Gerds T, Strub JR. Marginal adaptation of different types of all-ceramic partial coverage restorations after exposure to an artificial mouth. Br Dent J 2005;199:779-783; discussion 777. 55. Stappert CF, Guess PC, Gerds T, Strub JR. All-ceramic partial coverage premolar restorations. Cavity preparation design, reliability and fracture resistance after fatigue. Am J Dent 2005;18:275-280. 56. Stappert CF, Stathopoulou N, Gerds T, Strub JR. Survival rate and fracture strength of maxillary incisors, restored with different kinds of full veneers. J Oral Rehabil 2005;32:266-272. 57. Stappert CF, Att W, Thomas G, Strub JR. Fracture resistance of different partial-coverage ceramic molar restorations: An in vitro investigation. J Am Dent Assoc 2006;137:514-522. 58. Van Djiken JWV, Hasselrot L, Örmin A, Olofsson A-L. Restorations with extensive dentin/enamel-bonded ceramic coverage. A 5-year follow-up. Eur J Oral Sci 2001;109:222-229. 59. Yamada Y, Tsubota Y, Fukushima S. Effect of restoration method on fracture resistance of endodontically treated maxillary premolars. Int J Prosthodont 2004;17:94-98.
Clinical relevance: Since the present ceramic PCRs achieved, independent of preparation design, fracture resistance values similar to natural mandibular molars, lessinvasive, defect-oriented all-ceramic PCRs should be clinically considered. Prominent occlusal contact points should be avoided at ceramic restoration margins, since wear compromises the marginal integrity.
The Journal of Adhesive Dentistry
