REINFORCEMENT OF FIBER-REINFORCED COMPOSITE CROWNS WITH VARIANT MARGIN DESIGNS.
Fiber-reinforced composite restorations provide excellent esthetics; however, little is known regarding the influence of margin design on marginal accuracy and breaking strength for this type of crown. This study evaluated the effect of variations in tooth preparation design on the marginal accuracy and breaking strength of fiber-reinforced composite crowns. Three metal dies with a total convergence of 5-degrees and different margin designs (0.5-mm light chamfer, 1.0-mm deep chamfer, and 1.0-mm shoulder) were prepared. Sixty standardized crowns (FibreKor) were made on duplicated base metal alloy dies (n=20 for each margin design). Marginal accuracy was stereoscopically evaluated by measuring the distances between each of four pairs of indentations on the crowns and on the dies. The specimens were then compressively loaded to failure dies using a universal testing machine.
The data were analyzed with 1-way ANOVA followed by Ryan-Einot-Gabriel-Welsch multiple-range test (a =.05). Analysis of marginal accuracy and breaking strength disclosed a statistically significant difference for tooth preparation design (P<.001). The marginal adaptation of preparations with the 0.5-mm light chamfer (66 mm) and 1.0-mm deep chamfer (69 mm) were significantly better than preparations with a shoulder finish line (92 mm) (P<.001). The fracture strength of the preparations with the 0.5-mm light chamfer (15.8 MPa) and 1.0-mm deep chamfer (15.1 MPa) were significantly greater than those of the preparations with the 1.0-mm shoulder (13.7 MPa) (P<.001). Marginal accuracy of fiber-reinforced crowns was adversely affected by tooth preparation design. The marginal gaps were greater for the shoulder margin specimens than in the light or deep chamfer margin specimens. However, the fracture strength of the chamfer margins specimen was greater than that of the shoulder margin specimens.
Key words: Fiber-reinforced composite, margin design, breaking strength, marginal accuracy
Metal-ceramic restorations continue to be a popu-lar option for fixed prosthodontics, based on their clinical longevity and acceptable esthetics. However, concern has been expressed regarding corrosion of metal alloys and unwanted allergic or toxic side effects affecting patients and laboratory personnel.1 More-over, degradation of Magnetic Resonance Tomography (MRT) and Computer Tomography' (CT) images has been mentioned as a disadvantage of fixed intraoral metal restorations.2
Recently introduced high-strength ceramic restora-tions have proved popular due to their outstanding esthetics. These restorations allow light transmission and achieve an exceptionally life-like appearance. In addition, the discoloration of the gingiva caused by metallic systems can be avoided.3-5 However, improved esthetics have been associated with increased failure rates due to increased fracture properties of all ceramic crowns compared with metal ceramic crowns. More-over, increased costs and more complex cementation techniques have restricted extensive use of all ceramic crown systems.6-9
Demand continues within the dental profession for restorations that exhibit high strength, natural color, good wear resistance, marginal integrity, and ease of fabrication. With the introduction of new composite resins, including fiber-reinforced systems, the restora-tion or replacement of a single tooth or multiple teeth with a fiber-reinforced crown or metal-free partial dental prosthesis is now an option.10 Fiber-reinforced composite restorations have the potential to address some of the problems associated with conventional restorative materials. The systems have impressive mechanical properties, leading to extensive engineer-ing applications. Their strength-to-weight ratios are superior to those of most alloys. When compared to metals they offer many other advantages including the lack of corrosion, translucency, good bonding proper-ties and ease of repair. They also offer the potential for chairside and laboratory fabrication.11
Moreover, these materials have advantages over ceramic materials including higher flexural strength and lower opposing tooth wear; also intraoral repair is less complicated.12,13
Marginal accuracy and fracture resistance are considered crucial factors in the success and longevity of an indirect restoration.14 Studies examining the effect of the finish line on the marginal fit reported conflicting results. Pera et al15 reported that improved marginal fit was obtained with In-Ceram ceramic crowns fabricated on chamfer and 50-degree shoulder margins. However, other studies reported that cast crown fit was not influenced by the margin design.16,17 Also, it was shown that tooth preparations with a 1.2-mm shoulder margins produced the strongest Dicor crowns, while 0.8-mm deep chamfer margins produced the weakest restorations when cemented to metal dies.18,19 However, other studies reported that Dicor crowns luted with a resin luting agent was unaffected by the type of margin design prepared.20,21
Although several publications have characterized the materials science aspects of fiber-reinforced com-posites22-29, little mention has been made in the techni-cal literature on the breaking strength and marginal accuracy of fiber-reinforced composite crowns.13,25 The purpose of this investigation was to evaluate the effect of variations in tooth preparation design on the marginal accuracy and breaking strength of fiber-reinforced composite crowns. The null hypothesis was that margin design modification of tooth prepara-tions would have no influence on marginal accuracy and breaking strength of fiber-reinforced composite crowns.
Three metal master dies with 3 different cervical margins were designed to simulate complete fiber-reinforced composite crown preparations with the di-mensions (Fig 1). These comprised 0.5-mm light cham-fer margin, 1.0-mm deep chamfer margin and 1.0-mm shoulder margin (control) with sharp axiogingival line angle (Fig 2). Sixty impressions (20 impressions for each margin design) of the master metal dies were made with poly (vinylsiloxane) (Examix; GC America
Inc, Chicago, Ill) using a single-mix technique in indi-vidual polycarbonate trays (Ash Instruments, Potters Bar, UK). The impressions were poured with type IV die-stone (Jade stone; Whip Mix Corp, Louisville, Ky). A custom cellulose acetate crown index of an intact ivorine molar tooth was used to standardize the fiber-reinforced composite crown dimensions.
Die hardener (Surface hardener; Renfert GmbH, Hil-Zingen, Germany) and die separator (Picosep; Renfert GmbH) were applied to the stone dies. Die spacer (Picosep, Renfert GmbH) was applied to all dies. 1-mm thick copings made of Conquest/Sculpture (Jeneric/Pentron, Wallingford, Conn.) were photo poly-merized for 5 minutes on each die (Cure-light device). Then, one layer of the unidirectional-fiber pre-impreg-nated FiberKor (shade clear, 6 mm width) was manu-ally wrapped around the coping and further photo polymerized for five minutes. The crown was shaped using dentin and enamel facing material from Con-quest/Sculpture. The fabrication procedure was com-pleted with the cellulose acetate index. The completed restoration was post polymerized in the Conquestomat device for 10 minutes at 107deg C. After post polymeriza-tion, final finishing was performed with stone points, rubber, and wheel instruments (Polierset; Ivoclar Vivadent) following the manufacturer's recommenda-tion.
A total of 60 (20 for each margin design) FiberKor/ Sculpture crowns were fabricated.
For marginal accuracy measurements, the fin-ished crowns were fit passively to their respective metal master dies. Four pairs of index indentations were placed with a 1/2 round bur at equal distances around the circumference of each specimen and the metal master dies; these represented mesial, lingual, buccal, and distal locations. These indentations served as specific points for determining marginal discrepan-cies. A spring-loaded holding device with a force of 98 N which permitted axial rotation of the specimen was used to ensure that the crowns are fully seated on the master dies. Using a x100 magnification light micro-scope (Nikon Measurement, MM-11; Nikon Inc, Gar-den City, NY), direct measurements in micrometers of the marginal gap were measured. The measurements for each specimen were averaged. Gap distance was defined as the distance along a line perpendicular to the most cervical extent of the marginal level and the most cervical extension of the fiber-reinforced crown.
One investigator prepared all specimens and measured the discrepancies.
For breaking strength, each crown was luted to its master metal die with dual polymerizing resin luting agent (Nexus 2; Kerr Corp, Orange, Calif) according to the manufacturer's instructions. Luted crowns were light polymerized for 3 minutes under 4.9 N of axial loading (LTC; Chatillon LTC, Greensboro, NC). Light intensity was 730 mW/cm2, as measured by a radiom-eter (Optilux Model 100; SDS Kerr, Danbury, Conn) at a 10-mm distance from the specimens. Excess cement was cleaned off the restoration margin. The failure loads for luted crowns were determined with a [!-inch diameter cylindrical steel bar brought into contact with the center of the buccal and lingual triangular ridges in a universal testing machine (Model 4202; Instron Corp,
Canton, Mass) in compressive mode with a cross head speed of 0.05 mm/min. The maximum load it could withstand were recorded and divided by the surface area of the tooth preparation to obtain an estimate of the breaking strength in megapascals (Braking strength in MPa = load/surface area) The data were analyzed with 1-way analysis ANOVA followed by Ryan-Einot-Gabriel-Welsch multiple-range test (a =0.05).30
The 1-way ANOVA revealed a statistically signifi-cant difference among marginal accuracy (P<.001) and between breaking strength of each margin design (P<.001) (Table 1). The results of the marginal accuracy and the breaking strength are summarized in Table 2. The lowest mean marginal gap and standard deviation (SD) was obtained from preparations with the 0.5-mm light chamfer [66.2 (10.9) mm] and 1.0-mm deep cham-fer [69.7 (7.4) mm]. However, marginal gap (SD) for preparations with 1.0-mm shoulder margin [92.8 (15.8) mm] was significantly higher (P<.001). The highest mean breaking strength and SD was found for the preparations with the 0.5-mm light chamfer [15.8 (1.7) MPa] and 1.0-mm deep chamfer [15.1 (1.7) MPa] mar-gins. However, breaking strength and SD for prepara-tions with 1.0-mm shoulder margin [13.7 (2.1) MPa] was significantly lower (P less than .001).
TABLE 1: 1-WAY REPEATED MEASURE ANOVA FOR MARGINAL ACCURACY AND BREAKING STRENGTH
Margin gap###2###419.1###29.66 less than .001
Strength###2###22.2###6.59 less than .001
df = degree of freedom; MS = Mean square;###
P = Probabilities; F = Ratio###
TABLE 2: MARGIN GAP AND BREAKING STRENGTH AMONG DIFFERENT PREPARATION DESIGN (MEAN (SD)) (n = 10)
###Mean + Standard deviation
Margin design###Margin gap###Fracture
Light chamfer###66.2 (10.9)a###15.8 (1.7)c
Deep chamfer###69.7 (7.4)a###15.1 (1.7)c
Shoulder###92.8 (15.8)b###13.7 (2.1)d
Values with the same lower case letters are not significantly different at P less than .05.
The data support the null hypothesis of the study, that margin design modification of tooth preparations appeared most conductive to the development of better margin accuracy and high breaking strength of fiber-reinforced composite crowns. The majority of research-ers agree on the importance of margin accuracy and breaking strength for long-term success of restora-tions.1,4,5,7,12,13,15 In the current study, the marginal discrepancy of the fiber-reinforced composite crowns was similar to that of the all-ceramic crowns and was within a clinically acceptable level, 100 mm. However, the amount of discrepancies did not fall within the limit of 25 mm, implied by ADA specification No. 8.1
Various aspects of tooth preparation designs have been cited in the literature. However, considerable focus has been still directed towards the most appropri-ate margin design as innovative restorative systems are introduced. Manufacturers and authors offer differ-ent opinions as to the optimal form, but little scientific data is available. Lin et al28 reported that the finish line influenced the marginal adaptation of all-ceramic Procera crowns, while Pera et al15 demonstrated that improved marginal fit was obtained with In-Ceram ceramic crowns fabricated on chamfer and 50-degree shoulder tooth preparations compared with 90-degree shoulder margins. Gavelis et al29 suggested that the specific type of finish line helps the cement to escape and improving marginal adaptation in cast metal crowns. However, Syu et al16 reported that cast crown fit was not influenced by the design of the finish line. A similar result was reported in another study.21
Shearer et al4 showed no significant difference between chamfer and shoulder margins in the fit of In-Ceram crowns.
No definite criteria exist regarding what consti-tutes clinically acceptable marginal accuracy. McLean and von Fraunhofer6 proposed that a restoration would be successful if marginal gaps and cement thicknesses of less than 120 mm could be achieved. There have been many studies regarding the marginal fit of crowns. Testing Celay In-Ceram, Beschnidt et al5 reported mean marginal gaps of 78 mm in maxillary incisor crowns, which was in agreement with the results found in the present study. However, Groten et al14 reported a lower value of 18.3 mm. Beschnidt et al5 tested the marginal fit of IPS Empress 2 and found the marginal opening to be 62 mm, which was similar to the results of this study. Sulaiman et al8 and Gery et al9 evaluated the marginal gaps of conventional In-Ceram crowns, which were 160.66 mm and 123 mm respectively.
These results were also in accordance with this study. An explanation of the lack of agreement may be variation in the methods used by various investigators studying marginal accuracy. Sulaiman et al8 suggested the cause could be the use of different measuring instruments. Sample size and number of measurements per speci-men may also have contributed to the variation. The current study demonstrated that clinically acceptable mean marginal discrepancies could be achieved of all groups tested.
The maximum occlusal force has been reported in the literature to vary widely. The mean adult occlusal force reported to be about 400-800 N.1 Results of the current study exceed these limits. Moreover, the frac-ture strength was higher with a light or deep chamfer finish line than crowns with a shoulder finish line. This explained that the marginal bulk of fiber-reinforced crowns used did not contribute to the fracture strength compared to all-ceramic crowns18,19. This results were consistent with the results obtained for the Artglass crowns.22
The results obtained from this research are only introductory and comparative. A limitation was the use of direct method to measure the marginal accuracy. However, internal fit of the crowns was impossible to measure with the metal master dies. Although certain investigations focused on marginal fit, others4,8,9, also evaluated the internal fit of the crowns. Measuring the internal fit of artificial crowns requires cementing the crowns and sectioning the specimens. In the case of sectioning, the number of measurements per specimen is limited. The current study used a metal die that has merit in standardizing the preparation for all abut-ments. However, if natural teeth were used as the supporting models, the marginal discrepancies might have been reduced. Another limitation of this study was that the crowns were not subjected to an artificial aging process, such as thermal cycling which might affect the results. Also, the exact point of fracture was not determined.
Future investigations are needed to evaluate the long-term retention of posts and to con-firm the in vitro findings of the current study.
Mean marginal opening of each fiber-reinforced composite crown group was less than 100 mm, which was considered clinically acceptable. Lowest mean discrepancy was recorded for the two chamfer finish line groups. However, the fracture strength of the preparations with 1.0-mm shoulder margins produced the weakest restorations. However, the values were higher than those considered to be normal occlusal force.
Acknowledgement: The authors thank King Abdulaziz University, Jeddah, Saudi Arabia, for supplying the grant No. 429/018-9 and Dr William Johnston for his statistical assistance.
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ABDULHAMAID ABDULLA MAGHRABI, BDS, MS, PhD
Correspondence: Dr Abdulhamaid A. Maghrabi, Associate Professor, Section of Oral and Maxillofacial Rehabilitation. Faculty of Dentistry, King Abdulaziz University, Jeddah. PO Box 80157, Jeddah 21589, Saudi Arabia.
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|Publication:||Pakistan Oral and Dental Journal|
|Date:||Jun 30, 2010|
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