Dental Traumatology 2014; 30: 317–325; doi: 10.1111/edt.12095
Fracture resistance and failure mode of fatigued endodontically treated teeth restored with fiber-reinforced resin posts and metallic posts in vitro Fahad A. Alharbi1, Dan Nathanson2, Steven M. Morgano3, Nadim Z. Baba4 1 College of Dentistry, University of Dammam, Dammam, Saudi Arabia; 2Department of Restorative Sciences and Biomaterials, Goldman School of Dental Medicine, Boston University, Boston, MA, USA; 3Division of Postdoctoral Prosthodontics, Department of Restorative Sciences and Biomaterials, Goldman School of Dental Medicine, Boston University, Boston, MA, USA; 4Hugh Love Center for Research and Education in Technology, Loma Linda University School of Dentistry, Loma Linda, CA, USA
Key words: crown fracture; dental trauma; root fracture Correspondence to: Dr Nadim Z. Baba, DMD, MSD, Loma Linda School of Dentistry, Hugh Love Center for Research and Education in Technology, 11092 Anderson Street, Loma Linda, CA 92350, USA Tel.: (909) 558-4115 Fax: (909) 558-0916 e-mail: [email protected]
Abstract – Background: Fracture of restored endodontically treated teeth is a common complication. The mechanical properties of post systems may play a role in the incidence of tooth fracture. Aim: The purpose of this study was to evaluate in vitro the fracture resistance and pattern of fracture of endodontically treated teeth restored with three different post systems. Material and methods: Posts used were fiber-reinforced composite resin posts (FRC post), custom cast silver-palladium, and nickel-chromium posts and cores. A 3-point bending test was performed to calculate flexural strengths and elastic moduli of the specimens. Sixty extracted human maxillary canines were endodontically treated and divided into three groups (n = 20). All-ceramic crowns were fabricated and cemented with Variolink II resin cement. Ten specimens of each group were subjected to a constantly increasing load until fracture. The other 10 specimens were fatigued for 106 cycles in a custom-made fatigue machine. Recorded failure loads and modes were statistically compared with one-way ANOVA and TukeyHSD tests (a = 0.05). Results: The resistance to fracture of teeth restored with FRC posts, composite resin cores, and Empress II crowns was similar to that of teeth restored with cast posts and cores (P = 0.162). Supracrestal (above root/level of acrylic resin base) oblique fracture was the predominant mode of fracture associated with teeth restored with FRC posts (70%), while vertical root fractures were more common with teeth restored with cast posts and cores. Conclusions: Teeth restored with the FRC post system did not exhibit vertical root fractures and were less likely to show root fracture. Sixty to 80% of teeth restored with both types of cast posts and cores showed vertical and subcrestal root fractures.
Accepted 15 December, 2013
Loss of substantial coronal tooth structure as a result of dental caries, trauma, and endodontic therapy can affect the overall strength of a pulpless tooth. Several studies have indicated that the strength of the tooth is directly related to the remaining bulk of dentin (1–5). Additionally, an in vivo investigation of the pressoreceptive function of endodontically treated teeth has indicated a reduced tactile sensation with these teeth, which may affect a patient’s ability to detect functional overload (6). Most anterior teeth after endodontic treatment require restoration with crowns because of the presence of multiple restorations or the patient’s esthetic demands (7). When sufficient coronal tooth structure remains and preparation of the pulpless tooth is conservative, an artificial crown can be placed without a post (5). However, when the coronal portion of the tooth must be reconstructed to retain a crown, a post is commonly used to augment core retention (2, © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
8). The optimal post would provide core retention without creating undesirable stresses within the remaining tooth structure. Several materials have been used for fabricating endodontic posts. Metal alloys have been the material of choice for decades (9–11). However, several authors reported that the use of a post with a modulus of elasticity significantly greater than that of dentin might create stresses at cement interfaces, with the possibility of post separation or root fracture (12–15). The use of posts with high elastic moduli is potentially hazardous. The concentration of stresses in specific areas within the root may result in root fracture, which usually condemns the tooth to extraction. In recent years, fiberreinforced resin posts, zirconia posts, and woven-fiber composite resin material for posts and cores were introduced (16–21). Fiber-reinforced composite resin materials were reported to be an alternative option as a 317
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restorative dental material for making posts (20–24). The newer fiber-reinforced composite resin posts were reported to have retention to root canals similar to or better than commonly used prefabricated post systems (25). Failure of endodontically treated teeth may occur in the crown or in the root. When teeth are restored with cast crowns, fracture usually occurs around the margin extending into tooth structure. However, porcelain crowns may fracture without damage to tooth structure, and the failure mode of porcelain crowns is affected by the stiffness of the core foundation material (26–28). Lee and Wilson (29) reported that the fracture strengths were higher when all-ceramic crowns were supported with a stiff core material (Cobalt-Chromium alloy). Other researchers reported a strong influence of the elastic modulus of the foundation material on the fracture strengths of all-ceramic restorations (30–33). Vertical and deep root fractures have been associated with tapered cast posts and cores (21, 34–37). Some studies reported that tapered posts exhibited a wedging effect within the root (38, 39). Morgano and Milot (40) reported that cast posts should not produce a wedging effect if designed with a positive seat on coronal dentin. Furthermore, the fit of cast posts should be passive and pressure spots should be relieved. The length of the post is also an important factor that affects the distribution of stresses along the root (41) and may affect the incidence of vertical root fracture. The survival rate and the effect of endodontic posts on endodontically treated teeth subjected to intermittent loading was determined in several studies (4, 42–45). Pontius and Hutter (42) evaluated the survival rate and fracture resistance of maxillary central incisors restored with three different post systems. The authors reported vertical root fractures for specimens restored with metal posts and horizontal fractures for teeth restored with resin-infused ceramic posts. Akkayan and Gulmez (46) reported favorable (potentially restorable) fractures for teeth restored with quartz-fiber resin posts (8 of 10) and glass-fiber resin posts (6 of 10). Teeth restored with zirconium oxide posts (7 of 10) and titanium posts (10 of 10) produced catastrophic (nonrestorable) fractures. The authors categorized fracture patterns as favorable and catastrophic (non-favorable) based on their subjective opinion with regard to the restorability of the tooth. Horizontal or oblique tooth fractures at the crestal bone level were considered favorable (potentially restorable) while a more apical root fracture was considered catastrophic because the tooth could not possibly be restored. The aim of this in vitro study was to evaluate the effect of the elastic moduli of different post systems on the fracture resistance and fracturing pattern of endodontically treated teeth restored with ceramic crowns and fiber-reinforced composite resin posts and metal posts. The hypothesis is that the mechanical properties of tested post systems have no effect on the fracture resistance and fracturing pattern of endodontically treated teeth restored with fiber-reinforced composite resin posts or metal posts and ceramic crowns.
Table 1. Post systems tested Post system
FRC Postec Silver-palladium cast post-and-core Nickel-chromium cast post-and-core
Ivoclar Vivadent, Amherst, NY, USA Ney 76; Dentsply, York, PA, USA
Ticonium; Jeneric Pentron Inc., Wallingford, CT, USA
Material and methods
This study consisted of two parts. In the first part, the elastic moduli and flexural strengths of the tested post materials used for constructing test specimens were measured, whereas the second part investigated the effect of cyclic fatigue on the strength of endodontically treated teeth restored with ceramic crowns combined with either fiber-reinforced composite resin posts (FRC) or cast posts and cores made from two different casting alloys. Table 1 lists post materials and post systems tested. Part I: Elastic moduli and flexural strengths of post systems
A 3-point bending test was performed to calculate flexural strengths and moduli of elasticity for FRC Postec posts and the two cast-alloy posts. The FRC Postec posts (n = 5) were tested as provided from the manufacturer. Rectangular bars (1.5 9 1.9 9 19 mm) (n = 5) were made from each of the two cast alloys. For silver-palladium (SP) alloy bars, gypsum investment material (Beauty-Cast; Whip Mix Corp, Louisville, KY, USA) was mixed (0.30 ml g 1 water/powder ratio) under vacuum for 60 s and poured around the patterns under vibration. After setting, the casting ring was placed in a furnace to burn out the acrylic resin. Silver-palladium alloy (Ney 76, Dentsply, York, PA, USA) was melted and cast into mold. For nickel-chromium (NC) alloy bars, phosphate bonded investment (Power-Cast; Whip Mix Corp) was mixed (0.30 ml g 1 water/powder ratio) under vacuum for 60 s and poured around the patterns under vibration. After 45 min, the casting rings were placed in a furnace preheated to 315°C. Nickelchromium alloy (Ticonium, Jeneric Pentron Inc., Wallingford, CT, USA) was melted and cast into the mold. After casting, sprues were cut by using separating disks (Sullivan-Schein Dental, Melville, NY, USA). Widths and thicknesses of the bars were verified with the use of an electronic caliper (VWR Digital Caliper; VWR International, Buffalo Grove, Ill, USA). Each specimen was placed on a testing jig, which supported the specimen at two points, 9.6 mm apart. The load was applied at the center of the specimen at a cross-head speed of 0.5 mm min 1 by using a universal testing machine (Instron Model 4202; Instron Corp, Canton, MA, USA) until failure. Flexural strengths and elastic moduli were calculated. © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Fracture resistance/failure mode of fatigued Part II: Cyclic fatigue of restored endodontically treated teeth
Sixty extracted human maxillary canines (free of dental caries and cracks) were selected. After cleaning, crowns were sectioned horizontally, 2 mm coronal to the cementoenamel junction. Endodontic treatment was performed for all teeth. Cleaning and shaping of the root canals were accomplished and root canals were filled with gutta-percha points (Sullivan-Schein Dental, Melville, NY, USA) and a root canal sealer (Sealapex; SybronEndo Corp., Orange, CA, USA). A 9-mm deep and 1.5-mm wide post space was prepared in each canal with an engine reamer (Pesso Reamer, size 2; SullivanSchein Dental) followed by the corresponding Postec drill (Postec). Teeth were divided into three groups (n = 20) according to the type of post system used. FRC Postec posts (n = 20) were cemented with resin cement (Variolink II; IvoclarVivadent, Amherst, NY, USA) after cleaning and bonding of both the post space and the FRC posts. Post space walls were etched with 37% phosphoric acid (Total Etch; Ivoclar-Vivadent) for 15 s, then washed with water, and dried with paper points (Absorbent Points; Sullivan-Schein Dental). Excite bonding agent (Ivoclar-Vivadent) was applied to post space walls with a disposable brush (IvoclarVivadent) The 2-part dual polymerized resin cement (Variolink II; IvoclarVivadent, Amherst, NY, USA) was mixed and applied to the post space walls and resin posts. Posts were inserted and excess cement was removed with a dry disposable brush. The root surfaces were light-polymerized from four directions (labial, palatal, mesial, and distal) for 40 s in each direction. A radiograph was made to verify the seating of each post. For core fabrication, composite resin (Tetric Ceram) was incrementally applied to the tooth and around the FRC Postec post with a plastic instrument. After completion of the core, each tooth was prepared to receive a ceramic crown (Empress II, IvoclarVivadent, Amherst, NY, USA) with diamond rotary instruments (6856-31-018, 8856-31-018, 6368-31-023 and 8368-31023; Brasseler USA, Savannah, GA) in a high-speed handpiece (Ti-Max EL400; NSK America Corp, Schaumburg, Ill, USA) under cold-water spray. The preparation finish line was placed 2.0 mm apical to core/tooth junction. All teeth were held in a wet tissue during all procedures. Post-and-core patterns (n = 40) were directly made with an autopolymerizing acrylic resin (GC Pattern Resin, GC America Inc., Alsip, IL, USA) and prefabricated plastic patterns (Spee Dee Plastic Patterns; Pulpdent, Watertown, MA, USA). Teeth were held in a wet tissue during pattern fabrication and the post spaces were kept moist to avoid adherence of acrylic resin to dentin walls. Acrylic resin was applied incrementally to the plastic pattern with a disposable brush. After polymerization, a core pattern was contoured with fine diamond rotary instruments (8856-31-018 and 8368-31023; Brasseler USA). The height of the core portion of the patterns was maintained at 4.0 mm. Fabricated patterns were divided into two groups. One group was cast with silver-palladium alloy (Ney 76) and the second one was cast with the nickel-chromium alloy (Ticoni© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
um). All rings were allowed to bench cool, and sprues were cut with abrasive disks in a slow speed handpiece (NSK America Corp, Hoffman estates, IL, USA). After casting, the posts were fitted to their corresponding roots with the use of a silicone disclosing paste (Fit Checker; GC America Inc., Alsip, IL, USA). All cast posts were airborne particle abraded with 80micro meter using aluminum oxide, then washed with water and air-dried. Excite bonding agent was brushed on all cast posts and air-dried for 5 s. Resin cement (Variolink II, IvoclarVivadent, Amherst, NY, USA) was mixed according to the manufacturer’s instructions and applied to post space walls with lentulo-spiral paste fillers (Henry Schein Dental). Specimens were light polymerized from four directions (labial, palatal, mesial, and distal) for 40 s in each direction. Crown preparations were finalized for all specimens with fine diamond rotary instruments (8856-31-018 and 8368-31023; Brasseler USA). Working dies were prepared from an impression made with poly (vinyl siloxane) material and ceramic crowns (Empress II, IvoclarVivadent, Amherst, NY, USA) were fabricated. After receiving the crowns from the laboratory, the fit of all crowns was evaluated with a silicone disclosing paste (Fit Checker) and adjusted as required. Crowns were etched internally with 5% hydrofluoric acid (Ceramic Etching Gel, IvoclarVivadent, Amherst, NY, USA) for 2 min, then washed with water and dried. A primer (Monobond S) was applied to the intaglio surface of each crown and then dried. Roots with cemented posts and cores were etched for 10 s with 37% phosphoric acid (Total Etch, IvoclarVivadent, Amherst, NY, USA), then washed with water, and maintained in a moist condition. Excite bonding agent was applied and light polymerized for 20 s. Variolink II resin cement was mixed and applied inside each crown. Each crown was seated with finger pressure on the corresponding root, and then each surface (labial, palatal, mesial, and distal) was light polymerized for 40 s. Each restored tooth specimen was mounted in an aluminum base at a 45-degree angle to the vertical axis with an autopolymerizing acrylic resin (Formatray Resin; Kerr Corp). A plastic cup filled with water encircled each test specimen crown during the cyclic fatigue test. A custom-made fatigue machine (Biomaterials Division, Goldman School of Dental Medicine, Boston University, Boston, MA, USA) was used for intermittent load application (Fig. 1). The machine normally applies the load to specimens through an accurate movement of vertical arms. Specimens mounted in the aluminum base were supplemented with metal bars supported by two pins to allow an up/down movement. The anterior portion of each bar held a brass ball (2 9 2 mm diameter) that contacted the tooth in the center of the palatal surface 2 mm apical to incisal edge. The vertical arm of the fatigue machine applied load to the top of the metal bar and subsequently to the specimen. Ten specimens from each group were loaded 2-mm apical to the cusp tip with a 250-N load for 1 000 000 cycles. These specimens were evaluated every 100 000 cycles and at interim evaluations at each 250 000 cycles for premature failure.
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Fig. 1. Specimen preparation for fatigue testing. (a) Top view of test specimen mounted in aluminum base with plastic cup surrounding crown; (b) Side view of mounted tooth with horizontal metal bar attached to base with 2 pins. Metal bar holds small brass ball that contacts tooth crown 2 mm apical to incisal edge; (c,d), Mounted specimens were attached to cyclic fatigue machine where height of vertical arms was adjusted to contact the metal bar. When cyclic fatigue machine was turned on, vertical arms contacted metal bar intermittently.
Table 2. Description of different fracture types observed in restored teeth Fracture type
Fracture of artificial crown with or without core fracture Root fracture above root/level of acrylic resin base
Supracrestal root fracture Subcrestal root fracture Vertical fracture
Root fracture below root/level of acrylic resin base Fracture line running longitudinally along tooth
After the cyclic loading, specimens were subjected to a constantly increasing load to failure in the universal testing machine with a crosshead speed of 0.5 mm min 1. Ten specimens from each group were loaded to failure (without cyclic fatigue testing). These specimens were used as controls. Failure load and mode were recorded for each specimen. Failure mode was categorized into different categories as described in Table 2. One-way ANOVA and Tukey test-HSD (P = 0.05) were performed for calculated flexural strengths and elastic moduli. Survival-rate analysis was performed based on specimens that failed during cyclic fatigue loading. Fracture resistance of restored teeth was compared and analyzed based on the failure load of speci-
Fig. 2. Mean failure loads (Newtons) for teeth restored with FRC Postec post, silver-palladium cast post and core (CPCSP), and nickel-chromium cast post and core (CPCNC). Control: restored teeth were loaded to failure. Fatigued: restored teeth were intermittently loaded with 250N for 1 000 000 cycles prior to applying load to failure.
mens that survived the cyclic fatigue test. Statistical analyses were performed with 1-way ANOVA and Tukey test (P = 0.05) for specimens after cyclic fatigue. A Student t test was performed between each test and corresponding control groups (P = 0.05). Failure modes among the different groups were compared.
Table 3. Tukey test results for flexural strengths (MPa) and elastic moduli (GPa) of groups tested 95% Confidence interval Post systems
SP NC NC SP NC NC
Mean difference 177.4* 1130.4* 953.0* 72.5* 174.2* 101.8*
61.18 61.18 61.18 1.85 1.85 1.85