James L. Drummond

Degradation, Fatigue, and Failure of Resin Dental Composite Materials

The intent of this article is to review the numerous factors that affect the mechanical properties of particle- or fiber-filler-containing indirect dental resin composite materials. The focus will be on the effects of degradation due to aging in different media, mainly water and water and ethanol, cyclic loading, and mixed-mode loading on flexure strength and fracture toughness. Several selected papers will be examined in detail with respect to mixed and cyclic loading, and 3D tomography with multi-axial compression specimens. The main cause of failure, for most dental resin composites, is the breakdown of the resin matrix and/or the interface between the filler and the resin matrix. In clinical studies, it appears that failure in the first 5 years is a restoration issue (technique or material selection); after that time period, failure most often results from secondary decay.

Static and cyclic loading of fiber-reinforced dental resin

OBJECTIVE The aim of this study was to evaluate the flexure strength of unidirectional fiber-reinforced resins under static and cyclic loading with and without thermal cycling. METHODS The fiber-reinforced resin materials chosen for this project were commercially available endodontic posts and commercially procured bar samples. For all materials, controls for flexure strength were tested in air and in water using three-point loading. Specimens were thermal cycled between 7 and 63 degrees C for 6000 cycles. A staircase approach was used to determine the flexure fatigue limit and scanning microscopy was used to examine the microstructure. RESULTS The carbon/graphite fiber-reinforced resin posts and the glass FiberKor posts were significantly stronger than the ceramic (zirconia) and the other glass-reinforced resin materials. Thermal cycling caused a significant lowering (11-24%) of the flexure strength for each resin based post system. The ceramic post system decreased only by 2%. Further, for standard size glass fiber-reinforced resin bars, no significant differences between testing in air and water was observed, but a significant difference between static and cyclic loading was noted. SIGNIFICANCE The decreases in the strength property due to thermal cycling and the cyclic loading of these materials indicates that their utilization in the oral environment enhances their degradation, and potentially shortens their clinical life.

Comparative study of water sorption, solubility, and tensile bond strength of two soft lining materials

STATEMENT OF PROBLEM Two soft denture lining materials with distinct chemical compositions were evaluated to determine whether these compositional variations manifest themselves in property differences. PURPOSE This study evaluated and compared the water sorption, solubility, and tensile bond strength of a recently introduced silicone-based soft liner (Luci-sof) and a plasticized acrylic resin soft liner (Permasoft) using 2 processing techniques, laboratory-processed and autopolymerized at chairside, for the latter. MATERIAL AND METHODS For water sorption and solubility testing, 24 disks (45 mm in diameter and 1 mm in thickness) were prepared for each group, stored in distilled water at 37 degrees C, and tested after 1, 4, and 6 weeks. The disks used for the first week were again tested after 4 and 6 weeks of continuous storage. Their weight was recorded and sorption and solubility were calculated using 2 methods. Two PMMA cylinders (25 mm in diameter, 25 mm in length), joined together by the soft liner, were used to determine the bond strength in tension at a loading rate of 2 mm/min after 48 hours and 12 weeks aging in distilled water. RESULTS Permasoft had higher solubility (1.50% +/- 0.20% laboratory-processed and 1.42% +/- 0.16% autopolymerized at chairside) and sorption (2.45% +/- 0.36% laboratory-processed and 1. 76% +/- 0.08% autopolymerized at chairside) than Luci-sof (0.17% +/- 0.09% solubility and 0.41% +/- 0.17% sorption) after 6 weeks of aging. For tensile bond strength, there was no significant difference between each respective control and the 12-week bond strength for each material. However, Luci-sof (1.16 +/- 0.27 MPa) had a significantly higher tensile bond strength than Permasoft (0. 50 +/- 0.18 MPa laboratory-processed and 0.44 +/- 0.09 MPa autopolymerized at chairside). CONCLUSION On the basis of lower water sorption and solubility and higher tensile bond strength, Luci-sof may provide better clinical success.

The use of collagen cross-linking agents to enhance dentin bond strength

UNLABELLED Type I collagen is a major component of the hybrid layer, and improvement of its mechanical properties may be advantageous during bonding procedures. OBJECTIVE To investigate the effect of three different cross-linking agents (Glutaraldehyde [GD], Grape seed extract [GSE], and Genipin [GE]) on the tensile bond strength (TBS) of resin-dentin bonds. MATERIALS AND METHODS Sixty-four sound human molars were collected and their occlusal surfaces were ground flat to expose dentin. Dentin surfaces were etched using a phosphoric acid and then teeth were randomly divided according to the dentin treatment: Control group (no treatment), 5% GD, 6.5% GSE, or 0.5% GE. Teeth were restored either with One Step Plus or Adper Single Bond Plus adhesive systems and resin composite. After 24 h, teeth were sectioned to produce a cross-sectional surface area of 1.0 mm(2) and tested for tensile bond strength. Data were statistically analyzed using ANOVA and Fishers PLSD tests (p < 0.05). There was a statistically significant interaction between factors (treatment and adhesive p < 0.001). Treatment affected TBS (p < 0.0001), while no differences were observed between the adhesive systems (p = 0.6961). CONCLUSION Chemical modification to the dentin matrix promoted by GD and GSE, but not GE, resulted in increased bond strength. The application of selective collagen cross-linkers during adhesive restorative procedures may be a new approach to improve dentin bond strength properties.

Testing mode and surface treatment effects on dentin bonding

The goal of this project was to evaluate the effect of the following variables on shear dentin-bonding test results: mode of testing (cyclic fatigue versus static loading), surface treatments (32% phosphoric acid, 10% phosphoric acid, and no treatment [unetched]), and type of shear test (traditional planar versus push-out). All teeth were stored in distilled water and tested in a shear mode at a loading rate of 2 mm/ min. The specimens were loaded in static or cycled for 1000 cycles using a staircase approach or until fracture, whichever occurred first. On samples with etched dentin surfaces, the push-out test did not demonstrate a significant difference in measured bond strength when compared with results from the planar test, although sample preparation was more labor-intensive. The bond strength resulting from cyclic fatigue of the etched specimens was approximately 51% of the static loading value. Ten percent phosphoric acid was as effective as 32% phosphoric acid for dentin bonding. Finite-element analysis indicated that the traditional planar shear test produces flexure of the specimen and high tensile stress magnitudes within the resin bonding layer. The push-out test produces elevated compressive stresses localized in the composite along the circumference of the punch. Shear stresses in the resin bonding layer are similar for both testing methods at the same loading element contact force.

Fracture studies of selected dental restorative composites

OBJECTIVES The purpose of this study was to evaluate the flexure strength, elastic modulus, and fracture toughness (mode I, mode II, and mixed mode) of resin and four specially made dental restorative composite materials. METHODS Testing was done on prismatic bars in flexure and disk specimens in diametral compression. Fracture strengths were analyzed using Weibull statistics. Statistical analysis consisted of a one-way analysis of variance followed by a Tukey multiple means analysis for each of the materials. In addition, the fracture strengths were analyzed using Weibull statistics due to the brittle behavior exhibited by these materials. RESULTS The experimental results showed that the addition of fillers resulted in a significant three-fold increase in flexure modulus and a significant 30-50% increase in fracture toughness from the resin. As was indicated by the different Weibull modulus values, strength data obtained from four-point bending were not related with strength data from three-point bending. A straight notch vs. a relatively sharp V-notch gave higher fracture toughness values. Fracture toughness was dependent on the depth of a straight notch and was practically independent of the V-notch depth. Mode I and II fracture toughness in two composites (75Sr and 75Sr10) were carried out on precracked disk specimens in diametral compression. The results of mode I toughness were close to those obtained from the flexure testing. The mode II toughness values were greater than the mode I values by more than 30%. The data fit an equation of the form KI/KIC + (KII/KIIC)2 = 1(where KI, KII are the mode I and II stress intensity factors and KIC, KIIC are the respective critical values). SIGNIFICANCE Notching technique, testing configuration (three-point vs. four-point loading), and method of testing (bar vs. disk) have significant effect on the fracture properties.

Glass ionomer microleakage from preparations by an Er/YAG laser or a high-speed handpiece

OBJECTIVE The purpose of this study was to compare microleakage of glass ionomer materials (GIC) using two different modes of cavity preparation: a high-speed hand piece and an erbium/yttrium-aluminum-garnet (Er/YAG) laser. METHODS Eighty caries free permanent molar teeth had class V preparations cut on both the buccal and lingual surfaces. The high-speed hand piece was used to prepare a class V cavity preparation on the buccal surface and an Er/YAG laser was used to prepare a class V cavity preparation on the lingual surface. GIC or resin modified GIC was used as the restorative material. The teeth were thermocycled for 7000 cycles, placed in 2% basic fushin for 24h, sectioned in the center of each restoration, and analyzed under a stereomicroscope. RESULTS There was no statistical difference in microleakage between the two modes of cavity preparations. The gingival margins had more microleakage than the occlusal margins for both GIC materials and the resin modified GIC showed significantly more leakage than the conventional GIC. CONCLUSIONS The Er/YAG laser provided an equivalent method of tooth removal when evaluated for microleakage of GIC materials compared to a high-speed hand piece.

Weibull models for the statistical analysis of dental composite data: aged in physiologic media and cyclic-fatigued

The modulus of rupture or flexural strength of dental composites aged in distilled water and saline solution or cyclic-fatigued to fracture in distilled water was analyzed by use of Weibull statistics. Two- and three-parameter Weibull models were applied to the data. For the case of the dental composites aged for five distinct time periods, the most appropriate model was a two-parameter Weibull model for each separate aging time. For the cyclic-fatigue data, a three-parameter accelerated failure time model with a Weibull baseline distribution and scale parameter, depending on the variable cycle, was appropriate.

Rebonding of orthodontic brackets

OBJECTIVE To compare rebonding of orthodontic brackets based on the hypothesis that no difference would be found between the adhesive systems with respect to shear bond strength, mode of failure, and clinical failure rates. MATERIALS AND METHODS The three adhesive systems included two self-etch primers (Transbond and M-Bond) and a conventional phosphoric acid etch (Rely-a-Bond). The sample size was 20 premolars for each adhesive system. The shear bond strength was tested 24 hours after bracket bonding with the bonding/debonding procedures repeated two times after the first debonding. Bond strength, adhesive remnant index (ARI), and failure sites were evaluated for each debonding. Statistical analysis consisted of a two-way analysis of variance (ANOVA) followed by Scheffè analysis. The clinical portion evaluated 15 patients over a 12-month period. RESULTS The mean shear bond strengths after the first, second, and third debondings for Rely-a-Bond were 8.4 +/- 1.8, 10.3 +/- 2.4, and 14.1 +/- 3.3 MPa, respectively; for Transbond 11.1 +/- 4.6, 13.6 +/- 4.5, and 12.9 +/- 4.4 MPa, respectively; and for M-Bond 8.7 +/- 2.7, 10.4 +/- 2.4, and 12.4 +/- 3.4 MPa, respectively. After the three debondings the mean shear bond strength increased significantly from the first to the third debonding for Rely-a-Bond and M-bond (P </= .001), but did not change for Transbond (P = .199). CONCLUSIONS The original hypothesis is not rejected. The two self-etching primers showing higher or comparable bond strength to the conventional phosphoric etch with less adhesive remnant on the enamel surface after the first debonding. With repeated bonding/debonding, the differences in the bond strength, ARI, and failure site were not significantly different. There was no difference in the clinical performance of the three adhesive systems (P = .667).

Fatigue behaviour of dental composite materials.

OBJECTIVE The intent of the project was to evaluate the fatigue behavior of particle and fiber filled dental composites that are fabricated either directly or indirectly using a notched specimen with respect to the number of cycles until failure. METHODS The materials were five dental composites, three normally cured in the oral cavity (Restolux, Renew and Filtek Supreme), direct processing, and two laboratory produced (BelleGlass, and Tescera), indirect processing. The specimens were 3mmx3mmx25mm bars with a 0.75 or 1.0mm notch in the mid-span of the bars, polished with 320 grit SiC paper and aged for 6 months in air, distilled water, artificial saliva, and a 50/50 mixture of ethanol and distilled water. Testing was performed with a stress mean range of 5-49MPa, the maximum number of cycles was 100,000, and the number of cycles to failure was recorded. RESULTS For the specimens that failed, BelleGlass, Restolux, and Tescera were able to withstand a higher cyclic loaded stress than Renew and Filtek. The 50/50 by volume mixture of water and ethanol resulted in the lowest resistance to fatigue for all materials. CONCLUSIONS Of the dental composite materials investigated, the indirect processed and those with large particle fillers (higher weight percent filler) had better cyclic fatigue resistance than micro- and nano-particle fillers (lower weight percent filler).