Kenneth G. Boberick

Fatigue of Restorative Materials

Failure due to fatigue manifests itself in dental prostheses and restorations as wear, fractured margins, delaminated coatings, and bulk fracture. Mechanisms responsible for fatigue-induced failure depend on material ductility: Brittle materials are susceptible to catastrophic failure, while ductile materials utilize their plasticity to reduce stress concentrations at the crack tip. Because of the expense associated with the replacement of failed restorations, there is a strong desire on the part of basic scientists and clinicians to evaluate the resistance of materials to fatigue in laboratory tests. Test variables include fatigue-loading mode and test environment, such as soaking in water. The outcome variable is typically fracture strength, and these data typically fit the Weibull distribution. Analysis of fatigue data permits predictive inferences to be made concerning the survival of structures fabricated from restorative materials under specified loading conditions. Although many dental-restorative materials are routinely evaluated, only limited use has been made of fatigue data collected in vitro: Wear of materials and the survival of porcelain restorations has been modeled by both fracture mechanics and probabilistic approaches. A need still exists for a clinical failure database and for the development of valid test methods for the evaluation of composite materials.

Lifetime predictions for resin-based composites using cyclic and dynamic fatigue

Because dental restorative materials undergo fatigue in use, testing is often performed in the laboratory to evaluate material responses to cyclic loading. The purpose of this study was to compare the lifetime predictions resulting from two methods of fatigue testing: dynamic and cyclic fatigue. Model composites were made in which one variable was the presence of a silanizing agent, and specimens tested in 4-point flexure. Cyclic fatigue was carried out at a frequency of 5 Hz, while dynamic fatigue testing spanned seven decades of stress rate application. Data were reduced and the crack propagation parameters for each material were calculated from both sets of fatigue data. These parameters were then used to calculate an equivalent static tensile stress for a 5-year survival time. The 5-year survival stresses predicted by dynamic fatigue data were approximately twice those predicted by cyclic fatigue data. In the absence of filler particle silanization, the survival stress was reduced by half. Aging in a water-ethanol solution reduced the survival stresses by a factor of four to five. Cyclic fatigue is a more conservative means of predicting lifetimes of resin-based composites.

The effect of compressive cyclic loading on retention of a temporary cement used with implants.

Masticatory forces cause fatigue to cement-retained crowns and abutments and may adversely effect retention. The relation between the number of load cycles and the retentive forces is important. This study evaluated the effect of compressive cyclic loading on the retentive forces of a temporary cement used to retain implant crowns and the relationship between load cycles and retentive forces. Ten castings and implant abutments were cemented with zinc oxide-eugenol temporary cement. The retentive force necessary to dislodge the casting from the abutment was determined before and after the application of 2 Hz of vertical off-axis 3-mm sinusoidal-type compressive cyclic loading between 20 and 130 N for 500000, 1000000, and 5000000 cycles. These forces were equivalent to approximately 6 months, 1 year, and 5 years of human mastication. Data before and after the applied loading were analyzed with a paired sample t test (alpha = 0.05). The retentive forces of the 3 groups were analyzed by 1-way analysis of variance and post hoc by Scheffé multiple comparison (alpha = 0.05). The relation of the loading and the altered retentive forces were analyzed with the Pearson correlation coefficient. Compressive cyclic loading reduced the retentive forces significantly in all groups (P = .000). The retentive forces were reduced 16.75%, 18.73%, and 19.68% during the applied loading cycles of 500000, 1000000, and 5000000. All reduced retentive forces were not significantly different (P = .792). Although cyclic loading reduced the retentive forces, the increased cycles had little relationship (R = 0.119) to the decreased retentive forces of the temporary cement. The relationship between occlusal loading and retentive force can influence the choice of a temporary cement for a particular clinical situation.

Shade Matching by Dental Students

Dental students achieved a high identification rate for darker shades (P14 and P24) on the Portrait IPN shade guide, which represent the highest chroma and lowest value in the grayish and reddish yellow hue groups. A high identification rate was also obtained for P32, representing the lowest chroma and highest value in the reddish gray hue group. The data suggest that dental students had the most difficulty determining the correct hue group for unmarked shade tabs. Incorrect responses tended to remain in the same value and chroma range but were selected in a different hue group.

DIMENSIONAL STABILITY OF RECORD BASES FABRICATED FROM LIGHT-POLYMERIZED COMPOSITE USING TWO METHODS

STATEMENT OF PROBLEM Record bases fabricated from resin-based composite, which undergoes polymerization shrinkage during curing, can distort during curing to produce significant gaps along the posterior border. PURPOSE This study evaluated the pattern of gap formation that occurs over the palatal area of a maxillary record base fabricated from light-cured composite (Triad), and determined whether changes in the fabrication method affects the size and pattern of gap formation. Effects of composite width and passage of time on gap formation were also analyzed. MATERIAL AND METHODS Twenty record bases were fabricated with two methods: (1) according to the manufacturers directions, the entire surface of the record base was exposed to the curing light; and (2) the palatal area of the record base was covered with foil to prevent light penetration, and the base was light cured in three stages with readaptation of the uncured composite between stages. Ten record bases from each group were tested 1 hour after fabrication; the remaining 10 from each group were tested 7 days after fabrication. Measurements of adaptation were made at three points representing the right and left ridge crests and midpalatal areas. The first measurement was taken at the posterior border and subsequent measurements were made at 5 mm increments after reduction of the posterior border of the cast and record base with a model trimmer. RESULTS Dimensional changes were observed in both procedures. The largest discrepancy occurred at the posterior border of record bases fabricated using method 1. Mean gap formation at the posterior border in this group after 1 hour was 454, 729, and 334 microm and 483, 841, and 330 microm after 7 days at points A, B, and C, respectively. Record bases fabricated with procedure 2 demonstrated mean gap sizes at the posterior border after 1 hour of 219, 421, and 208 microm and 267, 403, and 192 microm after 7 days at points A, B, and C, respectively. No significant influence of composite width on gap size was found. CONCLUSION The average gap size due to stage curing (method 2) was never greater than the corresponding gap that arose from the use of the manufacturers recommendations (method 1). Limiting the amount of surface area exposed to the curing light and readapting the uncured composite to the cast between curing episodes can significantly reduce the mean gap size occurring at the posterior border.

Implant Replacement of Congenitally Missing Lateral Incisors: A Case Report

Implants can readily be placed and restored in congenitally missing maxillary lateral incisor sites with predictable results if surgical, periodontal, and prosthodontic conditions are favorable. A case report using dental implants to replace bilateral congenitally absent maxillary lateral incisors for teenage female identical twins is presented.

Use of a flexible cast for the indirect fabrication of provisional restorations

Efficient fabrication of a clinically acceptable provisional restoration for a fixed partial denture is an important part of treatment success. Fabrication of provisional restorations that uses the indirect technique produces accurate fitting provisional restorations without the chemical and thermal irritation associated with direct fabrication. With a typodont model, an indirect method is presented that uses an elastic cast for fabrication of multiple unit provisional restorations for fixed partial dentures. The cast is available within 6 minutes of impression making, can be trimmed with a sharp scalpel, and provides flexibility that allows easy separation of the acrylic provisional from the cast. The cast can also be used to evaluate the clinical acceptability of the preparations before impression making. This method has also been successfully used for the fabrication of acrylic provisional restorations for onlay preparations.

Microtensile Bond Strength of Resin-Resin Interfaces After 24-Hour and 12-Month Soaking

Abstract Evaluate the bond strengths of denture base-repair materials to minimize recurrent failure rate. Use microtensile bond strength (μTBS) testing to evaluate the interfacial bonding strength ...

Use of a flexible cast for fabrication of multiple post-coping overdenture restorations.

Direct intraoral fabrication of multiple post-coping restorations for an overdenture in the same arch can be a time-consuming chairside clinical procedure. In addition, accurate intraoral fabrication of resin patterns with intracrevicular margins is not always possible as a result of limited access. This article presents a direct-indirect method that uses an elastic cast for fabrication of multiple post-coping restorations with intracrevicular margins. Custom post patterns are fabricated directly in the root canal with autopolymerizing resin (Duralay). An impression is made of the remaining tooth structure, which also includes the post patterns, and a flexible cast is poured chairside. The cast is available within 6 to 8 minutes of impression making and can be trimmed similar to a stone die. The procedure also provides flexibility that allows separation of the cast from the impression without fracture of the custom post pattern or cast. The fabrication of the post-coping restoration can be completed in the laboratory, or if necessary, completed chairside and verified intraorally, before dismissing the patient.

Prosthodontic Self-treatment With Acrylic Resin Super Glue: A Case Report

A case history is presented of a patient who fabricated 3 prostheses from autopolymerizing acrylic resin intended for fingernail augmentation and then cemented them into her mouth with super glue. Patients must be warned not to attempt self-treatment for esthetics with self-fabricated prostheses because severe adverse and irreversible hard and soft tissue reactions may occur.