The International Journal of Prosthodontics, 02/1995

Clinical evidence indicated that the majority of fractures occur in prosthodontic structures do so after a period of many years. Such failures generally are not related to an ipisode of acute overload but result from fatigue failure. This paper reveiws the current knowledge of fatigue failure and test methods. An overview of published studies is given, and the authors suggest guidelines for future prosthodontic studies of this nature.

This study investigated a surface moistening technique in which the surface of an irreversible hydrocolloid impression material was smoothed using a moistened, gloved finger prior to impression making. To determine the effect of the procedure on the surface integrity of machined brass die, impressions with and without smoothed surfaces were made and examined. Similarly, multiple impressions were made for a single dentate subject, and stone casts were poured. These casts were assessed by three experienced clinicians. Results indicated that the moistened finger technique resulted in fewer surface bubbles and voids than did impressions made when the material was only loaded with a spatula and not smoothed.

This in vitro study examined the effects of environmental pH on elution of potentially toxic substances from heat-, light-, and dual- (chemical plus light) polymerized denture base resins. Eluates were prepared by daily transfer of disks to fresh buffers at pH 4.0, 5.0, and 6.8 over a 5-day period. Oral epithelial cells were plated in culture dishes in medium containing the eluate. After 24 hours, cellular RNA synthesis was assessed by measuring tritiated uridine uptake. Effects of materials were compared to identical cultures that contained the appropriate buffer without the eluate. The results indicate that the cytotoxic components leach out of the denture base resins in different amounts and at different rates, and the amount of leaching can be affected by pH.

Three impression materials, Impregum, Permadyne, and Reprosil, were tested with three restorative materials, Lodestar, Dispersalloy, and Concise. Polished cylindrical samples of each restorative material with and without a collar design were used for impression retention testing. A pullout test using an Instron machine was used to test the samples that were individually submerged in the impression materials until set. Results show similar behavior for Lodestar and Dispersalloy with and without a collar. Permadyne and Impregum were significantly more retentive with resin composite than was Reprosil. Resin composite was significantly more retentive than the metals. Impression materials may bond with restorative materials, significantly increasing resistance to impression removal.

Twenty edentulous patients had unsatisfacory dentures replaced in a student clinic, using conventional techniques, under consultant supervision. Three visual analogue scale records of self-assessed mandibular denture security were made while the dentures were fabricated, and two were made following placement of the new dentures to measure any changes occurring. The method appeared reliable, and significant improvements in security were recorded. The percent increase in security and the coefficient of variation of the preplacement visual analogue scale scores were each inversely proportional to the preplacement visual analogue scale scores. This relationship may be useful in the identification of patients with special problems.

This stdy made an in vitro comparison of the shear strength in a corrosive solution, the scanning electron microscopy of the processing steps, and the x-ray powder diffraction analyses of a newly developed opaque system (Biopaque) with a traditional opaque-system (Opaque P). Clinically, two traditional opaque systems (Opaque P and Vita-VMK-Paint-On 88 opaque) were compared to Biopaque over a 4 year period. A total of 218 metal ceramic units were placed for 29 patients. One hunded ten were veneered with Biopaque, 50 with Opaque P, and 58 with Vita-VMK-Paint-On 88. The technical failure rate of the metal ceramic restorations was 9.6%, and the biological failure rate 5.5%. There was no significant difference of the shear strength in a corrosive solution between Biopaque and the traditional opaque system (Opaque). The x-ray powder diffraction analyses revealed that only the base paste of Biopaque was a newly developed material. In this study, the new opaque system (Biopaque) showed the same in vitro and in vivo results as the other conventional opaque systems. However, at least 5 years of clinical evaluation are necessary before this new opaque system can be completely recommended.

This study evaluated the effect of disinfection methods on the dimensional stability, flexural properties, and microhardness of a heat-polymerized denture acrylic resin. A1- and 12-hour immersion period in 2% alkaline glutaraldehyde, and 3- and 15- minutes exposure to microwaves were employed as disinfection or sterilization procedures. Storage in water for 1 and 12 hours were used as the control. For each procedure 10 specimens were used. The results indicated that the linear changes observed were of no clinical importance. The same conclusion can be drawn for flexural properties and microhardness. The microwave method is a useful alternative to immersion disinfection.

Twenty five extracted human central incisors were divided into five groups nd prepared for complete cast crowns. Test teeth had cast dowel cores fabricated with the ferrule height varying from .5 to 2.0 mm in .5-mm increments. The five control teeth did not have cast dowel cores. A 4.0-kg load was applied to each of the restored teeth at an angle of 135 degrees to the long axis of each tooth. This load was applied cyclically at a rate of 72 cycles per minute. The load application point was predetermined by a waxing jig that was used to wax all crowns. The primary variable was the ferrule length. The independent variable was the number of load cycles required to create preliminary failure. Preliminary failure was defined here as the loss of the sealing cement layer between crown and tooth. An electrical resistance strain gauge was used to provide evidence of preliminary failure. The results of this study showed that the 0.5 mm and 1.0 mm ferrule lengths failed at a significantly lower number of cycles than the 1.5 mm and 2.0 mm ferrule lengths and control teeth.

Thirteen implants having external hexagonal extensions were evaluated for machining accuracy and consistency. A composite score was derived to evaluate overall manufacturing accuracy and consistency of the implants evaluated in this study. A technique to determine the rotational freedom between implant and abutment is described, and the coronal hexagonal matings of selected implants and abutments is evaluated and reported. Crossover use and combinations of products from different manufacturers was also evaluated for rotational freedom. Two nonexternal nexagonal extension implant configurations were measured for rotational movement as contrasting and comparative values.

The shear bond strength of two light-activated resin composite mateials to a preformed heat-polymerized resin material was measured. The substrate was a urethane dimethacrylate material designed for use as a direct bonded pontic. In the first phase of the study, the shear bond strength between the substrate and a urethane-based resin composite and seven surface treatments was evaluated. In the second phase, a Bis-GMA resin composite and three surface treatments were studied. The study showed that a bonding agent was essential for the achievement of an adequate bond strength between the light-activated and heat-polymerized resin. However, there was a significant difference in bond strength depending on the type of light-activated resin composite and bond agent used. A Bis-GMA based light-activated composite, together with its bond system (22.82 MPa), provided higher bond strength than a urethane-based resin composite and its bonding system (14.45 MPa). There was a direct relationship between cohesive failure and bond strength; as bond strength increased, the number of cohesive failures increased as well.

Successful porcelain repair requires conditioning of porcelain surfaces. Conditioning is intended to facilitate wetting by repair materials and improve interfacial bonding. The objective of this investigation was to determine the effects of selected surface treatments upon the wettability of a representative feldspathic procelain. Dynamic contact angle analysis and scanning electron microscopy were used to characterize the effects of such treatments. Standardized porcelain specimens were subjected to the following five treatment regiments: (1) control (no treatment); (2) airborne particle abrasion using 50 um aluminum oxide; (3) etching with ammonium bifluoride gel; (4) etching with acidulated phosphate fluoride gel; and (5) etching with hydrofluoric acid gel. Following treatment, specimens were cleansed and dried. Advancing contact angles were quantified using dynamic contact angle analysis. Mean falues and 95% confidence intervals were (in degrees): control, 63.8±2.7; ammonium bifluoride, 39.4 ±2.0; airborne particle abrading, 29.1±2.9; acidulated phosphate flouride, 24.9 ± 1.7; and hyudrofluoric acid 16.5 ± 1.2. Significant differences were found between all treatment groups (P=.05). Subsequent scanning electron microscopy examination of treated surfaces indicated lesser contact angles were associated with surfaces displaying deeper and wider grooves. Apparently, the resultant increase in surface area produces increased wettability. It is inferred that an increase insurface area may correspond to enhanced resin porcelain bonding.