Larry C. Breeding

Comparison of screw loosening, rotation, and deflection among three implant designs*

A common problem associated with single tooth implant restorations is abutment screw loosening. Manufacturers of implants have attempted to overcome this problem by incorporating antirotational design characteristics into their systems. Micromovement and torque levels required to loosen abutment screws for straight and angled antirotational screw-retained abutment/implant combinations from three different manufacturers were examined in this in vitro investigation. A custom-built machine was used and each sample was subjected to compressive horizontal reciprocal movements over a 25-degree incline for a simulated 1-month period. Data were generated that showed movements of the crown/abutment complex during force application. The amount of torque necessary to loosen the abutment screws before and after testing was also recorded and compared for each system. The results indicated no significant differences (p < 0.05) among all the straight and angled abutments for the variables studied.

Microwave disinfection of denture base materials colonized with Candida albicans

STATEMENT OF PROBLEM Infection of denture materials with Candida albicans is common and contributes to denture stomatitis. PURPOSE This 3-phase investigation examined: (1) the efficacy of microwave irradiation against C albicans colonized on 3 soft denture liners and 1 heat-polymerized denture base resin, and (2) the effect of this irradiation on the hardness of the materials tested. MATERIAL AND METHODS In phase 1, an experimental protocol was developed. Sterilized specimens from 2 denture base soft liners and 1 heat-polymerized acrylic resin denture base material (n = 45 each) were inoculated with C albicans. Two thirds of the specimens were irradiated in a 60 Hz microwave oven for 5 minutes (dry). C albicans growth was then assessed with streaked blood agar plates and thioglycollate broth. One third of the specimens were not irradiated and served as controls. Pretest and posttest Shore A hardness values were obtained and compared. For phase 2, 15 specimens from each material group were subjected to irradiation (while immersed in water) for 5 minutes; and, 15 from each material were subjected to 10- and 15-minute irradiation (dry), with subsequent sterility and change in hardness assessments completed as described in phase 1. In phase 3, 15 specimens from each material group were subjected to repeated 5-minute irradiation cycles (while immersed in water), and changes in hardness were examined. RESULTS Only the 5-minute irradiated specimens immersed in water were effectively sterilized, as verified by the thioglycollate assay. The effect of repeated 5-minute irradiation cycles resulted in a significant change in hardness of the PermaSoft specimens. CONCLUSIONS Five-minute irradiation, while immersed in water, killed all C albicans present on the materials tested; and, repeated 5-minute irradiation significantly affected the hardness of only the PermaSoft material.

Fatigue life of three core materials under simulated chewing conditions

There has been an increase in the use of prefabricated post systems to restore endodontically treated teeth. Various restorative materials are being used as core buildups on these posts. The purpose of this study was to compare three core materials that are used with prefabricated stainless steel posts. Two types of prefabricated posts were placed in extracted teeth, followed by core buildups in amalgam, composite resin, or glass ionomer. The teeth were prepared for full cast crowns with the margins of the crown preparation extending 0.5 to 1.0 mm below the margins of the core buildup. Crowns were fabricated and cemented with zinc phosphate cement. A custom-designed chewing machine was used to cyclically load the teeth with vertical and horizontal forces for one million cycles or until failure occurred. Results indicated highly significant differences in the survival of the post-core-crown restorations depending on which core buildup material was used. Amalgam cores had the lowest failure rate, followed by composite resin cores. All teeth restored with crowns over glass-ionomer core buildup failed. The type of prefabricated post used had no effect on the survival of the post-core-crown restorations regardless of the core buildup used.

Ablation of right and left ectopic atrial tachycardias using a three-dimensional nonfluoroscopic mapping system

We report our experience with mapping and ablation of right and left atrial tachycardia using a 3-dimensional nonfluoroscopic mapping system. Twenty-nine ectopic atrial tachycardias were successfully ablated. This novel mapping system has the potential to increase a successful cure of this arrhythmia by catheter ablation.

The curing potential of light-activated composite resin luting agents

This study determined the effect of different light exposure times on the polymerization of three composite resins used as cements (Heliosit, Visio-Fil, and Dual) through various thicknesses and shades of two preprocessed composite resins (Isosit and Visio-Gem). The Knoop hardness of the surface of the luting resin was measured after polymerization and selected samples were remeasured at 24 hours to evaluate continued cure. Results indicated that polymerization of light-activated composite resin luting agents cannot predictably be accomplished through a restoration exceeding 2 mm in thickness using a light exposure of 90 seconds or less. The combined chemical and light-activated cure of Dual cement recorded the highest percentage of acceptable hardness values.

The transverse strengths of three denture base resins reinforced with polyethylene fibers

This investigation compared the transverse strengths of three denture base resins with and without polyethylene reinforcement fibers. These fibers were incorporated in 10 specimens each of a high-impact strength resin, a rapid heat-polymerized resin, and a light-activated resin. Each specimen was broken on an Instron universal testing machine using a three-point load after processing and air drying. The values obtained were then compared with values previously recorded for the same type of specimens without incorporated fibers. Only the light-activated denture base resin specimens exhibited a significantly higher mean transverse strength after polyethylene fiber reinforcement.

Linear dimensional variability of three denture base resins after processing and in water storage

It has been shown that denture base resins shrink during curing and expand during storage in water. This investigation measured and compared linear dimensional changes of three denture base resins that occurred during processing and after storage in water for 30, 60, and 90 days. Triad, Accelar 20, and Lucitone 199 long- and short-cured resins were studied, and it was found that Accelar 20 resin exhibited the least curing shrinkage and Lucitone 199 resin short-cured exhibited the most. However, no significant differences occurred between the groups. After 90 days of water storage, the only resin that exhibited a shrinkage from the processed state was Accelar 20. All of the expansion or shrinkage changes were so small that they were not statistically significant and should not be clinically detectable.

The transverse strengths of three denture base resins

Ten specimens each of (1) Lucitone 199, short- and long-cured, (2) Accelar 20, and (3) Triad materials were broken using a 3-point load on an Instron Universal testing machine after processing and air drying. Five specimens of each resin also were broken after storage in deionized distilled water at 37 degrees C for 30, 60, and 90 days. Triad material demonstrated the lowest transverse strengths of all three materials overall. However, Triad material was unaffected by water storage. The other resins all showed decreased strengths with water storage.

The effect of simulated function on the retention of bar-clip retained removable prostheses

Patients wearing bar-clip retained removable prostheses may have loss of retention because of changes within the bar-clip assembly. This in vitro study recorded and compared the retention of one- and two-clip retained simulated mandibular complete denture prostheses before and after simulated function. Cast metal Hader bars and clip holders were used to make 10 one-clip and 10 two-clip specimen pairs. Tensile removal values before and after simulated function were recorded and compared by repeated-measures analysis of variance and Student tau tests (significance level 0.05). The results revealed that the use of two clips instead of one significantly increased retention of the simulated prosthesis. It was also found that there was a significant loss of retention after the specimens were placed on the bars and then removed once for both the one- and two-clip groups. Simulated function did not cause a significant change in retention for either group.

Accuracy of three interocclusal recording materials used to mount a working cast

When remaining occlusal contacts of a dental posterior quadrant have been prepared to receive complete or partial coverage restorations, it is necessary to use an interocclusal recording material to approximate the casts for laboratory mounting procedures. Inaccuracy in working cast approximation leads to occlusal discrepancies in the fabricated restorations that require correction by the dentist at the time of insertion. Traditionally waxes, impression compounds, plasters, and zinc oxide-eugenol pastes have been used intraorally to record maxillomandibular relationships. Elastomeric materials have also been advocated. Manufacturers of thermoplastic resins have recently suggested that these materials may be used with ease and accuracy for interocclusal records. This investigation measured and compared the accuracies of a thermoplastic resin, an acrylic resin, and a vinyl polysiloxane interocclusal recording material. A measuring method was developed in which a computerized axiograph was used to record positional errors in three planes, next to the posterior teeth, with the interocclusal recording materials listed.