Accuracy of casts generated from dual-arch impressions.

STATEMENT OF PROBLEM: Dual-arch trays are often used to make simultaneous impressions of a prepared tooth and the opposing teeth. Many dentists are concerned with the accuracy of the casts generated from this type of impression. PURPOSE: The purpose of this investigation was to compare the accuracy of stone casts of a prepared tooth generated using 2 types of dual-arch impression tray/impression material combinations. MATERIAL AND METHODS: The maxillary left first molar on a dentoform mounted on an articulator was prepared for a full-coverage gold crown. Ten impressions were made with either a plastic (P) or metal (M) tray and a polyether (PE) or vinyl polysiloxane (VPS) impression material. Each impression was cast in improved dental stone, and the buccolingual dimension of the die was measured at the midpoint of the buccal and lingual gingival margins. The prepared tooth (T) served as the control. The data were analyzed using a 2-factor analysis of variance (ANOVA) alpha=.05. RESULTS: The P/VPS combination (10.673 mm) produced the largest die, followed by P/PE (10.602), T (10.508), M/PE (10.484), and M/VPS (10.472). The 2-factor ANOVA showed a significant difference between the tray types but not between the impression materials. CONCLUSION: The metal trays produced dies smaller than the tooth, and the plastic trays produced dies that were larger.

An in vitro study of implant--tooth-supported connections using a robot test system.

Many unsolved problems in dental implant research concern the interfacial stress distributions between the implant components, as well as between the implant surface and contacting bone. To obtain a mechanical understanding of how vertical and horizontal occlusal forces are distributed in this context, it is crucial to develop in vitro testing systems to measure the force transmission between dental implants and attached prostheses. A new approach to such testing, involving a robotic system, is described in this investigation. The system has been designed to produce simulated mandibular movements and occlusal contact forces so that various implant designs and procedures can be thoroughly tested and evaluated before animal testing or human clinical trials. Two commonly used fixed prosthesis designs used to connect an implant and a tooth, a rigid connection and a nonrigid connection, were fabricated and used for experimental verification. The displacement and force distributions generated during simulated chewing activities were measured in vitro. Force levels, potentially harmful to human bone surrounding the connected dental implant and tooth, were analyzed. These results are useful in the design of prostheses and connecting components that will reduce failures and limit stress transfer to the implant/bone interface.

Tensile bond strengths of provisional luting agents used with an implant system.

STATEMENT OF PROBLEM: Retrievability of cemented implant-supported fixed prostheses is desirable. Retentive strengths of new provisional luting agents have not been reported. PURPOSE: This study compared the tensile bond strengths of 6 provisional luting agents when used with cemented superstructures and 1 implant system. MATERIAL AND METHODS: Ten castings were fabricated and randomly paired with abutment specimens. Castings were cemented and the assemblies were stored in distilled water at 37 degrees C for 48 hours. Tensile bond strength necessary to remove each casting was measured with a 500-kg load and a crosshead speed of 0.5 cm/min. Crown/abutment specimens were cleaned after testing, and the testing procedure was repeated for a total of 6 luting agents (n = 10). Data were analyzed with 1-way analysis of variance and Scheffé's analysis (alpha=.05). RESULTS: Tensile bond strengths ranged between 1.29 and 4.08 MPa. The lowest tensile bond strengths were found to be statistically similar between Temp Bond and Provilink luting agents. Neo-Temp luting agent exhibited the highest tensile bond strength (P <.01). CONCLUSIONS: Temp Bond and Provilink luting agents exhibited the lowest mean tensile bond strengths. Neo-Temp luting agent exhibited a tensile bond strength more than 3 times that of Temp Bond luting agent.

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.

In vitro mechanical property comparison of four resins used for fabrication of provisional fixed restorations.

PURPOSE: This study recorded and compared the flexural elastic moduli and moduli of rupture of four materials (Provipont DC resin, Triad provisional restorative material, Jet acrylic resin, and a 50:50 mixture of Jet acrylic resin and orthodontic resin) used to make provisional restorations. MATERIAL AND METHODS: Thirty-nine identical 63 x 10 x 3 mm specimens were made from each of the four materials. After 24 hours, 30 days, and 60 days of water storage at 37 degrees C (13 specimens each), standard three-point bend tests were conducted on an Instron universal testing machine at a crosshead speed of 0.5 cm/minute. Stress strain curves were generated, and values for the flexural elastic moduli and moduli of rupture were calculated. Data were subjected to two-way and one-way analyses of variance (alpha = 0.05). RESULTS: Provipont DC resin exhibited significantly higher flexural elastic moduli and moduli of rupture values at the 24-hour test time. However, Provipont DC resin exhibited the greatest decrease in these values over time.

Shear bond strengths of a two-paste system resin luting agent used to bond alloys to enamel.

STATEMENT OF PROBLEM: Adhesive resin luting agents are used to successfully bond the metal surfaces of fixed prostheses to teeth. Panavia 21 luting agent is a new addition to the series of Panavia adhesive resin luting agents. PURPOSE: This investigation measured the shear bond strengths of two types of alloy specimens (Olympia and Rexillium III) bonded to prepared human enamel (etched and unetched) with Panavia 21. MATERIAL AND METHODS: After a simulated porcelain firing sequence, the alloy specimens were bonded to the teeth and subjected to shear testing after water storage for 2 weeks, thermocycling for 500 cycles, and water storage for an additional 2 weeks. Data were analyzed with a two-way analysis of variance (alpha = 0.05). RESULTS: The shear bond strengths of Rexillium III and tinplated Olympia specimens bonded to prepared and unetched enamel were significantly lower than for specimens bonded to prepared and etched enamel with Panavia 21 luting agent.

The effect of metal surface treatment on the shear bond strengths of base and noble metals bonded to enamel.

Adhesive resin luting agents provide a way for bonding metal surfaces to teeth through a combination of micromechanical retention to the rough metal surface and chemical adherence to metal oxides. The purpose of this study was to measure the effect of metal alloy surface treatments that would produce different textures and oxide layers on the shear bond strength of three alloys luted to etched enamel with one adhesive resin luting agent (Panavia). After a simulated porcelain firing sequence, high noble (Olympia), noble (Jelstar), and base metal (Rexillium III) alloy specimens were subjected to one of the following treatments: (1) sandblasting and simulated glazing, (2) simulated glazing only, (3) simulated glazing and sandblasting, or (4) simulated glazing, sandblasting, and tin plating. The specimens were bonded to extracted teeth and subjected to shear testing after water storage for 2 weeks, thermocycling for 500 cycles, and water storage for an additional 2 weeks. Data were analyzed with a two-way analysis of variance (alpha = 0.05). The base metal specimens and the tin-plated high noble and noble metal specimen groups exhibited similar mean shear bond strengths that were greater than the other groups. Those high noble and noble metal alloys sandblasted after simulated porcelain firing cycles and before the simulated glaze cycle exhibited nonsignificant increases in shear bond strengths compared with the groups that were either sandblasted after the simulated glaze cycle or not sandblasted at all.

Surgical guide fabrication for an angled implant.

This article describes the steps necessary for determining ideal implant abutment position and, subsequently, the optimum implant location that would make this abutment position possible. Procedures necessary for fabricating a surgical guide are explained for placing an ITI angled implant.

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.

Transfer of gingival contours to a master cast.

This procedure provides a means for transferring intraorally formed gingival sulcular contours surrounding a single-tooth implant-supported provisional restoration to a master cast. When this procedure is used, the final fixed prosthesis may be made in the laboratory with gingival contours identical to those developed on the provisional restoration.

Mechanical considerations for the implant tooth-supported fixed partial denture.

The implant tooth-supported fixed partial denture presents a biomechanical design problem, because the implant is rigidly fixed within the alveolus, and the tooth is surrounded by a periodontal ligament that allows movement. Nonrigid fixed partial denture designs are advocated by some dentists as a method of compensating for this differential movement. Rigid fixed partial denture designs, however, are advocated by many clinicians. Studies have failed to show the advantage of one design over the other. This study developed an in vitro method for testing such prosthesis designs and measured movement of a natural tooth abutment during simulated function. The movement of the natural tooth abutment was not found to change substantially with the fixed partial denture designs tested.

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.

A bonded provisional fixed prosthesis to be worn after implant surgery.

The procedure described provides an esthetic means for inserting a provisional fixed restoration immediately after implant placement surgery. When this procedure is used, it is not necessary to prepare the abutment teeth adjacent to the surgical site for subsequent fixed restorations. This provisional restorative procedure may also be used immediately after extraction of teeth, when abutment tooth preparation is not desired or possible, or after stage II implant surgery when healing caps are inserted.

Mechanical properties of a light-polymerizing provisional restorative material with and without reinforcement fibers.

Interim restorations are vulnerable to inadvertent fracture during mastication. Autopolymerizing acrylic resins have traditionally been selected for fabrication of provisional restorations. Triad light-polymerizing tooth-colored acrylic resin was recently introduced as an alternative material for this procedure. This material does not contain methyl methacrylate monomer and permits an increased working time. Heavy occlusal forces may initiate cracks within these restorations, and propagation of these cracks may ultimately lead to failure. Various forms of reinforcement fibers are available and are marketed for strengthening dental resins used for provisional restorations. Investigators have demonstrated that the mechanical properties of acrylic resins may be improved with the incorporation of reinforcing fibers, but a published evaluation of fiber-reinforced light-polymerizing provisional restorative materials is lacking. This investigation recorded and compared two mechanical properties of one light-polymerizing provisional restorative material with and without incorporation of vertically and horizontally oriented woven, matted, polyethylene fibers. No significant difference in modulus of rupture was recorded between groups with and without fibers. The mean flexural elastic modulus of the group with the horizontally oriented fibers was significantly greater than the mean flexural elastic modulus of the specimens without incorporated fibers.

Comparison of shear bond strengths of two resin luting systems for a base and a high noble metal alloy bonded to enamel.

Researchers are investigating the use of noble metals for the fabrication of resin-bonded prostheses because of concerns about health hazards of nickel and beryllium in base metal alloys. Tin-plating has been advocated to improve the bond of resin luting agents to noble metal alloys. Some manufacturers have suggested that tin-plating is unnecessary to bond noble metal alloys to etched enamel with their products. In this study, Rexillium base metal and Olympia noble metal alloy specimens were bonded to extracted human teeth with the use of two resin luting agents (F21 and Panavia OP). One third of the noble metal specimens were tin-plated, one third were oxidized, and one third were oxidized and sandblasted. Each of the bonded specimens were thermocycled and subjected to a shear force until bond failure. The base metal specimens bonded with Panavia OP luting agent exhibited the greatest mean shear bond strengths. The tin-plating surface treatment significantly increased the mean shear bond strengths of Olympia noble metal specimens.

Use of light-polymerizing restorative materials in diagnostic cast modification procedures.

During the treatment-planning phase and before provisional restorations are made for the partially edentulous patient, it is often necessary to complete diagnostic cast modification procedures. Traditionally wax has been used for this purpose; therefore, it is necessary to duplicate the modified cast before fabrication of a vacuum-formed matrix. The cast duplication procedure can be eliminated by the use of light-polymerizing provisional restorative materials for cast modification instead of wax. Diagnostic casts for two patients are presented in this article, where techniques for using such a light-polymerizing material are described.

Custom impression trays. Part III: A stress distribution model.

All resins used to make custom impression trays exhibit plastic deformation at some force value; therefore it is important to compare the physical property values of such materials with the stresses to which impression trays are subjected during dental procedures. A simple mathematical model of a custom tray was developed to predict stress distributions in this final part of a three-part investigation. Experimental stress analysis of such a tray confirmed the accuracy of the model, which was then used to predict the maximum stress experienced by the tray during removal of a completed impression from the oral cavity. The results of this analysis indicated that these stresses would be significantly lower than the yield stress for a commonly used polymethyl methacrylate resin or a light-polymerized resin. The stresses were also sufficiently low for us to conclude that thermoplastic resins would not permanently deform; however, the stresses encountered in the experimental confirmation procedure were close to the yield stress values for these materials.

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.

Custom impression trays. Part II: Removal forces.

When choosing a material for making custom impression trays, it is important to understand the forces to which the tray will be subjected during removal of the completed impression from the oral cavity. Such forces have not been recorded in the dental literature. The purpose of Part II of this three-part series was to record these forces in vitro, using two different tray-removal methods. A polymethyl methacrylate custom tray was used during this study. Results from this investigation indicated that it is easier to remove a completed impression, made with a custom tray, by a single point of anterior force application (224 N) than by force application evenly around the tray (514 N). The recorded force values from this investigation will be used in Part III of this series.

The effect of custom tray material type and adhesive drying time on the tensile bond strength of an impression material/adhesive system.

The purpose of this investigation was to compare the tensile bond strength of one poly(vinyl siloxane) impression material/adhesive system to three different custom tray materials 10 minutes, 48 hours, and 7 days after adhesive application. One autopolymerizing (Fastray) and two light-polymerizing (Triad and Extoral) custom tray resins were examined. The Triad tray resin exhibited the significantly highest mean adhesive tensile bond strengths, and the Fastray resin exhibited the lowest. It was also observed that the 48-hour adhesive drying time group exhibited the highest mean adhesive tensile bond strengths for all the materials tested, and the 10-minute group exhibited the lowest.