Load transfer characteristics of unilateral distal extension removable partial dentures with polyacetal resin supporting components.

BACKGROUND: To photoelastically examine load transfer by unilateral distal extension removable partial dentures with supporting and retentive components made of the lower stiffness polyacetal resins. METHODS: A mandibular photoelastic model, with edentulous space distal to the right second premolar and missing the left first molar, was constructed to determine the load transmission characteristics of a unilateral distal extension base removable partial denture. Individual simulants were used for tooth structure, periodontal ligament, and alveolar bone. Three designs were fabricated: a major connector and clasps made from polyacetal resin, a metal framework as the major connector with polyacetal resin clasp and denture base, and a traditional metal framework I-bar removable partial denture. Simulated posterior bilateral and unilateral occlusal loads were applied to the removable partial dentures. RESULTS: Under bilateral and left side unilateral loading, the highest stress was observed adjacent to the left side posterior teeth with the polyacetal removable partial denture. The lowest stress was seen with the traditional metal framework. Unilateral loads on the right edentulous region produced similar distributed stress under the denture base with all three designs but a somewhat higher intensity with the polyacetal framework. CONCLUSIONS: The polyacetal resin removable partial denture concentrated the highest stresses to the abutment and the bone. The traditional metal framework I-bar removable partial denture most equitably distributed force. The hybrid design that combined a metal framework and polyacetal clasp and denture base may be a viable alternative when aesthetics are of primary concern.

Analysis of stress patterns in the human mandible.

Replicas of a dentate human mandible were prepared in photoelastic resin material. They were positioned in a supporting frame by means of struts, representing the principal muscles of mastication. Occlusal loading was simulated, and the stresses generated within the models were examined by three-dimensional photoelastic stress analysis.

Loading rate and temperature as variables in amalgam bending.

Three commercial dental amalgams of known dissimilar clinical properties were evaluated in pure blending at widely different loading rates and temperatures. Comparative data of fracture stress at 140 F rank these alloys according to their clinical fracture resistance; the phenomenon of creep may account for early marginal fracture prior to corrosion-induced fracture.

Effects of adhesion promoters on resin-enamel retention.

The adhesive effects of three adhesion promoters were compared utilizing three different filled resins on acid-etched human permanent teeth. The results demonstrated a significantly greater resistance to dislodgement of resins by shear forces when an NPG-GMA type of adhesion promoter was utilized. Type of resin did not influence resin retention.

Influence of a ferric chloride mordant solution on resin-dentin retention.

The effects of ferric chloride mordant solution, two cleansing solutions, and three chemical adhesion promoters were examined on resin-dentin adhesion. Adhesion was increased with the use of ferric chloride solution. An NPG-GMA or butyl acrylate-acrylic acid copolymer type of adhesion promoter also increased resistance to shear separation.

Fracture toughness of human enamel.

A microindentation technique was employed to estimate the fracture toughness of human enamel. A pattern of increasing fracture toughness values existed from incisal to cervical in incisor enamel. Statistical analysis indicated that the molar enamel was more brittle than either the canine or incisor enamel. It was observed that the cracks emanating from the corners of an indentation propagated preferentially, with the weakest path of fracture usually extending along the cervical-incisal axis.

A flexural method for evaluation of metal-ceramic bond strengths.

A test technique is presented for evaluating metal-ceramic bond strengths. The test piece consists of a four-point loaded metal beam with porcelain bonded to the central tension side of the beam. The test produced failures at predictable locations and was able to differentiate between metal surface treatments.

Effects of cement type and thickness on retention of serrated pins.

The retentive capacity of cemented pins was determined. Zinc phosphate, polycarboxylate, and cyanoacrylate were used to cement threaded pins (0.024 inch) into precise channels of 0.025, 0.026, and 0.027 inch. Highest retentive values were realized when zinc phosphate and polycarboxylate cements were used with the smallest pin-channel mismatch. Ethyl cyanoacrylate was least retentive and demonstrated no mismatch dependency.

Stress trajectories within the mandible under occlusal loads.

A human mandible was duplicated in birefringent resin. The distribution and effect of occlusal forces within the mandible was demonstrated utilizing birefringent resin models compared to natural specimens. The results of this study reinforce those theories that assign function a major role in bone remodeling in the mandible. Further, the condyle is seen to be a stress-bearing structure.

Evaluation of ultrasonic and sonic instruments for intraradicular post removal.

Ultrasonic instruments are a valuable asset for removing intraradicular posts from root canals before nonsurgical endodontic therapy. The purpose of this study was to evaluate the relative efficacy of post removal by ultrasonic and sonic devices. Fifty extracted teeth were instrumented and obturated. The canals were prepared and Parapost #5 posts were cemented with zinc phosphate cement. The teeth were divided into five groups, subjected to ultrasonic or sonic instrumentation and the time measured until post removal. Median values in minutes for the post extractions in each group were: Cavitron = 6.0, Enac = 8.3, and Neosonic = 41.2. Sonic instruments were generally unable to remove the posts. The results of this experiment indicate that the Cavitron and Enac ultrasonic units are significantly more efficient for removing posts than the Neosonic. Sonic instruments were not useful for post removal.

Critical analysis of the balanced force technique in endodontics.

The "balanced force" technique was introduced in 1985 and it was reported that the resistance of the dentin, as it circumferentially contacted the flutes of a file in a curved root canal, would be sufficient to override and mask any tendency of the file to straighten during rotational instrumentation. An alternative mechanism based on compressive force and file flexure is presented in this article. In a laboratory study, the apical force necessary to cause files of sizes #10 to #70 to bend and conform to an average canal curvature was determined. These measurements were then related to the apical forces applied to teeth when utilizing the Balanced Force technique in a simulated clinical setting. It was then proposed that the apically directed force necessary to prevent coronal movement of the file and to effect dentinal shearing during counterclockwise rotation placed the file in compression, flexing it to conform to the curvature of the canal. This explanation was consistent with the experimental data for the files used in this study up to size #60 for an average canal curvature and for average instrumentation forces.

A comparison of athletic mouthguard materials.

The development of protective mouthguards resulted from extensive injuries during participation in contact sports. This has produced a dramatic decrease in the incidence of injuries to the orofacial complex. Numerous materials and techniques have been used for mouthguard fabrication. The purpose of this investigation was to make an in vivo comparison of various thermoplastic mouthguard materials. Mouthguards were fabricated upon maxillary models of 40 members of the UCLA football team. The mouthguards were made of the following materials: poly (vinyl acetate-ethylene) copolymer clear thermoplastic; polyurethane; and laminated thermoplastic. Eleven measurements were taken for each mouthguard to demonstrate dimensional changes that occurred with wear. Statistical analyses determined differences between the types of mouthguards. The clear thermoplastic revealed less dimensional change than the polyurethane material. The laminated thermoplastic showed significantly less dimensional change than the other materials tested.

Accuracy of an electric torque-limiting device for implants.

The torque output of the Nobelpharma electric torque controller was determined using a special setup on an Instron test machine. The devices were held in a vice and oriented so that activation of the drivers caused a pure torsion effect. Measured torque levels generally varied significantly between individual devices. Further, significant differences generally existed between the nominal torque levels and the corresponding measured values. Torque application errors varied from target values by 1% to 165%. For the clinician to deliver appropriate torque levels, these data indicate that a simple means for chairside calibration and adjustment should be available.

Passivity of fit and marginal opening in screw- or cement-retained implant fixed partial denture designs.

The relationship of stress generation upon placement of cement-retained or screw-retained implant restorations has not been thoroughly investigated. Passivity of fit and marginal discrepancies of screw- and cement-retained implant fixed partial denture (FPD) designs were determined using a photoelastic model of a partially edentulous posterior mandibular arch with 3 screw-type implants. Buccal and lingual marginal openings, measured with a traveling microscope before cementation or screw tightening, revealed no statistical difference in adaptation between designs. Screw tightening caused a reduction in marginal opening (changes significant, P < .05). The opening with the cemented FPDs was similar before and after cementation. Photoelastic evaluation of the FPDs showed that cement-retained FPDs exhibited a more equitable stress distribution than did their screw-retained counterparts.

Moduli of rupture of layered dental ceramics.

OBJECTIVE: The purpose of this study was to determine the moduli of rupture of layered beams made of strong core materials veneered with weaker conventional feldspathic porcelain. METHODS: A three-point flexural test, and formulae derived especially for this purpose were used. Two systems were investigated. The first consisted of Vitadur N (Vita Zahnfabrik), a conventional feldspathic porcelain; and Dicor MGC (L.D. Caulk), a machinable glass ceramic. The second consisted of Vitadur N and In-Ceram (Vita Zahnfabrik), a strong reinforced aluminous porcelain. RESULTS: Two-way ANOVA and Tukey tests were used to evaluate the effect of numerous factors on the modulus of rupture. The results indicated that the material forming the tensile surface, the material forming the compressive surface and their interaction had a highly significant effect on modulus of rupture (p < 0.05). The effect of the material forming the tensile surface on modulus of rupture was of much greater magnitude than that of the compressive surface. Theoretical curves describing effects of the layers' elastic moduli and thickness on the force-bearing capacity of model beams were plotted. These indicated that for a wide range of thickness ratios and for a wide variety of elastic modulus ratios, the tensile material dominates the force-bearing capacity of layered beams except when a very much lower modulus material forms the compressive layer. SIGNIFICANCE: Layered prostheses made of strong cores veneered with weaker feldspathic porcelain may be prone to failure when the feldspathic surfaces are subjected to tensile force.

Torsion-axial force characteristics of SR-PLLA screws.

The torsion axial-force characteristics of biodegradable screws are central to their ability to generate interfragmentary compression when used as lag screws. The purpose of this investigation was to compare the torsion-axial force characteristics of prototype self-reinforced poly-L-lactide (SR-PLLA) screws with conventional titanium screws. Axial forces developed by incremental increases in the torque applied to the individual screws were measured in a test apparatus incorporating an Instron machine. For the SR-PLLA screws, the relationship between applied torque and axial force development was non-linear with a marked relaxation throughout the test range. The axial forces reached a maximum with increasing torque, after which failure of the screws occurred. The response curve for titanium screws of the same length demonstrated a steeper slope. No failures or force relaxation were observed with the titanium screws. The results of this investigation suggest that, at this time, the use of SR-PLLA screws for lag screw fixation should be restricted to low stress bearing areas.

An investigation of bonding systems for the polycarbonate crown restoration.

Tensile retentive strengths for six polycarbonate crown cementation systems were examined with use of extracted human teeth prepared with and without retentive grooves. From the results of this study, use of unfilled intermediary resins followed by polycarboxylate or zinc phosphate cements is recommended. Retentive grooves did not contribute to overall retention.

Effects of force direction on supporting bone during tooth movement.

Photoelastic stress analysis by means of simulated load application on model structures was used to visualize the forces that are induced within the supporting bone structures during various tooth movements. A comprehensive picture of the stress patterns produced by these forces, in addition to the degree of stress distribution, was obtained. The results of this study are relevant to the basic tooth movement procedutes that are used by the clinician in general practice.

Effect of application technique and dentin bonding agent interaction on shear bond strength.

This study evaluated the interaction of five clinical application techniques and the shear bond strength of four DBAs (OptiBond FL, Clearfil SE Bond, PQ1 and Prime & Bond NT). A hybrid resin composite (Herculite XRV restorative resin) was attached to human dentin surfaces using five application techniques: Group A--adhesive spread with a 3M brush for 30 seconds, followed by compressed air 0.5 cm from the surface for one second to remove the excess adhesive. Group B--adhesive spread with a 3M brush for 30 seconds, followed by compressed air 0.5 cm from the surface for three seconds to remove the excess adhesive. Group C--adhesive spread with 3M brush for 30 seconds, excess adhesive removed with a clean brush, two strokes side by side, no compressed air. Group D--adhesive spread with a Micro-applicator brush for 30 seconds followed by compressed air 0.5 cm from the surface for one second to remove the excess adhesive. Group E--adhesive spread with a Micro-applicator brush for 30 seconds, the excess adhesive removed with a clean brush, two strokes side by side and no compressed air. The specimens were stored in distilled water at 37 degrees C for 24 hours, followed by thermocycling between 5 degrees C and 55 degrees C for 1,000 cycles. The shear bond strengths were determined on a universal testing machine operating with a crosshead speed of 5 mm/minute. The fracture sites were examined by 20x stereo microscope to determine the type of failure that occurred during the debonding procedure. Bond strength data were compared with analysis of variance at a significance level of p<0.05. Post hoc comparisons of means were performed with t-tests with p-values adjusted for multiple comparisons. This in vitro study concluded that there was an interaction between the application technique and bonding agent tested. All DBAs utilized the one-second compressed air technique, which yielded the highest bond strengths.

Biological implications of dental materials.

Testing of the biologic effects of dental materials has lagged far behind the characterization of their mechanical and physical properties. Various biologic-type tests, whether in vitro or in vivo, have often produced conflicting or confusing data. The objective of this article was to bring together the results of various investigations to form guidelines for the safest use of dental materials. Since many interactions between dental materials and living tissue have not been definitively established, a conservative approach was taken. The clinician who heeds the data summarized for the dental materials presented here will have the greatest probability of obtaining a benign chemical response of soft and hard tissues.