The effect of artificial saliva on the rheological properties of tooth whitening systems.

This work was undertaken to explore the effect of saliva addition on the rheological properties of two contrasting tooth bleaching systems, one of which was a paste (Colgate Platinum) and the other a gel (Zaris, 3M ESPE). Using a dynamic stress rheometer with cone and plate geometry, it was shown that addition of artificial saliva reduced the apparent viscosity of each material. However, in some cases this was accompanied by an increase in elasticity. It is suggested that saliva may not have a deleterious effect on the ability of the materials to remain in the bleaching tray.

A clinical pilot study of the time-dependent composition of tooth bleaching systems.

This pilot study was undertaken to determine the compositional changes in tooth bleaching materials as a function of time in vivo. Ten patients were recruited and two bleaching systems were used - one a paste and the other a gel. Each material was placed in a custom bleaching tray and worn by each patient for each of four times - 15, 30, 60 and 120 min. The material was collected and chemically analysed for water by Karl Fischer titration and titrated for carbamide peroxide by the US Pharmacopoeia method. The paste material contained 18.66% water as supplied, and after 2 h this rose to between 28.6 and 64.4%. The gel material contained 2.85% water as supplied, and after 2 h this was diluted to between 28.5 and 73.4%. There was considerable difference in saliva uptake by the custom tray between patients. Most water uptake usually occurred within the first 30 min. Peroxide concentrations decreased in an approximately linear manner with time. There was a significant difference between the materials from baseline to 30 min and thereafter (P < 0.0009). This pilot study is an effective technique for chemical evaluation of bleaching materials. The effect of saliva is an important factor to consider, and is one that has hitherto not always been appropriately emphasized.

Rheological properties of vinyl polysiloxane impression pastes.

OBJECTIVES: Some vinyl polysiloxane impression (VPS) materials are claimed to be 'thixotropic' so that they will have 'non-drip' properties and not flow off an impression tray during insertion of the material into the mouth. This work was carried out to investigate rheological properties of VPS materials by determination of their flow properties under different experimental conditions relevant to their dental application. METHODS: The rheological characteristics of each component paste of two low viscosity VPS materials were assessed by a Dynamic Stress Rheometer using cone and plate geometry. RESULTS: Both pastes of one product (Imprint II) showed a yield stress of around 40 Pa in addition to some degree of true thixotropy. In contrast, both Examix pastes had no yield stress value, but the catalyst paste was markedly thixotropic, as shown by a drop in viscosity as a result of application of shear force over time. Both products had thixotropic properties. The Imprint II in addition showed a yield stress that would also contribute to the 'non-drip' properties. SIGNIFICANCE: Combination of yield stress and thixotropy will prevent the undesirable drip of the materials once it has been injected around the tooth preparation until the impression tray is loaded and seated. After seating the tray, a thixotropic material will not flow.

Development of a nondestructive compliance test for resilient denture liners.

PURPOSE: Resilient denture liners are prescribed for patients who cannot adjust to hard-based dentures because of a thin mucosa or severe alveolar ridge resorption. A nondestructive test to evaluate compliance of new soft liner materials will be useful in clinical trials. The purpose of this study was to evaluate a nondestructive compliance testing technique designed to characterize long-term, silicone-based resilient denture liner materials. MATERIALS AND METHODS: Samples of thicknesses of 1.1, 2.2, 3.3, and 4.4 mm of 2 materials (MPDS-SL [Lai Laboratories, Inc, Burnsville, MN] and Molloplast-B [Buffalo Dental, New York, NY]) were assessed for compliance using a closed-loop servohydraulic testing system, applying a 3 lb force following a squarewave pattern; force and position values were recorded using a storage oscilloscope. The oscilloscope values were analyzed using computer software to determine compliance values. The effect of material thickness was examined by testing wedges of the 2 materials. RESULTS: The testing technique used showed that differing thicknesses had significantly different compliance values (p <.0001). In the materials used to evaluate the technique, MPDS-SL behaved more elastically than did Molloplast-B (p <.0001). Material thicknesses beyond 2.2 mm did not increase compliance, although MPDS-SL had a steeper thickness-compliance curve than Molloplast-B. CONCLUSIONS: The method used to test compliance proved to be sufficiently sensitive to distinguish between 2 materials and between varying thicknesses. The sensitivity and nondestructive nature of this test show its suitability for clinical evaluation of resilient denture liners.

Operator-induced compressive axial forces during implant gold screw fastening.

STATEMENT OF PROBLEM: The gold screw of an implant is put under tension during fastening. An increase in operator-induced compressive axial force during fastening may diminish screw tension, lower the friction between the screw threads, and allow for increased tightening torque. PURPOSE: This study was undertaken to assess and compare the compressive axial forces and torques placed simultaneously on implant gold screws by persons with varying degrees of expertise. MATERIAL AND METHODS: A calibrated electric torque driver was used to fasten implant gold screws. Three groups of operators with various levels of implant experience (faculty [F; n = 4], prosthodontic residents [R; n = 4], and undergraduate dental students [S; n = 6]) were asked to repeatedly tighten and loosen a new gold abutment screw into a standard 3.75-mm diameter Branemark abutment. Compressive axial forces during torquing were assessed over the tightening time by means of a miniature load cell adapted to the electric torque driver. Each operator repeated the experiment 3 times after calibration. Within operator and between operator reliability were evaluated. RESULTS: Loosening compressive axial forces were always higher than tightening compressive axial forces, and peak torque was less on loosening than tightening. Faculty placed a smaller range of forces on the screws (mean = 3.29 N, SD +/- 1.45 N) than did the residents (mean = 2.74 N, SD +/- 1.96) or the students (mean = 3.01 N, SD +/- 2.54). CONCLUSION: The clinical experience of operators seems to influence their application of compressive axial force during gold screw tightening. Less torque during unfastening of gold screws seems to be related to increased axial loading.

In vivo evaluation of the surface of posterior resin composite restorations: a pilot study.

STATEMENT OF PROBLEM: Several methods have been used to determine the surface characteristics of resin composites in vivo and compare composite wear rates with enamel wear rates. PURPOSE: This pilot study describes the surface characteristics of resin composites and the wear of resin composites and enamel during 1 year of in vivo service. MATERIAL AND METHODS: A single Class II posterior resin composite restoration (Z100) was placed in 10 patients. Restored teeth and unrestored adjacent control teeth were measured for wear 4 times within the first year. A null point contact stylus profiler and fitting software were used to measure epoxy casts. Maximum depth of wear, average depth of wear, and characteristics of the restoration margin were determined. Paired t tests were used to compare the control and restored teeth, and ANOVA was used to assess the progression of wear over time (P<.05). RESULTS: After 1 year, maximum depth of wear over the entire preparation region was on average 204.8 microm (+/- 129.8), significantly greater than the 36. 8 microm (+/- 10.1) average maximum depth of wear of enamel at occlusal contact areas on control teeth (P=.009). Maximum depth of wear progressed over time (P=.009). Fracture of excess composite, commonly called flash fracture, occurred in 50% of the restored teeth extending over the preparation margin. CONCLUSION: Composite restorations wore significantly faster than enamel contact areas on control teeth. Also of concern were the marginal flash fractures that could facilitate secondary caries.

Rheological characteristics of tooth bleaching materials.

Tooth bleaching materials need to flow easily on insertion but should have high viscosity at low stresses to stay in place on the teeth. Some degree of elasticity may also aid retention on the teeth thereby maximizing efficacy. The present work was undertaken to study the comparative rheology of three tooth bleaching systems: two gels (Opalescence, Ultradent; Perfecta Trio, American Dental Hygienics) and a paste (Colgate Platinum, Colgate). A dynamic stress rheometer (Rheometrics Scientific) with cone and plate geometry was used, with the materials maintained at 37.0+/-0.1 degrees C with a vapour hood to minimize volatilization. Stress creep and recovery experiments were carried out. Steady shear viscosity for all three systems was high (>10(6) Pa s(-1)) for stresses <20 Pa. Between 100 and 200 Pa stress, all three materials showed a large drop in viscosity and flowed readily. The recovery portion of the data showed a marked difference where the elasticity of the gels was nearly two orders of magnitude higher than that of the paste. It was concluded that all materials would flow readily on insertion into the mouth and all have desirable high viscosity at low stress, but the paste material had the lowest elasticity. The effect of elasticity on performance needs to be determined clinically.

Histologic evaluation of the periodontium of abutment teeth in combination implant/tooth fixed partial denture.

The histologic response of the periodontal tissues of teeth rigidly joined to implants with a fixed partial denture was evaluated using light microscopy. The fourth premolar of a dog was connected to implants placed in the first and second premolar position with a fixed partial denture. The restored teeth were under function for periods of 6, 12, 18, and 24 months, with unrestored fourth premolars as controls. The histology of the periodontal ligament on the fourth premolar was found to be similar in the control and the restored teeth. The periodontal tissues contained a minimal amount of inflammatory cell infiltrate. The crestal bone was cortical in nature, showing no periodontal breakdown. The orientation of the periodontal fibers was easily determined, indicating that minimal remodeling had taken place. The number and morphology of the blood vessels were also similar in the control and the treated teeth. The lack of inflammation and stability of the periodontal tissue suggested that the use of combination implant-to-natural-teeth restorations with rigid joints in this animal model does not result in deleterious effects on the periodontal tissues and that the forces placed on the tissues are within the remodeling capabilities of the teeth.

Troubleshooting vinyl polysiloxane impressions.

For consistent results in impressions and restorations, the following four rules make a handy checklist: 1. Exclude as many variables as possible from your technique. 2. Review materials and techniques on a routine basis. 3. Discuss with laboratory technicians what they observe as they work with your impressions. 4. Pursue the satisfaction of keeping your skills at their highest level.

Judson C. Hickey Scientific Writing Award Winner. Fabrication of a guide for nonradiographic evaluation of bone contour.

The determination of bony contours at the site of implant placement aids the surgeon and the restoring dentist in treatment planning a patient's care. Although radiographic evaluations that use cross-sectional views are helpful, they are often expensive and not available in most dentists' offices. This article describes a technique for the fabrication of a guide for nonradiographic evaluation of bone contour. Bone contours are probed and transferred to a sectioned cast by the use of a guide.

Intrusion of teeth in the combination implant-to-natural-tooth fixed partial denture: a review of the theories.

This article reviews the literature dealing with the combination implant-to-natural-tooth-supported fixed partial denture. The restoration of masticatory function with a combination implant-to-natural-tooth fixed partial denture is associated with a variety of undesirable clinical sequelae, including the breakage of implant components, damage to the abutment teeth, or intrusion of the abutment teeth. Theories regarding intrusion of abutment teeth combined with implants for fixed partial dentures are only speculative. Several theories are presented to explain the intrusion of natural teeth in association with implant-to-natural-tooth fixed partial dentures. One of the first theories was based on the idea that a lack of normal stimulation of the periodontal ligament produces atrophy of the periodontal ligament and intrusion of the tooth. The remaining theories relate to excessive forces being placed on the natural tooth, resulting in movement of the tooth to a less stressful position. These forces are placed on the tooth by differential energy dissipation, mandibular flexion and torsion, flexion of the fixed partial denture framework, impaired rebound memory, debris impaction or microjamming, or ratchet effect related to the use of precision attachments. Based a review of the literature, a philosophy for treating combination implant-to-natural-tooth restorations is presented.

Fabrication of a guide for radiographic evaluation and surgical placement of implants.

The use of guides for radiographic evaluation and surgical placement of dental implants can improve the final outcome of treatment for patients receiving implants. This article describes a technique in which a guide is fabricated for radiographic evaluation of implant placement and also serves as a surgical guide for placement of the implants for a patient with severely worn dentition.

The role of the restorative dentist in the diagnostic and maintenance phases of implant therapy--Part I: Patient selection.

The objective of patient selection is to ensure that the patient's need and desire for dental treatment is consistent with the clinical findings and the clinician's ability to provide treatment. Considerable effort is required to gather and analyze the data necessary to determine whether implant therapy is feasible. Additional planning is required to determine if this is the best treatment modality. All aspects of dental treatment need to be considered and planned before the implant phase of treatment is contemplated. Our experience indicates that the potential need, and indeed necessity, for conventional dental treatment in patients seeking implant therapy is high. The general practitioner is the logical resource to fulfil the dental needs of the majority of these patients.

The role of the restorative dentist in the diagnosis and maintenance phases of implant therapy--Part II: Prosthetic planning.

Prosthetic planning is critical to the success of the diagnostic phase of implant therapy and is used to establish fixture numbers, position and the optimal occlusal scheme, based on the design of the definitive prosthesis. The prosthetic needs of the patient, anticipated functional and parafunctional forces, restoration fabrication and serviceability are all considered. A clear picture of the intended rehabilitation is provided to the surgical team and expedites the consultation between implant team members. Comprehensive diagnostic planning using mounted diagnostic casts and diagnostic wax-ups will provide definite parameters for fixture number, position and alignment, and permit the fabrication of radiographic and surgical templates as the treatment sequence progresses. Patients need to be informed of the time and expense involved in the diagnostic phase of implant therapy prior to the initiation of the planning process. The practitioner conducting the selection and planning phase is entitled and should expect financial remuneration commensurate with the time and effort required to achieve the objectives of this phase of implant therapy. The Planning Process Using mounted diagnostic casts and diagnostic wax-ups: 1. Establish fixture numbers and position; 2. Establish the optimal occlusal scheme; 3. Design the definitive prosthesis; 4. Fabricate a surgical template. Design Principles--Summary 1. The location and number of fixtures dictates the prosthetic design and influences restorative success. 2. A restoration must be designed to distribute forces to the fixtures without overloading the bone, and therefore be able to withstand anticipated functional and parafunctional loading. 3. Design for optimal function and esthetics with a regard for patient comfort and maintenance.