The effect of preparation height and taper on cement lute stress: a three-dimensional finite element analysis.

Three dimensional finite element models of an upper second premolar and molar with full veneer gold crown preparations were developed from extracted samples. The cement lute width was kept constant at 40 microm, but the height and preparation taper were varied. For both models the preparation height was either 1.5 mm (short preparation) or 3 mm (long preparation). The preparation taper was either 10 degree or 30 degree, giving a total of eight models. Each model was loaded with a 10 N horizontal load, a 10 N vertical load or a 10 N load distributed across the occlusal surface. The maximum shear stress and the maximum Von Mises' stress in the cement lute of each model were recorded. For the premolar, the maximum shear stresses ranged from 0.3-5.43 MPa and the maximum Von Mises' stress ranged from 1.44-14.98 MPa. For the molar, the maximum shear stresses ranged from 0.15-5.22 MPa and the maximum Von Mises' stress ranged from 0.3 7-15.02 MPa. The stress fields were consistently higher in the premolar with a 30 degree preparation taper compared to the 10 degree taper. The attainment of a cavity taper of 100 is still important to minimise stress in the cement lute and is particularly important in teeth with a lower preparation surface area such as a premolar

Development of cytomegalovirus (CMV) disease may be predicted in HIV-infected patients by CMV polymerase chain reaction and the antigenemia test.

OBJECTIVE: Cytomegalovirus (CMV) is a frequent opportunistic viral pathogen in patients with AIDS leading to retinitis and other serious manifestations. CMV disease may be successfully treated. Prophylactic antiviral therapy has been shown to reduce the risk of CMV disease if initiated early. We evaluated PCR and the antigenemia tests as methods for early detection of CMV disease. METHODS: Two-hundred HIV-seropositive subjects with CD4 T-cell counts below 100 x 10(6)/l were monitored with CMV polymerase chain reaction (PCR), the antigenemia test, blood cultures and CMV immunoglobulin (Ig) G and IgM titres every second month for 1 year. RESULTS: Thirty-eight patients (19%) developed CMV disease. The PCR test detected CMV DNA a median of 46 days before onset of disease. This was earlier than the median of 34 for the antigenemia test and a median of 1 day for CMV blood cultures. Univariate analysis showed that the CMV PCR, the antigenemia test and blood cultures all had significant predictive values for subsequent development of CMV disease with odds ratios (OR) of 30, 22 and 20. CMV serology had no predictive value. Multivariate analysis showed that the PCR method was superior to the other tests; OR: CMV PCR 10.0, antigenemia test 4.4 and CMV cultures 4.3. No clinical parameters had any significant predictive value in the stepwise multivariate model. CONCLUSIONS: The CMV PCR and the CMV antigenemia tests are both sensitive methods that may predict development of CMV disease up to several months prior to clinical disease. These methods make it possible to select patients at high risk for CMV disease and suitable for prophylactic therapy against CMV.

Elastic modulus of the periodontal ligament.

This study used a two-dimensional finite element mesh of a lower first premolar to model two different tooth loading systems which measured either the vertical or the horizontal displacements of this particular tooth. The elastic modulus of the periodontal ligament was varied in the finite element model until the horizontal and vertical displacements of the model correlated with the two experimental systems. It was found that an elastic modulus of 50 MPa gave good correlation between the finite element model and the experimental systems.

The influence of the restoration-tooth interface in light cured composite restorations: a finite element analysis.

This study examines the early shrinkage behaviour of dental composite resins, and in particular the interfacial stresses around the margins of a composite restoration. The development of stresses at the restoration-tooth interface can have a detrimental effect on the longevity of a restoration. The influence of this interface on the stress system generated in the tooth was examined using finite element analysis. The restoration-tooth interface was simulated using spring elements of varying spring constants (k = 1, 10(2), 10(4), 10(10) N/mm). Interfacial stresses varied from -0.15 to 0.42 MPa for a spring constant of 1 N/mm, and from -19 to 68 MPa for a spring constant of 10(10) N/mm. Correlations between stiffness at the restoration-tooth interface and higher shrinkage stresses due to restricted shrinkage were found. Interfacial failure at the upper and lower regions of the interface, as well as cuspal movements of the order of 2 microm were predicated for the model of the highest spring constant, 10(10) N/mm. The restoration-tooth interface modelled by the spring elements was seen to have a conclusive effect on the ensuing stress system, as well as the longevity of the restoration.

Working time of polymeric restorative materials.

The validity of determining the working times of polymeric restorative materials with the use of rheologic methods in the laboratory has been investigated by measuring the adaptation of polymeric restorative materials to artificial cavities when inserted at 30-second time intervals during their setting reactions. There was significant deterioration in adaptation during the working times which suggests that small changes in viscosity early in the setting reactions radically affect the materials' mobility.

A non-destructive method of evaluating the elastic properties of anterior restorative materials.

An ultrasonic pulse transit-time method was used to determine the Young's modulus, shear modulus, and Poisson's ratio of seven anterior-restorative materials. Small rectangular specimens were immersed in a water bath and ultrasonic pulses propagated through them. Two types of ultrasonic wave could be generated inside the specimen by altering the angle of incidence. Measurement of the pulse transit-time for these two waves enabled their velocities and, hence, the material's elastic constants to be calculated. The variation of these properties with time of conditioning and temperature was also determined.

Dynamic mechanical properties of resin-based filling materials.

The elastic moduli and mechanical damping of a range of polymeric restorative materials have been determined using a torsion pendulum. Composite materials have high moduli and low damping compared with unfilled resin. These materials have stable mechanical properties, but variation occurred as a result of different specimen conditioning methods. Non-destructive testing was valuable in that it allowed repeated testing of a single specimen.

Assessment of microleakage using a conductimetric technique.

A new method enables the dimensional changes of a range of anterior restorative materials to be demonstrated in an artificial cavity. The cavity wall-restoration interspace was incorporated into an electrochemical cell and the changes in the current passing through this cell reflected changes in the dimensions of the interspace.

The three-point beam test for the evaluation of dental porcelain.

The three-point beam test is simple and extremely sensitive to surface conditions. It appears to be ideally suited to the evaluation of dental porcelain, but specimen fabrication is difficult. In this study, specimens of porcelain fired on platinum foil have been constructed using an alumina former. The results showed excellent specimen consistency and reproducibility. Of the surfaces investigated, the use of tin-oxide-coated platinum foil did not result in an increase in transverse strength.

The polymerization shrinkage of composite resins.

The majority of methods used for the determination of volumetric polymerization shrinkage rely on mercury as the immersion medium. With current concern for industrial hygiene, the water-filled dilatometer may have advantages. The purpose of this study was to adapt this method for external-energy-cured materials. Six materials were investigated: Aurafill, Silar, Concise, Heliomolar, P-30, and Occlusin. Measurements were taken over one hour, and results ranged from 0.9 to 2.24 volume percent.

The effect of cuspal flexure on a buccal Class V restoration: a finite element study.

OBJECTIVES: Failure of Class V restorations is a clinical occurrence which is often blamed on inadequate moisture control. However, the effects of occlusal forces and cuspal movement may also have an effect. The aim of this study was to examine the effects that cuspal movement had on the shear forces around a buccal Class V restoration in a lower first premolar. METHOD: A lower first premolar restored with a buccal Class V composite restoration and a Class I composite or amalgam restoration were modelled using the finite element method. The width of the occlusal restoration was varied from 2.1 to 3.7 mm and the depth varied from 1.7 to 3.7 mm. RESULTS: Compared to the unrestored premolar, the presence of an occlusal preparation restored with composite increased the interfacial forces around the buccal Class V restoration by 1-67%. Similarly, the presence of an amalgam occlusal restoration increased the interfacial forces by 9-228%. CONCLUSIONS: It was found that the presence of an occlusal restoration increased cuspal movement, which in turn increased the shear forces around the buccal Class V cavity. This effect was more pronounced with increases in cavity depth compared to cavity width, and when amalgam was the occlusal restorative material.

Fatigue and wear of dental materials.

Fatigue and wear are two of the many basic physical processes which contribute to the degradation of restorative materials. The mechanisms of these are discussed in the context of general engineering and the dental environment. More work is required to define the processes involved in intraoral degradation of materials.

Luting agents for ceramic and polymeric inlays and onlays.

The requirements for luting agents for ceramic and polymeric inlays and onlays are reviewed and discussed. The current overriding consideration seems to be the chemical adhesion of the restoration to tooth substance and this is challenged as being an overambitious goal. The systems in use are principally the dual cure resin based cements and these have some disadvantages such as restricted and slow cure, chemical degradation and, most importantly, shrinkage on setting. This shrinkage, when the material is used in conjunction with a dentine bonding agent, is almost certain to guarantee the rupture of any bond formed. It is suggested that the objectives of the luting agent should be rethought with reference back to the well established principles of cast metal restorations.

Polymerization shrinkage of dental composite resins.

Aesthetic materials have always been a priority for the restoration of anterior teeth; increasingly, they have also gained prominence in the restoration of posterior teeth. This has been due to their advantages as an alternative to amalgam. Their drawbacks however, can include poor marginal adaptation, postoperative sensitivity and cuspal movement. These are particularly associated with the polymerization shrinkage accompanying the placement of composite resins. Consequently, a variety of methods have been used for determining the polymerization shrinkage. These range from dilatometer methods, specific gravity methods and deflecting disc systems to optical methods. In this work a unique method for the qualitative analysis of polymerization shrinkage was undertaken. This method utilized a miniature transducer and provided details of the shrinkage from within the material. The results indicated movement of material towards the initiating light, followed by a return movement away from it. The study was expanded to incorporate clinical aspects, whereby the composite resin was in direct contact with dental tissue, as in a restoration. Tests were performed with surface-treated cavity moulds, as in restoration placement, and without surface treatment. Results indicated that the shrinkage was highly dependent upon the region under investigation, as well as on the state of the surface.

Restoration of posterior teeth with composite resin. 1: Direct-placement composite.

Composite restorative materials were introduced to the dental profession in the early 1960s and quickly became the principal anterior restorative material. Following improvements in composite technology, these materials were also developed as aesthetic posterior materials but were initially hampered by problems of poor wear resistance. In this, the first of two articles on the uses of composite replacement materials, the development of composite resins are discussed and the problems of using direct-placement composite to restore posterior teeth are examined in detail. The second article will review the use of the composite inlay technique.

The restoration of posterior teeth with composite Resin. 2: Indirect-placement composite.

The first article of this series described the problems of restoring posterior teeth with direct-placement composite. This second article discusses the advantages and disadvantages of the composite inlay technique and examines the clinical provision of these restorations.