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

Capsular contracture after cosmetic breast implant surgery in Denmark.

The authors investigated the association between the occurrence of capsular contracture and implant and patient characteristics. All women with breast implants from 1977 to 1997 were identified from the files of two private plastic surgery clinics in Denmark. Information on implant and patient characteristics, surgical procedure, and complications was obtained through medical records and self-administered questionnaires. Of 754 women (1,572 implants), average age at implantation was 32 years. Implant types were silicone double lumen, textured, 31.2%; silicone single lumen, textured, 27.8%; silicone single lumen, smooth, 24.5%; silicone double lumen, smooth, 0.8%; and other or missing, 15.7%. Placement was submuscular for more than 90% of implants. Capsular contracture occurred in 7.9% of implanted breasts, on an average of 621 days postoperatively, with 51.6% being bilateral. Overall, 66.1% of capsular contractures were recorded within the first 12 months postoperatively, and 79.0% were recorded within 24 months. Double-lumen implants were associated with a significantly (p < 0.01) reduced occurrence of capsular contracture. In summary, capsular contracture typically occurs within the first 2 years of implantation. Host factors may be important because more than half the capsular contractures in the current study were bilateral. Occurrence of capsular contracture did not appear to be associated with implant surface or placement, occurrence of local complications, or patient characteristics, although these findings should be interpreted cautiously.

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.

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.

The effect of interfacial failure around a class V composite restoration analysed by the finite element method.

Partial failure around the tooth-composite interface of a class V restoration is common due to the effects of polymerization shrinkage. The effect that this has on the force distribution of the remaining intact interfaces has not been investigated. The aim of this study was to quantify the effect that partial failure of an isolated cavity wall interface had on the force distribution around the remaining intact interfaces of a class V composite restoration in a lower first premolar using a two-dimensional plane strain finite element model. Partial failure resulted in a 4-6-fold increase in peak tensile and shear forces compared to a tooth with a fully intact cavity wall interface. In some instances, the peak stresses were greater than the known bond strengths of composite to dentine.

The effect of curing light intensity and test temperature on the dynamic mechanical properties of two polymer composites.

The dynamic modulus of elasticity of two composite materials cured by three light intensities was investigated using a dynamic thermal and mechanical analyser at 37 degrees C and 60 degrees C. The lowest light intensity, 180 mW/cm2, was found to be unsatisfactory in that specimens were weak and mostly untestable. The higher intensities, 350 and 700 mW/cm2, cured the materials to slightly different extents with the elevated temperatures of testing leading to post-cure maturation of the materials. An interesting finding was that one material had a significantly higher modulus when tested at 37 degrees C in comparison with that found at 60 degrees C. In one material the higher light intensity resulted in significantly higher modulus values and these values were increased by post-curing. In the other material the higher light intensity did not result in higher modulus figures at either test temperature, and lower test temperature resulted in better post-cure maturation. These findings are most likely to be caused by differences in base monomer and activator/initiator concentrations. The results indicate that optimum polymerization may not be achieved in all materials by the same method, in particular, high curing light intensities may not achieve the most desirable results.

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.

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.

Static and dynamic moduli of composite restorative materials.

A series of polymeric composite restorative materials cured by primary and secondary methods was examined by static and dynamic testing. The static modulus of elasticity, dynamic and modulus and glass-transition temperature were determined over a three-month period with the intention of characterizing monomer conversion. Heat and pressure curing of one material gave good conversion, but not all combinations of blue-light irradiation achieved full conversion. The use of a curing oven, whether as the primary or secondary mechanism did not result in a significant improvement in modulus. The use of a dynamic mechanical thermal analyser provided good data and this type of equipment is recommended for the characterization of polymeric materials.

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.

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.

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.

Modelling the effects of enamel anisotropy with the finite element method.

The finite element method was used to model an in vitro tooth loading system. The dentine was modelled as a linear elastic and isotropic material, while the enamel was modelled as an anisotropic material to take account of the biological variation in prism angulation. The elastic modulus values of enamel and dentine used in this analysis were altered in order to replicate the movement of the in vitro system. It was found that a dentine modulus of 15 GPa and an enamel modulus of 80 GPa in the principal prism direction gave the best replication of cuspal movement.

Conversion of dual cure luting cements.

This investigation determined the efficiency and rate of cure of polymeric luting agents when cured under manufacturers' instructions with the appropriate visible light source, and under conditions where light was excluded and curing was reliant on the chemical reaction only. Indentation hardness was used as a reflection of conversion. There was a rapid increase in hardness immediately after light curing followed by a steady increase in hardness over the next 24 h. The non-irradiated chemically cured samples exhibited steadily increasing hardness over 24 h, but were too soft to test in the initial 30 min. Hardness values were extrapolated to obtain predicted values of flexural strength. Dual curing was more effective than chemical curing alone. The results suggest that the formulation of dual cure materials is a balance between high levels of conversion in all aspects of the restoration and colour instability due to amine degradation. Clinically there may be poorly cured areas in the deeper aspects of a deep cavity.

The elastic modulus of dentine determined by static and dynamic methods.

The purpose of this investigation was to determine a static elastic modulus of dentine using a three point beam test and a dynamic modulus in the frequency range of 0.1 Hz and 10 Hz across a temperature range of 27-37 degrees C. At body temperature, the mean static modulus was 8.6 GPa, (standard deviation 0.86 GPa). The dynamic test produced a range of modulus values. At 0.1 Hz the modulus ranged from 14.3 to 15.2 GPa; for 1.0 Hz the range was 14.6-15.5 GPa and for 10 Hz the range was 14.9-15.8 GPa. The results are of value in the design and analysis of restorative materials.

An adaptive finite-element approach for the analysis of dental restorations.

The present paper deals with the problem of stresses generated during the setting process of polymer tooth fillings. A finite element procedure capable of accurately predicting the stresses along an interface between two different materials is presented. Adaptation of the finite element mesh is carried out based on a simple error estimator. The procedure is tested against benchmark problems and then applied to polymer tooth fillings. The results of the study are relevant in understanding and improving the behaviour of dental restorations.

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.

Stresses generated by luting resins during cementation of composite and ceramic inlays.

The present study used the finite element method to model the stresses generated by a composite luting cement around a class I composite restoration and a ceramic inlay. In many cases maximum tensile stresses of greater than 20 MPa were found, and failure of the restoration-dentine and restoration-glass ionomer interfaces was predicted.