Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement

Objectives: To determine, by means of static fracture testing the effect of the tooth preparation design and the elastic modulus of the cement on the structural integrity of the cemented machined ceramic crown-tooth complex. 
Methods: Human maxillary extracted premolar teeth were prepared for all-ceramic crowns using two preparation designs; a standard preparation in accordance with established protocols and a novel design with a flat occlusal design. All-ceramic feldspathic (Vita MK II) crowns were milled for all the preparations using a CAD/CAM system (CEREC-3). The machined all-ceramic crowns were resin bonded to the tooth structure using one of three cements with different elastic moduli: Super-Bond C&B, Rely X Unicem and Panavia F 2.0. The specimens were subjected to compressive force through a 4 mm diameter steel ball at a crosshead speed of 1 mm/min using a universal test machine (Loyds Instrument Model LRX.). The load at the fracture point was recorded for each specimen in Newtons (N). These values were compared to a control group of unprepared/unrestored teeth. 
Results: There was a significant difference between the control group, with higher fracture strength, and the cemented samples regardless of the occlusal design and the type of resin cement. There was no significant difference in mean fracture load between the two designs of occlusal preparation using Super-Bond C&B. For the Rely X Unicem and Panavia F 2.0 cements, the proposed preparation design with a flat occlusal morphology provides a system with increased fracture strength. 
Significance: The proposed novel flat design showed less dependency on the resin cement selection in relation to the fracture strength of the restored tooth. The choice of the cement resin, with respect to its modulus of elasticity, is more important in the anatomic design than in the flat design. © 2013 Academy of Dental Materials.

Effect of the crown design and interface lute parameters on the stress-state of a machined crown-tooth system: A finite element analysis

The effect of preparation design and the physical properties of the interface lute on the restored machined ceramic crown-tooth complex are poorly understood. The aim of this work was to determine, by means of three-dimensional finite element analysis (3D FEA) the effect of the tooth preparation design and the elastic modulus of the cement on the stress state of the cemented machined ceramic crown-tooth complex. The three-dimensional structure of human premolar teeth, restored with adhesively cemented machined ceramic crowns, was digitized with a micro-CT scanner. An accurate, high resolution, digital replica model of a restored tooth was created. Two preparation designs, with different occlusal morphologies, were modeled with cements of 3 different elastic moduli. Interactive medical image processing software (mimics and professional CAD modeling software) was used to create sophisticated digital models that included the supporting structures; periodontal ligament and alveolar bone. The generated models were imported into an FEA software program (hypermesh version 10.0, Altair Engineering Inc.) with all degrees of freedom constrained at the outer surface of the supporting cortical bone of the crown-tooth complex. Five different elastic moduli values were given to the adhesive cement interface 1.8 GPa, 4 GPa, 8 GPa, 18.3 GPa and 40 GPa; the four lower values are representative of currently used cementing lutes and 40 GPa is set as an extreme high value. The stress distribution under simulated applied loads was determined. The preparation design demonstrated an effect on the stress state of the restored tooth system. The cement elastic modulus affected the stress state in the cement and dentin structures but not in the crown, the pulp, the periodontal ligament or the cancellous and cortical bone. The results of this study suggest that both the choice of the preparation design and the cement elastic modulus can affect the stress state within the restored crown-tooth complex.

Biocompatibility of Calcium Phosphate Bone Cement with Optimised Mechanical Properties

The broad aim of this work was to investigate and optimise the properties of calcium phosphate bone cements (CPCs) for use in vertebroplasty to achieve effective primary fixation of spinal fractures. The incorporation of collagen, both bovine and from a marine sponge (Chondrosia reniformis), into a CPC was investigated. The biological properties of the CPC and collagen-CPC composites were assessed in vitro through the use of human bone marrow stromal cells. Cytotoxicity, proliferation and osteoblastic differentiation were evaluated using lactate dehydrogenase, PicoGreen and alkaline phosphatase activity assays respectively. The addition of both types of collagen resulted in an increase in cytotoxicity, albeit not to a clinically relevant level. Cellular proliferation after 1, 7 and 14 days was unchanged. The osteogenic potential of the CPC was reduced through the addition of bovine collagen but remained unchanged in the case of the marine collagen. These findings, coupled with previous work showing that incorporation of marine collagen in this way can improve the physical properties of CPCs, suggest that such a composite may offer an alternative to CPCs in applications where low setting times and higher mechanical stability are important.

Critical evaluation of pulse-echo ultrasonic test method for the determination of setting and mechanical properties of acrylic bone cement: Influence of mixing technique

Currently there is no reliable objective method to quantify the setting properties of acrylic bone cements within an operating theatre environment. Ultrasonic technology can be used to determine the acoustic properties of the polymerising bone cement, which are linked to material properties and provide indications of the physical and chemical changes occurring within the cement. The focus of this study was the critical evaluation of pulse-echo ultrasonic test method in determining the setting and mechanical properties of three different acrylic bone cement when prepared under atmospheric and vacuum mixing conditions. Results indicated that the ultrasonic pulse-echo technique provided a highly reproducible and accurate method of monitoring the polymerisation reaction and indicating the principal setting parameters when compared to ISO 5833 standard, irrespective of the acrylic bone cement or mixing method used. However, applying the same test method to predict the final mechanical properties of acrylic bone cement did not prove a wholly accurate approach. Inhomogeneities within the cement microstructure and specimen geometry were found to have a significant influence on mechanical property predictions. Consideration of all the results suggests that the non-invasive and non-destructive pulse-echo ultrasonic test method is an effective and reliable method for following the full polymerisation reaction of acrylic bone cement in real-time and then determining the setting properties within a surgical theatre environment. However the application of similar technology for predicting the final mechanical properties of acrylic bone cement on a consistent basis may prove difficult.

Optimisation of a two-liquid component pre-filled acrylic bone cement system: A design of experiments approach to optimise cement final properties

The initial composition of acrylic bone cement along with the mixing and delivery technique used can influence its final properties and therefore its clinical success in vivo. The polymerisation of acrylic bone cement is complex with a number of processes happening simultaneously. Acrylic bone cement mixing and delivery systems have undergone several design changes in their advancement, although the cement constituents themselves have remained unchanged since they were first used. This study was conducted to determine the factors that had the greatest effect on the final properties of acrylic bone cement using a pre-filled bone cement mixing and delivery system. A design of experiments (DoE) approach was used to determine the impact of the factors associated with this mixing and delivery method on the final properties of the cement produced. The DoE illustrated that all factors present within this study had a significant impact on the final properties of the cement. An optimum cement composition was hypothesised and tested. This optimum recipe produced cement with final mechanical and thermal properties within the clinical guidelines and stated by ISO 5833 (International Standard Organisation (ISO), International standard 5833: implants for surgery—acrylic resin cements, 2002), however the low setting times observed would not be clinically viable and could result in complications during the surgical technique. As a result further development would be required to improve the setting time of the cement in order for it to be deemed suitable for use in total joint replacement surgery.

Sustainable concrete: design and testing

Producing concrete with secondary raw materials is an excellent way to contribute to a moresustainable world, provided that this concrete has at least the same performance during itsservice life as concrete made with the primary raw materials it replaces. Secondary rawmaterials for Light Weight (LW) aggregates (rigid polyurethane foams, shredded tire rubberand mixed plastic scraps) have been combined with secondary raw materials for the binder(fly ash, slag and perlite tailings) making sustainable concretes that were investigated fortheir suitability as LW, highly insulating concrete for four different types of applications.Compliance to desired engineering properties (workability, setting time) was not alwaysfeasible: it was mostly the low workability of the mixtures that limited their application.Contrary to well established cements, steering the workability by adding water was not anoption for these binders that rely on alkali-activation. Eight successful mixtures have beentested further. The results have shown that it is possible to produce a non-structuralsustainable concrete with good mechanical and thermal insulation properties.Design of concrete made with novel materials is currently not feasible without extensiveexperimentation as no design rules exist other than empirically derived rules based ontraditional materials. As a radical different approach, a flexible concrete mix design has beendeveloped with which the concrete can be modelled in the fresh and hardened state. Thenumerical concrete mix design method proves a promising tool in designing concrete forperformance demands such as elasticity parameters and thermal conductivity

'Heat-sensing' TRP channel expression in human odontoblasts

Background: Thermal changes in the oral cavity are a common trigger of dental pain. Several members of the transient receptor potential (TRP) super family of ion channels are believed to play a critical role in sensory physiology, where they act as transducers for thermal, mechanical and chemical stimuli. Objectives: The present study was designed to determine the expression and functionality of the TRPV1 channel in human odontoblasts. Methods: Cultured human odontoblasts were derived from dental pulp cells induced with 2 mM beta-glycerophosphate. Molecular and protein expression of TRPV1 was confirmed by PCR, western blotting and immunohistochemistry. Functional expression of the ‘heat-sensing' TRPV1 channel was investigated using a Ca2+ microfluorimetry assay in the presence of agonists/antagonists or with appropriate adjustment of the recording chamber temperature. Results: The odontoblastic phenotype of the cells was confirmed by the expression of the odontoblast markers dentin sialophosphoprotein (DSPP) and nestin. Expression of TRPV1 in human odontoblastic cells was confirmed by PCR, western blotting and immunohistochemistry. Odontoblasts were shown to respond to pharmacological agonists and to increasing temperature by an increase in intracellular Ca2+. Both the pharmacological and temperature responses could be blocked by specific antagonists. These results indicate that odontoblasts may sense heat via TRPV1. Conclusion: This study reports that TRPV1 is expressed by human odontoblasts and is activated by specific pharmacological agonists and by heat.
This work was supported by Research Grants from the Royal College of Surgeons of Edinburgh and the British Endodontic Society