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Biomedical subjects

R L Clarke

Publications and source records attributed to R L Clarke.

At least 55 records · Page 3Linked to original sources
PubMed1997

The transverse strength, reliability and microstructural features of four dental ceramics--Part I.

OBJECTIVES: The flexural strength, reliability and microstructure of a heat pressed ceramic system (Empress) was evaluated in comparison with three current frit materials used in ceramic crown and inlay construction. METHODS: Twenty bar specimens per material were constructed, overglazed and transverse tested at a crosshead speed of 0.5 mm/min. Etched and fractured bar specimens were viewed under a scanning electron microscope using secondary electron imaging. RESULTS: Mean strengths (MPa +/- SD) were: Empress, 117.3 +/- 31.7; Cerinate porcelain, 118.2 +/- 8.7; Corum porcetain, 92.9 +/- 13.3; and Alpha porcelain, 60.9 +/- 10.1. Results were highly significant (P < 0.001) with differences between all groups (P < 0.05) except Cerinate porcelain and Empress (P > 0.05). Weibull analysis of the results provided m-values and the predicted strength at the 1 and 5% probabilities of failure. Microstructural examination using secondary electron imaging showed leucite crystals 1.6-3.5 microns and associated microcracking in Corum porcelain and leucite crystal clusters (1.5 microns) in Empress. Cerinate porcelain exhibited a uniform distribution of fine leucite crystals (1 micron). Spherical porosity combined with crack pore combinations were noted in Vita Alpha porcelain. CONCLUSIONS: Empress glass ceramic was not stronger than Cerinate or more reliable than many of the frit materials. Uniform leucite distribution and fine crystal size may be associated with improved strength and reliability.

Acid Etching, Dental↗
PubMed1997

The biaxial flexural strength and reliability of four dental ceramics--Part II.

OBJECTIVES: The biaxial flexural strength and reliability of four dental ceramics including: Empress glass ceramic (EM), Cerinate porcelain (CE), Corum porcelain (CO) and Alpha porcelain (AL) were compared. METHODS: Twenty disc specimens per material were prepared overglazed and tested. The piston on three ball test was used to test the specimens in a universal testing machine at a crosshead speed of 0.15 mm/min. Weibull analysis provided m-values and the 1 and 5% probabilities of failure. RESULTS: Mean strengths (MPa +/- SD) were: EM, 133.5 +/- 21.5; CE, 109.1 +/- 11.3; CO, 119.8 +/- 19.2; and AL, 68.2 +/- 9.9. There was no statistical difference between EM or CO and CO and CE strengths (P > 0.05). Weibull m-values included: EM, 6.60; CE, 10.20; CO, 5.27; and AL, 6.93. Cerinate had the highest m-value (P < 0.01) and good dependability. CONCLUSION: Empress was not stronger or more reliable than many of the frit materials.

Aluminum Silicates↗
PubMed1997

Temperature rise recorded during lesion formation by high-intensity focused ultrasound.

Temperature rise was observed as a function of time in liver and dog prostate tissue ex vivo during heating with high-intensity focused ultrasound. The temperature rise was measured using a needle thermocouple placed at the focus. The temperature vs. time behaviour closely followed the predictions of a model based on bulk and surface heating. When the tissue temperature was raised above 50 degrees C, an increase in heating rate was seen. At higher temperatures, a point was reached at which a marked, irreversible change of tissue properties was observed, consistent with protein denaturation. The change was sometimes accompanied by a sudden further rise in temperature followed by an equally sudden fall. On dissection, regions of tissue damage (lesions) were seen, sometimes containing bubbles consistent with acoustic cavitation or vaporisation.

Animals↗
PubMed1997

Heterocyclic methacrylates for clinical applications-further studies of water sorption.

The room temperature polymerizing system comprising poly(ethyl methacrylate)-tetra hydrofurfuryl methacrylate (PEM/THFMA) has potential in orthopaedic and dental applications, and earlier work has shown it to have unusual water absorption characteristics. This aspect has been studied in further detail, by studying the water absorption behaviour from some biological solutions, and the effect of the addition of an antibiotic (gentamicin). For comparison purposes, a parallel system whereby tetrahydrofuryl methacrylate was replaced by hydroxyethyl methacrylate (PEM/HEMA), was studied. In the case of PEM/THFMA, water uptake was substantially reduced when absorption was carried out from solutions (from about 30% in water to about 1.5% in solutions of higher concentrations), and the corresponding diffusion coefficient increased (by a factor of several hundred). The addition of gentamicin increased uptake, but the extent of increase also decreased in solutions. It was concluded that uptake was related to the osmolarity of the external solution, and also on the presence of osmotic sites within the polymer; hence the uptake process appears to be governed by chemical potential considerations. At the higher uptakes, there was evidence of water clusters. In marked contrast, the uptake by the PEM/HEMA system was independent of the osmolarity of the external solutions, presumably due to the hydrophilic nature of HEMA.

Journal Article↗
PubMed1997

Synthesis of BISGMA derivatives, properties of their polymers and composites.

2,2-Bis[4-(2 hydroxy-3-methacryloyloxy propoxy) phenyl] propane (BisGMA) is commonly the main component of the organic matrix of dental filling materials. Derivatives of BisGMA were synthesized from the diglycidyl ether of Bisphenol A (DGEBA) by the parallel reaction with methacrylic acid (MAA) and isophthalic acid as well as mixtures of methacrylic anhydride with palmitic acid and acetic anhydride, respectively, whereby MAA was partially substituted by the latter components. By this technique the structure of BisGMA monomer could be varied with regard to weight content of C=C double bonds, the hydrophilicity of the molecule as well as its flexibility or stiffness. Free-radical initiated homopolymerization of the monomers was carried out at 80 degrees C. Composites, prepared from mixtures of monomers with triethylene glycol dimethacrylate (TEGDMA) filled with 76% silica were room temperature polymerized using both redox and photoinitiated techniques. The polymerization shrinkage, diffusion coefficients of water in the crosslinked polymer, and some thermal properties of the homopolymers were determined. Mechanical properties of the resulting polymers and composites are compared to those of BisGMA itself.

Journal Article↗
PubMed1997

"I am not a crook!".

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Financial Management↗
PubMed1996

THFMA in dental monomer systems.

This study investigates the physical properties of a range of dental resins containing up to 30% by weight of tetrahydrofurfuryl methacrylate (THFMA). The principal monomer was 2,2-bis-(4(2-hydroxy-3-methacryloyloxypropoxy)-phenyl)-propane (bis-GMA) or 1,6-bis-(methacryloxy-2-ethoxycarbony lamino)-2,4,4-trimethyl-hexane (UDMA). Heat-cured resins were tested for Young's modulus, flexural strength, refractive index, polymerization shrinkage and degree of conversion. The results indicated that THFMA has negligible effects on the mechanical properties of bis-GMA and UDMA and on the refractive index of UDMA, whilst the other measured properties present a strong relationship with the concentration of the additive. The dental relevance of the results is discussed and it is concluded that there is scope for further research on the use of THFMA as a comonomer in resin-based materials for dental applications.

Acrylic Resins↗
PubMed1995

Modification of intensity distributions from large aperture ultrasound sources.

The Fresnel-Kirchhoff diffraction theory has been used to model the acoustic fields produced by a variety of apodizations of large aperture, axially symmetric ultrasound sources. The forms of resulting intensity maxima and their resulting suitability for surgical lesioning of tissues are compared, through a "lesioning capability" and a "focal efficiency." Although only modest changes in intensity distribution are possible, some may have applications in making arrays of lesions for control of focal cancer and other pathologies.

Acoustics↗
PubMed1995

What about me?

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Contract Services↗
PubMed1995

Managed care and the business of risk management.

The emergence of managed care creates the need for a new paradigm of risk management because of its emphasis on comprehensive services and prospective payment. Information systems, contracts and pricing, utilization management, and outcome research are key elements in devising the new paradigm.

Capitation Fee↗
PubMed1994

Behavioral sensitivity of rats to extremely-low-frequency magnetic fields.

Work in our laboratory has revealed autonomic and/or behavioral sensitivity of mice, rats, and a domestic fowl to extremely-low-frequency (ELF) or nominally static magnetic (B) fields at flux densities between 250 and 1700 microT (rms). To extend our work, an automated exposure and data-acquisition system was used with the technique of conditional suppression to assess behavioral sensitivity to time-varying B fields. Each of five rats was exposed aperiodically to a B field during 3 min warning periods that terminated in a brief electric shock. The difference between rates of lever pressing during B-field warning periods and rates during immediately antecedent, 3 min control periods was analyzed at frequencies of 7, 16, 30, 60, and 65.1 Hz. To produce equivalent induced voltages in the rat at each frequency, graded flux densities were established that ranged from 1900 microT at 7 Hz to 200 microT at 65.1 Hz. Analysis of differences in lever-pressing rates revealed that in a given session of testing the rats would increasingly suppress responding when exposed to a B field, but this trend was independent of frequency. This experiment provides evidence of behavioral sensitivity by a mammal to an ELF magnetic field.

Analysis of Variance↗