Biomedical subjects
R L Clarke
Publications and source records attributed to R L Clarke.
HIPAA hype.
Explore the source record for details and available documents.
The water uptake of poly(tetrahydrofurfuryl methacrylate).
Poly(tetrahydrofurfuryl methacrylate) possesses some unique characteristics with respect to its biocompatibility and behaviour in water. The water uptake is high (>70%) and very slow (over 3 yr), but the material remains rigid throughout the process. The mechanism behind the uptake is in two stages; an initial Fickian stage, then as the matrix approaches saturation (about 3 wt%) a second clustering mechanism takes over. The rate of uptake of the second stage of the uptake is controlled by creep (or stress relaxation), and the chemical potential driving the uptake from clustering of the furfuryl rings of the polymer. If clustering or the creep is prevented (by appropriate co-polymerisation) the polymer behaves in an ideal, Fickian manner.
Viscoelastic properties of some soft lining materials. I--Effect of temperature.
The time dependent, viscoelastic characteristics of a number of dental soft liners (occasionally called resilient liners) were investigated using a dynamic mechanical analyser (DMA). The materials consisted of eight commercial products including methacrylate, silicone and phosphazine-based soft polymers as well as two experimental formulations. The DMA used was of the forced vibrational variety. The test conditions, such as frequency and temperature, were chosen to mimic in use (masticatory) conditions. A wide variety of behaviour was observed and accounted for. This included a seven fold range in the storage modulus (2.79-19.7 MPa) and a 50-fold range in the loss tangent (0.029-1.52) at 37 degrees C and 1 Hz. The materials could generally be divided into either rubbery behaviour with low modulus and loss tangent, i.e. the silicones, or leathery behaviour with both higher modulus and loss tangent, i.e. the methacrylates. A search of the literature did not find a universally preferred set of properties, although a possible link with residual ridge resorption is indicated.
Viscoelastic properties of some soft lining materials. II--Ageing characteristics.
A non-resonant forced vibration, dynamic mechanical analyser was employed to measure the viscoelastic characteristics of soft lining materials at 1 Hz, after storage in distilled water at mouth temperature for periods up to and including one year. The six commercial products included methacrylate, silicone, and phosphazine based polymers and the one experimental material was a methacrylate. Water sorption of the soft liners, recorded by change in sample mass, ranged from -4.39 to +48.57% and their solubilities from 0.13 to 13.58%, after one year. The heat-cured silicone was the most stable polymer in water in contrast to its autopolymerised counterpart. The excessive water uptake of this latter material resulted in a massive reduction in modulus. At the other extreme one methacrylate with a high plasticiser content hardened substantially after ageing (modulus changed from 5.87 to 72.3 MPa). Changes in loss tangent data were relatively small for all the polymers tested, even for materials with high water uptake. Reduced leaching and/or plasticiser content have led to a more stable generation of soft lining materials.
The biaxial flexural strength of two pressable ceramic systems.
The biaxial flexural strength of Optimal pressable ceramics and IPS Empress pressable ceramics were tested, compared and the micro-structures and compositions explored. The materials evaluated were Optimal shaded (Opcs) and unshaded (Opcus) ceramics and IPS Empress shaded (Ems) and unshaded (Emus) ceramics. Twenty-one disc specimens per material were prepared, heat-treated and tested. The piston on three-ball test ASTM F394-78 (1991) was used to test the specimens in a universal testing machine at a crosshead speed of 0.15 mm/min. Specimens were viewed in a scanning electron microscope and X-ray diffraction used to determine the phases present. Mean strengths (MPa +/- SD) were: Ems 120.1 +/- 20.5, Emus 135.8 +/- 16.0, Opcs 139.1 +/- 14.3 and Opcus 138.0 +/- 11.5. There was no statistically significant difference between Optimal shaded, Optimal unshaded and Empress unshaded strength values (p > 0.05). Empress shaded strength values were significantly lower than the other materials tested (p < 0.05). Weibull analysis provided m values: Ems 6.1, Emus 10.2, Opcs 12.8 and Opcus 13.9 and 1% and 5% probabilities of failure. Secondary electron imaging revealed a dense dispersal of leucite crystals in the glassy matrix of the Optimal ceramics of an average size 5.5 +/- 9.7 micron 2 for the Optimal shaded ceramic and 6.6 +/- 13.3 micron 2 for the Optimal unshaded ceramic. Leucite crystal agglomerates were evident for the Empress shaded material and a uniform distribution of fine leucite crystals (1.9 +/- 1.8 micron 2) for the Empress unshaded ceramic. Crystal and matrix microcracking were present in most of the material microstructures, together with porosity and tabular alumina platelets in the Optimal ceramics. X-ray diffraction revealed the presence of tetragonal leucite and small amounts of cubic leucite. Optimal ceramics and Empress unshaded ceramic provided higher strength and Weibull m values compared with Empress shaded ceramic. Inherent material defects were characterised and cubic leucite was identified.
Dental materials: 1997 literature review.
This review of the published literature on dental materials for the year 1997 has been compiled by the Dental Materials Panel of UK. It continues a series of annual reviews started in 1973. Emphasis has been placed upon publications, which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, ceramometallic restorations and resin-bonded bridges, ceramics, denture base resins and soft lining materials, impression materials, dental implant materials, orthodontic materials, biomechanics and image processing, resin composites, and casting investment materials and waxes). Three hundred and thirty three articles have been reviewed.
Production of harmonics in vitro by high-intensity focused ultrasound.
Experiments were performed to investigate the production of harmonics by high-intensity focused ultrasound (HIFU) produced by a spherical bowl; spherical radius 15 cm, frequency 1.7 MHz, as a function of beam power in excised bovine liver. The intensity of the nth harmonic, in both water and the tissue sample, varied approximately as the nth power of the incident intensity up to the point at which irreversible changes were produced in the sample. The greatest observed axial power absorption enhancement factor was approximately 6.3, and the greatest observed total absorbed power enhancement was approximately 2.3. These enhancements may have some effect on the onset of lesioning, but not much effect on its subsequent development. In water, at an intensity of about 120 W/cm2 at the focus, the -3-dB beam diameters of harmonic components were observed to vary approximately as the inverse square root of the harmonic number.
Profitability is no measure of viability.
Explore the source record for details and available documents.
Healthcare reform--round two.
Explore the source record for details and available documents.
Clarifying professional practice.
Explore the source record for details and available documents.
Providing capital for physician group practices: new opportunities for hospitals.
As physician group practices grow and consolidate, they have an increasing interest in developing close capital partnerships to ensure access to capital. Yet as many healthcare organizations have sought to divest poorly performing acquired physician practices, physicians have seen their pool of potential capital partners shrink. Under these conditions, hospitals have a new opportunity to present themselves to physician group practices as attractive capital partners. To understand the nature of this opportunity, one needs to know why group practices seek capital, how groups approach their investment strategies, and what criteria they use to compare prospective capital partners. To build stronger relationships with physicians, hospitals should focus on turning around their poorly performing acquired physician practices and pursue strategies such as collaborating with physician practice management companies and developing new models for partnering with physicians (e.g., special purchase agreements and more advanced management services organizations).
Dental materials: 1996 literature review.
This critical review of the published literature on dental materials for the year 1996 has been compiled by the Dental Materials Panel of the UK. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, resin-bonded bridges and ceramo-metallic restorations, ceramics, denture base resins and soft lining materials, impression materials, implants materials, orthodontic materials, biomechanics and image processing, resin composites and casting investment materials and waxes). Three hundred and thirteen articles have been reviewed.
Dental materials: 1996 literature review. Part 2.
This critical review of the published literature on dental materials for the year 1996 has been compiled by the Dental Materials Panel of the UK. It continues the series of annual reviews started in 1973 and published in the Journal of Dentistry. Emphasis has been placed upon publications which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, resin-bonded bridges and ceramo-metallic restorations, ceramics, denture base resins and soft lining materials, impression materials, implants materials, orthodontic materials, biomechanics and image processing, resin composites and casting investment materials and waxes). Three hundred and thirteen articles have been reviewed.
A 3-D finite-element model for computation of temperature profiles and regions of thermal damage during focused ultrasound surgery exposures.
Although there have been numerous models implemented for modeling thermal diffusion effects during focused ultrasound surgery (FUS), most have limited themselves to representing simple situations for which analytical solutions and the use of cylindrical geometries sufficed. For modeling single lesion formation and the heating patterns from a single exposure, good results were achieved in comparison with experimental results for predicting lesion size, shape and location. However, these types of approaches are insufficient when considering the heating of multiple sites with FUS exposures when the time interval between exposures is short. In such cases, the heat dissipation patterns from initial exposures in the lesion array formation can play a significant role in the heating patterns for later exposures. Understanding the effects of adjacent lesion formation, such as this, requires a three-dimensional (3-D) representation of the bioheat equation. Thus, we have developed a 3-D finite-element representation for modeling the thermal diffusion effects during FUS exposures in clinically relevant tissue volumes. The strength of this approach over past methods is its ability to represent arbitrarily shaped 3-D situations. Initial simulations have allowed calculation of the temperature distribution as a function of time for adjacent FUS exposures in excised bovine liver, with the individually computed point temperatures comparing favorably with published measurements. In addition to modeling these temperature distributions, the model was implemented in conjunction with an algorithm for calculating the thermal dose as a way of predicting lesion shape. Although used extensively in conventional hyperthermia applications, this thermal dose criterion has only been applied in a limited number of simulations in FUS for comparison with experimental measurements. In this study, simulations were run for focal depths 2 and 3 cm below the surface of pig's liver, using multiple intensity levels and exposure times. The results also compare favorably to published in vitro experimental measurements, which bodes well for future application to more complex problems, such as the modeling of multiple lesion arrays within complex anatomical geometries.
Compliance culprits.
Explore the source record for details and available documents.
Keeping pace: video communications in a managed care environment. Interview by Thomas A. Rasmussen.
A number of health care provider organizations, particularly those with geographically dispersed personnel, are utilizing the video medium to improve organizational performance. The key attributes of the video medium are uniformity of education delivery to multiple sites, the ability to deliver information in a timely manner, the ability to visually portray best practices for all to see, and the convenience for end-users. While the executives interviewed for this article aren't aware of any published research specifically documenting the linkage between video education and improved organizational performance, the panelists believe that informed health care professionals inherently contribute to improved organizational performance and that video education is a prudent investment of resources.