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Biomedical subjects

C D Roberts

Publications and source records attributed to C D Roberts.

At least 19 recordsLinked to original sources
PubMed2006

Pair production and optical lasers.

Electron-positron pair creation in a standing wave is explored using a parameter-free quantum kinetic equation. Field strengths and frequencies corresponding to modern optical lasers induce a material polarization of the QED vacuum, which may be characterized as a plasma of e+e- quasiparticle pairs with a density of approximately 10(20) cm-3. The plasma vanishes almost completely when the laser field is zero, leaving a very small residual pair density n(r) which is the true manifestation of vacuum decay. The average pair density per period is proportional to the laser intensity but independent of the frequency nu. The density of residual pairs also grows with laser intensity but n(r) proportional to nu(2). With optical lasers at the forefront of the current generation, these dynamical QED vacuum effects can plausibly generate 5-10 observable two-photon annihilation events per laser pulse.

Journal Article↗
PubMed2002

Quantum effects with an x-ray free-electron laser.

A quantum kinetic equation coupled with Maxwell's equation is used to estimate the laser power required at an x-ray free-electron laser (XFEL) facility to expose intrinsically quantum effects in the process of QED vacuum decay via spontaneous pair production. A 9 -TW-peak XFEL laser with photon energy of 8.3 keV could be sufficient to initiate particle accumulation and the consequent formation of a plasma of spontaneously produced pairs. The evolution of the particle number in the plasma will exhibit non-Markovian aspects of the strong-field pair production process, and the plasma's internal currents will generate an electric field whose interference with that of the laser leads to plasma oscillations.

Journal Article↗
PubMed2001

Pair creation and an x-ray free electron laser.

Using a quantum kinetic equation we study the possibility that focused beams at proposed x-ray free electron laser facilities can initiate spontaneous electron-positron pair production from the QED vacuum. Under conditions reckoned achievable at planned facilities, repeated cycles of particle creation and annihilation will take place in tune with the laser frequency. The peak particle number density is insensitive to this frequency, and one can anticipate the production of a few hundred particle pairs per laser period.

Journal Article↗
PubMed2000

Management of lawnmower injuries to the lower extremity in children and adolescents.

Lawnmower-associated trauma remains a substantial source of extremity injury in the pediatric and adolescent patient populations, producing complex wounds that require a combined orthopedic and plastic surgical approach. The authors review their experience with 16 patients, 2 to 17 years of age (mean age, 6.2 years), who were admitted to Duke University Medical Center for lower extremity lawnmower trauma between January 1988 and December 1999. The average hospitalization time was 13.5 days, and an average of 2.9 surgical procedures per patient were performed. Early debridement and bony fixation were carried out in all patients; 8 patients sustained traumatic amputations. Fifteen of 20 nonamputation fractures involved the foot and were managed with either closed reduction or K-wire fixation. Three of five long-bone fractures underwent external fixation. Wound closure was achieved with direct closure or skin grafting in the majority of patients. However, five microsurgical free flap transfers were required for extensive defect reconstruction of the foot (N = 4) and knee (N = 1). Adequate immediate debridement, fracture reduction, and early primary or if necessary secondary wound coverage including microsurgical free tissue transfer to prevent further damage and long-term disability in these type of devastating injuries is recommended.

Accidents, Home↗
PubMed1998

Wound management products--the evidence we need and the difficulties in obtaining it.

The management of complex wounds requires the expertise and co-ordinated effort of multiple disciplines in a variety of healthcare settings. Providers of wound care will be held increasingly responsible for the impact of clinical and economic outcomes for the patient, healthcare system and, ultimately, society as a whole. Collecting and analysing outcome data and comparing it to regional and national standards, will be the method which demonstrates the quality of service provided. This paper will review the design implications for conducting comparisons of therapies used in the management of wounds. Consideration will be given to overall design, sample size calculations, clinical setting, choice of comparator and mechanisms for exploring how efficacy and cost data can be linked. Evidence obtained from several prospective randomised clinical trials will be used as examples to support the theoretical considerations.

Clinical Nursing Research↗
PubMed1996

Data quality of the Drug Abuse Warning Network.

The purpose of this article was to assess the quality of data collected by the Drug Abuse Warning Network (DAWN), which reports drug abuse emergency department visits. The results of quality assurance studies at 36 sites were reviewed and interpreted. Data collection procedures are not consistent among hospitals and, along with personnel, regularly change within a hospital. Trained investigators reabstracted DAWN report forms at 24 sites and determined that only 57.4% of the cases that met DAWN case definition criteria had been reported; one of five cases had been reported at one site. The technique used in 11 (47.8%) of 23 hospitals to screen for potential DAWN cases detected only 36% of the cases found when all medical charts are examined. The investigators found discrepancies between reported and actual cases in 81.3% of the report forms reabstracted, with an average of 2.3 errors per form. Information as to the drug(s) involved was incorrect in 36.3% of the forms. Due to underreporting of drug abuse emergency department visits and poor quality data in DAWN report forms, DAWN estimates of drug activity must be viewed with caution. Furthermore, estimation of trends is risky, due to differences between emergency departments as to reporting systems and changes over time.

Community Networks↗
PubMed1995

The effects of non-nutritional factors on growth in cerebral palsy.

This study was designed to evaluate the effects of non-nutritional factors alone on the growth of children with cerebral palsy (CP). 20 children with hemiplegic CP, normal stature and normal triceps skinfold measurements had 16 different anthropometric measures performed on each side of the body (affected vs unaffected). All measures of breadth, circumference and length were significantly smaller on the affected side compared with the unaffected side. Skinfold measurements had a tendency to be larger on the affected side. These results suggest that non-nutritional factors related to disease severity have a significant influence on the growth of children with CP, even in the absence of malnutrition.

Activities of Daily Living↗
PubMed1994

Structure-activity relationships of C1 and C6 side chains of zaragozic acid A derivatives.

Systematic modification of the C6 acyl side chain of zaragozic acid A, a potent squalene synthase inhibitor, was undertaken to improve its biological activity. Simplification of the C6 side chain to the octanoyl ester has deleterious effects; increasing the linear chain length improves the in vitro activity up to the tetradecanoyl ester. An omega-phenoxy group is a better activity enhancer than an omega-phenyl group. A number of C6 carbamates, ethers, and carbonates were prepared and found to have similar activity profiles as the C6 esters. In the preparation of C6 ethers, C4 and C4,6 bisethers were also isolated; their relative activity is: C6 > C4 > C4,6. These C6 long-chain derivatives are subnanomolar squalene synthase inhibitors; they are, however, only weakly active in inhibiting hepatic cholesterol synthesis in mice. The C6 short-chain derivatives are much less active in vitro, but they all have improved oral activity in mice. Modification of the C1 alkyl side chain of the n-butanoyl analogue (ED50 4.5 mg/kg) did not improve the po activity further. A number of these C6 long-chain derivatives are also potent antifungal agents in vitro.

Animals↗
PubMed1994

Dose-response relationship in skin sensitization.

The dose-response relationship (challenge phase) of the skin sensitization response was investigated in previously sensitized Hartley guinea pigs. Larger numbers of animals were used per group at the lower doses so that statistically significant observations could be made. Model compounds known to be skin sensitizers were used: a strong sensitizer, dinitrochlorobenzene (DNCB), and a weaker sensitizer, p-phenylenediamine (PPDA). A gradation in response to changing DNCB doses was easily observed by using either the open epicutaneous test (OET) or the Buehler occlusive patch test. The Buehler test was used to study the dose-response relationship of DNCB sensitization. The sensitivity of the OET and Buehler test was judged not adequate to measure the dose response for PPDA, because at high doses a high incidence of responders was not obtained. Therefore, the maximization test was used to evaluate PPDA. Similar, non-linear dose-response curves were obtained with each compound. The higher doses produced a somewhat linear relationship, but at lower doses the curves flattened out and more slowly approached a zero response. Thus, for potent sensitizers, concentrations found in exposure situations might be in the linear portion of the dose-response curve. For weak responders, use concentrations might be in the shallow portion of the curve, where reactions would be underestimated if a linear dose-response curve were assumed.

Animals↗
PubMed1994

Effect of hemiplegia on skeletal maturation.

Children with cerebral palsy have been reported to have poor growth and delayed skeletal maturation, but it is unclear whether these effects are related to the underlying brain injury or to concomitant malnutrition. This study was designed to evaluate the effects of hemiplegic cerebral palsy on skeletal maturation and growth, with the unaffected side used as each subject's control. Bilateral hand-wrist radiographs were obtained for 19 children with spastic hemiplegia. Skeletal maturation was determined in a blinded fashion with the Fels method. The skeletal age of the affected (hemiplegic) side was less than that of the unaffected (control) side in all 19 subjects; the mean difference in skeletal age was 7.3 months (p < 0.001). The delay in skeletal maturation of the affected side correlated linearly with age and upper extremity function. These findings show that brain injury results in delayed skeletal maturation independent of malnutrition. This effect on skeletal maturation may explain, in part, the reason that some children with cerebral palsy grow poorly.

Adolescent↗
PubMed1991

Statistical model in tests for eye irritants.

A test used to classify substances for eye irritancy, as required by the Consumer Product Safety Commission, is performed on 1-3 groups of 6 albino rabbits in a sequential manner. When the statistical implications of the test are realized, it is possible for a substance to be classified as an irritant with fewer reactions than the number required for it to be classified as not an irritant. A procedure is given for correcting the inconsistency in the current test, and an alternative test, which considerably reduces the number of animals required, is proposed. Probability models and expected sample size calculations have been derived.

Animal Testing Alternatives↗
PubMed1979

Bacteriological quality control in human milk-banking.

The bacteriological quality of pooled human milk donated to the Oxford milk bank was analysed and the effects on bacteriology of sterilisation of the milk-collecting vessels in the home with hypochlorite solution and of Holder pasteurisation in a purpose-built human-milk pasteuriser were studied. Collecting milk in hypochlorite-sterilised vessels resulted in a significantly lower bacterial count of both pathogens and species of unlikely pathogenicity before pasteurisation and significantly increased the chance of pasteurisation giving a sterile product. Potentially pathogenic organisms grown in untreated milk were Escherichia coli, Staphylococcus aureus, and group B beta-haemolytic streptococci. Seven species of organisms of unlikely pathogenicity were also identified. Pasteurisation eliminated all potential pathogens from milk but did not reliably remove any of the species of unlikely pathogens. Banked human milk may be contaminated with bacteria which are known to be capable of producing lipases, proteases, and decarboxylases. Accurate pasteurisation, together with attention to the sterility of the collecting vessels, results in a bacteriologically safe product that retains many of the protective properties of raw milk.

Disinfection↗
PubMed1977

Acne vulgaris: an investigation into the number of anaerobic diphtheroids and members of the Micrococcaceae in normal and acne skin.

A quantitative study was made of the microflora of 174 acne and 68 non-acne subjects. Two groups of organisms were investigated, the anaerobic diphtheroids and members of the Micrococcaceae. The results showed high numbers of both groups of bacteria in skin bearing blackheads, papules or pustules and in non-acne adolescent skin. There were significantly lower numbers of bacteria in the pilosebaceous ducts of normal looking skin in acne areas and in pre-adolescent skin when compared with non-acne adolescent skin. It is suggested that increased numbers of bacteria alone do not predispose to acne, but that their interaction with the skin, which is a function of the localized skin environment, may be important.

Acne Vulgaris↗
PubMed1975

An explanation fro chloracne -- an industrial hazard.

Patients with chloracne were investigated and the results compared with data matched for age and site from patients with acne vulgaris. The ultrastructural appearances of pilo-sebaceous ducts in both groups were similar. The industrial group had significantly fewer C. acnes than the patients with ordinary acne. They also showed a significant increase in the surface lipid percentage composition of squalene, wax ester and cholesterol and this was associated with a significant decrease in the triglyceride free fatty acids and total triglyceride pool. We suggest that the acnegenic agent has produced its effect by modifying the metabolism of the pilo-sebaceous fluid.

Acne Vulgaris↗