Antibiotic prophylaxis for dentoalveolar surgery: is it indicated?

The document attached has been archived with permission from the Australian Dental Association. An external link to the publisher’s copy is included.Usually dentists in Australia give patients oral antibiotics after dentoalveolar surgery as a prophylaxis against wound infection. When this practice is compared to the principle of antibiotic prophylaxis in major surgery it is found to be at variance in a number of ways. In major surgery, the risk of infection should be high, and the consequences of infection severe or catastrophic, before antibiotic prophylaxis is ordered. If it is provided then a high dose of an appropriate spectrum antibiotic must be present in the blood prior to the first incision. Other factors which need to be considered are the degree of tissue trauma, the extent of host compromise, other medical comorbidities and length of hospitalization. Standardized protocols of administration have been determined and evaluated for most major surgical procedures. Dentoalveolar surgery is undoubtedly a skilled and technically challenging procedure. However, in contrast to major surgical procedures, it has a less than five per cent infection rate and rarely has severe adverse consequences. Dentoalveolar surgery should be of short duration with minimal tissue damage and performed in the dental chair under local anaesthesia. Controlled studies for both mandibular third molar surgery and placement of dental implants show little or no evidence of benefit from antibiotic prophylaxis and there is an adverse risk from the antibiotic. This review concludes that there is no case for antibiotic prophylaxis for most dentoalveolar surgery in fit patients. In the few cases where it can be considered, a single high preoperative dose should be given.B Lawler, PJ Sambrook, AN Gos

Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA

We present Very Long Baseline Array phase-referenced measurements of the parallax and proper motion of two pulsars, B0919+06 and B1857-26. Sub-milliarcsecond positional accuracy was obtained by simultaneously observing a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz. We discuss the merits of using weak close calibrator sources for VLBI observations at low frequencies, and outline a method of observation and data reduction for these type of measurements. For the pulsar B1919+06 we measure a parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is approximately 0.5 mas, an order of magnitude improvement over most previous determinations.Comment: 11 pages plus 4 figures. In press, Astronomical Journa

PKS B1400-33: an unusual radio relic in a poor cluster

We present new arcminute resolution radio images of the low surface brightness radio source PKS B1400-33 that is located in the poor cluster Abell S753. The observations consist of 330 MHz VLA, 843 MHz MOST and 1398 and 2378 MHz ATCA data. These new images, with higher surface brightness sensitivity than previous observations, reveal that the large scale structure consists of extended filamentary emission bounded by edge-brightened rims. The source is offset on one side of symmetrically distributed X-ray emission that is centered on the dominant cluster galaxy NGC 5419. PKS B1400-33 is a rare example of a relic in a poor cluster with radio properties unlike those of most relics and halos observed in cluster environments. The diffuse source appears to have had an unusual origin and we discuss possible mechanisms. We examine whether the source could be re-energized relic radio plasma or a buoyant synchrotron bubble that is a relic of activity in NGC 5419. The more exciting prospect is that the source is relic plasma preserved in the cluster gaseous environment following the chance injection of a radio lobe into the ICM as a result of activity in a galaxy at the periphery of the cluster.Comment: 26 pages, 8 figures, accepted for publication in the Astronomical Journa

Measurement of the Parallax of PSR B0950+08 Using the VLBA

A new technique has been developed to remove the ionosphere's distorting effects from low frequency VLBI data. By fitting dispersive and non-dispersive components to the phases of multi-frequency data, the ionosphere can be effectively removed from the data without the use of {\em a priori} calibration information. This technique, along with the new gating capability of the VLBA correlator, was used to perform accurate astrometry on pulsar B0950+08, resulting in a much improved measurement of this pulsar's proper motion ($\mu_{\alpha} = -1.6 \pm 0.4$ mas/yr, $\mu_{\delta} = 29.5 \pm 0.5$ mas/yr) and parallax ($\pi = 3.6 \pm 0.3$ mas). This puts the pulsar at a distance of $280 \pm 25$ parsecs, about twice as far as previous estimates, but in good agreement with models of the electron density in the local bubble.Comment: 5 pages, Latex with AASTEX. Accepted for publication in Ap

Tunnel junctions of unconventional superconductors

The phenomenology of Josephson tunnel junctions between unconventional superconductors is developed further. In contrast to s-wave superconductors, for d-wave superconductors the direction dependence of the tunnel matrix elements that describe the barrier is relevant. We find the full I-V characteristics and comment on the thermodynamical properties of these junctions. They depend sensitively on the relative orientation of the superconductors. The I-V characteristics differ from the normal s-wave RSJ-like behavior.Comment: 4 pages, revtex, 4 (encapsulated postscript) figures (figures replaced

The Sizes of 1720 MHz OH Masers: VLBA and MERLIN Observations of the Supernova Remnants W44 and W28

We have used the NRAO Very Long Baseline Array (VLBA) to image OH(1720 MHz) masers in the supernova remnants W28 and W44 at a resolution of 40 mas. We also used MERLIN to observe the same OH(1720 MHz) masers in W44 at a resolution of 290 x 165 mas. All the masers are resolved by these VLBA and MERLIN observations. The measured sizes range from 50 to 180 mas and yield brightness temperature estimates from 0.3--20 x 10**8 K. We investigate whether these measured angular sizes are intrinsic and hence originate as a result of the physical conditions in the supernova remnant shock, or whether they are scatter broadened sizes produced by the turbulent ionized gas along the line of sight. While the current data on the temporal and angular broadening of pulsars, masers and extragalactic soures toward W44 and W28 can be understood in terms of scattering, we cannot rule out that these large sizes are intrinsic. Recent theoretical modeling by Lockett et al. suggests that the physical parameters in the shocked region are indicative of densities and OH abundances which lead to estimates of sizes as large as what we measure. If the sizes and structure are intrinsic, then the OH(1720 MHz) masrs may be more like the OH(1612 MHz) masers in circumstellar shells than OH masers associated with HII regions. At two locations in W28 we observe the classical S-shapes in the Stokes V profiles caused by Zeeman splitting and use it to infer magnetic fields of order 2 milliGauss.Comment: 24 pages, 6 figures, accepted by Ap

The Radio Lifetime of Supernova Remnants and the Distribution of Pulsar Velocities at Birth

We have made VLA images of the fields around three young pulsars which have resulted in the discovery of two new supernova remnants and confirmation of a third. We argue that, in at least two cases and perhaps the third, the pulsars are physically associated with these supernova remnants. A review of all known young pulsars shows that the majority are associated with supernova remnants. We show that the typical density of the interstellar medium into which the supernova remnants are evolving has a density of 0.2 cm$^{-3}$ instead of the low value of 0.01 cm$^{-3}$ which had been calculated from other studies, and results in a considerably longer radio lifetime for supernova remnants. Both the morphology of the supernova remnants and the location of the pulsars imply that most of these young pulsars are born with large transverse velocities ($\sim$500 km s$^{-1}$). This high velocity mean in the distribution of pulsar velocities appears to be a general property of the pulsar population at birth, not seen in proper motion studies, due to selection effects. We explore the implications of this result as it relates to the origin of these velocities and the galactic distribution of pulsars. High velocity pulsars can escape their supernova remnant in a very short timescale, comparable to the lifetime of the remnant and may even play a role in extending the observable radio lifetime of the remnant. A significant fraction will be capable of escaping the disk of the Galaxy, producing an extended halo population.Comment: (apJ accepted). uuencoded, compressed postscript file. 23 pages. Hardcopies of figures and table available on request from [email protected]

Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138

We present three epochs of VLBA observations of Galactic HI absorption toward the quasar 3C~138 with resolutions of 20 mas (~ 10 AU). This analysis includes VLBA data from observations in 1999 and 2002 along with a reexamination of 1995 VLBA data. Improved data reduction and imaging techniques have led to an order of magnitude improvement in sensitivity compared to previous work. With these new data we confirm the previously detected milliarcsecond scale spatial variations in the HI opacity at the level of Delta(tau_{max}) =0.50 \pm 0.05. The typical size scale of the optical depth variations is ~ 50 mas or 25 AU. In addition, for the first time we see clear evidence for temporal variations in the HI opacity over the seven year time span of our three epochs of data. We also attempted to detect the magnetic field strength in the HI gas using the Zeeman effect. From this analysis we have been able to place a 3 sigma upper limit on the magnetic field strength per pixel of ~45 muG. We have also been able to calculate for the first time the plane of sky covering fraction of the small scale HI gas of ~10%. We also find that the line widths of the milliarcsecond sizescale HI features are comparable to those determined from previous single dish measurements toward 3C~138, suggesting that the opacity variations cannot be due to changes in the HI spin temperature. From these results we favor a density enhancement interpretation for the small scale HI structures, although these enhancements appear to be of short duration and are unlikely to be in equilibrium.Comment: 34 pages, 8 figures. Figures 3 & 4 are in color. Accepted to A