Diamond and methane formation from the chemical decomposition of polyethylene at high pressures and temperatures.

Polyethylene (C2H4)n was compressed to pressures between 10 and 30 GPa in a diamond anvil cell (DAC) and laser heated above 2500 K for approximately one second. This resulted in the chemical decomposition of the polymer into carbon and hydrocarbon reaction products. After quenching to ambient temperature, the decomposition products were measured in the DAC at pressures ranging from ambient to 29 GPa using a combination of x-ray diffraction (XRD) and small angle x-ray scattering (SAXS). XRD identified cubic diamond and methane as the predominant product species with their pressure-volume relationships exhibiting strong correlations to the diamond and methane equations of state. Length scales associated with the diamond products, obtained from SAXS measurements, indicate the formation of nanodiamonds with a radius of gyration between 12 and 35 nm consistent with 32-90 nm diameter spherical particles. These results are in good agreement with the predicted product composition under thermodynamic and chemical equilibrium.

Carbon clusters formed from shocked benzene.

Benzene (C6H6), while stable under ambient conditions, can become chemically reactive at high pressures and temperatures, such as under shock loading conditions. Here, we report in situ x-ray diffraction and small angle x-ray scattering measurements of liquid benzene shocked to 55 GPa, capturing the morphology and crystalline structure of the shock-driven reaction products at nanosecond timescales. The shock-driven chemical reactions in benzene observed using coherent XFEL x-rays were a complex mixture of products composed of carbon and hydrocarbon allotropes. In contrast to the conventional description of diamond, methane and hydrogen formation, our present results indicate that benzene's shock-driven reaction products consist of layered sheet-like hydrocarbon structures and nanosized carbon clusters with mixed sp2-sp3 hybridized bonding. Implications of these findings range from guiding shock synthesis of novel compounds to the fundamentals of carbon transport in planetary physics.

Surface wipe and bulk sampling of household dust: arsenic exposure in Cornwall, UK.

Dust elemental levels can be expressed as concentrations (bulk samples) or surface loadings (wipe samples). Wipe sampling has not been widely adopted for elements other than lead (Pb). In this study, 433 wipe samples from 130 households in south west England - a region of widespread, natural and anthropogenic arsenic contamination linked with previous mining activities-were analysed to (i) quantify loadings of arsenic (As); (ii) assess the quality of wipe data using QA/QC criteria; (iii) estimate, using published ingestion rates, human exposure to As in dust using loadings and concentrations from 97 bulk samples and (iv) comparatively assess the performance of wipe and bulk sampling using associations with As biomonitoring data (urine, toenails and hair). Good QC performance was observed for wipes: strong agreement between field duplicates, non-detectable contamination of field blank wipes and good reference material recoveries. Arsenic loadings exceeded an existing urban background benchmark in 67 (52%) households. No exceedances of tolerable daily As intake were observed for adult exposure estimates but infant estimates exceeded for 1 household. Infant estimates calculated using bulk concentrations resulted in 4 (3%) exceedances. Neither wipe nor bulk As metrics were sufficiently better predictors of As in biospecimens. Sampling strategies, analytical protocols, exposure metrics and assessment criteria require refinement to validate dust sampling methodologies.

Structures of riboswitch RNA reaction states by mix-and-inject XFEL serial crystallography.

Riboswitches are structural RNA elements that are generally located in the 5' untranslated region of messenger RNA. During regulation of gene expression, ligand binding to the aptamer domain of a riboswitch triggers a signal to the downstream expression platform. A complete understanding of the structural basis of this mechanism requires the ability to study structural changes over time. Here we use femtosecond X-ray free electron laser (XFEL) pulses to obtain structural measurements from crystals so small that diffusion of a ligand can be timed to initiate a reaction before diffraction. We demonstrate this approach by determining four structures of the adenine riboswitch aptamer domain during the course of a reaction, involving two unbound apo structures, one ligand-bound intermediate, and the final ligand-bound conformation. These structures support a reaction mechanism model with at least four states and illustrate the structural basis of signal transmission. The three-way junction and the P1 switch helix of the two apo conformers are notably different from those in the ligand-bound conformation. Our time-resolved crystallographic measurements with a 10-second delay captured the structure of an intermediate with changes in the binding pocket that accommodate the ligand. With at least a 10-minute delay, the RNA molecules were fully converted to the ligand-bound state, in which the substantial conformational changes resulted in conversion of the space group. Such notable changes in crystallo highlight the important opportunities that micro- and nanocrystals may offer in these and similar time-resolved diffraction studies. Together, these results demonstrate the potential of 'mix-and-inject' time-resolved serial crystallography to study biochemically important interactions between biomacromolecules and ligands, including those that involve large conformational changes.

Extended Lagrangian formulation of charge-constrained tight-binding molecular dynamics.

The extended Lagrangian Born-Oppenheimer molecular dynamics formalism [Niklasson, Phys. Rev. Lett., 2008, 100, 123004] has been applied to a tight-binding model under the constraint of local charge neutrality to yield microcanonical trajectories with both precise, long-term energy conservation and a reduced number of self-consistent field optimizations at each time step. The extended Lagrangian molecular dynamics formalism restores time reversal symmetry in the propagation of the electronic degrees of freedom, and it enables the efficient and accurate self-consistent optimization of the chemical potential and atomwise potential energy shifts in the on-site elements of the tight-binding Hamiltonian that are required when enforcing local charge neutrality. These capabilities are illustrated with microcanonical molecular dynamics simulations of a small metallic cluster using an sd-valent tight-binding model for titanium. The effects of weak dissipation on the propagation of the auxiliary degrees of freedom for the chemical potential and on-site Hamiltonian matrix elements that is used to counteract the accumulation of numerical noise during trajectories was also investigated.

Substituent effects on dynamics at conical intersections: alpha,beta-enones.

Femtosecond time-resolved photoelectron spectroscopy and high-level theoretical calculations were used to study the effects of methyl substitution on the electronic dynamics of the alpha,beta-enones acrolein (2-propenal), crotonaldehyde (2-butenal), methylvinylketone (3-buten-2-one), and methacrolein (2-methyl-2-propenal) following excitation to the S2(pipi*) state at 209 and 200 nm. We determine that following excitation the molecules move rapidly away from the Franck-Condon region, reaching a conical intersection promoting relaxation to the S1(npi*) state. Once on the S1 surface, the trajectories access another conical intersection, leading them to the ground state. Only small variations between molecules are seen in their S2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the S1 surface and the branching ratios to the products. Ab initio calculations used to compare the geometries, energies, and topographies of the S1/S0 conical intersections of the molecules are not able to satisfactorily explain the variations in relaxation behavior. We propose that the S1 lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S1/S0 conical intersection.

A randomized prospective study of posterolateral lumbar fusion. Outcomes with and without pedicle screw instrumentation.

STUDY DESIGN: A prospective evaluation of the clinical and radiographic outcomes of 71 patients who underwent lumbar fusion, with or without transpedicular instrumentation. The patients completed a questionnaire that determined pain relief, medication use, return to work, and overall satisfaction with surgery. OBJECTIVES: To explore the effect, if any, of instrumentation on the outcome of lumbar fusion surgery, according to reports of the patients, and whether there is a correlation between the radiographic determination of a solid fusion and the same patient-reported outcome. SUMMARY OF BACKGROUND DATA: The literature on this topic reports pseudarthrosis rates from 0% to 57% and good to excellent results from 56% to 95%. These studies provide no clear-cut recommendations concerning the effect of added lumbar instrumentation on patient-reported outcome in a prospective manner using concurrent control subjects. METHODS: The patients were randomized to groups with and without instrumentation after deciding to undergo a lumbar fusion and consenting to enter the study. Radiographs were obtained and questionnaires filled out at 6 weeks, 6 months, 1 year, and 2 years after surgery. RESULTS: There was no statistical difference in patient-reported outcome between the two groups. There was a slight nonsignificant trend toward increased radiographic fusion rate in the group with instrumentation that did not correlate with an increased patient-reported improvement rate. CONCLUSIONS: These results do not provide data that indicate a benefit in outcome from added instrumentation in elective lumbar fusions.

Neuropathologic changes with experimental spinal instrumentation: transpedicular versus sublaminar fixation.

Fifty-six mature beagles underwent lumbar spine destabilization, followed by fusion using four techniques. Spinal cord neuropathologic analysis was carried out to determine the number of abnormalities within each group. Group I (n = 14) had posterolateral bone grafting without instrumentation. Group IIa (n = 14) had Cotrel-Dubousset (CD) pedicle screws and rods. Group IIb (n = 14) had Steffee pedicle screws and plates. Group III (n = 14) had sublaminar wires and rods. All of the animals remained clinically neurologically normal throughout the 6 months of the study. The incidence of moderate to severe neuropathologic changes was 21% in Group I, 18% in Group II, and 64% in Group III. Thus, a significantly higher percentage of neuropathologic abnormalities occurred with sublaminar instrumentation than with no instrumentation (p = 0.027), or with transpedicular instrumentation (p = 0.027). In this controlled animal study, the theoretical advantage of pedicle screws, which should not violate the spinal canal, over sublaminar devices, which must enter the canal, was confirmed.

Influence of bone mineral density on the fixation of thoracolumbar implants. A comparative study of transpedicular screws, laminar hooks, and spinous process wires.

Posteriorly directed load to failure testing of four different types of spinal implants was performed in individual T5 to S1 vertebra harvested from seven fresh-frozen human cadaveric spines. The implants tested were: 1) Drummond spinous process wires, 2) Harrington laminar hooks, 3) Cotrel-Dubousset transpedicular screws, and 4) Steffee VSP transpedicular screws. The ultimate failure of each implant was compared with the bone mineral density of each vertebra to determine which implants, if any, were particularly advantageous in osteoporotic vertebrae. Before biomechanical testing, the spines were analyzed in vitro by dual photon absorptiometry to determine the bone mineral densities (gm/cm2) of each vertebra. The mean tensile loads to failure for each of the implants tested were as follows: Cotrel-Dubousset transpedicular screws: 345 Newtons; spinous process wire/button: 382 Newtons; Steffee transpedicular screws: 430 Newtons; and laminar hooks: 646 Newtons. The difference between the loads to failure for laminar hooks and the other implants was significant (P less than 0.05) using one-way analysis of variance. The overall correlation coefficient for bone mineral density with ultimate load to failure was 0.30 (P less than 0.001). The correlation coefficients were 0.47 (P less than 0.001) for spinous process wires alone; 0.096 (not significant) for laminar hooks alone; 0.37 (P less than 0.001) for Cotrel-Dubousset pedicle screws; and 0.48 (P less than 0.001) for Steffee pedicle screws. Of the four different implants tested, laminar hooks were most resistant to failure from posteriorly directed forces.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical evaluation of cervical spinal stabilization methods in a human cadaveric model.

The authors have previously reported in vitro testing of various posterior and anterior constructs (sublaminar, Rogers', and Bohlman's triple-wire wiring; AO hook plate fixation; and Caspar anterior plate fixation) in a bovine model with multiaxial biomechanical testing. This study was undertaken to evaluate the above constructs and other constructs in human cadaveric spines. Six subaxial human cervical spine specimens were biomechanically tested at the C5-C6 motion segment both intact and with a simulated distractive-flexion Stage 3 injury created at the C5-C6 level with complete disruption of the supraspinous ligament, interspinous ligament, ligamentum flavum, posterior longitudinal ligament, and facet joint capsules; with sufficient disruption of the intervertebral disc to allow a bilateral C5-C6 facet dislocation. The specimens were tested with a six-channel Bionix MTS 858 materials tester (M.T.S., Minneapolis, Minnesota) using cyclic loads to simulate cervical compression, flexion, extension, and rotation with measurements of axial load, axial displacement, torque, rotation, and anterior and posterior strains. Eight constructs were tested nondestructively: the intact spinal segment, sublaminar wiring, Rogers' wiring, Bohlman's wiring method (triple-wire technique), Roy-Camille posterior plate fixation, AO posterior hook-plate fixation, Caspar anterior plate fixation, and AO posterior hook-plate with Caspar anterior plate fixation. There was no significant difference in flexural stiffness and torsional stiffness between any of the constructs tested; however, there was a significant (P less than 0.05) increase in the posterior strain during flexion and axial loading tests between the Caspar plate construct and all other tested constructs, including the combined posterior and anterior plating construct. These differences persisted after cyclic testing of 100 cycles. Biomechanical testing demonstrated no significant differences between any of the posterior stabilization methods tested. Caspar anterior plating is clearly an inferior method of treating distractive flexion injuries of the cervical spine when compared with all posterior fixation techniques. Also, there is little biomechanical justification for the use of potentially dangerous sublaminar wire fixation and posterior plating methods in these injuries (with intact bony posterior elements), since the relatively safe interspinous wiring methods (Rogers' and Bohlman) are just as rigid as these other posterior fixation techniques.

Neuropathology with spinal instrumentation.

Neurohistologic examination of the spinal cord and cauda equina were compared for 28 beagles undergoing anterior and posterior spinal destabilization procedures--Group I (n = 7), destabilized operative controls; Group II (n = 7), posterolateral bone grafting; Group III (n = 7), Harrington distraction instrumentation and posterolateral fusion; and Group IV (n = 7), Luque rectangular instrumentation and posterolateral fusion. All dogs had appeared neurologically intact upon repeated examinations prior to death. Neurohistological abnormalities (Wallerian degeneration of the dorsal columns, corticospinal tracts, and nerve roots, focal cystic degeneration, and intraspinal central cavitation) occurred in only 1 of the 14 animals (7%) in Groups I and II (noninstrumented) and in 9 of the 14 animals (64%) in Groups III and IV (instrumented). This result is statistically significant (p less than 0.001). Transient sensory disturbances and radicular paresthesias have been described in clinical reports of spinal instrumentation. It is probable that subclinical neurologic injuries, such as intraspinal and nerve root infarction in posterior neural tissue, can occur with the use of sublaminar hooks or wires. The chondrodystrophic beagle spinal model in this study should be considered a "worst case situation," and the clinical incidence of neurohistologic changes is expected to be lower.

Management of femoral fractures and pseudofractures in adult hypophosphatasia.

Ten pseudofractures and six complete fractures occurred in five patients with adult hypophosphatasia. The four pseudofractures that were treated non-operatively progressed to complete fracture. The six pseudofractures and six complete fractures that were treated by internal fixation united. New fractures occurred in four patients after the removal of the internal fixation. We recommend that complete fractures, symptomatic pseudofractures, and progressive asymptomatic pseudofractures in patients with adult hypophosphatasia be treated with load-sharing internal-fixation devices, such as rigid intramedullary nails. These should not be removed unless it is absolutely essential. If removal is necessary, however, they should be replaced, preferably during the same anesthetic session. In our opinion, the guidelines developed from this study may be applied reasonably to patients with osteomalacia from causes other than hypophosphatasia, especially if medical therapy has been unsuccessful.

Macroepiphyseal dysplasia with symptomatic osteoporosis, wrinkled skin, and aged appearance: a presumed autosomal recessive condition.

We report our detailed investigation of a 7-1/2-year-old girl with short stature, aged appearance, decreased subcutaneous fat and muscle mass, dry coarse hair, foot deformities, macroepiphyses with prominent but lax joints, and osteoporosis with recurrent fractures who is the off-spring of first cousins. This constellation of abnormalities differs from previously reported cases where macroepiphyses were a prominent finding. Our patient appears, therefore, to have a new, autosomal recessively inherited, syndrome.

A thirty year study of breast cancer in a consecutive series of private patients. Is axillary nodal study a valuable index in prognosis?

A consecutive series of 435 patients with breast cancer followed from six to thirty years is presented. Radical mastectomy was the surgical procedure of choice and our experience indicated that the degree of axillary nodal involvement is a most important guide in establishing prognosis in this disease. The majority of recurrences became evident within the first five postoperative years, the next most hazardous period for the patient being the half decade between the fifth and tenth years after initial therapy. A second independent malignancy developed in 37 of the 275 pateints who died, and it was the cause of death in 7.6 per cent of this group. Among the 160 patients still living to date a second independent malignancy has already developed in 10. Bilateral breast cancer was observed thirty-eight times in this entire series of 435 patients, an incidence of 8.7 per cent.