Phosphorus in sediments of high-elevation lakes in the Sierra Nevada (California): implications for internal phosphorus loading

In high-elevation lakes of the Sierra Nevada (California), increases in phosphorus (P) supply have been inferred from changes in phytoplankton growth during summer. To quantify rates of sediment P release to high-elevation Sierran lakes, we performed incubations of sediment cores under ambient and reducing conditions at Emerald Lake and analyzed long-term records of lake chemistry for Emerald and Pear lakes. We also measured concentrations of individual P forms in sediments from 50 Sierra Nevada lakes using a sequential fractionation procedure to examine landscape controls on P forms in sediments. On average, the sediments contained 1,445 µg P g−1, of which 5 % was freely exchangeable, 13 % associated with reducible metal hydroxides, 68 % associated with Al hydroxides, and the remaining 14 % stabilized in recalcitrant pools. Multiple linear regression analysis indicated that sediment P fractions were not well correlated with soluble P concentrations. In general, sediments behaved as net sinks for P even under reducing conditions. Our findings suggest that internal P loading does not explain the increase in P availability observed in high-elevation Sierran lakes. Rather, increased atmospheric P inputs and increased P supply via dissolved organic C leaching from soils may be driving the observed changes in P biogeochemistry

The accuracy and precision of a micro computer tomography volumetric measurement technique for the analysis of in-vitro tested total disc replacements

Total disc replacements (TDRs) in the spine have been clinically successful in the short term, but there are concerns over long-term failure due to wear, as seen in other joint replacements. Simulators have been used to investigate the wear of TDRs, but only gravimetric measurements have been used to assess material loss. Micro computer tomography (μCT) has been used for volumetric measurement of explanted components but has yet to be used for in-vitro studies with the wear typically less than &lt; 20mm3 per 106 cycles. The aim of this study was to compare μCT volume measurements with gravimetric measurements and to assess whether μCT can quantify wear volumes of n-vitro tested TDRs. μCT measurements of TDR polyethylene cores were undertaken and the results compared with gravimetric assessments. The effects of repositioning, integration time, and scan resolution were investigated. The best volume measurement resolution was found to be ±3mm;3, at least three orders of magnitude greater than those determined for gravimetric measurements. In conclusion, the μCT measurement technique is suitable for quantifying in-vitro TDR polyethylene wear volumes and can provide qualitative data (e.g. wear location), and also further quantitative data (e.g. height loss), assisting comparisons with in-vivo and ex-vivo/i&gt; data. It is best used alongside gravimetric measurements to maintain the high level of precision that these measurements provide.</p

7.14 Wear:Total intervertebral disc prostheses

Total disc prostheses were introduced as an alternative to spinal fusion, the treatment goal being provision of not only relief from pain but also retention or restoration of motion, similar to that seen in other joint replacement surgery. Indeed, early designs were developed using the same materials and biomechanical principles as in hip and knee prostheses, implying that such devices involve the possible risks of wear-induced failure. As these devices reach medium-term implantation, cases of osteolysis and wear-related failure begin to emerge. It is therefore essential that wear is considered in the development of total disc prostheses. The complex biomechanics and differing environment of the spine create additional complications when modeling wear behavior and the effects that it may have on the body. This chapter provides a brief introduction to the anatomy and biomechanics of the spine and the degenerated disc, and the surgical considerations for total disc prostheses. The design of these devices is discussed, including key considerations of material selection, constraint, and wear. Wear simulation, including current international standards and simulators in use, and the wear of metal-polyethylene, metal-metal, and alternative bearing materials are examined in detail. Finally, the biological response to wear debris, specifically in the spine, is considered.</p