S.R. Smith

Nutrition of the intervertebral disc.

Study Design. A review of the literature on disc nutrition. Objectives. To summarize the information on disc nutrition in relation to disc degeneration. Summary of the Background Data. The disc is avascular, and the disc cells depend on diffusion from blood vessels at the disc’s margins to supply the nutrients essential for cellular activity and viability and to remove metabolic wastes such as lactic acid. The nutrient supply can fail due to changes in blood supply, sclerosis of the subchondral bone or endplate calcification, all of which can block transport from blood supply to the disc or due to changes in cellular demand. Methods. A review of the studies on disc blood supply, solute transport, studies of solute transport in animal and human disc in vitro, and of theoretical modeling studies that have examined factors affecting disc nutrition. Results. Small nutrients such as oxygen and glucose are supplied to the disc’s cells virtually entirely by diffusion; convective transport, arising from load-induced fluid movement in and out of the disc, has virtually no direct influence on transport of these nutrients. Consequently, there are steep concentration gradients of oxygen, glucose, and lactic acid across the disc; oxygen and glucose concentrations are lowest in the center of the nucleus where lactic acid concentrations are greatest. The actual levels of concentration depend on the balance between diffusive transport and cellular demand and can fall to critical levels if the endplate calcifies or nutritional demand increases. Conclusions. Loss of nutrient supply can lead to cell death, loss of matrix production, and increase in matrix degradation and hence to disc degeneration.

In vivo measurement of tissue metabolism in tendons of the rotator cuff: implications for surgical management.

We have undertaken an in vivo assessment of the tissue metabolism and cellular activity in torn tendons of the rotator cuff. Cellular oxygen consumption was measured in 13 patients undergoing mini-open repair of small, medium, large and massive full-thickness tears. Measurements were also taken from three control patients who were undergoing open stabilisation of the shoulder with grossly normal tendons. The level of oxygen and nitrous oxide was measured amperometrically using silver needle microelectrodes at the apex of the tear and 1.5 cm from its edge. With nitrous oxide indicating the degree of perfusion, oxygen consumption was calculated at each location to reflect cellular activity. All of the torn tendons had lower levels of cellular activity than the control group. This activity was lower still in the tissue nearest to the edge of the tear with the larger tears showing the lowest activity. This indicated reduced levels of tissue metabolism and infers a reduction in tendon viability. Our findings suggest that surgical repair of torn tendons of the rotator-cuff should include the more proximal, viable tissue, and may help to explain the high rate of re-rupture seen in larger tears.

Enhancing hollow fibre ultrafiltration using slug-flow — a hydrodynamic study☆

Abstract Gas—liquid slug flow is commonly employed as an enhancement technique for cross-flow ultrafiltration. This technique worked well on flat sheet and larger diameter membrane modules, but hydrodynamics and mass transfer, hence optimal operation, differs for the smaller diameter hollow fibre membranes where the technique proved less effective. This study investigates slug-flow hydrodynamics in capillary tubes in order to characterise the flow and predict optimal operation in hollow fibre membranes. Experiments investigated the effect of bubble size, bubble-supply frequency, operating pressure and capillary diameter on bubble shape and rise-velocity in non-porous tubes of 0.89 mm bore. Computational fluid dynamics (CFD) was employed to predict the flow behaviour inside capillaries. The CFD model and experimental results compared well. The CFD model yielded detailed information of the flow parameters and the flow patterns inside the capillaries and this allowed for better understanding of the hydrodynamics of the capillary tube slug-flow process.

Examination of the Mass–Heat Transfer Analogy for Two-Phase Flows in Narrow Channels: Comparison of Gas Bubble Enhancement of Membrane Separation and Heat Transfer to Vapour Bubbles in Boiling

The analogy between heat and mass transfer is frequently employed in single-phase flows. Here the analogy is examined for two processes involving two-phase flow in meso-channels with bores ≤1 mm: the injection of gas bubbles to enhance the ultrafiltration of large molecules in aqueous solutions using hollow fibre membranes and flow boiling in the confined-bubble regime. This paper considers the local mechanisms in these superficially similar processes, based on information from experiments and numerical simulations and discusses the extent to which they are indeed similar. It is concluded that there is no useful analogy between mass and heat transfer in these processes but they share a requirement for accurate estimation of the thickness of the liquid film between confined bubbles and the channel wall at high Reynolds numbers.