Interstitial bone stress distributions accompanying ingrowth of a screen-like prosthesis anchorage layer

Recent development of screen-like bonded weaves of titanium wire for orthopaedic implant anchorage affords a unique opportunity for analytic studies of porous ingrowth micromechanics. The regular geometry of individual wires and the periodicity of the mesh weave are exploited in a series of two-dimensional finite element models, mapping interstitial bone stress fields as a function of ingrowth depth and wire size, shape, and spacing.

When the depth of bone ingrowth was less than one wire diameter, peak bone stresses always occurred at the leading (i.e. deepest) edge of bone ingrowth, immediately adjacent to the wire. As ingrowth depth approached a full wire diameter, peak local bone stresses were 2–9 times the nominal applied host bone stress, with greater stresses occurring for lower screen weave densities. Within multiple screen layers, the top layer consistently experienced the peak stress and transmitted most of the applied load, regardless of the number of underlying screen layers surrounded by bone. Neither wire size variations nor partial wire flattening substantially affected general trends in stress predictions.     

Localization of Surface Structures During Procaryotic Differentiation: Role of Cell Division in Caulobacter crescentus

Asymmetric cell division in Caulobacter crescentus produces two cell types, a stalked cell and a new swarmer cell, with characteristics surface structures. We have examined the role of the cell cycle in the differentiation of these two cells using the adsorption of bacteriophage øLC72, the assembly of the polar flagellum, and stalk formation as assays for changes in surface morphology. Previous studies of this aquatic bacterium [17, 25] have suggested that the replicating chromosome acts as a 'clock' in timing the formation of the flagellar filament at one pole of the new swarmer cell. The analysis of conditional cell cycle mutants presented here extends these results by showing that DNA synthesis is also required for adsorption of phage øLC72 and, more importantly, they also suggest that a late cell division step is involved in determining the spatial pattern in which the phage receptors and flagella are assembled. We propose that this cell division step is required for formation of 'organizational' centers which direct the assembly of surface structures at the new cell poles, and for the polarity reversal in assembly that accompanies swarmer cell to stalked cell development.