Tensile Mechanics of the Knee Meniscus in the Context of Cracks: Failure and Fracture Mechanisms, Strain Concentrations, and the Effect of Specimen Shape

Knee meniscus tears (cracks) are a major cause of knee dysfunction and osteoarthritis, but little is known about how they grow or what effects they have on meniscus mechanics. The objective of this work was to investigate the mechanics and failure of crack-free and cracked meniscus in uniaxial tension, with specific attention to failure mechanisms (fracture and bulk rupture) and local strain concentrations. A finite element model was used to find a test configuration likely to cause fracture and crack propagation. Center cracks with a 45° crack–fiber angle were selected for producing large fiber stresses, and 90° edge cracks were selected for producing large inter-fiber shear stresses. The circumferential and radial tensile mechanics of the meniscus were quantified using ex vivo tensile testing. A fiber recruitment model was fitted to the test data, and a method was developed to quantify the inflection (yield) point and modulus based on the shape of the stress–strain curve. Comparison of tensile test specimen shapes showed that an expanded tab specimen shape produces more rapid and complete fiber recruitment, lesser yield strain, and greater peak stress (strength) than rectangle specimens, and, likely, dogbone specimens. Mechanical effects of meniscus cracks were quantified by comparing cracked and crack-free specimens in circumferential and radial tension. The cracks did not cause a decrease in peak stress, indicating fracture did not occur. However, significantly greater longitudinal strain and shear strain was found near the crack tip for circumferential tension specimens. In radial tension specimens, all strain field components were greater near the crack tip. Failure tended to proceed along fascicle boundaries. Circumferential specimens failed by widespread interdigitating fiber pull-out, which also caused crack deflection. Radial specimens failed by necking and fiber rotation. These data demonstrate the remarkable fracture toughness of the meniscus, but increased near-tip strain may cause sub-failure damage and dysfunction. These results provide functional targets for interventions to repair or regenerate the meniscus

The SOLAS air-sea gas exchange experiment (SAGE) 2004

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 58 (2011): 753-763, doi:10.1016/j.dsr2.2010.10.015.The SOLAS air-sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive Subpolar Zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a Lagrangian study quantifying key biological and physical drivers influencing the air-sea gas exchange processes of CO2, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom. A dual tracer SF6/3He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air-sea tracer exchange at the 10’s of km’s scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describes air-sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties, and wind speed were quantified to further assist the development of gas exchange models for high-wind environments. There was a significant increase in the photosynthetic competence (Fv/Fm) of resident phytoplankton within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary production and column-integrated chlorophyll a concentrations had only doubled relative to the unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions totalling 1.1 tonne Fe2+, this was a very modest response compared to the other mesoscale iron enrichment experiments. An investigation of the factors limiting bloom development considered co- limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst incident light levels and the initial Si:N ratio were the lowest recorded in all FeAX’s to date, there was only a small seed-stock of diatoms (less than 1% of biomass) and the main response to iron addition was by the picophytoplankton. A high rate of dilution of the fertilised patch relative to phytoplankton growth rate, the greater than expected depth of the surface mixed layer and microzooplankton grazing were all considered as factors that prevented significant biomass accumulation. In line with the limited response, the enhanced biological draw-down of pCO2 was small and masked by a general increase in pCO2 due to mixing with higher pCO2 waters. The DMS precursor DMSP was kept in check through grazing activity and in contrast to most FeAX’s dissolved dimethylsulfide (DMS) concentration declined through the experiment. SAGE is an important low-end member in the range of responses to iron addition in FeAX’s. In the context of iron fertilisation as a geoengineering tool for atmospheric CO2 removal, SAGE has clearly demonstrated that a significant proportion of the low iron ocean may not produce a phytoplankton bloom in response to iron addition.SAGE was jointly funded through the New Zealand Foundation for Research, Science and Technology (FRST) programs (C01X0204) "Drivers and Mitigation of Global Change" and (C01X0223) "Ocean Ecosystems: Their Contribution to NZ Marine Productivity." Funding was also provided for specific collaborations by the US National Science Foundation from grants OCE-0326814 (Ward), OCE-0327779 (Ho), and OCE 0327188 OCE-0326814 (Minnett) and the UK Natural Environment Research Council NER/B/S/2003/00282 (Archer). The New Zealand International Science and Technology (ISAT) linkages fund provided additional funding (Archer and Ziolkowski), and the many collaborator institutions also provided valuable support