The annulus fibrosus (AF) of the intervertebral disc (IVD) has a unique, complex structure. If engineered tissues for the IVD are to be successfully developed, it is essential that the constituent level mechanics of the tissues in their natural form are fully understood (Nerurkar, J. Biomech. 2010). Published finite element (FE) models of the IVD do not represent lamellae behaviour and are validated using bulk mechanics of the intervertebral joint. This study aims to develop models of the IVD that include representation of the lamellae structure of the AF and the behaviour of this tissue within the disc. Three FE models of a vertebra-disc-vertebra section were developed considering the following scenarios of the AF: Homogenous AF. Concentric rings representing AF's lamellae structure with frictionless contact between rings. Concentric rings with ‘interface’ elements representing the interlamellar space; properties were derived through calibration of a separate model of an AF tissue sample with histological studies of the AF (Gregory, J. Biomechs. 2009). Displacements, stiffness and disc bulge were compared with the literature. The properties derived for the interface elements were stiffer than those for the AF tissue. this is in agreement with in vitro studies that have examined the mechanisms by which the lamellae fail prior to the interlamellar interaction (Veres, Spine, 2010). The macro-scale performance of the disc was sensitive to how the interlamellar interactions were modelled. Disc stiffness reduced by 7.1% between the homogenous and frictionless models. Use of the interface model improved the agreement with the in vitro performance of the disc: 5.8% error was recorded for disc stiffness and 2.1% error for disc bulge. The mechanics of the lamellae within the AF changed significantly between the frictionless and interface models. The relative displacement of adjacent lamellae was reduced by 15% between the frictionless and interface models. This study shows that the representation of the lamina structure of the AF affects the mechanics of the whole disc. Discrepancies in the modelling of interlamellar mechanics could have a significant effect on the interpretation of several important aspects of the biomechanics of the IVD.METHODS
RESULTS & CONCLUSIONS
Negative pressure wound therapy (NPWT) and vessel loop assisted
closure are two common methods used to assist with the closure of
fasciotomy wounds. This retrospective review compares these two
methods using a primary outcome measurement of skin graft requirement. A retrospective search was performed to identify patients who
underwent fasciotomy at our institution. Patient demographics, location
of the fasciotomy, type of assisted closure, injury characteristics,
need for skin graft, length of stay and evidence of infection within
90 days were recorded.Introduction
Methods
Intervertebral disc function and dysfunction is governed by its structural architecture of concentric layers of highly ordered collagen fibres. This architecture is important at the mm scale for overall mechanical performance of the disc; and at the micron scale for mechano-transduction signalling pathways of the disc cells that are responsible for matrix maintenance and therefore disc health. To understand such mechanical behaviour 3-dimensional collagen fibre architecture must be quantified in intact intervertebral discs. Conventional imaging modalities lack either the spatial resolution (e.g. x-ray diffraction) or penetration (e.g. optical, electron or confocal laser microscopy) to yield mechanically important information. Preliminary studies of scanning acoustic microscopy (SAM) at 50 MHz visualises alternating layers of fibre texture, however exactly what is being imaged requires both explanation and validation. Three-dimensional SAM data sets obtained from intact discs were compared to polarised-light and scanning electron micrographs of individual layers of fibres, peeled by micro-dissection from discs. The dimensions of the structural features were measured and recorded. Optical and electron microscopy revealed that each layer consisted of highly oriented collagen fibres of diameter 5 μm with regularly spaced splits between fibres with a spacing of approximately 20–30 μm. The SAM data sets showed layers with a uniform highly oriented fibre texture that reversed between adjacent layers. Resolution of the texture was limited by the acoustic system to approximately 30 μm. It is clear that SAM at 50 MHz cannot resolve and therefore image individual collagen fibres. However, the regular defects in the fibre layers can be visualised and convey complete information about local collagen fibre architecture. SAM therefore provides an effective way of quantifying the fibrous structure of intact, hydrated, unfixed intervertebral discs.
