Intervertebral discs have a primarily mechanical role in transmitting loads through the spine. The disc is subjected to a combination of elastic, viscous and osmotic forces; previous 3D models of the disc have typically neglected osmotic forces. The fibrilreinforced poroviscoelastic swelling model, which our group has recently developed, is used to compute the interplay of osmotic, viscous and elastic forces in an intervertebral disc under axial compressive load. The unloaded 3D finite element mesh equilibrates in a physiological solution, and exhibits an intradiscal pressure of about 0.2 MPa. Before and after axial loading the numerically simulated hydrostatic pressure compares well with the experimental ranges measured. Loading the disc decreased the height of the disc and results in an outward bulging of the outer annulus. Fiber stresses were highest on the most outward bulging on the posterior-lateral side. The osmotic forces resulted in tensile hoop stresses, which were higher than typical values in a non-osmotic disc. The computed axial stress profiles reproduced the main features of the stress profiles, in particular the characteristic posterior and anterior stress which were observed experimentally.

Finite elem ent (FE )m odels have becom e an im portant toolto study load distribution in the healthy and degenerated disc. H ow ever,m odelpredictions require accurate constitutive law sand m aterialproperties.A s the m echanicalpropertiesofthe intervertebral disc are regulated by its biochem icalcom position and fiber-reinforced structure,the relationship betw een the constitutive behavior ofthe tissue and its com position requirescarefulconsideration.W hile num erousstudieshave investigated the annulusfibrosuscom pressive and tensileproperties,specificconditionsrequired todeterm inem odelparam etersfortheosm oviscoelasticm odelareunavailable.Therefore,the objectivesofthisstudy w ere(1)to com plem entthe existing m aterialtesting in thel iterature w ith confined com pression and tensile testson hum an annulus fibrosus and (2)to use these data,togetherw ith existing nucleus pulposus com pression data to tune a com position-based, osm oviscoelasticm aterialconstitutive law .The osm oviscoelasticm aterialconstitutive law and the experim entaldata w ere used to describe thefiberand nonfiberpropertiesofthehum an disc.Thecom pressivem aterialpropertiesofnorm aldisctissuew ereG m ¼ 1. Keywords: three-dim ensi onalfinite elem ent;confined com pressi on test;tensile test;vi scoelasticity;porom echanics;com position-based Finite elem ent (FE )m odels have becom e an im portant tool to study load distribution in the healthy and degenerated disc. H ow ever, m odel predictions require accurate constitutive law s and m aterialproperties.1 A s the m echanicalproperties ofthe intervertebraldisc are regulated by its biochem ical com position and fiberreinforced structure, the constitutive law describing this com pl ex tissue requires careful consideration. The annulus fibrosus (A F) consi sts of collagen, w hich provides tensile strength, and proteoglycans (PG s), w hi ch due to their strong sw el ling ability 3 provide tissue hydration. 4,5 This sw el ling tendency is am pl ified by the partialshielding ofthe w ater by the collagen. 4 A high intradiscal pressure in the nucleus is the consequence ofthe sw elling and is balanced by the external load and tensile stresses in the dense collagen fiber structure of the annulus. W e previously presented a three-dim ensi onal (3D ) osm oviscoelastic FE m odel 6,7 that accounts for the interdependency of the sw elling ability and the collagen prestressing. The m odel predicts intradiscal pressures in the order of 0. 1 to 0. 2 M Pa i n unloaded discs,in agreem ent w ith in vi vo m easurem ents.8 H ow ever,l ack ofexperim entaldata to determ ine som e of the m odel param eters lim i ts the applicability ofthis m odel.N um erous studies have reported the A F com pressive 9-13 and tensile properties, 11, 14-17 but specific conditions required to determ ine m odelparam eters for the osm oviscoelasticm odelare unavailable.In com pression, previous studies quantified the load distribution and shiftbetw een fibrillarand solid m atri cesin the annulus.H ow ever, i n these studies the ...

Because extrafibrillar water content dictates extrafibrillar osmolarity, we aimed to determine the influence of intra-and extrafibrillar fluid exchange on intradiscal pressures and stresses. As experimental results showed that extrafibrillar osmolarity affects intervertebral disc cell gene expression and crack propagation, quantification of the effects of changes in intra-and extrafibrillar fluid exchange is physiologically relevant. Therefore, our 3D osmoviscoelastic finite element (FE) model of the intervertebral disc was extended to include the intra-and extrafibrillar water differentiation. Two simulations were performed, one without intrafibrillar fluid and one with intrafibrillar fluid fraction as a function of the extrafibrillar osmotic pressure. The intrafibrillar fluid fraction as a function of the extrafibrillar osmotic pressure was exponentially fitted to human data and implemented into the model. Because of the low collagen content in the nucleus, no noticeable differences in intradiscal pressure estimation were observed. However, values of extrafibrillar osmolarity, hydrostatic pressure, and the total tissue stress calculated for the annulus were clearly different. Stresses, hydrostatic pressure, and osmolarity were underestimated when the intrafibrillar water value was neglected. As the loading increased, the discrepancies increased. In conclusion, the distribution of pressure and osmolarity in the disc is affected by intraand extrafibrillar water exchange. ß

Present research focuses on different strategies to preserve the degenerated disc. To assure long-term success of novel approaches, favorable mechanical conditions in the disc tissue are essential. To evaluate these, a model is required that can determine internal mechanical conditions which cannot be directly measured as a function of assessable biophysical characteristics. Therefore, the objective is to evaluate if constitutive and material laws acquired on isolated samples of nucleus and annulus tissue can be used directly in a wholeorgan 3D FE model to describe intervertebral disc behavior. The 3D osmo-poro-visco-hyper-elastic disc (OVED) model describes disc behavior as a function of annulus and nucleus tissue biochemical composition, organization and specific constituent properties. The description of the 3D collagen network was enhanced to account for smaller fibril structures. Tissue mechanical behavior tests on isolated nucleus and annulus samples were simulated with models incorporating tissue composition to calculate the constituent parameter values. The obtained constitutive laws were incorporated into the whole-organ model. The overall behavior and disc properties of the model were corroborated against in vitro creep experiments of human L4/L5 discs. The OVED model simulated isolated tissue experiments on confined compression and uniaxial tensile test and whole-organ disc behavior. This was possible, provided that secondary fiber structures were accounted for. The fair agreement (radial bulge, axial creep deformation and intradiscal pressure) between model and experiment was obtained using constitutive properties that are the same for annulus and nucleus. Both tissue models differed in the 3D OVED model only by composition. The composition-based modeling presents the advantage of reducing the numbers of material parameters to a minimum and to use tissue composition directly as input. Hence, this approach provides the possibility to describe internal mechanical conditions of the disc as a function of assessable biophysical characteristics.

Patients with Ankylosing Spondylitis starting on TNF-inhibiting therapy in Austria in 2007 were treated most often with Adalimumab, while Etanercept showed the lowest switching rate and the longest 1- and 2-year drug survival.

Physiological cyclic loading of the intervertebral disc generally occurs between 1 and 5 Hz [1]. However, most mechanical assessments of disc AF tissue have relied on uniaxial tensile tests under quasi-static conditions [2]. Furthermore, the few studies that have addressed AF viscoelasticity have reported only static viscoelastic properties such as creep or stress-relaxation [3]. As such, there exists almost no data characterizing the dynamic viscoelastic properties of AF tissue in tension. Such data would be critical for several applications: to elucidate the mechanical progression of intervertebral disc degeneration, to develop and validate structural finite element models, and to provide native tissue benchmarks for regenerative approaches that aim to restore mechanical function to diseased or degenerate tissue. Therefore, the objectives of this study are to: (1) quantify human AF tissue frequency-dependent and strain-dependent viscoelastic properties of human AF tissue in circumferential tension, and (2) determine the effects of disc degeneration on these properties.

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Intervertebral disc tissue consists of a fluid-filled extra-cellular matrix, in which living cells are sparsely dispersed. The mechanical function is highly dependent on the composition of the extra-cellular matrix, which primary consists of collagen fibrils and negatively charged proteoglycans. Due to the fixed charges of the proteoglycans (PG’s), the cation concentration inside the tissue is higher than physiological. This excess of ion particles leads to an osmotic pressure difference, which causes swelling of the tissue [1]. Because the intervertebral disc is gripped between two vertebrae, the swelling is constrained in vivo, resulting in a intradiscal pressure of 0.1 to 0.2 MPa in supine position. It has been shown that the osmotic pressure inside cartilaginous tissues is much higher than would be expected based on its FCD [2]. This is because part of the water in the tissue is absorbed by the collagen fibers. The proteoglycan molecules, because of their large size, are excluded from this intra-fibrillar space. This means that their effective concentrations are much higher in the extra-fibrillar space than if they were distributed uniformly throughout the entire matrix. Hence, the effective fixed charge density is higher than if computed from total tissue water content. A recent study demonstrates that intrafibrillar water increases osmolarity within the annulus fibrosus substantially [3]. On the other hand, Wognum et al. [4] showed by means of a physical and a numerical model of the disc that high osmolarity within the disc has a protective effect against crack propagation within the disc. Hence, the decrease in osmolarity associated with degeneration may be an explanation of (1) the growing number of cracks observed in the degenerating disc as well as (2) the poor correlation between external loading and crack propagation [5]. The purpose of the present study is to test the hypothesis of Wognum et al. [4] through direct observation of the deformation of annulus fibrosus tissue around discontinuities within its collagen network.