Mechanosensing at the Nuclear Envelope by Nuclear Pore Complex Stretch Activation and Its Effect in Physiology and Pathology.

Cell fate is correlated to mechanotransduction, in which forces transmitted by the cytoskeleton filaments alter the nuclear shape, affecting transcription factor import/export, cells transcription activity and chromatin distribution. There is in fact evidence that stem cells cultured in 3D environments mimicking the native niche are able to maintain their stemness or modulate their cellular function. However, the molecular and biophysical mechanisms underlying cellular mechanosensing are still largely unclear. The propagation of mechanical stimuli via a direct pathway from cell membrane integrins to SUN proteins residing in the nuclear envelop has been demonstrated, but we suggest that the cells' fate is mainly affected by the force distribution at the nuclear envelope level, where the SUN protein transmits the stimuli via its mechanical connection to several cell structures such as chromatin, lamina and the nuclear pore complex (NPC). In this review, we analyze the NPC structure and organization, which have not as yet been fully investigated, and its plausible involvement in cell fate. NPC is a multiprotein complex that spans the nuclear envelope, and is involved in several key cellular processes such as bidirectional nucleocytoplasmic exchange, cell cycle regulation, kinetochore organization, and regulation of gene expression. As several connections between the NPC and the nuclear envelope, chromatin and other transmembrane proteins have been identified, it is reasonable to suppose that nuclear deformations can alter the NPC structure. We provide evidence that the transmission of mechanical forces may significantly affects the basket conformation via the Nup153-SUN1 connection, both altering the passage of molecules through it and influencing the state of chromatin packing. Finally, we review the known correlations between a pathological NPC structure and diseases such as cancer, autoimmune disease, aging and laminopathies.

A Dual-Mode Bioreactor System for Tissue Engineered Vascular Models.

In the past decades, vascular tissue engineering has made great strides towards bringing engineered vascular tissues to the clinics and, in parallel, obtaining in-lab tools for basic research. Herein, we propose the design of a novel dual-mode bioreactor, useful for the fabrication (construct mode) and in vitro stimulation (culture mode) of collagen-based tubular constructs. Collagen-based gels laden with smooth muscle cells (SMCs) were molded directly within the bioreactor culture chamber. Based on a systematic characterization of the bioreactor culture mode, constructs were subjected to 10% cyclic strain at 0.5 Hz for 5 days. The effects of cyclic stimulation on matrix re-arrangement and biomechanical/viscoelastic properties were examined and compared vs. statically cultured constructs. A thorough comparison of cell response in terms of cell localization and expression of contractile phenotypic markers was carried out as well. We found that cyclic stimulation promoted cell-driven collagen matrix bi-axial compaction, enhancing the mechanical strength of strained samples with respect to static controls. Moreover, cyclic strain positively affected SMC behavior: cells maintained their contractile phenotype and spread uniformly throughout the whole wall thickness. Conversely, static culture induced a noticeable polarization of cell distribution to the outer rim of the constructs and a sharp reduction in total cell density. Overall, coupling the use of a novel dual-mode bioreactor with engineered collagen-gel-based tubular constructs demonstrated to be an interesting technology to investigate the modulation of cell and tissue behavior under controlled mechanically conditioned in vitro maturation.

A dynamic distention protocol for whole-organ bladder decellularization: histological and biomechanical characterization of the acellular matrix.

A combined physical-chemical protocol for whole full-thickness bladder decellularization is proposed, based on organ cyclic distention through repeated infusion/withdrawal of the decellularization agents through the urethra. The dynamic decellularization was intended to enhance cell removal efficiency, facilitating the delivery of detergents within the inner layers of the tissue and the removal of cell debris. The use of mild chemical detergents (hypotonic solution and non-ionic detergent) was employed to limit adverse effects upon matrix 3D ultrastructure. Inspection of the presence of residual DNA and RNA was carried out on decellularized matrices to verify effective cell removal. Histological investigation was focused on assessing the retention of adequate structural and functional components that regulate the biomechanical behaviour of the acellular tissue. Biomechanical properties were evaluated through uniaxial tensile loading tests of tissue strips and through ex vivo filling cystometry to evaluate the whole-organ mechanical response to a physiological-like loading state. According to our results, a dynamic decellularization protocol of 17 h duration with a 5 ml/min detergent infusion flow rate revealed higher DNA removal efficiency than standard static decellularization, resulting in residual DNA content < 50 ng/mg dry tissue weight. Furthermore, the collagen network and elastic fibres distribution were preserved in the acellular ECM, which exhibited suitable biomechanical properties in the perspective of its future use as an implant for bladder augmentation.

Electrical conditioning of adipose-derived stem cells in a multi-chamber culture platform.

In tissue engineering, several factors play key roles in providing adequate stimuli for cells differentiation, in particular biochemical and physical stimuli, which try to mimic the physiological microenvironments. Since electrical stimuli are important in the developing heart, we have developed an easy-to-use, cost-effective cell culture platform, able to provide controlled electrical stimulation aimed at investigating the influence of the electric field in the stem cell differentiation process. This bioreactor consists of an electrical stimulator and 12 independent, petri-like culture chambers and a 3-D computational model was used to characterize the distribution and the intensity of the electric field generated in the cell culture volume. We explored the effects of monophasic and biphasic square wave pulse stimulation on a mouse adipose-derived stem cell line (m17.ASC) comparing cell viability, proliferation, protein, and gene expression. Both monophasic (8 V, 2 ms, 1 Hz) and biphasic (+4 V, 1 ms and -4 V, 1 ms; 1 Hz) stimulation were compatible with cell survival and proliferation. Biphasic stimulation induced the expression of Connexin 43, which was found to localize also at the cell membrane, which is its recognized functional mediating intercellular electrical coupling. Electrically stimulated cells showed an induced transcriptional profile more closely related to that of neonatal cadiomyocytes, particularly for biphasic stimulation. The developed platform thus allowed to set-up precise conditions to drive adult stem cells toward a myocardial phenotype solely by physical stimuli, in the absence of exogenously added expensive bioactive molecules, and can thus represent a valuable tool for translational applications for heart tissue engineering and regeneration.

The self-association coiled-coil domain of PML is sufficient for the oncogenic conversion of the retinoic acid receptor (RAR) alpha.

In acute promyelocytic leukemia (APL) the retinoic acid receptor alpha (RARα) becomes an oncogene through the fusion with several partners, mostly with promyelocytic leukemia protein (PML), all of which have in common the presence of a self-association domain. The new fusion proteins, therefore, differently from the wild-type RARα, which forms only heterodimers with retinoic X receptor alpha, are also able to homo-oligomerize. The presence of such a domain has been suggested to be crucial for the leukemogenic potential of the chimeric proteins found in APL blasts. Whether or not any self-association domain is sufficient to bestow a leukemogenic activity on RARα is still under investigation. In this work, we address this question using two different X-RARα chimeras, where X represents the coiled-coil domain of PML (CC-RARα) or the oligomerization portion of the yeast transcription factor GCN4 (GCN4-RARα). We demonstrate that in vitro both proteins have transforming potential, and recapitulate the main PML-RARα biological properties, but CC-RARα is uniquely able to disrupt PML nuclear bodies. Indeed, in vivo only the CC-RARα chimera induces efficiently APL in a murine transplantation model. Thus, the PML CC domain represents the minimal structural determinant indispensable to transform RARα into an oncogenic protein.

Capsule endoscopy of the small bowel in the clinical practice: outpatient management is feasible and cheaper.

AIM: Four Italian regions have cost coding for outpatient capsule-endoscopy. Elsewhere it is performed in ordinary hospital admission. To identify, in a cohort of patients of a Gastroenterology Unit, those feasible for outpatient versus inpatient treatment; to analyze costs distribution in both management areas. METHODS: We retrospectively analysed 100 clinical records of admissions to A.O. San-Carlo-Borromeo, Milan between 2005-2008. Hospitalization criteria (at least 3): 1) occult/obscure gastrointestinal bleeding; 2) hemoglobin ≤ 8 gr/dL; 3) indication for blood transfusions; 4) urgent hospital admission. RESULTS: A total of 62 patients had urgent admission, 60 blood transfusions, 81 underwent EGD and colonoscopy, 8 enteroscopy and 5 surgery. Mean haemoglobin value was 8.67 g/dL. Capsule-endoscopy was positive in 70, uncertain in 8, negative in 22. Positive cases: 33 angiodyplasia, 18 ulcers/erosions, 13 polyps/masses, 5 overt bleeding, 1 celiac disease. 47/100 were appropriate as outpatient, saving 432 days of hospital stays. Admission coding was grouped into 7 DRGs (overall expense: 98,366 Euros). Considering EGD/colonoscopy outpatient costs and 1.100 euros as estimated value for capsule-endoscopy, the total expense could be 53.919. CONCLUSION: Outpatient capsule-endoscopy small bowel examination is feasible in half of the cases. It is cost saving, (about 45.000 Euros/100 patients), reducing inappropriate hospital stays.

Mechanisms of polymyxin B endotoxin removal from extracorporeal blood flow: molecular interactions.

The outer leaflet of Gram-negative bacteria membrane contains a great amount of lipopolysaccharides, also known as endotoxins, which play a central role in the pathogenesis of septic shock. It has been demonstrated that the polymyxin B (PMB) molecule has both antibacterial and antiendotoxin capabilities; in fact, it is able to compromise the bacterial outer membrane and bind lipopolysaccharides, thereby neutralizing its toxic effects. Extracorporeal hemoperfusion treatments based on cartridges containing PMB-immobilized fibers (Toraymyxin PMX-F; Toray Industries, Tokyo, Japan) are used to remove endotoxins circulating in the blood flow. In this study, we focused on the characterization of the interactions occurring in the formation of the PMB-endotoxin complex at the molecular level. In particular, the molecular mechanics approach was used to evaluate the interaction energy and eventually the interaction force between the two molecules. PMB was faced with five molecular portions of lipopolysaccharides differing in their structure. The interaction energy occurring for each molecular complex was calculated at different intermolecular distances and the binding forces were estimated by fitting interaction energy data. Results show that the short-range interactions between PMB and endotoxins are mediated mainly by hydrophobic forces, while in the long term, the complex formation is driven by ionic forces only. Maximum binding forces calculated via molecular mechanics for the PMB-endotoxin complex are in the range of 1.39-3.79 nN. Understanding the interaction mechanism of the single molecular complex is useful both in order to figure out the molecular features of such interaction and to perform higher scale level analysis, where such nanoscale detail is impractical but could be used to account for molecular behavior at a coarse level of discretization.

Mechanisms of polymyxin B endotoxin removal from extracorporeal blood flow: hydrodynamics of sorption.

The removal of blood endotoxins with the Toraymyxin extracorporeal sorption device exploits the capability of immobilized polymyxin B (PMB) to bind endotoxins stably with a high specificity. Although adsorption is a molecular-scale mechanism, it involves hydrodynamic phenomena in the whole range from the macroscopic down to the supramolecular scales. In this paper we summarize our experience with a computational, multiscale investigation of this device's hydrodynamic functionality. 3D computational fluid dynamics models were developed for the upper-scale studies. The flow behavior in the sorbent material was either modeled as a homogeneous Darcy's flow (macroscale study), or described as the flow through realistic geometrical models of its knitted fibers (mesoscale study). In the microscale study, simplified 2D models were used to track the motion of modeled endotoxin particles subjected to the competition of flow drag and molecular attraction by the fiber-grafted PMB. The results at each scale level supplied worst-case input data for the subsequent study. The macroscale results supplied the peak velocity of the flow field that develops in the sorbent. This was used in the mesoscale analysis, yielding a realistic range for the shear stresses in the fluid next to the fiber surface. With wall shear stresses in this range, endotoxin particle tracking was studied both in the vicinity of a single immobilized PMB molecule, and in the presence of a layer of PMB molecules evenly distributed at the fiber surface. Results showed that the capability to seize endotoxin molecules extends at least at a distance of 10-20 nm from the surface, which is one order of magnitude greater than the stable intermolecular bond characteristic distance. We conclude that a multiscale approach has the power to provide a comprehensive understanding, shedding light both upon the physics involved at each scale level and the mutual interactions of phenomena occurring at different scales.

Gastroenterology in Italian clinical practice. The RING Study: six years' data from gastroenterology departments.

AIM: Computer systems in hospitals provide information on the work of each single operative unit and the complexity of its caselist. However, in Italy, there is no official data-base for Gastroenterology Departments, to summarize their work. METHODS: The RING (Ricerca-INformatizzata-in-Gastroenterologia) study has collected, through a software made on purpose, 113 237 hospital discharge files (HDF) from 55 Italian hospital Gastroenterology Units, since 2001. This caselist provides a picture of the patients and is useful for clinical/management evaluation. RESULTS: Between January 2001 and December 2006, 55 Gastroenterology Units gathered 88240 HDF referring to ''ordinary admissions''. The male:female rate was 1:1, mean age was 61.3+/-18.5 years. Mean hospital stay was around eight days. Over the years there was a significant drop in DRG183 (miscellaneous digestive disorders-without complications) from 11.5% to 7.4% (P<0.0001), with no similar increase in DRG182 (with complications) which rose from 3.1% to 4.0%. Principal discharge diagnoses are post-hepatic and alcohol-related cirrhosis, hepatocarcinoma, acute pancreatitis, duodenal/gastric ulcer. CONCLUSIONS: The RING data show that the gastroenterologist has been working increasingly with patients whose pathologies would have been ''inappropriately'' treated surgically (DRGs 204 and 174). Inappropriate gastroenterological treatment seems to have decreased as well as the DRG183 with no apparent ''opportunistic'' compensatory increase in DRGs with complications, such as 182.

Management and outcomes of hepatic cirrhosis: Findings from the RING study.

BACKGROUND/AIM: Hepatic cirrhosis is a frequent reason for ordinary hospital admission (OA). The RING study collected hospital discharge files (HDF) from Italian hospital gastroenterology units (IGU). This caselist provides a broad picture of the patients admitted for this pathology. MATERIAL/METHODS: More than 50,000 HDF for OA were collected between 2001 and 2004 from 26 IGU. RESULTS: Eight thousand four hundred and eighty-seven HDF (16%) had a diagnosis of hepatic cirrhosis; Child-Pugh classes were 20.2% A, 34.8% B and 45.0% C. Patients' mean age was 63.7+/-12.1 years and 62.5% were male. A 61.1% of the cirrhosis cases had ascites, 29.9% portal-systemic encephalopathy, 29.2% hepatocellular carcinoma (HCC), 10% bleeding varices, 3.0% hepatorenal syndrome (HRS). Mortality for OA for cirrhosis was 5.7% versus 2.6% for other diagnoses. The proportion varied with the severity of the cirrhosis: 0% for Child A, 1.1% B, 10.5% C. Mortality was significantly associated with: Child-Pugh at admission (odds ratio: OR 9.2), HRS (OR 11.7), bleeding varices (OR 2.2), HCC (OR 1.8). CONCLUSIONS: Hepatic cirrhosis was found in 16% of the OA to IGU and mortality was double the rate for all the other pathologies in the same wards. Child-Pugh is a useful prognostic tool, higher classes implying a greater risk of death. HRS and bleeding varices were the complications with most influence on in-hospital mortality.

Multi-scale analysis of the toraymyxin adsorption cartridge. Part I: molecular interaction of polymyxin B with endotoxins.

Endotoxins or lipopolysaccharides are the main constituents of the outer leaflet of Gram-negative bacteria membrane and play a central role in the pathogenesis of the septic shock. Polymyxin B has both antibacterial and antiendotoxin capability; indeed it is able to destroy the bacterial outer membrane and bind endotoxin neutralizing its toxic effects. Cartridges containing polymyxin B-immobilized fibers (Toraymyxin PMX-F, Toray Industries, Japan) are used in extracorporeal hemoperfusion to remove circulating endotoxin. The aim of this study is the characterization of the polymyxin B-endotoxin system at the molecular level, thus providing quantitative evaluation of the binding forces exerted in the molecular complex. Polymyxin B was interfaced with five molecular models of lipopolysaccharides differing in their structure and molecular mechanics simulations were performed at different intermolecular distances aimed at calculating the interaction energies of the complex. Binding forces were calculated by fitting interaction energies data. Results show that in the short range the polymyxin B-endotoxin complex is mediated by hydrophobic forces and in the long range the complex is driven by ionic forces only. From a mechanical standpoint, polymyxin B-endotoxin complex is characterized by maximum binding forces ranging between 1.39 nN to 3.79 nN. The knowledge of the binding force behavior at different intermolecular distances allows further investigations at higher scale level (Part II).

Multi-scale analysis of the toraymyxin adsorption cartridge. Part II: computational fluid-dynamic study.

Extracorporeal endotoxin removal by means of the Toraymyxin device is based on the ability of polymyxin B to bind endotoxins with a high specificity. The endotoxins/polymyxin molecular interactions were computationally analyzed in a parallel work (Part I). In this paper we investigate with a multi-scale approach the phenomena involving blood and plasma fluid dynamics inside the device. The macro- and mesoscale phenomena were studied by means of 3D models using computational fluid dynamics. The flow behavior in the sorbent material was focused, modeling the sorbent as a homogeneous porous medium at the macroscale level, or accounting for the realistic geometry of its knitted fibers at the mesoscale level. A microscale model was then developed to analyze the behavior of endotoxin molecules subjected to the competition of flow drag and molecular attraction by fiber-grafted polymyxin B. The macroscale results showed that a very regular flow field develops in the sorbent, furthermore supplying the peak velocity to be input in the lower-scale model. The mesoscale analysis yielded the realistic range for wall shear stresses (WSSs) acting on fiber walls. With WSS values in the entire range, the results of the microscale analysis demonstrated that the capability of polymyxin B to capture endotoxin molecules from the flow extends at distances one order of magnitude greater than the characteristic distance of the stable intermolecular bond. We conclude that the use of an integrated, multi-scale analysis allows for a comprehensive understanding of the complex mechanisms involved in endotoxin sorption phenomena with immobilized polymyxin B.

Seasonal patterns of hospital treatment for inflammatory bowel disease in Italy.

AIM: It is still debated whether clinical flare-ups of chronic inflammatory bowel disease follow a seasonal pattern, and the various reports are based on general practitioners' records or hospital discharge charts. There are, however, no specific figures for treatment in hospital gastroenterology units, which serve as a reference point for these disorders. This study was therefore designed to investigate whether there is a seasonal pattern in admissions for inflammatory intestinal disease in Italy, differing from what is generally known about gastrointestinal pathologies, since there are no nation-wide figures on the subject. METHODS: The RING (Ricerca Informatizzata in Gastroenterologia) project is an observational study collecting hospital discharge forms from 22 centers in Italy. RESULTS: From winter 2000 to autumn 2003, the 22 gastroenterology units participating in the RING project discharged 32,357 patients following ordinary hospital admissions. Of these, 2,856 (8.8%) had a main diagnosis of inflammatory bowel disease: 1,541 Crohn's disease, and 1,315 ulcerative colitis. No seasonal patterns were detected for either category, or when the analysis was done by age, sex and site of disease. CONCLUSIONS: The most serious flare-ups of inflammatory bowel disease, i.e. those requiring routine hospital treatment, do not appear to follow any seasonal pattern, regardless of the site of the disease or the patient's age or sex.

A numerical and experimental study of compliance and collapsibility of preterm lamb tracheae.

Knowledge of the mechanical behaviour of immature tracheae is crucial in order to understand the effects exerted on central airways by ventilatory treatments, particularly of Total Liquid Ventilation. In this study, a combined experimental and computational approach was adopted to investigate the compliance and particularly collapsibility of preterm lamb tracheae in the range of pressure likely applied during Total Liquid Ventilation (-30 to 30 cmH2O). Tracheal samples of preterm lambs (n = 5; gestational age 120-130 days) were tested by altering transmural pressure from -30 to 30 cmH2O. Inflation (Si) and collapsing (Sc) compliance values were calculated in the ranges 0 to 10 cmH2O and -10 to 0 cmH2O, respectively. During the tests, an asymmetric behaviour of the DeltaV/V0 vs. P curves at positive and negative pressure was observed, with mean Si = 0.013 cmH2O(-1) and Sc = 0.053 cmH2O(-1). A different deformed configuration of the sample regions was observed, depending on the posterior shape of cartilaginous ring. A three-dimensional finite-element structural model of a single tracheal ring, based on histology measurements of the tested samples was developed. The model was parameterised in order to represent rings belonging to three different tracheal regions (craniad, median, caudal) and numerical analyses replicating the collapse test conditions were performed to evaluate the ring collapsibility at pressures between 0 and -30 cmH2O. Simulation results were compared to experimental data to verify the model's reliability. The best model predictions occurred at pressures -30 to -10 cmH2O. In this range, a model composed of median rings best interpreted the experimental data, with a maximum error of 2.7%; a model composed of an equal combination of all rings yielded an error of 12.6%.

Myosin head mechanical performance under different conformational change mechanisms.

The present paper puts forward a mathematical approach to model the conformational changes of the myosin head due to ATP hydrolysis, which determine the head swinging and consequent sliding of the actin filament. Our aim is to provide a simple but effective model simulating myosin head performance to be integrated into the overall model of sarcomere mechanics under development at our Laboratory (J. Biomech. 34 (2001) 1607). We began by exploring myosin head mechanics in recent findings about myosin ultrastructure, morphology and energetics in order to calculate the working stroke distance (WS) and the force transmitted to the actin filament during muscle contraction. Two different working stroke mechanisms were investigated, assuming that the swinging of the myosin head occurs either as a consequence of purely conformational changes (Science 261 (1993a) 58) or by thermally driven motion (ratchet mechanism) followed by conformational changes (Cell 99 (1999) 421). Our results show that force and WS values vary markedly between the two models. The maximum force generated is about 10 pN for the first model and 31 pN for the second model, and the WSs are about 13 and 4 nm, respectively. These results are then discussed and compared with published data. The experimental data used for comparison are scarce and non-homogeneous; hence, the final remarks do not lead to definite conclusions. In any event, relatively speaking, the first model is more coherent with experimental findings.

Inflammatory bowel disease and hospital treatment in Italy: the RING multi-centre study.

AIM: To explore the management of chronic inflammatory bowel disease, specifically Crohn's disease and ulcerative colitis, in Italian gastroenterology units. METHODS: The RING (Ricerca Informatizzata in Gastroenterologia) project is an observational study collecting hospital discharge forms from 56 centres. Factors associated with the length of hospital stay were studied using multivariate logistic regression. RESULTS: In 24 months starting from August 2000, out of 29,376 hospital discharge forms, 2131 (7.3%) were collected for inflammatory bowel disease (1163 for Crohn's disease and 968 for ulcerative colitis). The Crohn's disease and ulcerative colitis groups were compared according to demographic characteristics, diagnoses, procedures and hospital stay. In Crohn's disease, computed tomography/magnetic resonance imaging, x-rays/barium enema, number of procedures and number of diagnoses were significantly associated with a hospital stay longer than 10 days. In ulcerative colitis, this association was found for parenteral nutrition, malnutrition, computed tomography/magnetic resonance imaging and number of procedures. CONCLUSIONS: Crohn's disease was confirmed as a disabling disorder requiring more frequent hospital treatment than ulcerative colitis. For the latter, parenteral nutrition and malnutrition were related to a longer hospital stay. The number of procedures, especially abdominal computed tomography/magnetic resonance imaging, was a major item for both pathologies.

Pressure drop vs flow relationship in isolated preterm lamb tracheae.

Knowledge of immature tracheae mechanical behavior is fundamental in understanding the effects exerted on the upper airways by tidal liquid ventilation (TLV). Particularly, negative pressure can take place along the airways during expiration, which can cause airway collapse and flow limitation; therefore, representing a critical issue in preterm infant patients, whose airways are less stiff than adult ones. In this study, we investigated the expiratory pressure drop vs flow relationship of isolated preterm lamb tracheal samples to determine their hydraulic resistance, collapse pressure and collapse flow rate; a liquid flow through the samples was obtained by applying negative pressure at the outlet (cephalad) extremity of the tra-cheal sample, while keeping the inlet (caudal) extremity at atmospheric pressure. Histological analyzes were performed on the tracheal samples after each test session, in order to examine the morphological structure of the tracheal wall. Flow resistance tests demonstrated progressive lumen narrowing at increasing pressure drop (∆P=P in -P out ). The flow rate increased with ∆P un-til a plateau was reached, and then decreased, describing the onset of a collapse phenomenon; however, complete occlusion was not reached. The tracheal samples demonstrated a similar behavior to that of a Starling resistor during the collapse phase: when a critical ∆P was reached, collapse was observed starting at the outlet region, which was subjected to the greatest negative pressure, then propagating towards the caudal direction. (Journal of Applied Biomaterials & Biomechanics 2004; 2: 177-82).

Finite element analysis of a knee joint replacement during a gait cycle.

A three-dimensional (3-D) computer model was developed to predict, simultaneously, the knee joint replacement motions and the stresses occurring within the tibial insert under complex loading conditions. Friction contacts, polyethylene elastic- plastic behavior and soft tissue restraints were considered. Finite element analysis of the entire gait cycle was performed applying the displacement and loading conditions reported in the draft ISO 14243-1. The 3-D model was used to predict contact stresses and areas occurring within the polyethylene insert, when static loads of single instants on the gait cycle were applied. Results at 13% and 46% of the gait cycle were calculated with both entire cycle sim-ulation and single instant simulations. The predicted peak contact stress and contact area values were greater for single in-stant analyzes (respectively, 21% and 9%) if compared with those of the entire cycle simulation. In addition, single instant simulations were unable to predict accurately the contact region location, which was displaced in the anterior direction. Two different configurations of soft tissue restraints were compared; the first adopted from the draft ISO 14243-1 and the second from literature data. There were no significant differences observed between the two configurations. In particular, peak contact stresses followed the same trend, although the restraints of the literature data provided lower values in the last phase of the gait cycle. A preliminary verification of model reliability was performed comparing the tibial insert motions with data reported in the literature, under simplified loading conditions applied to the femoral component. (Journal of Applied Biomaterials & Biomechanics 2004; 2: 45-54).

Possible role of decorin glycosaminoglycans in fibril to fibril force transfer in relative mature tendons--a computational study from molecular to microstructural level.

Experimental studies on immature tendons have shown that the collagen fibril net is discontinuous. Manifold evidences, despite not being conclusive, indicate that mature tissue is discontinuous as well. According to composite theory, there is no requirement that the fibrils should extend from one end of the tissue to the other; indeed, an interfibrillar matrix with a low elastic modulus would be sufficient to guarantee the mechanical properties of the tendon. Possible mechanisms for the stress-transfer involve the interfibrillar proteoglycans and can be related to the matrix shear stress and to electrostatic non-covalent forces. Recent studies have shown that the glycosaminoglycans (GAGs) bound to decorin act like bridges between contiguous fibrils connecting adjacent fibril every 64-68 nm; this architecture would suggest their possible role in providing the mechanical integrity of the tendon structure. The present paper investigates the ability of decorin GAGs to transfer forces between adjacent fibrils. In order to test this hypothesis the stiffness of chondroitin-6-sulphate, a typical GAG associated to decorin, has been evaluated through the molecular mechanics approach. The obtained GAG stiffness is piecewise linear with an initial plateau at low strains (<800%) and a high stiffness region (3.1 x 10(-11)N/nm) afterwards. By introducing the calculated GAG stiffness in a multi-fibril model, miming the relative mature tendon architecture, the stress-strain behaviour of the collagen fibre was determined. The fibre incremental elastic modulus obtained ranges between 100 and 475 MPa for strains between 2% and 6%. The elastic modulus value depends directly on the fibril length, diameter and inversely on the interfibrillar distance. In particular, according to the obtained results, the length of the fibril is likely to play the major role in determining stiffness in mature tendons.