A three-dimensional finite element analysis of the combined behavior of ligaments and menisci in the healthy human knee joint

We present here a three-dimensional FE model of the healthy human knee that included the main structures of the joint: bones, all the relevant ligaments and patellar tendon, menisci and articular cartilages. Bones were considered to be rigid, articular cartilage and menisci linearly elastic, isotropic and homogeneous and ligaments hyperelastic and transversely isotropic. Initial strains on the ligaments and patellar tendon were also considered. This model was validated using experimental and numerical results obtained by other authors. Our main goal was to analyze the combined role of menisci and ligaments in load transmission and stability of the human knee. The results obtained reproduce the complex, nonuniform stress and strain fields that occur in the biological soft tissues involved and the kinematics of the human knee joint under a physiological external load.

Finite element analysis of the effect of meniscal tears and meniscectomies on human knee biomechanics

BACKGROUND

Many authors have suggested that the high levels of shear and tensile stresses that appear in the articular cartilage after meniscectomy are partly responsible for cartilage pathologies, such as osteoarthrosis.

METHODS

In this paper, we investigate the effect of meniscal tears and meniscectomies on the human knee joint. Solid models of the tibia, femur, menisci and cartilage were generated from MRI images. A three-dimensional finite element model was developed that included the femur, tibia, cartilage layers, menisci and ligaments. The femur and tibia were considered to be rigid, the articular cartilage and menisci to be linearly elastic, isotropic and homogeneous and the ligaments were modelled as hyperelastic. Three different situations were compared: a healthy tibio-femoral joint, a tibio-femoral joint with tears in one meniscus and a tibio-femoral joint after meniscectomy.

FINDINGS

The minimal principal stresses corresponding to a compressive load at 0 degrees flexion were obtained for the posterior zone of the medial meniscus and the corresponding region of the articular cartilage. Under an axial femoral compressive load, the maximal contact stress in the articular cartilage after meniscectomy was about twice that of a healthy joint.

INTERPRETATION

This fact could partially explain the cartilage damage and degeneration that have been observed after meniscectomy.

Biomechanical modeling of refractive corneal surgery

The aim of refractive corneal surgery is to modify the curvature of the cornea to improve its dioptric properties. With that goal, the surgeon has to define the appropriate values of the surgical parameters in order to get the best clinical results, i.e., laser and geometric parameters such as depth and location of the incision, for each specific patient. A biomechanical study before surgery is therefore very convenient to assess quantitatively the effect of each parameter on the optical outcome. A mechanical model of the human cornea is here proposed and implemented under a finite element context to simulate the effects of some usual surgical procedures, such as photorefractive keratectomy (PRK), and limbal relaxing incisions (LRI). This model considers a nonlinear anisotropic hyperelastic behavior of the cornea that strongly depends on the physiological collagen fibril distribution. We evaluate the effect of the incision variables on the change of curvature of the cornea to correct myopia and astigmatism. The obtained results provided reasonable and useful information in the procedures analyzed. We can conclude from those results that this model reasonably approximates the corneal response to increasing pressure. We also show that tonometry measures of the IOP underpredicts its actual value after PRK or LASIK surgery.

Sleep disturbances, socioeconomic status, and seizure control as main predictors of quality of life in epilepsy

Improving quality of life is the most important goal for patients with epilepsy. To recognize the factors associated with quality of life in patients with epilepsy in Mexico, we performed a cross-sectional survey using the Quality of Life in Epilepsy 31 (QOLIE-31) inventory to assess the quality of life of 401 adult patients with epilepsy at the National Institute of Neurology and Neurosurgery of Mexico. Clinical and demographical data were collected. Multiple regression was used to determine which factors affected quality of life in our patients. The variables that most strongly predicted a lower QOLIE-31 total score after multiple regression were sleep disorders (P<0.001), socioeconomic status (P<0.001), female gender (P=0.002), and high seizure frequency (P=0.001). In our study, neither depression nor time of evolution of epilepsy had significant influence on QOLIE-31 scores.

Mechanical behaviour of synthetic surgical meshes: finite element simulation of the herniated abdominal wall

The material properties of meshes used in hernia surgery contribute to the overall mechanical behaviour of the repaired abdominal wall. The mechanical response of a surgical mesh has to be defined since the haphazard orientation of an anisotropic mesh can lead to inconsistent surgical outcomes. This study was designed to characterize the mechanical behaviour of three surgical meshes (Surgipro®, Optilene® and Infinit®) and to describe a mechanical constitutive law that accurately reproduces the experimental results. Finally, through finite element simulation, the behaviour of the abdominal wall was modelled before and after surgical mesh implant. Uniaxial loading of mesh samples in two perpendicular directions revealed the isotropic response of Surgipro® and the anisotropic behaviour of Optilene® and Infinit®. A phenomenological constitutive law was used to reproduce the measured experimental curves. To analyze the mechanical effect of the meshes once implanted in the abdomen, finite element simulation of the healthy and partially herniated repaired rabbit abdominal wall served to reproduce wall behaviour before and after mesh implant. In all cases, maximal displacements were lower and maximal principal stresses higher in the implanted abdomen than the intact wall model. Despite the fact that no mesh showed a behaviour that perfectly matched that of abdominal muscle, the Infinit® mesh was able to best comply with the biomechanics of the abdominal wall.

A finite element simulation of the effect of graft stiffness and graft tensioning in ACL reconstruction

BACKGROUND

The anterior cruciate is the most frequent knee ligament to be totally disrupted. Surgical reconstruction of the anterior cruciate ligament is a common practice to treat the disability or chronical instability of knees due to anterior cruciate ligament insufficiency. Some of the factors that influence the success or failure of the anterior cruciate ligament reconstruction are the integrity of secondary restraints, the preoperative laxity of the knee, the status of the articular and meniscal cartilages, the selection of the graft material, the surgical technique, the graft tension and the postoperative rehabilitation.

METHODS

In this paper we present and discuss the results obtained with a three-dimensional finite element model of the human knee joint corresponding to different aspects of human anterior cruciate ligament reconstruction. In particular, this model was used to investigate the effect of graft stiffness and graft tensioning on the knee joint biomechanics. The initial graft tension was set to 0, 20, 40 or 60 N with the knee at 0 degrees , 30 degrees and 60 degrees of flexion. Three different stiffnesses corresponding to those of patellar tendon, gracilis and quadrupled semitendinosus grafts were analyzed.

FINDINGS

The resulting kinematics in each of these cases under an anterior load of 134 N was compared to that of the intact knee. The obtained results showed that, after reconstruction, the closest anterior tibial translation to that of the intact knee is obtained with a bone-patellar tendon-bone graft with a pretension of 60 N.

INTERPRETATION

However, this initial tension produces an important additional stress in the graft during the knee movement. This may cause problems in revascularization and remodelling during the postoperative healing process. A lower pretension of about 40 N should therefore be recommended in the present conditions.

Influence of the tunnel angle in ACL reconstructions on the biomechanics of the knee joint

BACKGROUND

A high tension in anterior cruciate ligament grafts affects both graft and knee functional performance. Clinical observations suggest that impingement of the graft against the posterior cruciate ligament might cause high graft tensions. Also, meniscal injury has been well documented in association with damage in the anterior cruciate ligament.

METHODS

In this paper, we present the results obtained in a three-dimensional finite element model of the human knee, corresponding to different aspects of anterior cruciate ligament reconstruction with bone-patellar tendon-bone grafts. This model was used to investigate the effect of the angle in the coronal plane of femoral and tibial tunnels. Firstly, graft tension was computed in a knee moved from 0 degrees to 60 degrees of flexion and the results were compared with experimental ones obtained by other authors. Secondly, the resulting kinematics under an anterior load of 134 N was compared to that of the intact knee.

FINDINGS

The obtained results showed that the closest anterior tibial translation to that of the intact knee was obtained with femoral and tibial tunnels with angles of 60 degrees. In this same case, a lower graft tension was also obtained. The results demonstrated noticeable increases in the meniscal stresses after anterior cruciate ligament reconstruction.

INTERPRETATION

Our results showed that impingement only depends on the femoral tunnel angle. On the contrary, laxity principally depends on the tibial tunnel angle. The angle of the femoral tunnel affects the graft tension while the tibial tunnel affects laxity, meniscal stresses and strains.

Experimental study and constitutive modelling of the passive mechanical properties of the porcine carotid artery and its relation to histological analysis: Implications in animal cardiovascular device trials

The present study focusses on the determination, comparison and constitutive modelling of the passive mechanical properties of the swine carotid artery over very long stretches in both proximal and distal regions. Special attention is paid to the histological and mechanical variations of these properties depending on the proximity to the heart. The results can have clinical relevance, especially in the research field of intravascular device design. Before the final clinical trials on humans, research in the vascular area is conducted on animal models, swine being the most common due to the similarities between the human and swine cardiovascular systems as well as the fact that the swine size is suitable for testing devices, in this case endovascular carotid systems. The design of devices usually involves numerical techniques, and an important feature is the appropriate modelling of the mechanical properties of the vessel. Fourteen carotid swine arteries were harvested just after sacrifice and cyclic uniaxial tension tests in longitudinal and circumferential directions were performed for distal and proximal samples. The stress-stretch curves obtained were fitted with a hyperelastic anisotropic model. Stress-free configuration states were also analyzed. Finally, human and swine samples were processed in a histological laboratory and images were used to quantify their microconstituents. The statistical analysis revealed significant differences between the mechanical behavior of proximal and distal locations in the circumferential but not in the longitudinal direction. Circumferential direction samples show clear differences both in residual stretches and tensile curves between the two locations, while the features of longitudinal specimens are independent of the axial position. The statistical analysis provides significant evidence of changes depending on the position of the sample, mainly in elastin and SMC quantification.

Neonatal exposure of male rats to estradiol benzoate causes rete testis dilation and backflow impairment of spermatogenesis

Estrogens administered to perinatal rodents cause spermatogenesis impairment; this study was undertaken to determine the mechanisms by which estrogens exert this effect. Neonatal male Wistar rats received estradiol benzoate (either 0.5 mg/5g BW or 1 mg/5g BW) and were killed at days 10, 22, 33, 45, and 60. Controls received vehicle. In tubule cross-sections of transverse sections of the right testes, 1) tubular diameter (TD) and seminiferous epithelium height (SEH) were measured, 2) normal and impaired spermatogenesis were classified in terms of the most advanced germ cell type present, including tubules lined by Sertoli cells only. A significant dose-dependent rise in the tubule percentage lined by Sertoli cells only at day 60 reflected spermatogenesis impairment. This was evidenced by the presence of multinucleated germ cells in a thin epithelium and sloughed into an enlarged tubular lumen, which was reflected in a significant dose-dependent increase in TD/SEH values from day 22 onward. TD was significantly greater and SEH significantly lower in tubular segments located at the cranial than the caudal halves of rat testes treated with the high (days 22, 33, and 60) and the low dose (day 33). This indicated distension in cranial tubular segments, perhaps due to the fact that these segments were the closest to the dilated rete testis. Consequently, they showed the highest TD/SEH values and the most regressive features of spermatogenesis (tubules lined by Sertoli cells only). In contrast, caudal segments in rat testes treated with the low dose showing TD/SEH values similar to controls displayed a delayed maturation of spermatogenesis coinciding with the late appearance of mature Leydig cells.

Computer simulation of damage on distal femoral articular cartilage after meniscectomies

It is commonly accepted that total or partial meniscectomies cause wear of articular cartilages that leads to severe damage in a period of few years. This also produces alteration of the biomechanical environment and increases articular instability, with a progressive and degenerative arthrosic pathology. Due to these negative consequences, total meniscectomy technique has been avoided, with a clear preference for partial meniscectomies. Despite the better results obtained with this latter technique, it has been demonstrated that the knee still suffers progressive long-term wear, which alters the properties of the surface of articular cartilage. In this paper, a phenomenological isotropic damage model of articular cartilage is presented and implemented in a finite element code. We hypothesized that there is a relation between the increase of shear stress and cartilage degeneration. To confirm the hypothesis, the obtained results were compared to experimental ones. It is used to investigate the effect of meniscectomies on articular damage in the human knee joint. Two different situations were compared for the tibio-femoral joint: healthy and after meniscectomy. The distribution of damaged regions and the damage level distribution resulted qualitatively similar to experimental results, showing, for instance that, after meniscectomy, significant degeneration occurs in the lateral compartment. A noteworthy result was that patterns of damage in a total meniscectomy model give better agreement to clinical results when using relative increases in shear stress, rather than an absolute shear stress criterion. The predictions for partial meniscectomies indicated the relative severity of the procedures.

Microstructural quantification of collagen fiber orientations and its integration in constitutive modeling of the porcine carotid artery

BACKGROUND

Mechanical characteristics of vascular tissue may play a role in different arterial pathologies, which, amongst others, requires robust constitutive descriptions to capture the vessel wall's anisotropic and non-linear properties.Specifically, the complex 3D network of collagen and its interaction with other structural elements has a dominating effect of arterial properties at higher stress levels.The aim of this study is to collect quantitative collagen organization as well as mechanical properties to facilitate structural constitutive models for the porcine carotid artery.This helps the understanding of the mechanics of swine carotid arteries, being a standard in clinical hypothesis testing, in endovascular preclinical trials for example.

METHOD

Porcine common carotid arteries (n=10) were harvested and used to (i) characterize the collagen fiber organization with polarized light microscopy, and (ii) the biaxial mechanical properties by inflation testing.The collagen organization was quantified by the Bingham orientation density function (ODF), which in turn was integrated in a structural constitutive model of the vessel wall.A one-layered and thick-walled model was used to estimate mechanical constitutive parameters by least-square fitting the recorded in vitro inflation test results.Finally, uniaxial data published elsewhere were used to validate the mean collagen organization described by the Bingham ODF.

RESULTS

Thick collagen fibers, i.e.the most mechanically relevant structure, in the common carotid artery are dispersed around the circumferential direction.In addition, almost all samples showed two distinct families of collagen fibers at different elevation, but not azimuthal, angles.Collagen fiber organization could be accurately represented by the Bingham ODF (κ1,2,3=[13.5,0.0,25.2] and κ1,2,3=[14.7,0.0,26.6]; average error of about 5%), and their integration into a structural constitutive model captured the inflation characteristics of individual carotid artery samples.Specifically, only four mechanical parameters were required to reasonably (average error from 14% to 38%) cover the experimental data over a wide range of axial and circumferential stretches.However, it was critical to account for fibrilar links between thick collagen fibers.Finally, the mean Bingham ODF provide also good approximation to uniaxial experimental data.

CONCLUSIONS

The applied structural constitutive model, based on individually measured collagen orientation densities, was able to capture the biaxial properties of the common carotid artery. Since the model required coupling amongst thick collagen fibers, the collagen fiber orientations measured from polarized light microscopy, alone, seem to be insufficient structural information. Alternatively, a larger dispersion of collagen fiber orientations, that is likely to arise from analyzing larger wall sections, could have had a similar effect, i.e. could have avoided coupling amongst thick collagen fibers.

STATEMENT OF SIGNIFICANCE

The applied structural constitutive model, based on individually measured collagen orientation densities, was able to capture the biaxial and uniaxial properties of the common carotid artery. Since the model required coupling amongst thick collagen fibers, an effective orientation density that accounts for cross-links between the main collagen fibers has been porposed. The model provides a good approximation to the experimental data.

Subcellular localization of tissue factor and human coronary artery smooth muscle cell migration

BACKGROUND

Tissue factor (TF) is the most relevant physiological trigger of thrombosis. Additionally TF is a transmembrane receptor with cell signaling functions.

OBJECTIVES

The aim of this study was to investigate TF subcellular localization, function and signaling in human coronary artery smooth muscle cell migration.

METHODS

Coronary arteries and primary cultures of vascular smooth muscle cells (HVSMC) were obtained from human explanted hearts. Wound repair and Boyden chamber assays were used to measure migration in vitro. TF-pro-coagulant activity (TF-PCA) was measured in extracted cellular membranes. Analysis of TF distribution was performed by confocal microscopy. A nucleofector device was used for TF and protease activated receptor 2 (PAR2) silencing. mRNA levels were analyzed by RT-PCR.

RESULTS

In migrating HVSMC TF translocates to the leading edge of the cells showing an intense patch-like staining in the lamellipodia. In the migrating front TF colocalizes with filamin (FLN) in the polarized lipid rafts. TF-PCA was increased in migrating cells. Silencing of the TF gene inhibits RSK-induced FLN-Ser-2152 phosphorylation, down-regulates CDC42, RhoA, and Rac1 protein expression and significantly inhibits cell migration. Silencing PAR2 also inhibits cell migration; however, silencing both TF and PAR2 induces a significantly higher effect on migration. Smooth muscle cells expressing TF have been identified in non-lipid-rich human coronary artery atherosclerotic plaques.

CONCLUSIONS

TF translocates to the cell front in association with cytoskeleton proteins and regulates HVSMC migration by mechanisms dependent and independent of factor (F)VIIa/PAR2. These results extend the functional role of TF to smooth muscle cell trafficking in vessel wall remodeling.

Tissue factor-Akt signaling triggers microvessel formation

BACKGROUND

Tissue factor (TF) and its signaling mediators play a crucial role in angiogenesis. We have previously shown that TF-induced endothelial cell (EC) CCL2 release contributes to neovessel formation.

OBJECTIVE

In this study, we have investigated the signaling pathways involved in TF-induced EC tube formation.

METHODS

The human microvascular endothelial cell line (HMEC-1) cultured onto basement membrane-like gel (Matrigel) was used to study TF signaling pathways during neovessels formation.

RESULTS

Inhibition of endogenous TF expression in ECs using siRNA resulted in inhibition of a stable tube-like structure formation in three-dimensional cultures, associated with a down-regulation of Akt activation, an increased phosphorylation of Raf at Ser(259) with a subsequent reduction of Raf kinase and a reduction of ERK1/2 phosphorylation ending up in Ets-1 transcription factor inhibition. Conversely, overexpression of TF resulted in an increase in tube formation and up-regulation of Akt protein. Moreover, immunoprecipitation of Akt and western blotting of the immunoprecipitates with anti-TF antibody revealed a direct interaction between TF and Akt. The effects of silencing TF were partially reversed by a PAR2 agonist that rescued tube formation, indicating that the TF-Akt pathway induces PAR2-independent effector signaling. Finally, enforced expression of Akt in TF-silenced ECs rescued tube formation in a Matrigel assay and induced Ets-1 phosphorylation.

CONCLUSIONS

In EC, TF forms a complex with Akt activating Raf/ERK and Ets-1 signaling induces microvessel formation.

Monomerization of C-reactive protein requires glycoprotein IIb-IIIa activation: pentraxins and platelet deposition

BACKGROUND

Pentraxins are inflammatory mediators linked to cardiovascular disease; however, their role in thrombosis remains to be fully elucidated.

AIMS

We investigated the role of pentraxins in thrombus formation on different vascular substrates under flow conditions.

METHODS

Native C-reactive protein (nCRP) and serum amyloid P (SAP) effects on thrombosis were evaluated under flow conditions on substrates placed in flat perfusion chambers. nCRP and dissociated monomeric CRP (mCRP) distributions were visualized by use of confocal microscopy. The effects of nCRP on vascular substrates were tested in the Badimon chamber.

RESULTS

mCRP, but not nCRP, induced a significant activation in platelet deposition, whereas SAP induced an activation only on fibrinogen-coated substrates. The effects of CRP on platelet deposition were significantly reduced by statin treatment. mCRP resulting from recirculation of blood containing nCRP over a thrombogenic vessel wall induced increased platelet deposition. Blocking glycoprotein IIb-IIIa prevented the effects of CRP dissociation and significantly reduced platelet deposition. Annexin V treatment did not block monomerization of CRP on activated platelets.

CONCLUSIONS

Under flow conditions, platelet deposited on all tested biological substrates support nCRP dissociation into mCRP. The effect is dependent on the thrombogenic potency of the substrate to trigger initial platelet deposition. Exposure of glycoprotein IIb-IIIa in the platelet surface supports nCRP dissociation. CRP monomerization was not dependent on the aminophospholipid exposed on the surface of activated platelets. The dissociated mCRP is trapped in the growing platelet aggregate and stimulates further platelet deposition. SAP increases platelet deposition only on fibrin monolayers. Therefore, pentraxins induce a platelet activation effect linking inflammation and thrombosis.

A structural approach including the behavior of collagen cross-links to model patient-specific human carotid arteries

In this work the mechanical response of the carotid arterial wall is studied. Some limitations of previous models of the arterial wall are overcomed and variability of the fitting problem is reduced. We review some experimental data from the literature and provide a constitutive model to characterize such data. A strain energy function is introduced including the behavior of cross-links between the main collagen fibers. With this function we are able to fit experimental data including information about the microstructure that previous models were not able to do. To demonstrate the applicability of the proposed model a patient-specific carotid artery geometry is reconstructed and simulated in a finite element framework, providing a microstructural description of the arterial wall. Our results qualitatively and quantitatively describe the experimental findings given in the literature fitting macroscopic mechanical tests and improving the features of previously developed models.

Viscoelastic properties of the passive mechanical behavior of the porcine carotid artery: influence of proximal and distal positions

The viscoelastic properties of porcine carotid tissue are investigated in this work. Experimental uniaxial stress relaxation tests along the longitudinal and circumferential directions of the vessel were performed for carotid strips extracted from 10 vessels. Directional and local differences--distal versus proximal position--in the tissue behavior were investigated. The experimental tests reveal a highly anisotropic, non-linear viscoelastic response and local dependence of the samples. The carotid artery shows anisotropic relaxation behavior for both proximal and distal samples. The highest stress relaxation was found in the circumferential tensile test for the highest applied strain at the distal position. For the circumferential direction, the relaxation stress was higher than in the longitudinal being at its highest in the distal position. These facts show that the stress relaxation is higher in the distal than in the proximal position. However, there are no differences between both positions in the longitudinal direction. In addition, a constitutive law that takes into account the fundamental features, including non-linear viscoelasticity, of the arterial tissue is proposed. The present results are correlated with the purely elastic response and the microstructural analysis of the tissue by means of histological quantification presented in a previous study.

Copper fractionation by SEC-HPLC and ETAAS: study of breast milk and infant formulae whey used in lactation of full-term newborn infants

This method will allow the determination of bound copper to low relative molecular mass compounds in milk. The milk whey obtained by ultracentrifugation was submitted to fractionation by size exclusion chromatography (SEC) on a TSK-Gel2000 (Toso Haas) column with a mobile phase of 0.2 M NH4NO3 + NH3, pH 6.7. Fractions of effluent corresponding to the protein peaks were collected and the copper content was determined by ETAAS. The method was sensitive (LOD 0.4 microgram l-1 and LOQ 1.5 micrograms l-1 in the fraction; LOQ 7.5-22.5 micrograms l-1 referred to the milk sample and depended on fraction volume) and precise (RSD +/- 10%). Media sample recoveries from the column were 101.2%. Cu was predominantly present in fractions corresponding to relative molecular mass 76 and 15 kDa of breast milk while copper was mostly found in fractions corresponding to 14 and 38 kDa of cow's milk-based infant formulae; moreover, copper was eluted in the relative molecular mass region < 6 kDa.