Biomedical subjects

P Vena

Publications and source records attributed to P Vena.

13 recordsLinked to original sources

PubMed2006

An axisymmetric computational model of skin expansion and growth.

Skin expansion is the principal technique used in plastic surgery to repair large cutaneous defects, typically after tumour removal, burn care, craniofacial surgery and post-mastectomy breast reconstruction. It allows a gain of new tissue by means of gradual expansion of a prosthesis, surgically implanted beneath the patient's skin. Nevertheless, wide clinical use is not supported by a deep quantitative knowledge of the phenomena occurring during the expansion. A finite element model of the skin expansion was developed to evaluate the stresses and the strains of the skin due to the expander inflation and validated by proper in vitro experiments; furthermore, a growth model based on the mechanical stimulus was implemented to estimate the skin area gain. The developed computational approach, composed of the skin expansion model interaction and the growth law, proved its validity to investigate skin expansion phenomena: its use suggests a new predictive tool to optimize clinical procedures and the expander devices' design.

Algorithms↗

PubMed2006

A constituent-based model for the nonlinear viscoelastic behavior of ligaments.

This paper presents a constitutive model for predicting the nonlinear viscoelastic behavior of soft biological tissues and in particular of ligaments. The constitutive law is a generalization of the well-known quasi-linear viscoelastic theory (QLV) in which the elastic response of the tissue and the time-dependent properties are independently modeled and combined into a convolution time integral. The elastic behavior, based on the definition of anisotropic strain energy function, is extended to the time-dependent regime by means of a suitably developed time discretization scheme. The time-dependent constitutive law is based on the postulate that a constituent-based relaxation behavior may be defined through two different stress relaxation functions: one for the isotropic matrix and one for the reinforcing (collagen) fibers. The constitutive parameters of the viscoelastic model have been estimated by curve fitting the stress relaxation experiments conducted on medial collateral ligaments (MCLs) taken from the literature, whereas the predictive capability of the model was assessed by simulating experimental tests different from those used for the parameter estimation. In particular, creep tests at different maximum stresses have been successfully simulated. The proposed nonlinear viscoelastic model is able to predict the time-dependent response of ligaments described in experimental works (Bonifasi-Lista et al., 2005, J. Orthopaed. Res., 23, pp. 67-76; Hingorani et al., 2004, Ann. Biomed. Eng., 32, pp. 306-312; Provenzano et al., 2001, Ann. Biomed. Eng., 29, pp. 908-214; Weiss et al., 2002, J. Biomech., 35, pp. 943-950). In particular, the nonlinear viscoelastic response which implies different relaxation rates for different applied strains, as well as different creep rates for different applied stresses and direction-dependent relaxation behavior, can be described.

Animals↗

PubMed2004

A discrete-time approach to the formulation of constitutive models for viscoelastic soft tissues.

This paper presents a novel approach to constitutive modeling of viscoelastic soft tissues. This formulation combines an anisotropic strain energy function, accounting for preferred material directions, to define the elastic stress-strain relationship, and a discrete time black-box dynamic model, borrowed from the theory of system identification, to describe the time-dependent behavior. This discrete time formulation is straightforwardly oriented to the development of a recursive time integration scheme that calculates the current stress state by using strain and stress values stored at a limited number of previous time instants. The viscoelastic model and the numerical procedure are assessed by implementing two numerical examples, the simulation of a uniaxial tensile test and the inflation of a thin tube. Both simulations are performed using parameter values based on previous experiments on preserved bovine pericardium. Parameters are then adjusted to investigate the sensitivity of the model. The hypotheses the model relies upon are discussed and the main limitations are stated.

Connective Tissue↗

PubMed2003

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.

Animals↗

PubMed2002

Effects of metal-inlay thickness in polyethylene cups with metal-on-metal bearings.

A way to prevent polyethylene wear in total hip replacements is to use metal-on-metal bearings. The cup design of these bearings may be a metal inlay in a polyethylene cup. However, these metal inlays are relatively thin and may deform on loading. The purpose of the current study was to determine whether these potential problems become actual for a realistic range of metal-inlay components having a thickness greater than 1 mm. For this purpose, the effects of thickness variation of a metal inlay in an ultrahigh molecular weight polyethylene cup were determined using three-dimensional finite element techniques. The results showed no indications for jamming of the bearing assuming a realistic inlay thickness (3-5 mm), even with a small clearance (25 microm). The metal inlay acted rigidly beyond a thickness of approximately 5 mm. Metal inlays thinner than 1.5 mm led to a considerable increase in contact area and a reduction in contact peak stress, which may be beneficial for the bearing performance. Currently, these thin liners have too many unknown characteristics and therefore the current authors recommend using rigid metal liners that have a thickness greater than 5 mm.

Biomechanical Phenomena↗

PubMed2001

Quantitative evaluation of the prosthetic head damage induced by microscopic third-body particles in total hip replacement.

The increase of the femoral head roughness in artificial hip joints is strongly influenced by the presence of abrasive particulate entrapped between the articulating surfaces. The aim of the present study is to evaluate the dependence of such damage on the geometry of the particles entrapped in the joint, with reference to the UHMWPE/chrome-cobalt coupling. Five chrome-cobalt femoral heads and their coupled UHMWPE acetabular cups, retrieved at revision surgery after a short period of in situ functioning, have been investigated for the occurrence of third-body damage. This was found on all the prosthetic heads, where the peak-to-valley height of the scratches, as derived from profilometry evaluations, ranged from 0.3-1.3 microm. The observed damage has been divided into four classes, related to the particle motion while being embedded into the polymer. Two kinds of particle morphology have been studied, spherical and prismatic, with size ranging from 5-50 microm. In order to provide an estimation of the damage induced by such particles, a finite element model of the third-body interaction was set up. The peak-to-valley height of the impression due to the particle indentation on the chrome-cobalt surface is assumed as an index of the induced damage. The calculated values range from 0.1-0.5 microm for spherical particles of size ranging from 10-40 microm. In the case of prismatic particles, the peak-to-valley height can reach 1.3 microm and depends both on the size and width of the particle's free corner, indenting the chrome-cobalt. As an example, a sharp-edged particle of size 30 microm can induce on the chrome-cobalt an impression with peak-to-valley height of 0.75 microm, when embedded into the polyethylene with a free edge of 5 microm facing the metallic surface. Negligible damage is induced, if a free edge of 7.5 microm is indenting the counterface. Our findings offer new support to the hypothesis that microscopic third-body particles are capable of causing increased roughening of the femoral head and provide a quantitative evaluation of the phenomenon.

Arthroplasty, Replacement, Hip↗

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