Paola Pachera

Investigation of the mechanical properties of the human crural fascia and their possible clinical implications

The mechanical properties of deep fasciae strongly affect muscular actions, development of pathologies, such as acute and chronic compartment syndromes, and the choice of the various fascial flaps. Actually, a clear knowledge of the mechanical characterization of these tissues still lacks. This study focuses attention on experimental tests of different regions of human crural fascia taken from an adult frozen donor. Tensile tests along proximal–distal and medial–lateral direction at a strain rate of 120xa0%/s were performed at the purpose of evaluating elastic properties. Viscous phenomena were investigated by applying incremental relaxation tests at total strain of 7, 9 and 11xa0% and observing stress decay for a time interval of 240xa0s. The elastic response showed that the fascia in the anterior compartment is stiffer than in the posterior compartment, both along the proximal–distal and medial–lateral directions. This result can explain why the compartment syndromes are more frequent in this compartment with respect to posterior one. Furthermore, the fascia is stiffer along the proximal–distal than along medial–lateral direction. This means that the crural fascia can adapt to the muscular variation of volume in a transversal direction, while along the main axis it could be considered as a structure that contributes to transmitting the muscular forces at a distance and connecting the different segments of the limb. The stress relaxation tests showed that the crural fascia needs 120xa0s to decrease stress of 40xa0%, suggesting a similar time also in the living so that the static stretching could have an effect on the fascia.

Synthetic surgical meshes used in abdominal wall surgery: Part II—Biomechanical aspects

This work reports the second part of a review on synthetic surgical meshes used for abdominal hernia repair. While material and structural characteristics, together with mesh-tissue interaction, were considered in a previous article (Part I), biomechanical behavior is described here in more detail. The role of the prosthesis is to strengthen the impaired abdominal wall, mimicking autologous tissue without reducing its compliance. Consequently, mesh mechanical properties play a crucial role in a successful surgical repair. The main available techniques for mechanical testing, such as uniaxial and biaxial tensile testing, ball burst, suture retention strength, and tear resistance testing, are described in depth. Among these methods, the biaxial tensile test is the one that can more faithfully reproduce the physiological loading condition. An outline of the most significant results documented in the literature is reported, showing the variety of data on mesh mechanical properties. Synthetic surgical meshes generally follow a non-linear stress-strain behavior, with mechanical characteristics dependant on test direction due to mesh anisotropy. Ex-vivo tests revealed an increased stiffness in mesh explants due to the gradual ingrowth of the host tissue after implant. In general, the absence of standardization in test methods and terminology makes it difficult to compare results from different studies. Numerical models of the abdominal wall interacting with surgical meshes were also discussed representing a potential tool for the selection of suitable prostheses.

Experimental investigation of the biomechanics of urethral tissues and structures.

What is the central question of this study? Prostheses for treatment of urinary incontinence elicit complications associated with an inadequate mechanical action. This investigation aimed to define a procedure addressed to urethral mechanical characterization. Experimental tests are the basis for constitutive formulation, with a view to numerical modelling for investigation of the interaction between the tissues and a prosthesis. What is the main finding and its importance? Horse urethra, selected for its histomorphometric similarity to human urethra, was characterized by integrated histological analysis and mechanical tests on the biological tissue and structure, leading to constitutive formulation. A non‐linear, anisotropic and time‐dependent response was found, representing a valid basis for development of a numerical model to interpret the functional behaviour of the urethra.Urinary dysfunction can lead to incontinence, with relevant impact on the quality of life. This severe dysfunction can be surgically overcome by using an artificial urinary sphincter. However, several complications may result from an inappropriate prosthesis functionality, in many cases due to an unsuitable mechanical action of the device on urethral tissues. Computational models allow the investigation of mechanical interaction between biological tissues and biomedical devices, representing a potential support for surgical practice and prosthesis design. The development of such computational tools requires experimental data on biological tissues and structures mechanics, which are rarely reported in the literature. The present activities aim at providing a procedure for the mechanical characterization of urethral tissues and structures. The experimental protocol includes the morphometric and histologic analysis of urethral tissues, the mechanical characterization of tissues response by tensile and stress relaxation tests and the evaluation of urethral structural behavior by inflation tests. Results from preliminary experimental activities are processed adopting specific model formulations, also providing the definition of parameters that identify elastic and viscous tissues behavior. Different experimental protocols, leading to a comprehensive set of experimental data, allow for a reciprocal assessment of reliability of the investigation approach. n nThis article is protected by copyright. All rights reserved

A numerical investigation of the healthy abdominal wall structures

The present work aims to assess, via numerical modeling, the global passive mechanical behavior of the healthy abdominal wall under the action of pressures that characterize different daily tasks and physiological functions. The evaluation of a normal range of intra-abdominal pressure (IAP) during activities of daily living is fundamental because pressure alterations can cause several adverse effects. At this purpose, a finite element model is developed from literature histomorphometric data and from diagnostic images of Computed Tomography (CT), detailing the different anatomical regions. Numerical simulations cover an IAP up to the physiological limit of 171 (0.0223MPa) mmHg reached while jumping. Numerical results are in agreement with evidences on physiological abdomens when evaluating the local deformations along the craniocaudal direction, the transversal load forces in different regions and the increase of the abdominal area at a IAP of 12mmHg. The developed model can be upgraded for the investigation of the abdominal hernia repair and the assessment of prostheses mechanical compatibility, correlating stiffness and tensile strength of the abdominal tissues with those of surgical meshes.

Constitutive Modeling of Time-Dependent Response of Human Plantar Aponeurosis

The attention is focused on the viscoelastic behavior of human plantar aponeurosis tissue. At this purpose, stress relaxation tests were developed on samples taken from the plantar aponeurosis of frozen adult donors with age ranging from 67 to 78 years, imposing three levels of strain in the physiological range (4%, 6%, and 8%) and observing stress decay for 240u2009s. A viscohyperelastic fiber-reinforced constitutive model with transverse isotropy was assumed to describe the time-dependent behavior of the aponeurotic tissue. This model is consistent with the structural conformation of the tissue where collagen fibers are mainly aligned with the proximal-distal direction. Constitutive model fitting to experimental data was made by implementing a stochastic-deterministic procedure. The stress relaxation was found close to 40%, independently of the level of strain applied. The agreement between experimental data and numerical results confirms the suitability of the constitutive model to describe the viscoelastic behaviour of the plantar aponeurosis.

MECHANICAL CHARACTERIZATION OF ANIMAL DERIVED GRAFTS FOR SURGICAL IMPLANTATION

Bioprostheses obtained from animal models are often adopted in abdominal surgery for repair and reconstruction. The functionality of these prosthetic implants is related also to their mechanical characteristics that are analyzed here. This work illustrates a constitutive model to describe the short-term mechanical response of PermacolTM bioprostheses. Experimental tests were developed on tissue samples to highlight mechanical non-linear characteristics and viscoelastic phenomena. Uni-axial tensile tests were developed to evaluate the strength and strain stiffening. Incremental uni-axial stress relaxation tests were carried out at nominal strain ranging from 10% to 20% and to monitor the stress relaxation process up to 400s. The constitutive model effectively describes the mechanical behavior found in experimental testing. The mechanical response appears to be independent on the loading direction, showing that the tissue can be considered as isotropic. The viscoelastic response of the tissue shows a strong decay of the stress in the first seconds of the relaxation process. The investigation performed is aimed at a general characterization of the biomechanical response and addresses the development of numerical models to evaluate the biomechanical performance of the graft with surrounding host tissues.

Biomechanical behavior of human crural fascia in anterior and posterior regions of the lower limb.

The present work focuses on the numerical modeling of the mechanical behavior of the crural fascia, the deep fascia enwrapping the lower limb muscles. This fascia has an important biomechanical role, due to its interaction with muscles during contraction and its association with pathological events, such as compartment syndrome. The mechanical response of the crural fascia is described by assuming a hyperelastic fiber-reinforced constitutive model, with families of fibers disposed according to the spatial disposition of the collagen network, as shown in histological analyses. A two-dimensional finite element model of a lower limb transversal section has been developed to analyze deformational behavior, with particular attention on interaction phenomena between crural fascia and enwrapped muscles. The constitutive model adopted for the crural fascia well fits experimental data taken along the proximal–distal and medial–lateral directions. The finite element analysis allows for interpreting the relation between change in volume and pressure of muscle compartments and the crural fascia deformation.

Computational modeling of abdominal hernia laparoscopic repair with a surgical mesh

PurposeAlthough new techniques and prostheses have been introduced in ventral hernia surgery, abdominal hernia repair still presents complications, such as recurrence, pain, and discomfort. Thus, this work implements a computational method aimed at evaluating biomechanical aspects of the abdominal hernia laparoscopic repair, which can support clinical research tailored to hernia surgery.MethodsA virtual solid model of the abdominal wall is obtained from MRI scans of a healthy subject. The mechanical behavior of muscular and fascial tissues is described by constitutive formulations with specific parameters. A defect is introduced to reproduce an incisional hernia. Laparoscopic repair is mimicked via intraperitoneal positioning of a surgical mesh. Numerical analyses are performed to evaluate the mechanical response of the abdominal wall in healthy, herniated and post-surgery configurations, considering physiological intra-abdominal pressures.ResultsDuring the deformation of the abdominal wall at increasing pressures, a percentage displacement increment up to 6% is found in the herniated condition, while the mechanical behavior of the repaired abdomen is similar to the healthy one. In the pressure range between 8 mmHg and 55 mmHg, the herniated abdomen shows an incremental stiffness differing of 7% with respect to the healthy condition, while the post-surgery condition shows an increase of the incremental stiffness up to 58%.ConclusionsThis computational approach may be exploited to investigate different aspects of abdominal wall surgical repair, including mesh mechanical characteristics and positioning. Numerical modeling offers a helpful support for selecting the best-fitting prosthesis for customize pre-surgery planning.

Investigation of interaction phenomena between crural fascia and muscles by using a three-dimensional numerical model

The focus of this work is the numerical modeling of the anterior compartment of the human leg with particular attention to crural fascia. Interaction phenomena between fascia and muscles are of clinical interest to explain some pathologies, as the compartment syndrome. A first step to enhance knowledge on this topic consists in the investigation of fascia biomechanical role and its interaction with muscles in physiological conditions. A three-dimensional finite element model of the anterior compartment is developed based on anatomical data, detailing the structural conformation of crural fascia, composed of three layers, and modeling the muscles as a unique structure. Different constitutive models are implemented to describe the mechanical response of tissues. Crural fascia is modeled as a hyperelastic fiber-reinforced material, while muscle tissue via a three-element Hill’s model. The numerical analysis of isotonic contraction of muscles is performed, allowing the evaluation of pressure induced within muscles and consequent stress and strain fields arising on the crural fascia. Numerical results are compared with experimental measurements of the compartment radial deformation and intracompartmental pressure during concentric contraction, to validate the model. The numerical model provides a suitable description of muscles contraction of the anterior compartment and the consequent mechanical interaction with the crural fascia.

Numerical modelling of crural fascia mechanical interaction with muscular compartments.

The interaction of the crural fascia with muscular compartments and surrounding tissues can be at the origin of different pathologies, such as compartment syndrome. This pathology consists in the onset of excessive intracompartmental pressure, which can have serious consequences for the patient, compromising blood circulation. The investigation of compartment syndrome etiology also takes into account the alteration of crural fascia mechanical properties as a cause of the syndrome, where the fascial stiffening would result in the rise of intracompartmental pressure. This work presents a computational approach toward evaluating some biomechanical aspects of the problem, within the context of a more global viewpoint. Finite element analyses of the interaction phenomena of the crural fascia with adjacent regions are reported here. This study includes the effects of a fascial stiffness increase along the proximal–distal direction and their possible clinical implications. Furthermore, the relationship between different pre-strain levels of the crural fascia in the proximal–distal direction and the rise of internal pressure in muscular compartments are considered. The numerical analyses can clarify which aspects could be directly implied in the rise of compartment syndrome, leading to greater insight into muscle-fascia mechanical phenomena, as well as promoting experimental investigation and clinical analysis of the syndrome.