In vivo experimental testing and model identification of human scalp skin

On the correlations of biomechanical properties of super-imposed temporal tissue layers and their age-, sex-, side- and post-mortem interval dependence

Obtaining biomechanical properties of biological tissues for simulation purposes or graft developments is time and resource consuming. The number of samples required for biomechanical tests could be reduced if the load-deformation properties of a given tissue layer could be estimated from adjacent layers or if the biomechanical parameters were unaffected by age, bodyside, sex or post-mortem interval. This study investigates for the first time potential correlations of multiple super-imposed tissue layers using the temporal region of the human head as an area of broad interest in biomechanical modelling. Spearman correlations between biomechanical properties of the scalp, muscle fascia, muscle, bone and dura mater from up to 83 chemically unfixed cadavers were investigated. The association with age, sex and post-mortem interval was assessed. The results revealed sporadic correlations between the corresponding layers, such as the maximum force (r = 0.43) and ultimate tensile strength (r = 0.33) between scalp and muscle. Side- and age-dependence of the biomechanical properties were different between the tissue types. Strain at maximum force of fascia (r = -0.37) and elastic modulus of temporal muscle (r = 0.26) weakly correlated with post-mortem interval. Only strain at maximum force of scalp differed significantly between sexes. Uniaxial biomechanical properties of individual head tissue layers can thus not be estimated solely based on adjacent layers. Therefore, correlations between the tissues' biomechanical properties, anthropometric data and post-mortem interval need to be established independently for each layer. Sex seems not to be a relevant influencing factor for the passive tissue mechanics of the here investigated temporal head tissue layers.

Identification of Young's modulus and equivalent spring constraint boundary conditions of the soft tissue with locally observed displacements for endoscopic liver surgery

In endoscopic surgery, the surgical navigation system needs to calculate the deformation of soft tissue by biomechanical model which requires elastic properties and boundary conditions. However, patient-specific elastic parameters and boundary conditions of soft tissue are hard to measure accurately from the preoperative images, especially the boundary conditions will change during the operation due to the ligament cutting. In addition, simple boundary conditions such as fixed constraints and free-force constraints are not physically adequate to simulate the elastic effect of ligaments attached to the liver. In this paper, we present a novel method to identify the Young's modulus and equivalent spring constraint boundary conditions of a locally observed soft tissue. Based on the spring constraint boundary condition, a two-step inverse algorithm is developed based on the finite element method (FEM) with integration of energy regularized Gauss-Newton (GN) method and l1-regularized method, which takes external forces and displacements of observable nodes as inputs. A series of numerical simulations and physical hydrogel phantom experiments were conducted. The results of simulation and physical experiments show that the Young's modulus and equivalent spring constraint boundary conditions identified by the proposed method agree well with their setup true values.

Load-deformation characteristics of acellular human scalp: assessing tissue grafts from a material testing perspective

Acellular matrices seem promising scaffold materials for soft tissue regeneration. Biomechanical properties of such scaffolds were shown to be closely linked to tissue regeneration and cellular ingrowth. This given study investigated uniaxial load-deformation properties of 34 human acellular scalp samples and compared these to age-matched native tissues as well as acellular dura mater and acellular temporal muscle fascia. As previously observed for human acellular dura mater and temporal muscle fascia, elastic modulus (p = 0.13) and ultimate tensile strength (p = 0.80) of human scalp samples were unaffected by the cell removal. Acellular scalp samples showed a higher strain at maximum force compared to native counterparts (p = 0.02). The direct comparison of acellular scalp to acellular dura mater and temporal muscle fascia revealed a higher elasticity (p < 0.01) and strain at maximum force (p = 0.02), but similar ultimate tensile strength (p = 0.47). Elastic modulus and ultimate tensile strength of acellular scalp decreased with increasing post-mortem interval. The elongation behavior formed the main biomechanical difference between native and acellular human scalp samples with elastic modulus and ultimate tensile strength being similar when comparing the two.

Mechanical characterisation of human and porcine scalp tissue at dynamic strain rates

Several biomedical applications require knowledge of the behaviour of the scalp, including skin grafting, skin expansion and head impact biomechanics. Scalp tissue exhibits a non-linear stress-strain relationship, anisotropy and its mechanical properties depend on strain rate. When modelling the behaviour of the scalp, all these factors should be considered in order to perform realistic simulations. Here, tensile tests at strain rates between 0.005 and 100 s^-1 have been conducted on porcine and human scalp in order to investigate the non-linearity, anisotropy, and strain rate dependence of the scalp mechanical properties. The effect of the orientation of the sample with respect to the Skin Tension Lines (STLs) was considered during the test. The results showed that anisotropy is evident in the hyperelastic response at low strain rates (0.005 s^-1) but not at higher strain rates (15-100 s^-1). The mechanical properties of porcine scalp differ from human scalp. In particular, the elastic modulus and the Ultimate Tensile Strength (UTS) of the porcine scalp were found to be almost twice the values of the human scalp, whereas the stretch at failure was not found to be significantly different. An anisotropic hyperelastic model (Gasser-Ogden-Holzapfel) was used to model the quasi-static behaviour of the tissue, whereas three different isotropic hyperelastic models (Fung, Gent and Ogden) were used to model the behaviour of scalp tissue at higher strain rates. The experimental results outlined here have important implications for those wishing to model the mechanical behaviour of scalp tissue both under quasi-static and dynamic loading conditions.

A model of human skin under large amplitude oscillatory shear

Skin mechanics is of importance in various fields of research when accurate predictions of the mechanical response of skin is essential. This study aims to develop a new constitutive model for human skin that is capable of describing the heterogeneous, nonlinear viscoelastic mechanical response of human skin under shear deformation. This complex mechanical response was determined by performing large amplitude oscillatory shear (LAOS) experiments on ex vivo human skin samples. It was combined with digital image correlation (DIC) on the cross-sectional area to assess heterogeneity. The skin is modeled as a one-dimensional layered structure, with every sublayer behaving as a nonlinear viscoelastic material. Heterogeneity is implemented by varying the stiffness with skin depth. Using an iterative parameter estimation method all model parameters were optimized simultaneously. The model accurately captures strain stiffening, shear thinning, softening effect and nonlinear viscous dissipation, as experimentally observed in the mechanical response to LAOS. The heterogeneous properties described by the model were in good agreement with the experimental DIC results. The presented mathematical description forms the basis for a future constitutive model definition that, by implementation in a finite element method, has the capability of describing the full 3D mechanical behavior of human skin.

Towards an in-plane methodology to track breast lesions using mammograms and patient-specific finite-element simulations

In breast cancer screening or diagnosis, it is usual to combine different images in order to locate a lesion as accurately as possible. These images are generated using a single or several imaging techniques. As x-ray-based mammography is widely used, a breast lesion is located in the same plane of the image (mammogram), but tracking it across mammograms corresponding to different views is a challenging task for medical physicians. Accordingly, simulation tools and methodologies that use patient-specific numerical models can facilitate the task of fusing information from different images. Additionally, these tools need to be as straightforward as possible to facilitate their translation to the clinical area. This paper presents a patient-specific, finite-element-based and semi-automated simulation methodology to track breast lesions across mammograms. A realistic three-dimensional computer model of a patient's breast was generated from magnetic resonance imaging to simulate mammographic compressions in cranio-caudal (CC, head-to-toe) and medio-lateral oblique (MLO, shoulder-to-opposite hip) directions. For each compression being simulated, a virtual mammogram was obtained and posteriorly superimposed to the corresponding real mammogram, by sharing the nipple as a common feature. Two-dimensional rigid-body transformations were applied, and the error distance measured between the centroids of the tumors previously located on each image was 3.84 mm and 2.41 mm for CC and MLO compression, respectively. Considering that the scope of this work is to conceive a methodology translatable to clinical practice, the results indicate that it could be helpful in supporting the tracking of breast lesions.

Scalp simulation - A novel approach to site-specific biomechanical modeling of the skin

OBJECTIVES

This study aimed to determine the hardness of the human scalp in vivo in order to identify an appropriate scalp simulant, from a range of commercially available silicone materials, for force impact assessment. Site-dependent variation in scalp hardness, and the applicability of contemporary skin simulants to the scalp were also considered.

MATERIALS AND METHODS

A Shore A-type durometer was used to collected hardness data from the scalps of 30 human participants (five males and five females in each of the three age categories: 18-30, 31-40, 41-50) and four commercially available silicones (light, medium, and heavy-bodied PVS, and duplication silicone). One-sample t-tests were used to compare the mean hardness of simulants to that of the scalp. Site-dependent variation in the hardness of the scalp was assessed using a mixed-model repeated measures ANOVA.

RESULTS

Mean human scalp hardness derived from participants was 20.6 Durometer Units (DU; SD = 3.4). Analysis revealed only the medium-bodied PVS to be an acceptable scalp simulant when compared to the mean hardness of the human scalp (p = 0.869). Scalp hardness varied significantly anteroposteriorly (with an observable linear trend, p < 0.001), but not mediolaterally (p = 0.271). Comparisons of simulants to site-specific variation in scalp hardness anteroposteriorly found the medium-bodied PVS to be only suitable in the central region of the scalp (p = 0.391). In contrast, the duplication silicone (p = 0.074) and light-bodied PVS (p = 0.147) were only comparable to the posterior region.

CONCLUSIONS

Contemporary skin simulants fail to accurately represent the scalp in terms of hardness. There is strong support for the use of medium-bodied PVS as a scalp simulant. Human scalp hardness varies significantly anteroposteriorly, but not mediolaterally, corresponding to regional anatomical variation within the scalp. A number of materials were identified as potential simulants for different regions of the scalp when more site-specific simulant research is required.

Intra- and inter-individual variability in the mechanical properties of the human skin from in vivo measurements on 20 volunteers

BACKGROUND/PURPOSE

The mechanical properties and behavior of the human skin in vivo are of medical importance, particularly to surgeons who have to consider the skin extension capabilities in the preparation of surgical acts. Variable data can be found in literature that result from diverse kinds of tests (in vivo, ex vivo, and postmortem) performed with different instruments.

METHODS

This paper presents the results of in vivo measurements performed on a cohort of 20 healthy volunteers with an ultralight homemade uniaxial extensometer. Different anatomical zones were explored under different directions of solicitation in order to document inter- and intra-individual variability as well as skin anisotropy.

RESULTS

The experimental data obtained are fitted with a phenomenological exponential model allowing the identification of three parameters characteristic of the tested skin behavior. These parameters can be related to the concept of skin extensibility used by surgeons.

CONCLUSION

The inter- and intra-variability observed on that cohort confirms the need for a patient-specific approach based on the in vivo measurement of the mechanical behavior of the human skin of interest. Even the direction of higher skin stiffness is found to be individual-dependent. The capability of the extensometer used in this study to fulfill such measurement needs is also demonstrated.

Biomechanics of oral mucosa

The prevalence of prosthodontic treatment has been well recognized, and the need is continuously increasing with the ageing population. While the oral mucosa plays a critical role in the treatment outcome, the associated biomechanics is not yet fully understood. Using the literature available, this paper provides a critical review on four aspects of mucosal biomechanics, including static, dynamic, volumetric and interactive responses, which are interpreted by its elasticity, viscosity/permeability, apparent Poisson's ratio and friction coefficient, respectively. Both empirical studies and numerical models are analysed and compared to gain anatomical and physiological insights. Furthermore, the clinical applications of such biomechanical knowledge on the mucosa are explored to address some critical concerns, including stimuli for tissue remodelling (interstitial hydrostatic pressure), pressure–pain thresholds, tissue displaceability and residual bone resorption. Through this review, the state of the art in mucosal biomechanics and their clinical implications are discussed for future research interests, including clinical applications, computational modelling, design optimization and prosthetic fabrication.

A polynomial hyperelastic model for the mixture of fat and glandular tissue in female breast

In the breast of adult women, glandular and fat tissues are intermingled and cannot be clearly distinguished. This work studies if this mixture can be treated as a homogenized tissue. A mechanical model is proposed for the mixture of tissues as a function of the fat content. Different distributions of individual tissues and geometries have been tried to verify the validity of the mixture model. A multiscale modelling approach was applied in a finite element model of a representative volume element (RVE) of tissue, formed by randomly assigning fat or glandular elements to the mesh. Both types of tissues have been assumed as isotropic, quasi-incompressible hyperelastic materials, modelled with a polynomial strain energy function, like the homogenized model. The RVE was subjected to several load cases from which the constants of the polynomial function of the homogenized tissue were fitted in the least squares sense. The results confirm that the fat volume ratio is a key factor in determining the properties of the homogenized tissue, but the spatial distribution of fat is not so important. Finally, a simplified model of a breast was developed to check the validity of the homogenized model in a geometry similar to the actual one.

Biomechanics of the Rhombic Transposition Flap

Objective

To develop a computational model of cutaneous wound closures comparing variations of the rhombic transposition flap.

Study Design

A nonlinear hyperelastic finite element model of human skin was developed and used to assess flap biomechanics in simulated rhombic flap wound closures as flap geometric parameters were varied.

Setting

In silico.

Methods

The simulation incorporated variables of transposition angle, flap width, and tissue undermining. A 2-dimensional second-order Yeoh hyperelastic model was fit to published experimental skin data. Resultant stress and strain fields as well as local surface changes were evaluated.

Results

For the rhombus defect, closure stress and strain were minimized for the transposition flap with a distal flap angle of 30° by recruiting skin from opposing sides of the defect. Alteration of defect dimensions showed that peak stress and principal strain were minimized with a square defect. Likelihood of a standing cutaneous deformity was driven by the magnitude of angle closure at the flap base. Manipulation of the transposition angle reoriented the primary skin strain vector. Asymmetric undermining decoupled wound closure tension from strain, with direct effects on boundary deformation.

Conclusions

The model demonstrates that flap width determines the degree of secondary tissue movement and impact on surrounding tissues. Transposition angle determines the orientation of maximal strain. Local flap design requires consideration of multiple factors apart from idealized biomechanics, including adjacent “immobile” structures, scar location, local skin thickness, and orientation of relaxed skin tension lines. Finite element models can be used to analyze local flap closures to optimize outcomes.

Biomechanics of the monopedicle skin flap

OBJECTIVE

The design and implementation of skin flaps remains a puzzle for the reconstructive surgeon. The objective of the present study is to use finite element (FE) analysis to characterize and understand the biomechanics of the monopedicle skin flap design.

STUDY DESIGN

The current study uses a nonlinear hyperelastic FE model of the human skin to understand the biomechanics of monopedicle-based flap designs as geometric flap parameters are varied.

SETTING

In silico.

SUBJECTS AND METHODS

The simulation included the displacement loading, stitching, and relaxation of various forms of the flap design. Stress and strain outcomes, previously correlated with scarring, necrosis, and blood perfusion, are reported for a basic monopedicle design as well as a number of modifications to this design.

RESULTS

The results suggest that the length of the monopedicle flap should not exceed 3 times the size of the defect, as the benefit in reducing principal strain (deformation) is diminished beyond this point. Further, to minimize skin strain, the ideal Burrow's triangle size can be described as proportional to flap length and inversely proportional to defect height, according to a linear function.

CONCLUSION

The ideal flap design should attempt to minimize not only the stress in the skin, but the size of the incisions and the degree of undermining. The results of our analyses provide guidance to increase the general understanding of monopedicle flap mechanics and provide context for the clinician and insight into designing a better monopedicle flap for individual situations.

Power type strain energy function model and prediction of the anisotropic mechanical properties of skin using uniaxial extension data

Many successful models to describe the biomechanical characteristics of planar biological soft tissues are based on strain energy function. However, the parameters in these models are determined by biaxial extension test, which might be difficult to exercise for certain types of soft tissue. This study presents a new constitutive model, the power type strain energy density function model (PTM), and a method to identify its material parameters for rabbit skin using uniaxial extension test of 4-direction strip samples. The abdominal skins from eight rabbits were taken to perform uniaxial tension tests in 7 different directions. The material parameters were identified for each subject based on any 4 out of 7 directions by applying some definite conditions of this issue. For each rabbit, the 35 groups of material parameters were consistent. The 7 material parameters in PTM were identified with root mean square errors <0.061. The results indicate that the material parameters of rabbit skin can be identified from uniaxial extension test data.

An experimental study of the mouse skin behaviour: Damage and inelastic aspects

Samples of male and female mice skin were tested under monotonic and cyclic loading to mechanically characterize the tissue for large deformations. Cyclic tests have shown a typical Mullins effect widely known for elastomers and other soft tissues. No statistical difference was found in the maximum stretch of the sample after the fifth loading cycle for male (1.26 +/- 0.035) and female (1.18 +/- 0.083). However, larger dispersion was obtained for the maximum stress for both genders, 0.61 +/- 0.16 MPa for male and 0.78 +/- 0.32 MPa for female. Results show the presence of inelastic strain and stress softening in the skin at large deformations. They also have shown how stress softening and residual strain change with the magnitude of the applied load. Good correlation was observed between the residual strain and the maximum strain previously attained by the sample during loading for all samples. However, the correlation was different between genders.

An EFE Model on Skin-Sleeve Interactions During Arm Rotation

Skin and garment constitute a dynamic contact system for human body comfort and protection. Although dermatological injuries due to fabric actions during human body movement are common, there is still no general guidance or standard for measuring or evaluating skin/garment contact interactions, especially, during intense sports. A three-dimensional explicit finite element (EFE) model combined with Augmented Lagrange algorithm (ALA) is developed to simulate interactions between skin and fabric during rotation of the arm. Normalized effective shear stresses at the interface between skin and the sleeve during the arm rotation are provided to reflect the severity of the interactions. The effects due to changes in fabric properties, fabric-skin gap, and arm rotation rate are also illustrated. It has been demonstrated from our predictions that factors such as elastic modulus, friction coefficients, density of fabric, and the initial gap between skin and fabric influence significantly the shear stress and thus the discomfort and even injury potential to skin during intensive body movement such as sports and military. Thus this study for the first time confirms quantitatively that poorly chosen fabric with inappropriate garment design renders adverse actions on human skin.

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.