New methodology for mechanical characterization of human superficial facial tissue anisotropic behaviour in vivo

Exploring uncertainty in hyper-viscoelastic properties of scalp skin through patient-specific finite element models for reconstructive surgery

Understanding skin responses to external forces is crucial for post-cutaneous flap wound healing. However, the in vivo viscoelastic behavior of scalp skin remains poorly understood. Personalized virtual surgery simulations offer a way to study tissue responses in relevant 3D geometries. Yet, anticipating wound risk remains challenging due to limited data on skin viscoelasticity, which hinders our ability to determine the interplay between wound size and stress levels. To bridge this gap, we reexamine three clinical cases involving scalp reconstruction using patient-specific geometric models and employ uncertainty quantification through a Monte Carlo simulation approach to study the effect of skin viscoelasticity on the final stress levels from reconstructive surgery. Utilizing the generalized Maxwell model via the Prony series, we can parameterize and efficiently sample a realistic range of viscoelastic response and thus shed light on the influence of viscoelastic material uncertainty in surgical scenarios. Our analysis identifies regions at risk of wound complications based on reported threshold stress values from the literature and highlights the significance of focusing on long-term responses rather than short-term ones.

Assessment of needle-tissue force models based on ex vivo measurements

Needle insertion is one of the most common procedures in clinical practice. Existing statistics reveal that success rates of needle insertions can be low, leading to potential complications and patient discomfort. Real-time imaging techniques like ultrasound and X-ray can assist in improving precision, but even experienced practitioners may face challenges in visualizing the needle tip. Researchers have proposed models of force interactions during needle insertions into biological tissue to enhance accuracy. This article presents an evaluation of the forces acting on intravenous needles during insertion into skin. The aim was to explore mathematical models, compare them with data from tests on animal specimens, and select the most suitable model for future research. The experimental setup involved conducting needle insertion tests on animal-originated cadavers, using the Brucker Universal Mechanical Tester device, which measured the force response during vertical movement of the needle. The research was divided into 2 stages. In Stage I, force measurements were recorded for both the insertion and extraction phases of the hypodermic needles. The measurements were conducted for several different needle sizes, speed and insertion angles. In Stage II, five different models were examined to determine how well they matched the experimental data. Based on the analysis of fit quality coefficients, the Gordon's exponential model was identified as the best fit to the measured data. The influence of needle size, insertion angle, and insertion speed on the measured force values was confirmed. Different insertion speeds revealed the viscoelastic properties of the tested samples. The presence of the skin layer affected the puncture force and force values for subsequent layers.

Applied use of biomechanical measurements from human tissues for the development of medical skills trainers: a scoping review

OBJECTIVE

The objective of this review was to identify quantitative biomechanical measurements of human tissues, the methods for obtaining these measurements, and the primary motivations for conducting biomechanical research.

INTRODUCTION

Medical skills trainers are a safe and useful tool for clinicians to use when learning or practicing medical procedures. The haptic fidelity of these devices is often poor, which may be because the synthetic materials chosen for these devices do not have the same mechanical properties as human tissues. This review investigates a heterogeneous body of literature to identify which biomechanical properties are available for human tissues, the methods for obtaining these values, and the primary motivations behind conducting biomechanical tests.

INCLUSION CRITERIA

Studies containing quantitative measurements of the biomechanical properties of human tissues were included. Studies that primarily focused on dynamic and fluid mechanical properties were excluded. Additionally, studies only containing animal, in silico , or synthetic materials were excluded from this review.

METHODS

This scoping review followed the JBI methodology for scoping reviews and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR). Sources of evidence were extracted from CINAHL (EBSCO), IEEE Xplore, MEDLINE (PubMed), Scopus, and engineering conference proceedings. The search was limited to the English language. Two independent reviewers screened titles and abstracts as well as full-text reviews. Any conflicts that arose during screening and full-text review were mediated by a third reviewer. Data extraction was conducted by 2 independent reviewers and discrepancies were mediated through discussion. The results are presented in tabular, figure, and narrative formats.

RESULTS

Data were extracted from a total of 186 full-text publications. All of the studies, except for 1, were experimental. Included studies came from 33 countries, with the majority coming from the United States. Ex vivo methods were the predominant approach for extracting human tissue samples, and the most commonly studied tissue type was musculoskeletal. In this study, nearly 200 unique biomechanical values were reported, and the most commonly reported value was Young's (elastic) modulus. The most common type of mechanical test performed was tensile testing, and the most common reason for testing human tissues was to characterize biomechanical properties. Although the number of published studies on biomechanical properties of human tissues has increased over the past 20 years, there are many gaps in the literature. Of the 186 included studies, only 7 used human tissues for the design or validation of medical skills training devices. Furthermore, in studies where biomechanical values for human tissues have been obtained, a lack of standardization in engineering assumptions, methodologies, and tissue preparation may implicate the usefulness of these values.

CONCLUSIONS

This review is the first of its kind to give a broad overview of the biomechanics of human tissues in the published literature. With respect to high-fidelity haptics, there is a large gap in the published literature. Even in instances where biomechanical values are available, comparing or using these values is difficult. This is likely due to the lack of standardization in engineering assumptions, testing methodology, and reporting of the results. It is recommended that journals and experts in engineering fields conduct further research to investigate the feasibility of implementing reporting standards.

REVIEW REGISTRATION

Open Science Framework https://osf.io/fgb34.

A computer based facial flaps simulator using projective dynamics

BACKGROUND AND OBJECTIVES

Interactive surgical simulation using the finite element method to model human skin mechanics has been an elusive goal. Mass-spring networks, while fast, do not provide the required accuracy.

METHODS

This paper presents an interactive, cognitive, facial flaps simulator based on a projective dynamics computational framework. Projective dynamics is able to generate rapid, stable results following changes to the facial soft tissues created by the surgeon, even in the face of sudden increases in skin resistance as its stretch limit is reached or collision between tissues occurs. Our prior work with the finite element method had been hampered by these considerations. Surgical tools are provided for; skin incision, undermining, deep tissue cutting, and excision. A spring-like "skin hook" is used for retraction. Spring-based sutures can be placed individually or automatically placed as a row between cardinal sutures.

RESULTS

Examples of an Abbe/Estlander lip reconstruction, a paramedian forehead flap to the nose, a retroauricular flap reconstruction of the external ear, and a cervico-facial flap reconstruction of a cheek defect are presented.

CONCLUSIONS

Projective dynamics has significant advantages over mass-spring and finite element methods as the physics backbone for interactive soft tissue surgical simulation.

Experimental characterisation of porcine subcutaneous adipose tissue under blunt impact up to irreversible deformation

A deeper understanding of the mechanical characteristics of adipose tissue under large deformation is important for the analysis of blunt force trauma, as adipose tissue alters the stresses and strains that are transferred to subjacent tissues. Hence, results from drop tower tests of subcutaneous adipose tissue are presented (i) to characterise adipose tissue behaviour up to irreversible deformation, (ii) to relate this to the microstructural configuration, (iii) to quantify this deformation and (iv) to provide an analytical basis for computational modelling of adipose tissue under blunt impact. The drop tower experiments are performed exemplarily on porcine subcutaneous adipose tissue specimens for three different impact velocities and two impactor geometries. An approach based on photogrammetry is used to derive 3D representations of the deformation patterns directly after the impact. Median values for maximum impactor acceleration for tests with a flat cylindrical impactor geometry at impact velocities of 886 mm/s, 1253 mm/s and 2426 mm/s amount to 61.1 g, 121.6 g and 264.2 g, respectively, whereas thickness reduction of the specimens after impact amount to 16.7%, 30.5% and 39.3%, respectively. The according values for tests with a spherically shaped impactor at an impact velocity of 1253 mm/s are 184.2 g and 78.7%. Based on these results, it is hypothesised that, in the initial phase of a blunt impact, adipose tissue behaviour is mainly governed by the behaviour of the lipid inside the adipocytes, whereas for further loading, contribution of the extracellular collagen fibre network becomes more dominant.

The effect of skin tension, needle diameter and insertion velocity on the fracture properties of porcine tissue

Using metal needles to penetrate skin tissue is common in medical treatments for the delivery of medication or minimally invasive surgery. In most applications the fracture properties of skin tissue is not important as the human surgeon has full control over the needle. Given that robotically controlled surgeries and self applied medical devices have become increasingly popular, a better understanding of the fracture properties and how to mathematically model the fracture process is needed. Experiments measuring the force required to fracture porcine skin tissue were done while varying the applied skin tension, needle insertion speed and needle diameter. The applied skin tension was found to have the greatest influence on the fracture properties, while the insertion speed was found to have a negligible impact. The variance in experimental results was not well explained by the three independent variables alone, suggesting that additional parameters influence the fracture process.

Friction properties of in vivo human skin from visualized friction testing

Investigations on mechanical behaviors of intravital human skin are of significance in various fields. However, due to the great complexity and the individual variation of human skin, traditional experimental mechanics often fails to work in such research objects. In this study, the friction property considering the skin-uplift effect of human skin was in vivo studied experimentally and theoretically. An in situ and noninvasive friction experiment was performed in vivo on human skin, where the projected contact morphology was captured through a novel specially developed optical system. According to the contact morphology, a model taking uplift resistance into account is proposed based on Greenwood model, in which the contact area was depicted as a combination of two ellipses to better characterize the skin deformation. Moreover, since the model degrades into Greenwood model in small deformation, it can be considered as an extension from the perspective of small deformation to large deformation. Based on the model, the adhesion friction and deformation friction have been separated according to the ratio of indentation depth to probe radius. The results show that the friction property of skin varies with the indentation depth changing, and the deformation friction is positively correlated with the ratio of indentation depth to probe radius.

In vivo stress relaxation of human scalp

OBJECTIVE

Conduct a first in vivo study on the large deformation stress relaxation behavior of the human scalp.

METHODS

This study was conducted during Mohs micrographic surgery of the scalp of 14 patients aged 59-90 with wounds initially ranging from 9 to 41 mm wide. The initial wound diameter was measured under zero applied force. Then, the force required to close each wound using a single size 1 nylon suture and a SUTUREGARD suture retention device was measured, after which the suture was then locked in the retention device at fixed displacement. At time points of 300 s, 600 s, and 1800 s, the suture retention device was released, and the wound opening was again recorded at zero force, and the force required to close the wound was recorded.

RESULTS

The average wound closure force relaxed by 44% and 65% after 300 s and 1800 s, respectively. Average wound width decreased 30% and 42%, after 300 s and 1800 s, respectively, due to creep deformation. Furthermore, all wounds relaxed to be below 15 N of closure force after 600 s, which is considered the maximum clinically acceptable force. A relaxation time of ∼270 s and a threshold force for creep of ∼5 N was found.

SIGNIFICANCE

Results of this study provide the first quantitative clinical guidance for efficient scalp closure of large wounds by creep deformation and stress relaxation. Furthermore, the methodology developed here can be used as a basis for future in vivo studies of the stress relaxation and creep deformation of human scalp, which in turn can provide data for the development and validation of constitutive models for scalp deformation.

Location-specific mechanical response and morphology of facial soft tissues

The facial tissue of 9 healthy volunteers (m/f; age: 23-60y) is characterized at three different locations using a procedure combining suction measurements and 18MHz ultrasound imaging. The time-dependent and multilayered nature of skin is accounted for by adopting multiple loading protocols which differ with respect to suction probe opening size and rate of tissue deformation. Over 700 suction measurements were conducted and analyzed according to location-specific mechanical and morphological characteristics. All corresponding data are reported and made available for facial tissue analysis and biomechanical modeling. Higher skin stiffness is measured at the forehead in comparison to jaw and parotid; these two regions are further characterized by lower creep deformation. Thicker tissue regions display a tendency towards a more compliant and less dissipative response. Comparison of superficial layer thickness and corresponding mechanical measurements suggests that connective tissue density determines the resistance to deformation in suction experiments.