Scar Zones: Region-Specific Differences in Skin Tension May Determine Incisional Scar Formation

Incidence of medical adhesive-related skin injury: a reduction by changing posture

OBJECTIVE

Medical adhesive-related skin injuries (MARSI), defined as skin damage associated with the use of medical adhesive products or devices, are a common and under-reported condition that compromises skin integrity. The prevention and management of MARSI that can occur around the needle insertion site of a chest wall implantable port in hospitalised patients with a tumour remain challenging issues. The aim of this study was to explore whether the incidence of MARSI could be reduced by changing the body position during dressing changes.

METHOD

Participants were recruited between May 2019 and November 2020 in the oncology department of a tertiary hospital. Patients were randomly assigned to Group AB (supine followed by semi-recumbent position) and Group BA (semi-recumbent followed by supine position) with a standard intervening recovery interval of 21-28 days. Assessments for typical MARSI included itching, the combination of erythema and oedema, and blisters in the port area, and were graded according to the level of severity.

RESULTS

The itch intensity was significantly lower in phase B (semi-recumbent) compared to phase A (supine) (2.35±1.985 versus 5.31±1.332, respectively; p<0.01). Similarly, the severity of erythema and oedema was less severe when comparing phase B to phase A: grade 0 (64.9% versus 10.5%, respectively); grade 1 (28.1% versus 19.3%, respectively); grade 2 (3.5% versus 7.0%, respectively); grade 3 (1.8% versus 45.6%, respectively); and grade 4 (1.8% versus 17.5%, respectively) (Z=5.703; p<0.01). Blisters were found far less frequently in phase B than phase A (1.8% versus 56.1%, respectively; p<0.01).

CONCLUSION

The study provided statistically significant evidence that patients in a semi-recumbent position receiving dressing at a chest wall implantable port had fewer and less severe injection site MARSI than when in a supine position.

DECLARATION OF INTEREST

The authors have no conflicts of interest to declare.

Insights and mechanics-driven modeling of human cutaneous impact injuries

Cutaneous damage mechanisms related to dynamic fragment impacts are dependent on the impact angle, impact energy, and fragment characteristics including shape, volume, contact friction, and orientation. Understanding the cutaneous injury mechanism and its relationship to the fragment parameters is lacking compromising damage classification, treatment, and protection. Here we develop a high-fidelity dynamic mechanics-driven model for partial-thickness skin injuries and demonstrate the influence of fragment parameters on the injury mechanism and damage sequence. The model quantitatively predicts the wound shape, area, and depth into the skin layers for selected impact angles, kinetic energy density, and the fragment projectile type including shape and material. The detailed sequence of impact damage including epidermal tearing that occurs ahead of the fragments initial contact location, subsequent stripping of the epidermal/dermal junction, and crushing of the underlying dermis are revealed. We demonstrate that the fragment contact friction with skin plays a key role in redistributing impact energy affecting the extent of epidermal tearing and dermal crushing. Furthermore, projectile edges markedly affect injury severity dependent on the orientation of the edge during initial impact. The model provides a quantitative framework for understanding the detailed mechanisms of cutaneous damage and a basis for the design of protective equipment.

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.

An Overview of Scaffolds and Biomaterials for Skin Expansion and Soft Tissue Regeneration: Insights on Zinc and Magnesium as New Potential Key Elements

The utilization of materials in medical implants, serving as substitutes for non-functional biological structures, supporting damaged tissues, or reinforcing active organs, holds significant importance in modern healthcare, positively impacting the quality of life for millions of individuals worldwide. However, certain implants may only be required temporarily to aid in the healing process of diseased or injured tissues and tissue expansion. Biodegradable metals, including zinc (Zn), magnesium (Mg), iron, and others, present a new paradigm in the realm of implant materials. Ongoing research focuses on developing optimized materials that meet medical standards, encompassing controllable corrosion rates, sustained mechanical stability, and favorable biocompatibility. Achieving these objectives involves refining alloy compositions and tailoring processing techniques to carefully control microstructures and mechanical properties. Among the materials under investigation, Mg- and Zn-based biodegradable materials and their alloys demonstrate the ability to provide necessary support during tissue regeneration while gradually degrading over time. Furthermore, as essential elements in the human body, Mg and Zn offer additional benefits, including promoting wound healing, facilitating cell growth, and participating in gene generation while interacting with various vital biological functions. This review provides an overview of the physiological function and significance for human health of Mg and Zn and their usage as implants in tissue regeneration using tissue scaffolds. The scaffold qualities, such as biodegradation, mechanical characteristics, and biocompatibility, are also discussed.

Expert recommendations on supportive skin care for non-surgical and surgical procedures

A thorough knowledge of non-surgical procedures (laser, peelings, injections, threads) and surgical procedures (combined surgeries and skin grafts), including contraindications and potential risks and side effects, (e.g. infection, hypopigmentation, hyperpigmentation, and scarring) is essential to be able to reduce their incidence and ensure the patient receives the most benefit from the procedure. Individuals with darker skin and of high Fitzpatrick phototype are at higher risk of dyschromias, notably melasma and post-inflammatory hyperpigmentation, which may be treated using aesthetic procedures but may also arise as a complication of some procedures. A group of experts in cosmetic surgery and dermatology reviewed the published literature and discussed recommendations for optimizing outcomes with practical advice on supportive skincare before, during and after non-surgical or surgical procedures. A broad-spectrum sunscreen with a high sun protection factor against UVB and high protection against UVA, especially long UVA, is essential for all treatment modalities for the prevention and potential improvement of pigmentation disorders. Supportive skin care management to prepare, cleanse and protect the skin and post-procedure skin care with healing and anti-inflammatory ingredients are recommended to speed up regeneration and wound healing whilst minimizing scarring and downtime. Additionally, adjunctive skin care to procedures with antioxidant, anti-ageing and lightening properties may enhance skin benefits.

The effects of mechanical force on fibroblast behavior in cutaneous injury

Wound healing results in the formation of scar tissue which can be associated with functional impairment, psychological stress, and significant socioeconomic cost which exceeds 20 billion dollars annually in the United States alone. Pathologic scarring is often associated with exaggerated action of fibroblasts and subsequent excessive accumulation of extracellular matrix proteins which results in fibrotic thickening of the dermis. In skin wounds, fibroblasts transition to myofibroblasts which contract the wound and contribute to remodeling of the extracellular matrix. Mechanical stress on wounds has long been clinically observed to result in increased pathologic scar formation, and studies over the past decade have begun to uncover the cellular mechanisms that underly this phenomenon. In this article, we will review the investigations which have identified proteins involved in mechano-sensing, such as focal adhesion kinase, as well as other important pathway components that relay the transcriptional effects of mechanical forces, such as RhoA/ROCK, the hippo pathway, YAP/TAZ, and Piezo1. Additionally, we will discuss findings in animal models which show the inhibition of these pathways to promote wound healing, reduce contracture, mitigate scar formation, and restore normal extracellular matrix architecture. Recent advances in single cell RNA sequencing and spatial transcriptomics and the resulting ability to further characterize mechanoresponsive fibroblast subpopulations and the genes that define them will be summarized. Given the importance of mechanical signaling in scar formation, several clinical treatments focused on reducing tension on the wound have been developed and are described here. Finally, we will look toward future research which may reveal novel cellular pathways and deepen our understanding of the pathogenesis of pathologic scarring. The past decade of scientific inquiry has drawn many lines connecting these cellular mechanisms that may lead to a map for the development of transitional treatments for patients on the path to scarless healing.

Topological Distribution of Wound Stiffness Modulates Wound-Induced Hair Follicle Neogenesis

In the large full-thickness mouse skin regeneration model, wound-induced hair neogenesis (WIHN) occurs in the wound center. This implies a spatial regulation of hair regeneration. The role of mechanotransduction during tissue regeneration is poorly understood. Here, we created wounds with equal area but different shapes to understand if perturbing mechanical forces change the area and quantity of de novo hair regeneration. Atomic force microscopy of wound stiffness demonstrated a stiffness gradient across the wound with the wound center softer than the margin. Reducing mechanotransduction signals using FAK or myosin II inhibitors significantly increased WIHN and, conversely, enhancing these signals with an actin stabilizer reduced WIHN. Here, α-SMA was downregulated in FAK inhibitor-treated wounds and lowered wound stiffness. Wound center epithelial cells exhibited a spherical morphology relative to wound margin cells. Differential gene expression analysis of FAK inhibitor-treated wound RNAseq data showed that cytoskeleton-, integrin-, and matrix-associated genes were downregulated, while hair follicular neogenesis, cell proliferation, and cell signaling genes were upregulated. Immunohistochemistry staining showed that FAK inhibition increased pSTAT3 nuclear staining in the regenerative wound center, implying enhanced signaling for hair follicular neogenesis. These findings suggest that controlling wound stiffness modulates tissue regeneration encompassing epithelial competence, tissue patterning, and regeneration during wound healing.

Mechanical Strain Drives Myeloid Cell Differentiation Toward Proinflammatory Subpopulations

Objective: After injury, humans and other mammals heal by forming fibrotic scar tissue with diminished function, and this healing process involves the dynamic interplay between resident cells within the skin and cells recruited from the circulation. Recent studies have provided mounting evidence that external mechanical forces stimulate intracellular signaling pathways to drive fibrotic processes. Innovation: While most studies have focused on studying mechanotransduction in fibroblasts, recent data suggest that mechanical stimulation may also shape the behavior of immune cells, referred to as "mechano-immunomodulation." However, the effect of mechanical strain on myeloid cell recruitment and differentiation remains poorly understood and has never been investigated at the single-cell level. Approach: In this study, we utilized a three-dimensional (3D) in vitro culture system that permits the precise manipulation of mechanical strain applied to cells. We cultured myeloid cells and used single-cell RNA-sequencing to interrogate the effects of strain on myeloid differentiation and transcriptional programming. Results: Our data indicate that myeloid cells are indeed mechanoresponsive, with mechanical stress influencing myeloid differentiation. Mechanical strain also upregulated a cascade of inflammatory chemokines, most notably from the Ccl family. Conclusion: Further understanding of how mechanical stress affects myeloid cells in conjunction with other cell types in the complicated, multicellular milieu of wound healing may lead to novel insights and therapies for the treatment of fibrosis.

Disrupting mechanotransduction decreases fibrosis and contracture in split-thickness skin grafting

Burns and other traumatic injuries represent a substantial biomedical burden. The current standard of care for deep injuries is autologous split-thickness skin grafting (STSG), which frequently results in contractures, abnormal pigmentation, and loss of biomechanical function. Currently, there are no effective therapies that can prevent fibrosis and contracture after STSG. Here, we have developed a clinically relevant porcine model of STSG and comprehensively characterized porcine cell populations involved in healing with single-cell resolution. We identified an up-regulation of proinflammatory and mechanotransduction signaling pathways in standard STSGs. Blocking mechanotransduction with a small-molecule focal adhesion kinase (FAK) inhibitor promoted healing, reduced contracture, mitigated scar formation, restored collagen architecture, and ultimately improved graft biomechanical properties. Acute mechanotransduction blockade up-regulated myeloid CXCL10-mediated anti-inflammation with decreased CXCL14-mediated myeloid and fibroblast recruitment. At later time points, mechanical signaling shifted fibroblasts toward profibrotic differentiation fates, and disruption of mechanotransduction modulated mesenchymal fibroblast differentiation states to block those responses, instead driving fibroblasts toward proregenerative, adipogenic states similar to unwounded skin. We then confirmed these two diverging fibroblast transcriptional trajectories in human skin, human scar, and a three-dimensional organotypic model of human skin. Together, pharmacological blockade of mechanotransduction markedly improved large animal healing after STSG by promoting both early, anti-inflammatory and late, regenerative transcriptional programs, resulting in healed tissue similar to unwounded skin. FAK inhibition could therefore supplement the current standard of care for traumatic and burn injuries.

Thermally damaged porcine skin is not a surrogate mechanical model of human skin

Porcine skin is considered a de facto surrogate for human skin. However, this study shows that the mechanical characteristics of full thickness burned human skin are different from those of porcine skin. The study relies on five mechanical properties obtained from uniaxial tensile tests at loading rates relevant to surgery: two parameters of the Veronda-Westmann hyperelastic material model, ultimate tensile stress, ultimate tensile strain, and toughness of the skin samples. Univariate statistical analyses show that human and porcine skin properties are dissimilar (p < 0.01) for each loading rate. Multivariate classification involving the five mechanical properties using logistic regression can successfully separate the two skin types with a classification accuracy exceeding 95% for each loading rate individually as well as combined. The findings of this study are expected to guide the development of effective training protocols and high-fidelity simulators to train burn care providers.

Risk factors in the prognosis of flap techniques for pilonidal sinus disease based on magnetic resonance imaging and clinical parameters

BACKGROUND

Postoperative complications and recurrence are major diffficulties in the flap techniques for the treatment of pilonidal sinus (PS), however, the risk factors remain unclear. While magnetic resonance imaging (MRI) offers the highest soft tissue resolution, few studies have applied MRI to investigate the basic parameters of PS.

METHODS

A total of 100 patients receiving Limberg flap (LF) or Karydakis flap (KF) surgery at the Sixth Affiliated Hospital of Sun Yat-Sen University were retrospectively analyzed, and the median follow-up period was 42 (range, 20-90) months. We performed a multivariate logistic analysis on the clinicopathological parameters and MRI data to identify risk factors for complications and recurrence.

RESULTS

The basic parameters of PS were obtained by MRI analysis. The multivariate analysis revealed a large longitudinal sinus diameter (OR = 1.020, 95%CI = 1.000-1.041) and sacrococcygeal dermal thickness (OR = 1.680, 95%CI = 1.142-2.472) to be independent risk factors for early complications. Meanwhile, a small sacrococcygeal fat thickness (OR = 0.923, 95%CI = 0.864-0.987) and a high BMI (OR = 1.291, 95%CI = 1.067-1.563) are independent risk factors for late complications and recurrence, respectively.

CONCLUSION

We used MRI to measure the basic parameters of PS accurately, including size, volume, location and some key points of the surrounding tissues, and identified, besides the selection of surgical approach, some specific basic parameters of PS might be the risk factors for complications and recurrence after flap techniques.

Mimicking the Hierarchical Organization of Natural Collagen: Toward the Development of Ideal Scaffolding Material for Tissue Regeneration

Biological materials found in living organisms, many of which are proteins, feature a complex hierarchical organization. Type I collagen, a fibrous structural protein ubiquitous in the mammalian body, provides a striking example of such a hierarchical material, with peculiar architectural features ranging from the amino acid sequence at the nanoscale (primary structure) up to the assembly of fibrils (quaternary structure) and fibers, with lengths of the order of microns. Collagen plays a dominant role in maintaining the biological and structural integrity of various tissues and organs, such as bone, skin, tendons, blood vessels, and cartilage. Thus, "artificial" collagen-based fibrous assemblies, endowed with appropriate structural properties, represent ideal substrates for the development of devices for tissue engineering applications. In recent years, with the ultimate goal of developing three-dimensional scaffolds with optimal bioactivity able to promote both regeneration and functional recovery of a damaged tissue, numerous studies focused on the capability to finely modulate the scaffold architecture at the microscale and the nanoscale in order to closely mimic the hierarchical features of the extracellular matrix and, in particular, the natural patterning of collagen. All of these studies clearly show that the accurate characterization of the collagen structure at the submolecular and supramolecular levels is pivotal to the understanding of the relationships between the nanostructural/microstructural properties of the fabricated scaffold and its macroscopic performance. Several studies also demonstrate that the selected processing, including any crosslinking and/or sterilization treatments, can strongly affect the architecture of collagen at various length scales. The aim of this review is to highlight the most recent findings on the development of collagen-based scaffolds with optimized properties for tissue engineering. The optimization of the scaffolds is particularly related to the modulation of the collagen architecture, which, in turn, impacts on the achieved bioactivity.

New strategy of modulating incision tension: A wound tension offloading device applied before surgery

Wound tension plays a key role in the process of wound healing and scar formation. Tension offloading devices have been reported to reduce postsurgical scar formation. This study aims to determine whether the application of a tension offloading device preoperatively would result in superior attenuation of scar genesis in comparison to traditional methods. Randomized, controlled trials were performed on 12 patients, 4 patients were treated both preoperatively and postoperatively, while the other 4 were treated only postoperatively. The remaining 4 patients did not receive any sort of intervention. The overall performance was analyzed over 6 months period. The skin elasticity coefficient improved significantly with the application of a tension-offloading device. Compared with control group, patients who received treatment via the device displayed a better result in scar width and regression of color. It was also shown that the use of a device in the group with twin pre-op and post-op intervention resulted in a reduction of the wound healing period in comparison to the post-op group. Application of a tension-offloading device preoperatively can reduce tensile forces acting on the incision, thereby resulting in faster wound healing and enhanced efficacy on postsurgical reapplication. The effectiveness of the device in preventing hypertrophic scar is likely to be improved by long-term application after operation.

Modified Buried Vertical Mattress Suture Versus Buried Intradermal Suture: A Prospective Split-Scar Study

BACKGROUND

The modified buried vertical mattress suture (MBVMS) is believed to provide excellent outcomes by relieving the tension on wound edges. However, clinical data on the topic remain sparse and inadequate.

OBJECTIVE

To compare the cosmetic results of the MBVMS and the buried intradermal suture (BIS) in chest wounds using a split-scar model.

MATERIALS AND METHODS

Twenty patients participated in the study. One randomly selected half of each chest wound was closed with the MBVMS; the other half was closed with the BIS. Immediately, postoperatively, the maximum degree of wound eversion was obtained. After 3 months, the wound complication rates were recorded, and the aesthetic appearance of each scar was evaluated by the Patient and Observer Scar Assessment Scale (POSAS), the Vancouver Scar Scale (VSS), the visual analog scale (VAS), and scar width.

RESULTS

The MBVMS yielded a greater mean postoperative eversion height and width (p < .05); lower POSAS, VSS, and VAS scores (p < .05); and a narrower scar width (p < .05) than did the BIS.

CONCLUSION

Compared with the BIS, the MBVMS provided significantly increased wound eversion immediately, postoperatively, and improved aesthetic outcomes at the end of the 3-month follow-up period.

Personalized Computational Models of Tissue-Rearrangement in the Scalp Predict the Mechanical Stress Signature of Rotation Flaps

OBJECTIVE

To elucidate the mechanics of scalp rotation flaps through 3D imaging and computational modeling. Excessive tension near a wound or sutured region can delay wound healing or trigger complications. Measuring tension in the operating room is challenging, instead, noninvasive methods to improve surgical planning are needed.

DESIGN

Multi-view stereo allows creation of 3D patient-specific geometries based on a set of photographs. The patient-specific 3D geometry is imported into a finite element (FE) platform to perform a virtual procedure. The simulation is compared with the clinical outcome. Additional simulations quantify the effect of individual flap parameters on the resulting tension distribution.

PARTICIPANTS

Rotation flaps for reconstruction of scalp defects following melanoma resection in 2 cases are presented. Rotation flaps were designed without preoperative FE preparation.

MAIN OUTCOME MEASURE

Tension distribution over the operated region.

RESULTS

The tension from FE shows peaks at the base and distal ends of the scalp rotation flap. The predicted geometry from the simulation aligns with postoperative photographs. Simulations exploring the flap design parameters show variation in the tension. Lower tensions were achieved when rotation was oriented with respect to skin tension lines (horizontal tissue fibers) and smaller rotation angles.

CONCLUSIONS

Tension distribution following rotation of scalp flaps can be predicted through personalized FE simulations. Flaps can be designed to reduce tension using FE, which may greatly improve the reliability of scalp reconstruction in craniofacial surgery, critical in complex cases when scalp reconstruction is essential for coverage of hardware, implants, and/or bone graft.

Mechanotransduction in Wound Healing and Fibrosis

Skin injury is a common occurrence and mechanical forces are known to significantly impact the biological processes of skin regeneration and wound healing. Immediately following the disruption of the skin, the process of wound healing begins, bringing together numerous cell types to collaborate in several sequential phases. These cells produce a multitude of molecules and initiate multiple signaling pathways that are associated with skin disorders and abnormal wound healing, including hypertrophic scars, keloids, and chronic wounds. Studies have shown that mechanical forces can alter the microenvironment of a healing wound, causing changes in cellular function, motility, and signaling. A better understanding of the mechanobiology of cells in the skin is essential in the development of efficacious therapeutics to reduce skin disorders, normalize abnormal wound healing, and minimize scar formation.

Percutaneous Fasciotomies versus Traditional Keystone Flap: Evaluating Tension in Complex Wound Closure

Supplemental Digital Content is available in the text.

The keystone flap is a popular reconstructive option for closure of cutaneous defects. Traditionally, this is a perforator-based fasciocutaneous advancement flap that uses both skin incision and fascial release. We propose a limited skin incision technique that utilizes percutaneous fasciotomies to accomplish wound closure.

Applying the Chinese Wood Joinery Mortise-and-Tenon Principle to Repair Widening Surface Scars

We applied the classical Chinese wood joinery mortise-and-tenon principle to repair widening surface scars caused by incision tension. Along the outer margin of surface scars, the top half of the scar tissue was cut and removed. The authors designed serial tenon structures on the retained dermal surface of the scar and a series of corresponding mortise structures in the dermal tissue on the opposite side of the incision. Finally, the mortise and tenon structures were integrated and sutured, resulting in tensionless closure. Thirty-two surface scars were repaired with this method. The follow-up time ranged from 6 to 24 months. The incisions healed in the form of fine linear scars. No widening scars were observed in this series. The proposed mortise-and-tenon scar repair technique can effectively reduce incision tension and thus reduce scar formation at the incision site. The authors recommend this technique as an alternative effective method for revising widening surface scars.

Prise en charge chirurgicale du carcinome basocellulaire

The incidence of basal cell carcinoma (BCC) is increasing as the population is aging and doubles every ten years. Surgery is the first-line treatment of BCC. Dermatological surgery is an oncological skin surgery whose first objective is to obtain a complete resection of the tumor. Its aim is also to reconstruct the defect using the optimal repair technique for the best cosmetic and scarring outcome and without functional impairment. The dermatological approach with the "oncological reading" of cutaneous tumors constitutes the essential preliminary time to the diagnosis of BCC and the identification of its limits. The perfect knowledge of the security margins in accordance with the guidelines allows a complete excision and a reconstruction in one stage under local anesthesia in the majority of cases. The surgical treatment must use 3D histology techniques or micrographic surgery to manage difficult cases of aggressive BCC in high risk zone or recurrence. Management of very aggressive BCC or locally advanced BCC is discussed in a multidisciplinary consultation by assessing the benefit/risk ratio of the surgical treatment and by identifying the appropriate surgeon after documenting the tumor, its operability and patient's adherence to the surgical treatment. © 2018. Published by Elsevier Masson SAS. All rights reserved. Cet article fait partie du numéro supplément Prise en charge des carcinomes basocellulaires difficiles à traiter réalisé avec le soutien institutionnel de Sun Pharma.

Steps in Mechanotransduction Pathways that Control Cell Morphology

It is increasingly clear that mechanotransduction pathways play important roles in regulating fundamental cellular functions. Of the basic mechanical functions, the determination of cellular morphology is critical. Cells typically use many mechanosensitive steps and different cell states to achieve a polarized shape through repeated testing of the microenvironment. Indeed, morphology is determined by the microenvironment through periodic activation of motility, mechanotesting, and mechanoresponse functions by hormones, internal clocks, and receptor tyrosine kinases. Patterned substrates and controlled environments with defined rigidities limit the range of cell behavior and influence cell state decisions and are thus very useful for studying these steps. The recently defined rigidity sensing process provides a good example of how cells repeatedly test their microenvironment and is also linked to cancer. In general, aberrant extracellular matrix mechanosensing is associated with numerous conditions, including cardiovascular disease, aging, and fibrosis, that correlate with changes in tissue morphology and matrix composition. Hence, detailed descriptions of the steps involved in sensing and responding to the microenvironment are needed to better understand both the mechanisms of tissue homeostasis and the pathomechanisms of human disease.

Incorporating mechanical strain in organs-on-a-chip: Lung and skin

In the last decade, the advent of microfabrication and microfluidics and an increased interest in cellular mechanobiology have triggered the development of novel microfluidic-based platforms. They aim to incorporate the mechanical strain environment that acts upon tissues and in-vivo barriers of the human body. This article reviews those platforms, highlighting the different strains applied, and the actuation mechanisms and provides representative applications. A focus is placed on the skin and the lung barriers as examples, with a section that discusses the signaling pathways involved in the epithelium and the connective tissues.

Modified Subcutaneous Buried Horizontal Mattress Suture Compared With Vertical Buried Mattress Suture

BACKGROUND

Wound tension reduction is still a challenge to surgeons. Over the years, many techniques have been proposed to avoid this issue. In this paper, we present a new suture technique.

OBJECTIVES

To investigate the tension-reduction effectiveness of the modified subcutaneous buried horizontal mattress suture compared with the vertical buried mattress suture technique.

METHODS

Two suture techniques, the vertical buried mattress suture (group A) and the modified subcutaneous buried horizontal mattress suture (group B), were performed on paired samples of symmetrical skin flaps. An equal pulling force was applied to each paired sutured flap, and the dehiscences of the samples in the two groups were compared. Then, after the periodic mechanical pulling force was recorded, the dehiscences were compared again.

RESULTS

The dehiscences of the vertical buried mattress suture samples (group A) were much wider than their corresponding samples. Modified subcutaneous buried horizontal mattress suture samples (group B) remained well closed with no or minimal dehiscence, under various tensions, even after mechanical pulling force was applied.

CONCLUSION

The modified subcutaneous buried horizontal mattress suture greatly decreased the tension on the dermis and subcutaneous tissue in our in vitro experiment.

[How to optimize scarring in dermatologic surgery?]

Scarring is the response elicited by the skin surface to injury and loss of tissue material. Wound healing takes place through a complex natural repair system consisting of vascular, inflammatory and proliferative phenomena, followed by a remodelling and cell apoptosis phase. This incredible repair system is inevitable, but sometimes unpredictable due to individual differences based on multiple factors. The scar is the objective criterion of a skin surgery, both for the patient and the dermsurgeon. It is therefore crucial to establish with the patient during the preoperative consultation, the size and positioning of the expected scar, taking into account the oncologic, anatomic and surgical constraints. Scars can ideally blend into normal skin, but may also give rise to various abnormalities. We can manage and prevent these abnormalities by mastering initial inflammation, that may induce hyperpigmentation and hypertrophy. Early massage using cortocosteroid topic or anti-inflammatory moisturizers may be effective. Random individual scarring may be minimized by a dynamic personalized accompanying scarring.

The Incompatibility of Living Systems: Characterizing Growth-Induced Incompatibilities in Expanded Skin

Skin expansion is a common surgical technique to correct large cutaneous defects. Selecting a successful expansion protocol is solely based on the experience and personal preference of the operating surgeon. Skin expansion could be improved by predictive computational simulations. Towards this goal, we model skin expansion using the continuum framework of finite growth. This approach crucially relies on the concept of incompatible configurations. However, aside from the classical opening angle experiment, our current understanding of growth-induced incompatibilities remains rather vague. Here we visualize and characterize incompatibilities in living systems using skin expansion in a porcine model: We implanted and inflated two expanders, crescent, and spherical, and filled them to 225 cc throughout a period of 21 days. To quantify the residual strains developed during this period, we excised the expanded skin patches and subdivided them into smaller pieces. Skin growth averaged 1.17 times the original area for the spherical and 1.10 for the crescent expander, and displayed significant regional variations. When subdivided into smaller pieces, the grown skin patches retracted heterogeneously and confirmed the existence of incompatibilities. Understanding skin growth through mechanical stretch will allow surgeons to improve-and ultimately personalize-preoperative treatment planning in plastic and reconstructive surgery.

Interfacial growth during closure of a cutaneous wound: stress generation and wrinkle formation

A biomechanical growth model for the proliferation stage of cutaneous wound healing is developed emphasizing the emergence of stress and wrinkled skin during the healing process. The healing is assumed to be primarily driven by growth at the wound edge (i.e. the interface between the wound and the skin) leading to incompatible growth strains. A closed form solution of the boundary value problem is obtained using a Varga hyperelastic membrane model for both the skin and the wound. The nature of the solution is explored for various parametric values of the skin tension, healing rate, edge incompatibility, wrinkled region radius, and wound stiffness. The obtained results for the stress field, wrinkling, and rate of healing are qualitatively in good agreement with the existing experimental observations.

A mechanistic insight into the mechanical role of the stratum corneum during stretching and compression of the skin

The study of skin biophysics has largely been driven by consumer goods, biomedical and cosmetic industries which aim to design products that efficiently interact with the skin and/or modify its biophysical properties for health or cosmetic benefits. The skin is a hierarchical biological structure featuring several layers with their own distinct geometry and mechanical properties. Up to now, no computational models of the skin have simultaneously accounted for these geometrical and material characteristics to study their complex biomechanical interactions under particular macroscopic deformation modes. The goal of this study was, therefore, to develop a robust methodology combining histological sections of human skin, image-processing and finite element techniques to address fundamental questions about skin mechanics and, more particularly, about how macroscopic strains are transmitted and modulated through the epidermis and dermis. The work hypothesis was that, as skin deforms under macroscopic loads, the stratum corneum does not experience significant strains but rather folds/unfolds during skin extension/compression. A sample of fresh human mid-back skin was processed for wax histology. Sections were stained and photographed by optical microscopy. The multiple images were stitched together to produce a larger region of interest and segmented to extract the geometry of the stratum corneum, viable epidermis and dermis. From the segmented structures a 2D finite element mesh of the skin composite model was created and geometrically non-linear plane-strain finite element analyses were conducted to study the sensitivity of the model to variations in mechanical properties. The hybrid experimental-computational methodology has offered valuable insights into the simulated mechanics of the skin, and that of the stratum corneum in particular, by providing qualitative and quantitative information on strain magnitude and distribution. Through a complex non-linear interplay, the geometry and mechanical characteristics of the skin layers (and their relative balance), play a critical role in conditioning the skin mechanical response to macroscopic in-plane compression and extension. Topographical features of the skin surface such as furrows were shown to act as an efficient means to deflect, convert and redistribute strain-and so stress-within the stratum corneum, viable epidermis and dermis. Strain reduction and amplification phenomena were also observed and quantified. Despite the small thickness of the stratum corneum, its Young׳s modulus has a significant effect not only on the strain magnitude and directions within the stratum corneum layer but also on those of the underlying layers. This effect is reflected in the deformed shape of the skin surface in simulated compression and extension and is intrinsically linked to the rather complex geometrical characteristics of each skin layer. Moreover, if the Young׳s modulus of the viable epidermis is assumed to be reduced by a factor 12, the area of skin folding is likely to increase under skin compression. These results should be considered in the light of published computational models of the skin which, up to now, have ignored these characteristics.

Application of finite element modeling to optimize flap design with tissue expansion

BACKGROUND

Tissue expansion is a widely used technique to create skin flaps for the correction of sizable defects in reconstructive plastic surgery. Major complications following the inset of expanded flaps include breakdown and uncontrolled scarring secondary to excessive tissue tension. Although it is recognized that mechanical forces may significantly impact the success of defect repair with tissue expansion, a mechanical analysis of tissue stresses has not previously been attempted. Such analyses have the potential to optimize flap design preoperatively.

METHODS

The authors establish computer-aided design as a tool with which to explore stress profiles for two commonly used flap designs, the direct advancement flap and the double back-cut flap. The authors advanced both flaps parallel and perpendicular to the relaxed skin tension lines to quantify the impact of tissue anisotropy on stress distribution profiles.

RESULTS

Stress profiles were highly sensitive to flap design and orientation of relaxed skin tension lines, with stress minimized when flaps were advanced perpendicular to relaxed skin tension lines. Maximum stresses in advancement flaps occurred at the distal end of the flap, followed by the base. The double back-cut design increased stress at the lateral edges of the flap.

CONCLUSIONS

The authors conclude that finite element modeling may be used to effectively predict areas of increased flap tension. Performed preoperatively, such modeling can allow for the optimization of flap design and a potential reduction in complications such as flap dehiscence and hypertrophic scarring.

A novel vacuum assisted closure therapy model for use with percutaneous devices

Long-term maintenance of a dermal barrier around a percutaneous prosthetic device remains a common clinical problem. A technique known as Negative Pressure Wound Therapy (NPWT) uses negative pressure to facilitate healing of impaired and complex soft tissue wounds. However, the combination of using negative pressure with percutaneous prosthetic devices has not been investigated. The goal of this study was to develop a methodology to apply negative pressure to the tissues surrounding a percutaneous device in an animal model; no tissue healing outcomes are presented. Specifically, four hairless rats received percutaneous porous coated titanium devices implanted on the dorsum and were bandaged with a semi occlusive film dressing. Two of these animals received NPWT; two animals received no NPWT and served as baseline controls. Over a 28-day period, both the number of dressing changes required between the two groups as well as the pressures were monitored. Negative pressures were successfully applied to the periprosthetic tissues in a clinically relevant range with a manageable number of dressing changes. This study provides a method for establishing, maintaining, and quantifying controlled negative pressures to the tissues surrounding percutaneous devices using a small animal model.

Mechanotransduction and fibrosis

Scarring and tissue fibrosis represent a significant source of morbidity in the United States. Despite considerable research focused on elucidating the mechanisms underlying cutaneous scar formation, effective clinical therapies are still in the early stages of development. A thorough understanding of the various signaling pathways involved is essential to formulate strategies to combat fibrosis and scarring. While initial efforts focused primarily on the biochemical mechanisms involved in scar formation, more recent research has revealed a central role for mechanical forces in modulating these pathways. Mechanotransduction, which refers to the mechanisms by which mechanical forces are converted to biochemical stimuli, has been closely linked to inflammation and fibrosis and is believed to play a critical role in scarring. This review provides an overview of our current understanding of the mechanisms underlying scar formation, with an emphasis on the relationship between mechanotransduction pathways and their therapeutic implications.

[Early hypertrophic scar after surgery on the nasal region: value of long-acting corticosteroid injections]

BACKGROUND

"Pincushioning" is a complication of post-surgical scarring following use of transposition flaps particularly when surgery is performed on the nasal region. The transposition flap technique is very useful for the repair of certain defects of the tip of the nose, the medial canthus or of the ala nasi. The aim of this study is to define the clinical characteristics of this scarring dystrophy, which we propose to call "early hypertrophy scarring", to clarify the nature thereof and to assess the efficacy of intralesional injection of corticosteroids at the first signs of hypertrophy.

PATIENTS AND METHOD

A prospective, open, non-comparative, single-centre study examined the clinical and histological characteristics of early hypertrophy scarring and the effectiveness of therapy with one or two injections of corticosteroids performed on the 15th day post-operatively and optionally repeated at D45 depending on the outcome. From January 2011 to January 2013, 12 consecutive patients with early hypertrophy scarring were included (ten men and two women - mean age: 64 years). All had undergone surgery for basal cell carcinoma under local anaesthesia with one-stage repair by means of a rhombic flap or a bilobed flap located in the nasal area. Scars were injected strictly intra-lesionally with triamcinolone acetate (40 mg/1 mL) until whitening occurred. A single injection was performed in three cases of rhombic flap while a second injection was given at D45 in the remaining nine cases.

RESULTS

Complete regression of the early hypertrophy scarring was obtained in ten of the 12 patients by D90. Incomplete regression was observed but with a marked improvement in the other two patients.

DISCUSSION

Early hypertrophy scarring is distinguished by its clinical characteristics of hypertrophic or keloid scars. Biopsy performed in two cases showed the fibrous but non-fatty nature of early hypertrophy scarring. Biomechanical factors particular to the nasal region and the transposition flap technique could account for the early and excessive collagen production causing early hypertrophy scarring. Early injection of corticosteroids, which was consistently effective in our study, could represent a simple treatment for early hypertrophy scarring, thus avoiding surgical correction. These preliminary results in a small number of patients require confirmation by a comparative, multicentre, prospective controlled study.

The role of wound healing and its everyday application in plastic surgery: a practical perspective and systematic review

BACKGROUND

After surgery it is often recommended that patients should refrain from strenuous physical activity for 4-6 weeks. This recommendation is based on the time course of wound healing. Here, we present an overview of incisional wound healing with a focus on 2 principles that guide our postoperative recommendations: the gain of tensile strength of a wound over time and the effect of mechanical stress on wound healing.

METHODS

A systematic search of the English literature was conducted using OVID, Cochrane databases, and PubMed. Inclusion criteria consisted of articles discussing the dynamics of incisional wound healing, and exclusion criteria consisted of articles discussing nonincisional wounds.

RESULTS

Experiments as early as 1929 laid the groundwork for our postoperative activity recommendations. Research using animal models has shown that the gain in tensile strength of a surgical wound is sigmoidal in trajectory, reaching maximal strength approximately 6 weeks postoperatively. Although human and clinical data are limited, the principles gained from laboratory investigation have provided important insights into the relationship among mechanical stress, collagen dynamics, and the time course of wound healing.

CONCLUSION

Our postoperative activity recommendations are based on a series of animal studies. Clinical research supporting these recommendations is minimal, with the most relevant clinical data stemming from early motion protocols in the orthopedic literature. We must seek to establish clinical data to support our postoperative activity recommendations so that we can maximize the physiologic relationships between wound healing and mechanical stress.

A Mechanomodulatory Device to Minimize Incisional Scar Formation

OBJECTIVE

To mechanically control the wound environment and prevent cutaneous scar formation.

APPROACH

We subjected various material substrates to biomechanical testing to investigate their ability to modulate skin behavior. Combinations of elastomeric materials, adhesives, and strain applicators were evaluated to develop topical stress-shielding devices. Noninvasive imaging modalities were utilized to characterize anatomic site-specific differences in skin biomechanical properties in humans. The devices were tested in a validated large animal model of hypertrophic scarring. Phase I within-patient controlled clinical trials were conducted to confirm their safety and efficacy in scar reduction in patients undergoing abdominoplasty surgery.

RESULTS

Among the tested materials and device applicators, a polymer device was developed that effectively off-loaded high tension wounds and blocked pro-fibrotic pathways and excess scar formation in red Duroc swine. In humans, different anatomic sites exhibit unique biomechanical properties that may correlate with the propensity to form scars. In the clinical trial, utilization of this device significantly reduced incisional scar formation and improved scar appearance for up to 12 months compared with control incisions that underwent routine postoperative care.

INNOVATION

This is the first device that is able to precisely control the mechanical environment of incisional wounds and has been demonstrated in multiple clinical trials to significantly reduce scar formation after surgery.

CONCLUSION

Mechanomodulatory strategies to control the incisional wound environment can significantly reduce pathologic scarring and fibrosis after surgery.

Systems-based approaches toward wound healing

Wound healing in the pediatric patient is of utmost clinical and social importance because hypertrophic scarring can have aesthetic and psychological sequelae, from early childhood to late adolescence. Wound healing is a well-orchestrated reparative response affecting the damaged tissue at the cellular, tissue, organ, and system scales. Although tremendous progress has been made toward understanding wound healing at the individual temporal and spatial scales, its effects across the scales remain severely understudied and poorly understood. Here, we discuss the critical need for systems-based computational modeling of wound healing across the scales, from short-term to long-term and from small to large. We illustrate the state of the art in systems modeling by means of three key signaling mechanisms: oxygen tension-regulating angiogenesis and revascularization; transforming growth factor-β (TGF-β) kinetics controlling collagen deposition; and mechanical stretch stimulating cellular mitosis and extracellular matrix (ECM) remodeling. The complex network of biochemical and biomechanical signaling mechanisms and the multiscale character of the healing process make systems modeling an integral tool in exploring personalized strategies for wound repair. A better mechanistic understanding of wound healing in the pediatric patient could open new avenues in treating children with skin disorders such as birth defects, skin cancer, wounds, and burn injuries.