Skin anisotropy: Finding the optimal incision line for volar forearm in males and females

Abstract

Purpose

Proper understanding of skin biomechanics, viscoelasticity and investigation of skin tension vectors is necessary to find optimal incision lines. Great tension across a healing wound after any surgical procedure might lead to forming hypertrophic scars. The aim of the study was to investigate tension lines in volar forearm skin in young males and females, in order to ensure best incision line.

Methods

Five biomechanical and viscoelastic parameters were measured using a hand-held myotonometer: Oscillation Frequency [Hz], Dynamic Stiffness [N/m], Logarithmic Decrement of tissue's natural oscillation, Mechanical Stress Relaxation Time [ms], and Creep. Measurements were taken in four different directions; Along Forearm, Across Forearm, Along Langer's Line and Across Langer's Line.

Results

Significant main effects for direction were found for Oscillation Frequency (p < 0.001, η² = 0.371) [Hz], Dynamic Stiffness (p < 0.001, η² = 0.522) [N/m], Logarithmic Decrement (p < 0.001, η² = 0.083), Mechanical Stress Relaxation Time (p < 0.001, η² = 0.494) [ms] and Creep (p < 0.001, η² = 0.480). For each parameter except for logarithmic decrement results obtained Along Langers Line and Across Forearm were significantly different to Across Langers Line and Along Forearm (p < 0.001, d = −2.76 – 2.66). Significant main effects for sex were found for logarithmic decrement Along Forearm (p < 0.001, d = 1.698) and Across Langer's Line (p = 0.021, d = 1.697).

Conclusions

Our results suggested that optimal incision line for this age group in males and females could potentially be performed diagonally i.e. Across Langer's Line or parallel i.e. Along Forearm to forearm axis. These directions would provide the lowest tension across a healing wound and possibly minimalize the risk of hypertrophic scarring post incision.

Introduction

Volar forearm is one of the most investigated areas in skin research (Mazzarello et al., 2018). It was previously confirmed that photo-aging and aging process leads to changes in skin biomechanical properties, such as decrease of skin elasticity (Krueger et al., 2011; Mazzarello et al., 2018; Nomura et al., 2017; Ohshima et al., 2013). The Volar forearm is less exposed to sunlight, therefore the impact of photo-aging process is lower (Nomura et al., 2017) and the skin is not as thick as other areas of the body e.g. forehead or cheeks (Woo et al., 2014). Despite the complex, heterogeneous structure of the skin and the fact that each layer (stratum corneum, epidermis and dermis) is characterized by different intrinsic properties, they work in harmony to maintain homeostasis (Agache and Vatchon, 2017a). Consequently, it is justified to investigate mechanical properties of skin globally, as if it was a homogenous material (Agache and Vatchon, 2017a).

Skin is an anisotropic tissue which behaves differently depending on the direction of an applied force (Agache and Vatchon, 2017a; Graham et al., 2019). Tensile testing to rupture of porcine skin tissue had been conducted to evaluate the sensitivity of the skin elasticity and fracture-related properties to varying tissue direction (Wong et al., 2016). It was found that the elastic modulus and fracture strength vary significantly with the tissue direction. It has been previously reported that age affects the skin biomechanical and viscoelastic properties, as well as its directionality (Rosado et al., 2017; Pawlaczyk et al., 2013; Thieulin et al., 2020). The sensitivity of skin biomechanical properties to age may be underpinned by variation in collagen composition and structure. Age-related changes in collagen fibril cross-sectional area fraction could affect the elastic modulus and fracture strength of tendons (Goh et al., 2008); age-related changes in the resilience and fracture toughness could be directed by changes in the frequency distribution of collagen fibril sizes (Goh et al., 2012). In older groups skin mechanical behaviour differs significantly depending on force direction, while in younger populations the differences are less visible (Thieulin et al., 2020). With regard to viscoelastic-related properties, tensile testing to rupture of porcine skin tissue for varying strain rate had been conducted (Wong et al., 2016). It was found that some but not all the mechanical properties were sensitive to strain rate. For example, while elastic modulus increases and strain to rupture decreases with increases in strain rate, the fracture strength was not sensitive to variation in strain rate. Therefore, investigating skin anisotropy is an appropriate approach to characterize skin aging (Gahagnon et al., 2012). Moreover, proper understanding of skin biomechanics, viscoelasticity and investigation of skin tension vectors is necessary to find optimal incision lines (Son and Harijan, 2014). Great tension across a healing wound after any surgical procedure might lead to forming hypertrophic scars (Son and Harijan, 2014). Skin tension lines were originally described by Karl Langer in 1861 (Langer, 1978; LANGER, 1861). Langer used 2,5 cm spike to puncture holes into skin of cadavers. He noticed that holes were ellipsoid shaped, which helped him to determine direction of the lines. Langer's Cleavage Lines indicate direction of increased tension in skin and are still widely used by clinicians to plan incision direction (Carmichael, 2014). While Langer's observations describe static behaviour of skin, Kraissl based his investigation on wrinkles, which are perpendicular to underlying muscles, appearing on skin with movement (Kraissl and Conway, 1949). Borges' relaxed skin tension lines (RSTL) can be identified by pinching the skin, the largest wrinkle designate RSTL (Borges, 1989). Striae distensae reflects natural anti-tension lines of the skin, and occurs perpendicular to the optimal skin incision lines (Lemperle et al., 2014). However, all these observations are not based on quantitative measurements. Recently in order to establish the most effective way for surgical excisions a new computerised tensiometer was used to map biodynamic excisional skin tension (BEST) lines (Paul, 2017, 2017, 2018a, 2018b; Paul et al., 2016, 2017) but BEST lines are indicated for excisional surgeries not to incisional procedures (Paul et al., 2016). Therefore, there is still lack of scientific evidence concerning best incision lines.

It is important to develop methods dedicated for clinical use that will enable the quick and accurate assessment of many different skin parameters in a non-invasive manner (Agache P., 2017; Agache and Vatchon, 2017a; Rodrigues, EEMCO Group, 2001; Piérard, EEMCO Group, 1999; Rodrigues and Fluhr, EEMCO Group., 2020). Such an approach would contribute to evaluation of skin condition and diagnosis, as well as, provide information regarding optimal incision lines (Agache P., 2017). MyotoPRO has been used in recent research to accurately and quickly measure skin stiffness in healthy subjects (Dellalana et al., 2018), in patients with cutaneous chronic graft-versus-host-disease (cGVHD) to evaluate cutaneous sclerosis (Chen et al., 2018) and to assess post caesarean section scars (Gilbert et al., 2020). Recent research has revealed greater reliability of this device for assessment of skin stiffness using L-shape probes (Rosicka et al., 2020). Moreover, it was previously reported that inter-observer and intra-observer ICC (Inter-Class Correlation) values showed great or excellent reliability of the MyotonPRO for stiffness measurement (Dellalana et al., 2018) as well as all other parameters (Gilbert et al., 2020). However it was noted that differences between skin stiffness among different body areas might be associated with tension lines (Rosicka et al., 2020). At present there is limited understanding as how the skin behaves depending on the direction of an applied force. Investigating the mechanical and viscoelastic properties of the volar forearm skin according to the direction of applied force would inform clinicians of appropriate surgical procedures.

Therefore, the main purpose of this study was to investigate tension lines in volar forearm skin in young males and females, in order to ensure best incision direction. This study will provide novel information about skin anisotropy and how force direction influences skin mechanical and viscoelastic properties. We hypothesised that Langer's lines are the lowest tension lines in volar forearm skin. Furthermore, we hypothesise that lower tension may be a result of lower stiffness and greater elasticity of the tissue, with increased recovery time of the tissue and a higher creep value.