Changes in the Biomechanical Properties of Human Skin in Hyperthermic and Hypothermic Ranges

Introduction. The prevalence of thermal skin injuries is high. Despite new research findings, skin burns and acute cold-contact injuries, together with resulting tissue damage, are not entirely understood. In particular, little is known about how these types of injuries alter the biomechanical properties of skin. Objective. This study evaluates hyperthermic- and hypothermic-induced alterations in the biomechanical properties of human skin using a skin elasticity measurement device. Materials and Methods. In 54 cases, local hypothermia (15°C and 5°C) and local hyperthermia (40°C and 45°C) were induced at the palmar forearm of healthy participants. The biomechanical properties of skin were measured using the skin elasticity measurement device before and after each temperature change at 2 different depths. Results. The skin firmness, pliability, retraction, and elasticity/calculated elasticity showed a continuous decrease in values with decreasing skin temperatures in total skin measurements and an increase in values with increasing skin temperatures in the upper layer and total skin measurements. Conclusions. As per the results, the investigators believe these hyperthermic- and hypothermic-induced alterations in biomechanical skin properties are due to increased blood flow, in addition to a reversible increase in interstitial and intracellular fluid contents, thermal contraction, and expansion of collagen and elastic fibers, all of which are precursors to irreversible damage.

Introduction

The clinical determination of the exact depth of a thermal injury and clinical evaluation of such an injury are a diagnostic challenge; hence, the initial evaluation of a thermal injury is frequently imprecise. In addition, the extent of the injury is difficult to predict, and it may take some time until demarcation of nonviable tissue appears.¹ Although new research findings have improved the understanding of thermal injuries,^2,3 the exact mechanism of these injuries and resulting tissue damage remain unclear. Furthermore, as yet, there is no effective device for objective measurements of thermal injury depth.⁴ Bartell et al⁵ first described noninvasive quantification of biomechanical skin properties using a hand-held device. Since then, research^6,7 on thermal injuries generally has focused on scar formation or comparisons of different therapeutic concepts.

Thermal skin injury leads to denaturation of collagen in fiber structures, the viscous ground matrix, and intracellular components.⁸ Changes in collagen and elastic fiber structures are measurable as alterations in skin elasticity. Thus, the investigators hypothesized that evaluating thermally induced changes in skin elasticity could improve the understanding of burn and cold injuries, including the tissue damage induced by such injuries, and aid assessments of thermal injury depth.

Many noninvasive methods exist to evaluate skin elasticity parameters in skin injuries, scars, and aging. These include the Dermal Torque Meter (Product A; Dia-Stron, Inc, Clarksburg, NJ), Dermaflex (Cortex Technology, Hadsund, Denmark), DermaLab (Cortex Technology), Reviscometer (Product B; Courage+Khazaka electronic GmbH, Cologne, Germany), and Cutometer Dual MPA 580 (skin elasticity measurement device; Courage+Khazaka electronic GmbH) instruments, among which the skin elasticity measurement device is most commonly used.^9,10 Product A applies constant torque and measures rotational deformation and recovery of skin.^9,11 It has been used to assess cutaneous scars for wounds treated with cultured skin substitutes and autologous keratinocytes and fibroblasts.^9,11 Product B evaluates skin isotrophy by sheer wave propagation and is used more frequently than the other devices.¹⁰ The skin elasticity measurement device is commonly used to evaluate burn scars and the results of split-thickness skin graft transplantations.^12-14 To date, studies^12-14 have focused on the quality of healed burn wounds and scars or compared different treatment concepts.

The skin elasticity measurement device provides a noninvasive method to evaluate skin elasticity. Skin firmness (Uf) and elasticity/calculated elasticity (Ue) parameters are linked to the stretching of collagen and elastic fibers.¹⁵Skin retraction (Ur) is particularly important in elastic fiber stretching.¹⁵ Other parameters, such as delayed distention (Uv), depend on the interstitial fluid content in the fibrous network.¹⁵ The skin elasticity measurement device has different probe apertures that enable measurements of various skin components. A small 2-mm probe measures the mechanical properties of the upper skin layers, and a 6-mm probe measures the mechanical properties of total skin, including the dermis and hypodermis.¹⁶

In the investigators’ previous studies,^17-19 they objectively evaluated skin elasticity parameters as a function of burn injury depth and acute cold-contact injury using the skin elasticity measurement device in a miniature pig model. The results revealed complex changes in the skin’s biomechanical properties in acute thermal injuries. In these studies,^17-19 skin Uf, skin overall elasticity (Ua), and skin Ue showed a continual decrease with an increasing depth of burn injury in first-, second-, and third-degree burn injuries. Similarly, these parameters exhibited a continuous decrease with increasing depth of acute cold injury in first-, second-, and third-degree injuries. As porcine skin has many similarities to human skin, the authors hypothesized that depth-dependent changes in biomechanical skin properties could aid depth assessments of thermal injuries in humans.

The aim of this study was to evaluate hyperthermic- and hypothermic-induced alterations in the biomechanical properties of human skin using the skin elasticity measurement device.

Materials and Methods

Experimental participants

Healthy volunteers were recruited after obtaining the approval of the authors’ local ethics committee at the University of Tübingen (Tübingen, Germany). The study complied with ethical standards for human experimentation, as stated in the 1975 Declaration of Helsinki, and all participants provided written informed consent before entry into the study.

In total, 54 volunteers were enrolled in the study (34 women, 20 men). The mean age was 26.9 years (range, 18–61 years). Eight of 54 (14.81%) participants were smokers and 46 (85.19%) were nonsmokers. The exclusion criteria consisted of history of cardiovascular diseases, diabetes mellitus, arterial hypertension, and skin diseases.

Experimental devices

A TSA-II NeuroSensory Analyzer (thermal stimuli device; Medoc Ltd, Ramat Yishai, Israel) was used to generate standardized thermal stimuli. The TSA-II thermode (aluminum bar: 30 mm × 30 mm) was placed on the participant’s skin (predefined area) to heat or cool it to the exact predefined temperatures for an exact predefined time. Thus, no further thermal stimuli device was necessary.

To evaluate biomechanical properties of the skin, the aforementioned skin elasticity measurement device was used (Cutometer Dual MPA 580) as employed in the investigators’ previous studies of thermal injury in a miniature pig model.^17-19 This skin elasticity measurement device consisted of a main unit and 2 handheld probes, with either a 2-mm or 6-mm aperture suction head. The suction head is placed on the skin, negative pressure is created in the suction head, and the skin is drawn into the apertures. The skin penetration depth is determined by a noncontact optical system inside the suction head. Upper skin layers are deformed using the smaller aperture of the probe, and total skin is deformed by the larger aperture.^15,20

In the biomechanical measurements, the time-strain mode (modus 1) was used, with a 450-mbar load applied for 2 seconds (on-time) followed by a relaxation time of 2 seconds (off-time). For skin elasticity analyses, 3 parameters (Uf, Ua, and Ur) were directly measured using the device, and 1 parameter (Ue) was calculated, as the Ue value is not displayed numerically by the device (Figure 1). The values of these parameters are generally assumed to be representative of skin elasticity in biomechanical measurements^15,21:

Uf = R0: maximal skin extension; firmness of skin. This value represents the passive behavior of the skin to force, with lower values representing higher firmness.
Ua = R8: pliability; overall elasticity. This value denotes complete relaxation after the force is removed. With an approximation towards R0, elasticity increases.
Ur: This denotes retraction/relaxation of the skin.
Ue: Calculated by (R7 × R0)/R5.

Experimental protocol

Each healthy participant was seated for at least 10 minutes in the same temperature-controlled examination room (21°C ± 1°C; relative humidity 36%–42%). The patient’s heart rate, blood pressure, and skin temperature of the palmar forearm then were measured. Measurements were taken with the patient’s forearm, palmar side up, on a desk. To minimize possible fatigue, 2 different skin areas were used. First, the middle between the patient's elbow and wrist was marked. Then, 2 additional marks were made on the same forearm: one mark 2 cm distal and one mark 2 cm proximal of the middle of the forearm. The palmar forearm was chosen because research has shown smaller age-related changes in skin elasticity in this area than in other areas of the body.²²

The first skin area was cooled to skin-temperature measurement of 15°C and then to 5°C within 1 minute. Afterwards, the second skin area was heated to 40°C and then to 45°C within 1 minute. These temperatures and durations were chosen because they do not lead to burn or cold injury and demonstrate temperature pain thresholds.^23-25 The biomechanical properties of the skin were measured before and after each temperature change using 2 different probes (2-mm and 6-mm apertures).

Statistical analysis

To determine normality, a Shapiro-Wilk test was used. A Friedman test then was conducted to evaluate differences between groups (statistical significance: P ≤ .001). A Wilcoxon signed-rank test was used for comparing 2 related samples (statistical significance: P ≤ .05). The significance level was determined using the Holm-Bonferroni method. Spearman’s rank correlation test was conducted to test the association between 2 ranked variables (P ≤ .05). A Mann-Whitney U test assessed significant changes between smokers and nonsmokers.

Results

The Shapiro-Wilk test confirmed the normality of the skin elasticity measurement device values obtained with the 2-mm aperture. However, the values with the 6-mm aperture did not consistently demonstrate normality. Thus, Wilcoxon’s signed-rank test was used. In 1 case, the experiment was aborted due to pain caused by the thermal stimuli.

Control variables

The mean cardiac frequency at the time of the examination was 68.3 bpm (range, 50–98 bpm). The mean blood pressure was 121/74 mm Hg, and the initial mean body temperature at the test site was 35.1°C (range, 34.0–36.9°C).

Spearman’s rank correlation test revealed a correlation only between age and Ur values for the skin elasticity measurement device 6-mm probe at a skin temperature of 40°C (P = .286). Age had no influence on any other values measured (P < .05). There was no correlation between blood pressure and skin elasticity measurement device values. However, there was a correlation between body temperature (Ur at 15°C; Uf, Ua, Ue, and Ur at 5°C), heart rate (Uf at 40°C; Uf, Ua, Ue, and Ur at 45°C), and skin temperature (Ur at 15°C; Uf, Ua, Ue, and Ur at 5°C; Ue at 40°C).

As compared with values obtained before the experiment, only the Ue value in superficial layers differed in smokers versus nonsmokers, with significantly lower values found among smokers.

Cooled skin: upper layers

The evaluation of skin elasticity measurement device data obtained using the handheld probe with a suction head of 2 mm revealed a statistically significant increase in Uf at skin temperatures of 15°C and at 5°C. Values for Ua, Ue, and Ur did not show a statistically significant increase as compared with those of healthy skin. However, there was a statistically significant change in Ua and Ur values obtained at 15°C versus those observed at 5°C (Figures 2, 3).

Cooled skin: total skin

The evaluation of skin elasticity measurement device data obtained using the handheld probe with a suction head of 6 mm revealed no statistically significant changes in Uf, Ua, Ue, or Ur values at a skin temperature of 15°C as compared with values for untreated skin. At a skin temperature of 5°C, Ue and Ur showed a statistically significant decrease (Figures 2, 3).

Heated skin: upper layers

At a skin temperature of 40°C, Uf, Ua, and Ue values exhibited a statistically significant increase as compared with those for untreated skin. Likewise, at a skin temperature of 45°C, these values displayed a statistically significant increase. Values for Ur increased significantly only at a skin temperature of 45°C (Figures 3, 4).

Heated skin: total skin

There was a statistically significant increase in Ue and Ur values at 40°C and 45°C than those obtained for healthy skin. In contrast, Uf and Ua values showed a statistically significant increase at 45°C and 40°C, respectively (Figures 3, 4).

Discussion

The alteration of biomechanical skin properties is a function of damaged or temporarily restricted tissue structures. The effects of changes in biomechanical skin properties in human skin induced by thermal injury have remained unclear. In the present study, the authors objectively evaluated biomechanical parameters of human skin as a function of thermal injury using a skin elasticity measurement device.

Five parameters are derived from the skin elasticity measurement device deformation-time curve: Uf, Ua, Ur, Ue, and Uv. Draaijers et al¹⁵ reported that the skin elasticity measurement device values were highly correlated with each other and therefore suggested that the use of 1 parameter was sufficient for evaluations of scar elasticity. However, Ue, Uf, and Ua are generally measured in skin elasticity analyses because these parameters are assumed to be the most representative skin elasticity measurement device values of skin elasticity.^15,21

It is well-known that Uf and Ue parameters are linked to the stretching of elastic and collagen fibers.¹⁵ The biomechanical properties of skin are less dependent on skin thickness than on regional differences in dermal and hypodermal collagen, elastic fiber structure, and viscous ground matrix.²⁶ Differences in these skin parameters depend on age, sex, body site, genetic factors, skin diseases, and sun exposure.^15,27

A number of previous studies evaluated the precision and validity of skin elasticity measurement device-based measurements for pathological skin conditions such as scar formation.^15,28 Data on the use of the skin elasticity measurement device for evaluations of physiological skin conditions are rare, with most studies focusing on skin ageing.^20,26,29-31 In a literature review, the investigators found no studies of the use of the skin elasticity measurement device to evaluate skin changes due to thermal stimuli.

The results of the present study revealed increases in Uf, Ua, and Ue values for cooled upper skin layers at skin temperatures of 15° and 5°C but only marginal increases in these values for total skin at 15°C and decreases at 5°C. Acute cold-contact injury can result in ice crystal formation, protein changes, plasma membrane impairment, and a decrease in interstitial and intracellular fluid contents.^32-34 The authors expect that these irreversible changes did not occur during the cooling of skin to 15° or 5°C in the present experiment because of the limited exposure time (ie, 1 minute) and suggest that the increased elasticity in upper skin layers at a skin temperature of 15°C was due to increased blood flow and a reversible rise in interstitial and intracellular fluid contents. At a skin temperature of 5°C, the investigators assume that blood flow decreased due to cold-induced vasoconstriction and thermal contraction of collagen and elastic fibers, both of which are precursors to irreversible damage. Possibly, these changes initially affected the upper skin layers during cooling, whereas they had less of an effect on total skin and deeper layers. Hence, correlating the findings of a quantitative analysis of the microcirculation of the skin with the results obtained using the skin elasticity measurement device would be highly interesting.

Regarding the heated skin, the values for upper layers and total skin showed an increase in all parameters at 40°C and 45°C. The increase in skin elasticity may be due to a thermal-induced widening of collagen and elastic fibers without any structural damage, such as denaturation, and an increase in intravascular and extravascular water content and hyperemia in thermally stimulated skin.³⁵

The present results are in accordance with those of the previous investigations conducted by the study authors in a miniature pig model.^17-19 In these studies, the authors evaluated the biomechanical properties of porcine skin as a function of burn injury depth and acute cold-contact injury by inflicting real burn and cold-contact injuries. Skin Uf, Ua, and Ue values decreased continuously with increasing depths of burn injury and acute cold injury. A comparison between the miniature pig and human model is not possible, as the porcine skin was significantly damaged due to dermal injury. However, as porcine skin has many similarities to human skin, the investigators hypothesized that the depth-dependent changes in biomechanical skin properties may be transferable to humans. They assumed the decreased skin elasticity in real burn injuries in their miniature pig model was due to direct tissue damage, with depth-dependent protein changes and cellular impairment. In contrast, the heating of human skin resulted in increased skin elasticity in the present study. Moritz et al²³ demonstrated that an application time of 2 hours was necessary to inflict a first-degree burn injury at a surface temperature of 45°C. Therefore, the injury induced at 45°C using an aluminum bar (30 mm × 30 mm) for about 1 minute in this human model did not create real burn injury and probably no permanent protein impairment. Heating above 45°C and cooling below 5°C is not possible in a human model due to pain and irreparable damage.

The results of the present study demonstrate thermally induced changes in biomechanical skin properties in both hyperthermic and hypothermic ranges. To expand the knowledge of thermal wounds, measurements of depth-dependent changes in skin elasticity in human burn and cold-contact injuries are necessary. Such studies have not yet been conducted using the skin elasticity measurement device. However, depth-dependent skin changes in thermal injuries have been examined using ocular micrometers, different tensile tests, or in vitro experiments.^36-39 These studies detected destruction of collagen, blood vessels, follicular epithelial cells, and mesenchymal cells. Another study⁸ reported depth-dependent changes in stainability in heat-denatured collagen in burnt skin. In the present study, the investigators did not compare the results of the measurements obtained using the skin elasticity measurement device with those obtained using other objective methods, such as ocular micrometer measurements.

Limitations

Despite careful planning, this study has some limitations. Although sun exposure can influence skin elasticity, the authors did not consider this factor in the assessment. In addition, they did not assess the hydration status of the participants, a factor that also can affect elasticity. To evaluate the influence of age and smoking on skin elasticity, a more heterogeneous experimental group would have been helpful. In further studies, these limitations should be addressed.

Based on the results of this study, the authors are currently preparing a clinical study to evaluate the skin elasticity measurement device for the assessment of thermal injury depth in human patients. However, this study will be subject to some limitations, such as a limited amount of cold-contact injuries and lack of standardized wounds.

Conclusions

Important skin elasticity values (Uf, Ua, Ur, and Ue) showed a continuous decrease with decreasing skin temperatures in total skin and an increase in values with increasing skin temperatures in upper layers and total skin. The investigators hypothesize that the changes in heated and cooled skin are due to increased blood flow, in addition to a reversible increase in interstitial and intracellular fluid contents, thermal contraction, and expansion of collagen and elastic fibers, all of which are precursors of irreversible damage. The authors believe skin elasticity measurements obtained using the study device may be valuable for depth assessments of burn wounds and cold-contact wounds and that such measurements can aid skin injury management decisions.

Acknowledgments

Affiliation: Department of Plastic, Reconstructive, Hand and Burn Surgery, BG-Trauma Center, Eberhard Karls University Tuebingen, Germany

Correspondence: Manuel Held, MD, Department of Plastic, Reconstructive, Hand and Burn Surgery, BG-Trauma Center, Schnarrenbergstr. 95, 72076 Tuebingen, Germany; ManuelHeld@hotmail.com

Disclosure: The authors disclose no financial or other conflicts of interest.

References

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