Use of a novel Indentometer to evaluate skin induration in localized scleroderma and psoriasis vulgaris

Abstract

Introduction

Diseased skin in localized scleroderma (LS) and plaque psoriasis (PPs) is characterized by induration that can be evaluated by non‐invasive bioengineering methods. In this study, we applied a new measurement device based on indentometry to determine the changes of skin mechanical properties in patients with LS and PPs.

Material and methods

A total of 30 sclerodermatous plaques in 12 patients with LS and 46 psoriatic plaques in 19 patients with PPs were measured with Indentometer IDM 800 (Courage + Khazaka, Cologne, Germany). The device measures the penetration depth of the probe indenter (pin) into the skin in mm. We used two probes with pin diameters 3 and 5 mm, respectively. The stiffer the skin, the less deep is the displacement by the indenter. The smaller the diameter, the deeper the pin will go into the skin when using the same force. The measurements were made on diseased skin and in adjacent normal skin served as control. In addition, the psoriatic plaques were measured before and after treatment.

Results

The sclerodermatous skin and the psoriatic skin showed lower Indentometer values compared to adjacent normal skin as measured with both probes. This suggests that diseased skin is stiffer than healthy skin. The values of psoriatic plaques increased after treatment applied that correlates with the clinical improvement. The Indentometric readings significantly negatively correlated with clinical scores of skin induration. There was a significant correlation between the measurements with probe 3 mm and probe 5 mm.

Conclusion

The non‐invasive method used is appropriate for objective and quantitative determination of the degree of skin induration in LS and PPs as well as for assessment of the disease evaluation and treatment efficacy.

1. INTRODUCTION

Mechanical properties of the skin are one of the most important its property. They can be studied non‐invasively using bioengineering devices based on the application of external influence to the skin surface and the measurement of the force of resistance or the changes occurring as functions of time or the force of action. In connection with the direction of the applied force, two main groups of methods are distinguished ¹ :

1. Methods applying an effect perpendicular to the skin surface: suction method, indentometry, levarometry, and balistometry.

2. Methods applying an effect parallel to the skin surface: twistometry, extension method and shear waves propagation method.

These methods are the basis of several commercially available devices. The search in PubMed shows that the most popular test method is the suction method, and the most commonly used device is the Cutometer. ²

Indentometry is less popular. Devices based on indentometry exert vertical pressure on the skin surface. A ball or flat disk is applied to the surface of the skin, and a vertical force causes the tissue to deform. The stiffness/hardness/firmness/softness of the skin is determined by measuring the depth of the depression at a certain pressure for a standard time. ¹ , ² Until recently the data obtained are interpreted by using the Hertz theory of contact mechanics and by calculation of Young's modulus of elasticity. In the past years, three commercially available devices are introduced for fast and comfortable indentometric measurement of skin mechanical properties. The Durometer (Rex Gauge, IL, USA) is the international standard for the hardness measurement of rubber, plastic, and other non‐metallic materials and was the first instrument applied to measure skin hardness. Results are obtained in Durometry units, and the lower numbers indicate less resistance and softer materials. ² , ³ , ⁴

The Indentometer IDM 800 (Courage and Khazaka, Cologne, Germany) and the SkinFibroMeter (Delfin Technologies, Finland) are newer and little known to researchers. ² Separately, both devices were applied successfully to evaluate the lymphedema of the lower limb. ⁵ , ⁶

In this study we used for the first time a novel measurement device based on indentometry (Skin Indentometer IDM 800) to investigate the skin induration of diseased skin in patients with localized scleroderma (LS) and plaque psoriasis (PPs).

2. MATERIAL AND METHODS

2.1. Patients

Two groups of patients were studied. The first one includes 12 consecutive patients with LS (three males, nine females, mean age 54.5 ± 15.7 years). A total of 30 sclerodermatous plaques were measured. The second one consists of 19 consecutive patients with PPs (10 male, nine females, mean age 44.1 ± 15.0 years). A total of 46 psoriatic plaques were measured before and after treatment. The diagnosis of LS and PPs was confirmed by skin histopathology examination. In all psoriatic patients, identical treatment was applied including 10‐day therapy with NB UVB phototherapy, topical corticosteroids and emollients.

2.2. Skin score

Severity of the cutaneous induration of 22 LS and 42 PPs plaques was rated according to a four‐point scale (0: normal skin, 1: mild, 2: moderate and 3: severe skin induration).

2.3. Skin indentometry

Skin induration was measured using a novel device Indentometer IDM 800 (Courage + Khazaka, Cologne, Germany). The device measures the penetration depth of the probe indenter (pin) into the skin in mm. We used two probes with pin diameters 3 and 5 mm, respectively. The stiffer the skin, the less deep is the displacement by the indenter. The smaller the diameter, the deeper the pin will go into the skin when using the same force. The measurements were made on diseased skin and in adjacent normal skin served as control. Measurements of psoriatic plaques were performed after removal of the scales with 10% salicylic ointment. For each skin area, indentometry was assessed four times, and the mean value was considered as the final result. The environmental conditions were room temperature 22–25 degrees and humidity 40%–50%.

2.4. Statistics

The number of measured sclerodermatous and psoriatic plaques and corresponding control sites was in accordance with the sample size requirements for in vivo assessment of biomechanical properties of the human skin. ² Results were expressed as mean ± standard deviation. Before selecting a statistical method, the data were checked for normality by Sharpio–Wilk method. ² All data series except for the LS control sites measured by probe 3 mm and psoriasis plaques before treatment measured by both probes showed normal distribution. The differences in the parameters between the diseased and control healthy skin were evaluated using Student's t‐test for unpaired data and Mann–Whitney test. The differences in the parameters of psoriatic plaques before and after treatment were evaluated using Student's t‐test for paired data and Wilcoxon signed rank test. The correlation between the values measured with probe 3 mm and probe 5 mm was assessed with Pearson correlation test. In addition, we analyze the clinical scoring by means of Wilcoxon signed rank test and the relationship between the skin score and Indentometer values by means of Spearman's rho test. SPSS v.17 software was used for performing statistical analysis. A p‐value of <0.05 was considered as statistically significant.

2.5. Ethics

The study was approved by the committee of scientific ethics of the Medical University of Plovdiv (Approval No 7/2021). The investigation was in accordance with the Helsinki Declaration, and all measurements were made after giving informed consent.

3. RESULTS

3.1. Patients with LS

The sclerodermatous skin showed lower Indentometer values compared to adjacent normal skin. The depth penetration with probe 3 mm was 1.46 ± 0.30 mm versus 1.87 ± 0.35 mm (p < 0.001) and with probe 5 mm – 1.18 ± 0.33 mm versus 1.49 ± 0.35 mm (p < 0.001). This suggests that diseased skin is much stiffer than healthy skin (Figure 1). A significant correlation between the measurements with probe 3 mm and probe 5 mm was found on diseased (R = 0.7315; p < 0.05) and control (R = 0.8145; p < 0.05) skin. Using both probes, low indentometric values correlated with severe induration of sclerodermatous skin (p < 0.001) (Figure 2).

FIGURE 1.

Skin induration values of localized scleroderma and control healthy skin. (n = 30)

FIGURE 2.

Relationship of indentometric values with clinical skin score of sclerodermatous plaques (n = 22; p < 0.001 Spearman's rho)

3.2. Patients with PPs

Before treatment, psoriatic skin showed lower Indentometer values compared to adjacent normal skin. The depth penetration with probe 3 mm was 1.42 ± 0.26 mm versus 1.78 ± 0.25 mm (p < 0.0001) and with probe 5 mm was 1.22 ± 0.23 versus 1.48 ± 0.26 (p < 0.0001). This suggests that diseased skin is stiffer than healthy skin.

After treatment applied, the values of psoriatic plaques significantly increased compared to initial values but without reaching the values of normal skin. The depth penetration with probe 3 mm was 1.54 ± 0.25 mm versus 1.42 ± 0.26 mm (p < 0.0001) and with probe 5 mm – 1.37 ± 0.21 mm versus 1.22 ± 0.23 mm (p < 0.0001). This suggests that the treatment applied results in a decrease in skin induration that is associated with clinical lesion improvement (mean induration score 2.3 ± 0.5 before vs. 1.5 ± 0.5 after treatment; p < 0.001).

Values of the healthy skin no significantly changed after treatment. The depth of penetration with probe 3 mm was 1.76 ± 0.25 mm versus 1.78 ± 0.25 mm (p = 0.674) and with probe 5 mm – 1.52 ± 0.23 versus 1.48 ± 0.26 (p = 0.412).

After treatment and using both probes, the significant differences between diseased and control skin remains (Figure 3).

FIGURE 3.

Skin induration values of psoriatic plaques and control healthy skin, before and after treatment (n = 46). Significant differences were found between psoriatic/control skin, before and after treatment, as well as between values of psoriatic plaques before/after treatment

A significant correlation between the measurements with probe 3 mm and probe 5 mm on diseased and control skin was found, too.

Using both probes, low indentometric values correlated with severe skin induration of the psoriatic plaques (p < 0.001) (Figure 4).

FIGURE 4.

Relationship of indentometric values with clinical skin score of psoriatic plaques before treatment (n = 42; p < 0.001 Spearman's rho)

4. DISCUSSION

In this study, we applied for the first time a novel skin indentometer to evaluate the skin induration in patients with LS and PPs. The mechanical properties of the skin in these two diseases have long been of interest to researchers and have been studied mainly by two methods – indentometry (Durometer) and suction method (Dermaflex, Cutometer).

Seyger et al. ⁷ for the first time compared the Durometer values readings to clinical scores in patients with morphea. They found that the durometer measurements had low inter‐ and intraobserver variability; however, they showed poor correlation with clinical skin scores (modified skin score). In another study, the same authors observed normalization of durometer readings in patients with morphea after treatment with low‐dose methotrexate. ⁸

Using a skin suction elastometer (Dermaflex), Serup and Northeved ⁹ determined the changes in tensile distensibility, hysteresis and resilient distension of diseased skin in patients with morphea.

A newer suction device (Cutometer) has been used in two studies for assessment of ultraviolet A1 phototherapy in the treatment of LS. Andres et al., ¹⁰ and De Rie et al. ¹¹ found that the elasticity of treated skin increased compared to control skin after treatment applied.

In contrast to morphea, more measurements of the mechanical properties of the skin have been made in systemic sclerosis (SSc). From the first ¹² to the last, ¹³ all studies demonstrate that the indentometry by means of a Durometer is a reliable method to assess skin hardness.

Unlike durometry, measurements with Cutometer allow the determination of more mechanical parameters of the sclerodermatous skin grouped in three categories ‐ distension, elasticity and viscoelasticity. Studies have shown that the method is appropriate for objective and quantitative assessment of skin involvement in patients with edematous and indurative phases of systemic sclerosis. ¹⁴ Moreover, the increased skin viscoelasticity parameter is proposed as the possible sign for the very early diagnosis of SSc. ¹⁵

Our results showed that the novel Indentometer IDM 800 used was able to distinguish diseased from normal skin which is a prerequisite for its future use to assess the effect of the treatment applied in LS.

In regard to the investigation of skin mechanical properties in psoriasis, the situation is similar. We did not find indentometry studies of psoriatic plaques in the available literature.

Serup and Northeved ¹⁶ using a suction device (Dermaflex) found lower skin extensibility and higher hysteresis and resilient distension of the psoriatic plaques compared to regional control.

Borroni et al. ¹⁷ also used the Dermaflex A to study the changes in the psoriatic plaques induced by psoralen plus ultraviolet A therapy and found a reduction in the distensibility and elasticity of the skin.

There were only two studies on skin mechanical properties in psoriasis vulgaris performed by means of Cutometer. Dobrev ¹⁸ found that psoriatic plaques were characterized by lower skin distensibility and elasticity, and a higher viscoelastic to elastic ratio compared with control skin. Delayed skin distension and hysteresis measured using an 8‐mm diameter probe were also increased. These changes were related to the increased skin thickness of psoriatic plaques. After treatment, the mechanical parameters of psoriatic plaques approached the values of adjacent control skin. In their study, Kim et al. ¹⁹ found that the cutometer R9 parameter (hysteresis) significantly correlated with the Psoriasis Symptom Inventory score.

Using Indentometer, we confirmed that psoriatic skin has an increased induration that normalizes after treatment applied. There was a significant negative correlation between the Indentometric values and the clinical scoring of the induration of LS and PPs plaques. In both studies performed by us, the results obtained with Indentometer probes 3 mm and 5 mm were identical.

5. CONCLUSION

The novel non‐invasive indentometric device used is appropriate for objective and quantitative determination of the degree of skin induration in LS and PPs as well as for assessing the disease evaluation and treatment efficacy.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Ivanova Z, Aleksiev T, Dobrev H, Atanasov N. Use of a novel Indentometer to evaluate skin induration in localized scleroderma and psoriasis vulgaris. Skin Res Technol. 2022;28:317–321. 10.1111/srt.13134