Skin indentation firmness and tissue dielectric constant assessed in face, neck, and arm skin of young healthy women

Purpose: Our goal was to test the hypothesis that skin firmness correlates with skin hydration. Methods: Dermal water was assessed by tissue dielectric constant (TDC) at 0.5 mm (TDC0.5) and 2.5 mm (TDC2.5) depths on four face sites and two arm sites of 35 women (25.0 ± 1.6 years). Firmness was determined by force (mN) to indent skin to 0.3 mm (F0.3) and 1.3 mm (F1.3). Results: F0.3 was similar among face sites (avg = 16.2 ± 7.2 mN) but F1.3 varied (avg = 32.5 ± 4.1 mN). TDC2.5 was similar among face sites (avg = 37.7 ± 4.2) but TDC0.5 varied (avg = 36.2 ± 4.8). F1.3 of arm sites was similar (avg = 60.2 ± 18.6 mN) and both greater than F1.3 of neck (28.3 ± 7.1 mN) and face. Regression analysis showed a near-zero correlation between forces and TDC at all sites. Conclusion: The near-zero correlation may be due to low skin interstitial hydraulic resistance to mobile water movement in healthy young skin. If true, then conditions in which dermal hydraulic conductance is reduced as in lymphedematous, diabetic, or aged skin are more likely show the hypothesized relationship. Our findings provide normalized reference values and suggest that such persons are an important population to study to test for a possible skin water–indentation force relationship and its utilization for early diagnosis.