Skin mechanics can influence monofilament threshold

NewsBot · Jun 11, 2015

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Thresholds of skin sensitivity are partially influenced by mechanical properties of the skin on the foot sole.
Strzalkowski ND, Triano JJ, Lam CK, Templeton CA, Bent LR
Physiol Rep. 2015 Jun;3(6). pii: e12425. doi: 10.14814/phy2.12425

Across the foot sole, there are vibration and monofilament sensory differences despite an alleged even distribution of cutaneous afferents. Mechanical property differences across foot sole sites have been proposed to account for these differences. Vibration (VPT; 3 Hz, 40 Hz, 250 Hz), and monofilament (MF) perception threshold measurements were compared with skin hardness, epidermal thickness, and stretch response across five foot sole locations in young healthy adults (n = 22). Perceptual thresholds were expected to correlate with all mechanical property measurements to help address sensitivity differences between sites. Following this hypothesis, the MedArch was consistently found to be the thinnest and softest site and demonstrated the greatest sensitivity. Conversely, the Heel was found to be the thickest and hardest site, and was relatively insensitive across perceptual tests. Site differences were not observed for epidermal stretch response measures. Despite an apparent trend of elevated sensory threshold at harder and thicker sites, significant correlations between sensitivity measures and skin mechanical properties were not observed. Skin hardness and epidermal thickness appeared to have a negligible influence on VPT and minor influence on MF within this young healthy population. When normalized (% greater or smaller than subject mean) to the subject mean for each variable, significant positive correlations were observed between MF and skin hardness (R(2) = 0.422, P < 0.0001) and epidermal thickness (R(2) = 0.433, P < 0.0001) providing evidence that skin mechanics can influence MF threshold. In young healthy adults, differences in sensitivity are present across the foot sole, but cannot solely be accounted for by differences in the mechanical properties of the skin.
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NewsBot · Aug 30, 2017

Skin Hardness and Epidermal Thickness Affect the Vibration Sensitivity of
the Foot Sole
Yves Jammes et al
Clin Res Foot Ankle 2017, 5:3

Objective: The cutaneous mechanoreceptors of the foot sole detect the changes in the application of mechanical
loads on the plantar surface during gait and standing, and contribute to controlling the standing balance and postural
reflexes in healthy subjects. A local thickening of the foot sole skin occurs in response to repetitive load application.
We hypothesized that an elevated skin hardness of the foot sole could reduce its mechano sensitivity.
Methods: In healthy subjects, we quantified the sensation produced by different amplitudes of vibratory
stimulations at two frequencies (25 and 150 Hz). The vibration threshold was determined on the 1st or 2nd, and
5th metatarsal heads, and the heel at each vibration frequency. The Stevens power function (Ψ=k.Φn
) allowed to
obtain regression equations between the estimate (Ψ) of the vibratory stimuli and their physical magnitude (Φ).
Any increase in the absolute k value (all were negative) indicated a reduced sensitivity to the lowest loads. The n
coefficient measured the global perception. The highest skin hardness (Shore) was measured on the 5th metatarsal
head and the heel. In some subjects, superficial skin abrasion of the 5th metatarsal head was performed and the
vibration sensitivity was tested again.
Results: The vibration threshold was significantly higher at the level of the 5th metatarsal head and the heel. The
k value was significantly higher at the 25 and 150 Hz frequencies for the 5th metatarsal head, and only at 25 Hz for
the heel. At both vibration frequencies, negative correlations were obtained between the k values and skin hardness.
After skin abrasion, the n coefficient was significantly higher at both vibration frequencies.
Conclusion: Skin hardness affects the foot sole mechano sensitivity and could alter the control of posture during
standing and walking. This indicates that foot care by podiatrist are relevant to improve posture control.
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