Vibration exposure of the glabrous skin of the human hand

Abstract: An occupational exposure to hand-arm vibration can cause a complex of neurological, vascular and musculo-skeletal disturbances, known as the 'vibration syndrome'. However, the underlying pathophysiological mechanisms are not at all clear. Early signs of an incipient vibration syndrome are often intermittent disturbances in the cutaneous sensibility of the fingers, i.e. numbness and/or tactile paresthesias. At later stages, a vasoconstrictive phenomenon appears, usually as episodes of finger blanching.When using a vibratory tool, all mechanical energy entering the body has to be transmitted through, or absorbed by, the glabrous skin in contact with the handle. Therefore, the aims of this study was to investigate: (i) mechanicalresponses of the skin to vibrations, (ii) the response properties of cutaneous mechanoreceptors to vibrations, and (iii) influences of vibration exposure on touch perception.It was found by measuring the mechanical point impedance (0.02-10 kHz) that the skin is easy to make vibrate within the range of 80 to 200 Hz. Within or close to this range are the dominant frequencies of many vibratory tools. Thus, strong mechanical loads, such as compressive and/or tensile strain, can appear in the skin which, in turn, may induce temporary or permanent injuries.Recordings of impulses in single mechanoreceptive afferents, while the skin as exposed to vibrations, were obtained using needle electrodes inserted into the median nerve. The 4 types of mechanoreceptive afferents (FA I, FA II, SA I, and SA II) in the glabrous skin exhibited different response characteristics to vibrations. The FA I units were most easily excited at vibratory frequencies between ca 8 and 64 Hz and the FA II units between ca 64 and 400 Hz. The SA units were most sensitive at lower frequencies. At high stimulus amplitudes, such as may occur while using vibratory tools, a considerable overlap existed between the frequency ranges at which the units were exited. Evidence was also provided, that mechanical skin stimuli produced by edges of a vibrating object, compared to flat surfaces, more vigorously excited the FA I and particularly the SA I units. Thus, a marked edge enhancement, essential for tactile gnosis and precision manipulation, seems to exist already within the peripheral nervous system.Acure impairment of tactile sensibility caused by vibrations, proved to be due to a reduced sensitivity of the mechanoreceptive afferents. A loss of manual dexterity a'vi an increased risk for accidents may therefore appear, both during and after a vibration exposure.Percussive tools, high speed drills and ultrasonic devices are known to generate mechanical energy at frequencies above 1 kHz, i.e. frequencies usually not felt. At these frequencies, it is known that most of the energy, entering the body, is absorbed by the skin. Therefore, it was investigated whether a long-term exposure to high-frequency vibration may have a detrimental effect on the cutaneous sensitivity. One group of dentists and one of therapists, professionally exposed to high-frequency vibrations, were studied with regard to vibrotactile thresholds in their hands. The study showed that deleterious effects on tactile sensibility, at local exposure to high frequency vibration, can not be excluded.