Vibration, an oscillatory motion about a fixed point, can enter the body at any point that is in contact with a vibrating surface including the hands (hand-arm vibration), feet (foot-transmitted vibration), or buttock and back (whole-body vibration). Adverse health effects, associated with vibration exposure are partially dependent on the primary point of entry to the body; therefore, it is important to document the source of the occupational exposure and the transmission route. For example, when driving a motorized vehicle a worker is exposed to hand-arm vibration (HAV) through contact with the steering wheel, whole-body-vibration (WBV) through the seat, and foot-transmitted vibration (FTV) through contact with the floor. In another scenario, a worker using a pneumatic power-tool may be solely exposed to HAV from the tool; however, if the tool is attached to a platform then the worker may also have concurrent FTV exposure. Similarly, some workers may only have FTV exposure, for example those who work on platforms that vibrate (i.e. crusher platform; drilling platform, bolting platform, etc.).

Several researchers have estimated occupational exposure to WBV including Bovenzi [1] who suggested 4–7% of the workforce in North America and Europe were exposed annually to harmful levels of WBV, and Wasserman [2] who reported 7 million workers in the United States had annual exposure to WBV. More recently Palmer and colleagues [3], estimated that 7.2 million men and 1.8 million women in Great Britain are exposed to occupational WBV. To date, published epidemiological data has not classified FTV independent from general WBV. Therefore, we do not know how many people from the estimated WBV exposure groups are primarily exposed to FTV. Epidemiological data are available for HAV exposure, suggesting 1.7–5.8% of workers in the United States, Canada and European countries are exposed to occupational HAV [4]. Furthermore, the prevalence of vibration white finger is estimated at 0–5% amongst workers using vibrating tools in warm climates and jumps up to 80–100% amongst workers exposed in countries at higher latitudes with colder climates [4,5].

Health effects associated with WBV have been well documented and include low-back pain, spinal degeneration, neck problems, headaches, nausea, gastrointestinal tract problems, disturbed sleep, and autonomic nervous system dysfunction [6–8]. HAV exposure causes a condition referred to as hand-arm-vibration syndrome (HAVS). HAVS is associated with vascular, neurological, and musculoskeletal problems of the hand-arm system [9]. Exposure to FTV may cause an analogous syndrome in the lower extremities; however, little is known about the characteristics of occupational FTV or clinical implications with prolonged exposure. The extent of disability caused by vibration exposure is variable and may depend on the magnitude, frequency, and duration of exposure; however, a clear dose response relationship has yet to be proven [4]. Low-back pain is the most common complaint amongst workers exposed to WBV, which can lead to lost work time depending on the severity of the disorder [10,11]. The severity of HAVS is generally classified using the Stockholm Workshop Scales (SWS) [12]. The SWSs are based on symptomatic history and generally require additional objective testing to establish a diagnosis of HAVS and provide accurate staging. The highest vascular component score on the SWS describes frequent blanching attacks affecting most fingers accompanied by trophic skin changes. The highest SWS score for the sensorineural component describes persistent numbness with tingling accompanied by reduced tactile discrimination and manual dexterity impairment. FTV can result in vascular and sensorineural changes in the feet; however, the SWS does not lend itself to staging of vibration white foot (VWFt) and no similar staging system has been developed to date to classify severity of FTV related disease.

The objective of this paper is to review the current state of knowledge of health risks associated with FTV, including evidence from animal studies, clinical studies and field studies. Case data from a field study in mining are presented to document the characteristics of FTV and discuss the associated risk of VWFt.

Foot- and hand-transmitted vibration can be referred to as segmental vibration because the transmission of energy from the vibrating source to the body is limited to a single region or segment of the body. In workers, most studies examining the effects of segmental vibration have focused on vibration transmission to the hands. Skin biopsy samples collected from the fingers of workers have demonstrated that repeated, long-term exposure to vibration can result in damage to the arteries of the fingers and hands. This damage is characterized by thickening of the vascular smooth muscle, and fibrosis of the internal lining of blood vessels (i.e., the intima [13]). There is also nerve damage in these samples that is characterized by fibrosis in the region around peripheral nerves, a thinning or loss of the myelin sheath, and a reduction in peripheral nerves [14]. Although these studies demonstrate the morphological changes associated with vascular and sensorineural dysfunction in workers, additional data are needed to understand the mechanisms underlying these changes.

Animal models have been used to study the damage and mechanisms underlying vibration-induced injuries to the vascular, sensorineural and skeletal muscle systems. The models have included both single and repeated exposures of the fore and hind limbs of rats, mice or rabbits, or tails of rats to vibration using frequencies ranging from 30–1000 Hz with accelerations of approximately 2–7 g [15–18]. These studies have demonstrated that the morphological and physiological effects of both acute and repetitive vibration on these systems are similar to the effects seen in humans [13–20]. Furthermore, the effects are in part dependent upon the frequency of the vibration exposure [15,18, 21,22].

A single session of exposure to vibration (4 h in duration) using a broad range of frequencies has shown that blood flow is reduced in exposed appendages of animals [21]. These acute changes in blood flow may be the result of an increased responsiveness to α2C adrenoreceptor-mediated vaso-constriction [23] and/or a reduced responsiveness to acetylcholine (ACh)-mediated vasodilation [24]. These functional effects appear to be mediated in part by vibration-induced increases in oxidative stress within vessels [24]. Morphological changes after a single vibration exposure include the presence of vacuoles in the smooth muscle and endothelial layers of peripheral arteries [22].

Exposure to repeated bouts of vibration results in fibrosis of the intima (internal lining of arteries comprised of endothelial cells), and obstruction of the lumen in peripheral vessels [16,17], as well as remodeling of arteries characterized by a thickening of the vascular smooth muscle wall and reduction in the diameter of the lumen [20]. Repeated exposures also result in a reduced responsiveness of arteries to ACh-mediated vasodilatation and increases in oxidative activity and the expression of remodeling factors [17,20].

Vibration also induces morphological and physiological changes in peripheral nerve function in animal models. Single exposures to vibration of a single frequency (i.e., in the 30–1000 Hz range), have been associated with a reduction in nerve conduction velocity [21], edema and myelin disruption in nerves [18, 25,26], and a transient increase in the sensitivity of peripheral nerves to transcutaneous electrical stimulation at 2000 Hz [27]. Exposure to a single bout of impact vibration (i.e. acceleration shock pulse applied to one point) resulted in swelling in nerve terminals and an increased sensitivity to heat on the day of the exposure in rats exposed to tail vibration. By day 4 post-exposure, there was a reduction in PGP9.5 (a marker of nerve fibers) in nerve terminals, myelin disruption, and a reduced sensitivity to a heat stimulus in tails exposed to impact vibration [28]. In animal models, repetitive exposure results in damage to the myelin sheath around axons [29], reductions in axoplasmic transport [30], a reduced ability of nerves to regenerate after dissection [31], and a permanent reduction in nerve conduction velocity [32]. Similar morphological and functional changes in humans [14] are most likely responsible for the long-term changes in sensorineural function seen in workers exposed to segmental vibration [33–35].

Fewer studies have examined the effects of segmental vibration on skeletal muscle. Single exposures to vibration do not appear to have any effects on skeletal muscle morphology [36]. However, repeated exposure to vibration results in muscle fiber degeneration and regeneration, the presence of central nuclei [36,37] and increases in the cross-sectional areas of slow twitch muscle fibers [38]. Skeletal muscle atrophy also occurs and is accompanied by limb weakness leading to walking difficulty in rats [39]. If similar changes in skeletal muscle and peripheral nerves occur in humans, it is reasonable to assume that they could lead to longer-term changes in motor function and muscle weakness.

Vibration-induced injuries appear to be associated with the amount of tissue stress and strain experienced during exposure to vibration [40]. A research team at the National Institute for Occupational Safety and Health (Morgantown, USA) recently characterized the physical (i.e., biodynamic) response of the rat tail to vibration. This was done by securing the tail to a vibrating platform, exposing the tail to different vibration frequencies, and comparing the displacement of the tail with the displacement of the platform at the various exposure frequencies [41]. The ratio of the tail displacement to the platform displacement, which is referred to as vibration transmissibility, is one measure that can be used to describe the biodynamic response. In an additional study, the frequency-dependent biodynamic response of the tail was correlated with changes in vascular physiology [15]. The results of these studies demonstrated that indices of vascular remodeling and function occurred more rapidly at vibration frequencies that generated the greatest biodynamic response [15]. The results of modeling studies investigating vibration power absorption in the tail suggest this is because tissue stress and strain is greatest at the frequencies that generate the greatest response [41]. These findings are consistent with the results of human studies on the hand-arm system [40], and suggest that characterizing the biodynamic response of a body segment to vibration may be critical for determining the risk of developing a vibration-induced injury.

Vibration white finger (VWF), the vascular component of hand-arm vibration syndrome (HAVS), is a well documented occupational form of secondary Raynaud’s phenomenon first reported by Loriga in 1911 [42]. More recently, a condition descriptively termed “vibration white foot” (VWFt) or “vibration-induced white toes” (VWT) has been recognized. VWT is analogous to vascular HAVS but manifests as Raynaud’s phenomenon in the toes arising after sufficient exposure to foot-transmitted vibration. To date, the published literature addressing cases of VWT in workers without hand-arm vibration syndrome is limited to two case reports [43,44], though a greater number of studies have reported VWT in workers with HAVS after mixed exposure to both hand and foot-transmitted vibration [45–47]. That Raynaud’s phenomenon in the toes could occur in workers with direct foot-transmitted vibration is supported from a pathophysiological standpoint, given that both systemic and local mechanisms have been implicated in the hand-arm vibration syndrome [48].

In 1994, Tingsgard et al. [43] reported a case of a 46-year-old mink farmer presenting with cold-induced blanching in the toes of the left foot. The farmer had a twelve-year history of vibration exposure to his left foot from resting the foot on a vibrating pedal while driving a small wagon. In addition to the unilateral Raynaud’s phenomenon observed in the toes, further work-up additionally showed a subtle neuropathy in the affected foot. Other causes were excluded, and the worker was diagnosed with VWT. A second case of VWT has more recently been reported. Thompson et al. described the case of an underground miner with 18 years of experience who presented with isolated symptoms of blanching and pain in the toes after at least four years of FTV exposure from the operation of vehicle-mounted underground bolting machines [44]. Neurological and musculoskeletal examinations of both the upper and lower extremities were normal. Vascular flow in the digits was measured using photocell plethysmography. This technique involves placement of photocell devices around the digits, which detect systolic and diastolic volumetric changes in the blood vessels. Low or absent systolic variations in pulse waveforms post cold stress demonstrates decreased blood flow secondary to vasospasm. Using a standardized protocol, digital photocell plethysmography of the hands and feet of the worker after a 2-minute 10°C cold immersion bath was conducted to determine if the worker had evidence of vascular impairment in the feet and/or hands. Results from the cold provocation digital photocell plethysmography showed significant cold induced vasospasm in the toes with normal findings in the fingers. After excluding other potential causes for vascular abnormalities in the feet, the worker’s condition was attributed to past segmental vibration exposure to the feet [44].

Reports have also documented vasospastic disease in the feet of workers exposed to both hand and foot-transmitted vibration, though interpretation of the aetiology of the foot symptoms in these workers is complicated by concurrent vascular HAVS [45–47]. This is due to the fact that workers with documented vascular HAVS in the hands often present with concurrent vasospasm in the toes, even in the absence of FTV exposure [49,50]. In these cases, the underlying mechanism seems to primarily be a centrally mediated increase in sympathetic tone [51], though it is possible that other systemic effects including altered circulating vasoactive mediators such as endothelin may also contribute [52]. Interestingly, there is some evidence that workers with mixed hand and foot-transmitted vibration exposure show more severe foot symptoms than workers with hand vibration exposure alone [46]. Hedlund et al. studied 27 miners exposed to hand-transmitted vibration, with a subset of these workers also having concurrent exposure to FTV secondary to drilling on raised platforms with a dominant frequency of 40 Hz [46]. Six of the 27 miners experienced white toes and all six with white toes were in the FTV group. This study demonstrated a statistically significant association between Raynaud’s phenomenon in the toes and a history of FTV [46]. The two published case-reports of VWT [45,47] also support this association, with both reports describing workers with a history of mixed hand and foot vibration exposure having prominent foot symptoms.

Research from animal, clinical and field studies indicate exposure to segmental vibration can cause damage to peripheral nerves and blood vessels. Furthermore, results from animal model studies suggest the negative health effects are frequency dependent and associated with tissue stress and strain experienced when exposed to vibration. Results from field and clinical studies suggest exposure to FTV can result in vibration white feet/toes which is analogous to hand-arm vibration syndrome. To date there have been only two published case reports of VWT independent of a diagnosis of HAVS. Furthermore, workers who are exposed to both HAV and FTV appear to have more severe foot symptoms. However, further research is required to understand the link between occupational exposure to FTV and the development of VWT and/or VWFt. In particular, more research is required to determine the magnitude, frequency, and duration of FTV exposure associated with a clinical diagnosis of VWFt.

Animal models have demonstrated vibration-induced alterations in peripheral vascular and nerve physiology, which have given clinicians a better understanding of the changes that occur in the human physiological system when it is exposed to vibration. Clinical and epidemiological studies support the contention that workers with significant segmental vibration exposure to the upper and/or lower extremities can develop cold-induced vasospasm in the toes. In general, in workers exposed to HAV, symptoms of cold-induced finger vasospasm usually seem to be most severe in the directly exposed hands as a result of local vibration-induced trauma, while the non-exposed feet tend to have less severe symptoms best attributed to centrally mediated mechanisms (increased sympathetic tone and altered circulating vasospastic mediators). In workers with direct vibration exposure to the feet, risk of developing VWT is increased. Both the magnitude and, perhaps more importantly, the dominant frequency of the FTV seems to be an important determinant of risk, with research to date suggesting higher frequencies (40 Hz) to carry greater risk with respect to VWFt or VWT compared to lower frequency distributions. Further, study is required to delineate the magnitude of the problem and to better characterize and control foot-transmitted vibration as an occupational hazard.