Finger receptor dysfunction in dental technicians exposed to high-frequency vibration.

Finger receptor dysfunction in dental technicians exposed to high-frequencyvibration. Scand J Work Environ Health 1989;15:339- 344. The effects of high-frequency vibration (up to 40 kHz) on digital nerve function werestudied in ten dental technicians and ten age-matched referents. Nerveconduction velocities, including fractionated antidromic measurements over the carpal tunnel, showed no difference between the groups. In the group of dental technicians the difference between the responselatency of the mechanical and electricstimuli in the median nerve distally on the fingers of the right hand was slightly higher than in the reference group, and there fore distal nerve or receptor dysfunction was suggested. Vibration warming and cooling thresholds were significantlyincreased and thus revealed damage to both myelinated and unmyelinated fibers in the fingers of subjects exposed to high-frequency vibration.

Dent al technician s o ften co m plai n of numb ness, pares thesia, a nd pain in th e han ds. By working with high -sp eed gri ndi ng machi nes, th ey ar e exposed to high -frequen cy vibr a tio n up to 40 kHz. Den tist s a nd ultrasonic th er ap ists exp osed to high -frequen cy vibration from ult rahi gh-speed handpieces have been show n to acquire a deficient vibration pe rception (1, 2).
Long-lasting exposure to low er freq uency vib ration from drill s, chi ppi ng hammers, and grinding machines in th e me ch an ical ind ustry a nd worksho ps a nd fro m chai n saws a mong lumberjack s ofte n induces neuropathic sig ns in the hands (3,4). In such subjects a bno rmal hand an d finger perc epti on ca n be dem onstra ted for vib rat ion (5, 6) a nd othe r modalit ies such as temperat ure (6)(7)(8). Epide mio logic st udies have suggested an increased ris k of carpal tu nnel syndrome among vibrat io n-exposed worke rs (9)(10)(11). E lectrophysiolo gica l stud ies have sh own defects in nerve conduction as fa r pro ximal as th e ca rpal tunnel in such patients (unpublished observat ion s). The pathophysiolo gical mech an ism s behind these effects are still unclear. Low-fre quency vibra tion is transmitted pr o xim all y from the a rea o f ski n co n tact with th e ha ndheld to ol a nd mig ht produce dam a ge to nerves through swelling of th e end one ur ium a nd impaired mi cro cir-Reprint requests to: Dr U Hjortsberg, Yrkesmedicinska kliniken, Allmanna sjukhuset, S-214 01 Malmo, Sweden. cula tion, as suggested fro m our experime nta l wo rk (12). W ith high-frequency vibra tion the mechanical energy is a bso rbed in the ski n; thus no proxi ma l ne rve dam age is exp ecte d.
The purpose of our study was to investig a te d ist al senso ry perception a nd nerve conduction in the hands of dental technicians expos ed for a lon g time to hi ghfreq uen cy vibration. Our hypothesis wa s th at the highfrequ ency vibra tio n mi ght interfere wit h the function of the distal ner ve endings and re cepto rs but shoul d no t affect the more pr o xim al d igital nerve fibers.

Subjects
T en men bet ween 26 a nd 62 (mean 44) yea rs o f age wh o had wor ked as dent al technician s fo r 7 to 43 (mean 27) yea rs participated in the study. M ost o f the dental tec hnicia ns complained abo ut paresth esia a nd numbness o f the hands. Acco rd ing to th e senso rineural stagi ng proposed in Stockho lm in 1986 (5) , two were in stage 0, lacking se nsorine ural finge r sympt o ms; six were in stage 1, ha ving inte rm itte nt numbness; on e was in stage 2, ha vin g pe rsiste nt numbness; and one was in stage 3, with redu ced m anipulative dexterit y. Sta ging o f th eir vascula r rea ct ivity according to th e revised Taylor-Pelmear scale (13) revealed six subjects in stage 0, ha ving no attacks of fing er blanch ing; two in stage 1, ha ving occasio nal blan ching attack s a ff ecting th e fin gerti ps; and two in stage 3, having freq uent attacks of all phalanges of most fingers. The temperature reco ver y ti me after han d cooling was a bnorma l fo r four su bjects a nd no rm al for six (ta ble 1).
In parallel , ide ntical examinatio ns wit h the sa me equi pme nt were made o n 10 age-m atch ed heal th y men lacking symptoms from their hands and witho ut occupa tio na l exp osure to vib ra tio n .

Exposure
All the exp o sed su bjects worked fo r 2-6 h / wor kday with high -frequency grinding equipment. They all were professionally a ctive at the time of the investigation but had not been exposed to vibration on the exam ination d a y, whe n te sting was made between 1300 and 1400.
During th e last 30 yea rs grindi ng equipment ha s beco me more efficient through incr ea ses in sp eed from IS 000 to 30 000 revolutions/min . A ll the expo sed subjects worked with h igh-speed gr ind ers; eigh t wer e specia lized in grind ing Vitall ium '" sk eleto n .
Measurem ent s on drills and work objects were m ade Vibration ex pos u re a ro se not only from th e handheld grinder s, but a lso from the handheld wo rkpiece (figure I). Th e weighted accelerat ion s (ISO 5349) were 2-4 m/s'. Above 1.2 kH z th e accelera tio ns ra nged fro m 3 to 500 mi s' (130-175 dB) . Comparison with a low-speed g rinde r is shown in figur e 2.

Neurophysiological methods
The senso ry nerve co nd uctio n velocity in th e medi an nerve wa s m ea su red in th e fo rearm, across th e carpal tu nne l, an d from the palm to th e middle fin ger with a n a ntidro mic technique ( IS , un published o bser vatio ns). Th e nerve response was reco rded with ring electrodes around th e proximal and distal interphalan geal joints (acti ve electro d e proximal) . The nerve was stimulated with surface elect rodes at the elb o w, 2 cm proxim al to the wri st crea se, and in the palm.
T he thres ho lds fo r warming a nd co o ling were dete rm ined sepa ra tely with Th errnot est" (So me dic In c) eq uipment with a base-lin e temperature o f th e P eltier clement of 30°C . The subject was in structed to pu sh a button a t th e ea rliest sensa tio n o f either wa rm th or col d. The cha nges in temper atu re we re plo tt ed , a nd the th resh old wa s de te rmi ned fro m th e wr itten curve of severa l trials. The fingert ips of d igits I I a nd V we re teste d bilater ally.
Vibr ation thresholds were d etermined for the sa m e fingertips with vib ra meter equipment (So medic Inc) ( 16). The pr o be vibrating a t 100 Hz was placed o nto the vo la r surface of ea ch fin ger , whic h was restin g o n a pa d filled with rice . The va lues ha ve been exp ressed as vibr at ion a m plit udes (urn) a nd a re ave rages o f thr esho lds to in cr ea sin g a nd decrea sin g amplitudes of vib ra tio n in rep eat ed tri al s. There was some association between the years of work as a dental technician and the vibration thresholds [r, 0.52 (NS) and r s 0.77 (P < 0.01) for right digits II and V, respectively; r s 0.55 (P<0.05) and r s 0.80 (P<O.OI) for left digits II and V, respectively]. However, age was significantly intercorrelated with the vibration thresholds and exposure time.
No significant correlation between vibrogram abnormality and years of exposure was found (r s 0.11-0.24). The timing of nerve impulse generation in the distal nerve terminals was determined by the difference in latency between the mechanical and electrical stimulation of tactile afferents from the fingertip of digit III (I7). The action potentials of the sensory nerve were recorded with surface electrodes over the median nerve at the wrist. Electrical stimulation (duration 0.2 ms) was applied with a wire electrode around the nail root (cathode) and around the fingertip 1 cm more distally (anode). Mechanical stimulation was applied with a tactile stimulator (Somedic Inc) delivering brief mechanical pulses (amplitude 800 urn, rise time 500 um/rns, duration IOms) to the nail of the same finger. A neurographic device (Medelec 92B) was triggered from the tactile stimulator, and 100 traces were averaged. The latency difference between the responses recorded with tactile and electrical stimulation (T-E difference) was calculated for each subject.
From the IO dental technicians, vibrograms were also made according to the method described by Lundborg et al (18). The differences between the exposed and unexposed groups were evaluated with Wilcoxon's test for paired data. Associations were tested with the Spearman Rank correlation test.

Results
The results of the neurographic tests are summarized in table 2, and the results of the perception tests for vibration and temperature are presented in table 3. There was no indication of any interference with nerve conduction at any segment of the median nerve in the exposed group. The conduction velocity across the carpal tunnel was even somewhat higher in the exposed group than in the referents.
In contrast with the conduction velocities, there were consistent differences between the exposed subjects and the referents in the perception of all three sensory modalities tested (table 3). The differences were slightly greater for the right hand than for the left, and for vibration and warming the changes were greater for digit II than for digit V.
The T-E differences showed an increase for the right hand (table 2). The difference did not, however, reach the level of formal statistical significance (P = 0.056). No difference was found for the left hand.
The distribution of values obtained for the four discriminating variables for digit II of the right hand are shown in figure 3. There was a large overlap of values between the exposed subjects and the referents, and the best separation can be seen for vibration, eight exposed subjects being outside the normal range, and for cooling, five subjects being outside the normal range.
The vibrogram examination showed marked abnormality (stage 3) in three subjects, slight abnormality (stage I) in three, and normal curves for four. There was a slight correlation between the vibration threshold and vibrogram abnormality for digit II [Spearman

Discussion
Our main results were the increases in vibration thresholds and the limits for warming and cooling and an increased difference in the digital nerve response to tactile and electric stimuli at the fingertip. Thus the lesion was not restricted to the group of receptors activated by vibration exposure. In addition thin afferent s mediating cold (A-delta) a nd warmth (C fiber s) were affected. The se catego ries have no specialized recepto r organs, and , since no change in digital nerve conduction velocity was detected , the nerve fiber terminal s are the probable ta rgets o f the lesion.
The dental technicians complained about clumsiness and loss of fine finger dexter ity. Loss of manipulative dexterit y has previously been reported among subjects expo sed to low-frequency vibra tion (19). Vibrationinduced neuropathy in the hand has been addressed by several authors, however usuall y with reference to the occurrence of symptoms similar to the carpal tunnel syndro me (9, 11). A decrease in nerve conduction 342 velocities has been demonstrated for the digital and median nerve of patients exposed to low-frequency vibration (12). The dental technicians examined in the present study had normal nerve conduction over the carpal tunnel. Abnormal temperature and vibration thre sholds are also reported to be caused by lowfrequen cy vibration (6,12). Involv ement of the digital nerves and the med ian nerve at th e carpa l tunnel level requires transmission of the vibration energ y through the tissues to this level. Low-frequency vibration generated by tools requi ring a full handgrip is absorbed and dampened in the whole hand-wrist system and might therefore even affect the ner ves in the carpal tunnel area. Howe ver, with the use of grinding instruments utilizing high-frequency vibration, the vibration is damped in the skin, and therefore exposed subjects might develop a lesion pattern diffe rent from that of th ose exposed to low-frequency vibration (12,20). Vibration at higher frequencies has been sho wn to transmit mor e energy to the hand than an eq ual level of lower frequencies weighted according to ISO 5349 (21,22).
Since dental technicians hold firmly onto their work objects with a forced pinch grip, vibration energy is readily transmitted to the skin. Furthermore skin tissue anoxia might add to the harmful effect of vibration. Resonance in frequencies below 1000 Hz could also be considered a possible mechanism for the injury.
Our results suggest that high-frequency vibration only induces a distal nerve and/or receptor injury. Low-frequency vibration induces combined distal and proximal nerve lesions in thick myelinated and thin unmyelinated fibers (23). Our own experimental studies showing ultrastructural changes and axonal disorganization primarily in small unmyelinated fibers of nerves in the hind legs of rats exposed to experimental vibration support this mechanism of nerve injury (24).
Vibration skin receptors are very sensitive to vibration in the frequency of 100 to 300 Hz, which is the resonance frequency of the skin (20). Thus vibration above this frequency might not give adequate warning information about noxious vibration exposure. Some dental technicians told that they raised the rotation speed when they experienced finger tingling. Thus they escaped the irritating effect of the vibration but were still exposed to its presumably harmful effects.
The differences in results regarding perception for vibration within one frequency only (100 Hz) versus a range of frequencies from 8 to 500 Hz (vibrogram test) may be explained by the uneven distribution of damage in various receptor or nerve fiber groups. Vibration-induced neuropathy may sometimes interfere at first with perception within the higher frequencies (18). If there is an involvement at 500 Hz but not at 100 Hz, the problem will not be detected by the onefrequency vibrometer technique.
Since the examinations were made after about 20 h after the last exposure to vibration, the sensory changes should not be acute changes only, but are presumably permanent effects of the exposure to vibration.
The material in this study is small, and there is an overlap between the vibration-exposed group and the referents. However, the results indicate that, while conduction velocity measurements are of limited help for verifying sensory dysfunction in subjects exposed to high-frequency vibration, the described psychophysical techniques described, combined with the calculation of the TE-difference time, might represent valuable new diagnostic tools in this respect. Larger groups must however be studied before one can select proper tests and define normal limits for screening subjects exposed to high-frequency vibration.
According to the 1986 norm of ISO/DIS 5349, vibration harmful for the hand-arm is weighted in frequencies from 6.3 to 1250 Hz. Since acceleration on the dentaldrills and small work objects is high in the frequencies above 1000 Hz and reduced tactile sensitivity has been reported for subjects exposed to highfrequency vibrat io n , a standard should be set for frequencies above 1000 Hz.