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The effect of two sites of high frequency vibration on cutaneous pain threshold
133
Pain, 25 (1986) 133-138 Elsevier
PAI 00871
The Effect of Two Sites of High Frequency Vibration on Cutaneous Pain Threshold L. Sherer *, Jo Ann Clelland ** Patricia O’Sullivan Daniel M. Doleys * and Betiy Canan ** Management Center, Brookwood Medical Center, Division ofAMl, Birmingham,
*** ,
Carolyn * Pain
AL 35209, ** Division of Physical Therapy, School of Community and Allied Health, University of Alabama at Birmingham, Birmingham, AL 35294, *** School of Nursing, University of Alabama at Birmingham, Birmingham, AL 35294 (U.S.A.) (Received
7 May 1985, revised received 20 September
1985, accepted
10 October
1985)
Summary The purpose of this study was to evaluate the effect of two sites of high frequency vibration on experimentally produced pain thresholds. Subjects were assigned to one of two experimental groups. Vibration was applied proximal to the site of pain threshold measurement in one group and distal to the measurement site in the other group. The cutaneous pain threshold was measured at the ulnar aspect of the wrist in both groups prior to, during, and following 5 rnin of vibration. Subjects were 30 right-handed, Caucasian males with a negative history of upper extremity dysfunction. A repeated measures analysis of variance (ANOVA) was used to analyze the data. There was a sig~ficant interaction between vibration site and time of pain threshold measurement. Post hoc analysis of that interaction indicated that a significant difference between experimental groups occurred only during vibration; the distal group values were significantly higher than the proximal group values (P < 0.03). For the distal group, pain threshold values were significantly higher during vibration than pre vibration and post vibration (P < 0.05). In the proximal group, there was no significant difference in pain threshold values across the 3 time periods. The results of this study indicate that vibration applied distal to the site of pain can provide temporary analgesia.
Address correspondence to: Carolyn L. Sherer, M.S.. Attn.: Pain Management Medical Center, 2022 Medical Center Drive, Birmingham, AL 35209, U.S.A.
0304”3959/86/$03.50
Q 1986 Elsevier Science Publishers
B.V. (Biomedical
Division)
Center,
Brookwood
134
Introduction
Peripheral conditioning techniques involve sensory modification and have evolved since the proposal of the gate theory of pain by Meizack and Wall in 1965 [4,7,12]. One form of peripheral conditioning which is effective for pain control is conventional transcutaneous electrical nerve stimulation (TENS) [8]. Vibration is another form of sensory stimulation that has been shown to relieve pain [lo]. Research indicates that application of vibration elevates the pain threshold in normal subjects [11,15,17] even though vibration decreases the amount of noxious stimulation required to elicit severe pain [l,ll]. Vibration results in decreased subjective clinical pain if that pain is animal to moderate in degree ]5,6,9,14] but is less effective for management of severe clinical pain 1141, The vibration frequency that has resulted in elevated skin pain thresholds has varied from 60 Hz to 240 Hz [11,15,17]. The amplitude of vibration that has been associated with elevated pain thresholds is often not clearly reported but seems to have ranged from 0.02 to 4.8 mm [15,17]. In previous studies, cutaneous pain threshold measurements were recorded during the vibratory stimulation with the duration of the stimulation required to effect a change in threshold not clearly stated, though significant changes in pain threshold appear to occur within 3 min [li,lS]. The sites of vibration have been proximal to pain [15], circumferential to the noxious stimulus [17], or directly over the pain or the site of pain threshold measurement [ll]. The efficacy of these stimulation sites has not been studied and, therefore, the optimal site for vibration application is unknown. The purpose of this study was to evaluate the effect of high frequency vibration on pain thresholds in normal adult males when the vibration was applied proximal and distal to the site of pain threshold measurement. Electrical stimulation has been used as the source of experimental pain in previous vibration studies [11,15] and has the following advantages: the stimulus is simple to use, to reproduce, and to express in physical terms, milliamperes (mA); electrical stimulation has the least chance of damaging tissue; and the ‘pricking pain’ sensation produced is easily defined and recognized [13]. Pain threshold was used to evaluate experimental pain in this study and was defined as the lowest electrical current which, at a fixed frequency and pulse width, first evoked a pricking pain sensation [2]. Method
Subjects The subjects were 30 right-handed Caucasian males, ages 18-39 years, with no history of pain or dysfunction of either upper extremity. Instrumentation The TECA CH3 Chronaxie Meter *, which produces a measurable low voltage * TECA Corp., White Plains, NY, U.S.A.
135
DC current, was used as the source of noxious stimulation. Characteristics of the stimulating current used were 5 msec duration square waves at 100 Hz delivered once per second. A 2 mm x 2 mm rubber electrode served as the stimulating electrode, and a 7.5 cm x 10 cm pad moistened with water served as the dispersive electrode. A Wahl hand-held vibrator was used ** . The frequency of vibration was 120 Hz, the amplitude 1.5 mm. Determination of cutaneous pain threshold Prior to beginning the experiment, the investigator trained the subjects to recognize the pin-prick sensation on the left upper extremity. The experiment was performed on the right upper extremity. The cutaneous pain threshold was determined by stimulating the skin over the pisiform bone. The stimulating electrode was taped to the ulnar aspect of the hand over the pisiform bone and the dispersive electrode was placed on the upper back. The intensity of the chronaxie meter was systematically increased by 0.5 mA increments, allowing 1 set to pass before the intensity was raised an additional increment. Subjects informed the investigator when they first experienced a distinct painful pin-prick sensation. Three threshold recordings were obtained and averaged to determine the mean cutaneous pain threshold for each subject. Procedure A table of random numbers was used to assign subjects to 1 of 2 groups: a distal group, which received vibration distal to the pisiform bone on the hypothenar eminence, or a proximal group, which received vibration distal to the head of the ulna on the anteromedial aspect of the forearm. The first set of measurements was taken before the subject received any vibratory stimulation. Each subject then received vibration within a 2.5 cm X 5 cm rectangular area immediately distal to one of the previously specified bony landmarks. The vibrator, placed in direct contact with the skin, was moved in a clockwise manner within the rectangle at a speed of 1 rev/set, and constant firm pressure was maintained throughout vibration. After 3 min of vibration, a second set of cutaneous pain threshold measurements was taken while vibration continued. The vibration was discontinued after a total of 5 min, which was approximately the same time that the second set of pain threshold measurements was completed. Two minutes after cessation of vibration a final set of pain threshold values was taken.
Results The pain threshold data are summarized in Table I. A repeated measures analysis of variance (ANOVA) with one between-group factor (site of vibration) and one repeated factor (time of pain threshold measurement) was used to analyze the data. The results are displayed in Table II. The assumptions for performing the repeated measures ANOVA analysis were met. ** Wahl Clipper Corp., Sterling, IL, U.S.A.
136 TABLE
I
THE MEAN VIBRATION
CUTANEOUS PAIN THRESHOLD (mA) PRIOR TO, DURING, AND FOLLOWING IN TWO EXPERIMENTAL GROUPS (PROXIMAL AND DISTAL) x
S.D.
Pre-vibration Proximal Distal
4.99 5.06
1.90 2.13
During vibration Proximal Distal
5.17 7.12
1.57 2.74
2 mm post vibration Proximal Distal
5.43 5.40
1.84 1.62
There was no significant difference in pain threshold between the two sites of vibration when averaged across the 3 time periods (see Fig. 1). There was a significant interaction between vibration site and time of pain threshold measurement. A simple main effects test was conducted on the difference between the two sites at each of the 3 times during which pain threshold measurements were taken. The results show a significant difference (t = 2.35. df 28, P -c 0.03) between experimental groups only during the second set of pain threshold measurements, i.e., during vibration. The pain thresholds in the distal group were significantly higher during vibration than were those of the proximal group. A single factor repeated measures ANOVA showed a significant change in pain threshold across time for the distal group (F = 7.77, df = 2, 28, P < 0.002). Post hoc analysis using the Ne~an-Keu~s procedure with repeated measures 1191 indicated that the pain threshold during vibration was significantly higher (P < 0.05) than either pre- or post-vibration values, but that there was no significant difference
TABLE
II
ANALYSIS
OF VARIANCE
SUMMARY Degrees of freedom
Sum of squares
MS
F
P
Between subjects Site Error
1 28
9.51 255.74
9.51 9.13
1.04
0.316
Within subjects Time Site X time Error
2 2 56
18.65 17.93 13.46
9.33 8.96 1.31
7.11 6.83
0.002 0.002
Source of variance
137
7.0 8.8 = 6.6 g 6.4 12 6.2 ;
’
t -
(0% ProxknatGroup - fl 8 ‘\ Distal Group- .
,
;;-
p 5.6 c 5.4 ‘ii D, 5.2 5.0 4.6 4 PWvibration
DWitlQ vibration
Postvibration
Fig. 1. Graphic display of the mean cutaneous pain threshold values prior to, during, and 2 min post vibration in proximal and distal experimental groups.
between the pre- and post-vibration pain threshold values. Visual assessment of the data indicated an increase in proximal group scores over the 3 time periods; however, a single factor repeated ANOVA indicated that this elevation was statistically non-sig~fic~t (F= 2.00, df = 2, 28, P c 0.15).
Discussion
Although the pain threshold increased during vibration in both groups, only the elevation in the distal group was statistically significant. A previous study reported an increase in pain thresholds when vibration was applied proximal to pain stimulation 1151.The opposite results occurred during the present study. One reason may be that the distance between the vibration and the site of pain threshold measurements differed in the proximal and distal groups. The subjects in the distal group were vibrated on the hypothenar eminence, adjacent to the pisiform bone (site of pain threshold measurements} and the proximal group was stimulated 20-30 cm from the pisiform bone near the ulnar groove. The findings of the present study differ from reports in the literature regarding the use of conventional high frequency TENS. Pertovaara [16] and Wheeler [18] compared the effect of TENS on pain thresholds when electrode placement was proximal and distal to the site of pain threshold measurement; both reported a significant of threshold only in the group receiving TENS proximally. The length of time of exposure of the experimental condition has varied among vibration studies. Some studies did not clearly report the duration of vibration required to effect a change f15,17], although two studies have indicated a change in threshold within 3 min [l,ll]. Similarly, in the present study, elevation of the distal group occurred after 3 mm of vibration. Two minutes after cessation of vibration there was no significant after-effect in this study.
138
Although one other study had suggested an after-effect 2 min post vibration [ll], our results were compatible with the findings of Pertovaara who reported no elevation of pain threshold immediately post vibration [15] and with Bini who found maximal post-vibratory hypoalgesia to be 60-70 set [l].
Acknowledgements We wish to thank Carol Rosenstiel
and Michele
Parr for technical
assistance.
References 1 Bini, G., Cruccu, G., Hagbarth, K.. Schady, W. and Torebjbrk, E., Analgesic effect of vibration and cooling on pain induced by intraneural electrical stimulation, Pain, 18 (1984) 239-248. 2 Bonica, J.J., The Management of Pain, Lea and Febiger, Philadelphia, PA, 1953, pp. 76-84. 3 Ekblom, A. and Hansson, P., Effects of conditioning vibratory stimulation on pain threshold of the human tooth, Acta physiol. stand., 114 (1982) 601-604. 4 Eriksson, M.B.E., Sjolund, B.H. and Nielzen, S., Long term results of peripheral conditioning stimulation as an analgesic measure in chronic pain, Pain, 6 (1979) 335-347. 5 Hansson, P. and Ekblom. A., Acute pain relieved by vibratory stimulation. Letter, Brit. dent. J., 151 (1981) 213. 6 Hansson, P. and Ekblom, A., Transcutaneous electrical nerve stimulation (TENS) as compared to placebo TENS for the relief of acute orofacial pain, Pain, 15 (1983) 157-165. 7 Lipton, S., Relief of Pain in Clinical Practice, Blackwell Scientific Publications, London. 1979, pp. 249-259. 8 Long, D.M. and Hagfors, N., Electrical stimulation in the nervous system: the current status of electrical stimulation of the nervous system for the relief of pain, Pain, 1 (1975) 109-123. 9 Lundeberg, T., Ottoson, D., Hakansson, S. and Meyerson, B.A., Vibratory stimulation for the control of intractable chronic orofacial pain. In: J.J. Bonica. U. Lindblom and A. Iggo (Eds.), Advances in Pain Research and Therapy, Vol. 5, Raven Press, New York, 1983, pp. 555-561. 10 Melzack, R., The Puzzle of Pain, Basic Books, New York, 1973, pp. 110-111. 11 Melzack, R. and Wall, P.D., Masking and metacontrast phenomena in the skin sensory system, Exp. Neurol., 8 (1963) 35-46. 12 Melzack, R. and Wall, P.D., Pain mechanisms: a new theory, Science, 150 (1965) 971-978. 13 Notermand, S.L.H., Measurement of the pain threshold determined by electrical stimulation and its clinical application. In: M. Weisenberg (Ed.), Pain: Clinical and Experimental Perspective, Mosby, St. Louis, MO, 1975, pp. 72-87. 14 Ottoson, D., Ekblom, A. and Hansson, P., Vibratory stimulation for the relief of pain of dental origin, Pain, 10 (1981) 37-45. 15 Pertovaara, A., Modification of human pain threshold by specific tactile receptors. Acta physiol. stand., 107 (1979) 339-341. 16 Pertovaara, A., Experimental pain and transcutaneous electrical nerve stimulation at high frequency. Appl. Neurophysiol., 43 (1980) 290-297. 17 Wall, P.D. and Cronly-Dillon, J.R., Pain, itch and vibration, Arch. Neurol. (Chic.), (1960) 365-375. 78 Wheeler, J.B., The effect of conventional TENS placement on cutaneous pain threshold in normal adult males, Unpublished thesis, University of Alabama in Birmingham, Birmingham, AL, 1983. 19 Winer, B.J., Statistical Principles in Experimental Design, McGraw-Hill, New York, 1971, pp. 528-529.
Pain, 25 (1986) 133-138 Elsevier
PAI 00871
The Effect of Two Sites of High Frequency Vibration on Cutaneous Pain Threshold L. Sherer *, Jo Ann Clelland ** Patricia O’Sullivan Daniel M. Doleys * and Betiy Canan ** Management Center, Brookwood Medical Center, Division ofAMl, Birmingham,
*** ,
Carolyn * Pain
AL 35209, ** Division of Physical Therapy, School of Community and Allied Health, University of Alabama at Birmingham, Birmingham, AL 35294, *** School of Nursing, University of Alabama at Birmingham, Birmingham, AL 35294 (U.S.A.) (Received
7 May 1985, revised received 20 September
1985, accepted
10 October
1985)
Summary The purpose of this study was to evaluate the effect of two sites of high frequency vibration on experimentally produced pain thresholds. Subjects were assigned to one of two experimental groups. Vibration was applied proximal to the site of pain threshold measurement in one group and distal to the measurement site in the other group. The cutaneous pain threshold was measured at the ulnar aspect of the wrist in both groups prior to, during, and following 5 rnin of vibration. Subjects were 30 right-handed, Caucasian males with a negative history of upper extremity dysfunction. A repeated measures analysis of variance (ANOVA) was used to analyze the data. There was a sig~ficant interaction between vibration site and time of pain threshold measurement. Post hoc analysis of that interaction indicated that a significant difference between experimental groups occurred only during vibration; the distal group values were significantly higher than the proximal group values (P < 0.03). For the distal group, pain threshold values were significantly higher during vibration than pre vibration and post vibration (P < 0.05). In the proximal group, there was no significant difference in pain threshold values across the 3 time periods. The results of this study indicate that vibration applied distal to the site of pain can provide temporary analgesia.
Address correspondence to: Carolyn L. Sherer, M.S.. Attn.: Pain Management Medical Center, 2022 Medical Center Drive, Birmingham, AL 35209, U.S.A.
0304”3959/86/$03.50
Q 1986 Elsevier Science Publishers
B.V. (Biomedical
Division)
Center,
Brookwood
134
Introduction
Peripheral conditioning techniques involve sensory modification and have evolved since the proposal of the gate theory of pain by Meizack and Wall in 1965 [4,7,12]. One form of peripheral conditioning which is effective for pain control is conventional transcutaneous electrical nerve stimulation (TENS) [8]. Vibration is another form of sensory stimulation that has been shown to relieve pain [lo]. Research indicates that application of vibration elevates the pain threshold in normal subjects [11,15,17] even though vibration decreases the amount of noxious stimulation required to elicit severe pain [l,ll]. Vibration results in decreased subjective clinical pain if that pain is animal to moderate in degree ]5,6,9,14] but is less effective for management of severe clinical pain 1141, The vibration frequency that has resulted in elevated skin pain thresholds has varied from 60 Hz to 240 Hz [11,15,17]. The amplitude of vibration that has been associated with elevated pain thresholds is often not clearly reported but seems to have ranged from 0.02 to 4.8 mm [15,17]. In previous studies, cutaneous pain threshold measurements were recorded during the vibratory stimulation with the duration of the stimulation required to effect a change in threshold not clearly stated, though significant changes in pain threshold appear to occur within 3 min [li,lS]. The sites of vibration have been proximal to pain [15], circumferential to the noxious stimulus [17], or directly over the pain or the site of pain threshold measurement [ll]. The efficacy of these stimulation sites has not been studied and, therefore, the optimal site for vibration application is unknown. The purpose of this study was to evaluate the effect of high frequency vibration on pain thresholds in normal adult males when the vibration was applied proximal and distal to the site of pain threshold measurement. Electrical stimulation has been used as the source of experimental pain in previous vibration studies [11,15] and has the following advantages: the stimulus is simple to use, to reproduce, and to express in physical terms, milliamperes (mA); electrical stimulation has the least chance of damaging tissue; and the ‘pricking pain’ sensation produced is easily defined and recognized [13]. Pain threshold was used to evaluate experimental pain in this study and was defined as the lowest electrical current which, at a fixed frequency and pulse width, first evoked a pricking pain sensation [2]. Method
Subjects The subjects were 30 right-handed Caucasian males, ages 18-39 years, with no history of pain or dysfunction of either upper extremity. Instrumentation The TECA CH3 Chronaxie Meter *, which produces a measurable low voltage * TECA Corp., White Plains, NY, U.S.A.
135
DC current, was used as the source of noxious stimulation. Characteristics of the stimulating current used were 5 msec duration square waves at 100 Hz delivered once per second. A 2 mm x 2 mm rubber electrode served as the stimulating electrode, and a 7.5 cm x 10 cm pad moistened with water served as the dispersive electrode. A Wahl hand-held vibrator was used ** . The frequency of vibration was 120 Hz, the amplitude 1.5 mm. Determination of cutaneous pain threshold Prior to beginning the experiment, the investigator trained the subjects to recognize the pin-prick sensation on the left upper extremity. The experiment was performed on the right upper extremity. The cutaneous pain threshold was determined by stimulating the skin over the pisiform bone. The stimulating electrode was taped to the ulnar aspect of the hand over the pisiform bone and the dispersive electrode was placed on the upper back. The intensity of the chronaxie meter was systematically increased by 0.5 mA increments, allowing 1 set to pass before the intensity was raised an additional increment. Subjects informed the investigator when they first experienced a distinct painful pin-prick sensation. Three threshold recordings were obtained and averaged to determine the mean cutaneous pain threshold for each subject. Procedure A table of random numbers was used to assign subjects to 1 of 2 groups: a distal group, which received vibration distal to the pisiform bone on the hypothenar eminence, or a proximal group, which received vibration distal to the head of the ulna on the anteromedial aspect of the forearm. The first set of measurements was taken before the subject received any vibratory stimulation. Each subject then received vibration within a 2.5 cm X 5 cm rectangular area immediately distal to one of the previously specified bony landmarks. The vibrator, placed in direct contact with the skin, was moved in a clockwise manner within the rectangle at a speed of 1 rev/set, and constant firm pressure was maintained throughout vibration. After 3 min of vibration, a second set of cutaneous pain threshold measurements was taken while vibration continued. The vibration was discontinued after a total of 5 min, which was approximately the same time that the second set of pain threshold measurements was completed. Two minutes after cessation of vibration a final set of pain threshold values was taken.
Results The pain threshold data are summarized in Table I. A repeated measures analysis of variance (ANOVA) with one between-group factor (site of vibration) and one repeated factor (time of pain threshold measurement) was used to analyze the data. The results are displayed in Table II. The assumptions for performing the repeated measures ANOVA analysis were met. ** Wahl Clipper Corp., Sterling, IL, U.S.A.
136 TABLE
I
THE MEAN VIBRATION
CUTANEOUS PAIN THRESHOLD (mA) PRIOR TO, DURING, AND FOLLOWING IN TWO EXPERIMENTAL GROUPS (PROXIMAL AND DISTAL) x
S.D.
Pre-vibration Proximal Distal
4.99 5.06
1.90 2.13
During vibration Proximal Distal
5.17 7.12
1.57 2.74
2 mm post vibration Proximal Distal
5.43 5.40
1.84 1.62
There was no significant difference in pain threshold between the two sites of vibration when averaged across the 3 time periods (see Fig. 1). There was a significant interaction between vibration site and time of pain threshold measurement. A simple main effects test was conducted on the difference between the two sites at each of the 3 times during which pain threshold measurements were taken. The results show a significant difference (t = 2.35. df 28, P -c 0.03) between experimental groups only during the second set of pain threshold measurements, i.e., during vibration. The pain thresholds in the distal group were significantly higher during vibration than were those of the proximal group. A single factor repeated measures ANOVA showed a significant change in pain threshold across time for the distal group (F = 7.77, df = 2, 28, P < 0.002). Post hoc analysis using the Ne~an-Keu~s procedure with repeated measures 1191 indicated that the pain threshold during vibration was significantly higher (P < 0.05) than either pre- or post-vibration values, but that there was no significant difference
TABLE
II
ANALYSIS
OF VARIANCE
SUMMARY Degrees of freedom
Sum of squares
MS
F
P
Between subjects Site Error
1 28
9.51 255.74
9.51 9.13
1.04
0.316
Within subjects Time Site X time Error
2 2 56
18.65 17.93 13.46
9.33 8.96 1.31
7.11 6.83
0.002 0.002
Source of variance
137
7.0 8.8 = 6.6 g 6.4 12 6.2 ;
’
t -
(0% ProxknatGroup - fl 8 ‘\ Distal Group- .
,
;;-
p 5.6 c 5.4 ‘ii D, 5.2 5.0 4.6 4 PWvibration
DWitlQ vibration
Postvibration
Fig. 1. Graphic display of the mean cutaneous pain threshold values prior to, during, and 2 min post vibration in proximal and distal experimental groups.
between the pre- and post-vibration pain threshold values. Visual assessment of the data indicated an increase in proximal group scores over the 3 time periods; however, a single factor repeated ANOVA indicated that this elevation was statistically non-sig~fic~t (F= 2.00, df = 2, 28, P c 0.15).
Discussion
Although the pain threshold increased during vibration in both groups, only the elevation in the distal group was statistically significant. A previous study reported an increase in pain thresholds when vibration was applied proximal to pain stimulation 1151.The opposite results occurred during the present study. One reason may be that the distance between the vibration and the site of pain threshold measurements differed in the proximal and distal groups. The subjects in the distal group were vibrated on the hypothenar eminence, adjacent to the pisiform bone (site of pain threshold measurements} and the proximal group was stimulated 20-30 cm from the pisiform bone near the ulnar groove. The findings of the present study differ from reports in the literature regarding the use of conventional high frequency TENS. Pertovaara [16] and Wheeler [18] compared the effect of TENS on pain thresholds when electrode placement was proximal and distal to the site of pain threshold measurement; both reported a significant of threshold only in the group receiving TENS proximally. The length of time of exposure of the experimental condition has varied among vibration studies. Some studies did not clearly report the duration of vibration required to effect a change f15,17], although two studies have indicated a change in threshold within 3 min [l,ll]. Similarly, in the present study, elevation of the distal group occurred after 3 mm of vibration. Two minutes after cessation of vibration there was no significant after-effect in this study.
138
Although one other study had suggested an after-effect 2 min post vibration [ll], our results were compatible with the findings of Pertovaara who reported no elevation of pain threshold immediately post vibration [15] and with Bini who found maximal post-vibratory hypoalgesia to be 60-70 set [l].
Acknowledgements We wish to thank Carol Rosenstiel
and Michele
Parr for technical
assistance.
References 1 Bini, G., Cruccu, G., Hagbarth, K.. Schady, W. and Torebjbrk, E., Analgesic effect of vibration and cooling on pain induced by intraneural electrical stimulation, Pain, 18 (1984) 239-248. 2 Bonica, J.J., The Management of Pain, Lea and Febiger, Philadelphia, PA, 1953, pp. 76-84. 3 Ekblom, A. and Hansson, P., Effects of conditioning vibratory stimulation on pain threshold of the human tooth, Acta physiol. stand., 114 (1982) 601-604. 4 Eriksson, M.B.E., Sjolund, B.H. and Nielzen, S., Long term results of peripheral conditioning stimulation as an analgesic measure in chronic pain, Pain, 6 (1979) 335-347. 5 Hansson, P. and Ekblom. A., Acute pain relieved by vibratory stimulation. Letter, Brit. dent. J., 151 (1981) 213. 6 Hansson, P. and Ekblom, A., Transcutaneous electrical nerve stimulation (TENS) as compared to placebo TENS for the relief of acute orofacial pain, Pain, 15 (1983) 157-165. 7 Lipton, S., Relief of Pain in Clinical Practice, Blackwell Scientific Publications, London. 1979, pp. 249-259. 8 Long, D.M. and Hagfors, N., Electrical stimulation in the nervous system: the current status of electrical stimulation of the nervous system for the relief of pain, Pain, 1 (1975) 109-123. 9 Lundeberg, T., Ottoson, D., Hakansson, S. and Meyerson, B.A., Vibratory stimulation for the control of intractable chronic orofacial pain. In: J.J. Bonica. U. Lindblom and A. Iggo (Eds.), Advances in Pain Research and Therapy, Vol. 5, Raven Press, New York, 1983, pp. 555-561. 10 Melzack, R., The Puzzle of Pain, Basic Books, New York, 1973, pp. 110-111. 11 Melzack, R. and Wall, P.D., Masking and metacontrast phenomena in the skin sensory system, Exp. Neurol., 8 (1963) 35-46. 12 Melzack, R. and Wall, P.D., Pain mechanisms: a new theory, Science, 150 (1965) 971-978. 13 Notermand, S.L.H., Measurement of the pain threshold determined by electrical stimulation and its clinical application. In: M. Weisenberg (Ed.), Pain: Clinical and Experimental Perspective, Mosby, St. Louis, MO, 1975, pp. 72-87. 14 Ottoson, D., Ekblom, A. and Hansson, P., Vibratory stimulation for the relief of pain of dental origin, Pain, 10 (1981) 37-45. 15 Pertovaara, A., Modification of human pain threshold by specific tactile receptors. Acta physiol. stand., 107 (1979) 339-341. 16 Pertovaara, A., Experimental pain and transcutaneous electrical nerve stimulation at high frequency. Appl. Neurophysiol., 43 (1980) 290-297. 17 Wall, P.D. and Cronly-Dillon, J.R., Pain, itch and vibration, Arch. Neurol. (Chic.), (1960) 365-375. 78 Wheeler, J.B., The effect of conventional TENS placement on cutaneous pain threshold in normal adult males, Unpublished thesis, University of Alabama in Birmingham, Birmingham, AL, 1983. 19 Winer, B.J., Statistical Principles in Experimental Design, McGraw-Hill, New York, 1971, pp. 528-529.