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Vibrometry of industrial workers: A case study
International Journal of lndustrial Ergonomics, 11 (1993) 341-345
341
Elsevier Case study
Vibrometry of industrial workers" A case study Regina Neese and Stephan
Konz
Department of Industrial Engineering, Kansas State University, Manhattan, KS 66506, USA
(Received October 27, 1992; accepted in revised form March 11, 1993)
Abstract Vibrometry was performed on 165 factory workers and supervisors in 18 different job classifications. A low vibrometry score indicates insensitivity to vibrations and thus potential compression neuropathy. Males had significantly higher scores than females. Subjective estimates of the job's repetitiveness, force requirement and wrist angle movement did not correlate significantly with the vibrometry score but pace did (self-paced jobs had higher scores). Questions relating to pain in the hands and numbness in the hands were significant predictors of the score. The conventional vibrometry score is the total from seven frequencies. Analysis shows only the top frequencies (125, 250, 500 Hz) are needed and possibly just 250 and 500 Hz.
Relevance to industry Vibrometry is a significant advance in the diagnosis of carpal tunnel syndrome. The case study indicates it may be possible to simplify the vibrometry test (thus reducing its cost). In addition, self-pace work may have less potential for cumulative trauma than machine-paced work.
Keywords Vibrometry; Carpal tunnel syndrome; tests for CTS.
Introduction V i b r o m e t r y is a r e c e n t l y d e v e l o p e d t e c h n i q u e to e v a l u a t e p e o p l e for c o m p r e s s i o n n e u r o p a t h y o f the m e d i a n nerve - i.e., c a r p a l t u n n e l s y n d r o m e (CTS). T h e first study was in S c a n d i n a v i a in 1986 ( L u n d b o r g et al.). In t h e U S A , J e t z e r et al. (1987) d e v e l o p e d t h e J e t z e r i n d e x to score c a r p a l t u n n e l s y n d r o m e . A d d i t i o n a l w o r k is by J e t z e r (1989) a n d G r u n e r t et al. (1990). T h e basic c o n c e p t is to p l a c e t h e m i d d l e finger on a v i b r a t i n g stylus. Physiologically the l a r g e m y e l i n a t e d g r o u p A b e t a fibers t h a t m e d i a t e touch a n d v i b r a t o r y p e r c e p t i o n a r e m o r e sensitive to c o m p r e s s i o n t h a n a r e o t h e r fibers such as the
small g r o u p C fibers t h a t m e d i a t e pain. A series of d i f f e r e n t f r e q u e n c i e s is p r e s e n t e d (8, 16, 31.5, 63, 125, 250 a n d 500 Hz). T h e p e r s o n indicates by p u s h i n g a b u t t o n w h e n t h e v i b r a t i o n can a n d can not b e d e t e c t e d . T h e p r o c e d u r e ( a n d d i s p l a y e d o u t p u t ) is very similar to a u d i o m e t r i c testing o f sound. A p e r s o n having CTS will b e less sensitive to v i b r a t i o n - especially in the 125, 250 a n d 500 H z bands. In this study, v i b r o g r a m s w e r e o b t a i n e d for 165 i n d u s t r i a l e m p l o y e e s o f o n e firm. T h e goal was to see if e m p l o y e e s in r e p e t i t i v e j o b s d i f f e r e d from t h o s e in n o n - r e p e t i t i v e jobs.
Method Correspondence to: S. Konz, Department of Industrial Engi-
neering, Kansas State University, Manhattan, KS 66506, USA.
T h e e m p l o y e e s w e r e 165 factory w o r k e r s a n d supervisors from six d i f f e r e n t d e p a r t m e n t s ; they
0169-8141/93/$06.00 © 1993 - Elsevier Science Publishers B,V. All rights reserved
342
R. Neese, S. Konz / Vibrometry case study
were in 18 d i f f e r e n t j o b classifications. T h e age r a n g e was 19 to 63; 122 w e r e f e m a l e a n d 43 male; 9 were left h a n d e d a n d 156 right h a n d e d . T h e e m p l o y e e s c o m p l e t e d an i n f o r m e d consent form, a m e d i c a l history q u e s t i o n n a i r e , a description of s y m p t o m s a n d m o t i o n s p e r f o r m e d d u r i n g the j o b a n d t h e n v i b r o m e t r y scores w e r e d e t e r m i n e d for each h a n d . A Bruel a n d K j a e r v i b r o m e t e r ( m o d e l 9627) was used. T h e v i b r a t i o n stylus unit was p l a c e d on p a d s of v i b r a t i o n d a m p e n i n g m a t e r i a l (to minimize v i b r a t i o n ) in a w o o d e n box. A 25 m m hole was cut in the box top to expose the stylus. P a d d i n g was p l a c e d on top of the box for the forearm. T h e test t o o k p l a c e in a q u i e t r o o m in t h e m e d i c a l d e p a r t m e n t ; subjects w o r e e a r p h o n e s to m i n i m i z e e n v i r o n m e n t a l noise. T h e y p l a c e d t h e i r a r m in the n e u t r a l p o s i t i o n on the box top with the m i d d l e finger on the stylus. T h e y w e r e told not to w a t c h the h a n d a n d " a s soon as you b e g i n to feel the stylus vibrate, p u s h the b u t t o n on t h e h a n d c o n t r o l a n d do not r e l e a s e it until you no l o n g e r feel v i b r a t i o n in y o u r finger". ( T h e result is a s a w - t o o t h p a t t e r n . ) A series of v i b r a t i o n s was i n d u c e d at 8, 16, 31.5, 63, 125, 250 a n d 500 Hz. T h e test always b e g a n with the left h a n d a n d the s e q u e n c e always was from low to high frequency; they w e r e not given p r a c t i c e sessions. T h e tests were a d m i n i s t e r e d by two " l i g h t d u t y " e m p l o y e e s of the firm. T h e e x p e r i m e n t e r t r a i n e d t h e m a n d c h e c k e d t h e i r testing p r o c e d u r e daily. T h e e x p e r i m e n t e r also assessed e a c h j o b for d e g r e e of r e p e t i t i o n , force a n d wrist deviation, using a subjective scale (0 = none, 5 = average, a n d 10 = high). T h e j o b s also w e r e classified as e i t h e r " s e l f - p a c e d " or " m a c h i n e p a c e d " , d e p e n d ing on w h e t h e r the o p e r a t o r or t h e m a c h i n e cont r o l l e d t h e initiation of e a c h cycle. T h e v i b r o g r a m ' s J e t z e r index was d e t e r m i n e d by Dr. J e t z e r ( O c c u p a t i o n a l M e d i c i n e Consultants, M i n n e a p o l i s , MN). F o r e a c h frequency, the m e a n r e s p o n s e dB was d e t e r m i n e d by averaging the high a n d low value of the " s a w t o o t h p a t t e r n " . F o r 8 H z the m e a n might b e 105 dB. T h e n the d i f f e r e n c e b e t w e e n t h e m e a n a n d 160 dB (i.e., no r e s p o n s e to a very p o w e r f u l v i b r a t i o n ) was d e t e r m i n e d . This d i f f e r e n c e (55 dB) was d i v i d e d by 10 to get a score of 5.5 for that frequency. Scoring
was to the n e a r e s t tenth. T h e n the scores for all seven f r e q u e n c i e s w e r e t o t a l e d - giving, for exa m p l e , a total score ( J e t z e r index) o f 30. A score b e l o w 20 i n d i c a t e s insensitivity to v i b r a t i o n ( a n d thus p o t e n t i a l c o m p r e s s i o n n e u r o p a t h y ) a n d a score above 30 i n d i c a t e s sensitivity to v i b r a t i o n ( a n d thus " n o r m a l i t y " ) .
Results M e a n scores w e r e c a l c u l a t e d for e a c h hand; the right h a n d m e a n was 35.2 a n d left h a n d m e a n was 35.7. T h e right a n d left h a n d scores w e r e significantly c o r r e l a t e d ( r 2 = 56%). T a b l e 1 shows that males h a d significantly h i g h e r scores t h a n f e m a l e s - especially in the h i g h e r frequencies; t h a t is, f e m a l e s w e r e less sensitive to vibration. Since a low score is b a d , this is
Table 1 Mean scores for males and females in individual frequencies, and critical frequencies Hand
Frequency
Right
8.0 16.0 31.5 63.0 125.0 250.0 500.0
5.8 5.5 5.2 5.7 5.9 5.6 4.1
5.9 5.3 4.8 5.1 5.4 4.9 3.5
* * *
total
37.1
34.6
*
critical3 critical2
15.6 9.6
13.8 8.4
* *
8.0 16.0 31.5 63.0 125.0 250.0 500.0
5.9 5.2 4.9 5.7 5.9 5.6 4.1
5.8 5.2 4.7 5.1 5.4 4.9 3.5
* * * *
total
37.7
35.0
critical3 critical2
15.5 9.7
13.7 8.3
Left
Males
Females
* *
* indicates differences between males and females significant at the p < 0.05 level. total = (8 + 16 + 31.5 + 63 + 125 + 250 + 500) Hz critical2 = (250 + 500) Hz critical3 = (125 + 250 + 500) Hz
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R. Neese, S. Konz / Fibrometry case study
comparable with the common experience of higher carpal tunnel syndrome rates among females. Since there seemed to be a frequency effect, two additional scores were calculated. Critical2 is the mean of the 250 and 500 Hz score and Critical3 is the mean of the 125, 250 and 500 Hz score. The significant gender differences for Critical2 and Critical3 as well as for total show that it may be possible to test just the upper 2 or 3 bands instead of all 7. Four questions from the questionnaire were analyzed since they suggested possible compression neuropathy. They were: - Do you now have numbness or tingling in your hands? - Do you now or have you in the past year had pain/weakness in your hands? - Do you now have or have you in the past year had pain or numbness in your hands at night? - Have you ever been diagnosed or treated for carpal tunnel syndrome? They could circle N (scored zero), yes for left or right hand (scored 1) or yes for both hands (scored
2). Table 2 shows the r 2 value for linear regression between the various variables and the vibrometry score of the preferred hand for 7 frequencies (SCROEP), for top 3 frequencies (CRIT3P) and for top 2 frequencies (CRIT2P).
Table 2 The linear coefficient of determination (r 2) is given for various variables for SCOREP, CRIT3P, and CRIT2P. Values above 0.026 are statistically significant (p < 0.05) Variables
SCOREP
CRIT3P CRIT2P
0.137 0.014 0.006 0.001
0.136 0.021 0.002 0.003
0.144 0.034 0.000 0.005
0.054 0.050 0.041 0.000
0.069 0.053 0.047 0.000
0.065 0.049 0.040 0.000
0.039 0.012
0.055 0.021
0.063 0.022
Task
Pace Force Angle Repetitions Experience questions
Numbness Pain Pain at night Carpal tunnel synd. Person
Gender Age
Table 3 Multiple correlation coefficient (r 2) for SCOREP, CRIT3P and CRIT2P Criterion
Variables
Cumulative r 2
SCOREP
Pace Pain Pain sq Age
13.7% 17.0 21.7 23.5
CRIT3P
Pace Pain Pain sq Age
13.6 17.2 21.4 24.3
CRIT2P
Pace Pain Numbness Numbness sq
14.4 17.8 20.9 25.1
The best single predictor of the vibrometry score was Pace (either self or machine; self paced is better) with 13.7% of the variance explained. Next were the numbness question (r 2 = 5.4%), pain (r 2 = 5.0%) and pain at night r 2 = 4.1%). Gender only had r 2 = 3.9%. The regressions also were run for the criterion C R I T 3 P and CRIT2P. In general, r 2 values were higher for CRIT3P and C R I T 2 P than for SCORE. This implies that they are more sensitive than using the vibrometry score for all 7 frequencies. If the lower frequencies can be omitted, the cost of vibrometry testing would be reduced. Table 2 indicates that, of the task variables, only pace is important. The experience questions are good predictors with numbness the best, then pain, then pain at night. Asking if a person has carpal tunnel syndrome was not worthwhile. Females had lower vibration sensitivity scores; that is, they were less able to detect vibration than males. Next the S T E P W l S E multiple regression program was used to see how well vibrometry scores could be predicted from combinations of variables. New variables were created by squaring the variables of pain, numbness, numbness at night and age to see if the effects are linear. Table 3 shows, for SCOREP, that pace enters first, and then pain, pain squared and then age. Using all four variables the r z = 23.5%. For CRIT3P, the variables are pace, pain, and pain squared and age with r 2 = 2 4 . 3 % . For
R. Neese, S. Konz / Vibrometry case study
344
Table 4 Jobs, number of people tested in each job, pace (self or machine), subjective score for repetitions, force and angle, vibrogram mean and critical3 scores and their grouping (p < 0.05). The rows are ordered on the mean vibrometry score Job title
N
Subjective score Reps
Force
Angle
CBF operator Scrapper Press operator Management
7 19 7 10
10 10 4 0
8 10 8 0
Batch operator Handwork P. oper. Packager
20 8 6 30
8 9 8 7
Overwrap. co. P. packer Stock hand. B. operator
10 6 5 19
8 5 2 9
Pace
Vibrometry score Total
Group
Crit3
Group
5 10 8 0
S S S S
40.0 38.2 38.2 36.9
A AB AB ABC
17.1 16.2 16.0 14.7
A AB AB ABC
8 2 8 4
5 7 10 5
S S S M
36.5 35.4 35.4 35.1
ABC ABC ABC ABC
14.6 14.1 14.3 13.8
ABC ABC ABC ABC
5 3 9 4
5 5 6 8
M M S M
33.2 32.8 31.5 30.3
BC BC BC C
12.9 13.6 12.4 11.1
ABC ABC BC C
CRIT2P, the variables are pace, pain, numbness and numbness squared with r 2 25.10~. Gender does not appear as one of the top four predictors for any of the three vibrometry scores. Upon examination, gender was significantly correlated with the experience scores so when the experience scores entered the model, gender added little new information. Next, vibrometry scores were analyzed for a relation to the 18 job classifications. The results were not significant. However, upon closer examination some jobs had a very small number of people in them. In addition, the previous analysis indicated males had higher scores than females. Therefore the analysis was rerun omitting the six jobs with 3 or less people in them. In addition, =
gender and age were added as covariates. Vibrogram scores were related to these 12 jobs; see Table 4. Table 4 gives the 12 jobs, some job characteristics, and the total and Critical3 scores for the right hand.
Discussion Vibrometry is able to distinguish among people with repetitive trauma problems. However it seems that it is not necessary to analyze all seven frequencies as most of the information is contained in the responses to frequencies of 125, 250 and 500 Hz and perhaps just the frequencies of 250 and 500 Hz.
Table 5 Ergonomic priorities survey. Scale (score) each job, then prioritize jobs. Investigate highest score job first Criterion
Score 0 ....
5 ....
10
No. of repetitions
Non repetitive
250/day
500/day
Amount of force
No force applied
~< 3 kg
>~ 7 kg
Pinch grip required to perform job? Wrist flexion/extension Manual percent of worker cycle Self vs. machine pace Hand vibration
No No flexion or extension 0% Self pace None
yes-500/day 45 ° deviation 75% 50/50 self/machine 1 h/day
1000/day /> 90 ° 100% Machine pace over 2 h / d a y
Gloves
No
Yes
R. Neese, S. Konz / ~brometry case study
T h e "job experience" questions gave better estimates of the vibrometry score. Pain was the best question. T h a t is, if a person said they had pain in their hands, their vibrogram score was lower. T h e significant pain squared term (which had a plus sign vs. the negative sign for pain) indicates that the vibrometry vs. pain scores flattens out as pain increases rather than rising m o r e steeply. N u m b n e s s and n u m b n e s s squared also had the same curve shape with the curve flattening with higher n u m b n e s s values. A g e also affected vibrometry score with lower scores for older workers. A l t h o u g h vibrometry can show which individuals have CTS symptoms, engineers and m a n a g e r s would like to be able to predict which job factors cause the problem. W h e n trying to predict the response to vibrometry, most task variables, with the exception of pace, have low correlations. People with machine-paced jobs had lower vibrometry scores than people with self-paced jobs. Subjective estimates by the experimenter of force, angle, and the repetitions had low correlations with the vibrometry scores. T h u s job characteristics had some ability to differentiate the scores but there was considerable overlap. In addition the values for reps, force, angle and pace are c o n f o u n d e d . Thus we do not know how important each score is in predicting the vibrometry score. Part of the problem is that the job characteristics used in our survey were not very precise. Table 5 is an a t t e m p t to define these job characteristics m o r e precisely. T h e revised form has
345
words a d d e d to give the analyst a better estimate of various scale values. For example, for the criterion of repetitions, a value of 2 5 0 / d a y would be given a value of 5. In addition, criteria were a d d e d for pinch grips, manual percent of the cycle, self vs. machine pace, vibration, and use of gloves.
Acknowledgements T h e authors thank Dr. T. Jetzer for providing a vibrometer and analysis of the vibrometry scores and Zenith D a t a Suppliers for use of a portable computer.
References Grunert, B., Wertsch, J., Matloub, H. and McCallum-Burke, S., 1990. Reliability of sensory threshold measurement using a digital vibrogram. J. of Occupational Medicine, 32(2): 100-102. Jetzer, T., 1989. The use of vibration testing in the early evaluation of workers with carpal tunnel syndrome. In: Proceedings of the Fifth International Conference on Hand-Arm Vibration, Kanazawa, Japan, 1989. Jetzer, T., Conrad, J. and Heithoff, K., 1987. The role of scanning and vibrometry testing in the diagnostic evaluation of carpal tunnel syndrome. Proceedings of the VolvoIFSSH Conference on the Prevention of Brachial Injuries and Cumulative Trauma Disorders, Stockholm, 1987. Lundborg, G., Lie-Stenstrom, A., Sollerman, C., Stromberg, T. and Pykko, I., 1986. Digital vibrogram: A new diagnostic tool for sensory testing in compression neuropathy. The J. of Hand Surgery, llA, 5: 693-699.
341
Elsevier Case study
Vibrometry of industrial workers" A case study Regina Neese and Stephan
Konz
Department of Industrial Engineering, Kansas State University, Manhattan, KS 66506, USA
(Received October 27, 1992; accepted in revised form March 11, 1993)
Abstract Vibrometry was performed on 165 factory workers and supervisors in 18 different job classifications. A low vibrometry score indicates insensitivity to vibrations and thus potential compression neuropathy. Males had significantly higher scores than females. Subjective estimates of the job's repetitiveness, force requirement and wrist angle movement did not correlate significantly with the vibrometry score but pace did (self-paced jobs had higher scores). Questions relating to pain in the hands and numbness in the hands were significant predictors of the score. The conventional vibrometry score is the total from seven frequencies. Analysis shows only the top frequencies (125, 250, 500 Hz) are needed and possibly just 250 and 500 Hz.
Relevance to industry Vibrometry is a significant advance in the diagnosis of carpal tunnel syndrome. The case study indicates it may be possible to simplify the vibrometry test (thus reducing its cost). In addition, self-pace work may have less potential for cumulative trauma than machine-paced work.
Keywords Vibrometry; Carpal tunnel syndrome; tests for CTS.
Introduction V i b r o m e t r y is a r e c e n t l y d e v e l o p e d t e c h n i q u e to e v a l u a t e p e o p l e for c o m p r e s s i o n n e u r o p a t h y o f the m e d i a n nerve - i.e., c a r p a l t u n n e l s y n d r o m e (CTS). T h e first study was in S c a n d i n a v i a in 1986 ( L u n d b o r g et al.). In t h e U S A , J e t z e r et al. (1987) d e v e l o p e d t h e J e t z e r i n d e x to score c a r p a l t u n n e l s y n d r o m e . A d d i t i o n a l w o r k is by J e t z e r (1989) a n d G r u n e r t et al. (1990). T h e basic c o n c e p t is to p l a c e t h e m i d d l e finger on a v i b r a t i n g stylus. Physiologically the l a r g e m y e l i n a t e d g r o u p A b e t a fibers t h a t m e d i a t e touch a n d v i b r a t o r y p e r c e p t i o n a r e m o r e sensitive to c o m p r e s s i o n t h a n a r e o t h e r fibers such as the
small g r o u p C fibers t h a t m e d i a t e pain. A series of d i f f e r e n t f r e q u e n c i e s is p r e s e n t e d (8, 16, 31.5, 63, 125, 250 a n d 500 Hz). T h e p e r s o n indicates by p u s h i n g a b u t t o n w h e n t h e v i b r a t i o n can a n d can not b e d e t e c t e d . T h e p r o c e d u r e ( a n d d i s p l a y e d o u t p u t ) is very similar to a u d i o m e t r i c testing o f sound. A p e r s o n having CTS will b e less sensitive to v i b r a t i o n - especially in the 125, 250 a n d 500 H z bands. In this study, v i b r o g r a m s w e r e o b t a i n e d for 165 i n d u s t r i a l e m p l o y e e s o f o n e firm. T h e goal was to see if e m p l o y e e s in r e p e t i t i v e j o b s d i f f e r e d from t h o s e in n o n - r e p e t i t i v e jobs.
Method Correspondence to: S. Konz, Department of Industrial Engi-
neering, Kansas State University, Manhattan, KS 66506, USA.
T h e e m p l o y e e s w e r e 165 factory w o r k e r s a n d supervisors from six d i f f e r e n t d e p a r t m e n t s ; they
0169-8141/93/$06.00 © 1993 - Elsevier Science Publishers B,V. All rights reserved
342
R. Neese, S. Konz / Vibrometry case study
were in 18 d i f f e r e n t j o b classifications. T h e age r a n g e was 19 to 63; 122 w e r e f e m a l e a n d 43 male; 9 were left h a n d e d a n d 156 right h a n d e d . T h e e m p l o y e e s c o m p l e t e d an i n f o r m e d consent form, a m e d i c a l history q u e s t i o n n a i r e , a description of s y m p t o m s a n d m o t i o n s p e r f o r m e d d u r i n g the j o b a n d t h e n v i b r o m e t r y scores w e r e d e t e r m i n e d for each h a n d . A Bruel a n d K j a e r v i b r o m e t e r ( m o d e l 9627) was used. T h e v i b r a t i o n stylus unit was p l a c e d on p a d s of v i b r a t i o n d a m p e n i n g m a t e r i a l (to minimize v i b r a t i o n ) in a w o o d e n box. A 25 m m hole was cut in the box top to expose the stylus. P a d d i n g was p l a c e d on top of the box for the forearm. T h e test t o o k p l a c e in a q u i e t r o o m in t h e m e d i c a l d e p a r t m e n t ; subjects w o r e e a r p h o n e s to m i n i m i z e e n v i r o n m e n t a l noise. T h e y p l a c e d t h e i r a r m in the n e u t r a l p o s i t i o n on the box top with the m i d d l e finger on the stylus. T h e y w e r e told not to w a t c h the h a n d a n d " a s soon as you b e g i n to feel the stylus vibrate, p u s h the b u t t o n on t h e h a n d c o n t r o l a n d do not r e l e a s e it until you no l o n g e r feel v i b r a t i o n in y o u r finger". ( T h e result is a s a w - t o o t h p a t t e r n . ) A series of v i b r a t i o n s was i n d u c e d at 8, 16, 31.5, 63, 125, 250 a n d 500 Hz. T h e test always b e g a n with the left h a n d a n d the s e q u e n c e always was from low to high frequency; they w e r e not given p r a c t i c e sessions. T h e tests were a d m i n i s t e r e d by two " l i g h t d u t y " e m p l o y e e s of the firm. T h e e x p e r i m e n t e r t r a i n e d t h e m a n d c h e c k e d t h e i r testing p r o c e d u r e daily. T h e e x p e r i m e n t e r also assessed e a c h j o b for d e g r e e of r e p e t i t i o n , force a n d wrist deviation, using a subjective scale (0 = none, 5 = average, a n d 10 = high). T h e j o b s also w e r e classified as e i t h e r " s e l f - p a c e d " or " m a c h i n e p a c e d " , d e p e n d ing on w h e t h e r the o p e r a t o r or t h e m a c h i n e cont r o l l e d t h e initiation of e a c h cycle. T h e v i b r o g r a m ' s J e t z e r index was d e t e r m i n e d by Dr. J e t z e r ( O c c u p a t i o n a l M e d i c i n e Consultants, M i n n e a p o l i s , MN). F o r e a c h frequency, the m e a n r e s p o n s e dB was d e t e r m i n e d by averaging the high a n d low value of the " s a w t o o t h p a t t e r n " . F o r 8 H z the m e a n might b e 105 dB. T h e n the d i f f e r e n c e b e t w e e n t h e m e a n a n d 160 dB (i.e., no r e s p o n s e to a very p o w e r f u l v i b r a t i o n ) was d e t e r m i n e d . This d i f f e r e n c e (55 dB) was d i v i d e d by 10 to get a score of 5.5 for that frequency. Scoring
was to the n e a r e s t tenth. T h e n the scores for all seven f r e q u e n c i e s w e r e t o t a l e d - giving, for exa m p l e , a total score ( J e t z e r index) o f 30. A score b e l o w 20 i n d i c a t e s insensitivity to v i b r a t i o n ( a n d thus p o t e n t i a l c o m p r e s s i o n n e u r o p a t h y ) a n d a score above 30 i n d i c a t e s sensitivity to v i b r a t i o n ( a n d thus " n o r m a l i t y " ) .
Results M e a n scores w e r e c a l c u l a t e d for e a c h hand; the right h a n d m e a n was 35.2 a n d left h a n d m e a n was 35.7. T h e right a n d left h a n d scores w e r e significantly c o r r e l a t e d ( r 2 = 56%). T a b l e 1 shows that males h a d significantly h i g h e r scores t h a n f e m a l e s - especially in the h i g h e r frequencies; t h a t is, f e m a l e s w e r e less sensitive to vibration. Since a low score is b a d , this is
Table 1 Mean scores for males and females in individual frequencies, and critical frequencies Hand
Frequency
Right
8.0 16.0 31.5 63.0 125.0 250.0 500.0
5.8 5.5 5.2 5.7 5.9 5.6 4.1
5.9 5.3 4.8 5.1 5.4 4.9 3.5
* * *
total
37.1
34.6
*
critical3 critical2
15.6 9.6
13.8 8.4
* *
8.0 16.0 31.5 63.0 125.0 250.0 500.0
5.9 5.2 4.9 5.7 5.9 5.6 4.1
5.8 5.2 4.7 5.1 5.4 4.9 3.5
* * * *
total
37.7
35.0
critical3 critical2
15.5 9.7
13.7 8.3
Left
Males
Females
* *
* indicates differences between males and females significant at the p < 0.05 level. total = (8 + 16 + 31.5 + 63 + 125 + 250 + 500) Hz critical2 = (250 + 500) Hz critical3 = (125 + 250 + 500) Hz
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R. Neese, S. Konz / Fibrometry case study
comparable with the common experience of higher carpal tunnel syndrome rates among females. Since there seemed to be a frequency effect, two additional scores were calculated. Critical2 is the mean of the 250 and 500 Hz score and Critical3 is the mean of the 125, 250 and 500 Hz score. The significant gender differences for Critical2 and Critical3 as well as for total show that it may be possible to test just the upper 2 or 3 bands instead of all 7. Four questions from the questionnaire were analyzed since they suggested possible compression neuropathy. They were: - Do you now have numbness or tingling in your hands? - Do you now or have you in the past year had pain/weakness in your hands? - Do you now have or have you in the past year had pain or numbness in your hands at night? - Have you ever been diagnosed or treated for carpal tunnel syndrome? They could circle N (scored zero), yes for left or right hand (scored 1) or yes for both hands (scored
2). Table 2 shows the r 2 value for linear regression between the various variables and the vibrometry score of the preferred hand for 7 frequencies (SCROEP), for top 3 frequencies (CRIT3P) and for top 2 frequencies (CRIT2P).
Table 2 The linear coefficient of determination (r 2) is given for various variables for SCOREP, CRIT3P, and CRIT2P. Values above 0.026 are statistically significant (p < 0.05) Variables
SCOREP
CRIT3P CRIT2P
0.137 0.014 0.006 0.001
0.136 0.021 0.002 0.003
0.144 0.034 0.000 0.005
0.054 0.050 0.041 0.000
0.069 0.053 0.047 0.000
0.065 0.049 0.040 0.000
0.039 0.012
0.055 0.021
0.063 0.022
Task
Pace Force Angle Repetitions Experience questions
Numbness Pain Pain at night Carpal tunnel synd. Person
Gender Age
Table 3 Multiple correlation coefficient (r 2) for SCOREP, CRIT3P and CRIT2P Criterion
Variables
Cumulative r 2
SCOREP
Pace Pain Pain sq Age
13.7% 17.0 21.7 23.5
CRIT3P
Pace Pain Pain sq Age
13.6 17.2 21.4 24.3
CRIT2P
Pace Pain Numbness Numbness sq
14.4 17.8 20.9 25.1
The best single predictor of the vibrometry score was Pace (either self or machine; self paced is better) with 13.7% of the variance explained. Next were the numbness question (r 2 = 5.4%), pain (r 2 = 5.0%) and pain at night r 2 = 4.1%). Gender only had r 2 = 3.9%. The regressions also were run for the criterion C R I T 3 P and CRIT2P. In general, r 2 values were higher for CRIT3P and C R I T 2 P than for SCORE. This implies that they are more sensitive than using the vibrometry score for all 7 frequencies. If the lower frequencies can be omitted, the cost of vibrometry testing would be reduced. Table 2 indicates that, of the task variables, only pace is important. The experience questions are good predictors with numbness the best, then pain, then pain at night. Asking if a person has carpal tunnel syndrome was not worthwhile. Females had lower vibration sensitivity scores; that is, they were less able to detect vibration than males. Next the S T E P W l S E multiple regression program was used to see how well vibrometry scores could be predicted from combinations of variables. New variables were created by squaring the variables of pain, numbness, numbness at night and age to see if the effects are linear. Table 3 shows, for SCOREP, that pace enters first, and then pain, pain squared and then age. Using all four variables the r z = 23.5%. For CRIT3P, the variables are pace, pain, and pain squared and age with r 2 = 2 4 . 3 % . For
R. Neese, S. Konz / Vibrometry case study
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Table 4 Jobs, number of people tested in each job, pace (self or machine), subjective score for repetitions, force and angle, vibrogram mean and critical3 scores and their grouping (p < 0.05). The rows are ordered on the mean vibrometry score Job title
N
Subjective score Reps
Force
Angle
CBF operator Scrapper Press operator Management
7 19 7 10
10 10 4 0
8 10 8 0
Batch operator Handwork P. oper. Packager
20 8 6 30
8 9 8 7
Overwrap. co. P. packer Stock hand. B. operator
10 6 5 19
8 5 2 9
Pace
Vibrometry score Total
Group
Crit3
Group
5 10 8 0
S S S S
40.0 38.2 38.2 36.9
A AB AB ABC
17.1 16.2 16.0 14.7
A AB AB ABC
8 2 8 4
5 7 10 5
S S S M
36.5 35.4 35.4 35.1
ABC ABC ABC ABC
14.6 14.1 14.3 13.8
ABC ABC ABC ABC
5 3 9 4
5 5 6 8
M M S M
33.2 32.8 31.5 30.3
BC BC BC C
12.9 13.6 12.4 11.1
ABC ABC BC C
CRIT2P, the variables are pace, pain, numbness and numbness squared with r 2 25.10~. Gender does not appear as one of the top four predictors for any of the three vibrometry scores. Upon examination, gender was significantly correlated with the experience scores so when the experience scores entered the model, gender added little new information. Next, vibrometry scores were analyzed for a relation to the 18 job classifications. The results were not significant. However, upon closer examination some jobs had a very small number of people in them. In addition, the previous analysis indicated males had higher scores than females. Therefore the analysis was rerun omitting the six jobs with 3 or less people in them. In addition, =
gender and age were added as covariates. Vibrogram scores were related to these 12 jobs; see Table 4. Table 4 gives the 12 jobs, some job characteristics, and the total and Critical3 scores for the right hand.
Discussion Vibrometry is able to distinguish among people with repetitive trauma problems. However it seems that it is not necessary to analyze all seven frequencies as most of the information is contained in the responses to frequencies of 125, 250 and 500 Hz and perhaps just the frequencies of 250 and 500 Hz.
Table 5 Ergonomic priorities survey. Scale (score) each job, then prioritize jobs. Investigate highest score job first Criterion
Score 0 ....
5 ....
10
No. of repetitions
Non repetitive
250/day
500/day
Amount of force
No force applied
~< 3 kg
>~ 7 kg
Pinch grip required to perform job? Wrist flexion/extension Manual percent of worker cycle Self vs. machine pace Hand vibration
No No flexion or extension 0% Self pace None
yes-500/day 45 ° deviation 75% 50/50 self/machine 1 h/day
1000/day /> 90 ° 100% Machine pace over 2 h / d a y
Gloves
No
Yes
R. Neese, S. Konz / ~brometry case study
T h e "job experience" questions gave better estimates of the vibrometry score. Pain was the best question. T h a t is, if a person said they had pain in their hands, their vibrogram score was lower. T h e significant pain squared term (which had a plus sign vs. the negative sign for pain) indicates that the vibrometry vs. pain scores flattens out as pain increases rather than rising m o r e steeply. N u m b n e s s and n u m b n e s s squared also had the same curve shape with the curve flattening with higher n u m b n e s s values. A g e also affected vibrometry score with lower scores for older workers. A l t h o u g h vibrometry can show which individuals have CTS symptoms, engineers and m a n a g e r s would like to be able to predict which job factors cause the problem. W h e n trying to predict the response to vibrometry, most task variables, with the exception of pace, have low correlations. People with machine-paced jobs had lower vibrometry scores than people with self-paced jobs. Subjective estimates by the experimenter of force, angle, and the repetitions had low correlations with the vibrometry scores. T h u s job characteristics had some ability to differentiate the scores but there was considerable overlap. In addition the values for reps, force, angle and pace are c o n f o u n d e d . Thus we do not know how important each score is in predicting the vibrometry score. Part of the problem is that the job characteristics used in our survey were not very precise. Table 5 is an a t t e m p t to define these job characteristics m o r e precisely. T h e revised form has
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words a d d e d to give the analyst a better estimate of various scale values. For example, for the criterion of repetitions, a value of 2 5 0 / d a y would be given a value of 5. In addition, criteria were a d d e d for pinch grips, manual percent of the cycle, self vs. machine pace, vibration, and use of gloves.
Acknowledgements T h e authors thank Dr. T. Jetzer for providing a vibrometer and analysis of the vibrometry scores and Zenith D a t a Suppliers for use of a portable computer.
References Grunert, B., Wertsch, J., Matloub, H. and McCallum-Burke, S., 1990. Reliability of sensory threshold measurement using a digital vibrogram. J. of Occupational Medicine, 32(2): 100-102. Jetzer, T., 1989. The use of vibration testing in the early evaluation of workers with carpal tunnel syndrome. In: Proceedings of the Fifth International Conference on Hand-Arm Vibration, Kanazawa, Japan, 1989. Jetzer, T., Conrad, J. and Heithoff, K., 1987. The role of scanning and vibrometry testing in the diagnostic evaluation of carpal tunnel syndrome. Proceedings of the VolvoIFSSH Conference on the Prevention of Brachial Injuries and Cumulative Trauma Disorders, Stockholm, 1987. Lundborg, G., Lie-Stenstrom, A., Sollerman, C., Stromberg, T. and Pykko, I., 1986. Digital vibrogram: A new diagnostic tool for sensory testing in compression neuropathy. The J. of Hand Surgery, llA, 5: 693-699.