International Journal of Epidemiology © Oxford Unrvernty Press 1975
Vol. 4, No. 3 Printed in Great Britain
An Epidemiological Study of the Relationship Between Occupations and Acute Herniated Lumbar Intervene bra I Discs* JENNIFER L. KELSEY1
INTRODUCTION
There has been much discussion of the role that occupations play in the aetiology of herniated lumbar intervertebral discs. Some surgeons (1-3) have felt that jobs involving heavy physical labour predispose to herniated lumbar discs, while others (4-5) have suggested that sedentary occupations increase the risk. Despite these diverse opinions, which are based primarily on clinical.impression, to our knowledge no epidemiological studies have previously been undertaken of the relationship between occupations and herniated lumbar discs. Using data from an epidemiological study in the New Haven, Connecticut, area (6), this paper examines the risk for acute herniated lumbar intervertebral discs among people in different types of occupations.
niated lumbar intervertebral discs to characteristics of two groups of people of the same sex and similar age without herniated lumbar discs. Cases were ascertained from people in the age group 20-64 years residing in the New Haven Standard Metropolitan Statistical Area who had lumbar X-rays taken during the period June 1971 to May 1973 at the three hospitals in this area and at the office of two of the private radiologists in New Haven. Lumbar X-rays alone are of course of limited value in diagnosing herniated discs, but the assumption was made that most people with severe low back or sciatic pain would have lumbar X-rays taken, so that many with herniated lumbar disc could be ascertained from among these patients. In order to determine which of them were in fact likely to have herniated discs, they were interviewed within a few weeks of the time they were X-rayed and their medical records were reviewed. During the interview, in addition to being asked about demographic characteristics, occupational history, and exposure to other possible risk factors, they were asked about symptoms and were given a few simple diagnostic tests by the interviewers. On the basis of the symptoms and signs noted during the interview, the radiologist's report (which could be used to exclude people with other conditions which could be bringing about the same symptoms and signs), and the surgeon's observations (in cases in which surgery was performed), the people having low back X-rays were divided into
METHODS
The general approach in this study involved comparing characteristics of people with recent her• Supported by USPHS Grant 5-RO1-AM-15397 from tne National Institute of Arthritis, Metabolism and Digestive Diseases. 1 Department of Epidemiology and Public Health, Yale University School of Medicine, 60 College Street, New Haven, Connecticut 06510, U.S.A. t Supported by Career Development Award 1-KO4-NS-70502 from the National Institute of Neurological Diseases and Stroke. This paper was presented at the Rheumatic Complaints session of the Seventh International Scientific Meeting of the International Epidemiological Association, held at the University of Sussex, England, August 1974.
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Kelsey, J. (Dept. Epidemiology and Public Heattn, Yale University School of Medicine, 60 College Street, New Haven, Connecticut 06510, U.S.A.). An epidemiological study of the relationship between occupations and acute herniated lumbar intervertebral discs. Int. J. Epid. 1975, 4:197-205. A case-control study of the epidemiology of acute herniated lumbar intervertebral discs in the New Haven, Connecticut, area indicates that sedentary occupations increase the risk for herniated lumbar discs, particularly among those aged 35 years and older who have had sedentary jobs for several years. People in occupations requiring prolonged driving of motor vehicles, such as truck drivers, appear to be at particularly high risk. The results of this study provide no evidence that people whose jobs involve heavy manual labour are at greater risk for acute herniated lumbar disc than others of their age and sex.
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• The descriptions 'ruptured', 'free fragments', 'herniated', 'prolapsed', 'bulging', and 'extruded' are included, but not disc degeneration without evidence of nerve root involvement. The one patient whose medical record indicated that the herniated disc was seen at surgery but who did not have symptoms consistent with a herniated disc is not included as a case.
within three years in the other hospital; the ages of all other patients were matched within ten years. No one who would have been classified as a surgical, probable, or possible case or who had previously had a herniated lumbar disc or chronic low back pain could serve as a control. In addition, the controls had to have sought medical care for a condition which they had had for no more than one year, so that, like the cases, they had recently acquired their disease. When a potential control did not meet these criteria, another person of the appropriate age and sex admitted to the same service was chosen. The second control group was composed of people who had low back X-rays taken and were thus interviewed in the course of ascertaining cases, but who were not classified as surgical, probable, or possible cases and who had not had their symptoms for more than one year. Because of the different characteristics of people going to various hospital services, comparisons of these cases and controls were made in seven separate groups for males and six groups for females according to the hospital service. The separate four-fold tables resulting from these comparisons were then combined by the Mantel-Haenzel procedure (7) in order to test for overall statistical significance and to estimate relative risk. The age distributions of cases and controls in each group were similar, so they did not have to be compared in separate age groups. The same questionnaire and diagnostic tests were administered by carefully trained nonmedical interviewers to all cases and controls. Most interviewing was done in the homes of the subjects, but when it was possible to interview them in the hospital this was done. The overall response rate was 79 per cent for those having low back X-rays and 77 per cent for matched controls. Information was obtained on all jobs which the respondent had held for at least a year since he left school. Questions were asked about what the specific occupation was; how many years he had been employed in it; how many hours per week he usually worked; whether he sat none of the time, a little of the time, about half of the time, most of the time, or all the time; if he sat half the time or more, what kind of chair he usually sat in; whether he did any lifting, pushing, pulling, or carrying; and if he lifted, about how many times a day he did this and what was the approximate weight of the objects he usually lifted. A total of 217 pairs was obtained (89 females, 128 males) for the comparison of cases and matched
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those who might have herniated lumbar discs and those who probably did not. Among those considered likely to have herniated lumbar disc, the following diagnostic criteria were applied: 1. Surgical cases were those in which all of the following three criteria were fulfilled: (a) the surgeon stated on the hospital chart that he saw the herniated disc during surgery;* (b) the patient gave evidence in his answers to the questionnaire that his pain was distributed along the sciatic nerve; and (c) the patient had a positive straight leg raising test and/or the symptoms of increased pain in the low back or along the sciatic nerve when stretching or extending his leg from a sitting position and/or the symptom of increased pain along the sciatic nerve when coughing or sneezing. 2. Probable cases were similar to the surgical except that the herniation need not have been observed at surgery. Included were cases in which the sciatic pain was felt in both the thigh and lower leg and cases in which there was sciatic pain in part of the leg and numbness in another part. 3. Possible cases differed from the probable in that the sciatic pain was only in the thigh or the lower leg but not in both. Also, if the leg was numb so that the distribution of pain was unknown but the symptoms and signs were otherwise consistent with a herniated disc, the person was classified as a possible case. Only those who had developed symptoms within the previous year were included in the case-control comparison, since it was desired to find out about exposure to possible risk factors prior to the onset of the disc problem rather than subsequent to it. Two control groups of people without known herniated discs were also interviewed so that their occupational histories and exposure to other risk factors could be compared to those of the cases. The first group was a matched control group. Each surgical, probable, and possible case was matched to the next person of the same sex and about the same age who was admitted to the same hospital service or to the same radiologist's office for a condition not related to the spine. Ages of patients admitted to the emergency room were matched within two years in two hospitals and
RELATIONSHIP BETWEEN OCCUPATIONS A N D ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
than pairs in which the case sat less than half the time and the control sat half the time or more. With the sexes combined, the relative risk is estimated to be 1-58. In Table II, the pairs are divided into those in which the case was 35 years of age or older and those in which the case was younger than 35. It may be seen that the association between sedentary occupations and herniated lumbar discs is seen only among those in the older age group, in which the estimate of relative risk is 2-40. The proportions of cases and unmatched controls who sat half or more of the time on the job are compared in Table i n . In most of the groups, the proportion of cases is higher than the proportion of controls; when the individual groups are combined by the Mantel-Haenzel procedure, the estimate of relative risk, 1 -70, is very close to that for the cases and matched controls. When this was examined by age group (data are not shown here) it was again found that the trend was stronger in those age 35 and over. Thus, both comparisons suggest that sedentary occupations are conducive to the development of herniated lumbar discs, especially among those aged 35 years and older, and that, more specifically, those who sit half of the time or more on their jobs have about a 60-70
RESULTS
Table I compares cases and matched controls according to whether they spent half or more of their time sitting while at the job they had at the time they developed their symptoms. It can be seen that among both males and females there were more pairs in which the case sat half of the time or more and the control sat less than half the time • Matched controls could not be obtained for six of the cases seen by private radiologists because of problems related to confidentiality of names of people seeking medical care from private physicians.
TABLE I
Cases and matched controls according to whether they sat half the time or more on their job ( > i ) or sat less than half the time on their job ( < i ) , by sex and type of case* Number of pairs Type of case
Case >\ Control > i
Case > i Control •< i
Case i
Case i) or sat less than half the time on their job ( < J) by sex and age of cast* Number of pairs Sex and age of case
Relative risk
Case >i Control < i
Case i
Case < i Control i Control > i
RELATIONSHIP BETWEEN OCCUPATIONS AND ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
201
TABLE IV
Male cases and matched controls according to whether they had a job in which they sat half the time or more in a motor vehicle (MV) or did not have a job in which they sat half the time or more in a motor vehicle (NMV) by type of case Number of pairs CaseMV Control NMV
Case NMV Control MV
Case NMV Control NMV
Total
Surgical Probable Possible
0 0 0
6 8 8
4 2 2
52 29 17
62 39 27
Al] cases
0
22
8
98
128
Relative risk — 2-75 X1, - 5-63, p = 02 X1 value is for paired observations (8).
and the cases did not, leading to an estimate of relative risk of 2 • 75. If the data are subdivided by age (not shown here), the difference is seen both in those aged 35 years and older and those under 35. Among truck drivers Table V indicates that the relative risk is even higher, 4-67. The data for male cases and unmatched controls, although not presented here, also show differences between the proportion of cases and of controls whose jobs involve sitting while driving motor vehicles and who were truck drivers, with estimates of relative risk of 3 • 14 and 2-26, respectively. Thus, not only are people with sedentary occupations at increased risk for herniated lumbar disc, but those who sit while driving motor vehicles are at especially high risk, In fact, if persons who sit half or more of the time in motor vehicles are removed from the analysis, the overall association with sitting is reduced. Next, consideration is given to the role of lifting in the aetiology of herniated lumbar discs.
It should be mentioned that when the respondents were asked what they were doing when their symptoms started, lifting was the most common answer given by males, accounting for 16 per cent of the responses, and the fifth most common answer from the females, making up 9 per cent of the answers. However, since most people can do ordinary lifting without experiencing a disc herniation, this says little about the role of lifting as a predisposing or underlying factor. Of more relevance is a comparison of lifting done on the job by cases and controls. Table VI shows the numbers of cases and matched controls according to whether they did any lifting or no lifting on their job. It can be seen that among the males, cases and controls were equally likely to lift; among the females, cases were somewhat more likely to lift than controls, but this did not reach statistical significance. Comparisons of the proportions of cases and unmatched controls who did any lifting on the job may be seen in Table VII. There is no
TABLE V
Male cases and matched controls according to whether their occupation was truck driver (TD) or not truck driver (NTD), by type of case Number of pairs Type of case
CaseTD Control TD
CaseTD Control NTD
Control NTD CaseTD
Case NTD Control NTD
Total
Surgical Probable Possible
1 0 0
8 2 4
1 1 1
52 36 22
62 39 27
Total
1
14
3
110
128
Relative risk = 4-67 X « 1 - 5 - 8 8 , p < 02 X* value is for paired observations (8).
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CaseMV Control MV
Type of case
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY TABLE VI
Cases and matched controls according to whether they did any lifting (L) or did not do any lifting (NL) on their job, by sex and type of case*
Number of pairs Type of case
CaseL Control L
Case L Control NL
Relative risk
xt
CaseNL Control L
CaseNL Control NL
9 7 5
9 4 5
7 5 1
100 1-75 100
21
18
13
117
010
Males 29 17 12
Total
58
Surgical
Females Surgical Probable Possible
5 8 0
9 7 3
3 7 1
3 7 4
Total
13
19
11
14
1-73
1-63
Total, both sexes
71
40
29
27
1-38
1-45
• Includes only pairs in which both members were employed during the year in which the symptoms began. t x* values are for paired observations (8).
indication that male cases did any more lifting than the controls, and just a suggestion that female cases were less likely than controls to have jobs in which they lifted. The cases and controls who did any lifting on their jobs were asked about the weight of what they usually lifted, in five broad categories ranging from less than 5 pounds to 50 pounds or more, and about the frequency with which they usually lifted it, again in five broad categories ranging from less than once a day to 20 or more times per day. Among both males and females and in comparisons of cases with both matched and unmatched controls, there was no tendency for cases to lift heavier objects than controls or to lift more frequently than the controls. Table VIII, for instance, shows the comparison of male cases and matched controls according to the weight of what was lifted; it can be seen that there are about as many pairs in which the case lifted heavier objects than the control as pairs in which the control lifted heavier objects than the case. Respondents were also asked whether they did any pushing, pulling or carrying on their jobs. Comparisons of cases with the two control groups gave no indication that jobs requiring pushing, pulling, or carrying either increased or decreased the risk for herniated lumbar discs.
DISCUSSION
In interpreting the results of this study, it must be kept in mind that these cases were not a probability sample of persons with herniated lumbar intervertebral discs since this study population is weighted towards people who were hospitalized for their disc problems. The controls were also people who sought medical care for various conditions, and it would be impossible to describe the population from which either control group came. However, it is felt that with so little known about the epidemiology of herniated lumbar intervertebral discs, it was reasonable to undertake an exploratory study such as this based on people seeking medical care rather than on a more general population. In any event, it is hoped that the cases and controls are subject to similar selective factors and that differences between these cases and controls are indicative of differences between people with and without herniated lumbar discs in a more general population. In addition, it should be stressed that a large number of individual occupations and many possible risk factors besides those relating to occupations were examined in this study. When this many variables are considered, it is likely that some associations will occur by chance, so further investi-
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Probable Possible
:
RELATIONSHIP BETWEEN OCCUPATIONS AND ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
203
TABLE VII
Proportion of cases and unmatched controls who did any lifting on their Job, by sex and hospital service* Cases
Controls
Number in group
proportion who lift
Number in group
proportion who lift
I. Yale-New Haven in-patients n . St. Raphael in-patients IH. Yale-New Haven emergency room patients IV. St. Raphael emergency room patients V. Other Yale and St. Raphael patients VI. Private radiologists' patients VII. Veterans Administration patients
52 20 16 13 4 4 13
0-731 0-700 0-813 0-846 0-500 0-500 0-769
25 21 72 67 11 13 27
0-761 0-571 0-722 0-716 0-545 0-692 0-815
Females I. Yale-New Haven in-patients IL St. Raphael in-patients IIL Yale-New Haven emergency room patients IV. St. Raphael emergency room patients V. Other Yale and St. Raphael patients VI. Private radiologists' patients
16 17 12 11 9 3
0-500 0-647 0-500 0-364 0-667 0-333
6 10 50 60 29 9
0-833 0-600 0-540 0-583 0-724 0-222
Hospital service Males
TABLE v m
Male cases and matched controls according to weight of what was lifted on the job* Controls
Cases
Total
0 501b.
0
5 1b.
12 2 3 5 6 4
0 0 0 0 0 2
32
2
5-9 lb.
10-24 lb.
25-49 lb.
>501b.
Total
0 0 0 0 1 3
5 1 1 0 6 4
2 1 0 2 3 4
10 2 1 4 8 14
29 6 5 11 24 31
4
17
12
39
106
Z — 0-119 by Wilcoxon matched-pairs signed-ranks test. * Includes pairs in which both members were employed during the year in which the symptoms began. gations are clearly needed to confirm or refute the leads provided in this study. The association between sedentary occupations, especially those which involve driving motor vehicles, and herniated lumbar discs would appear to be real since it was seen in both sexes and in comparison between cases and both control groups. The strength of this association in those aged 35 and older and the lack of association in those who are under that age suggest that a
certain amount of time in sedentary occupations is necessary for an effect to be seen. In fact, a comparison (not shown here) of the number of years cases and controls have had jobs in which they spend half or more of the time sitting confirms this suggestion, for the increased risk is seen only among those who have had sedentary occupations for about five years or more, and appears to become greater with increasing number of years at a sedentary job. Further evidence of an aetiological
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X* tests of association calculated by Mantel-Haenzel procedure: Males only, x* = 0-403, p > 10, estimate of relative risk « 1-20. Females only, *• — 0-922, p > -10, estimate of relative risk = 0-67. Males and females, *• = 0-017, p > -10, estimate of relative risk — 0-94. * Includes those who were employed during the year in which the symptoms began.
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butable to chance. Other considerations which decrease the likelihood that the association is real are that if the comparison is limited to pairs in which both the case and control worked 20 hours per week or more, the relative risk was estimated to be only about 1-25, and that there were no associations between the weight of what was lifted or the frequency with which it was lifted and the development of herniated lumbar discs. Furthermore, the female cases were somewhat less likely to have jobs involving lifting than the unmatched controls. Thus, although the possibility of a relationship between lifting and herniated lumbar discs in females cannot be ruled out, the present study provides little evidence of such an association in either sex. It is also possible that the rough indicators of lifting used in this study were not sensitive enough to detect a difference between cases and controls, and, indeed, this is an additional reason for further more detailed study. It would seem, nevertheless, that if lifting on the job were an important factor, some association would have been found. Hult (13) and Friberg and Hirsch (14) also concluded that heavy work is not a fundamental cause, although they were concerned with the 'lumbagosciatica syndrome* and with disc degeneration in patients seen at a clinic for back problems, respectively, rather than specifically with herniated lumbar intervertebral discs. This is not to imply that lifting might not play a role in certain other types of back problems, such as muscle strain, but heavy manual labour would not appear to be an important underlying factor in the aetiology of herniated lumbar intervertebral discs. Finally, it should again be emphasized that since this study is apparently the first of its kind, and since it had certain limitations in the study population used, the response rate, the types of questions asked, and the large number of associations examined, it will be important to have' these findings tested in other epidemiological investigations.
REFERENCES
(1) Goodsell, J. O.: Correlation of ruptured lumbar disc with occupation. Clin. Orthop. 50: 225, 1967. (2) Howorth, M. B.: A Textbook of Orthopedics. Philadelphia: Saunders, 1953. (3) Spurting, R. G.: Lesions of the Lumbar Intervertebral Disc. Springfield: Thomas, 1953. (4) Semmes, R. E.: Ruptures of the Lumbar Intervertebral Disc. Springfield: Thomas, 1964.
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role for too much sitting comes from another part of this study, in which it was found that the length of time males reported that they usually sat on weekends was also associated with herniated lumbar discs. The reason for the association with sedentary occupations cannot be ascertained from this type of study. However, two possible explanations are apparent: one that prolonged sitting has an adverse effect, and the other that physical activity is beneficial. It is known that sitting puts more pressure on the discs than either standing or lying down (9-11), and it would seem reasonable that prolonged stress from sitting for long periods of time would increase the probability of a disc herniation. A beneficial effect of physical activity is also plausible, since it would presumably improve the general condition of the muscles and ligaments of spine and thus provide more support for the discs. There was little evidence from this study that physical activity at or away from work decreased the risk for lumbar disc herniations, but this could be attributable to the difficulty of assessing physical activity, especially away from work. In any event, the relative importance of prolonged' sitting compared to lack of exercise remains unknown. The additional risk associated with jobs involving driving motor vehicles was not anticipated when the study began, but can be clearly seen in the comparisons of occupations of cases and controls. In fact, the one single occupation which stood out as having a particularly high risk for herniated lumbar discs in this relatively small study was that of truck driver. Again, the exact reason for this association remains unknown; however, driving for long periods of time requires prolonged sitting in a seat with insufficient support for the low back, with legs extended, in a position subject to the vibration from the road and to mechanical stress from starting and stopping, so it is not surprising that it is associated with an increased risk for disc herniation. In addition, respondents were asked in another part of the questionnaire whether they drove a car; significantly more cases than controls answered affirmatively, thus providing further support for the role of driving in the aetiology of herniated lumbar discs (12). This study gave no evidence of an increased risk for herniated lumbar discs among males who did lifting on their jobs, and little indication of this among the females. The slight positive association between lifting and herniated lumbar discs when female cases were compared to matched controls should be noted, although it is most likely attri-
RELATIONSHIP BETWEEN OCCUPATIONS A N D ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
(10) Caillet, R.: Low Back Syndrome. Philadelphia: Davis, 1968. (11) Nachemson, A.: In vivo discometry in lumbar discs with irregular radiograms. Ada Orthop. Scand. 36: 418, 1965. (12) Kelsey, J. L. and Hardy, R. J.: Driving of motor vehicles as a risk factor for acute herniated lumbar intervertebral disc. Amer. J. Epid. In press. (13) Hult, L.: Cervical, dorsal, and lumbar spinal syndromes. Acta Orthop. Scand., Suppl. 17, 1954. (14) Friberg, S. and Hirsch, C : Anatomical and clinical studies of lumbar disc degeneration. Acta Orthop. Scand. 19: 222, 1950.
{revised version received 11 June 1975)
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(5) Coventry, M. B.: Introduction of Symposium, including anatomy, physiology, and epidemiology. / . Bone Jt. Surg. [Amer.] 50: 167, 1968. (6) Kelsey, J. L. and Ostfeld, A- M.: Demographic characteristics of persons with acute herniated lumbar intervertebral disc. / . Chronic Dis. 28: 37, 1975. (7) Mantel, N. and Haenszel, W.: Statistical aspects of the analysis of data from retrospective studies of disease. / . Nail. Cancer Inst. 22: 719, 1959. (8)Snedecor, G. W. and Cochran, W. G.: Statistical Methods. Ames, Iowa: Iowa State University Press, 1967. (9) Keegan, J. J.: Alterations of the lumbar curve related to posture and seating. / . Bone Jt. Surg. [Amer.] 35: 589, 1953.
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Vol. 4, No. 3 Printed in Great Britain
An Epidemiological Study of the Relationship Between Occupations and Acute Herniated Lumbar Intervene bra I Discs* JENNIFER L. KELSEY1
INTRODUCTION
There has been much discussion of the role that occupations play in the aetiology of herniated lumbar intervertebral discs. Some surgeons (1-3) have felt that jobs involving heavy physical labour predispose to herniated lumbar discs, while others (4-5) have suggested that sedentary occupations increase the risk. Despite these diverse opinions, which are based primarily on clinical.impression, to our knowledge no epidemiological studies have previously been undertaken of the relationship between occupations and herniated lumbar discs. Using data from an epidemiological study in the New Haven, Connecticut, area (6), this paper examines the risk for acute herniated lumbar intervertebral discs among people in different types of occupations.
niated lumbar intervertebral discs to characteristics of two groups of people of the same sex and similar age without herniated lumbar discs. Cases were ascertained from people in the age group 20-64 years residing in the New Haven Standard Metropolitan Statistical Area who had lumbar X-rays taken during the period June 1971 to May 1973 at the three hospitals in this area and at the office of two of the private radiologists in New Haven. Lumbar X-rays alone are of course of limited value in diagnosing herniated discs, but the assumption was made that most people with severe low back or sciatic pain would have lumbar X-rays taken, so that many with herniated lumbar disc could be ascertained from among these patients. In order to determine which of them were in fact likely to have herniated discs, they were interviewed within a few weeks of the time they were X-rayed and their medical records were reviewed. During the interview, in addition to being asked about demographic characteristics, occupational history, and exposure to other possible risk factors, they were asked about symptoms and were given a few simple diagnostic tests by the interviewers. On the basis of the symptoms and signs noted during the interview, the radiologist's report (which could be used to exclude people with other conditions which could be bringing about the same symptoms and signs), and the surgeon's observations (in cases in which surgery was performed), the people having low back X-rays were divided into
METHODS
The general approach in this study involved comparing characteristics of people with recent her• Supported by USPHS Grant 5-RO1-AM-15397 from tne National Institute of Arthritis, Metabolism and Digestive Diseases. 1 Department of Epidemiology and Public Health, Yale University School of Medicine, 60 College Street, New Haven, Connecticut 06510, U.S.A. t Supported by Career Development Award 1-KO4-NS-70502 from the National Institute of Neurological Diseases and Stroke. This paper was presented at the Rheumatic Complaints session of the Seventh International Scientific Meeting of the International Epidemiological Association, held at the University of Sussex, England, August 1974.
197
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Kelsey, J. (Dept. Epidemiology and Public Heattn, Yale University School of Medicine, 60 College Street, New Haven, Connecticut 06510, U.S.A.). An epidemiological study of the relationship between occupations and acute herniated lumbar intervertebral discs. Int. J. Epid. 1975, 4:197-205. A case-control study of the epidemiology of acute herniated lumbar intervertebral discs in the New Haven, Connecticut, area indicates that sedentary occupations increase the risk for herniated lumbar discs, particularly among those aged 35 years and older who have had sedentary jobs for several years. People in occupations requiring prolonged driving of motor vehicles, such as truck drivers, appear to be at particularly high risk. The results of this study provide no evidence that people whose jobs involve heavy manual labour are at greater risk for acute herniated lumbar disc than others of their age and sex.
198
INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
• The descriptions 'ruptured', 'free fragments', 'herniated', 'prolapsed', 'bulging', and 'extruded' are included, but not disc degeneration without evidence of nerve root involvement. The one patient whose medical record indicated that the herniated disc was seen at surgery but who did not have symptoms consistent with a herniated disc is not included as a case.
within three years in the other hospital; the ages of all other patients were matched within ten years. No one who would have been classified as a surgical, probable, or possible case or who had previously had a herniated lumbar disc or chronic low back pain could serve as a control. In addition, the controls had to have sought medical care for a condition which they had had for no more than one year, so that, like the cases, they had recently acquired their disease. When a potential control did not meet these criteria, another person of the appropriate age and sex admitted to the same service was chosen. The second control group was composed of people who had low back X-rays taken and were thus interviewed in the course of ascertaining cases, but who were not classified as surgical, probable, or possible cases and who had not had their symptoms for more than one year. Because of the different characteristics of people going to various hospital services, comparisons of these cases and controls were made in seven separate groups for males and six groups for females according to the hospital service. The separate four-fold tables resulting from these comparisons were then combined by the Mantel-Haenzel procedure (7) in order to test for overall statistical significance and to estimate relative risk. The age distributions of cases and controls in each group were similar, so they did not have to be compared in separate age groups. The same questionnaire and diagnostic tests were administered by carefully trained nonmedical interviewers to all cases and controls. Most interviewing was done in the homes of the subjects, but when it was possible to interview them in the hospital this was done. The overall response rate was 79 per cent for those having low back X-rays and 77 per cent for matched controls. Information was obtained on all jobs which the respondent had held for at least a year since he left school. Questions were asked about what the specific occupation was; how many years he had been employed in it; how many hours per week he usually worked; whether he sat none of the time, a little of the time, about half of the time, most of the time, or all the time; if he sat half the time or more, what kind of chair he usually sat in; whether he did any lifting, pushing, pulling, or carrying; and if he lifted, about how many times a day he did this and what was the approximate weight of the objects he usually lifted. A total of 217 pairs was obtained (89 females, 128 males) for the comparison of cases and matched
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those who might have herniated lumbar discs and those who probably did not. Among those considered likely to have herniated lumbar disc, the following diagnostic criteria were applied: 1. Surgical cases were those in which all of the following three criteria were fulfilled: (a) the surgeon stated on the hospital chart that he saw the herniated disc during surgery;* (b) the patient gave evidence in his answers to the questionnaire that his pain was distributed along the sciatic nerve; and (c) the patient had a positive straight leg raising test and/or the symptoms of increased pain in the low back or along the sciatic nerve when stretching or extending his leg from a sitting position and/or the symptom of increased pain along the sciatic nerve when coughing or sneezing. 2. Probable cases were similar to the surgical except that the herniation need not have been observed at surgery. Included were cases in which the sciatic pain was felt in both the thigh and lower leg and cases in which there was sciatic pain in part of the leg and numbness in another part. 3. Possible cases differed from the probable in that the sciatic pain was only in the thigh or the lower leg but not in both. Also, if the leg was numb so that the distribution of pain was unknown but the symptoms and signs were otherwise consistent with a herniated disc, the person was classified as a possible case. Only those who had developed symptoms within the previous year were included in the case-control comparison, since it was desired to find out about exposure to possible risk factors prior to the onset of the disc problem rather than subsequent to it. Two control groups of people without known herniated discs were also interviewed so that their occupational histories and exposure to other risk factors could be compared to those of the cases. The first group was a matched control group. Each surgical, probable, and possible case was matched to the next person of the same sex and about the same age who was admitted to the same hospital service or to the same radiologist's office for a condition not related to the spine. Ages of patients admitted to the emergency room were matched within two years in two hospitals and
RELATIONSHIP BETWEEN OCCUPATIONS A N D ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
than pairs in which the case sat less than half the time and the control sat half the time or more. With the sexes combined, the relative risk is estimated to be 1-58. In Table II, the pairs are divided into those in which the case was 35 years of age or older and those in which the case was younger than 35. It may be seen that the association between sedentary occupations and herniated lumbar discs is seen only among those in the older age group, in which the estimate of relative risk is 2-40. The proportions of cases and unmatched controls who sat half or more of the time on the job are compared in Table i n . In most of the groups, the proportion of cases is higher than the proportion of controls; when the individual groups are combined by the Mantel-Haenzel procedure, the estimate of relative risk, 1 -70, is very close to that for the cases and matched controls. When this was examined by age group (data are not shown here) it was again found that the trend was stronger in those age 35 and over. Thus, both comparisons suggest that sedentary occupations are conducive to the development of herniated lumbar discs, especially among those aged 35 years and older, and that, more specifically, those who sit half of the time or more on their jobs have about a 60-70
RESULTS
Table I compares cases and matched controls according to whether they spent half or more of their time sitting while at the job they had at the time they developed their symptoms. It can be seen that among both males and females there were more pairs in which the case sat half of the time or more and the control sat less than half the time • Matched controls could not be obtained for six of the cases seen by private radiologists because of problems related to confidentiality of names of people seeking medical care from private physicians.
TABLE I
Cases and matched controls according to whether they sat half the time or more on their job ( > i ) or sat less than half the time on their job ( < i ) , by sex and type of case* Number of pairs Type of case
Case >\ Control > i
Case > i Control •< i
Case i
Case i) or sat less than half the time on their job ( < J) by sex and age of cast* Number of pairs Sex and age of case
Relative risk
Case >i Control < i
Case i
Case < i Control i Control > i
RELATIONSHIP BETWEEN OCCUPATIONS AND ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
201
TABLE IV
Male cases and matched controls according to whether they had a job in which they sat half the time or more in a motor vehicle (MV) or did not have a job in which they sat half the time or more in a motor vehicle (NMV) by type of case Number of pairs CaseMV Control NMV
Case NMV Control MV
Case NMV Control NMV
Total
Surgical Probable Possible
0 0 0
6 8 8
4 2 2
52 29 17
62 39 27
Al] cases
0
22
8
98
128
Relative risk — 2-75 X1, - 5-63, p = 02 X1 value is for paired observations (8).
and the cases did not, leading to an estimate of relative risk of 2 • 75. If the data are subdivided by age (not shown here), the difference is seen both in those aged 35 years and older and those under 35. Among truck drivers Table V indicates that the relative risk is even higher, 4-67. The data for male cases and unmatched controls, although not presented here, also show differences between the proportion of cases and of controls whose jobs involve sitting while driving motor vehicles and who were truck drivers, with estimates of relative risk of 3 • 14 and 2-26, respectively. Thus, not only are people with sedentary occupations at increased risk for herniated lumbar disc, but those who sit while driving motor vehicles are at especially high risk, In fact, if persons who sit half or more of the time in motor vehicles are removed from the analysis, the overall association with sitting is reduced. Next, consideration is given to the role of lifting in the aetiology of herniated lumbar discs.
It should be mentioned that when the respondents were asked what they were doing when their symptoms started, lifting was the most common answer given by males, accounting for 16 per cent of the responses, and the fifth most common answer from the females, making up 9 per cent of the answers. However, since most people can do ordinary lifting without experiencing a disc herniation, this says little about the role of lifting as a predisposing or underlying factor. Of more relevance is a comparison of lifting done on the job by cases and controls. Table VI shows the numbers of cases and matched controls according to whether they did any lifting or no lifting on their job. It can be seen that among the males, cases and controls were equally likely to lift; among the females, cases were somewhat more likely to lift than controls, but this did not reach statistical significance. Comparisons of the proportions of cases and unmatched controls who did any lifting on the job may be seen in Table VII. There is no
TABLE V
Male cases and matched controls according to whether their occupation was truck driver (TD) or not truck driver (NTD), by type of case Number of pairs Type of case
CaseTD Control TD
CaseTD Control NTD
Control NTD CaseTD
Case NTD Control NTD
Total
Surgical Probable Possible
1 0 0
8 2 4
1 1 1
52 36 22
62 39 27
Total
1
14
3
110
128
Relative risk = 4-67 X « 1 - 5 - 8 8 , p < 02 X* value is for paired observations (8).
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CaseMV Control MV
Type of case
202
INTERNATIONAL JOURNAL OF EPIDEMIOLOGY TABLE VI
Cases and matched controls according to whether they did any lifting (L) or did not do any lifting (NL) on their job, by sex and type of case*
Number of pairs Type of case
CaseL Control L
Case L Control NL
Relative risk
xt
CaseNL Control L
CaseNL Control NL
9 7 5
9 4 5
7 5 1
100 1-75 100
21
18
13
117
010
Males 29 17 12
Total
58
Surgical
Females Surgical Probable Possible
5 8 0
9 7 3
3 7 1
3 7 4
Total
13
19
11
14
1-73
1-63
Total, both sexes
71
40
29
27
1-38
1-45
• Includes only pairs in which both members were employed during the year in which the symptoms began. t x* values are for paired observations (8).
indication that male cases did any more lifting than the controls, and just a suggestion that female cases were less likely than controls to have jobs in which they lifted. The cases and controls who did any lifting on their jobs were asked about the weight of what they usually lifted, in five broad categories ranging from less than 5 pounds to 50 pounds or more, and about the frequency with which they usually lifted it, again in five broad categories ranging from less than once a day to 20 or more times per day. Among both males and females and in comparisons of cases with both matched and unmatched controls, there was no tendency for cases to lift heavier objects than controls or to lift more frequently than the controls. Table VIII, for instance, shows the comparison of male cases and matched controls according to the weight of what was lifted; it can be seen that there are about as many pairs in which the case lifted heavier objects than the control as pairs in which the control lifted heavier objects than the case. Respondents were also asked whether they did any pushing, pulling or carrying on their jobs. Comparisons of cases with the two control groups gave no indication that jobs requiring pushing, pulling, or carrying either increased or decreased the risk for herniated lumbar discs.
DISCUSSION
In interpreting the results of this study, it must be kept in mind that these cases were not a probability sample of persons with herniated lumbar intervertebral discs since this study population is weighted towards people who were hospitalized for their disc problems. The controls were also people who sought medical care for various conditions, and it would be impossible to describe the population from which either control group came. However, it is felt that with so little known about the epidemiology of herniated lumbar intervertebral discs, it was reasonable to undertake an exploratory study such as this based on people seeking medical care rather than on a more general population. In any event, it is hoped that the cases and controls are subject to similar selective factors and that differences between these cases and controls are indicative of differences between people with and without herniated lumbar discs in a more general population. In addition, it should be stressed that a large number of individual occupations and many possible risk factors besides those relating to occupations were examined in this study. When this many variables are considered, it is likely that some associations will occur by chance, so further investi-
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Probable Possible
:
RELATIONSHIP BETWEEN OCCUPATIONS AND ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
203
TABLE VII
Proportion of cases and unmatched controls who did any lifting on their Job, by sex and hospital service* Cases
Controls
Number in group
proportion who lift
Number in group
proportion who lift
I. Yale-New Haven in-patients n . St. Raphael in-patients IH. Yale-New Haven emergency room patients IV. St. Raphael emergency room patients V. Other Yale and St. Raphael patients VI. Private radiologists' patients VII. Veterans Administration patients
52 20 16 13 4 4 13
0-731 0-700 0-813 0-846 0-500 0-500 0-769
25 21 72 67 11 13 27
0-761 0-571 0-722 0-716 0-545 0-692 0-815
Females I. Yale-New Haven in-patients IL St. Raphael in-patients IIL Yale-New Haven emergency room patients IV. St. Raphael emergency room patients V. Other Yale and St. Raphael patients VI. Private radiologists' patients
16 17 12 11 9 3
0-500 0-647 0-500 0-364 0-667 0-333
6 10 50 60 29 9
0-833 0-600 0-540 0-583 0-724 0-222
Hospital service Males
TABLE v m
Male cases and matched controls according to weight of what was lifted on the job* Controls
Cases
Total
0 501b.
0
5 1b.
12 2 3 5 6 4
0 0 0 0 0 2
32
2
5-9 lb.
10-24 lb.
25-49 lb.
>501b.
Total
0 0 0 0 1 3
5 1 1 0 6 4
2 1 0 2 3 4
10 2 1 4 8 14
29 6 5 11 24 31
4
17
12
39
106
Z — 0-119 by Wilcoxon matched-pairs signed-ranks test. * Includes pairs in which both members were employed during the year in which the symptoms began. gations are clearly needed to confirm or refute the leads provided in this study. The association between sedentary occupations, especially those which involve driving motor vehicles, and herniated lumbar discs would appear to be real since it was seen in both sexes and in comparison between cases and both control groups. The strength of this association in those aged 35 and older and the lack of association in those who are under that age suggest that a
certain amount of time in sedentary occupations is necessary for an effect to be seen. In fact, a comparison (not shown here) of the number of years cases and controls have had jobs in which they spend half or more of the time sitting confirms this suggestion, for the increased risk is seen only among those who have had sedentary occupations for about five years or more, and appears to become greater with increasing number of years at a sedentary job. Further evidence of an aetiological
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X* tests of association calculated by Mantel-Haenzel procedure: Males only, x* = 0-403, p > 10, estimate of relative risk « 1-20. Females only, *• — 0-922, p > -10, estimate of relative risk = 0-67. Males and females, *• = 0-017, p > -10, estimate of relative risk — 0-94. * Includes those who were employed during the year in which the symptoms began.
204
INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
butable to chance. Other considerations which decrease the likelihood that the association is real are that if the comparison is limited to pairs in which both the case and control worked 20 hours per week or more, the relative risk was estimated to be only about 1-25, and that there were no associations between the weight of what was lifted or the frequency with which it was lifted and the development of herniated lumbar discs. Furthermore, the female cases were somewhat less likely to have jobs involving lifting than the unmatched controls. Thus, although the possibility of a relationship between lifting and herniated lumbar discs in females cannot be ruled out, the present study provides little evidence of such an association in either sex. It is also possible that the rough indicators of lifting used in this study were not sensitive enough to detect a difference between cases and controls, and, indeed, this is an additional reason for further more detailed study. It would seem, nevertheless, that if lifting on the job were an important factor, some association would have been found. Hult (13) and Friberg and Hirsch (14) also concluded that heavy work is not a fundamental cause, although they were concerned with the 'lumbagosciatica syndrome* and with disc degeneration in patients seen at a clinic for back problems, respectively, rather than specifically with herniated lumbar intervertebral discs. This is not to imply that lifting might not play a role in certain other types of back problems, such as muscle strain, but heavy manual labour would not appear to be an important underlying factor in the aetiology of herniated lumbar intervertebral discs. Finally, it should again be emphasized that since this study is apparently the first of its kind, and since it had certain limitations in the study population used, the response rate, the types of questions asked, and the large number of associations examined, it will be important to have' these findings tested in other epidemiological investigations.
REFERENCES
(1) Goodsell, J. O.: Correlation of ruptured lumbar disc with occupation. Clin. Orthop. 50: 225, 1967. (2) Howorth, M. B.: A Textbook of Orthopedics. Philadelphia: Saunders, 1953. (3) Spurting, R. G.: Lesions of the Lumbar Intervertebral Disc. Springfield: Thomas, 1953. (4) Semmes, R. E.: Ruptures of the Lumbar Intervertebral Disc. Springfield: Thomas, 1964.
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role for too much sitting comes from another part of this study, in which it was found that the length of time males reported that they usually sat on weekends was also associated with herniated lumbar discs. The reason for the association with sedentary occupations cannot be ascertained from this type of study. However, two possible explanations are apparent: one that prolonged sitting has an adverse effect, and the other that physical activity is beneficial. It is known that sitting puts more pressure on the discs than either standing or lying down (9-11), and it would seem reasonable that prolonged stress from sitting for long periods of time would increase the probability of a disc herniation. A beneficial effect of physical activity is also plausible, since it would presumably improve the general condition of the muscles and ligaments of spine and thus provide more support for the discs. There was little evidence from this study that physical activity at or away from work decreased the risk for lumbar disc herniations, but this could be attributable to the difficulty of assessing physical activity, especially away from work. In any event, the relative importance of prolonged' sitting compared to lack of exercise remains unknown. The additional risk associated with jobs involving driving motor vehicles was not anticipated when the study began, but can be clearly seen in the comparisons of occupations of cases and controls. In fact, the one single occupation which stood out as having a particularly high risk for herniated lumbar discs in this relatively small study was that of truck driver. Again, the exact reason for this association remains unknown; however, driving for long periods of time requires prolonged sitting in a seat with insufficient support for the low back, with legs extended, in a position subject to the vibration from the road and to mechanical stress from starting and stopping, so it is not surprising that it is associated with an increased risk for disc herniation. In addition, respondents were asked in another part of the questionnaire whether they drove a car; significantly more cases than controls answered affirmatively, thus providing further support for the role of driving in the aetiology of herniated lumbar discs (12). This study gave no evidence of an increased risk for herniated lumbar discs among males who did lifting on their jobs, and little indication of this among the females. The slight positive association between lifting and herniated lumbar discs when female cases were compared to matched controls should be noted, although it is most likely attri-
RELATIONSHIP BETWEEN OCCUPATIONS A N D ACUTE HERNIATED LUMBAR INTERVERTEBRAL DISCS
(10) Caillet, R.: Low Back Syndrome. Philadelphia: Davis, 1968. (11) Nachemson, A.: In vivo discometry in lumbar discs with irregular radiograms. Ada Orthop. Scand. 36: 418, 1965. (12) Kelsey, J. L. and Hardy, R. J.: Driving of motor vehicles as a risk factor for acute herniated lumbar intervertebral disc. Amer. J. Epid. In press. (13) Hult, L.: Cervical, dorsal, and lumbar spinal syndromes. Acta Orthop. Scand., Suppl. 17, 1954. (14) Friberg, S. and Hirsch, C : Anatomical and clinical studies of lumbar disc degeneration. Acta Orthop. Scand. 19: 222, 1950.
{revised version received 11 June 1975)
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(5) Coventry, M. B.: Introduction of Symposium, including anatomy, physiology, and epidemiology. / . Bone Jt. Surg. [Amer.] 50: 167, 1968. (6) Kelsey, J. L. and Ostfeld, A- M.: Demographic characteristics of persons with acute herniated lumbar intervertebral disc. / . Chronic Dis. 28: 37, 1975. (7) Mantel, N. and Haenszel, W.: Statistical aspects of the analysis of data from retrospective studies of disease. / . Nail. Cancer Inst. 22: 719, 1959. (8)Snedecor, G. W. and Cochran, W. G.: Statistical Methods. Ames, Iowa: Iowa State University Press, 1967. (9) Keegan, J. J.: Alterations of the lumbar curve related to posture and seating. / . Bone Jt. Surg. [Amer.] 35: 589, 1953.
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