a The National Institute of Public Health, Section for Epidemiology, PO Box 4404, Torshov, 0403 Oslo, Norway.
b National Insurance Administration, Drammensvn. 60, 0271 Oslo, Norway.
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Abstract |
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Methods In a prospective study, data on all new disability pensioners with osteoarthritis in Norway during the two follow-up periods, 19711980 and 19811990, were analysed by logistic regression. The study include data on all subjects living in Norway and registered as 5056 years old and employed either in the census collected in 1970 or in the census of 1980.
Results Manual workers have nearly twice the probability of becoming a disability pensioner with osteoarthritis compared to professionals after adjusting for part-time work, income, level of education, marital status and gender. Adjusted for other risk factors, the probability of becoming a disability pensioner with osteoarthritis was three times higher in the 1980s compared to the 1970s.
Conclusion The relatively strong association between manual work and disability pensioning with osteoarthritis suggests difficulties in adjusting manual work patterns for a person with osteoarthritis, which may have increased during the study period as implied by the separate effect of the 1980s.
Keywords Disability pension, osteoarthritis, multivariate logistic regression, prospective studies, risk factors, manual work
Accepted 5 January 2000
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Introduction |
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In Norway in 1990 disability pensioners with osteoarthritis (DPOA) comprised 4.3% of all disability pensioners (5.0% of females and 3.5% of males). In 1990 new DPOA cases were 0.65 per 1000 men 1666 years old without a disability pension and 0.92 per 1000 women 1666 years old (1991).2 For 50 59-year-old men and women there has been an increase in the annual incidence of disability pensioning with osteoarthritis over the period 19681990 (unpublished work by the same authors).
Risk factors for becoming a disability pensioner with a musculoskeletal diagnosis which have been published are female sex,24 old age,2,4,5 not being married,3,5,6 low level of education,47 low socioeconomic status,811 low income,6 working as shop assistant, nurse aide or charlady among women,12 and heavy occupations among men.5,10,13
The risk factors reported in previous studies are highly interrelated; people with a low level of education more often have poorly paid, physically demanding jobs than do those with a high level of education.14 Married women more often work part time than do single women15 and part-time work is more common in physically demanding jobs. The occupations associated with increased risk of becoming DPOA for women are physically demanding, require little education, and are associated with part-time work. If manual work remains a risk factor after adjusting for the other factors, the nature of this association will be discussed.
The objective of this study is, based on prospective data of those employed and 5056 years old at the start of a follow-up period, to assess if manual work is an independent predictor of becoming DPOA during the periods 19711980 and 19811990, after adjusting for working part time, income, level of education, marital status and gender. We also wanted to examine any difference in occurrence or impact of risk factors of becoming DPOA between the periods 19711980 and 19811990 after having adjusted for type of work, education, part-time work, income, marital status and gender.
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Method |
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Pensioners can be granted a reduced disability pension according to judged degree of incapacity (5090%). Of all new DPOA in 1990, 43.6% of females and 18.3% of males held a reduced disability pension.2
Our analyses include the total population of subjects in Norway without a disability pension who were registered as 5056 years old and considered as employed either in the 1970 census or in the 1980 census. Using the data registered by the National Insurance Administration (NIA), those who had been granted a public disability pension during a 10-year follow-up period after the census and still held this pension by the end of the year the pension was granted, were identified. All the new disability pensioners with a main diagnosis of osteoarthritis, both with a reduced and a full disability pension, were included as cases.
The lower age limit was chosen because disability pension with osteoarthritis is very rare for people younger than 50. As those registered as older than 56 years in the census should be censored when they were granted a retirement pension during the follow-up period, the upper age limit was chosen as 56 years.
The 1970 census comprised 168 793 men and 171 996 women 5056 years old in 1970 and the 1980 census comprised 152 015 men and 152 038 women at the same age in 1980. Of these, 154 711 (91.2%) men and 82 889 (48.2%) women were employed with known occupation, income and hours worked in 1970. The corresponding numbers for 1980 were 121 617 (80.0%) for men and 85 826 (56.5%) for women. The number of new DPOA among the employed in the selected age group during the follow-up period 19711980 was 1085 men and 1029 women. The corresponding numbers for 19811990 were 2058 and 2566.
As we wanted to assess the risk related to manual work adjusted for income and part-time work, people not gainfully employed in the censuses were excluded from the analyses. The unemployment rate in Norway was low both in 1970 and 1980, but increased from 1983.16
Measures
A disability pension is applied for at the local insurance office, which requests a health certificate from the applicant's physician. The local insurance office then judges whether the patient meets the criteria for being granted a disability pension.17 If the local insurance office questions the diagnosis or the patient's health status, it can ask for an examination by a specialist or can have the patient examined by a physician employed by the NIA. The applicant will have had one year with sickness benefit and usually at least one year of rehabilitation before a disability pension is applied for.
Health certificates are evaluated by the NIA, and illnesses, injuries and defects are classified according to the International Classification of Diseases (ICD). The NIA used ICD-7 (Seventh Revision) during the period 19671982, ICD-8 during the period 19831986, and ICD-9 during the period 19871993. The diagnoses registered according to ICD-8 have been recoded to ICD-9 codes by the NIA.
Osteoarthritis was identified as the codes 7230 in ICD-7 and 715 in ICD-9.
The predictor variables from the census were marital status, duration of education, income, hours worked the previous year, and occupation. Marital status was classified as never married, married, widow(er), divorced or separated, the last three categories being recoded into previously married. Duration of education was recorded as total years. The variable 'number of hours worked the previous year', was dichotomized as part time (<1300) and full time. The occupation coded according to Nordic Classification of Occupations,18 was recoded into three groups, manual workers, including both skilled and unskilled labourers, routine non-manual workers and professionals.18,19 These groups have very different work environments, the labourers lift and bend more often than the two other groups and the professionals are more free to define their working pace.20
Personal identification numbers were removed from the data before being released for research.
Analysis
Cumulative incidences for each of the 10-year follow-up periods were calculated with 95% CI for all registered subjects, employed 5056 years old people without a disability pension, at the start of the follow-up period. Population attributable proportion was calculated as [(incidence in population) (incidence in unexposed group)]/(incidence in population).21
Logistic regression was chosen as the tool of analysis. The odds ratio can be interpreted as an approximate relative risk when the events are rare (<10%). To deal with the problem of right censured data, people dying or becoming a disability pensioner with any other diagnosis than osteoarthritis were excluded from these analyses.22
As no one could be included in the study from both censuses, the data from both periods (19711980 and 19811990) were joined into one dataset with a period variable (0 if 19711980 and 1 if 19811990).
The size of the risk may depend on the level of a third variable and this made it necessary to test for interaction between the chosen variables. It was decided that if there was a significant interaction effect between i.e. gender and the other variables in the analysis then sex-wise re-analyses would be undertaken. A corresponding procedure was followed for the other variables until all variables were tested for significant interaction terms with the remaining variables, and there seemed to be no need for further re-analysis.
Analyses with age entered as a continuous variable compared with analyses in which different age groups were entered as dummy variables show no non-linear effect of age, implying that age could be entered as a single, continuous variable without loss of information. The same proved to be true for education.
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Results |
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The pooled data were tested for significant interaction effects between the variables in Table 2. There were significant interaction effects between gender and each of the following variables: age, education, type of work, part-time/full-time work and income. The data for men and women were re-analysed separately. Similar tests were done for interaction effects between marital status and the other variables in the model, and interaction effects were significant. The period variable was similarly tested and also showed significant interaction effects with remaining variables in the model. New multivariate analyses were therefore conducted stratified by gender, marital status and period.
In stratified multivariate analyses manual work was a risk factor for becoming DPOA for both men and women during the 1970s and the 1980s (Table 3).
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Discussion |
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The data from the census of the whole population of Norway were edited by Statistics Norway, which secures high data quality. Disability pensioners dying the same year the disability pension was granted were excluded, but the general mortality in the age group we studied is low, and osteoarthritis is not a fatal disease. The classification of osteoarthritis by the physicians has probably not changed over the period. A Danish study found just 2% misclassification of the disease (referred to in ref. 23).
The increasing unemployment of the 1980s might have increased the number of applicants for disability pension.24 Of those employed and 5056 years old in 1970, 20.6% become a disability pensioner during 19711980. Of those employed and 5056 years old in 1980, 30.6% become a disability pensioner during 19811990. The increase in DPOA, as well as general disability pensioning, might be associated with the increasing unemployment with physicians using the diagnosis of osteoarthritis for applicants who have become unemployed. The increase in DPOA was larger than the general increase in disability pensioning, as new DPOA in the studied population was 3.4% of all new male disability pensioners during 19711980 and 6.2% during 19811990; the corresponding increase for women was from 5.8% to 9.6%.
By comparing the new disability pensioners in the years 1971 and in 1981 in the same way, we find that the proportion of DPOA of all new disability pensioners was larger in 1981, 2.8% for men and 6.2% for women, than in 1971, 1.6% for men and 3.7% for women. Thus the relatively stronger increase in the number of DPOA is not just a result of the unemployment in the 1980s, as unemployment in Norway did not increase until 1983.
Some occupations have previously been shown to be associated with osteoarthritis.13,23,2528 Data on occupation registered in the censuses might not be suitable for identifying these occupations as people may have changed occupation to avoid painful tasks29 or continue to be employed is spite of less physical fitness.30 However, as we have data on subjects 5056 years old in Norway, we consider change in occupation for this group to be of little importance.
The association between manual work and becoming DPOA might in part be due to poor health in general, which is associated with low social class i.e. manual work.14,31,32 Lower social class often implies low income, and low income is strongly associated with poor health.33 We found separate effects of each of the variables associated with social class (education, type of work and, in most groups, income) after adjusting for the others. These results demonstrate that the three variables not only mediate the effect of social status, but also contribute with separate, unique effects.
The higher risk of becoming DPOA among manual workers during the 10-year follow-up period might reflect that some manual occupations cause osteoarthritis, because of the higher workload. But the literature is inconclusive as to risk factors for both hip and knee osteoarthritis for women and hip osteoarthritis for men.34 Heredity35 and sports35,36 seem to be more important risk factors than workload. Obesity also seems to predict osteoarthritis.35,37
The apparent increased risk might also reflect that a manual worker with a musculoskeletal disorder like osteoarthritis might have less possibility of adjusting the work environment to make it possible to remain in work in spite of reduced physical capacity.38
Even if we can improve the work environment for manual workers to reduce the risk of DPOA, the number of new DPOA might not be reduced in all subgroups of the population. The association between manual work and DPOA is, for instance, weak among never married women (Table 3). Manual work considered as the only risk factor is probably insufficient, and a combination of risk factors, as in Table 4
, may provide a better view of the problem. The proportion of people in this age group doing manual work is decreasing (Table 1
). The population attributable proportion for manual work is also decreasing and is relatively small for women. The size of the population attributable proportion for women might reflect that typical female professional occupations, e.g. registered nurse, are physically demanding.
The threefold increase in becoming DPOA in the 1980s after adjusting for group, age and gender, imply that there are factors that influence the probability of becoming DPOA that were not included in the present study. Such factors might be less willingness to invest in adjustment of workplace, larger demands on the level of physical ability to stay employed, increased competition due to increased unemployment or poorer health or less tolerance of pain among those born 19241930 compared to those born 19141920.
However, the proportion becoming DPOA was only 4.0% among women and 3.0% among men during the period 19811990 even among people with the highest risk, those with 7 years of education, who worked part time in manual work.
The results, however, cannot answer to what extent manual work causes osteoarthritis and to what extent the increased disability pensioning among manual workers is a result of differential possibilities of coping with different types of work and should be looked into in other studies.
Often musculoskeletal diagnoses are treated as one group in epidemiological studies,29,39,40 implying that the various diseases have more or less the same epidemiological risk factors. Different musculoskeletal disorders have different aetiology and affect various parts of the body. The results from this study apply to osteoarthritis and may not be generalizable to other musculoskeletal diagnoses. Similar studies of disability pension with other musculoskeletal diagnoses should be conducted.
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Acknowledgments |
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References |
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