Collection of information
As part of the Trøndelag Health Study, the survey HUNT2 was carried out in Norway in 1995-1997, and the survey HUNT3 was conducted approximately 11 years later in 2006-2008 [20, 21]. All residents at the time in the former Nord-Trøndelag county aged ≥ 20 years were invited to participate in each survey. They were requested to fill in questionnaires on health status and to take part in clinical consultations, which included measurement of height and weight.
One question in the HUNT2 and HUNT3 questionnaires was formulated in this way: “During the last year, have you suffered from pain and/or stiffness in your muscles and joints that has lasted for at least 3 consecutive months?ˮ Each participant answering yes was given the following question: “Where did you have these complaints?” Several body regions were listed. Respondents answering yes to the first question and including the lower back as a relevant region were regarded as having chronic LBP [22].
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In HUNT2 the participants gave information about use of MHT by answering this question: “Have you ever used medicines containing oestrogen?” Examples of common brand names were displayed, and it was emphasised that the question did not refer to OC use. The questionnaire distinguished between use of tablets or patches and use of cream or suppositories. In each case, the respondents were requested to indicate whether they had now, previously or never been engaged in the kind of use considered and to specify duration of usage. Finally, current users were requested to supply the particular brand name of the product. Use of tablets or patches was regarded as systemic MHT while cream or suppositories represented local use. The proportion of women providing information about possible local use was much lower than for systemic use, and only systemic use is considered in the present study. On the basis of brand names, current systemic MHT use was either classified as containing oestrogen only or as representing a combination of oestrogen and progestin.
The women provided information about use of OC by answering the following question: ”Have you ever taken contraceptive pills, including mini-pills?” Women who had ever used OC were then asked whether they were still OC users. They were also requested to indicate the duration of OC use.
Women participating in HUNT2 gave information on age at menarche by answering the question “How old were you when you started menstruating?” The participants also gave information regarding physical activity in leisure time, smoking, duration of education and childbirths. In addition, they provided information used for computing Hospital Anxiety and Depression Scale (HADS) scores [23].
Study design
Use of MHT
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The study of associations with MHT was restricted to women in the age range 40-69 years. The corresponding target population in HUNT2 comprised 20,765 women. Of these, 17,568 attended the HUNT2 survey (Fig. 1). A total of 4574 women were excluded from this study because of missing information on chronic LBP or MHT, and 20 women were excluded because they were pregnant when the survey was carried out. Information on chronic LBP and use of MHT in HUNT2 was collected from the remaining 12,974 women included in the cross-sectional study, corresponding to a participation rate of 62%.
The 8967 women who did not report chronic LBP in HUNT2 were included in the 11-year follow-up (Fig. 1). Information about residence status was collected from national registries and linked by the unique personal identification numbers being used in Norway. During follow-up 451 women died, and 223 left the county. A total of 2286 women who lived in the county at the time of HUNT3 did not participate in HUNT3 or did not supply information on chronic LBP. Thus, 6007 women were included in the analysis of risk of chronic LBP after follow-up, representing 72% of the women remaining in the county and 67% of the original cohort.
Use of OC
The target population in HUNT2 comprised 37,503 women in the age interval 20-69 years. Of these, a total of 28,520 women attended the HUNT2 survey (Fig. 2). Information on chronic LBP and use of OC in HUNT2 was collected from 23,593 women, corresponding to a participation rate of 63%. This data set formed the basis of the cross-sectional study of associations with use of OC.
The subset of 17,508 women who did not report chronic LBP in HUNT2 were included in the cohort study (Fig. 2). During the period of follow-up, 569 women died and 1085 left the county. A total of 5268 women did not participate or did not supply information on chronic LBP. Altogether 10,586 women were available for analysis of risk of chronic LBP after follow-up, representing 67% of the remaining women resident in the county and 60% of the original cohort.
Variables
Use of systemic MHT as reported in HUNT2 was considered in 3 categories as never, former and current use. Duration of systemic MHT use included 5 categories corresponding to never use, 1 month-2 years, 3-5 years, 6-8 years and ≥ 9 years. For type of systemic MHT among current users, categories represented therapy based on oestrogen only or combination therapy, in addition to never use. Use of OC was also categorised as never, former and current use. Duration of OC use included 5 categories, corresponding to never use, 1 month-4 years, 5-9 years, 10-14 years and ≥ 15 years. In all situations, never use was regarded as reference category.
Body mass index (BMI), defined as weight/height2 and computed in kg/m2, was subdivided into 3 groups: < 25, 25-29.9, ≥ 30. For physical activity in leisure time, including going to work, the first category represented those engaged in light activity only or hard physical activity (leading to sweating or being out of breath) < 1 h per week. Other categories represented hard physical activity 1-2 and ≥ 3 h per week. The information about physical activity collected in HUNT2 was verified by a reliability and validity study of a subsample [24]. Education was grouped according to duration as ≤ 9, 10-12 and ≥ 13 years. Categories of cigarette smoking represented current daily smoking, previous daily smoking and never daily smoking.
Age at menarche was categorised into 7 groups: ≤ 11, 12, 13, 14, 15, 16, ≥ 17 years. A particular variable was introduced to take into account both nulliparity and age at first childbirth (in 5-year categories) among parous women. Women who were pregnant at the time of HUNT2 constituted a separate category. Total HADS scores were categorised into 5 intervals, 0-4, 5-9, 10-14, 15-19 and ≥ 20, to obtain a relatively detailed representation of psychological factors.
Statistical analysis
Relationships in the data set analysed between use of systemic MHT or OC and other potential risk factors were described in an exploratory approach by tabulating mean values or frequency distributions over categories of MHT and OC. To assess the extent of differential participation at end of follow-up, similar tabulations were performed among the women who were residents of Nord-Trøndelag at the time of follow-up but did not participate, and among those who moved out of the county or died during follow-up.
Generalised linear modelling for binomial observations with a log link was applied to both cross-sectional and cohort data to study associations between use of MHT or OC and chronic LBP. The initial analysis incorporated adjustment for age only. Additional adjustment was then introduced for other relevant factors known to be risk factors for LBP and suspected to be associated with use of MHT or OC. This involved BMI [25,26,27], physical activity in leisure time [28,29,30], education [31,32,33] and smoking [34,35,36]. In view of the associations established previously in this data set [18, 19], adjustment was also made for age at menarche, nulliparity and age at first childbirth. All variables adjusted for except age were regarded as categorical. The continuous non-linear effect of age [25] was represented by a cubic polynomial.
In a minor subset of the data, information about potential confounders was not available, and analyses with comprehensive adjustment were based on a lower number of women than purely age-adjusted analyses. In particular, HADS scores could not be determined among 1833 women (9.3%) of the 19,637 women included in the cross-sectional adjusted analysis of associations with use of OC. Moreover, HADS scores were not available for 883 (9.7%) of the 9113 women in the adjusted cohort analysis. As HADS scores express essential psychological components potentially associated with risk of LBP [37], use of MHT [38] and OC [39], additional adjustment for HADS was made in particular sensitivity analyses. To facilitate comparison, similar analyses were also carried out with no adjustment for HADS including only the women with known HADS scores.
Separate tests were performed for interaction between use of MHT or OC and each factor adjusted for and between use of MHT and OC, in the main cross-sectional and cohort analyses with comprehensive adjustment.
All statistical analyses were carried out using IBM SPSS version 26 (IBM Corp., Armonk, New York).
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