Methods
Participants
Cross-sectional data were obtained from adolescents aged (mean (standard deviation)) 17.0 (0.3) years, 52.8% who were female, participating in the West Australian Pregnancy Cohort "Raine" Study (http://www.rainestudy.org.au). This long-term project began as a pregnancy cohort in which 2,900 women attending antenatal clinics at a tertiary level obstetric hospital in Perth, Western Australia were enrolled between 1989 and 1991. 2,868 children born to 2,804 mothers remained with the study to form the Western Australian Pregnancy Cohort (Raine) Study. 1475 of the original participants completed some aspect of the 17 year follow-up (three questionnaires and a physical examination), and 1307 (88.6%) completed the paper questionnaire covering LBP prevalence, LBP impacts, and the SF-36 Version 1 Health Survey. A comparison of the Raine cohort at birth compared to the general Western Australian population using the Western Australian Maternal and Child Health Research Database found the Raine cohort to be at higher risk in terms of prenatal and perinatal characteristics and to be overly representative of socially disadvantaged families, whilst a comparison of the characteristics of participants versus non-participants 14 years after cohort inception found that socially disadvantaged families were less likely to remain in the cohort to age 14 years. However, when participating families in the 17 year follow-up were compared to the Western Australian population of families with 15 to 17 year old children it was found that the Raine sample had a lower proportion of rural dwelling families (18.4% versus 33.9%, p < 0.001), a slightly higher proportion of urban dwelling families in high socioeconomic status neighbourhoods (23.6% versus 20.6%), and a slightly lower proportion of families with a combined family income of less than AUS$25,000 (7.9% versus 10.8%). Ethnicity in the cohort is predominantly Caucasian (93%). Ethical approval was granted from Curtin University Human Research Ethics Committee and the West Australian Department of Health Ethics Committee. Consent was obtained from the adolescents' guardians.
Measures
The prevalence of LBP and its impact, and the presence of mid-back pain (MBP) and neck and shoulder pain (NSP), were ascertained from the questions in Table 1, which along with the SF-36 Version 1 were administered as part of a larger questionnaire examining multiple lifestyle and medical dimensions. 1246 subjects provided valid SF-36 and pain data, while 1283 subjects provided valid impact and pain data. MBP and NSP were included in the catagorisation of pain given the potential significant impact that multiple bodily pain areas may have on the interpretation of LBP related impact.
Pain
The experience of LBP was ascertained via the Nordic LBP questionnaire, with modifications based on subsequent research. LBP in the sample was characterised by the formation of three mutually exclusive LBP prevalence groups according to answers to questions P1 to 3 in Table 1; i) no current LBP (NO_LBP; 'No' to P1), ii) current non-chronic LBP (CNC_LBP: 'Yes'to P1, and 'No' to P2 and P3) iii) current chronic LBP (CC_LBP: 'Yes'to P1, and 'Yes' to P2 or P3). Pain comorbidities in other spinal regions, regardless of LBP status, was characterised by formation of three mutually exclusive groups according to answers to P4 and P5 in Table 1; i) No MBP or NSP ('No' to P4 and P5) ii) MBP or NSP ('Yes to one only of P4 and P5) and iii) MBP and NSP ('Yes to both P4 and P5).
Specific Impacts of Low Back Pain
LBP impacts were ascertained from questions I1 to I5 in Table 1. These represent previously validated questions that capture different aspects of impact associated with LBP. In those subjects with current LBP, a count variable indicating number of LBP impacts experienced was also derived.
Health-related Quality of Life
The SF-36 is a widely used HRQOL questionnaire. This study used the SF-36 Version 1 which has been used in 130 Australian studies and validated in several. The SF-36 is a generic instrument for assessment of HRQOL consisting of 36 items measuring: physical functioning, role physical, bodily pain, general health, vitality, social functioning, role emotional, and mental health. Factor analytic studies in a number of populations have confirmed the existence of two higher order factors representing physical and mental health factors, the Physical Component Summary (PCS) and Mental Component Summary (MCS) measures. PCS and MCS measures were calculated using Australian factor weightings, based on the Australian National Bureau of Statistics 1995 Australian National Health Survey dataset and scored on a 0–100 scale and normalised to have a mean of 50 and a standard deviation of 10, based the 1995 Australian National Health Survey data.
Data Analysis
Chi-squared tests were performed to test associations between LBP group and gender, and LBP group and other spinal pain areas. In those subjects with current LBP only, chi-squared tests were performed to test associations between LBP group and specific LBP impacts, and chi-squared test for linear trend of association between LBP group and count of LBP impact.
In all subjects, differences in LBP groupings between SF-36 health status scores were estimated using linear regression models with SF36 scores as the dependent variable and the three category variable 'LBP group' parameterised by two dummy predictor variables, one representing CC-LBP and one representing CNC-LBP. Final estimates of LBP group differences were adjusted for gender and presence of other spinal pain areas. Statistical significance was adjusted to α = 0.008 to account for multiple tests of 2 outcomes and 3 pain groups. Interactions between gender and LBP group, and between other spinal regions (MBP, NSP) and LBP group were tested with statistical significance at α = 0.05. Model diagnostics confirmed homogeneity of residual variance and absence of influential cases. Statistical analysis was performed with Stata/IC 10.1 for Windows (Statacorp LP, College Station TX).