4.1 Main findings
This study sought to examine the associations between childhood and current socio-economic position, social mobility and LBP outcomes. The main findings suggest, first that low parental socio-economic position remains a risk factor for radiating LBP and sciatica even after taking into account own education. Second, own education had the most consistent associations with radiating LBP. Third, stable lower socio-economic position and downward mobility are strongly associated with radiating LBP particularly among men.
These results confirm the importance of life-course epidemiology in understanding social inequalities in health (Ben-Shlomo and Kuh, 2002; Kuh and Ben-Shlomo, 2004; Kuh et al., 2005; Power et al., 2007). Adverse childhood conditions have been assumed to affect health in adulthood directly (latency model) or indirectly through adult conditions (pathway model), or to have cumulative effects together with circumstances in adulthood (cumulative model) (Ben-Shlomo and Kuh, 2002; Kuh and Ben-Shlomo, 2004; Kuh et al., 2005). In this study, we focused on the latency and cumulative models.
Studies focusing on life-course socio-economic differences in LBP are almost non-existent, and a study among young British adults failed to find an association between manual social class at birth and LBP in early adulthood (Power et al., 2001). However, adverse childhood conditions have been associated with poor physical functioning (Mäkinen et al., 2006) and disability retirement (Harkonmäki et al., 2007). Childhood socio-economic circumstances such as family income reflect, e.g., childhood living conditions and affect morbidity in adulthood independent of adult socio-economic position (Galobardes et al., 2004). Although childhood and adult circumstances can be distinct dimensions, their effects on health in adulthood have both been shown to persist (Hertzman et al., 2001). It is therefore necessary to simultaneously focus on childhood and current circumstances when examining socio-economic inequalities in health. Our results are in accordance with the earlier evidence and support both the latency and the cumulative models, albeit the associations varied by different socio-economic indicators and types of LBP.
Gender difference with respect to having been raised in a farming environment as a risk factor for low back disorders was clear in our study. While girls may have helped more in household activities, boys who were raised in a farm may have participated in the fieldwork with heavy lifting and other physically demanding tasks. These results suggest that early exposures to physically strenuous work may adversely affect the spine and contribute to low back disorders at follow-up. Long-term effects of early exposures have been proposed also previously (Boshuizen et al., 1990).
Although our findings confirmed the association between own education and back pain (Latza et al., 2000), we focused on the associations between childhood socio-economic position and LBP outcomes that remained independent of own education. Additionally, the importance of social mobility needs to be highlighted as the risk of radiating LBP was particularly high among those with stable lower socio-economic position and downward social mobility as measured by educational level of participants and their parents. A recent study found that stable low socio-economic position was associated with pain interfering with work (Lacey et al., 2013). However, childhood socio-economic position in that study was retrospective and based on age when the respondent left school, the study included only older participants and pain was based on a single item. Albeit using a different socio-economic indicator, our findings are further in line with earlier evidence showing that those with stable manual class tend to have poorer physical health (Power et al., 2007) and higher mortality (Hart et al., 1998).
It is plausible that persistent adverse socio-economic circumstances show the strongest associations. One might also assume mobility from low to higher social position (high education) to be protective of low back disorders, e.g., via moving to a job with lower physical demands and better job control; however, this was not found in our study. It is possible that the association between upward social mobility and increased risk of sciatica among men in our study is a chance finding due to a very small number of men in this particular group. Alternatively, early life harmful exposures might also continue to affect the later risk of sciatica also among men with upward social mobility, suggesting long latency.
Childhood socio-economic differences were found for radiating LBP and sciatica. Three groups of risk factors exerting their effects via different pathophysiological pathways could be suggested. These include physical load (biomechanical pathway), health behaviours (mainly via a metabolic pathway) and psychosocial factors (perception of symptoms, coping, etc.). Early physical exposures may cause damage to the spine, inducing a degenerative process and lead to radiating LBP or sciatica. Additionally, those with lower socio-economic position can be more prone to accidental injuries (Laflamme et al., 2009) that again might lead to degeneration and increased risk of low back disorders. Those with lower socio-economic position are also more likely to smoke as well as be physically inactive and overweight. Smoking and overweight have also been identified as risk factors of LBP (Leboeuf-Yde, 1999, 2000; Shiri et al., 2010a). As our outcomes were self-reported, contribution of psychosocial factors cannot be ruled out (Schneider et al., 2005). For example, perception of pain could differ between socio-economic groups. Women also tend to report more symptoms than men (Schneider et al., 2006).
Finally, low back disorders are prevalent and a major cause of work disability (Hagen et al., 2000; Griffith et al., 2012; Saastamoinen et al., 2012). This highlights the societal and public health significance of novel evidence about the determinants of such disorders in childhood and early adulthood. Furthermore, to help employees maintain their work ability, it is important to identify the risk groups for different types of LBP, tackle their causes at an early phase, and promote health and well-being of employed populations and young people entering the labour market.
4.3 Methodological considerations
This study had some limitations. First, LBP outcomes were self-reported. However, pain is a subjective condition, and most epidemiological studies rely on self-reported data (Dionne, 2010). Second, the numbers did not allow for a more detailed analysis of critical and sensitive periods by examining different age cohorts. Future studies could elaborate the sensitive periods and conduct the analyses in different age strata. Third, additional indicators of own socio-economic position such as occupational class and income could shed light on the persistence and course of socio-economic inequalities in LBP. However, in this cohort of young adults, education was considered as the most suitable indicator of socio-economic position. Fourth, childhood pain could also be related to experience of pain in adulthood (Macfarlane, 2010) and this was not assessed in this study. Further elaboration of mediating factors such as own BMI, smoking, and other health behaviours or physical workload was beyond the scope of this study and could be addressed in further studies.
Strengths of the study include a long follow-up of women and men from childhood and adolescence to young adulthood and early middle age. Childhood socio-economic data were based on reports from baseline and thus not sensitive to bias typical to prior studies using retrospective data on childhood socio-economic position (Hardt and Rutter, 2004). Additionally, multiple socio-economic circumstances were examined to shed light on their independent effects. This is of importance as various indicators of socio-economic position are not interchangeable but capture different domains of socio-economic circumstances (Braveman et al., 2005). The opportunity to include own education is a special strength, as we could examine both the independent, direct effects of childhood socio-economic position to LBP outcomes and mediating effects of own education. The inclusion of own education also provided the opportunity to examine social mobility using parental and own education to indicate changes and stability in socio-economic position across life course. A further strength is the inclusion of different LBP outcomes, as many previous studies have only focused on non-specific or any back pain. We not only distinguished between the two main types of LBP, non-specific and radiating LBP, but also studied a clinically defined low back disorder, i.e., sciatica. Focus on these two types of LBP and sciatica helped increase understanding whether they share similar or different risk factors across life course, and highlighted the need to separate non-specific LBP from specific low back disorders.
Childhood socio-economic position contributes to low back disorders in adults. The associations, however, vary for different LBP outcomes and by socio-economic indicator. The found associations remained after considering own education and potential confounders, suggesting that low back disorders have early origins in childhood families and exposures in particular in children with low socio-economic position. Own education contributes to particularly the risk of radiating LBP. Additionally, stable lower socio-economic position and downward social mobility as measured by education are risk factors for radiating LBP. Gender differences likely reflect different exposures and differences in life-course risk factor trajectories among women and men. To prevent low back disorders, childhood socio-economic circumstances, cumulative adversity and long latency of early exposures should be considered.