Introduction

Research into the epidemiology and treatment of low back pain has exponentially increased over the past few decades, and at least matches the increase in output in other research fields. Fig. 1 shows the increase in the number of randomised controlled trials in back pain patients (cited in Medline) over the past 30 years, which increased from less than 30 in the period from 1983 to 1987 to more than 650 over the past 5 years. The graph also highlights the increasing proportion of trials conducted in primary care, workplace settings and the community, emphasising the rising awareness that low-back pain has wide impact in the community, and is mostly managed within primary and occupational healthcare.

Randomised controlled trials in low back pain (publications cited in Medline); trials in primary care, workplace or community settings are highlighted.

Low back pain is a challenge to both clinicians and researchers, due to the wide variability in clinical presentation; lack of consensus regarding diagnostic criteria or clinical classification; large variation in course and prognosis; and limited success in identifying effective treatments. However, increasing research efforts has generated an expanding body of evidence on the epidemiology, prognosis and treatment of back pain, which has led to new insights and changes in thinking regarding the classification and management of back pain. Four themes emerge from these developments that cut across several topics: (1) the increasing awareness of the impact of low back pain on population health; (2) the realisation that the current classification of low back pain (in terms of acute/subacute or chronic pain) is not adequate, leading to the proposition of new approaches to describing and investigating the course and prognosis of back pain; (3) the need to better understand the mechanisms or pathways that explain impact and long-term outcomes in patients with back pain; and (4) the potential of individualised or stratified models of care to improve patient outcomes and efficiency of care for back pain. In this chapter we will discuss the developments across these four themes, and how these may influence future direction in back pain research.

Increasing awareness of the population impact of low-back pain

The large personal, economic and societal burden of low-back pain has been emphasised in numerous publications, but until recently good quality and up-to-date systematic reviews of the prevalence and impact of low back pain were lacking. The recently published meta-analysis by Hoy et al. summarised evidence from 165 studies in 54 countries, presenting a pooled estimate of the mean (SD) point prevalence of activity-limiting low back pain of 11.9 (2.0)%, and 1-month prevalence 23.2 (2.9)% after adjusting for methodological variation between studies. The authors emphasise the heterogeneity between studies in terms of variation in the definition of back pain, risk of bias (representativeness of the sample and response rates), and study setting. They highlight the fact that the evidence is still very much weighted towards studies conducted in developed countries and adults of working age, with available studies showing a lower prevalence of back pain in developing countries and in older people. The lower prevalence of back pain in low- and middle-income countries could be the result of higher levels of physical activity, higher pain tolerance, or shorter height in these populations , but differences in health priorities, limited access to healthcare, or lack of health insurance or disability compensation systems may also lead to a lower reporting of back pain in these populations. Other studies have also reported a decreasing prevalence of back pain in older populations , but as discussed by Dionne et al. this may be explained by a range of factors, including decreased pain perception or increased tolerance or acceptance of pain in older age, the accumulation of other health problems which may take priority, increasing influence of mental health problems such as depression or cognitive impairment, and exclusion from studies of individuals living in nursing homes. Studies investigating pain in older people indicate that the incidence of more severe, disabling pain continues to increase in older age , and that back pain has larger impact in older age .

Awareness of the public health impact of back pain has been boosted by the recent publication of the results of the Global Burden of Disease (GBD) projects, which featured back pain as the condition with the highest impact in terms of years lived with disability . Buchbinder et al. have discussed the difficulties of defining impact and constructing disability weights for low back pain in the GBD projects, which were related to inconsistent definitions of back pain, limited data from developing countries, lack of data on incidence, duration and recurrences of back pain, and limited data on the distribution of severity of back pain in populations. This led to wide uncertainty of estimates, and only allowed the use of a definition of back pain in terms of duration (more or less than 3 months) and presence of leg pain. Buchbinder et al. highlight the importance of more sophisticated descriptors of back pain for population-based studies, and in previous work have identified dimensions of back pain impact that are rarely measured in population-based research, such as loss of independence, worry about the future, and negative or discriminatory actions by others .

Increasing awareness of the population impact of low-back pain

The large personal, economic and societal burden of low-back pain has been emphasised in numerous publications, but until recently good quality and up-to-date systematic reviews of the prevalence and impact of low back pain were lacking. The recently published meta-analysis by Hoy et al. summarised evidence from 165 studies in 54 countries, presenting a pooled estimate of the mean (SD) point prevalence of activity-limiting low back pain of 11.9 (2.0)%, and 1-month prevalence 23.2 (2.9)% after adjusting for methodological variation between studies. The authors emphasise the heterogeneity between studies in terms of variation in the definition of back pain, risk of bias (representativeness of the sample and response rates), and study setting. They highlight the fact that the evidence is still very much weighted towards studies conducted in developed countries and adults of working age, with available studies showing a lower prevalence of back pain in developing countries and in older people. The lower prevalence of back pain in low- and middle-income countries could be the result of higher levels of physical activity, higher pain tolerance, or shorter height in these populations , but differences in health priorities, limited access to healthcare, or lack of health insurance or disability compensation systems may also lead to a lower reporting of back pain in these populations. Other studies have also reported a decreasing prevalence of back pain in older populations , but as discussed by Dionne et al. this may be explained by a range of factors, including decreased pain perception or increased tolerance or acceptance of pain in older age, the accumulation of other health problems which may take priority, increasing influence of mental health problems such as depression or cognitive impairment, and exclusion from studies of individuals living in nursing homes. Studies investigating pain in older people indicate that the incidence of more severe, disabling pain continues to increase in older age , and that back pain has larger impact in older age .

Awareness of the public health impact of back pain has been boosted by the recent publication of the results of the Global Burden of Disease (GBD) projects, which featured back pain as the condition with the highest impact in terms of years lived with disability . Buchbinder et al. have discussed the difficulties of defining impact and constructing disability weights for low back pain in the GBD projects, which were related to inconsistent definitions of back pain, limited data from developing countries, lack of data on incidence, duration and recurrences of back pain, and limited data on the distribution of severity of back pain in populations. This led to wide uncertainty of estimates, and only allowed the use of a definition of back pain in terms of duration (more or less than 3 months) and presence of leg pain. Buchbinder et al. highlight the importance of more sophisticated descriptors of back pain for population-based studies, and in previous work have identified dimensions of back pain impact that are rarely measured in population-based research, such as loss of independence, worry about the future, and negative or discriminatory actions by others .

Re-defining back pain

The classification of back pain has posed many challenges, given the fact that a specific patho-anatomical cause of the back pain can only be established in a minority of patients. The large majority of patients with ‘non-specific’ low back pain form a heterogeneous group with widely varying presentations of the condition. A classification based on duration of the current episode (acute: less than 6 weeks; sub-acute: 6–12 weeks; chronic: more than 12 weeks) has been used for many years to distinguish between patients with favourable or poor prognosis, to present treatment recommendations in guidelines, to define eligibility for trials, or to classify back pain studies in systematic reviews. However, realisation has grown that a classification based on episode duration alone does not provide the information required to describe the impact of back pain, accurately estimate prognosis or to make optimal treatment decisions . Dunn et al. demonstrate that evidence from prospective cohort studies does not support a model of back pain characterised by a series of individual, unrelated episodes, but more clearly points towards back pain as a long-term condition. A recent systematic review of the natural history of back pain, which included seven population-based studies with up to 28 years follow-up concluded that the status of low-back pain in individuals is relatively stable over time, with the majority reporting persistent or recurrent symptoms over long-periods of time . These findings are confirmed by recent studies investigating trajectories of back pain based on monthly or weekly assessments of pain and disability, which identified distinct back pain trajectories of differing severity and impact, but most of which are stable over long periods of time with only mild fluctuations . Although the course of back pain has mainly been studied in adults, back pain may have its onset early in life. Based on 59 studies Calvo-Muñoz et al. estimated the mean point prevalence of back pain in children and adolescents at 12% (95% CI: 90. to 15.9%), with increasingly higher prevalence rates being reported in more recent and better quality studies. The early onset of the condition and its long-term course confirm the need for a life-time perspective when investigating and managing back pain, and re-defining back pain as a long-term condition rather than managing individual episodes of back pain.

Back pain can be presented along a continuum from incidental, mild episodes of pain to a chronic health condition strongly interfering with daily life. People who experience multiple episodes of back pain seem to accumulate risk of chronic disabling pain, and appear more susceptible to the influence of a range of prognostic factors. For example, in a population-based cohort study with 12 years follow-up, Brage et al. found that emotional distress was a predictor of long-term low back disability, but only in individuals with previous back pain. Accumulation over the life course of pain episodes, but also of exposures to physical, psychological, social or environmental stressors (allostatic load ) may all contribute to the risk of developing chronic disabling back pain . A challenge for research is to investigate this accumulating load over the life-course, and how this may be balanced by individuals’ resources (resilience) to deal with such adversities . Such knowledge would provide better insight into the reasons why some individuals are more likely than others to experience more chronic or severe trajectories of back pain.

Back pain is often more than just pain in the back. Hartvigsen et al. summarise the findings from numerous studies which indicate that the majority of people with back pain have pain elsewhere, and that the extent of pain is strongly associated with reduced physical functioning, feelings of anxiety and depression, work absence, and reduced quality of life. For example, a Norwegian survey showed that only 16.8% of responders ( n = 3179) reported localised musculoskeletal pain, whereas 53% reported pain in more than one site. Using data from a national comorbidity survey ( n = 5692) Von Korff et al. found that the prevalence of chronic back pain was more than three times as high among persons who reported other chronic pain conditions as those without these conditions: 34.1 vs 9.6%. If multisite pain is that common, the location of pain, whether in the back or elsewhere may be of little importance when assessing the consequences of pain. Using data from a population-based study in the Netherlands Fig. 2 presents the proportion of individuals with increased anxiety and depressive symptoms among those with either back pain ( n = 256), headache ( n = 287), abdominal pain ( n = 96), or hand pain ( n = 129). These were mutually exclusive groups, although they could have other co-occurring symptoms. The results show similar levels of anxiety and in particular depression regardless of the location of pain. Distress was more strongly associated with the extent of pain (widespread pain) than with pain location.

Increased anxiety and depression scores (HADS > 7) in people (population-based sample in the Netherlands) reporting no pain ( n = 346), back pain ( n = 256), headache ( n = 287), abdominal pain ( n = 96) or hand pain ( n = 129) or chronic widespread pain ( n = 322).

Other studies have also shown similarities across regional pain problems in terms of the course of pain and the type of prognostic factors, identifying generic predictors of outcome across different regional musculoskeletal pain conditions . A prognostic score for estimating risk of chronic disabling pain developed originally in back pain patients has been shown to accurately predict outcome in people with knee pain and across a range of musculoskeletal pain sites . Hartvigsen et al. therefore propose a stepwise approach to the diagnostic classification and management in patients with musculoskeletal pain, regardless of pain location, of which clinical and cost-effectiveness could be investigated in future research.

Understanding the complexity of back pain

There has been increasing interest in the role of psychological and social factors in the onset and progression of low-back pain over the past 30 years, and this has resulted in an increasing number of trials investigating interventions that assess or address psychosocial factors in back pain patients (see Fig. 3 ). However, the results of these trials have often been disappointing, showing only small to moderate effects of a wide range of treatments for back pain [e.g. Refs. ].

Randomised controlled trials in low back pain (publications listed in Medline); trials addressing or assessing psychosocial factors have been highlighted.

Several authors have argued that we should move beyond the ‘black box’ approach of pragmatic trials and increase our understanding of theory and the mechanisms underlying proposed interventions. Pincus et al. argue that sub-optimal results of intervention studies may be the results of low competency or fidelity in delivery of interventions, or inadequate matching of interventions to specific patient problems. They feel that strong theoretical guidance is essential in the design, selection, and delivery of treatment methods that specifically target known processes of pathology. When delivering an intervention study, additional data collection before, during and after the treatment period will allow a more thorough evaluation of the processes that may explain response to treatment in trial participants (mediation analysis). The methods to support treatment mediation analysis are still being developed, but offer strong opportunities to better understand the treatment processes and mechanisms and have the potential to further improve the design or delivery of interventions for back pain .

A strong theoretical or empirical basis for intervention studies requires sufficient knowledge of the role of biological, psychological and social factors in the onset and progression of low-back pain. Research into social factors is limited compared with the large body of evidence on biological and psychological aspects of back pain. With the exception of the psychosocial and physical work environment which has been extensively investigated in occupational settings [e.g. Refs. ], there is less evidence on the importance of social support and influences of partner, family and friends, social roles, and social factors in communication with health professionals . The literature shows inconsistency regarding concept definitions, but social support from family, friends and social groups in general appears to facilitate a more rapid recovery from back pain . The evidence, however, is not consistent and the associations between social factors, emotional responses, and clinical aspects of the pain condition are complex. For example, in a prospective study among primary care patients with acute or subacute back pain, solicitous partner responses (e.g. getting the person in pain to rest, taking over their jobs and duties) were associated with a favourable course of back pain , whereas in another, cross-sectional study solicitous partner responses were associated with higher levels of disability, although only in those with low levels of depression. Campbell et al. suggest that solicitous behaviours from partners may be a barrier to recovery by increasing disability but secondly facilitate recovery by reducing depression. Further research is clearly needed to understand the longitudinal relationships between social influences, quality of relationships, emotional well-being and pain or disability. The influence of social factors may vary across the life course . For example, associations have been found between back pain reported by children and lifetime back pain prevalence in their parents. Genetic factors, shared environmental factors or social learning of illness behaviour during childhood may explain these associations .

The increasing interest in psychological and social factors should not distract from the potential importance of clinical or pathological aspects of back pain, for example in defining appropriate indications for surgery . Understanding how biological, psychological and social dimensions interact to determine onset and progression of back pain is essential to generate better options for the management of back pain. The need for the development and validation of methods for a better clinical and multidimensional assessment in back pain patients has been highlighted by several authors in this issue of Best Practice and Research Clinical Rheumatology . This may vary from a brief screening tool to be used for prognostic stratification in primary care to more extensive and detailed assessment in physiotherapy practice, secondary care services or pain rehabilitation clinics.