Low-back pain

Handbook of Clinical Neurology, Vol. 131 (3rd series) Occupational Neurology M. Lotti and M.L. Bleecker, Editors © 2015 Elsevier B.V. All rights reserved

Chapter 21

Low-back pain FRANCESCO S. VIOLANTE*, STEFANO MATTIOLI, AND ROBERTA BONFIGLIOLI Occupational Health Unit, Department of Medical and Surgical Sciences, University of Bologna, Bologna, Italy

INTRODUCTION Low-back pain is one of the most common painful conditions experienced by human bs throughout their life. Lifetime prevalence has been reported to be as high as 84% (Balague´ et al., 2012), depending on the case definition used, and no age group is spared, even children (Calvo-Mun˜oz et al., 2013). Low-back pain is also an occupational disorder and, probably, the most common among occupational disorders worldwide (Punnett et al., 2005). Although low-back pain is not a lethal condition, it was estimated at the third rank among all diseases by disability-adjusted life-years in 2010 in the USA (US Burden of Disease Collaborators, 2013), after ischemic heart disease and chronic obstructive pulmonary disease, and at the first rank by years lived with disability. It also ranked high (13th) globally for the same year, in disability-adjusted life-years (Murray et al., 2012). This chapter will provide an overview of modern concepts of low-back pain (in general) and will outline some distinctive features of work-related low-back pain.

DEFINITION AND ATTRIBUTES OF LOWBACK PAIN It has been written that “low back pain is neither a disease nor a diagnostic entity of any sort” (Ehrlich, 2003): nevertheless, this condition is a leading cause of disability and it consumes a great amount of health resources (Chou et al., 2007). Low-back pain is defined as “pain, muscle tension or stiffness localized below the costal margin and above the inferior gluteal folds, with or without referred leg pain”

(Airaksinen et al., 2006; van Tulder et al., 2006; Chou, 2011). Low-back pain also stimulates a number of scientific studies: while this chapter was being written, a simple PubMed search with the string “low-back pain” retrieved more than 24 000 articles (up from 176 in the year 1983 to 1723 in 2013 – a 10-fold increase in 30 years). One of the first successful efforts at categorizing work-related low-back pain was made by the Quebec Task Force on Spinal Disorders (1987), which classified 11 categories of this disorder, based on symptoms or status. Presently, clinical guidelines (Hegmann, 2011) usually call for distinguishing two main categories of low-back pain: 1.

2.

nonspecific low-back pain, which is defined as a condition attributable to no recognizable known specific pathology (including low-back pain deemed to be of mechanical origin) specific low-back pain, which is defined as a condition attributable to a recognizable known specific pathology (e.g., infection, tumor, fracture, inflammatory process, radicular syndrome).

Nonspecific low-back pain accounts for the vast majority (85%) of cases of this condition (van Tulder et al., 2006): most of these cases will recover spontaneously within a couple of weeks (Quebec Task Force on Spinal Disorders, 1987). However, in some cases pain and disability have a longer duration, so low-back pain is currently classified, with regard to duration of symptoms, as: ●

acute: an episode of low-back pain lasting less than 6 weeks

*Correspondence to: Francesco S. Violante, M.D., Occupational Health Unit, University of Bologna – Sant’Orsola Malpighi Hospital, via Palagi 9, 40138 Bologna, Italy. Tel: +39-051-636-2611, E-mail: [email protected]

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F.S. VIOLANTE ET AL. subacute: duration between 6 and 12 weeks EPIDEMIOLOGY OF LOW-BACK PAIN ● chronic: duration more than 12 weeks. While this chapter was being written, a simple PubMed The distinction between nonspecific/specific and acute/ search with the string “low back pain/epidemiology” subacute/chronic low-back pain is not only useful for (Medical Subject Headings: MeSH) retrieved more than epidemiologic studies, but also (mainly) for choosing 1700 articles: more than 100 were systematic reviews the appropriate strategy for the diagnosis and treatment about different aspects of this multifaceted condition. of the disorder. Before providing an overview of occupational low-back The most distinctive feature of low-back pain is its pain, however, it is useful to summarize some of the recurrence: 24–87% of individuals who have an episode available epidemiologic evidence about this condition of low-back pain will suffer a recurrence within 1 year in the general population. (Stanton et al., 2009). The variability of recurrence estiIt is worth noting that what is presently known about mates is probably due to the lack of a common definition the epidemiology of low-back pain is based on studies of an episode of low-back pain, namely, when an episode which reflect the interpretation of this condition in the is finished and when a new one has commenced (de Vet past. Traditionally, low-back pain has been viewed as a et al., 2002). result of wear and tear of the spine throughout the life Based on a systematic review of published studies (hence the emphasis on spine pathology, and especially (Stanton et al., 2009), it has been proposed that a new epidisc disease), or as a consequence of an injury, especially sode of low-back pain is observed if: at the workplace. As a result, a great number of studies have been performed on low-back pain in adults, focus● the patient has been painfree for at least 30 days and ing on prevalence and incidence of the condition, per● has been experiencing pain for at least 24 hours and sonal risk factors, and antecedents of low-back pain ● the pain has an intensity at least equal to a defined episodes. minimal clinically important change on a chosen pain The traditional view of low-back pain, however, has intensity scale and been recently challenged in different ways. ● the pain is associated with a functional limitation at The view that low-back pain results from wear and least equal to a defined minimal clinically important tear of the spine and, mainly, from disc disease has been change on a chosen functional limitation scale. challenged by several lines of evidence: Some practitioners still label low-back pain cases ● Even young children suffer from low-back pain, in according to the presumed cause of the pain (discothe absence of any underlying spine pathology. genic, facetogenic, sacroiliac), but in most cases this ● Intervertebral discs in women are usually in better is an unsupported assumption (see section on condition than in men of the same age group, but, pathophysiology, below). on average, women report low-back pain more freIn order to better standardize the diagnosis of lowquently than men. back pain for epidemiologic studies, some researchers ● Disc disease is very frequent in the totally asymphave proposed “consensus definitions” based on the tomatic adult population. location of the symptoms, and their frequency, duration, ● Studies on identical twins suggest that genetic facand severity (Dionne et al., 2008). tors seem to influence disc degeneration as much In addition, a number of instruments have been proas (or more than) age and biomechanical overload. posed for grading the severity of low-back pain (Ostelo et al., 2008) and the associated disability (Smeets However, as most of the studies so far published are et al., 2011). based on the traditional view of low-back pain, we will Based on the presently available knowledge (Axe´n first summarize the information on low-back pain and and Leboeuf-Yde, 2013; Dunn et al., 2013), it is possible disc disease prevalence and incidence in the general popto conceive low-back pain as a condition (or, maybe, a set ulation, and the relevant risk factors. After that, we will of different conditions) which can assume a diverse clinsummarize the information on work-related low-back ical picture along a continuum which has on one extreme pain and disc disease. the individuals who will (almost) never experience an episode of low-back pain throughout their life, and at the Epidemiology of low-back pain in the general other extreme the subjects who will suffer from continpopulation uous low-back pain for most days in their life and, in between, subjects who will suffer from repeated epiAccording to a systematic review of studies about the sodes of low-back pain, variable for frequency, duraprevalence of low-back pain in adults (Hoy et al., tion, and severity (who, however, will be painfree 2012), the point prevalence of low-back pain (subjects between those episodes). who report low-back pain at a point in time), based on ●

LOW-BACK PAIN 243 estimates, has a mean of 18.3%, with a standard deviation of 11.7. One-month prevalence (145 estimates) has a mean of 30.8% and a standard deviation of 12.7, 1-year prevalence (271 estimates) has a mean of 38% and a standard deviation of 19.4, and lifetime prevalence (133 estimates) has a mean of 38.9% and a standard deviation of 24.3. The large variation in these estimates reflects the heterogeneity of the studies and the lack of use of a uniform case definition for an episode of low-back pain. According to a systematic review of studies on the first-time incidence of low-back pain in adults (Taylor et al., 2014), the summary pooled estimate for community-based populations was 26%, with 24–28% as 95% confidence interval. A similar value (27%) was obtained in studies that investigated only subjects who were painfree at baseline. Low-back pain seems to be common also among young people: according to a meta-analysis of studies on the prevalence of low-back pain (Calvo-Mun˜oz et al., 2013), the mean point prevalence obtained from 10 studies was 12% (95% confidence interval: 9–15.9), the mean period prevalence at 1 week obtained from six studies was 17.7% (95% confidence interval: 12.4–24.7), and the mean period prevalence at 1 year obtained from 13 studies was 33.6% (95% confidence interval: 26.9–41), whereas the mean lifetime prevalence obtained from 30 studies was 39.9% (95% confidence interval: 34.2–45.9). According to the authors, lifetime prevalence exhibited a positive, statistically significant relationship with the mean age of the participants in the samples and with the publication year of the studies. The most recent studies showed higher prevalence rates than the oldest ones, and studies with better methodology exhibited higher lifetime prevalence rates than studies that were methodologically poor. Incidence of low-back pain in children has also been studied. According to a study (Nissinen et al., 1994), the incidence of low-back pain in a group of 859 children (mean age at the start and end of the follow-up 11.8–13.8 years) was 18.4% in girls and 16.9% in boys. Another study (Jones et al., 2003), based on an original cohort of 1046 schoolchildren, aged 11–14 years at baseline, identified as being free of low-back pain, reported new onset of low-back pain in 12.5% of children at age 11, 16% at age 12, 23.8% at age 13, and 24.1% at age 14 years. The proportion of children reporting low-back pain was 20.1% among schoolgirls and 16.9% among schoolboys. Prevalence of low-back pain in children does not show striking differences from corresponding values in the adult population and, as reported earlier, challenges the idea that low-back pain is mostly an adult-acquired condition due to wear and tear of the spine. On the other hand, the incidence of low-back pain in children seems to be lower than in adults, up to the age of 12 years.

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Factors associated with low-back pain in the general population A number of factors have been studied in association with low-back pain in the general population (Hoy et al., 2012): ● ● ●

Female prevalence of low-back pain is higher than among males across all age groups. Prevalence increases with age until middle age, decreasing afterwards. Prevalence in high-income countries seems to be higher than in middle- and low-income countries.

Obesity and smoking have been found to be associated with low-back pain in twin studies (Ferreira et al., 2013). However, the magnitude of the pooled odds ratios from cohort studies for both smoking (Shiri et al., 2010a) and obesity (Shiri et al., 2010b) was between 1.3 and 1.5. Nonoccupational strenuous physical activity has been found to be associated with an increased risk of low-back pain, whereas leisure time activity has been found to be protective (Heneweer et al., 2011). Although their role is still uncertain, a variety of psychosocial factors seem to be associated with low-back pain, possibly with a predictive value greater than the results of nuclear magnetic resonance (Pincus et al., 2002; Carragee et al., 2005). More than “causing” lowback pain, psychosocial factors could be implicated in the transition to a chronic pain status (Ramond et al., 2011). A number of factors have been studied in association with low-back pain also in children: in a systematic review of the studies examining risk factors for first episode of low-back pain in children (Hill and Keating, 2010), the authors reported that 13 of the 47 risk factors studied were significantly associated with a first episode of low-back pain. However no risk factor was found to be associated with future low-back pain in children in more than one study and inconsistency in definitions of low-back pain, predefined recall periods, and methods used to collect and analyze data limit conclusions that can be drawn about factors that identify children at risk of developing low-back pain.

Epidemiology of lumbar disc disease in the general population Intervertebral disc degeneration is a concept for which in medicine, up to the present time, there is no widely agreed definition. Conceptually, disc degeneration may be defined as the product of the process of deterioration of intervertebral discs (spanning the lifetime) associated with remodeling of the same discs and adjoining vertebrae, including adaptation of disc structure to variations of physical load or the occasional lesion.

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Operationally, disc degeneration is defined by the diagnostic method used, which at the present time is mostly nuclear magnetic resonance (Battie´ and Videman, 2006). Since 2012, “intervertebral disc degeneration” is a MeSH term, defined as “degenerative changes in the intervertebral disc due to aging or structural damage, especially to the vertebral end-plates” (http://www.ncbi. nlm.nih.gov/mesh/68055959). Lately, lumbar disc degeneration has been studied in the general population by means of nuclear magnetic resonance: unfortunately, the absence of a generally agreed standard makes it difficult to compare the results of different studies and probably explains the wide range of reported results. As an example, lumbar disc bulging prevalence in asymptomatic subjects has been reported to be between 10% and 80% (and higher), and the prevalence of annular tears has been reported to range between 6% and 56% (Battie´ et al., 2004). The prevalence of disc degeneration is already elevated in young adults: a study in subjects between 20 and 22 years of age has reported that almost half had at least one degenerated disc and a quarter had a bulging disc (Takatalo et al., 2009). Epidemiology of lumbar disc herniation suffers from the same limitations of disc degeneration study: incidence and prevalence vary according to the case definition (surgery, magnetic resonance imaging) and they seem to have the same distribution of disc degeneration in relation to age and gender, although lumbar disc herniation seems to be much rarer. Among adults, prevalence has been estimated to range between 1% and 3% (Andersson, 1997), whereas incidence has been estimated in adult males at around 1.7% (Jørgensen et al., 2013). Lumbar disc herniation is well known (although rare) even in infancy (Martı´nez-Lage et al., 2003; Haidar et al., 2010). This is not a new finding, as the presence of lumbar disc degeneration in children has been known for more than a century (Beneke, 1897): specimens from autopsies or surgical procedures have demonstrated the presence of vertebral cartilage pathology or annular tears of the discs in children between 3 and 10 years of age. Sciatica is the symptom more often associated with lumbar disc herniation: lifetime prevalence has been estimated at 12.2–26%, annual prevalence has been estimated at 2.2–19.5%, and point prevalence has been estimated at 1.6–4.8% (Konstantinou and Dunn, 2008). Incidence of sciatica seems to be variable as well, with reported values in the range 0.65–37% (Cook et al., 2014).

Factors associated with lumbar disc disease in the general population Recent studies point to a changing view of the determinants of lumbar disc degeneration (Battie´ et al., 2009). The prevalent view of lumbar disc disease determinants at the beginning of the 1990s (Frymoyer, 1992) was: “Among the factors associated with its occurrence are age, gender, occupation, cigarette smoking, and exposure to vehicular vibration. The contribution of other factors such as height, weight, and genetics is less certain.” Ten years later (Ala-Kokko, 2002), the view had dramatically changed: Even though several environmental and constitutional risk factors have been implicated in this disease, their effects are relatively minor, and recent family and twin studies have suggested that sciatica, disc herniation and disc degeneration may be explained to a large degree by genetic factors. Although the relative contribution of genetic and environmental factors is not precisely known, the current evidence about determinants of lumbar disc degeneration can be summarized as follows. Genetic factors play an important role in lumbar disc degeneration and this role seems to be more important for the lumbar discs between L1 and L4 (Battie´ et al., 1995). Age is another important factor: frequency of lumbar disc degeneration increases with increasing age (Miller et al., 1988). Female gender seemed to be protective for discs (Miller et al., 1988): recent evidence suggests that the actual progression rate of disc degeneration with age among women and men is converging (Lee et al., 2012). Body weight, smoking, and height seem also to be associated with lumbar disc degeneration (Wahlstr€om et al., 2012).

Epidemiology of low-back pain in the occupational population The association between manual material handling and low-back pain has been known since ancient times: Bernardino Ramazzini (1700), in his book on workers’ diseases, describes the different diseases attributed to the porter’s occupation. The scientific evidence of a relation between lowback pain and occupational factors was reviewed by a US National Institute for Occupational Safety and Health (NIOSH) expert group (Bernard, 1997), which

LOW-BACK PAIN concluded that there was strong evidence of workrelatedness: A causal relationship is shown to be very likely between intense or long-duration exposure to the specific risk factor(s) and musculoskeletal disorder when the epidemiologic criteria of causality are used. A positive relationship has been observed between exposure to the specific risk factor and musculoskeletal disorder in studies in which chance, bias, and confounding factors could be ruled out with reasonable confidence in at least several studies linking “lifting/forceful movement” and “whole body vibration” to back disorders. Evidence of workrelatedness, e.g., Some convincing epidemiologic evidence shows a causal relationship when the epidemiologic criteria of causality for intense or long-duration exposure to the specific risk factor(s) and musculoskeletal disorder are used. A positive relationship has been observed between exposure to the specific risk factor and musculoskeletal disorder in studies in which chance, bias, and confounding factors are not the likely explanation was found between “awkward posture” and “heavy physical work” and back disorders. No evidence of work-relatedness was found between “static work posture” and back disorders. Since the substantial NIOSH review, a number of studies have been published concerning occupational low-back pain epidemiology, including systematic revisions of these studies. A systematic review (Hoogendoorn et al., 1999) including 28 cohort and three case-referent studies (no cross-sectional study was included) concluded for the existence of strong evidence for manual materials handling, bending and twisting, and whole-body vibration as risk factors for back pain. The evidence was moderate for patient handling and heavy physical work, and no evidence was found for standing or walking, sitting, sports, and total leisure-time physical activity. Another systematic review (Bakker et al., 2009), which used stricter inclusion criteria and chose only 18 cohort studies, found strong evidence that leisuretime sport or exercises, sitting, and prolonged standing/walking were not associated with low-back pain. Evidence for associations in leisure-time activities (e.g., do-it-yourself home repair, gardening), wholebody vibration, nursing tasks, heavy physical work, and working with one’s trunk in a bent and/or twisted

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position and low-back pain was conflicting. However, of the 12 studies concerning “heavy physical work,” five reported a statistically significant increase in the risk of low-back pain and seven reported some increase in lowback pain risk as well, although not statistically significant. A systematic review (Ribeiro et al., 2012) did not find a clear dose–response relationship for work posture exposures and low-back pain. Limited evidence was found for trunk range of motion and duration of sustained flexed posture as risk factors for low-back pain. No evidence was found for frequency of trunk flexion as a risk factor for low-back pain. A meta-analysis of mechanical workplace risk factors and low-back pain using individual participant data (Griffith et al., 2012) found small to moderate odds ratios for the association of mechanical exposures and low-back pain, although the relationships were complex. The odds ratios for posture exposures ranged from 1.1 to 2.0, whereas for force exposure they ranged from 1.4 to 2.1. The magnitudes of the odds ratios differed according to the definition of low-back pain, and heterogeneity was associated with both study-level and individual-level characteristics. Low-back pain has been particularly studied in some occupational groups: among occupations in which women are the majority, an increased risk of low-back pain has been consistently reported in nurses (Yassi and Lockhart, 2013), with risk estimates in the range 1.2–5.5. Among occupations in which men are the majority, an increased risk of low-back pain has been consistently reported in construction workers (Boschman et al., 2011), with odds ratios in the range 2.3–3.0. Similar evidence has been reported for farmers (Osborne et al., 2012).

Factors associated with low-back pain in the occupational population Manual material handling has been traditionally associated with low-back pain. However, manual material handling is not a single risk factor but, rather, a common task which can result in an overload of the spine (which is, ultimately, the risk factor for low-back pain). Lifting heavy loads, especially in an awkward posture or with bending and twisting of the trunk, may result in an overload of the spine: as manual lifting is a very common task, it is not surprising that sometimes the terms manual material handling and manual lifting are used interchangeably. An overload of the spine may also be the result of pushing and pulling heavy loads: all these issues

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are thoroughly reviewed in an excellent manual (Chaffin et al., 2006). The relation between low-back pain prevalence and lifting has been studied using the NIOSH lifting index (see section on management of work-related low-back pain, below) as an approximate measure of the exposure. In a cross-sectional study (Waters et al., 1999), the odds ratio of low-back pain was 1.04 for workers engaged in tasks in which the lifting index value was below 1, 1.96 for lifting index values between 1 and 2, 2.20 for lifting index values between 2 and 3, and 1.09 for lifting index above 3 (the highest category of exposure). The authors concluded that more data were needed for rigorous examination of the correlation between the lifting index and risk of low-back pain and that more research was needed to investigate the possible effects of psychosocial and personal factors on the reports of low-back pain that had been shown in other studies. The same group of authors published a similar study 12 years later (Waters et al., 2011) obtaining, more or less, the same results (odds ratio of low-back pain was 1.06 for workers engaged in tasks in which the lifting index value was below 1, 1.22 for lifting index values between 1 and 2, 1.88 for lifting index values between 2 and 3, and 1.36 for lifting index above 3). Their conclusion was that, as found in the first data collection, the risk in the highest exposure group (lifting index above 3) was less than in the group with exposure lifting index values between 2 and 3. As they noted previously, it is possible that this may be due to problems with the predictive power of the equation: however, they concluded that this was not likely the case, but probably due to a combination of “worker selection” and “survivor” effects. Nonetheless, they also warned that, as the lifting index increases, workers with low-back pain are more likely to believe that their disorder is due to the repeated activity that they do on the job. The reduction in self-reported low-back pain in highest estimated exposure groups seems to be confirmed also in later longitudinal studies (Garg et al., 2014a, b). Exposure to whole-body vibrations (mainly as a result of driving heavy equipment vehicles) has been studied as a risk factor for the development of low-back pain and there are several reviews on the subject: a meta-analysis of 18 observational studies reported a meta-relative risk of 2.21 in drivers of heavy equipment vehicles as compared to those not exposed (Waters et al., 2008). However, the authors stated that the methodologic qualities of the published studies ranged from marginal to average and that prospective cohort studies were urgently needed to confirm the results and that the biologic plausibility should be further explored.

Psychosocial factors at work seem to be also associated with musculoskeletal pain in general and also with low-back pain: a longitudinal study of a large working population (Sterud and Tynes, 2013) found that odds ratios for low-back pain and high job demands were 1.41 (95% confidence interval: 1.16–1.72) and for low job control were 1.26 (95% confidence interval: 1.01–1.57). The same magnitude results had been reported in a systematic review and meta-analysis on the same issue (Lang et al., 2012). Contrary to popular opinion, the epidemiologic literature does not support the notion that sitting while at work is associated with low-back pain (Hartvigsen et al., 2000).

Epidemiology of lumbar disc disease in the occupational population There are to date no systematic studies on lumbar disc disease in the occupational population, mainly due to the fact that, as noted above, intervertebral disc degeneration is a concept for which in medicine, up to the present time, there is no widely agreed definition. So far, most of the available evidence about the epidemiology of lumbar disc disease in the occupational population comes from studies which have examined the occupational risk factors for this outcome (see below).

Factors associated with lumbar disc disease in the occupational population Manual material handling and heavy work (which usually includes manual material handling) have been found to be associated with lumbar disc disease in several studies. A large cohort study of Swedish construction workers (Wahlstr€om et al., 2012) found an increased risk for hospitalization due to lumbar disc disease for several occupational groups compared with white-collar workers and foremen. Occupational groups with high biomechanical loads had the highest risks: for example, the relative risk (RR) for concrete workers was 1.55 (95% confidence interval: 1.29–1.87). A taller stature was consistently associated with increased risk. The RR for a man of 190–199-cm height was 1.55 (95% confidence interval: 1.30–1.86) compared with a man of 170–179-cm height. Body weight and smoking were also risk factors, but weaker than height. Workers in the age span of 30–39 years had the highest RR (1.87; 95% confidence interval: 1.58–2.23) compared with those aged 20–29 years, whereas men aged 60–65 years had a lower RR (0.86; 95% confidence interval: 0.68–1.09). A cohort study of Danish male workers followed for 33 years (Sørensen et al., 2011) found that the strongest

LOW-BACK PAIN predictor of hospitalization for herniated lumbar disc disease was frequent strenuous physical activity at work: compared with unexposed subjects, the hazard ratio was 3.9 (95% confidence interval: 1.82–8.38). Also, body height was a significant predictor, whereas body weight was only insignificantly associated with herniated lumbar disc disease. Exposure to whole-body vibration has been linked to lumbar disc degeneration, however, a systematic review (Bible et al., 2012) which included five retrospective cohort and two cross-sectional studies which had used imaging modalities for the assessment of lumbar disc disease found that, although mixed results and conclusions were found, the majority of studies did not identify an association between whole-body vibration exposure and an abnormal spinal imaging finding indicating damage of the spine. The authors also stressed that each included study had limitations secondary to quantifying whole-body vibration exposure accurately, both as a single encounter and as a total exposure over years. A major concern about the interpretation of the available studies about occupational factors and lumbar disc disease is that, so far, no study actually examined the relation with the putative occupational factor and the specific level of disc disease. Apparently, the outcome studied was “any disc disease at any lumbar level,” whereas biomechanics provide evidence that lumbar load during manual material handling, for example, is very different for different discs in the lumbar spine, even adjacent ones. Based on measures of compressive forces on intervertebral discs in healthy volunteers, available since the 1960s (Nachemson and Morris, 1963), biomechanical models have been developed to estimate loads on lumbar spine during manual lifting: according to these models, “the L5–S1 disc incurs the greatest moment in lifting activities because it most often has the largest moment arm relative to any load in the hands” (Chaffin et al., 2006, p. 131). According to these models, also shear forces acting on lumbar disc during lifting varies with the disc level (Arjmand et al., 2011): for example, it has been estimated that, whereas axial compression forces acting on L4–L5 and L5–S1 discs during manual lifting are somewhat comparable, disc shear reactive forces are much greater on L5–S1 disc (range 129–1437 N) than on L4–L5 disc (range 3–571 N). As biomechanical knowledge suggests that “exposure” of lumbar discs to axial and shear forces during manual material handling (and, possibly, to wholebody vibration) is not the same for each level, future studies should be designed to provide evidence linking

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manual material handling (and whole-body vibration) with disc disease at specific anatomic levels. Some evidence about specific patterns of lumbar disc disease seems to emerge (Cheung et al., 2010).

PATHOPHYSIOLOGYOF WORK-RELATED LOW-BACK PAIN AND DISC DISEASE It is not the scope of this chapter to provide a comprehensive review of low-back pain: however, to introduce the diagnosis and management of work-related low-back pain, it is useful to summarize some concepts about pathophysiology of low-back pain in general. As stated previously, low-back pain is a symptom (similarly to pain in any particular body region) and not a particular disease: that is, the pain is a consequence of different pathologic processes, which result in this common symptom. The human spine is a complex structure which maintains our erect position and has the ability to move in different directions in space: it is also subject to different forces acting on our body while we stand, move, lift, carry, or push/pull loads. Many structures of the body located in the region where low-back pain is felt are capable of originating pain (Vora et al., 2010). In a given case, however, it may be difficult to trace the pain to one (or more) specific structure(s), without the use of invasive tests. Intervertebral discs are composed of a central nucleus pulposus surrounded by a tough annulus fibrosus, and they are avascular. Nutrition of the disc is believed to be provided by terminal vessels to the vertebral endplate which supply the outer surface of the annulus fibrosus: the inner portion of the disc depends on diffusion for nutrition and metabolite accumulation or removal. Intervertebral discs are innervated and capable of producing pain, possibly by different mechanisms (mechanical or biochemical), and pain may be acute, as demonstrated by pain associated with discography, or chronic, as in disc degeneration (Bogduk, 2012). Why degenerated discs do not produce pain in a number of subjects remains a matter of research. Disc degeneration has been traditionally viewed as a preceding condition for disc herniation: however, recent experimental evidence seems not to support this view (Lama et al., 2013), as earlier investigations demonstrated that disc herniation could not be produced by compression alone, but that it was necessary to first remove the posterior part of the intervertebral motion segment and then load the disc in compression and flexion (Chaffin et al., 2006, p. 32). Each lumbar vertebra has two posterolateral zygapophysial joints, commonly referred to as facet joints: these

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joints articulate with the corresponding facets of the superior and inferior vertebrae. Work-related low-back pain could theoretically result from degenerative changes in the facet joints of the lumbar vertebrae due to repeated excessive load (as a consequence of manual material handling). However, there are no studies which have examined this specific hypothesis and facet osteoarthritis seems to be more common at the L4–L5 level (Kalichman et al., 2009) and not at L5–S1 level, which is the spinal segment subjected to the highest load during manual lifting (Chaffin et al., 2006). Other structures which may be a source of pain in the low back are the sacroiliac joint–ligament complex, the ligaments of the lumbosacral spine, the muscles of the lumbosacral spine, the thoracolumbar fascia (Vora et al., 2010), and the peridural membrane of the spinal canal (Ansari et al., 2012). As well as low-back pain, sciatica is not a diagnosis but a symptom: the traditional view was that it was a result of compression on a nerve root due to an herniated disc, but current opinion is that inflammation is also necessary (Valat et al., 2010).

DIAGNOSIS OF WORK-RELATED LOW-BACK PAIN Diagnosis of work-related low-back pain is extensively covered by a number of guidelines (Hegmann, 2011): in this chapter we will only review some specific issues which are peculiar to the diagnosis of work-related low-back pain. In order to do this, we will first summarize current recommendations on the diagnosis of lowback pain in general.

the legs or perineum will prompt investigation for a more serious neurologic condition. Physical examination is considered a sufficient diagnostic workup in a case of acute low-back pain (and sciatica) without red flags: early magnetic resonance imaging use in acute, work-related, disabling low-back pain is considered to be associated with a strong iatrogenic effect, regardless of radiculopathy status (Webster et al., 2013). Pharmacologic treatment of low-back pain should be based on the use of analgesic drugs, with acetaminophen as a first-line treatment: if control of pain is not satisfactorily achieved, nonsteroidal anti-inflammatory drugs are used and, in some cases, opioids. Most cases of acute low-back pain will resolve in less than 2 weeks (Quebec Task Force on Spinal Disorders, 1987): if the case does not resolve within 4–6 weeks, different options should be considered and diagnostic imaging is indicated (Hegmann, 2011). Yellow flags have been developed as warning signals for the physician for the possible development of chronic pain: in most cases, this evolution seems to be linked to psychosocial factors (Chou and Shekelle, 2010), although biologic issues should not be overlooked (Hancock et al., 2011). Yellow flags (Nicholas et al., 2011) have been categorized as: ●

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Diagnosis of low-back pain in general The presentation of a case of low-back pain to the physician is usually “acute”: the patient will report the onset of a pain in the back (extending to a leg, in some cases) which developed in a short time (sometimes abruptly, in other cases over a period of a few hours). Although in most cases the physician will face a case of nonspecific low-back pain (which in most cases is benign and self-limiting), it is currently recommended that the physician always search for signs of a possible severe underlying condition (red flags). History may reveal a major trauma and pain which reduce spontaneously in the supine position (vertebral fracture), whereas a first episode of pain after 50 years of age, a history of cancer, weight loss, symptoms such as fever, chills, a recent bacterial infection, and immunosuppression, will raise the suspicion of cancer or infection. Severe or progressive sensory alteration or weakness, bladder or bowel dysfunction, or evidence of neurologic deficit in

beliefs, appraisals, and judgments: unhelpful beliefs about pain, such as indication of injury as uncontrollable or likely to worsen, expectations of poor treatment outcome, delayed return to work emotional responses: distress not meeting criteria for diagnosis of mental disorder, worry, fears, anxiety pain behavior (including pain coping strategies): avoidance of activities due to expectations of pain and possible reinjury, overreliance on passive treatments (hot packs, cold packs, analgesics).

Diagnosis of work-related low-back pain By definition, work-related low-back pain is theoretically a case in which occupational activities have been the cause of the episode or have significantly contributed to the episode. In this sense, the diagnosis of workrelated low-back pain is essentially an etiologic diagnosis, which can be summarized as: is this case of nonspecific low-back pain due to the patient’s job? It’s a difficult question to answer, so we will review some issues which can be considered in order to provide a rational basis to the diagnosis (for simplicity, we will discuss the issue of acute nonspecific low-back pain, but the concepts used are pertinent also to the other outcomes, previously cited).

LOW-BACK PAIN In some cases workers will present with a history of an inciting event that produced immediate low-back pain, such as lifting and/or twisting the trunk while holding a heavy object: in this case, the work-relatedness of the (bona fide) episode is easy to ascertain. In other cases the worker will present without a history of an inciting event that produced immediate lowback pain, but it is known that the worker belongs to a category in which epidemiology has shown an increased risk of low-back pain (nurses, construction workers): in such cases, we can resort to epidemiology to evaluate the a priori probability of facing a case of work-related lowback pain. If we know, for example, that the category to which the worker belongs has an RR for the outcome greater than 2 (compared to unexposed workers), than the attributable fraction (i.e., the proportion of cases caused by the exposure) is greater than 50%, according to the formula RR – 1/RR. This a priori probability, however, is attributable to every case, including the cases caused by the natural background. The higher the RR within the group of workers to which the case belongs, the higher the a priori probability that a random case belongs to the group of cases caused by exposure (for example, if the RR for the outcome in a population of exposed workers equals 5, 80% of the cases in the that population are produced by the exposure and only 20% by the natural background). In the case of low-back pain, sciatica, and disc disease, as we have previously discussed, the outcome is common in the unexposed population and the RR is not particularly elevated among exposed subjects (below 2, in most cases of manual lifting, for example) so that an exclusive reliance on general epidemiology alone would probably bring one to an exclusion of workrelatedness in most cases. In order to achieve a sounder base for the diagnosis of work-relatedness of an individual case of low-back pain, it becomes crucial to have an epidemiologic picture of the specific population to which that case belongs (ideally, the specific workplace, provided that it includes a sufficient number of subjects). Careful consideration should also be given to the nonwork-related risk factors for the outcome in question, bearing in mind that, as a rule of thumb, RR for personal factors is usually not lower than that of occupational factors.

MANAGEMENT OF WORK-RELATED LOW-BACK PAIN The management of work-related low-back pain integrates the clinical treatment with some specific issues

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relating to work: thus, we will review some concepts about the treatment of the condition, the assessment of fitness for work, and the prevention of low-back pain.

Treatment of work-related low-back pain Medical treatment of work-related low-back pain does not differ from the general treatment of the condition (analgesics to control the pain, advice to stay active to avoid deconditioning the musculature). However, in the case of work-related low-back pain, additional issues should be considered in order to: ● ●

avoid the transition of the subject to a chronic case facilitate the return to work as soon as possible.

To reach these goals the occupational physician should consider not only the yellow flags, already mentioned, but also the so-called blue flags and black flags (Main and Burton, 2000). Blue flags include perceptions about the relationship between work and health such as belief that work is too onerous and likely to cause further injury and/or belief that workplace supervisor and workmates are unsupportive, whereas black flags were termed the more observable characteristics of the workplace and nature of the work, as well the insurance and compensation system under which workplace injuries are managed, such as legislation restricting options for return to work, conflict with insurance staff over injury claim, overly solicitous family and healthcare providers, or heavy work, with little opportunity to modify duties. A recent review on the “flags” subject concluded that: Consistent evidence has been found to support the role of various psychological factors in prognosis, although questions remain about which factors are the most important, both individually and in combination, and how they affect outcomes. Published early interventions have reported mixed results, but, overall, the evidence suggests that targeting yellow flags, particularly when they are at high levels, does seem to lead to more consistently positive results than either ignoring them or providing omnibus interventions to people regardless of psychological risk factors. Psychological risk factors for poor prognosis can be identified clinically and addressed within interventions, but questions remain in relation to issues such as timing, necessary skills, content of treatments, and context. In addition, there is still a need to elucidate mechanisms of change and better integrate this understanding into the broader context of secondary prevention of chronic pain and disability (Nicholas et al., 2011).

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As the prognosis of low-back pain is related to the clinical picture of the case, the individual and psychologic characteristics, as well as the work and social environment (Hayden et al., 2010), it is important that the necessary attention is paid to all these issues to achieve the best possible outcome for the individual and society, especially in the worker who has still problems 6–12 weeks after the onset of the episode of low-back pain.

Assessment of fitness for work Whether or not a single case of low-back pain is deemed to be work-related, the issue of fitness for work will be crucial. The advice to stay active implies that workers affected by low-back pain remain in their job or return to it very soon, even if this may result in some low-back pain. In some cases, however, the job may require exceptional levels of fitness (e.g., emergency rescue services) or unavoidable manual loading of heavy loads: in these (infrequent) cases, return to work may require the temporary modification of the duties of the worker, to facilitate return to work. As any prolonged period off work raises the chance that symptoms will became chronic, it is preferable to devise temporary “light” duties or a pattern of work to encourage uninterrupted employment or early return to work (Palmer et al., 2013). The issue of work restrictions for low-back pain is a debated one. Although is common practice to issue work restrictions for low-back pain, there is not much evidence about the effectiveness of this practice. A study on the outcome of work restriction for low-back pain in a utility company (Hiebert et al., 2003) found that restrictions were given to 43% of workers, the median duration of restricted duty was 32.5 days and, for 22% of workers, restricted duty was never lifted. Sickness absence duration did not differ between those who had received restrictions and those who had not (adjusted hazard ratio, 1.12); recurrence appeared less likely to occur among those who had work restrictions in their initial episode, but, the difference was not statistically significant. The authors concluded that no evidence of an association between a prescription of work restriction and early return to work was found. The utility of working restriction in subjects who are engaged in manual lifting, even after spine surgery, had already been questioned (Carragee et al., 1999), suggesting that no postoperative activity restrictions after limited discectomy allowed shortened time to return to work and that complication rates appeared comparable to those reported in the literature for patients under postoperative restrictions.

Prevention of work-related low-back pain Historically, the risk associated with manual lifting has been addressed, initially setting limits on the load which could be lifted manually, without consideration of the specific conditions in which the load was lifted. For example, in 1943 the Bureau of Labor Standards of the US Department of Labor published the Bulletin No. 11 – A Guide to the Prevention of Weight Lifting Injuries: the bulletin recommended a maximum lifting weight of 50 lb (23 kg) for men and 25 lb (11 kg) for women. Up to the late 1970s, many national legislations included maximum weights for manual lifting for male and female workers. A major shift occurred in 1981 with the publication of the NIOSH Work Practices Guide for Manual Lifting, which used the available biomechanical evidence from three different approaches (biomechanical, physiologic, and psychophysical) “for recommending load limits given different task factors.” The NIOSH equation was updated 10 years later (Waters et al., 1993) and published in 1994 (Waters et al., 1994). The criteria used to design the new equation were: ● ● ●

biomechanical: maintain L5–S1 compression forces below 3400 N psychophysical: loads are acceptable “to 75% of females and about 99% of males” physiologic: limit energy expenditure to values ranging from 2.2 to 4.7 kcal/min depending on the duration and vertical displacement of the lifts.

Subsequent evaluation, however, has found that the multiplicative nature of the equation actually produces recommended weight limits which are even lower than those based on the more protective criteria on which the equation is based (Potvin, 2014). The revised NIOSH equation is probably, to date, the most well-known approach to the design of safe manual lifting tasks (although it is often used as a risk evaluation tool). Although lifting is a principal component of manual material handling, manual pushing/pulling of a load is also common (carrying is less frequent in most jobs). In order to avoid work-related low-back pain (and disc disease and sciatica), guidelines can be followed to design jobs in such a way that all the components of manual material handling are within safe limits. The revised NIOSH equation, as shown above, provides a very conservative guidance for the design of manual lifting jobs: according to recent evidence (Potvin, 2014), the revised NIOSH lifting equationrecommended weight limit was found to be much more conservative than would be expected if it strictly adhered

LOW-BACK PAIN to its stated biomechanical, psychophysical, and physiologic criteria. The recommended weight limit was generally consistent with the biomechanical criteria (3400 N of lumbar compression force) for lifts below knuckle height, but resulted in compression forces that were much lower than the criterion at higher lifting heights. In moderate lifting frequencies, the average NIOSHrecommended weight limit is acceptable to more than 95% of female workers according to the psychophysical criterion. At the highest frequencies, the recommended weight limit was found to be an average of 63% of the values based on either the psychophysical or physiologic criteria. For pushing/pulling tasks, psychophysically based tables have been compiled and updated based on studies published in 1978 and 1991 (Snook, 1978; Snook and Ciriello, 1991). Recently, prediction equations of acceptable pushing/pulling forces (initial and sustained) have been published, to be used on a wider range of frequency of exertions, distance, and handle height (Garg et al., 2014c). It should be remembered, however, that psychophysically based guidance is not based on health risks, but on acceptability of the task by subjects.

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