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Lifestyle Risk Factors Increase the Risk of Hospitalization for Sciatica: Findings of Four Prospective Cohort Studies

      Abstract

      Background

      The purpose of this study is to assess the effects of lifestyle risk factors on the risk of hospitalization for sciatica and to determine whether overweight or obesity modifies the effect of leisure-time physical activity on hospitalization for sciatica.

      Methods

      We included 4 Finnish prospective cohort studies (Health 2000 Survey, Mobile Clinic Survey, Helsinki Health Study, and Young Finns Study) consisting of 34,589 participants and 1259 hospitalizations for sciatica during 12 to 30 years of follow-up. Sciatica was based on hospital discharge register data. We conducted a random-effects individual participant data meta-analysis.

      Results

      After adjustment for confounding factors, current smoking at baseline increased the risk of subsequent hospitalization for sciatica by 33% (95% confidence interval [CI], 13%-56%), whereas past smokers were no longer at increased risk. Obesity defined by body mass index increased the risk of hospitalization for sciatica by 36% (95% CI 7%-74%), and abdominal obesity defined by waist circumference increased the risk by 41% (95% CI 3%-93%). Walking or cycling to work reduced the risk of hospitalization for sciatica by 33% (95% CI 4%-53%), and the effect was independent of body weight and other leisure activities, while other types of leisure activities did not have a statistically significant effect.

      Conclusions

      Smoking and obesity increase the risk of hospitalization for sciatica, whereas walking or cycling to work protects against hospitalization for sciatica. Walking and cycling can be recommended for the prevention of sciatica in the general population.

      Keywords

      Clinical Significance
      • Current smoking increases the risk of hospitalization for sciatica by 33%, whereas past smokers are no longer at increased risk.
      • Obesity defined by body mass index increases the risk of hospitalization for sciatica by 36%, and abdominal obesity defined by waist circumference increases the risk by 41%.
      • Walking or cycling to work reduces the risk of hospitalization for sciatica by 33%.

      Introduction

      Sciatica involves pain that radiates from the lower back along the sciatic nerve to the back of the thigh and down the leg, and is accompanied by clinical findings suggestive of compression or irritation of the lumbosacral nerve root.
      • Koes B.W.
      • van Tulder M.W.
      • Peul W.C.
      Diagnosis and treatment of sciatica.
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      • Zafonte R.D.
      Sciatica.
      A herniated lumbar disc with nerve root compression is the most common cause of sciatica.
      • Koes B.W.
      • van Tulder M.W.
      • Peul W.C.
      Diagnosis and treatment of sciatica.
      • Ropper A.H.
      • Zafonte R.D.
      Sciatica.
      The prevalence of clinically verified sciatica in the general population ranges between 2% and 5%.
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      Prevalence and risk factors of disk-related sciatica in an urban population in Tunisia.
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      Serum lipids in relation to sciatica among Finns.
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      • Impivaara O.
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      Determinants of sciatica and low-back pain.
      Sciatica is a more persistent and disabling condition than other low back syndromes.
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      Patients with sciatica still experience pain and disability 5 years after surgery: a systematic review with meta-analysis of cohort studies.
      The majority of sciatica cases are, however, treated conservatively, and only a small proportion of the cases eventually need surgery.
      • Koes B.W.
      • van Tulder M.W.
      • Peul W.C.
      Diagnosis and treatment of sciatica.
      • Ropper A.H.
      • Zafonte R.D.
      Sciatica.
      • Valat J.P.
      • Genevay S.
      • Marty M.
      • Rozenberg S.
      • Koes B.
      Sciatica.
      In general, sciatica is a relatively uncommon cause for hospitalization.
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      Smoking and overweight as predictors of hospitalization for back disorders.
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      Occupational and other predictors of herniated lumbar disc disease-a 33-year follow-up in the Copenhagen male study.
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      Risk factors for hospitalization due to lumbar disc disease.
      Among metal industry workers, 4.4% of men and 5.9% of women had been hospitalized for cervical, thoracic, or lumbar intervertebral disc disorder during a 27-year follow-up.
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      • Luukkonen R.
      • Kirjonen J.
      Smoking and overweight as predictors of hospitalization for back disorders.
      Moreover, among male construction workers, 0.8% had been hospitalized for lumbar disc disease during a 16-year follow-up,
      • Wahlström J.
      • Burstrom L.
      • Nilsson T.
      • Järvholm B.
      Risk factors for hospitalization due to lumbar disc disease.
      and among men working in private or public companies, 1.7% had been hospitalized for a herniated lumbar disc during a 33-year period.
      • Sorensen I.G.
      • Jacobsen P.
      • Gyntelberg F.
      • Suadicani P.
      Occupational and other predictors of herniated lumbar disc disease-a 33-year follow-up in the Copenhagen male study.
      The etiology of sciatica is multifactorial.
      • Koes B.W.
      • van Tulder M.W.
      • Peul W.C.
      Diagnosis and treatment of sciatica.
      Of lifestyle risk factors, overweight and obesity,
      • Shiri R.
      • Lallukka T.
      • Karppinen J.
      • Viikari-Juntura E.
      Obesity as a risk factor for sciatica: a meta-analysis.
      and smoking
      • Shiri R.
      • Falah-Hassani K.
      The effect of smoking on the risk of sciatica: a meta-analysis.
      increase the risk of sciatica, but the role of leisure-time physical activity in sciatica is uncertain.
      • Shiri R.
      • Falah-Hassani K.
      • Viikari-Juntura E.
      • Coggon D.
      Leisure-time physical activity and sciatica: a systematic review and meta-analysis.
      A meta-analysis
      • Shiri R.
      • Lallukka T.
      • Karppinen J.
      • Viikari-Juntura E.
      Obesity as a risk factor for sciatica: a meta-analysis.
      showed that overweight increases the risk of clinically verified sciatica by 12% and hospitalization for sciatica by 16%. For obesity, the excess risk is 31% for sciatica and 38% for hospitalization due to sciatica. Moreover, another meta-analysis
      • Shiri R.
      • Falah-Hassani K.
      The effect of smoking on the risk of sciatica: a meta-analysis.
      found that current smoking increases the risk of clinically verified sciatica by 35%, and hospitalization or surgery due to sciatica by 45%. Smoking cessation reduces the excess risk, and past smokers are 9%-10% more likely to have sciatica than never smokers.
      • Shiri R.
      • Falah-Hassani K.
      The effect of smoking on the risk of sciatica: a meta-analysis.
      Furthermore, a recent meta-analysis of a limited number of prospective cohort studies found that a high level of leisure-time physical activity protects against lumbar radicular pain by 16%. Leisure-time physical activity had, however, no protective effect on clinically verified sciatica.
      • Shiri R.
      • Falah-Hassani K.
      • Viikari-Juntura E.
      • Coggon D.
      Leisure-time physical activity and sciatica: a systematic review and meta-analysis.
      Most of the previous studies on the role of lifestyle risk factors in clinically verified sciatica used a cross-sectional design, or were case control studies conducted among selected populations. The previous systematic reviews
      • Shiri R.
      • Lallukka T.
      • Karppinen J.
      • Viikari-Juntura E.
      Obesity as a risk factor for sciatica: a meta-analysis.
      • Shiri R.
      • Falah-Hassani K.
      The effect of smoking on the risk of sciatica: a meta-analysis.
      • Shiri R.
      • Falah-Hassani K.
      • Viikari-Juntura E.
      • Coggon D.
      Leisure-time physical activity and sciatica: a systematic review and meta-analysis.
      identified only a limited number of prospective cohort studies on this topic. There may be reverse causation between leisure-time physical activity and sciatica. Individuals with lumbar radicular pain may limit their leisure activities because of fear of pain.
      • Leeuw M.
      • Goossens M.E.
      • Linton S.J.
      • Crombez G.
      • Boersma K.
      • Vlaeyen J.W.
      The fear-avoidance model of musculoskeletal pain: current state of scientific evidence.
      Cross-sectional studies are more prone to reverse causation than prospective cohort studies. Moreover, there is a vicious cycle between obesity and physical inactivity.
      • Pietiläinen K.H.
      • Kaprio J.
      • Borg P.
      • et al.
      Physical inactivity and obesity: a vicious circle.
      Physical inactivity contributes to weight gain, and decreased level of physical activity can be a consequence of obesity.
      • Pietiläinen K.H.
      • Kaprio J.
      • Borg P.
      • et al.
      Physical inactivity and obesity: a vicious circle.
      A prospective cohort study found that physical inactivity increases the risk of lumbar radicular pain in abdominally obese individuals, but not in persons with normal waist circumference.
      • Shiri R.
      • Solovieva S.
      • Husgafvel-Pursiainen K.
      • et al.
      The role of obesity and physical activity in non-specific and radiating low back pain: the Young Finns study.
      It is unknown whether overweight or obesity modifies the effect of leisure-time physical activity on sciatica. The aim of this study was to investigate the effects of lifestyle risk factors on hospitalization for sciatica by conducting an individual participant data meta-analysis of 4 prospective cohort studies. Furthermore, we determined whether overweight or obesity modifies the effect of leisure-time physical activity on hospitalization for sciatica.

      Methods

      Population

      Health 2000 Survey

      A representative sample of people aged 30 years or older living in Finland in 2000-2001 was recruited using a 2-stage cluster-sampling design.
      Of 7977 individuals, 6986 (87.6%) were interviewed, and 6354 (79.7%) participated in the health examination.
      We excluded 124 patients who had been hospitalized for sciatica 5 years prior to the baseline survey between 1996 and 2000 based on register data, and 126 cases of probable sciatica based on clinical examination at baseline. The final sample consisted of 6413 participants with data on at least leisure-time physical activity and were followed-up until December 31, 2013. All participants signed a written informed consent, and the Ethics Committee for Epidemiology and Public Health of the Hospital District of Helsinki and Uusimaa, Finland approved the study.

      Mobile Clinic Survey

      The study was carried out by the Social Insurance Institution's Mobile Clinic Unit between 1973 and 1976.
      • Reunanen A.
      • Aromaa A.
      • Pyorala K.
      • Punsar S.
      • Maatela J.
      • Knekt P.
      The Social Insurance Institution's coronary heart disease study. baseline data and 5-year mortality experience.
      Participants aged 20 years or older living in Finland participated in the baseline examination (N = 19,518, participation rate = 83%). Individuals (n = 97) whose first hospitalization for sciatica was 5 years prior to the baseline examination were excluded from the analysis. The final population comprised 19,343 participants with data on variables of interest, and they were followed-up until December 31, 2011. The participants were fully informed about the use of the collected data for research purposes, and voluntary participation was according to the principles of the World Medical Association Declaration of Helsinki.

      Helsinki Health Study

      The study was conducted among the employees of the City of Helsinki aged 40 to 60 years at baseline in 2000-2002 (N = 8960, response rate = 67%).
      • Lahelma E.
      • Aittomäki A.
      • Laaksonen M.
      • et al.
      Cohort profile: the Helsinki Health Study.
      The register linkages were made for those with a written consent for combining their survey responses with the register data (74%).
      • Lahelma E.
      • Aittomäki A.
      • Laaksonen M.
      • et al.
      Cohort profile: the Helsinki Health Study.
      • Laaksonen M.
      • Aittomäki A.
      • Lallukka T.
      • et al.
      Register-based study among employees showed small nonparticipation bias in health surveys and check-ups.
      The participants (n = 25) who had been hospitalized for sciatica 4 years prior to the baseline study were excluded from the analysis, leaving 6288 participants with data on variables of interest. The participants were followed-up until December 31, 2013. The ethics committees of the health authorities of the City of Helsinki, and the Department of Public Health, University of Helsinki approved the study.

      Young Finns Study

      The baseline study included children and adolescents aged 3, 6, 9, 12, 15, or 18 years (N = 4320) living in 5 Finnish university cities in 1980 (N = 3596, response rate = 83%).
      • Shiri R.
      • Solovieva S.
      • Husgafvel-Pursiainen K.
      • et al.
      The association between obesity and the prevalence of low back pain in young adults: the Cardiovascular Risk in Young Finns Study.
      The follow-up studies were carried out in 1983, 1986, 2001, 2007, and 2011. The baseline population for the current study included 2620 participants (73% of those who participated at baseline) who took part in the 2001 follow-up study.
      • Shiri R.
      • Solovieva S.
      • Husgafvel-Pursiainen K.
      • et al.
      The association between obesity and the prevalence of low back pain in young adults: the Cardiovascular Risk in Young Finns Study.
      The age of the participants ranged between 24 and 39 years at baseline in 2001. Twenty-one participants who had been hospitalized for sciatica 4 years prior to the baseline study in 2001 were excluded from the analysis. Finally, 2545 participants were included in the current study and were followed-up until December 31, 2014. The study protocol was approved by the local ethics committees and all participants gave a written informed consent.

      Outcome

      The data on hospitalizations for sciatica were obtained for all cohorts from the Finnish Hospital Discharge Register covering all Finnish hospitals. The register is held by the National Institute for Health and Welfare. In all 4 studies, the following 5 International Classification of Diseases (ICD)-10 codes on sciatica were included: 1) M511: lumbar and other intervertebral disc disorders with radiculopathy; 2) M512: other specified intervertebral disc displacement; 3) M543: sciatica; 4) M544: lumbago with sciatica; and 5) G551: nerve root and plexus compressions in intervertebral disc disorders. Additionally, in the Mobile Clinic Survey, the data on sciatica prior to 1996 were based on the following ICD-8 codes (353.99, 725.10, and 725.19) and ICD-9 codes (7225A, 7227C, and 7228C).

      Independent Variables

      Information on age, sex, education, occupation, or exposure to physical workload factors, smoking, and leisure-time physical activity was gathered by the home interview in the Health 2000 Survey and by a self-administered questionnaire in 3 other studies. Weight and height were measured in 3 studies and were self-reported in the Helsinki Health Study. Moreover, waist and hip circumferences were measured in the Health 2000 Survey and Young Finns Study. Physicians diagnosed chronic back disorders at baseline in the Health 2000 Survey,
      and low back pain was self-reported in 3 other studies.
      In the Health 2000 Survey, information on the nature, frequency, and duration of leisure-time physical activity (Supplementary Table 1, available online), in the Mobile Clinic Survey data on nature and duration of leisure-time physical activity (Supplementary Table 2, available online), in the Helsinki Health Study data on duration and intensity of physical activity (Supplementary Table 3, available online), and in the Young Finns Study, information on frequency, intensity, and duration of physical activity during leisure time (Supplementary Table 4, available online) was collected at baseline. A metabolic equivalent of task (MET) index
      • Ainsworth B.E.
      • Haskell W.L.
      • Herrmann S.D.
      • et al.
      2011 Compendium of Physical Activities: a second update of codes and MET values.
      was computed for the Young Finns Study and Helsinki Health Study. We used the tertile distribution of physical activity to classify the participants into low, moderate, or high level of leisure-time physical activity. Furthermore, information on commuting to work by walking or cycling was gathered in the Health 2000 Survey and Young Finns Study.

      Statistical Analysis

      We defined overweight as body mass index (BMI) value between 25.0 and 29.9 kg/m2, and obesity as BMI value ≥30 kg/m2. We also defined overweight as waist circumference between 94 and 101.9 cm for men and between 80 and 87.9 cm for women, and abdominal obesity as waist circumference ≥102 cm for men and ≥ 88.0 cm for women.
      • National Institute for Health and Clinical Excellence (NICE)
      Obesity: the prevention, identification, assessment and management of overweight and obesity in adults and children. London: NICE.
      We ran Cox proportional hazards regression models. The data on deaths during the follow-up periods were obtained from Statistics Finland. Participants who died during the follow-up periods were censored. For the Health 2000 Survey, we conducted a survey data analysis using Stata's svy prefix command (StataCorp, LLC, College Station, Texas). In all studies we controlled the estimates for age, sex, education, occupation, or exposure to workload factors, smoking, overweight and obesity, leisure-time physical activity, and low back pain at baseline. Age did not modify the association between lifestyle factors and hospitalization for sciatica. Furthermore, the Helsinki Health Study and the Young Finns Study did not have statistical power for sex-specific analysis. We therefore did not perform age- or sex-specific analyses in each individual study. We performed stratified analyses to determine whether overweight or obesity modifies the effect of leisure-time physical activity on hospitalization for sciatica. We tested for violation of proportional hazards assumption, and the analyses were conducted in the absence of violation. We used a 2-stage random-effects individual participant data meta-analysis.
      • Burke D.L.
      • Ensor J.
      • Riley R.D.
      Meta-analysis using individual participant data: one-stage and two-stage approaches, and why they may differ.
      We assessed the presence of heterogeneity across the studies by the I2 statistics.
      • Higgins J.
      • Green S.
      Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011], The Cochrane Collaboration.
      We used Stata, version 13, and SAS, version 9.4 (SAS Institute Inc., Cary, NC) for the analyses.

      Results

      The characteristics of the included studies are presented in Table 1. The sample size of the studies ranged between 2545 and 19,343, and the mean follow-up time ranged between 12 and 30 years. The cumulative incidence of hospitalization for sciatica varied from 0.9% to 4.7%. Altogether, 34,589 participants and 1259 events were included in the individual participant data meta-analyses.
      Table 1Characteristics of Individual Participant Data Meta-Analysis of Four Population-Based Prospective Cohort Studies on the Effects of Lifestyle Risk Factors on Hospitalization for Sciatica
      CharacteristicHealth 2000 SurveyMobile Clinic SurveyHelsinki Health StudyYoung Finns Study
      Sample64131934362882545
      Events2609015741
      Mean follow-up time (years)11.630.012.513.8
      Age at baseline, range30-9920-9240-6024-39
      Female, %53497855
      Overweight, %41353431
      Obesity, %22121512
      Ever smoking, %49474742
      Low level of leisure activity, %28263426
      High level of leisure activity, %17123442
      Hospitalization for sciatica, %4.24.70.91.6
      The results of original studies are presented in Supplementary Table 5, Supplementary Table 6, Supplementary Table 7, Supplementary Table 8, Supplementary Table 9, Supplementary Table 10, Supplementary Table 11, Supplementary Table 12, available online. An individual participant data meta-analysis (Table 2) showed that overweight/obesity defined either by BMI or waist circumference increases the risk of hospitalization for sciatica after controlling for age, sex, education, occupation, smoking, leisure-time physical activity, and low back pain at baseline. The included studies, however, showed inconsistent results on the association between obesity defined by BMI and sciatica. There were only 3 events in obese participants in the Helsinki Health Study. Therefore, for this study we report the results for overweight/obesity only. The Health 2000 Survey and Young Finns Study found a significant positive association, whereas in the Mobile Clinic Survey and Helsinki Health Study, no association was found. Limiting the follow-up period to <14 years in the Mobile Clinic Survey yielded a larger hazard ratio (HR) for obesity (1.22; 95% confidence interval [CI], 0.88-1.71), and the pooled HR also reached statistical significance for obesity (HR = 1.36; 95% CI, 1.07-1.74, I2 = 0%).
      Table 2Individual Participant Data Meta-Analysis of Four Prospective Cohort Studies on the Effects of Lifestyle Risk Factors on Hospitalization for Sciatica
      CharacteristicnEventHR95% CII2, %
      Body mass index
       Normal17,1356191
       Overweight or obesity17,1106351.191.01-1.410
        Overweight10,0124581.311.14-1.490
        Obesity40281511.270.92-1.7441
        Obesity
      Limiting the follow-up time to <14 years in the Mobile Clinic Survey.
      40281221.361.07-1.740
      Waist circumference
       Normal3334921
       Overweight or obesity48011931.341.01-1.770
        Overweight1987731.160.84-1.620
        Obesity28141201.411.03-1.930
      Smoking
       Never18,4355851
       Ever15,7466581.201.08-1.340
        Past63402341.110.94-1.310
        Current83334031.331.13-1.5610
      Leisure activity
       Low96823201
       Moderate18,0987491.120.87-1.4445
       High66691890.960.76-1.2120
      Walking or cycling to work
       No66492401
       Yes1651470.670.47-0.960
      CI = confidence interval; HR = hazard ratio.
      Adjustment for age, sex, education, occupation, leisure time physical activity, overweight/obesity, smoking and back pain at baseline.
      * Limiting the follow-up time to <14 years in the Mobile Clinic Survey.
      Current smoking at baseline increased the risk of hospitalization for sciatica by 33% (95% CI, 13%-56%), whereas past smokers were no longer at increased risk. Commuting to work by walking or cycling reduced the risk of hospitalization for sciatica by 33% (95% CI 4%-53%), while other types of leisure activities had no significant effect on hospitalization for sciatica (Table 2). The protective effect of commuting to work by walking or cycling on sciatica was independent of body weight and other leisure-time physical activities, whereas other leisure-time physical activities had no significant effect on hospitalization for sciatica in either normal weight or overweight/obese persons (Table 3).
      Table 3Individual Participant Data Meta-Analysis on the Effect of Leisure-Time Physical Activity on Hospitalization for Sciatica in Normal Weight and Overweight/Obese Participants
      Leisure-Time Physical ActivityNormalOverweight or obese
      nEventHR95% CII2, %nEventHR95% CII2, %
      Overweight and obesity defined by body mass index
       Leisure activity
        Low4203143153461761
        Moderate90533731.140.79-1.653089383721.050.87-1.260
        High37761031.210.66-2.20652756840.880.67-1.140
       Walking or cycling to work
        No260976137961611
        Yes816220.810.49-1.340757230.600.38-0.960
      Overweight and obesity defined by waist circumference
       Leisure activity
        Low7251811432531
        Moderate1476410.960.55-1.68024451111.050.52-2.1040
        High1045331.500.91-2.480849260.420.06-3.1075
       Walking or cycling to work
        No240073137821571
        Yes782150.550.31-0.980765290.770.51-1.170
      CI = confidence interval; HR = hazard ratio.
      Adjustment for age, sex, education, occupation, smoking and back pain at baseline.

      Discussion

      This individual participant data meta-analysis suggests that current smoking and obesity increase the risk of hospitalization due to sciatica by 33%-41%, and commuting to work by walking or cycling protects against hospitalization for sciatica by about 33%. Other leisure-time physical activities had no significant effect on hospitalization for sciatica.
      The findings of the current study are in line with our earlier meta-analyses.
      • Shiri R.
      • Lallukka T.
      • Karppinen J.
      • Viikari-Juntura E.
      Obesity as a risk factor for sciatica: a meta-analysis.
      • Shiri R.
      • Falah-Hassani K.
      The effect of smoking on the risk of sciatica: a meta-analysis.
      • Shiri R.
      • Falah-Hassani K.
      • Viikari-Juntura E.
      • Coggon D.
      Leisure-time physical activity and sciatica: a systematic review and meta-analysis.
      The effect sizes are also similar to those of the meta-analyses. However, the previous studies were mostly cross-sectional or case control studies, and only a limited number of cohort studies focused on clinically verified sciatica. Additionally, we found that walking or cycling to work, independent of body weight and other leisure activities, protects against hospitalization for sciatica. In our earlier meta-analyses
      • Shiri R.
      • Falah-Hassani K.
      • Viikari-Juntura E.
      • Coggon D.
      Leisure-time physical activity and sciatica: a systematic review and meta-analysis.
      • Shiri R.
      • Falah-Hassani K.
      Does leisure time physical activity protect against low back pain? Systematic review and meta-analysis of 36 prospective cohort studies [e-pub ahead of print].
      we found that moderate or high level of leisure-time physical activity protects against lumbar radicular pain
      • Shiri R.
      • Falah-Hassani K.
      • Viikari-Juntura E.
      • Coggon D.
      Leisure-time physical activity and sciatica: a systematic review and meta-analysis.
      and chronic low back pain
      • Shiri R.
      • Falah-Hassani K.
      Does leisure time physical activity protect against low back pain? Systematic review and meta-analysis of 36 prospective cohort studies [e-pub ahead of print].
      by 11%-16% only, supporting the current result that the protective effects of physical activity on most low back outcomes are minor.
      We found no association between overweight/obesity and hospitalization for sciatica in the Helsinki Health Study. The finding can be due to selection bias. It could also be a chance finding, as the number of events was low. The rate of sciatica was, nonetheless, lower than expected in the study population. The cumulative rate of sciatica was 2.0% in all the 41,495 employees of the City of Helsinki aged 40 years or older during a 13-year follow-up, while the rate was only 0.9% in the current study population. This suggests that the employees at lower risk of sciatica have been recruited into the study. However, our analysis showed that overweight or obese employees were more likely to consent to register linkage. Furthermore, in this female-dominated study, weight and height were self-reported. Women underreport their weight and overreport their height, resulting in an underestimation of BMI
      • Merrill R.M.
      • Richardson J.S.
      Validity of self-reported height, weight, and body mass index: findings from the National Health and Nutrition Examination Survey, 2001-2006.
      and diluting the association between overweight/obesity and hospitalization for sciatica.

      Mechanisms

      In our earlier reports
      • Shiri R.
      • Lallukka T.
      • Karppinen J.
      • Viikari-Juntura E.
      Obesity as a risk factor for sciatica: a meta-analysis.
      • Shiri R.
      • Falah-Hassani K.
      The effect of smoking on the risk of sciatica: a meta-analysis.
      • Shiri R.
      • Falah-Hassani K.
      • Viikari-Juntura E.
      • Coggon D.
      Leisure-time physical activity and sciatica: a systematic review and meta-analysis.
      we discussed some possible underlying mechanisms through which lifestyle risk factors contribute to sciatica. Smoking reduces perfusion around the intervertebral discs,
      • Kauppila L.I.
      • McAlindon T.
      • Evans S.
      • Wilson P.W.
      • Kiel D.
      • Felson D.T.
      Disc degeneration/back pain and calcification of the abdominal aorta. A 25-year follow-up study in Framingham.
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      • Viikari-Juntura E.
      • Leino-Arjas P.
      • et al.
      The association between carotid intima-media thickness and sciatica.
      increases production and release of inflammatory cytokines in intervertebral discs,
      • Nemoto Y.
      • Matsuzaki H.
      • Tokuhasi Y.
      • et al.
      Histological changes in intervertebral discs after smoking and cessation: experimental study using a rat passive smoking model.
      • Oda H.
      • Matsuzaki H.
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      • Wakabayashi K.
      • Uematsu Y.
      • Iwahashi M.
      Degeneration of intervertebral discs due to smoking: experimental assessment in a rat-smoking model.
      slows down the healing process,
      • Haugen A.J.
      • Brox J.I.
      • Grovle L.
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      • Wang D.
      • Nasto L.A.
      • Roughley P.
      • et al.
      Spine degeneration in a murine model of chronic human tobacco smokers.
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      • Singh A.
      • Wiseman M.
      • Goodship A.
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      Obesity may be involved in the development of sciatica by increasing the secretion of leptin and inflammatory mediators from excess adipose tissue,
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      Leisure-time physical activity and regular walking or cycling to work are associated with adiposity and 5 y weight gain in middle-aged men: the PRIME Study.

      Strengths and Limitations

      This study had several strengths. First, 3 of the included studies were population based, and one was focused on an employed population subsample. Second, the response rate to the baseline survey was high in the 3 studies, and they can therefore be considered representative samples of their target populations. Third, height and weight were measured in all but one study. Fourth, we used national administrative register data to similarly and objectively define sciatica in all 4 cohorts. The outcome was based on comparable and international ICD codes, and were derived from the same register data holder. Fifth, using register data avoided loss to follow-up. This register covers almost all of Finland, and the accuracy of the codes is, in general, good.
      • Sund R.
      Quality of the Finnish Hospital Discharge Register: a systematic review.
      Sixth, in all studies we were able to exclude the sciatica cases that had been hospitalized 4 years prior to the baseline surveys.
      The study also had some limitations. First, none of the studies objectively measured physical activity at baseline. The included studies used somewhat different questions to assess the level of leisure-time physical activity. Moreover, commuting to work could not be distinguished from other leisure-time physical activities in 2 studies, and only one study collected data on the intensity of physical activity. Second, 2 studies had low statistical power, and we were not able to conduct a reliable sex-specific analysis. Third, the coverage of the Finnish Hospital Discharge Register has been above 80%
      • Sund R.
      Quality of the Finnish Hospital Discharge Register: a systematic review.
      and improved to above 95% only in more recent years.
      • Sund R.
      Quality of the Finnish Hospital Discharge Register: a systematic review.
      Fourth, the 2 largest studies assessed physical activity and measured weight and height at only one point in time. We were therefore unable to consider changes in the risk factors during the relatively long follow-up period. Some participants may have increased or decreased their level of physical activity, and some overweight participants at baseline may have gained more weight and become obese, and alternatively, some obese participants might have lost weight during the follow-up period.

      Conclusions

      Smoking and obesity increase the risk of hospitalization for sciatica, while walking or cycling to work reduces the risk. Walking and cycling can be recommended for the prevention of sciatica in the general population. The public health implications of commuting to work by walking or cycling can be substantial.

      Acknowledgments

      We thank Noora Kartiosuo, Jukka Kontto, and Olli K. Pietiläinen for the register data management.

      Appendix

      Supplementary Table 1Questions on Physical Activity in the Health 2000 Survey
      Item
      How much do you exercise and strain yourself physically in your leisure time?
       In my leisure time I read, watch TV, and do other activities in which I do not move much and which do not strain me physically
       In my leisure time I walk, cycle, and move in other ways at least 4 hours per week
       In my leisure time I exercise at least 3 hours per week
       In my leisure time I practice regularly several times per week for competition
      How often do you exercise in your leisure time so that you are at least slightly out of breath and sweating?
       Daily
       4-6 times a week
       2-3 times a week
       Once a week
       2-3 times a month
       Few times a year or even more rarely
      How many minutes do you walk or cycle on your daily way to and from work?
       I do not work or I work at home
       I use a motor vehicle for the entire trip
       <15 minutes a day
       From 15 to 29 minutes a day
       From 30 to 59 minutes a day
       From 1 to 2 hours a day
       2 hours or longer a day
      Supplementary Table 2Question on Physical Activity in the Mobile Clinic Survey
      Item
      How often and how much do you exert yourself during leisure time in a usual week?
       None or little
       Walking, cycling, or related light activities ≥ 4 hours/week
       Ball games, jogging, or related activities ≥ 3 hours/week
       Regular vigorous exercise
      Supplementary Table 3Question on Physical Activity in the Helsinki Health Study
      Item
      The following question is about your leisure-time physical activity. If you think of the previous 12 months, how vigorous have your physical activities been and how often have you engaged in such activities?
      Walking or equivalent
      Brisk walking or equivalent
      Light jogging or equivalent
      Running or equivalent
       Not at all
       Altogether less than half an hour per week
       Altogether ½-1 hour per week
       Altogether 2-3 hours per week
       Altogether 4 hours or more per week
      Supplementary Table 4Construction of the MET Index in the Young Finns Study
      ItemScore
      How much breathlessness and sweating do you experience when you engage in physical activity and sport?
       Not at all2
       Moderate amount4
       A lot8
      How often do you engage in rigorous physical activity?
       Not at all0
       Once a month or more0.25
       Once a week1.0
       2-3 times a week2.5
       4-6 times a week5.0
       Every day7.0
      How long is duration of individual session of exercise or sports?
       <20 minutes0.17
       20-40 minutes0.50
       40-60 minutes0.83
       More than 60 minutes1.33
      MET index, range0-74.5
      Commuting to work
      How do you travel to and from work in summer?
       Own car
       Public transport
       Walking
       Cycling
      How do you travel to and from work in winter?
       By own car
       By public transport
       Walking
       Cycling
      MET = metabolic equivalent of task.
      Supplementary Table 5Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica by Lifestyle Risk Factors in the Health 2000 Survey
      CharacteristicnEvent% of SciaticaModel 1Model 2Model 3
      HR95% CIHR95% CIHR95% CI
      Frequency of leisure-time physical activity leading to breathlessness or sweating (times per week)
       <11654573.711
       11052393.80.880.58-1.340.800.53-1.22
       2-31975854.31.040.73-1.490.980.68-1.40
       ≥41649784.91.290.94-1.771.180.85-1.64
      Leisure activity
       Low1889603.4111
       Moderate34011534.61.210.90-1.631.140.85-1.531.290.97-1.73
       High1070474.31.070.73-1.561.020.70-1.481.210.84-1.76
      Walking or cycling to work
       <15 minutes a day47362084.5111
       15-29 minutes a day581183.10.580.36-0.940.590.36-0.960.580.35-0.97
       ≥30 minutes a day558254.40.860.56-1.310.830.54-1.270.830.54-1.28
      Body mass index
       Normal2410913.9111
       Overweight25521004.01.140.85-1.531.170.85-1.601.170.85-1.61
       Obesity1409695.01.471.04-2.091.461.01-2.131.491.02-2.18
      Waist circumference
       Normal1943753.9111
       Overweight1570674.31.200.86-1.671.200.85-1.681.180.83-1.66
       Obesity24361094.61.391.00-1.911.370.99-1.911.370.98-1.91
      Smoking
       Never33511163.6111
       Past1328523.91.190.85-1.661.120.78-1.591.050.73-1.49
       Occasional336175.21.300.77-2.191.270.73-2.201.170.66-2.09
       Current1363755.51.521.14-2.021.581.17-2.121.581.16-2.14
      CI = confidence interval.
      Model 1: Adjustment for age and sex.
      Model 2: model 1 + level of educational, workload factors, and chronic back disorder at baseline.
      Model 3: model 2 + leisure activity, commuting to work, body mass index (or waist circumference) and smoking.
      Supplementary Table 6Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica by Lifestyle Risk Factors in the Mobile Clinic Survey
      CharacteristicnEvent% of SciaticaModel 1Model 2Model 3
      HR95% CIHR95% CIHR95% CI
      Leisure activity
       Low50292324.61.001.001.00
       Moderate11,9005614.70.980.84-1.140.950.81-1.110.960.82-1.12
       High24141084.50.760.60-0.960.780.61-0.980.810.64-1.02
      Body mass index
       Normal10,2464834.71.001.001.00
       Overweight67543425.11.351.17-1.571.311.13-1.521.321.14-1.53
       Obesity2343763.21.060.82-1.361.050.81-1.351.050.82-1.35
      Smoking
       Never10,3194234.11.001.001.00
       Past35021674.81.180.97-1.431.120.93-1.361.110.92-1.35
       Current55223115.61.351.15-1.581.231.05-1.451.231.05-1.45
      CI = confidence interval.
      Model 1: Adjustment for age (continuous) and sex.
      Model 2: model 1 + education, occupation, and back pain at baseline.
      Model 3: model 2 + leisure time physical activity, body mass index and smoking.
      Supplementary Table 7Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica By Lifestyle Risk Factors in the Helsinki Health Study
      CharacteristicnEvent% of SciaticaModel 1Model 2Model 3
      HR95% CIHR95% CIHR95% CI
      Leisure activity
       Low2117150.7111
       Moderate2006241.21.670.88-3.191.690.88-3.231.680.87-3.21
       High2165180.81.160.58-2.301.190.60-2.371.180.59-2.38
      Body mass index
       Normal3202311.0111
       Overweight/obesity3086260.80.880.52-1.500.830.49-1.420.850.49-1.46
      Smoking
       Never3330250.8111
       Past1510151.01.330.70-2.541.290.68-2.471.300.68-2.48
       Current1448171.21.590.85-2.951.510.80-2.831.520.80-2.87
      CI = confidence interval.
      Model 1: Adjustment for age and sex.
      Model 2: model 1 + education, occupation, and back pain at baseline.
      Model 3: model 2 + leisure time physical activity, body mass index and smoking.
      Supplementary Table 8Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica by Lifestyle Risk Factors in the Young Finns Study
      CharacteristicnEvent% of SciaticaModel 1Model 2Model 3
      HR95% CIHR95% CIHR95% CI
      Frequency of leisure-time physical activity leading to breathlessness or sweating (times per week)
       <1656132.011
       1726101.40.700.31-1.600.870.37-2.02
       2-3834121.40.740.33-1.610.890.40-2.00
       ≥432961.80.960.36-2.540.980.34-2.81
      Leisure activity (MET)
       Low647132.0111
       Moderate791111.40.700.31-1.560.840.37-1.920.800.30-2.15
       High1020161.60.790.38-1.650.920.43-1.990.920.37-2.30
      Walking or cycling to work in both summer and winter
       No1913321.7111
       Yes51240.80.510.18-1.460.590.20-1.680.350.08-1.50
      Body mass index
       Normal1277141.1111
       Overweight706162.32.030.97-4.211.670.78-3.541.920.85-4.34
       Obesity27662.21.940.74-5.081.660.63-4.342.030.75-5.55
      Waist circumference
       Normal1391171.2111
       Overweight41761.41.150.45-2.930.870.32-2.381.020.36-2.86
       Obesity378112.92.411.11-5.211.860.83-4.131.770.72-4.37
      Daily smoking
       Never1435211.5111
       Ever1073212.01.360.74-2.491.050.55-2.011.090.49-2.44
      CI = confidence interval; MET = metabolic equivalent of task.
      Model 1: Adjustment for age and sex.
      Model 2: Model 1 + educational status, and low back pain at baseline.
      Model 3: Model 2 + leisure activity, walking or cycling to work, waist circumference (or body mass index) and smoking.
      Supplementary Table 9Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica for the Levels of Leisure-Time Physical Activity According to Body Mass Index and Waist Circumference in the Health 2000 Survey
      Leisure-Time Physical ActivityNormalOverweight or Obese
      nEvent% of SciaticaHR95% CInEvent% of SciaticaHR95% CI
      Overweight and obesity defined by body mass index
       Leisure activity
        Low613132.311241474.01
        Moderate1314564.31.700.90-3.222084964.71.090.76-1.56
        High467224.62.041.08-3.83602254.00.870.54-1.41
       Walking or cycling to work
        <15 minutes a day1690664.0130391424.81
        ≥15 minutes a day538213.80.840.50-1.41599223.70.630.39-1.02
       Overweight and obesity defined by waist circumference
       Leisure activity
        Low416153.711206443.91
        Moderate1074383.60.980.54-1.7721861075.01.280.90-1.82
        High445224.91.410.82-2.43581244.00.960.56-1.64
       Walking or cycling to work
        <15 minutes a day1397604.4131701434.61
        ≥15 minutes a day491142.80.570.31-1.04627284.50.800.52-1.22
      CI = confidence interval.
      Adjustment for age, sex, level of educational, workload factors, smoking and chronic back disorder at baseline.
      Supplementary Table 10Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica for the Levels of Leisure-Time Physical Activity According to Body Mass Index in the Mobile Clinic Survey
      Leisure-Time Physical ActivityNormal BMIOverweight or Obese
      nEvent% of SciaticaHR95% CInEvent% of SciaticaHR95% CI
      Low24491204.91.0025801124.31.00
      Moderate63033014.80.930.75-1.1555972604.61.020.81-1.27
      High1494624.10.750.55-1.03920465.00.890.62-1.26
      BMI = body mass index; CI = confidence interval.
      Adjustment for age (continuous), sex, education, occupation, smoking, and back pain at baseline.
      Supplementary Table 11Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica for the Levels of Leisure Time Physical Activity According to Body Mass Index in the Helsinki Health Study
      Leisure-Time Physical ActivityNormal BMIOverweight or Obese
      nEvent% of SciaticaHR95% CInEvent% of SciaticaHR95% CI
      Low85670.81126180.61
      Moderate1052131.21.640.65-4.12954111.21.730.69-4.32
      High1294110.91.160.44-3.0187170.81.200.43-3.32
      BMI = body mass index; CI = confidence interval.
      Adjustment for age (continuous), sex, education, occupation, smoking and back pain at baseline.
      Supplementary Table 12Adjusted Hazard Ratios (HR) of Hospitalization for Sciatica for the Levels of Leisure-Time Physical Activity According to Body Mass Index and Waist Circumference in the Young Finns Study
      Leisure-Time Physical ActivityNormalOverweight or Obese
      nEvent% of SciaticaModel 1Model 2nEvent% of SciaticaModel 1Model 2
      HR95% CIHR95% CIHR95% CIHR95% CI
      Overweight and obesity defined by body mass index
       Leisure activity
        Low28531.01126493.411
        Moderate38430.80.760.15-3.760.780.15-3.9230351.70.490.16-1.470.670.22-2.09
        High52181.51.430.38-5.421.620.42-6.2436361.70.490.17-1.380.530.17-1.62
      Walking or cycling to work in both summer and winter
        No919101.111757192.511
        Yes27810.40.360.04-2.820.440.05-3.4815810.60.260.03-1.940.270.04-2.07
      Overweight and obesity defined by waist circumference
       Leisure activity
        Low30931.01122694.011
        Moderate40230.80.790.16-3.930.850.17-4.2825941.50.390.12-1.280.560.17-1.91
        High600111.81.930.54-6.942.140.59-7.8126820.80.190.04-0.880.120.02-1.01
      Walking or cycling to work in both summer and winter
        No1003131.311612142.311
        Yes29110.30.300.04-2.340.360.05-2.7813810.70.330.04-2.540.330.04-2.56
      CI = confidence interval.
      Model 1: Adjustment for age and sex.
      Model 2: Adjustment for age, sex, educational status, and low back pain at baseline.

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