You are here

  • Home
  • Archive
  • Volume 74, Issue 11
  • Inguinal hernia repair among men in relation to occupational mechanical exposures and lifestyle factors: a longitudinal study

Article Text

Article menu

Download PDFPDF

Workplace
Original Article
Inguinal hernia repair among men in relation to occupational mechanical exposures and lifestyle factors: a longitudinal study
  1. Marie Vestergaard Vad1,2,
  2. Poul Frost2,
  3. Jacob Rosenberg3,
  4. Johan Hviid Andersen1,
  5. Susanne Wulff Svendsen1
  1. 1 Danish Ramazzini Centre, Department of Occupational Medicine, Regional Hospital West Jutland—University Research Clinic, Herning, Denmark
  2. 2 Danish Ramazzini Centre, Department of Occupational Medicine, Aarhus University Hospital, Aarhus, Denmark
  3. 3 Department of Surgical Gastroenterology, Herlev Hospital, University of Copenhagen, Herlev, Denmark
  1. Correspondence to Dr Marie Vestergaard Vad, Danish Ramazzini Centre, Department of Occupational Medicine, Aarhus University Hospital, Noerrebrogade 44, Building 2C, DK-8000 Aarhus C, Denmark; marivd{at}rm.dk

Abstract

Objectives To evaluate exposure–response relationships between occupational mechanical exposures and first-time lateral and medial inguinal hernia repair and effects of lifestyle factors. To estimate if occupational mechanical exposures advance the repairs.

Methods This longitudinal study was based on a cohort of men aged 18–65 years with questionnaire data from the Musculoskeletal Research Database at the Danish Ramazzini Centre. We estimated occupational mechanical exposures using a job exposure matrix. First-time inguinal hernia repairs from 1998 to 2014 were identified in the Danish Hernia Database. We used Cox regression analyses and calculated excess fractions among the exposed and rate advancement periods (RAPs).

Results Among 17 967 men, we identified 382 lateral and 314 medial repairs. The risk of lateral repairs increased with time spent standing/walking with an HR of 1.45 (95% CI 1.12 to 1.88) for ≥6 hours/day versus <4 hours/day, corresponding to an excess fraction of cases of 31% in the group with ≥6 hours/day. This group had a RAP of 6.7 (95% CI 2.6 to 10.8) years. Medial repairs were not associated with occupational mechanical exposures. A body mass index ≥30 kg/m2 showed lower HRs for both repair types. Leisure-time physical activity and smoking status were not related to any of the outcomes.

Conclusions Assuming a causal relationship, the results suggest that around 30% of all first-time lateral inguinal hernia repairs in the highest exposure category would be preventable if the time spent standing/walking could be reduced from ≥6 to <4 hours/day. The repairs might even be postponed by 6–7 years.

  • body mass index
  • groin hernia
  • herniorrhaphy
  • lifting
  • rate advancement period
  • physical activity
  • smoking
  • standing
  • work

https://doi.org/10.1136/oemed-2016-104160

Statistics from Altmetric.com

    Request Permissions

    If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.

    What this paper adds

    • Little is known on occupational risk factors for inguinal hernia.

    • This study, which included lifestyle factors, corroborated a relationship between standing/walking and lateral but not medial repair.

    • 30% of first-time lateral repairs among men exposed to standing/walking ≥6 hours/day would be preventable, if the exposure could be reduced to <4 hours/day. The repairs might even be postponed by 6–7 years.

    Introduction

    Inguinal hernia affects 25%–30% of all men during their lifetime and inguinal hernia repair is one of the most common procedures in general surgery. Except in case of serious comorbidity, surgical treatment is usually recommended if the hernia is symptomatic.1 In Denmark, around 10 000 surgeries are performed each year, and around 90% of the procedures are performed among men.2 The high number of surgical procedures entails considerable individual and societal costs. In a recent review, we concluded that little is known about the effect of occupational mechanical exposures.3 Previous studies were flawed by insufficient exposure assessment and unspecific outcome assessment; in particular, no distinction was made between lateral and medial hernias.3

    Since the review, we have conducted a nationwide register-based follow-up study where we found that occupational mechanical exposures in terms of hours/day spent standing/walking, total load lifted per day and daily frequency of lifting loads weighing ≥20 kg were risk factors for lateral, but not medial, inguinal hernia.4 The study controlled for socioeconomic status but did not directly address potential confounding factors such as a high body mass index (BMI), smoking and leisure-time physical activity.4

    A high BMI has previously been assumed to be a risk factor for inguinal hernia, but recent studies have suggested that a high BMI is inversely related to the risk of inguinal hernias.5 6 This might be due to lower incidence because visceral fat may act as a barrier against protrusion of the hernia sack or detection bias because it is more difficult for the patients to become aware of a hernia or more difficult to diagnose a hernia clinically. Relative contraindications for surgery among the obese may also play a part.7 8 Smoking has been reported to be a risk factor for incisional hernia and recurrent inguinal hernia, which may be due to changes in collagen metabolism.9–11 On the other hand, smoking has been negatively associated with inguinal hernia formation, which remains unexplained.12–15 Physical activity may increase the intra-abdominal pressure, which could lead to herniation through an already weak point in the abdominal wall.15

    Several studies have shown a monotonous increase in the incidence of inguinal hernia repair with age,8 12 which means that calculation of rate advancement periods (RAPs) would be applicable.16–18 RAPs measure how much earlier in life the outcome occurs among the exposed. Accelerated occurrence is not reflected in traditional epidemiological estimates of relative risk or excess fraction of cases.19 To our knowledge, no studies have addressed the question, whether inguinal hernia repair is performed earlier in life with increasing occupational mechanical exposures. If occupational mechanical exposures are associated with earlier repair of inguinal hernia, this may be of public health concern, for one thing because of a 10% risk of chronic postoperative pain.20

    We aimed to evaluate the hypothesis that exposure–response relationships exist between hours/day spent standing/walking, total load lifted per day and daily frequency of lifting loads weighing ≥20 kg and the risk of first-time lateral, but not medial, inguinal hernia repair, when controlled for lifestyle factors. Additional aims were to evaluate effects of lifestyle factors and assess RAPs.

    Methods

    Design and study cohort

    We performed a follow-up study using the Musculoskeletal Research Database (MRD) at the Danish Ramazzini Centre. When we originally described the MRD, it contained questionnaire data from 39 590 persons, who had participated in at least one of nine studies of Danish working populations, which were conducted from 1993 to 2004.21 Since then, questionnaire data from 4325 participants in a general practice study from 2008 have been added, including 1858 men who reported that they were occupationally active.22 In the nine original studies, the proportion that responded was ≥70%; in the 10th study, this proportion was 60%.22 The MRD now contains questionnaire data from 18 455 men. Of note, the MRD does not contain a representative sample of the Danish working age population; rather, the cohort was established to over-represent high occupational mechanical exposures, while still covering the whole exposure spectrum and ensuring exposure contrast.

    We restricted the population of the present study to men aged 18–65 years at start of follow-up (birth years 1932 to 1990, both years included). A total of 146 men participated in 2 of the 10 original studies in the MRD. For these men, we chose the most informative questionnaire in the present context. We excluded men with missing information on both occupational title and code according to the Danish version of the International Standard Classification of Occupations from 1988 (D-ISCO 88). We also excluded men who underwent inguinal hernia repair before start of follow-up (see below) and men with a recurrent operation as their first registration in the Danish Hernia Database. Information on death and emigration was obtained from the Danish Civil Registration System.23 The Danish Data Protection Agency approved the MRD (record number 1-16-02-616-4) and the present study (record number 1-16-02-241-12). In Denmark, register and questionnaire studies do not require approval by committees on biomedical research ethics.

    Exposure assessment

    Individual exposure estimates were obtained by connecting information on occupational title (or D-ISCO 88 code if the occupational title was not available) from the MRD with exposure estimates from a job exposure matrix (JEM), the Lower Body JEM.18 The JEM cross-tabulates 121 homogeneously exposed job groups with the mean of five experts’ ratings of the number of hours/day spent standing/walking, the total load lifted (kg/day) and the daily frequency of lifting loads weighing ≥20 kg in each job group. Standing/walking was categorised as <4 (reference), 4–<6 and 6–≤7.3 hours/day.24 The daily frequency of lifting loads weighing ≥20 kg was categorised as <2 (reference), 2–<11 and 11–≤89 lifts/day. The total load lifted per day was categorised as 0 (reference), >0–<1000 and 1000–≤4900 kg/day.24

    Outcome assessment

    First-time inguinal hernia repair between start and end of follow-up (see below) was retrieved from the Danish Hernia Database, which covers inguinal hernia repairs in public and private hospitals with a completeness of >90%.2 25 26 Inguinal hernia repairs were categorised as lateral, medial or other (pantaloon or unspecified hernias) and as left-sided or right-sided. In case of surgery for a left-sided and a right-sided hernia on the same day, the outcome was counted once if the hernias had the same anatomical type (n=10) and twice otherwise (n=47).

    Lifestyle factors and other covariates

    Information on lifestyle factors was obtained from the MRD. Age at start of follow-up was categorised as 18–<30 (reference), 30–<40, 40–<50, 50–<60 and 60–<66 years. BMI was calculated as weight divided by height squared and categorised as <25 (reference), 25–<30 and 30–≤63 kg/m2. Smoking status was categorised as never smoker (reference), ex-smoker and current smoker. Leisure-time physical activity was categorised as <2 hours/week of light activity (reference), 2–4 hours/week of light activity, light activity >4 or strenuous activity 2–4 hours/week and strenuous activity >4 hours/week (participants were asked to mark the category that fitted best). Three of the questionnaires provided examples: light activity could be taking small walks, riding a bike at a steady pace and light garden work, while strenuous activity could be walking fast, biking at high speed and competitive sports. One of the original studies with 1871 male participants (slaughterhouse workers) did not ask about smoking, height or weight, and one study with 2053 male participants (machinists, car mechanics and house painters) did not ask about leisure-time physical activity. D-ISCO 88 main group (1–9) was used as a proxy for socioeconomic status and was categorised as self-employed and academics (groups 1–2), white-collar workers (groups 3–5) and blue-collar workers (groups 6–9).

    Statistical analysis

    We performed crude, age-adjusted and multivariable Cox proportional hazard analyses to estimate HRs for lateral and medial inguinal hernia repair, respectively. The number of follow-up years was calculated from the date of filling in the MRD questionnaire, the participant’s 18th year birthday or 1 January 1998, whichever came last, until the date of first-time inguinal hernia repair (irrespective of type) or censoring on the date of death or emigration, the participant’s 66th year birthday, or 31 December 2014, whichever came first.

    We performed two-by-two correlation analysis between the exposure variables. Since we expected the exposure variables to be highly correlated, we a priori decided to analyse one occupational mechanical exposure at a time. We also decided not to include socioeconomic status in the analyses because we used D-ISCO 88 codes to estimate socioeconomic status and therefore expected collinearity with the exposure variables. The fully adjusted analyses included the following covariates categorised as described above: age at start of follow-up, BMI, smoking status, leisure-time physical activity and year of data collection in the original study. Supplementary analyses of smoking intensity and pack-years of smoking were performed for five and three of the original studies where the necessary information was available.

    To minimise the problem of missing information on lifestyle factors, particularly in highly exposed job groups, we used multiple imputation to generate 20 copies of the data27 28 using age at start of follow-up, socioeconomic status, year of data collection in the original study, number of hours/day spent standing/walking, total load lifted per day, daily frequency of lifting loads weighing ≥20 kg and the respective outcome. We also performed complete case analyses. The excess fraction among the exposed was calculated as (HR-1)/HR multiplied by the number of exposed cases and converted to per cent. To evaluate whether age could be included as a linear term for calculation of RAPs, we modelled the age effect using linear, quadratic and cubic age terms and compared model fit using likelihood ratio tests. A monotonous relationship with age was observed for both repair types and comparison of a model with linear age terms and a model with all three age terms showed that data conformed to a simple linear model (p=0.3217 for lateral and p=0.7860 for medial repair). Thus, we were able to calculate RAPs.16–18 For all models, we ensured that the proportional hazards assumption was met by means of a global test, which also comprised a variable-by-variable test.29 30 We used STATA 13 (StataCorp LP, College Station, Texas, USA).

    Results

    Figure 1 shows the flow-chart of the study. A total of 17 967 men contributed to the analyses with 201 312 person-years, corresponding to an average follow-up time of 11.2 years per person. The mean age at start of follow-up was 43.3 (SD 10.7) years. During follow-up, 382 lateral, 314 medial and 41 pantaloon or unspecified hernia repairs occurred. For lateral and medial hernia repair, the incidence rate was 19 and 16 per 10 000 person-years, respectively. The mean age at surgery was 46 years for both types of repair.

    Figure 1
    Figure 1

    Flowchart. D-ISCO 88, Danish version of the International Standard Classification of Occupations from 1988.

    Table 1 shows characteristics of the cohort according to categories of standing/walking. Some of the most frequent job titles in the group with 4–<6 hours/day spent standing/walking at work were technician, salesman, farmer, carpenter and electrician, and some of the most frequent job titles in the group with 6–≤7.3 hours/day were meat cutter, motor vehicle mechanic and machine operator. As expected, the percentage of blue-collar workers increased with increasing exposure to standing/walking. The correlation coefficient between total load lifted per day and time spent standing/walking was 0.68, and between the two lifting exposures, it was 0.96. The percentages in table 1, which include imputed values, are almost identical to the corresponding percentages in online supplementary table S1, which are based on non-missing data. As seen in the online supplementary table S1, the highest numbers with missing information on lifestyle factors occurred in the group with 6–≤7.3 hours/day spent standing/walking. Information on BMI was missing for 2290 men, while information on smoking was missing for 2743 and leisure-time physical activity for 2364.

    Supplementary Material

    Supplementary Table 1
    View this table:
    Table 1

    Characteristics of the study cohort

    For lateral hernia repair, table 2 shows HRs in relation to each of the three occupational mechanical exposures. For hours/day spent standing/walking, a significant relation was found for 6–≤7.3 hours/day with a 45% increase in the risk of lateral repair compared with the group with <4 hours/day. For total load lifted per day and daily frequency of lifting loads weighing ≥20 kg, the fully adjusted HRs did not reach statistical significance, but a trend was suggested for both measures of lifting. Based on the fully adjusted HRs, the excess fraction of cases was 31% among participants who spent 6–≤7.3 hours/day standing/walking, which means that 31% of the repairs in this exposure group should be preventable if the exposure could be reduced to <4 hours/day. According to the complete case analysis, the risk of lateral inguinal hernia repair increased with time spent standing/walking with a fully adjusted HR of 1.29 (95% CI 1.00 to 1.68) for 6–≤7.3 hours/day as compared with <4 hours/day. For medial hernia repair, we found no associations with occupational mechanical exposures as shown in table 3.

    View this table:
    Table 2

    HRs of first-time lateral inguinal hernia repair in relation to occupational mechanical exposures and covariates

    View this table:
    Table 3

    HRs of first-time medial inguinal hernia repair in relation to occupational mechanical exposures and covariates

    As seen in tables 2 and 3, age was a risk factor for both lateral and medial hernia repair with similar HRs for the two types of repair. A BMI ≥30 kg/m2 was associated with lower HRs for both lateral and medial repair. Leisure-time physical activity and smoking status were not related to any of the outcomes. For smoking intensity, the HR for 1–19 cigarettes/day was 0.62 (95% CI 0.33 to 1.16) and the HR for 20–58 cigarettes/day was 0.48 (95% CI 0.24 to 0.97). The HRs for 1–19 and 20–100 pack-years of smoking were 0.78 (95% CI 0.33 to 1.87) and 0.50 (95% CI 0.20 to 1.25).

    For the group that spent 6–≤7.3 hours/day standing/walking, we found a RAP of 6.7 years (95% CI 2.6 to 10.8) as compared with the group that spent <4 hours/day standing/walking. In other words, lateral inguinal hernia repair was performed 6–7 years earlier in life.

    Discussion

    In the group that spent ≥6 hours/day standing/walking at work, we found a 45% higher risk of lateral inguinal hernia repair than in the group that spent <4 hours/day standing/walking, when controlled for lifestyle factors. The fully adjusted HRs for both measures of lifting did not reach statistical significance, but a trend was suggested. For medial hernia repair, we found no associations with occupational mechanical exposures. A BMI ≥30 kg/m2 showed lower HRs for both repair types. Leisure-time physical activity and smoking status were not related to any of the outcomes. Assuming a causal relationship, the results suggested that around 30% of all first-time lateral inguinal hernia repairs in the highest exposure group would be preventable, if the time per day spent standing/walking could be reduced from ≥6 to <4 hours/day. Additionally, the repairs might be postponed by 6–7 years.

    We have previously found exposure-–response relationships between lateral inguinal hernia repairs and occupational mechanical exposures in a nationwide register-based cohort study, which comprised around 1.5 million men.4 The present cohort partly overlapped with the much larger nationwide cohort, but the participants did not constitute a random sample. They differed from the nationwide cohort with respect to the distribution of age (43 vs 36 years at start of follow-up) and occupational mechanical exposures; for example, 9% of the nationwide cohort spent >5 hours/day standing/walking versus 40% that spent ≥6 hours/day in the present study. Moreover, the exposure assessment was based on high-quality job titles in the MRD, where several of the original study populations were selected based on job title information21 instead of registered DISCO-88 codes, and we were able to adjust for and evaluate effects of lifestyle factors. We also calculated RAPs. Thus, the present study contributes new information to the existing evidence base.

    The study benefited from a longitudinal design, independent assessment of specific occupational mechanical exposures and a distinction between first-time lateral and medial inguinal hernia repair. Exposure and outcome assessment was based on JEM and register data across all studies, which means that any effect of methodological differences between the original studies would be limited to lifestyle factors. We adjusted for year of data collection in the original study as a measure against this potential source of confounding.

    Patients with inguinal hernia with high occupational mechanical exposures may be offered surgical treatment more often if they experience more frequent and/or more serious symptoms than patients with low exposures. However, there is no obvious reason to believe that work-related symptom aggravation would occur specifically for patients with lateral inguinal hernia, which means that detection bias probably cannot explain the difference between the two hernia types. We are currently investigating the possibility of differential preoperative pain status for patients with medial and lateral hernia depending on occupational mechanical exposures.

    The exposures were only estimated once, which entails exposure misclassification to the extent that participants changed jobs so that their exposures declined or increased during follow-up, but people tend to stay in jobs with similar exposures over time and formation of an inguinal hernia is an unlikely reason for a job change. Any changes in exposure levels would therefore be non-differential and lead to attenuation of the effect estimates.

    The expert-rated exposure estimates were not validated against estimates obtained by observation or technical measurements, which means that the results may not accurately reflect the risk of surgery associated with the exact exposure categories that we applied. On the other hand, the validity of the JEM was originally assessed by two external experts, who in general agreed with the ranking of exposures,18 and the JEM has previously shown good predictive validity in studies of total hip replacement31 and surgery for varicose veins.24 When the Lower Body JEM was constructed, a comprehensive minimally exposed group was created to ensure that few occupational titles with low exposures would be incorrectly classified into a high exposure category18 32 (ie, to ensure a high specificity); therefore, we were not able to construct a group with ‘zero’ exposure. To the extent that these occupational titles were more than minimally exposed, exposure–response relationships based on the JEM would underestimate true associations. In the present study, self-employed, academics and white-collar workers constituted almost 90% of the group that was minimally exposed to standing/walking, which means that their mean exposures may have been well below 4 hours/day. To fulfil the preventive potential suggested by the present study, it may therefore be necessary to lower the exposure to standing/walking to well below less than 4 hours/day.

    One of the original studies had no information on leisure-time physical activity and one study had no information on BMI and smoking. Since data were not missing completely at random as the participants in these studies all belonged to the highest exposure category of standing/walking, complete case analysis would entail a risk of underestimating the effects of occupational mechanical exposures. Accordingly, sensitivity analyses omitting participants with missing information on lifestyle factors showed slightly lower HRs. This was the reason why we applied multiple imputation.

    We have previously found exposure–response relationships between lateral inguinal hernia repair and exposures accrued within a 5-year time window.4 This may represent effects of daily exposure intensities rather than cumulative effects since the time window was fixed and people tend to stay in jobs with similar exposures over time (see above). The effect estimates of the present study were similar to those of our previous study, where we found an OR of 1.43 (95% CI 1.34 to 1.51) for the highest cumulative exposure category of standing/walking.4 In that study, we reported a preventive potential at the population level of 15%. The preventive potential in the most highly exposed group was 31% ((1.43–1)/1.43),4 which corresponds closely to the findings of the present study. In our previous study, we also found a possible effect of lifting exposures.4 The present study is in accordance with this finding even though the results did not reach statistical significance. We are aware of two other studies with results that seem to be consistent with a specific relationship between occupational mechanical exposures and lateral inguinal hernia.3 One of these studies found an association between heavy lifting and lateral hernia;33 the other found that a sudden onset was more likely among patients with lateral hernia than among patients with medial hernia and that a sudden onset was more likely among patients with heavy work.34 The mechanism behind lateral hernia formation could be that increased abdominal pressure while standing/walking leads to a protrusion of the abdominal content through the inguinal canal.4 An upright position may be a prerequisite for the canal to open, whereas sitting may preclude hernia formation—this could be investigated in laboratory settings. Any cumulative effects might be explained by a gradual widening of the pre-existing opening over years with high exposures, which would lead to a gradual increase in the daily risk of hernia formation.

    We found that a high BMI was associated with a reduced risk of first-time lateral and medial inguinal hernia repair. The finding was based on few cases in the category with a BMI ≥30 kg/m2, but the association was consistent with findings in several previous studies.6 7 14 35 A high amount of visceral fat may have a protective effect by prohibiting protrusion of a hernia sack, even though a high BMI has been related to a high intra-abdominal pressure.36 Alternatively, the association may be explained by bias because inguinal hernias are underdiagnosed in persons with a high BMI or because persons with a high BMI are less likely to be offered surgical treatment for an inguinal hernia due to BMI-related comorbidity. In any case, our results regarding occupational mechanical exposures were adjusted for BMI.

    Previous studies have yielded inconsistent results regarding smoking as a risk factor for inguinal hernias, and few studies have addressed leisure-time physical activity.12 35 We found no relation between smoking status and leisure-time physical activity and the risk of lateral or medial hernia repair. In most of the original studies in the MRD, the information on smoking was limited to smoking status, which means that we may have overlooked any effect, but the supplementary analyses based on information on smoking intensity and pack-years of smoking also did not suggest apositive  association. Our results do not preclude an increased risk of inguinal hernia in relation to specific sports such as weightlifting.37

    If corroborated in other study populations, our results suggest an untapped potential for prevention in terms of number and timing of lateral inguinal hernia repairs, if the number of hours/day spent standing/walking can be reduced for men in the most highly exposed group. This may even have other benefits since prolonged standing/walking has also been associated with, for example, leg discomfort38 and varicose veins.24 Our results cannot be generalised to women, but the results regarding relationships between occupational mechanical exposures and inguinal hernia repairs should be generalisable to other male populations.

    In conclusion, this study, where we were able to include lifestyle factors, corroborated our previous observation of a relationship between ≥6 hours/day spent standing/walking at work and the risk of first-time lateral, but not medial inguinal hernia repair. A high BMI was associated with a lower risk of both types of repair, while smoking status and leisure-time physical activity were unrelated to the outcomes. Assuming a causal relationship, around 30% of all first-time lateral inguinal hernia repairs in the highest exposure group would be preventable, if the time spent standing/walking could be reduced from ≥6 to <4 hours/day. The repairs might even be postponed by 6–7 years.

    References

    1. ↵
      2. Simons MP ,
      3. Aufenacker T ,
      4. Bay-Nielsen M , et al
      . European Hernia Society guidelines on the treatment of inguinal hernia in adult patients. Hernia 2009;13:343–403.doi:10.1007/s10029-009-0529-7
      OpenUrlCrossRefPubMedWeb of Science
    2. ↵
      2. Bay-Nielsen M ,
      3. Kehlet H ,
      4. Strand L , et al
      . Quality assessment of 26,304 herniorrhaphies in Denmark: a prospective nationwide study. Lancet 2001;358:1124–8.doi:10.1016/S0140-6736(01)06251-1
      OpenUrlCrossRefPubMedWeb of Science
    3. ↵
      2. Svendsen SW ,
      3. Frost P ,
      4. Vad MV , et al
      . Risk and prognosis of inguinal hernia in relation to occupational mechanical exposures--a systematic review of the epidemiologic evidence. Scand J Work Environ Health 2013;39:5–26.doi:10.5271/sjweh.3305
      OpenUrl
    4. ↵
      2. Vad MV ,
      3. Frost P ,
      4. Bay-Nielsen M , et al
      . Impact of occupational mechanical exposures on risk of lateral and medial inguinal hernia requiring surgical repair. Occup Environ Med 2012;69:802–9.doi:10.1136/oemed-2012-100787
      OpenUrlAbstract/FREE Full Text
    5. ↵
      2. Zendejas B ,
      3. Ramirez T ,
      4. Jones T , et al
      . Incidence of inguinal hernia repairs in Olmsted County, MN: a population-based study. Ann Surg 2013;257:520–6.doi:10.1097/SLA.0b013e31826d41c6
      OpenUrlCrossRefPubMed
    6. ↵
      2. Ravanbakhsh S ,
      3. Batech M ,
      4. Tejirian T
      . Increasing Body Mass Index is inversely related to groin hernias. Am Surg 2015;81:1043–6.
      OpenUrl
    7. ↵
      2. Zendejas B ,
      3. Hernandez-Irizarry R ,
      4. Ramirez T , et al
      . Relationship between body mass index and the incidence of inguinal hernia repairs: a population-based study in Olmsted County, MN. Hernia 2014;18:283–8.doi:10.1007/s10029-013-1185-5
      OpenUrl
    8. ↵
      2. de Goede B ,
      3. Timmermans L ,
      4. van Kempen BJ , et al
      . Risk factors for inguinal hernia in middle-aged and elderly men: results from the Rotterdam Study. Surgery 2015;157:540–6.doi:10.1016/j.surg.2014.09.029
      OpenUrlPubMed
    9. ↵
      2. Henriksen NA ,
      3. Mortensen JH ,
      4. Sorensen LT , et al
      . The collagen turnover profile is altered in patients with inguinal and incisional hernia. Surgery 2015;157:312–21.doi:10.1016/j.surg.2014.09.006
      OpenUrl
    10. ↵
      2. Henriksen NA ,
      3. Yadete DH ,
      4. Sorensen LT , et al
      . Connective tissue alteration in abdominal wall hernia. Br J Surg 2011;98:210–9.doi:10.1002/bjs.7339
      OpenUrlCrossRefPubMed
    11. ↵
      2. Sorensen LT ,
      3. Friis E ,
      4. Jorgensen T , et al
      . Smoking is a risk factor for recurrence of groin hernia. World J Surg 2002;26:397–400.doi:10.1007/s00268-001-0238-6
      OpenUrlCrossRefPubMedWeb of Science
    12. ↵
      2. Ruhl CE ,
      3. Everhart JE
      . Risk factors for inguinal hernia among adults in the US population. Am J Epidemiol 2007;165:1154–61.doi:10.1093/aje/kwm011
      OpenUrlCrossRefPubMedWeb of Science
    13. ↵
      2. Liem MS ,
      3. van der Graaf Y ,
      4. Zwart RC , et al
      . Risk factors for inguinal hernia in women: a case-control study. The Coala Trial Group. Am J Epidemiol 1997;146:721–6.
      OpenUrlPubMedWeb of Science
    14. ↵
      2. Rosemar A ,
      3. Angerås U ,
      4. Rosengren A
      . Body mass index and groin hernia: a 34-year follow-up study in Swedish men. Ann Surg 2008;247:1064–8.doi:10.1097/SLA.0b013e31816b4399
      OpenUrlCrossRefPubMed
    15. ↵
      2. Bendavid R
      . The unified theory of hernia formation. Hernia 2004;8:171–6.doi:10.1007/s10029-004-0217-6
      OpenUrlPubMed
    16. ↵
      2. Seidler A ,
      3. Euler U ,
      4. Bolm-Audorff U , et al
      . Physical workload and accelerated occurrence of lumbar spine diseases: risk and rate advancement periods in a German multicenter case-control study. Scand J Work Environ Health 2011;37:30–6.doi:10.5271/sjweh.3121
      OpenUrlCrossRefPubMed
    17. ↵
      2. Brenner H ,
      3. Gefeller O ,
      4. Greenland S
      . Risk and rate advancement periods as measures of exposure impact on the occurrence of chronic diseases. Epidemiology 1993;4:229–36.doi:10.1097/00001648-199305000-00006
      OpenUrlCrossRefPubMedWeb of Science
    18. ↵
      2. Rubak TS ,
      3. Svendsen SW ,
      4. Andersen JH , et al
      . An expert-based job exposure matrix for large scale epidemiologic studies of primary hip and knee osteoarthritis: The Lower Body JEM. BMC Musculoskelet Disord 2014;15:204.doi:10.1186/1471-2474-15-204
      OpenUrlCrossRefPubMed
    19. ↵
      2. Greenland S
      . Relation of probability of causation to relative risk and doubling dose: a methodologic error that has become a social problem. Am J Public Health 1999;89:1166–9.doi:10.2105/AJPH.89.8.1166
      OpenUrlCrossRefPubMedWeb of Science
    20. ↵
      2. Fitzgibbons RJ ,
      3. Forse RA
      . Clinical practice. Groin hernias in adults. N Engl J Med 2015;372:756–63.doi:10.1056/NEJMcp1404068
      OpenUrl
    21. ↵
      2. Svendsen SW ,
      3. Dalbøge A ,
      4. Andersen JH , et al
      . Risk of surgery for subacromial impingement syndrome in relation to neck-shoulder complaints and occupational biomechanical exposures: a longitudinal study. Scand J Work Environ Health 2013;39:568–77.doi:10.5271/sjweh.3374
      OpenUrlCrossRefPubMedWeb of Science
    22. ↵
      2. Jensen JC ,
      3. Haahr JP ,
      4. Frost P , et al
      . Do work-related factors affect care-seeking in general practice for back pain or upper extremity pain? Int Arch Occup Environ Health 2013;86:799–808.doi:10.1007/s00420-012-0815-z
      OpenUrl
    23. ↵
      2. Pedersen CB
      . The Danish Civil Registration System. Scand J Public Health 2011;39:22–5.doi:10.1177/1403494810387965
      OpenUrlCrossRefPubMedWeb of Science
    24. ↵
      2. Tabatabaeifar S ,
      3. Frost P ,
      4. Andersen JH , et al
      . Varicose veins in the lower extremities in relation to occupational mechanical exposures: a longitudinal study. Occup Environ Med 2015;72:330–7.doi:10.1136/oemed-2014-102495
      OpenUrlAbstract/FREE Full Text
    25. ↵
      2. Burcharth J ,
      3. Liljekvist MS ,
      4. Pommergaard HC , et al
      . Cumulative incidence and registry validation of groin hernia repair in a 34-year nationwide cohort. Dan Med J 2016;63:A5284.
      OpenUrl
    26. ↵
      2. Friis-Andersen H ,
      3. Bisgaard T
      . The Danish Inguinal Hernia database. Clin Epidemiol 2016;8:521–4.doi:10.2147/CLEP.S99512
      OpenUrl
    27. ↵
      2. Sterne JA ,
      3. White IR ,
      4. Carlin JB , et al
      . Multiple imputation for missing data in epidemiological and clinical research: potential and pitfalls. BMJ 2009;338:b2393.doi:10.1136/bmj.b2393
      OpenUrlFREE Full Text
    28. ↵
      2. White IR ,
      3. Royston P
      . Imputing missing covariate values for the Cox model. Stat Med 2009;28:1982–98.doi:10.1002/sim.3618
      OpenUrlCrossRefPubMedWeb of Science
    29. ↵
      2. Grambsch PM ,
      3. Therneau TM
      . Proportional hazards tests and diagnostics based on weighted residuals. Biometrika 1994;81:515–26.doi:10.1093/biomet/81.3.515
      OpenUrlCrossRefWeb of Science
    30. ↵
      2. Schoenfeld D
      . Partial residuals for the proportional hazards regression model. Biometrika 1982;69:239–41.doi:10.1093/biomet/69.1.239
      OpenUrlCrossRefWeb of Science
    31. ↵
      2. Rubak TS ,
      3. Svendsen SW ,
      4. Søballe  K , et al
      . Total hip replacement due to primary osteoarthritis in relation to cumulative occupational exposures and lifestyle factors: a nationwide nested case-control study. Arthritis Care Res 2014;66:1496–505.doi:10.1002/acr.22326
      OpenUrlCrossRef
    32. ↵
      2. Armstrong BG
      . Effect of measurement error on epidemiological studies of environmental and occupational exposures. Occup Environ Med 1998;55:651–6.doi:10.1136/oem.55.10.651
      OpenUrlAbstract/FREE Full Text
    33. ↵
      2. Salcedo-Wasicek MC ,
      3. Thirlby RC
      . Postoperative course after inguinal herniorrhaphy. A case-controlled comparison of patients receiving workers' compensation vs patients with commercial insurance. Arch Surg 1995;130:29–32.
      OpenUrlCrossRefPubMedWeb of Science
    34. ↵
      2. Sanjay P ,
      3. Woodward A
      . Single strenuous event: does it predispose to inguinal herniation? Hernia 2007;11:493–6.doi:10.1007/s10029-007-0253-0
      OpenUrlCrossRefPubMed
    35. ↵
      2. Carbonell JF ,
      3. Sanchez JL ,
      4. Peris RT , et al
      . Risk factors associated with inguinal hernias: a case control study. Eur J Surg 1993;159:481–6.
      OpenUrlPubMedWeb of Science
    36. ↵
      2. Cobb WS ,
      3. Burns JM ,
      4. Kercher KW , et al
      . Normal intraabdominal pressure in healthy adults. J Surg Res 2005;129:231–5.doi:10.1016/j.jss.2005.06.015
      OpenUrlCrossRefPubMedWeb of Science
    37. ↵
      2. Davis PR
      . The causation of herniae by weight-lifting. Lancet 1959;2:155–7.doi:10.1016/S0140-6736(59)90563-X
      OpenUrlPubMedWeb of Science
    38. ↵
      2. Waters TR ,
      3. Dick RB
      . Evidence of health risks associated with prolonged standing at work and intervention effectiveness. Rehabil Nurs 2015;40:148–65.doi:10.1002/rnj.166
      OpenUrl
    View Abstract

    Footnotes

    • Contributors All authors have contributed to this study according to the ICMJE recomendations.

    • Funding The study was funded by The Danish Working Environment Fund; Project number: 2013002392/5.

    • Competing interests None declared.

    • Provenance and peer review Not commissioned; externally peer reviewed.