腰椎椎間盤退化性疾病與終生累積負重之相關性研究

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國立台灣大學公共衛生學院職業醫學與工業衛生研究所博士論文

Institute of Occupational Medicine and Industrial Hygiene College of Public Health

National Taiwan University Doctoral Dissertation

腰椎椎間盤退化性疾病與終生累積負重之相關性研究 Relationship between Lumbar Disc Degeneration Diseases and

Life Time Cumulative Lifting Load

洪瑜嬬 Yu-Ju Hung

指導教授：郭育良教授黃耀輝教授

Advisor: Yue-Liang Guo, M.D., Ph.D. Yaw-Huei Hwang, Ph.D.

中華民國 104 年 01 月

Jan, 2015

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國立台灣大學博士學位論文口試委員會審定書

腰椎椎間盤退化性疾病與終生累積負重之相關性研究

Relationship between Lumbar Disc Degeneration

Diseases and Life Time Cumulative Lifting Load

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誌謝

在此論文完成之際，首先誠摯的感謝我的指導教授郭育良博士，是老師的帶領使我一窺學術的殿堂，老師不厭其煩耐心的教導，與不時的討論並指點我正確的方向，使我在這些年中獲益匪淺。還記得曾經與老師從晚上十一點線上討論與跑統計到凌晨四點!老師對學術的嚴謹與熱情令我敬仰與佩服，其身教與言教更是我終生學習的典範。

本論文得以完成實應歸功於許多人的付出，尤其是秉鈺與慧茹學姊、紋娟、

振翔、曉玲、馥戎、柄偉、銘杰等多位參與收案調查的工作人員，以及台大影醫部施庭芳主任、陳邦斌醫師、沈玲君小姐與魏姐，多虧你們的協助，果菜市場的終身累積暴露調查才能順利完成。感謝黃耀輝老師、陳保中老師、

劉紹興老師與蕭淑銖老師給予的指導與建議，使得本論文能夠更加完善。

謝謝儷萍這些年來的並肩作戰、陪伴與體諒，冠含溫柔無私的幫助及扶持，

啟信醫師溫厚穩定人心的啟發，大夥们在學術上的討論、生活上經驗與困難的互相分享與鼓勵，豐富了我的人生，我銘感在心。感謝 Dickens 先生，是您促成了果菜市場族群的聯繫與調查，您的支持與護惜，是我繼續走下去的動力，您是我肩上天使。

最後，謹以此文獻給我摯愛的雙親洪敏元醫師與李隴華女士。您們長久以來全力的支持與體諒，是我生活上與精神上最大後盾，這份成果與榮耀應歸於您們，感恩上天，生而為您們的女兒是我最大的福氣。

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摘要

前言：

腰椎椎間盤退化性疾病是導致下背疼痛的重要原因之一，在台灣與歐美各國的職業補償統計顯示其高盛行率與發生率所導致的相關失能，造成了醫療與產業成本提高，對於國家的經濟發展是一項沉重的負擔。過去的研究結果顯示職業性負重是椎間盤退化的重要危險因子之一，然而，因為其影響因子之多重性，職業暴露劑量之定量仍有其困難，與椎間盤退化之劑量反應關係尚無定論。因此本研究針對特定高危險性暴露的工作族群，進行詳細的終生累積負重調查，嘗試定量究竟多少終生暴露劑量的搬運重量會造成傷害;且更進一步的調查是否過去所有的負重皆對於傷害的發生有影響? 抑或單次負重中存在有著閾值, 超過此值後的累積負重才具危害效應? 男女性的閾值是否不同? 除了負重之外，椎間盤的形態學(高度與寬度)與椎間盤突出是否具有相關性?若有，是否可以藉由量測椎間盤的高度與寬度來預測椎間盤突出? 本研究期望能以此結果應用於職場上作為制定保護勞工健康的累積負重參考基準和預防疾病的發生。

方法：

本研究設計為橫斷性研究，個案來源為 20-65 歲之間的果菜市場搬運工作人員，作為高危險性暴露的工作族群，以及以國立台灣大學附設醫院內科的門診感冒病患，作為一般工作族群。每位受試者均接受一份問卷調查、腰椎

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磁共振攝影(MRI)與工作姿勢的模擬取相。為了獲得個案的終身累積負重暴露，研究人員詳細詢問個案過去工作中的搬運重量與時間，現場取相個案所示範的搬運動作，並應用腰椎受力評估軟體(3D SSPP)預測每一個搬運姿勢下的腰椎受力，最終相加所有的腰椎受力與執行搬運的時間乘積，此總和值即為個案的終身累積負重暴露，單位為牛頓×小時(Newton×hour (Nh))。腰椎核磁共振攝影的檢查項目包括五節腰椎之椎間盤缺水(Dehydration)、纖維盤破裂(Annulus tear)、椎間盤變薄(Disc height narrowing)、突出(Bulging or protruding)與脊椎滑脫症(Degenerative spondylolithesis、

Spondylolytic spondylolithesis)、椎間孔狹窄(Foramina narrowing)、神經根壓迫(Nerve root compression)，和椎間盤的高度與寬度。統計分析方法以邏輯斯迴歸模式檢視終生累積負重暴露與每一節腰椎之椎間盤退化疾病的相關性。以四種檢驗最適配方程式的統計法來比較各項負重閾值計算下的終生累積負重對椎間盤突出的發生有最佳的預測力。以 ROC 曲線下的面積大小比較二種預測椎間盤突出的預測力: Model 1 以年齡、性別、身高、體重作為危險因子，Model 2 以椎間盤的高度、寬度、年齡、性別、身高、體重作為危險因子。

結果：

共有 715 位自願者參與本研究，最後進行資料分析者為 553 位。研究結果顯示，終身累積負重與腰椎椎間盤退化疾病之間具有顯著相關，同時並呈現有

暴露劑量-效應模式。其中，高負重暴露者(> 8.9×10⁶Nh)相較於低負重者(<

4×10⁵Nh)其第五節腰椎發生椎間盤缺水的危險性是 2.5 倍(AOR=2.5,

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CI=1.5,4.1)，椎間盤變薄的危險性是 4.1 倍(AOR=4.1,95% CI=1.9,10.1);

中度負重暴露者(4×10⁵-8.9×10⁶Nh)相較於低負重者發生椎間盤突出(Bulging)

的危險性是 2.1 倍(AOR=2.1,95% CI=1.3,3.3)。超過閾值以上的負重才計入終生累積暴露的計算下，男性使用單次負重 3000 牛頓，女性 2800 牛頓，作為閾值的終生累積暴露值對 L4-S1 椎間盤突出有最好的預測度。針對腰椎後三節，椎間盤的高度、寬度與椎間盤突出具有相關性; 比較以年齡、性別、

身高、體重作為危險因子的預測方程式(Model 1)和再加入椎間盤的高度、寬度作為危險因子的預測方程式(Model 2) ，發現後者的預測力較佳。

結論：

本研究顯示終生累積負重與椎間盤退化疾病之間具有劑量–效應關係，並定量出特定終生累積負重值對椎間盤退化的發生具有危險性; 男性單次負重閾值 3000 牛頓，所計算的終生累積暴露值對 L4-S1 椎間盤突出有最好的預測度，女性為 2800 牛頓。以椎間盤的高度、寬度、年齡、性別、身高、體重等危險因子構成的預測方程式可以用來預測椎間盤突出的發生。本研究計算腰椎終生累積負重的模式與預測椎間盤突出之方程式可作為職場上累積負重暴露與預防疾病發生的參考。

關鍵字：

椎間盤退化性疾病、終生累積負重、椎間盤突出、閾值、橫斷性研究、腰椎核磁共振攝影、預測方程式

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Abstract

Background and Objective: Lumbar disc degeneration (LDD) has been related to heavy physical loading. However, the quantification of the exposure has been controversial and the dose-response relationship with the LDD has not been established. It is also unclear whether a specific threshold value exists in each lifting load, the accumulation above which best predicts lumbar disc protrusion, or on the other hand, all lifting load should be accumulated. In a clinical setting, the radiographic diagnosis of disc condition typically requires magnetic resonance imaging (MRI), which is less readily available than plain radiograph is in most primary care facilities. If the relationship between reduced disc height and disc bulging or protrusion was existed, useful insight can be obtained to guide further direction of patient evaluation. The purposes of this study are to investigate the dose-response relationship between lifetime cumulative lifting load and LDD; to determine the optimal threshold value of lumbar compression load in each lifting, which allowed for best prediction for disc protrusion while lifetime cumulative load was calculated; and to determine the association between disc morphology and disc bulging or protrusion.

Method: This is a cross-sectional study. Every participant received assessments with a questionnaire, MRI of the lumbar spine and lumbar disc compression load.

MRI assessments included disc dehydration, annulus tear, disc height narrowing, bulging, protrusion, extrusion, sequestration, degenerative and spondylolytic spondylolisthesis, foramina narrowing, and nerve root compression on each

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software system. We sum up all lifting exposure to the calculation for examining the association between lifetime cumulative lifting load and LDD; and sum up only lifting load greater than proposed thresholds for determining the optimal threshold value of lumbar compression load in each lifting .For accumulation above different thresholds, predictive capabilities for disc protrusion were compared using four statistical values, (1) Area under the curve of a receiver operating characteristic curve, (2) R², (3) Akaike information criterion, and (4) Bayesian information criterion. The intervertebral disc height and disc depth were measured. Logistic regression analysis was applied to identify the association between anthropometric factors, disc morphology factors, and disc

bulging/protrusion. Model 1 was constructed using anthropometric variables to investigate the capacity for predicting disc bulging/protrusion. Model 2 was constructed using anthropometric variables and disc morphology variables. The ability of the models to discriminate between participants with and without disc bulging/protrusion was evaluated using a receiver operating characteristic curve.

Result: A total of 553 participants were recruited in this study and categorized into tertiles by cumulative lifting load, i.e., <4.0 × 10⁵, 4.0 × 10⁵–8.9 × 10⁶, and >= 8.9

× 10⁶Newton× hours. The risk of LDD increased with cumulative lifting load. The best dose-response relations was found at the L5–S1 disc level, in which high cumulative lifting load was associated with elevated odds ratios of 2.5 (95% CI 1.5–4.1) for dehydration, and 4.1(95% CI 1.9–10.1)for disc height narrowing comparing to low lifting load. Participants exposed to intermediate lifting load had increased odds ratios of 2.1(95% CI 1.3–3.3) for bulging comparing to low lifting load. The tests for trend were significant. For men, 3000 Newton for each lifting

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task was the optimal threshold value for predicting L4-S1 disc protrusion, whereas for women, 2800 Newton was optimal. Total of 452 MRI scans were analyzed for the morphology study. Age, body weight, body height, disc height, and disc depth were significantly associated with disc bulging/protrusion. The

area-under-the-curve (AUC) statistics of Model 2 were significantly better than Model 1 at the L3-L4 (p<.05) and L4-L5 level (p<.05) but not at the L5-S1 level.

Conclusions: The results suggest a dose-response relationship between

cumulative lifting load and LDD. Cumulative lifting load predicted L4-S1 disc protrusion best when the threshold value was set at 3000 Newton for men, and 2800 Newton for women. The results showed an association between disc

morphology and disc bulging/protrusion at the L3-L4, L4-L5, and L5-S1 level. We also developed a model by using anthropometric factors and disc morphology to predict disc bulging/protrusion.

Key words: Lifetime cumulative lifting load; Lumbar disc degeneration; MRI;

Dose-response relationship; disc morphology, disc protrusion; threshold value

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List of Tables

Part I. Dose-Response Relationship between Lumbar Disc Degeneration and Life Time Cumulative Lifting Load ... 82 Table 1.Demographic characteristics of the study participants ... 82 Table 2. Prevalence of disc-related degenerative findings on MRI images of the lumbar spine in the study ... 84 Table 3. The association between disc degeneration and life-time lifting exposure ( Newton × hours , Nh) among upper lumbar level$ ... 85 Table 4. The association between disc degeneration and life-time lifting exposure ( Newton × hours, Nh) among lower lumbar level# ... 86 Supplementary Table 1. Percentage agreement for intra-reader reliability of all MRI variables ... 89 Supplementary Table 2. Kappa value for intra-reader reliability of all MRI

variables ... 90 Part II. Threshold Values of Lumbar Load in Lifting for Calculating Lifetime Cumulative Load to Predict Disc Protrusion ... 91 Table 1. Demographic characteristics of the study participants ... 91 Table 2. Performance of predictive abilities for L4-S1 disc protrusion as measured by area-under-curve (AUC) of receiver-operator characteristic (ROC) curve, R-square, Akaike information criterion (AIC), and Bayesian information criterion (BIC) of cumulating lifetime lifting load using various threshold values in male participants ... 93 Table 3. Performance of predictive abilities for L4-S1 disc protrusion as measured by area-under-curve (AUC) of receiver-operator characteristic (ROC) curve, R-square, Akaike information criterion (AIC), and Bayesian information criterion (BIC) of cumulating lifetime lifting load using various threshold values in female participants ... 94 Table 4. The association (as shown by adjusted odds ratio, aOR) between L4-S1 disc protrusion and lifetime cumulative lifting load when only lift loads above different threshold values were calculated in male participants ... 95 Table 5. The association (as shown by adjusted odds ratio, aOR) between L4-S1 disc protrusion and lifetime cumulative lifting load when only lift loads above different threshold values were calculated in female participants ... 96 Part III. Prediction of Lumbar Disc Bulging or Protrusion Based on

Anthropometric Factors and Disc Morphology ... 97 Table 1. Demographic characteristics of the study participants (n=452) ... 97 Table 2. The disc morphology factors of the L3-L4, L4-L5, and L5-S1 levels ... 98

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Table 3. Intrareader reliability of disc bulging/protrusion in MRI by percentage agreement ... 98 Table 4. Intrareader and interreader reliability of disc height and disc depth

measurement by interclass correlation coefficients (ICC) ... 98 Table 5. The association between anthropometric factors and disc bulging or protrusion, and anthropometric factors with disc morphology and disc bulging or protrusion at the L3-L4, L4-L5, and L5-S1 levels by logistic regression ... 99

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List of Figures

Part I. Dose-Response Relationship between Lumbar Disc Degeneration and Life Time Cumulative Lifting Load ... 100 Figure1. Flow diagram of the participants selection process in the study ... 100 Part II. Threshold Values of Lumbar Load in Lifting for Calculating Lifetime Cumulative Load to Predict Disc Protrusion ... 101 Figure 1 (a). The AUC statistic distrubution of L4-S1 disc protrusion with

proposed threshold values in male participants ... 101 Figure 1 (b). The R Square values of L4-S1 disc protrusion with proposed

threshold values in male participants ... 102 Figure 1 (c). The AIC values of L4-S1 disc protrusion with proposed threshold values in male participants ... 103 Figure 1 (d). The BIC values of L4-S1 disc protrusion with proposed threshold values in male participants ... 104 Figure 2 (a). The AUC statistic distrubution of L4-S1 disc protrusion with

proposed threshold values in female participants ... 105 Figure 2 (b). The R Square values of L4-S1 disc protrusion with proposed

threshold values in female participants ... 106 Figure 2 (c). The AIC values of L4-S1 disc protrusion with proposed threshold values in female participants ... 107 Figure 2 (d). The BIC values of L4-S1 disc protrusion with proposed threshold values in female participants ... 108 Figure 3. Receiver-operating characteristic curves for the prediction of L4-S1 disc protrusion in male participants by models of different threshold of lifting load. . 109 Figure 4. Receiver-operating characteristic curves for the prediction of L4-S1disc protrusion in female participants by models of different threshold of lifting load.

... 110 Part III.Prediction of Lumbar Disc Bulging or Protrusion Based on

Anthropometric Factors and Disc Morphology ... 111 Figure 1. Receiver-operating characteristic curves for the prediction of L3-L4 disc bulging/protrusion by model 1 and model 2... 111 Figure 2. Receiver-operating characteristic curves for the prediction of L4-L5 disc bulging/protrusion by model 1 and model 2... 112 Figure 3. Receiver-operating characteristic curves for the prediction of L5-S1 disc bulging/protrusion by model 1 and model 2... 113

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Chapter 1 Introduction

Lumbar disc degeneration (LDD) is associated with heavy physical loading [1-9].

Some individuals who experience degenerative changes in the discs may present with symptoms of low back pain (LBP) [6, 10, 11]. The substantial economic burden and productivity loss caused by LBP have become considerable societal problems. Comprehensive investigations of the lifetime cumulative load on lumbar discs that results in various LDD on each disc level are rarely conducted. Beside, only few studies have analyzed the dose-response relationship between physical loading and LDD. Establishing such a dose-response relationship is difficult because of suboptimal exposure assessments and a relative lack of definitive imaging findings regarding LDD. Therefore, understanding the dose-response relationship between physical loading and LDD can provide valuable information regarding safe lifting load for designing work tasks with relatively low risks of low back injury.

Among the disc degeneration conditions, herniated intervertebral disc (HIVD) is one of the most commonly diagnosed abnormalities associated low back pain and sciatica [12]. It has been listed as an occupational disease and compensated in many countries, such as Denmark, France, Germany, United States, and Taiwan

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[7]. A crucial question is whether a specific threshold value exists in each lifting load, the accumulation above which best predicts HIVD, or on the other hand, all lifting load should be accumulated. A review of the literature revealed several recommended threshold lifting load values, but those might not be practicable for calculating the cumulative effects for several reasons. First, they were examined for a single spontaneous lift and the career-long effects of repeated lifting were not considered. Second, most of them were proposed for preventing low back pain, not for HIVD. Third, the current 3400N recommended values do not appear to be optimal because more than 50% of work-related low back injuries are attributed to tasks involving the manual handling of materials [13]. Fourth, uniform liftload limits are not generalizable across ethnicity and sex. Hence, it is essential to determine the optimal threshold value of liftload per lift for calculating the lifetime cumulative load in order to prevent HIVD in Taiwan.

Beside the risk factor of lifting load to disc herniation, we attempt to discover if there is association between disc morphology and disc protrusion. In the clinical setting, the radiographic diagnosis for disc herniation usually requires MRI, which is less readily available in most of the primary care facilities. Plain spine X-ray has difficulty providing information on disc conditions. Only reduction in disc height is visible in radiographs [14]. Since plain films are frequently obtained on patients

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with back pain, if the relationship could be established between reduced disc height and disc herniation, useful insight could be obtained to guide further directions of patient evaluation.

Accordingly, the purposes of this study were (1) to examine the dose-response relationship between lifetime cumulative lifting load and various LDD on each lumbar disc level; (2) to determine whether any threshold value existed to predict disc protrusion when calculating lifetime cumulative lifting load; and if so, what would have been the best threshold value; (3) to examine whether disc

morphology can provide information useful for the prediction of disc

bulging/protrusion, while controlling for anthropometric factors such as age, gender, body height, and body weight, which have been associated with LDD or disc herniation [15-20]. Furthermore, if such relationship is present, this study also aims to establish a predicting model using anthropometric factors and disc

morphology to predict disc bulging/protrusion.

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Chapter 2 Literature Review

A. Epidemiology of Low Back Pain and Intervertebral Disc Degeneration

Low back pain is a major public health problem in western industrialized societies.

According to a systematic literature review of population prevalence studies of low back pain between 1966 and 1998, the point prevalence ranged from 12% to 33%, 1-year prevalence ranged from 22% to 65%, and lifetime prevalence ranged from 11% to 84% [21]. It also places an enormous economic burden on society;

its total cost, including direct medical costs, insurance, lost production and disability benefits, is estimated at £12 billion per annum in the UK and 1.7% of the gross national product in the Netherlands[22, 23]. Back pain is apparent the most prevalent and costly musculoskeletal disorders (MSDs) among United State (U.S.) industries. The total cost was estimated to be 50~100 billion in 1990. It also accounted one forth of the workers’ compensation claims and one third of the compensation costs[24]. According to the U.S. Bureau of Labor Statistics, 11 to 13 million people developed LBP in 2000, and approximately $100 billion were spent on treating this symptom [25]. A nationwide study in Taiwan reported prevalence of low back and waist pain to be 18.3% in male workers and 19.7% in female workers [26]. According to the National Health Insurance Bureau report,

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more than 2.14 million patients sought medical care for back pain in 1998. The medical cost exceeded 3 billion New Taiwan Dollars. The direct and indirect cost associated with low back pain is tremendous.

Low back pain may arise from a spectrum of conditions, e.g. strain and sprain, osteoarthritis, degenerative disc disease, inflammatory spondylitis etc. Disc degeneration of the lumbar spine is considered as one of the underlying factors of LBP, but controversy still prevails about the relationship. In some magnetic resonance imaging (MRI) studies an association has been found [11, 27-29]

although degenerative changes have been found to be common in asymptomatic people as well [29, 30]. Luoma found an increased risk of LBP was found in relation to disc dehydration and disc bulge[28]. In a meta-analysis study, Endean found disc protrusion, nerve root compression, disc dehydration and annulus tear were associated with LBP [11]. Among disc degeneration conditions, herniated nucleus pulpolus, or herniated intervertebral disc (HIVD) is one of the most commonly diagnosed abnormalities associated low back pain and sciatica [12].

Disc protrusion has been listed as an occupational disease and compensated in many countries, such as Denmark, France, Germany, United States, and Taiwan [2].

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B. Risk Factor of Intervertebral Disc Degeneration

I. Definition of Intervertebral Disc Degeneration

Studies had pointed to there are two main challenges in epidemiology related to disc degeneration [5, 31]. First, there is no standard definition of disc degeneration, thus the systems of measurement vary between studies and lead to complicate comparisons. Second, measures of disc degeneration often lack adequate reliability and precision. Definitions have not been uniform, to some extent because the phenomenon is not well understood. Disc degeneration is a product of lifelong degradation with synchronized remodeling of discs and neighboring vertebrae, including simultaneous adaptation of the disc structures to changes in physical loading and responses to the occasional injury. Generally, disc degeneration is defined largely by the method of evaluation. For large population samples, the currently preferred method of evaluation is magnetic resonance imaging.

II. Prevalence of Disc Degeneration

Reported prevalences vary widely between samples and studies. The range of reported prevalences for asymptomatic subjects was as follows: 10% to 81% for bulging, 3% to 63% for protrusion, 0% to 24% for extrusion, 20% to 83% for reduction in signal intensity, 3% to 56% for disc narrowing, and 6% to 56% for

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anular tears. Prevalences for subjects not selected of absence of back pain were as follows: 22% to 48% for bulging, 0% to 79% for protrusion, 1% to 55% for extrusion, 0% sequestration, 9% to 86% for reduction in signal intensity, 15% to 53% for disc narrowing, and 15% for anular tears [5]. Differences between studies in subjects’ age, disc levels and exposure to risk factors may have contributed to the variations in prevalence rates reported.

III. Anthropometric Factors to Disc Degeneration

The mechanisms for the degenerative changes in the disc are poorly understood, but aging is the biggest determinant. There have been many epidemiological studies over the past 30 years [5, 18-20, 29, 32]. In Battié‘s study [5], it showed that various degenerative findings were associated with increasing age from thirty-five to seventy years among 116 men. Videman had indicated LDD including disc dehydration, bulge and disc height narrowing show an increasing prevalence with increasing age [18]. Another study reported that increasing age correlated with a higher prevalence of disc bulge [20]. Twomey showed the intervertebral disc become more convex in the old age [32]. In a review article, Miller et al reported an increase in disc degeneration from 16% at age 20 to about

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autopsy specimens [19]. Age is found to be strongly associated with lumbar disc degeneration.

With respect to gender, men was found degenerative changes earlier than in women by approximately ten years [31]. Miller et al reported that lumbar disc degeneration appeared already in 11- to 19-year-old males and 10 years later in females [19]. An epidemiologic case-control study to identify risk factors for acute prolapsed lumbar intervertebral disc showed that the ratio of men to women was 1.5 to 1 among surgical cases [33]. Some studies have indicated that tallness is a factor associated with an increased risk of herniation [15, 17], but Kelsey’s studies failed to support such relationship [33, 34]. In Hrubec’s results, he reported body height and body weight were positively associated with the risk of disc herniation diagnosed in United States Army hospital [15]. In a study on disc herniation, men with a height of 180 cm or more showed a relative risk of 2.3 and women with a height of 170 cm or more 3.7, compared with those who were more than 10 cm shorter. The author reported that body height may be an important contributors to the herniation of lumbar intervertebral disc [17].

The only chemical exposure associated with disc degeneration is cigarette

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smoking. In kelsey’ study, cigarette smoking in the past year was associated with an increased risk for prolapsed disc [33]. Cigarette smoking was reported that only explain 2% of the variance in disc degeneration from lumbar magnetic resonance images when studying monozygotic twin siblings who were highly exposed to a lifetime smoking history (32 pack-years in mean) [35]. In another study of monozygotic twins study, no significant association between disc degeneration and smoking was found [6].

IV. Disc Morphology Factors to Disc Degeneration

Several studies have showed that disc morphology changes were observed in disc degeneration. In some cases, the degree of disc degeneration has been commonly assessed by the disc height decrease rather than by signal intensity change in the nucleus pulposus on MRI [36]. It has long been clinical experience that patients with disc bulging or protrusion have disc space narrowing [37]. Degeneration of the intervertebral disc is associated with progressive changes in disc morphology, matrix composition and properties [14]. The decrease in the intervertebral disc space would constrict the intervertebral foramen sufficiently to cause entrapment or compression of the spinal nerve root. A 1 mm narrowing of the intervertebral

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area [38]. Tibrewal showed that patients with disc herniation had reduced disc height compared with the normal although the differences did not reach statistical significance [37]. The reason might be because of the smaller samples size in this study and the greater anatomic variation at the L5-S1 disc level. Brinckmann and Grootenboer found a disc height reduction and an increase in disc bulge occur in proportion to the amount of disc tissue removed [39]. In another study, the authors found fracture and discectomy result in an increase of the radial disc bulge and a decrease of the disc height [40]. These studies revealed that there was a relation between disc height and disc bulge. According to Natarajan’s study, it suggested that changes in disc volume or disc area might be more rational to disc bulging than decrease of disc height [41].

Several studies had reported that disc height or disc depth was related to age [18, 32, 41-44]. Natarajan found there is a decline of disc height after the fifth decade of life [41]. Amonookuofi showed that disc height and diameter vary significantly in the different age groups [42]. The sizes of disc increase as a person age [42].

Age and axial disc size were reported account for more of the explained variance (6%) in disc height narrowing [16]. In another study, the maximum disc height was greater in the older (50-60 years) than in the younger individuals (20-30 years)

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[44]. It was presumed to be a result of the microfracture of the endplate during adult life, which leads to a more concave form of the intervertebral disc [44].

However, Koeller had different observation that the average disc height is almost independent of age [45]. The height of the intervertebral disc is influenced by several factors. Age and the grade of disc degeneration also influence the disc height. Both factors are related, as the incidence and degree of disc degeneration increase considerably with age [46].

V. Genetics Factors to Disc Degeneration

In recent years, a dramatic advance has been shifted to the genetic influences on the risk for disc degeneration. In one review article, Ala-Kokko noted that environmental factors may explain only a small portion of disc degeneration and concluded that “genetic factors play an important role in disc pathology

[degeneration], and perhaps a major one”[47]. Two of the first systematic analyses of familial aggregation of disc degeneration were conducted with monozygotic twin pairs [6, 48]. Results from these studies demonstrated substantial familial aggregation in terms of the extent and location of disc degeneration. One of the studies assessed the degree of similarities in degenerative findings by spinal level in the lumbar discs of 20 pairs of monozygotic twins from 36 to 60 years of age, relative to what would be expected by chance based on the prevalence of the findings by level among all 40 subjects [48]. Results suggested a substantial

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the other study published in 1995, lumbar MRIs of 115 pairs of male MZ twins were assessed to investigate the relative effects of environmental exposures commonly suspected as risk factors for disc degeneration, age and familial aggregation on disc bulging and disc height narrowing [6]. In a multivariable analysis of the T12–L4 region, physical loading exposures explained 7% of the variance in summary disc degeneration scores among the 230 subjects; this rose to 16% with the addition of age and to 77% with the addition of a variable

representing familial aggregation. In the L4–L5 and L5-S1 region, physical

loading measures explained only 2% of the variance in disc degeneration summary scores in multivariable analysis. The portion of the variance in lower-lumbar disc degeneration scores explained rose to 9% with the addition of age and to 43% with the addition of familial aggregation. Significantly more of the variance in

degeneration remained unexplained in the lower lumbar region, as compared to the upper lumbar region, and is likely the result of mechanical forces interacting with spinal anthropometrics in such a way as to have a disproportional effect on the lower lumbar levels. This study provided the first estimate of the relative

importance of specific environmental agents and overall familial influences, which include genetic factors [48].

VI. Occupational Exposure and Disc Degeneration

Previous studies had shown the relation between lumbar disc degeneration and occupational risk factors such as heavy lifting, forward bending, awkward posture and whole body vibration, particularly heavy physical loading have been the main

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suspected risk factors [1-9]. The association of mechanical load on spinal structures and back pain has been reported [34, 49-51]. The L4-S1 lumbar discs usually have the highest prevalence of disc degeneration than L1-L4 discs, suggesting the role of lifetimes physical exposure in disc pathogenesis because aging and genetic effect could be expected to affect all discs similarly [6]. The traditional view as to the causes of the disc degeneration was as the result of

“wear and tear” on the disc from daily exposures to physical loading or biomechanical forces [31]. During loading the disc deforms and loses height gradually. As the disc changes its composition because of ageing or degeneration, the response of the disc to mechanical loads also changes. With a loss of

proteoglycan and thus water content, the nucleus can no longer respond as efficiently. This change results in uneven stresses across the endplate and the annulus fibres, and, in severe cases of degeneration, the inner fibres may bulge inward when the disc is loaded. It may lead to abnormal stresses on other disc structures, eventually causing more severe condition. Disc height narrowing affects other spinal structures, such as muscles and ligaments, and, in particular, leads to an increase in pressure on the facet joints, which may be the cause of the degenerative changes seen in the facet joints of spines with abnormal discs [45].

With respect to whole-body vibration or driving, a case-control study found the

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greater the number of hours spent in a motor vehicle, the higher the risk of having disc protrusion [33]. A study of forty-five pairs of monozygotic twins who were highly exposed to motorized vehicles and associated whole-body vibration did not find an association between lumbar disc degeneration and lifetime driving

histories. The current evidence suggests no notable effect of driving on disc degeneration [6] .

C. Dose-Response Relationship between Cumulative Lifting Load and Disc Degeneration

In the cumulative or repetitive injury model of intervertebral disc degeneration, physical loading or biomechanical forces on the discs, particularly through occupational physical demands, have been the main suspected risk factors [52, 53] . Cumulative loading can be defined as one of the following: the accumulated demands on the spine during the duration of activity; loads build up over the period of a work shift; or the accumulation of loading throughout a worker’s lifetime [54]. Numerous studies had documented the association between cumulative spinal load and low back injuries [54-58]. Based on an “injury”' paradigm model, it implies that overloading results in structural damage which leads to disc degeneration causing symptomatic conditions. This model did not examine the biological capacity of the musculoskeletal system to adapt to external

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exposures. Biological tissues are viscoelastic in nature, and prolonged loads may results in cumulative fatigue, which reduces their stress-bearing capacity. Such changes may reduce the threshold stress which the tissue fail. It is considerable that the history of exposure to physical load may decrease the threshold for precipitation of back injuries or disc degeneration, as well as the peak load at which the injuries precipitate [58].

In spite of significant association between certain occupational exposures and intervertebral disc disease, a threshold for occupational exposure or a

dose-response relationship have not been established. Only few studies have shown a dose-response relationship between physical workload and lumbar disc degeneration. Kelsey’s study indicated that subjects lifting objects more than 11.3 kg (25 lb) over 25 times per day had more than three times the risk for acute prolapsed lumbar intervertebral disc compare to persons without lifting [7].

Hofmann revealed an association between length of employment in nurses with high spinal load and the risk of disc herniation. Seilder showed a postive dose-response relation between cumulative lumbar load and lumbar disc herniation (through manual materials handling and/or intensive load postures) [1-3]. The odds ratio (OR) of herniation for men with a sum of exposure of

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>21.51 ×10⁶ Nh verse subjects with a sum of up to 5.0 ×10⁶ Nh was 3.4 [3].

D. Threshold Value of Lifting Load to Disc Degeneration

Threshold value is considered the value above which the risk or probability of injury increases significantly. It is necessary to judge the amount of load required by the work. Information on the back force requirements of the work can be used to plan and assess interventions to decrease the number of work-related injuries.

A review of the literature revealed several recommended threshold liftload values, although most of them were used in the prevention of low back injury [54, 55, 59-64]. Chaffin and Park found low back injury incidence rates of 5% and 10%

among workers (n = 411) when the estimated compressive force at L5/S1 was higher than respectively 2500 N and 4500 N [62]. For jobs with predicted

compressive force at L5/S1 between 4500 N and 6800 N, the authors found a rate of back injuries more than 1.5 times higher than for jobs with predicted

compressive force lower than 4500 N [65]. The National Institute for Occupational Safety and Health (NIOSH) suggested that if spinal compression exceeds

approximately 3400 N, workers would be at an increased risk of low back injury [66]. NIOSH guidelines for compression are based on the studies of Evans and

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Sonoda [67]. The results of these studies show that even though the intervertebral discs do not rupture, microfractures of the vertebral cartilage endplates of cadavers of subjects under 40 years old start to happen when applying on average 6700 N of axial load (1500 pounds, approximately 680 Kg). When the spines were from subjects 60 or more years old, the microfractures started to happen when applying average axial loads of 3400 N [67]. The major limitation of NIOSH 1981

guidelines is that the cutpoints are based on cadaver studies with large standard deviations, and the living structures threshold to compression injury for different people might differ. Even NIOSH questions the value of 3400 N, NIOSH opinion is that this value “may not protect the entire workforce” [65]. In addition, the guidelines are based on studies of axial compression only and do not take into account the cumulative effect and temporal characteristics of the exertions over time on the viscoelastic tissues of the body [58]. The compression guidelines proposed by NIOSH are widely used, however, as suggested by different studies, they are probably inaccurate and when followed may expose the workforce to demands exceeding its capacity.

Norman et al studied more than 10,000 automotive assembly workers [55]. When the authors compared a sub-group of 104 cases (with low back injury) with 130

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controls (without low back injury) the peak shear force on L4/L5 (odds ratio of 2.3) emerged as the strongest factor followed by peak compression force on L4/L5 (odds ratio of 1.9). The mean peak compression load of the auto-assembly workers who reported low back pain was 3423 N. This value was statistically different (p < 0.001) form the mean value found for the group who did not report low back pain (2733 N). Jager and Luttman compared the results from their proposed biomechanical model for low back axial compression with the literature regarding lumbar compression strength [60]. The average ultimate axial

compression strength (total of 307 lumbar segments) reported by the authors was 4400 N (standard deviation 1900).

Only few studies of threshold value to disc protrusion were reported. Hutton and Adams found a mean value of 10249 N as being representative of the ultimate compressive axial force of intervertebral discs of cadavers of males between 22 and 46 years old [63]. They found that more than 40% of the intervertebral disks prolapsed when 5400 N of axial load was applied to flexed spines (simulated by wedging vertebral bodies)[63]. Additionally, in another study the authors observed trabecular fractures in the intervertebral discs when an average repetitive axial load of 3800 N was applied to simulated hyperflexed spines [59]. However, the

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reference values being used to-date do not seem to be optimally effective.

Evidence of this inadequacy is given by the low success achieved so far in controlling work-related low back injures.

E. Exposure Assessment Methods

I. Exposure assessment methods

It has been proposed that mechanical exposure during physical work should be described by three main dimensions: (1) Intensity —intensity of the force, (2) Repetitiveness—the frequency of shifts between force levels and (3)

Duration—the time the physical activity is performed. Any attempt to quantify exposure should include all the three dimensions for a worker being assessed. A wide range of exposure assessment methods has been identified and categorized as self-reports, observational methods and direct measurements [68]. Self-reports from workers such as interviews and questionnaires can be used to collect demographic data, occupational data on workplace exposure. A major problem with these methods is that worker perceptions of exposure have been found to be imprecise and unreliable. Direct measures such as laboratory methods used motion analysis systems, electromyography and accelerometry to achieve comprehensive information, but they could not be generalized to worksites and were limited to

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and researchers. However, the validity and reliability in measuring mechanical exposure are increased [50].

II. Three-Dimensional Static Strength Prediction Program (3D SSPP)

Numerous methods have been developed for estimating the disc compression load.

Direct measurement of lumbar spine load through in vivo studies is rare because of concerning about the ethical issue that it should implant a transducer or sensor into the disc. The first intradiscal pressure data was reported by Nachemson during the 1960s and was the important reference for rehabilitation medicine and workplace recommendations [69]. In 1998, Hans-Joachim conducted another intradiscal pressure measurement with one volunteer performing various activities and found good correlation with Nachemson’s data [70]. However, this type of study is rarely attempted because of the ethical considerations regarding such an invasive

procedure. Presently it is not feasible to directly monitor the loads imposed on the spine structure and tissues while workers are performing an occupationally related task in the workplace. Instead, indirect measures such as computerized

biomechanical modeling is considered the most precise method and typically used for estimating the disc compression load. The Three-Dimensional Static Strength Prediction Program (3D SSPP) was a biomechanical model developed by the

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Center for Ergonomics at the University of Michigan. This program is normally applicable to the analysis of “slow” movements used in heavy material handling tasks since the biomechanical computations assume that the effects of acceleration and momentum are negligible. However, it accounts for internal and external forces occurring in and on the body. The subjects’ anthropometric data were part of the U.S. industrial database used by the University of Michigan Center for Ergonomics to develop the 3D SSPP software [71]. Jang conducted a field study to investigate spinal compression force of nursing tasks in a hospital setting by utilizing 3D SSPP [51].The results showed consistency with Marras’s laboratory based study by using the EMG-assisted model [72]. The 3D SSPP was further utilized as a gold standard to the HCBCF (Hand-calculated back compress force) estimation model for ergonomic evaluation of 600 lifting tasks [73]. The 3DSSPP was also used as a measurement tool in several studies [74]. Among the

measurement methods, 3-dimention Static Strength Prediction Program (3D SSPP, Center for Ergonomics, University of Michigan) software system was considered a more quantitative tool to estimate spinal load.

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Chapter 3 Material and Method

Part I. Dose-Response Relationship between Lumbar Disc Degeneration and Life Time Cumulative Lifting Load

Study Population

We conducted a cross-sectional study. To analyze workers from a broad spectrum of lifting exposures, the participants in this study were recruited from 2

populations. The group that carried heavy load comprised members of the San Chung Fruit and Vegetable Wholesale Market in Taiwan. Most of these workers load and unload fruit boxes almost every day; thus, lifting is a common task at their workplace. Patients who sought treatment in the Internal Medicine Clinic of the National Taiwan University Hospital (NTUH) and were diagnosed with upper respiratory infections (URI), mostly the common cold, were recruited as the background population. During recruitment, the wholesale market workers and the walk-in clinic patients were not informed of the hypothesis of the study. They were invited to participate in a survey regarding spine and bone disorders. The inclusion criteria for the study were an age between 20–65 years and at least 6 months of working experience. A person was excluded if he or she had been previously diagnosed with cancer, psychiatric conditions, spinal tumors, inflammatory spondylopathy, compression fracture, or major back trauma. We pooled these 2

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populations to examine the effects of lifting on LDD, and the entire population was categorized into tertiles according to lumbar cumulative lifting load. Figure1 shows the participant selection process implemented in this study. Before

participating in the study, all workers and patients received written and oral information regarding the study procedures and potential adverse effects, and signed informed consent forms. The study protocols were reviewed and approved by the Institutional Review Board of the NTUH.

Data Collection

Every participant was assessed by using a questionnaire and obtaining MRI images of the lumbar spine. The demographic and occupational data of the participants were obtained from the extensive, structured questionnaire. For each participant, a complete occupational history and a history of back pain as well as information on job tasks, driving and riding experience, leisure activities, drinking, and smoking were collected. The participants reviewed each job held since they entered the workforce. The requested information included job titles, working tenures, body weights at each job, descriptions of tasks, lifting exposure at work (such as estimates of the most common weights lifted or carried), the frequency and duration of lifting or carrying, the number of working hours per day, and the

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number of working days per week. A structured interview was implemented to provide the participants with adequate time for assessing the relevant work tasks in each job in their occupational history. The trained interviewers used common milestones in life to help the participants recall the necessary information. The participants were encouraged to recall their body weights during the period of each job. When the job period was longer than 5 years, the average body weight during this job period was used. Cigarette exposure was calculated in pack-years by multiplying the number of packs of cigarettes smoked daily by the number of smoking years.

Estimation of Lifetime Compression Load on Lumbar Disc

Regarding the estimation of lifetime compression load, the participants recalled all of the jobs that they held after completing schooling. When a person performs a lifting task, the compression load on the spinal disc is increased. Therefore, work tasks involving the manual materials handling were used to represent the

compression load for each job. Specific objects that had been lifted or carried regularly were described, and participants subsequently answered questions concerning the weight, frequency, and duration of each task. The participants performed a typical material handling task to simulate the positions and weights

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encountered at each job. Lifting activity was divided into a sequence of static postures including the initial lift-up, transferring, and unloading postures, and each posture was analyzed. The frontal and lateral views of each lifting posture were photographed according to a standardized photography procedure work sheet. To generalize the compression load into the cumulative lifting exposure in Newton × hours (Nh), the following method was used for representing the compression load of each job. A participant was instructed to choose an empty box of a size similar to those of objects typically carried at work. Bottles of water were placed in the box until the total weight was similar to those of the typical objects, and the resulting weight was used as an estimate of the typical weight carried for that specific job. Subsequently, the participant was instructed to demonstrate simulated working postures, including lift-up, transferring, and unloading postures, by using the empty box, and photographs of these postures were captured. The initial position of the weight lifting task was defined as the lift-up posture, the final position was defined as the unloading posture, and the action of transferring

material while walking was defined as the transferring posture. Although the initial and final lifting positions may have varied during a typical day of materials

handling on the job, the selected typical tasks, including the simulated positions and weights, were used to calculate the compression load to represent the job. The

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compression load on the lumbar disc during lifting was estimated using the 3 Dimension Static Strength Prediction Program (3DSSPP, Center for Ergonomics, University of Michigan) software system [51, 73]. The 3DSSPP was used to predict the static strength requirements for tasks such as lifts, pushes, and pulls during each work period. Anthropometric data such as the gender, height, body weight, carried load, and working posture photograph of each participant were input into the 3DSSPP system to predict the compression load on the lumbar disc.

In addition, the angle of the body can be adjusted automatically by using the system. To evaluate the intra-rater and inter-rater reliability of lumbar load estimation by using the 3DSSPP, photos of the simulated work conditions of the 60 study participants were repeatedly evaluated in 2 rounds, and the second round of evaluation was conducted 4 weeks after the first.

To investigate the actual cumulative lifting exposure, the participants recalled details regarding lift-up time (t_lift-up), transporting time (ttransporting), and unloading time (t_unload) of each lifting task at their jobs. Hence, in this study, the lifting exposure of each task was defined as the sum of the products of the lift-up force (F_lift-up) and lift-up time (t_lift-up), transporting force (Ftransporting) and transporting time (ttransporting), and unloading force (F_unload) and unloading time (t_unload). The cumulative

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compression load calculation method used in this study was modified from that used by Seidler [1-3]. However, unlike Seidler, we used the 3DSSPP to estimate the lumbar compression load. For each job described, the load on the lumbar disc was calculated as the product of the compression load and the duration of lifting in hours. The lifetime cumulative load (Nh) for each participant was then estimated by summing the loads on the lumbar disc from all jobs. The calculation can be expressed as the following equation:

Cumulative lifting load =

∑ [(Flift-up*t_lift-up + Ftransporting*ttransporting + F_unload*t_unload)/3600 * frequency of lifting/day * working

days/year * working year]

F: compression load on the lumbar disc t: time (second)

According to the findings of Siedler, all workloads from the past contribute to LDD [3]. Therefore, the lifetime cumulative load for each participant was estimated by summing each load on the lumbar disc from all jobs. In previous studies, the lifetime exposure was typically estimated using the number of working hours per day [1-3]. However, in practical working environments, workers do not lift for 8 hours daily; therefore, the results might have been overestimated in previous studies. By contrast, the detailed investigation and calculation methods

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used in this study were implemented for calculating precise cumulative lifting exposure values.

The researchers visited the fruit market to obtain a video recording of the working conditions and lifting processes, and observed that the weight lifted per unit of fruit was rather regular, thus simplifying the calculation process. The video recording was rated separately by using the 3DSSPP, which yielded results consistent with those from the recollections of the fruit market workers. The reproducibility of the lifting measurements was tested 6 months after the initial interview with the help of 25 participants. The lifting measurements of their current jobs were used for reliability testing. These measurements included the weight lifted, lift-up time, frequency of lifting per day, and tenure at the job. We observed that most of the participants’ lift-up time was almost equal to their unloading time, and that the transporting time was zero. Therefore, the reliability of the transporting and unloading time were not examined. After observing and recording the fruit workers’ practices, we determined that pushing or pulling is not a common task for the majority of fruit market workers because they typically drive an electric pedicab to transfer fruit boxes. Therefore, the lumbar compression load of pushing and pulling were not assessed.

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Magnetic Resonance Imaging Equipment and Protocol

The LDD was assessed using MRI. All MRI examinations were obtained at the NTUH by using a GE 1.5-T unit (General Electric Medical Systems, Milwaukee, WI) and a spine array coil (5 × 11 in.). The study involved 4 spin-echo sequences:

an axial localizer (spoiled gradient), sagittal views with a repetition time and echo time (TR/TE) of 500/minimum full ms and 3350/110 ms, and an axial view with a TR/TE of 5325/110 ms. The slice thickness was 4 mm for sagittal and axial sequences, and the field of view was 28 and 20 cm for the sagittal and axial images, respectively. The T1-weighted axial sequences were stacked slices extending from the inferior aspect of T12 through the inferior aspect of S1. The T1-weighted axial and sagittal images exhibited 2 excitations, and the

T2-weighted sagittal images exhibited one excitation.

Definition of the Degenerative Disc Related Magnetic Resonance Imaging Findings

Each intervertebral disc from L1–L2 to L5–S1 was evaluated for disc dehydration, annulus tear, disc height narrowing, disc bulging, protrusion, extrusion,

sequestration, degenerative and spondylolytic spondylolisthesis, foramina

narrowing, and nerve root compression. An experienced radiologist performed the evaluation based on standard images and according to written instructions. The

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radiologist was blinded to the participants’ medical histories and occupational exposure statuses. Disc dehydration was defined as T2-weighted signal intensity loss from the intervertebral disc [75]. Annular tears are separations between annular fibers, the avulsion of fibers from their vertebral body insertions, or breaks through fibers that extend radially, transversely, or concentrically, involving one or more layers of the annular lamellae [76]. According to the Farfan method [77], disc height can be measured as the mean of the ventral and dorsal distances between the contours of the adjacent vertebral bodies. Reduction of disc height was defined as a disc narrower than the upper disc that it was normal [20]. Disc bulging was defined as the presence of disc tissue that is circumferentially (50%–100%) beyond the edges of the ring apophyses. Protrusion was present if the greatest distance, in any plane, between the edges of the disc material beyond the disc space was more than the distance between the edges of the base in the same plane. Extrusion was present when, in at least one plane, any one distance between the edges of the disc material beyond the disc space was greater than the distance between the edges of the base, or when no continuity existed between the disc material beyond the disc space and that within the disc space. Extrusion may be further specified as sequestration if the displaced disc material has completely lost continuity with the parent disc [76]. Spondylolytic spondylolisthesis was

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identified in a lateral projection as an anterior displacement with a break of the pars interarticularis. Degenerative spondylolisthesis was defined as with an intact pars interarticularis, and spondylolytic spondylolisthesis involves the separation of the posterior aspect of the vertebral body from the anterior body [75]. The

intrareader reliability regarding the presence or absence of each MRI variable was determined as the average reliability of 5 lumbar discs of the 60 participants evaluated on 2 occasions within 3 months.

Statistical Analysis

All statistical analyses were conducted using JMP 5.0 (SAS Company). For the evaluation of the occurrence of LDD among the lifting group, a logistic regression was conducted, adjusting for potential risk factors including age, gender, body mass index (BMI), and smoking. To calculate trend analyze, the lifting exposure was included as interval-scaled variables in the logistic regression model. Power calculation in this study that with alpha error of 0.05, twice the risk compared with the reference group, a prevalence <3.5% in degenerative-disc-related MRI findings in each lifting load group (data not shown) could not achieve statistical power of 80%. Therefore, we did not further examine the relationship between the lifting exposure and these MRI variables (prevalence <3.5%). A Bonferroni correction for

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multiple comparisons was performed, and P values <.0042 and <.0083 indicated significance for the upper and lower lumbar region, respectively. The

reproducibilities of the modified calculation of the compression load and lifting measurements were analyzed by using SPSS (16.0 for Windows) to compute intraclass correlation coefficients (ICC). Percentage agreement was used to assess the intrareader reliability of the MRI variables.

Part II. Threshold Values of Lumbar Load in Lifting for

Calculating Lifetime Cumulative Load to Predict Disc Protrusion

Study Population

This study is a further investigation of the previous study. Recruitment of the participants, measurements of the work exposure, and imaging studies of the lumbar spines were detailed in part I. To obtain a broad spectrum of lifting exposures, the participants were recruited from 2 populations: (1) walk-in clinic patients and (2) workers who carry heavy loads. Patients visited the Internal Medicine Clinic of one National University Hospital and diagnosed with upper respiratory infections (URI), mostly the common cold, were recruited as the background population. The group that carried heavy loads were workers of one fruit and vegetable wholesale market. Lifting is a common task for these workers.

During recruitment, the market workers and the walk-in clinic patients were not

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informed of the hypothesis of the study. They were invited to participate in a study regarding spine and bone disorders. The inclusion criteria of this study were between 20 and 65 years and at least 6 months of working experience. Participants previously diagnosed with compression fracture, major back trauma, inflammatory spondylopathy, spinal tumors, cancer, or psychiatric conditions were excluded. We combined these 2 populations to examine the effects of lifting on disc protrusion.

Data Collection

Each participant was asked to complete a questionnaire and to obtain MRI of the lumbar spine. The demographic and occupational data were obtained from an extensive, structured questionnaire. A detailed structured interview with adequate time was implemented to the participants for assessing the relevant work tasks in each job held since they entered the workforce including a complete occupational history, job titles, working tenures, body weights at each job, descriptions of tasks, lifting exposure at work (eg, estimates of the most common weights lifted),

frequency and duration of lifting, numbers of working hours per day and working days per week. The trained interviewers used common milestones in life to help the participants recall the necessary information. The participants were encouraged to recall their body weights during the period of each job. When the job period

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was longer than 5 years, the average body weight during this job period was used.

Cigarette exposure was calculated in pack-years by multiplying the number of packs of cigarettes smoked daily by the number of smoking years.

Estimation of Lumbar Disc Compression Load and Calculation of Lifetime Cumulative Lifting Load on the Lumbar Disc

Regarding the estimation of lifetime exposure, the participants recalled all of the jobs held after completing schooling, and the weight, frequency, and duration of each task. The participants performed a typical material handling task to simulate the positions and weights encountered at each job. Lifting activity was divided into a sequence of static postures, including the initial lift-up, transferring, and

unloading postures, and each posture was analyzed. The initial position of the weight lifting task was defined as the lift-up posture, the final position was defined as the unloading posture, and the action of transferring material while walking was defined as the transferring posture. Although the initial and final positions of lifting may have varied during a typical day of materials handling on the job, the selected typical tasks, including the simulated positions and weights, were used to calculate the compression load to represent the job. The compression load on the lumbar disc during lifting was estimated using the 3D Static Strength Prediction Program (3DSSPP, Center for Ergonomics, University of Michigan, Ann Arbor,

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Michigan) software system . Anthropometric data such as sex, height, body weight, carried weight, and working posture photograph of each participant were input into the 3DSSPP system to predict the compression load on the lumbar disc. To evaluate the intrarater and interrater reliability of lumbar load estimation by using the 3DSSPP, photographs of the simulated work conditions of the 60 study participants were repeatedly evaluated in 2 rounds, with the second round of evaluation was conducted 4 weeks after the first round.

To investigate the actual cumulative lifting exposure, the participants recalled details regarding lift-up time (tlift-up), transporting time (ttransporting), and unloading time (tunload) of each lifting task at their jobs. Hence, in this study, the lifting exposure of each task was defined as the sum of the products of the lift-up force (Flift-up) and lift-up time, transporting force (Ftransporting) and transporting time, and unloading force (Funload) and unloading time. Only those lift-up forces greater than proposed threshold value were added into lifetime exposure. For each job

described, the lifting exposure was calculated as the product of the lifting load and the duration of lifting in hours (Newton × hour, Nh). The lifetime cumulative load for each participant was then calculated by summing the lifting exposure on the lumbar disc from all jobs.

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Threshold Value of Lifting Load

The threshold value in this study was defined as exposure with a lifting load above this proposed value was considered as contributed to disc protrusion over an entire career life, and was included in the lifetime cumulative calculation. The proposed threshold values were set at zero Newton (N), and at 100 N increments from 2000 to 4000 N. For example, if the threshold value is set as 3400 N, only lifting load above 3400 N per lift will be included in the calculation. And, when the threshold value is set at 0 N, every lifting load generated from each activity will be included in the calculation. The calculation can be expressed as the following equation:

Cumulative lifting load =

∑ [(Flift-up*tlift-up + Ftransporting*ttransporting + Funload*tunload)/3600 * frequency of

lifting/day * working days/year * working year]

where F represents the lifting load on the lumbar disc and t represents time (seconds).

The reproducibility of the lifting measurements was tested 6 months after the initial interview with the help of 25 participants. Their current jobs were used for

腰椎椎間盤退化性疾病與終生累積負重之相關性研究

國立台灣大學公共衛生學院職業醫學與工業衛生研究所 博士論文

Institute of Occupational Medicine and Industrial Hygiene College of Public Health

National Taiwan University Doctoral Dissertation

腰椎椎間盤退化性疾病與終生累積負重之相關性研究 Relationship between Lumbar Disc Degeneration Diseases and

Life Time Cumulative Lifting Load

洪瑜嬬 Yu-Ju Hung

指導教授：郭育良 教授 黃耀輝 教授

Advisor: Yue-Liang Guo, M.D., Ph.D. Yaw-Huei Hwang, Ph.D.

中華民國 104 年 01 月

Jan, 2015

國立台灣大學博士學位論文 口試委員會審定書

腰椎椎間盤退化性疾病與終生累積負重之 相關性研究

Relationship between Lumbar Disc Degeneration

Diseases and Life Time Cumulative Lifting Load

誌謝

摘要

Abstract

目 錄

List of Tables

List of Figures

Chapter 1 Introduction

Chapter 2 Literature Review

A. Epidemiology of Low Back Pain and Intervertebral Disc Degeneration

B. Risk Factor of Intervertebral Disc Degeneration

C. Dose-Response Relationship between Cumulative Lifting Load and Disc Degeneration

D. Threshold Value of Lifting Load to Disc Degeneration

E. Exposure Assessment Methods

Chapter 3 Material and Method

Part I. Dose-Response Relationship between Lumbar Disc Degeneration and Life Time Cumulative Lifting Load

Part II. Threshold Values of Lumbar Load in Lifting for

Calculating Lifetime Cumulative Load to Predict Disc Protrusion

國立台灣大學公共衛生學院職業醫學與工業衛生研究所博士論文

指導教授：郭育良教授黃耀輝教授

國立台灣大學博士學位論文口試委員會審定書

腰椎椎間盤退化性疾病與終生累積負重之相關性研究

目錄