Abstract
Background Lumbar disk 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.
Objective The purpose of this study was to investigate the dose-response relationship between lifetime cumulative lifting load and LDD.
Design This was a cross-sectional study.
Methods Every participant received assessments with a questionnaire, magnetic resonance imaging (MRI) of the lumbar spine, and estimation of lumbar disk compression load. The MRI assessments included assessment of disk dehydration, annulus tear, disk height narrowing, bulging, protrusion, extrusion, sequestration, degenerative and spondylolytic spondylolisthesis, foramina narrowing, and nerve root compression on each lumbar disk level. The compression load was predicted using a biomechanical software system.
Results A total of 553 participants were recruited in this study and categorized into tertiles by cumulative lifting load (ie, <4.0 × 105, 4.0 × 105 to 8.9 × 106, and ≥8.9 × 106 Nh). The risk of LDD increased with cumulative lifting load. The best dose-response relationships were found at the L5–S1 disk level, in which high cumulative lifting load was associated with elevated odds ratios of 2.5 (95% confidence interval [95% CI]=1.5, 4.1) for dehydration and 4.1 (95% CI=1.9, 10.1) for disk height narrowing compared with low lifting load. Participants exposed to intermediate lifting load had an increased odds ratio of 2.1 (95% CI=1.3, 3.3) for bulging compared with low lifting load. The tests for trend were significant.
Limitations There is no “gold standard” assessment tool for measuring the lumbar compression load.
Conclusions The results suggest a dose-response relationship between cumulative lifting load and LDD.
Lumbar disk degeneration (LDD) is associated with heavy physical loading.1–9 Some individuals who experience degenerative changes in the disks may have symptoms of low back pain (LBP).6,10,11 In US industries, LBP is the most prevalent and costly musculoskeletal disorder; moreover, because it is a major cause of work absenteeism, it accounts for a large proportion of occupational disability costs.12 According to the US Bureau of Labor Statistics, 11 to 13 million people developed LBP in 2000, and approximately $100 billion was spent on treating this symptom.13 The substantial economic burden and productivity loss caused by LBP have become considerable societal problems. In 2001, 65% of all reported cases of low back injuries were caused by overexertion, whereas 60% occurred during lifting.13 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. However, 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. Regarding exposure assessment, the suspected major risk factors for LDD are physical loading or biomechanical forces on the disks, and the most relevant risk factor is the lifetime cumulative dose.14 Regarding outcome measurement, the results are typically based on 2 or 3 degenerative disk–related conditions or on self-reported LBP.
In recent decades, magnetic resonance imaging (MRI) has been considered the most sensitive technique for detecting disk degeneration4 and is an ethically acceptable method for large population samples. Lumbar disk degeneration can be most clearly described using MRI, thus clarifying the dose-response relationship between physical loading and LDD. Determining the prevalence rate of specific disk degeneration by using MRI and assessing the relationships between lumbar spine levels and various findings can provide insights into the etiopathogenesis of disk degeneration.15,16 Comprehensive investigations of the lifetime cumulative load on lumbar disks that result in various LDDs on each disk level are rarely conducted. Therefore, the purpose of this study was to examine the dose-response relationship between various LDDs and the lifetime cumulative load on each lumbar disk level caused by manual lifting.
Method
Participants
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: (1) workers who carry heavy loads and (2) walk-in clinic patients. The group that carried heavy loads 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 and were diagnosed with upper respiratory infections (URIs), 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 and 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 populations to examine the effects of lifting on LDD, and the entire population was categorized into tertiles according to lumbar cumulative lifting load. The Figure 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.
Flow diagram of the participants' selection process in the study. URI=upper respiratory infection, MRI=magnetic resonance imaging.
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 (eg, estimates of the most common weights lifted or carried), frequency and duration of lifting or carrying, number of working hours per day, and 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 the Lumbar Disk
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 disk 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 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 newtons × 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 that 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 compression load on the lumbar disk during lifting was estimated using the 3D Static Strength Prediction Program (3DSSPP, Center for Ergonomics, University of Michigan, Ann Arbor, Michigan) software system.17,18 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 sex, 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 disk. In addition, the angle of the body can be adjusted automatically by using the system. 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 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. The cumulative compression load calculation method used in this study was modified from that used by Seidler and colleagues.1–3 However, unlike Seidler and colleagues, we used the 3DSSPP to estimate the lumbar compression load. For each job described, the load on the lumbar disk 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 disk from all jobs. The calculation can be expressed as the following equation:
where F represents the compression load on the lumbar disk and t represents time (seconds).
According to the findings of Siedler et al,3 all workloads from the past contribute to LDD. Therefore, the lifetime cumulative load for each participant was estimated by summing each load on the lumbar disk 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 used in this study were implemented for calculating precise cumulative lifting exposure values.
We 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 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 time and unloading time was 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 was not assessed.
Magnetic Resonance Imaging Equipment and Protocol
The LDD was assessed using MRI. All MRI examinations were conducted at National Taiwan University Hospital using a GE 1.5-T unit (General Electric Medical Systems, Milwaukee, Wisconsin) and a spine array coil (12.7 × 27.9 cm [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 milliseconds and 3,350/110 milliseconds, and an axial view with a TR/TE of 5,325/110 milliseconds. 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 Disk–Related MRI Findings
Each intervertebral disk from L1–L2 to L5–S1 was evaluated for disk dehydration, annulus tear, disk height narrowing, disk 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 radiologist was blinded to the participants' medical histories and occupational exposure statuses.
Disk dehydration was defined as T2-weighted signal intensity loss from the intervertebral disk.19 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.20 According to the Farfan method,21 disk height can be measured as the mean of the ventral and dorsal distances between the contours of the adjacent vertebral bodies. Reduction of disk height was defined as disk height that is less than that of the disk height of the disk above.16 Disk bulging was defined as the presence of disk 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 disk material beyond the disk 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 disk material beyond the disk space was greater than the distance between the edges of the base or when no continuity existed between the disk material beyond the disk space and that within the disk space. Extrusion may be further specified as sequestration if the displaced disk material has completely lost continuity with the parent disk.20 Spondylolytic spondylolisthesis was identified in a lateral projection as an anterior displacement with a break of the pars interarticularis. Degenerative spondylolisthesis was defined as displacement of one vertebral body relative to the next inferior vertebral body with an intact pars interarticularis, and spondylolytic spondylolisthesis involves the separation of the posterior aspect of the vertebral body from the anterior body.19 The intrareader reliability regarding the presence or absence of each MRI variable was determined as the average reliability of 5 lumbar disks of the 60 participants evaluated on 2 occasions within 3 months.
Data Analysis
All statistical analyses were conducted using JMP 5.0 (SAS Institute Inc, Cary, North Carolina). For the evaluation of the occurrence of LDD among the lifting group, a logistic regression was conducted, adjusting for potential risk factors, including age, sex, body mass index (BMI), and smoking. To calculate tests for trend, the lifting exposure was included as an interval-scaled variable in the logistic regression model. For power calculation in this study with an alpha error of .05, twice the risk compared with the reference group, a prevalence <3.5% in degenerative disk–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 multiple comparisons was performed, and P values <.0042 and <.0083 indicated significance for the upper and lower lumbar regions, respectively. The reproducibility of the modified calculation of the compression load and lifting measurements was analyzed using SPSS version 16.0 for Windows (SPSS Inc, Chicago, Illinois) to compute intraclass correlation coefficients (ICCs). Percentage of agreement was used to assess the intrareader reliability of the MRI variables.
Role of the Funding Source
This study was supported by a grant from the Taiwan National Health Research Institute (NHRI-98∼100A1-PDCO-0108111).
Results
Of the 715 eligible people, 162 were excluded from this study for the following reasons: 84 people had cancer, 16 people had psychiatric conditions, 13 people had spinal tumors, 4 people had inflammatory spondylopathy, 18 people had compression fractures, and 27 people experienced major back trauma (Figure). A total of 393 fruit market workers (mean age=51.2 years, SD=10.0) and 160 walk-in clinic patients (mean age=49.3 years, SD=11.6) with URIs were included in the analysis in the study; 252 participants were men, and 301 participants were women. The demographic characteristics of the participants are shown in Table 1. The BMI was calculated as weight in kilograms divided by length in meters squared (kg/m2). The fruit market workers (25.3±3.5 kg/m2) exhibited higher BMI values compared with the walk-in clinic patients (25.3 kg/m2, SD=3.5, and 23.6 kg/m2, SD=3.4, respectively), and most participants (75.6%) had more than 15 years of work experience.
Demographic Characteristics of the Study Participantsa
The cumulative lifting load was categorized into tertiles (ie, <4.0 × 105, 4.0 × 105−8.9 × 106, and ≥8.9 × 106 Nh). There were 185, 184, and 184 participants in the low, intermediate, and high lifting load groups, respectively. The fruit market workers were exposed to higher lifting loads than the walk-in clinic patients. Low back pain during the previous 6 months was reported by approximately 83.6% of the participants. The prevalence rate of LBP among the fruit market workers (86.3%) was higher than that among the walk-in clinic patients (76.7%). The ICCs for intrarater and interrater reliability of a modified calculation of the compression load, excluding transporting and unloading time, were .998 and .992, respectively. The reproducibility of lifting measurements was high for lifting weights (ICC=.945), frequency of lifting per day (ICC=.914), and working tenure (ICC=.943) and moderate for lifting time (ICC=.743). The percentage agreement of intrareader reliability for the MRI variables ranged from .833 to 1.000, as shown in eTable 1. The kappa values of intrareader reliability for the MRI variables are shown in eTable 2.
The prevalence rates of LDD are shown in Table 2. The most prevalent conditions were dehydration and the bulging of disks. Dehydration was most common at level L4–L5 (69.1%), followed by L5–S1 (63.7%), L3–L4 (54.4%), L2–L3 (38.5%), and L1–L2 (20.2%). Disk bulging was most common at level L4–L5 (61.8%), followed by L3–L4 (46.1%), L5–S1 (45.4%), L2–L3 (26.4%), and L1–L2 (7.8%). Among the conditions, the most prevalent site of disk height narrowing, spondylolytic spondylolisthesis, and nerve root compression was the L5–S1 level. The other disk conditions, including disk dehydration, annulus tears, disk bulging, protrusion, extrusion, degenerative spondylolisthesis, and foramina narrowing, were most frequently observed at the L4–L5 level. No disk sequestration was observed in this study.
Prevalence of Disk-Related Degenerative Findings on Magnetic Resonance Imaging Scans of the Lumbar Spine in the Study
Table 3 depicts the relationship between the lifetime cumulative lifting load and LDD among the upper lumbar levels, including L1–L2, L2–L3, and L3–L4. Regarding disk dehydration, the participants in the high lifting load group had increased adjusted odds ratios (AORs) at the L2–L3 and L3–L4 levels (AOR=1.9, 95% confidence interval [95% CI]=1.2, 3.2, and AOR=2.1, 95% CI=1.2, 3.5, respectively) compared with those in the low lifting load group. After a Bonferroni correction was implemented, the association between lifting load and dehydration remained statistically significant at the L3–L4 level. In addition, the trend analysis was significant (P<.0083). For disk bulging, the association was statistically significant at the L2–L3 and L3–L4 levels (AOR=2.2, 95% CI=1.3, 3.8, and AOR=2.0, 95% CI=1.3, 3.4, respectively), and the trend analysis was significant (P<.0083). Annulus tear, disk height narrowing, protrusion, extrusion, sequestration, degenerative and spondylolytic spondylolisthesis, foramina narrowing, and nerve root compression were not included in the statistical analysis because of a low prevalence among the MRI findings (<3.5%), which limited the analytical power for detecting statistical differences.
Association Between Disk Degeneration and Life-Time Lifting Exposure (Newtons × Hours [Nh]) Among Upper Lumbar Levela
Table 4 shows the data regarding the association between lifetime cumulative lifting load and LDD among the lower lumbar levels, including L4–L5 and L5–S1. After Bonferroni correction, the high lifting load group was associated with disk dehydration at the L4–L5 and L5–S1 levels (AOR=3.1, 95% CI=1.8, 5.5, and AOR=2.5, 95% CI=1.5, 4.1, respectively), and the trend analysis was significant (P<.0042). After Bonferroni correction, the association between disk height narrowing at the L5–S1 level and both the intermediate and high lifting load groups was significant compared with the low lifting load group (AOR=3.7, 95% CI=1.7, 9.0, and AOR=4.1, 95% CI=1.9, 10.1, respectively), and the trend analysis was significant (P<.0042). Regarding disk bulging, the associations with the intermediate lifting load group were significant at both the L4–L5 and L5–S1 levels compared with the low lifting load group (AOR=2.0, 95% CI=1.3, 3.2, and AOR=2.1, 95% CI=1.3, 3.3, respectively). After a Bonferroni correction was performed, no association between the lifting load and annulus tears, protrusion, or foramina narrowing was observed. Disk extrusion, sequestration, degenerative and spondylolytic spondylolisthesis, and nerve root compression were not analyzed because of their low prevalence among the MRI findings (<3.5%). In summary, the optimal dose-response relationships between the cumulative lifting load and LDD were observed at the L5–S1 level.
Association Between Disk Degeneration and Life-Time Lifting Exposure (Newtons × Hours [Nh]) Among Lower Lumbar Levelsa
Discussion
This was a cross-sectional study conducted to examine whether the lifetime cumulative lifting load causes dose-dependent LDD. The effects of lifetime cumulative lifting load include disk dehydration, disk height narrowing, and disk bulging. These effects have been observed among people exposed to cumulative lifting loads of 4.0 × 105 to 8.9 × 106 Nh as presentations of disk bulging at the L4–L5 and L5–S1 levels and disk height narrowing at the L5–S1 level. When the cumulative lifting load exceeds 8.9 × 106 Nh, disk dehydration has been observed at the L3–L4, L4–L5, and L5–S1 levels. These results suggest that a dose-response relationship exists between the lifetime cumulative lifting load and LDD.
Based on our research, only a few studies have described a dose-response relationship between physical loading and LDD.1–3,7 Kelsey et al7 indicated that people who lifted objects heavier than 11.3 kg (25 lb) more than 25 times per day exhibited more than 3 times the risk for developing acute prolapsed lumbar intervertebral disks than did people who did not. Seilder and colleagues1–3 showed that a positive dose-response relationship exists between lumbar disk herniation and the cumulative lumbar load through manual material handling. The odds ratio (OR) was 1.7 for men in the middle exposure group (5–21.51 × 106 Nh), whereas the OR was 2.4 for women in the second-highest exposure group (4.04–14.47 × 106 Nh).3 In our study, the workers exposed to intermediate lifting loads (4.0 × 105–8.9 × 106 Nh) were 2.1 times more likely to have disk bulging at L5–S1 compared with those exposed to low lifting load (<4 × 105 Nh). We determined that workers exposed to intermediate lifting loads exhibited a 3.7-fold likelihood of experiencing disk height narrowing at L5–S1, which is consistent with the findings of Seilder et al.3 In our study, observable health effects exerted on the intervertebral disks were observed among the lower lumbar levels.
Direct measurement of lumbar spine load when conducting in vivo studies requires implanting a transducer or sensor into the disk. This type of study is rarely attempted because of the ethical considerations regarding such an invasive procedure. Numerous methods have been developed for estimating the disk compression load. Among these methods, computerized biomechanical modeling is considered the most precise method for estimating the disk compression load. The 3DSSPP was established based on several biomechanical studies,16,22–27 and anthropometric data from a US industrial database were applied in estimating the lumbar disk compression load28; this method has been used in field investigations.17,18,29,30 The advantage of this computerized biomechanical model is its capability for estimating the disk compression load within a single exertion. This model was validated by comparing it with 4 optimization models, and high correlation rates were obtained (r>.8).22 Moreover, the model has been used as the standard model for estimating the disk compression load.18 The limitation of 3DSSPP is that it cannot be used for simulating dynamic exertions. Therefore, in this study, the work tasks were divided into sequences of static postures, and each posture was analyzed.
Compared with previous studies that focused on only 1 or 2 disk degenerations1,2,7 at specific lumbar levels, this study evaluated various LDD and examined each lumbar level. The results indicated that varying lifting loads appear to exert different effects for various LDDs and at different lumbar levels. For example, disk bulging caused by carrying intermediate loads was observed at the L2–L3 and L3–L4 levels, and bulging caused by carrying high lifting loads was detected at the L4–L5 and L5–S1 levels. However, dehydration was observed only in the group that carried high lifting loads. Disk height narrowing was detected in both groups that carried intermediate and high lifting loads, but only at the L5–S1 level. Regarding the most prevalent sites, the study results indicated that disk height narrowing, spondylolytic spondylolisthesis, and nerve root compression were mostly detected at the L5–S1 level and that the other LDDs were common at the L4–L5 level. These results are consistent with those of previous studies.5,6,16,31 Generally, most studies on LDD have observed that the effects occur more frequently and severely at lower levels than at upper levels.5,6,16,31 Systemic factors such as age, smoking habits, and genetics are expected to have similar effects at all lumbar levels.
The observation that severe degeneration frequently occurred at the L4–L5 and L5–S1 levels supported the hypothesis that mechanical loading may play a crucial role in disk pathogenesis.16 Although many of the ORs were statistically significant, they were minor (<3.0), suggesting that the associations between the lifting load and degenerative disk–related MRI findings were not strong. Based on the literature, several critical risk factors, such as hereditary factors and age, may lead to the development of degenerative disks.9,32,33 Nevertheless, the significant ORs identified in this study suggested that lifting exposure contributes to the development of degenerative disks and should not be ignored.
The walk-in clinic patients with URI were recruited to incorporate a background population that was minimally exposed to lifting load. The ideal study participants would have been from one industry that involved a broad spectrum of lifting exposure. However, most fruit market workers were exposed to heavy lifting, except for a small percentage of administrative workers. Thus, recruiting a group of participants with low lifting exposure as a comparison group enhanced this study. Because these walk-in patients were employed in various occupations, they were predominantly grouped into the low lifting exposure tertile. In addition, URIs are among the most common conditions in the general population. Therefore, these walk-in patients with URI were regarded as representative of the general population.
Limitations
This study had several limitations. Because this was a cross-sectional study, it was subject to the healthy worker survivor effect. For example, disk bulging at the L4–L5 and L5–S1 levels was observed more frequently among the participants who lifted intermediate loads than among those who lifted high lifting loads. Moreover, disk sequestration was not observed in this study. In addition, the prevalence of disk extrusion and spondylolytic spondylolisthesis was lower compared with that reported in previous studies.31,34 This lower prevalence might have occurred because severely affected workers had left their jobs. Consequently, based on the MRI findings, several degenerative disk–related conditions were not analyzed because of low prevalence.
Another limitation was the reliance on the participants' memories regarding their occupational history and relevant work tasks from several decades previously. Although the repeatability of self-reported and specific current job tasks was examined and determined to be satisfactory, the reliability of the information pertaining to previous jobs was difficult to determine. To enhance reliability, a structured interview was administered to provide the participants with adequate time for examining the work details of their previous jobs. The trained interviewers used common milestones in life to help the participants recall the necessary details. The trained interviewers captured the working simulation photos by following a standard procedure. Several studies have indicated that, compared with direct measurements, the validity of self-reported data is lacking.35–37 By contrast, Pope et al38 demonstrated the accuracy of self-reported manual material handling activities and presented satisfactorily accurate results regarding frequency, duration, and amplitude. Direct measurements obtained using work or laboratory simulations yield the most accurate information; however, using such methods in retrospective studies involving relatively large sample sizes is impractical.
Another limitation was not including pushing and pulling tasks in load determination. We observed that the fruit market workers did not typically practice pushing and pulling, potentially causing the lifetime cumulative load to be underestimated among the other participants who performed pushing or pulling tasks in their jobs. These factors were not considered to have generated bias in our findings because pushing and pulling involve exerting much smaller compression loads on the lumbar disks compared with lifting. Similarly, if the participants of the low lifting load group were exposed to higher lifting loads than recorded, these higher lifting loads potentially caused random errors and several values regarding the relationship between lifting exposure and LDD to be underestimated. Another limitation was that exposure to lifting during leisure and home activities was not considered, potentially causing misclassification and error in the cumulative lifting load estimates.
Moreover, the differences between the study groups may have confounded the association between cumulative lifting load and LDD. The sex and age distributions of the participants from the 2 groups were similar. The fruit market workers lifted heavier loads and exhibited higher BMIs and smoking durations and lower education levels compared with the walk-in clinic patients. The lifting exposure patterns among the fruit market workers were more consistent compared with those of the walk-in clinic patients. When we grouped the participants into tertiles, the possibility of misclassification was considered acceptable. In addition, from a statistical point of view, such misclassification is unlikely to cause an overestimation of the ORs, and an underestimation of the actual results is more likely to occur. The BMI was associated with our findings regarding disk dehydration and bulging. Age is strongly associated with LDD,5,33 and degenerative changes in the lumbar spine are observed approximately 10 years earlier in men than in women.32 Smoking has been associated with LDD39; however, findings regarding smoking have not been consistent. Education level has probably no effect on LDD. People with lower education levels are more likely to choose physically demanding work than people with higher education levels, thereby exposing themselves to high levels of lifting. Therefore, we adjusted age, sex, BMI, and smoking habits to minimize the possible confounding that might occur. After the adjustment, the ORs were decreased compared with the crude ORs, suggesting that these adjusted factors influence the outcome; therefore, the effects of lifting can be detected.
In conclusion, the results suggest a dose-response relationship between cumulative lifting load and LDD. Based on the MRI findings, the effects include disk dehydration, disk height narrowing, and disk bulging, especially at the lower lumbar levels. The lifting load apparently exerts different effects on various LDDs, as well as on different disk levels.
The Bottom Line
What do we already know about this topic?
Lumbar disk degeneration (LDD) is associated with heavy lifting loads. However, the evidence for the dose-response relationship between lifetime cumulative lifting load and LDD is limited.
What new information does this study offer?
This study uses a precise method to estimate a person's lifetime cumulative lifting load on lumbar disks. The study found dose-dependent relationships between lifting load and disk bulging and dehydration among the L2–S1 disks, as well as between lifting load and annulus tears, disk height narrowing, disk protrusion, and foramina narrowing on the L5–S1 disk.
If you're a patient, what might these findings mean for you?
Previous exposure to cumulative lifting loads of more than 8.9 million newton-hours could put people at high risk for developing LDD.
Footnotes
Ms Hung, Dr Shih, Dr Liou, and Dr Guo provided concept/idea/research design. Ms Hung and Dr Guo provided writing. Ms Hung, Dr Chen, Ms Ma, Ms Huang, and Dr Guo provided data collection. Ms Hung, Dr Chen, Ms Ma, and Ms Huang provided data analysis. Ms Hung, Dr Shih, Ms Ma, and Dr Guo provided project management. Dr Guo provided fund procurement. Dr Shih and Dr Guo provided facilities/equipment. Dr Ho provided institutional liaisons. Dr Shih, Dr Hwang, and Dr Guo provided consultation (including review of manuscript before submission). The authors thank Dr Dickens Chen for his valuable contributions to the recruitment of participants from the fruit market.
The Institutional Review Board of the National Taiwan University Medical Center approved the study.
This study was supported by a grant from the Taiwan National Health Research Institute (NHRI-98∼100A1-PDCO-0108111).
- Received March 10, 2013.
- Accepted June 15, 2014.
- © 2014 American Physical Therapy Association
References
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