Study Population
This was a retrospective study. The protocol and consent forms of the study were reviewed and approved by the National Taiwan University Hospital Research Ethics Committee (NTUH-REC No.:200805047R). The method of recruiting the participants, measurements of work exposure, and imaging studies of the participants’ lumbar spines are detailed elsewhere [8]. To obtain a broad spectrum of lifting exposures, the following number of participants from 2 populations were recruited: (1) 263 walk-in clinic patients and (2) 452 workers who carry heavy loads. The patients visited the Internal Medicine Clinic of National Taiwan University Hospital, were diagnosed with upper respiratory infections (URIs), which were mostly the common cold, and were recruited as the initial study population. The group that carried heavy loads included workers from one fruit and vegetable wholesale market. Lifting is a daily, routine task for these workers. During recruitment, the market workers and the walk-in patients were not informed of the hypothesis of the study. They were invited to participate in an investigation regarding spine and bone disorders. The inclusion criteria were an age between 20 and 65 years and at least 6 months of working experience. Participants diagnosed with cancer, psychiatric conditions, spinal tumours, inflammatory spondylopathy, compression fractures, or major back trauma were excluded [8]. We combined the participants in the 2 populations to examine the effects of lifting on disk protrusion. 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. Of these eligible 715 subjects, 162 were excluded from this study for the following reasons: 84 people experienced cancer, 27 people had major back trauma, 18 people had compression fractures, 16 people had psychiatric conditions, 13 people had spinal tumours, and 4 people had inflammatory spondylopathy.
Data Collection
Each participant was asked to complete a questionnaire and to undergo a magnetic resonance imaging (MRI) scan of the lumbar spine. A detailed structured interview was administered to the participants to assess the relevant work tasks in each job held since they entered the workforce. The occupational history included the participant’s job titles, tenures, body weights at each job, descriptions of the tasks, loads carried, lifting frequencies and durations, working hours per day and working days per week. The participants were encouraged to recall their body weights during the time they worked at each job. When the job period was longer than 5 years, the average body weight during this job period was used.
Estimation of the Lumbar Disk Compression Load
A method of estimating lumbar disk compression load has been published previously [8]. Regarding the estimation of lifting exposure, the participants recalled all of the jobs they had held after completing schooling and the weight carried during, frequency of, and duration of each task. The participants performed a typical material handling task to simulate the positions and weights encountered at each job. The lifting activity was divided into a sequence of static postures, including the initial lift-up, transferring, and unloading postures, and each posture was analysed. The initial position in 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 the 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 experienced during the job. The compression load on the disk during lifting was estimated using the 3D Static Strength Prediction Program (3D SSPP, University of Michigan, Michigan) [19,20]. Anthropometric data, such as the participant’s sex, height, body weight, and carried weight, and a photograph of each participant in the working posture were input into the system to predict the lumbar load. To evaluate the intrarater and interrater reliability of the lumbar load estimated by 3D SSPP, 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.
Definition of the Threshold in this Study
The threshold in this study was defined as the lumbar load per lifting movement, and loads above this proposed value were considered to contribute to disk protrusion over an entire career and were included in the cumulative lifting exposure calculation. In other words, the calculation neglected all exposures with a lumbar load below the threshold. The proposed threshold values were set at zero Newtons (N) and all 100 N increments from 2000 to 4000 N.
Calculation of the Cumulative Lifting Exposure on the Lumbar Disk
The method of calculating the cumulative lifting exposure on the lumbar disk was modified from the Mainz-Dortmund dose model (MDD), which is based on overproportional weighting of the lumbar disc compression force relative to the corresponding duration of lifting, and has been applied in several studies [5,6,7,8].
To investigate the actual cumulative lifting exposure, the participants recalled details regarding the 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 product of the lift-up lumbar load (Flift-up) and the corresponding lift-up time, the product of the transporting lumbar load (Ftransporting) and the corresponding transporting time, and the product of the unloading lumbar load (Funload) and the corresponding unloading time. Only the lumbar loads larger than the proposed threshold were eligible for inclusion in the cumulative exposure calculation. For example, if the threshold is set at zero N, every load lifted in each activity is included in the calculation. When the threshold is set to be 3400 N, only exposures with a lumbar load per lift above 3400 N are eligible for inclusion in the calculation. For each job described, the lifting exposure was calculated as the sum of the product of the lifting load and the corresponding duration of lifting in hours (Newtons × hours, Nh). The cumulative lifting exposures for each participant were then calculated by summing the lifting exposures on the lumbar disk across all jobs.
The calculation can be expressed as the following equation:
Cumulative lifting exposure (Newtons × hours) =
Σ [(F lift-up(Newtons) × t lift-up(seconds) + F transporting(Newtons) × t transporting(seconds) + F unload(Newtons) × t unload(seconds)) × 1(minute)/60(seconds) × 1(hour)/60(seconds) × frequency of lifting/day × working days/year × working year]
where F represents the lifting load (Newtons) on the lumbar disk and t represents the duration of lifting the load (seconds).
An example of the calculation of the sum of the lifting exposures is given below:
One male worker with a body height of 170 cm and weight of 75 kg reported that he had carried object 1 (weighing 10 kg; lifted it up for 1 second, walked with it for 30 seconds, and unloaded for 1 second) 100 times a day and object 2 (weighting 20 kg; lifted it up for 1.5 seconds, walked with it for 10 seconds, and unloaded for 1.5 seconds) 50 times a day, for the 220 days a year that he worked. The total work tenure was 20 years.
The lumbar force estimated by 3D SSPP
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Estimated lumbar force (Newtons)
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Duration of one task (seconds)
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weight
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lift-up
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walking
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unload
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lift-up
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walking
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unload
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Object 1
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10 kg
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3713
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1667
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3713
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1
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30
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1
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Object 2
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20 kg
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4858
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2343
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4858
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1.5
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10
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1.5
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Calculation of the cumulative lifting exposure:
- When the threshold is set at zero N, the cumulative exposure is as follows:
{[(3713 ×1) + (1667 ×30) + (3713 ×1)] × 100 + [(4858 ×1.5) + (2343 ×10) + (4858 ×1.5)] ×50} ×220 ×20/3600 =9.3 ×106 (Newton-hours)
The result of the calculation indicated that this person should be included in the intermediate lifting group (1.8 ×106~1.6 ×107 Newton-hours).
- When the threshold is set at 3000 N, the cumulative exposure is as follows:
{[(3713 ×1) + (1667 ×30) + (3713 ×1)] × 100 + [(4858 ×1.5) + (2343 ×10) + (4858 ×1.5)] ×50} ×220 ×20/3600 = 1.79 ×106 (Newton-hours)
The result of the calculation indicated that this person should be included in the intermediate lifting group (2.5 ×105~5.6 ×106 Newton-hours).
- When the threshold is set at 3400 N, the cumulative exposure is as follows:
{[(3713 ×1) + (1667 ×30) + (3713 ×1)] × 100 + [(4858 ×1.5) + (2343 ×10) + (4858 ×1.5)] ×50} ×220 ×20/3600 = 1.79 ×106 (Newton-hours)
The result of the calculation indicated that this person should be included in the intermediate lifting group (0~4 ×106 Newton-hours).
- When the threshold is set at 4000 N, the cumulative exposure is as follows:
{[(3713 ×1) + (1667 ×30) + (3713 ×1)] × 100 + [(4858 ×1.5) + (2343 ×10) + (4858 ×1.5)] ×50 } ×220 ×20/3600 = 8.9 ×105 (Newton-hours)
The result of the calculation indicated that this person should be included in the intermediate lifting group (0~4 × 106 Newton-hours).
We divided the participants into low, intermediate and high cumulative lifting load groups with even distributions of males and females according to the cumulative lifting exposure categories. However, only the lift-up forces larger than the proposed threshold were included in the calculation. Thus, as the threshold increased, more participants were categorized in the low lifting group. Therefore, males with a cumulative exposure above the 3000 N and zero N thresholds were categorized into the low, intermediate, and high tertiles. For those with a cumulative exposure above the 3400 N and 4000 N thresholds, the low group included the participants with zero Nh, and the remaining males with cumulative loads above zero Nh were split between the intermediate and high groups. On the other hand, in the female population, those with a cumulative exposure above the zero N threshold were divided into low, intermediate, and high tertiles. For those with a cumulative exposure above the 2800 N, 3400 N, and 4000 N thresholds, the low group included those with zero Nh, and the remaining females with cumulative loads above zero Nh were divided between the intermediate and high groups.
The reproducibility of the lifting measurements was tested 6 months after the initial interview with 25 participants. Their current jobs were used for reliability testing. These measurements included the working tenure, lifting weights, frequency of lifting per day, and lift-up time for the job. After observing and recording the fruit workers’ practices, we found that most of the participants’ lift-up times were nearly equal to their unloading times and that the transporting times were zero. Therefore, the reliability of the transporting time and unloading time were not examined. In addition, 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 load of pushing and pulling was not assessed.
Each intervertebral disk at L4–L5 to L5–S1 was evaluated for disk bulging, protrusion, extrusion, and sequestration using MRI. All MRI examinations were conducted at the National Taiwan University Hospital. The MRI equipment and protocol, definitions of the disk conditions, and evaluation of intrarater reliability regarding the presence or absence of protrusion are described in a previous study [8]. Two radiologists were responsible for the image interpretation and were blinded to the lifting exposure status of the participants.
Data Analysis
The reproducibility of the calculation of the lifting load and lifting measurements was analysed using SPSS version 16.0 for Windows (SPSS Inc, Chicago, Illinois), and intraclass correlation coefficients (ICCs) were computed. Kappa was used to assess the intrarater reliability of disk protrusion on MRI. Logistic regression analysis using JMP 5.0 (SAS Institute Inc., Cary, North Carolina) was used to identify the association between the cumulative lifting load and disk protrusion at either of the lower disk levels, namely, the L4-L5 and L5-S1 disks. Each variable was examined to determine its influence on disk protrusion and was considered to be a potential confounder if there was a statistically significant association (p<0.05). Thus, the logistic regression was adjusted for the participant’s age, BMI, and history of smoking. Odds ratios (ORs) and 95% confidence intervals were calculated by logistic regression analysis. To determine the best threshold of the lifting load, four statistical values were used to compare the outcome (L4-S1disk protrusion) and the cumulative lifting load with different thresholds, namely, (1) the area under the curve (AUC) of a receiver operating characteristic (ROC) curve, (2) coefficient of determination (R2), (3) Akaike information criterion (AIC), and (4) Bayesian information criterion (BIC). We compared the AUC in various models that were plotted using MedCalc for Windows Version 9.2.1.0 (MedCalc Software, Mariakerke, Belgium). Models with higher AUC values were considered the optimal models. The amount of the cumulative lifting load variable that was explained by various threshold values in the model was evaluated based on the R2 statistic. The AIC and BIC were obtained using SAS Version 9.1 (SAS Institute Inc.). The AIC is closely related to the BIC. Given a set of candidate models for the data, the preferred model is the one with the smallest AIC value, and the same concept applies to the BIC.