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). Recruitment of the participants, measurements of the work exposure, and imaging studies of the lumbar spines were detailed elsewhere [8]. To obtain a broad spectrum of lifting exposures, the participants were recruited from 2 populations: (1) 263 walk-in clinic patients and (2) 452 workers who carry heavy loads. Patients visited the Internal Medicine Clinic of National Taiwan 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 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 between 20 and 65 years old and at least 6 months of working experience. Participants diagnosed with several health conditions described previously were excluded [8]. We combined these 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 and published 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 tumors, and 4 people had inflammatory spondylopathy.
Data Collection
Each participant was asked to complete a questionnaire and to obtain magnetic resonance imaging (MRI) of the lumbar spine. A detailed structured interview was implemented to the participants for assessing the relevant work tasks in each job held since they entered the workforce. The occupational history included job titles, tenures, body weights at each job, descriptions of tasks, carry load, lifting frequency and duration, working hours per day and working days per week. 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.
Estimation of Lumbar Disk Compression Load
The method of lumbar disk compression load estimation has been published previously [8]. 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 disk during lifting was estimated using the 3D Static Strength Prediction Program (3DSSPP, University of Michigan, Michigan) software system [19,20]. Anthropometric data such as sex, height, body weight, carried weight, and working posture photograph of each participant were input into the system to predict lumbar load. To evaluate the intrarater and interrater reliability of lumbar load estimation by 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.
Definition of Threshold in This Study
The threshold value in this study was defined as lumbar load per lift and above this proposed value was considered as contributed to disk protrusion over an entire career life, and was included in the lifetime cumulative calculation. In other words, the calculation neglected all exposures with a lumbar load below the threshold. The proposed threshold values were set at zero Newton (N), and at 100N increments from 2000 to 4000N.
Calculation of Lifetime Cumulative Lifting Exposures on the Lumbar Disk
The calculation method of lifetime cumulative lifting exposures 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 was applied by several studies [5,6,7,8].
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 lumbar load (Flift-up) and the corresponding lift-up time, the products of the transporting lumbar load (Ftransporting) and the corresponding transporting time, and the products of the unloading lumbar load (Funload) and the corresponding unloading time. Only those lumbar loads greater than proposed threshold value were eligibled into the lifetime cumulative exposure calculation. For example, if the threshold value is set at 0 N, every lifting load generated from each activity will be included in the calculation. And, when the threshold value is set as 3400 N, only exposures with a lumbar load per lift above
3400 N will be eligibled in the calculation. For each job described, the lifting exposure was calculated as the sum of product of the lifting load and the corresponding duration of lifting in hours (Newton × hour, Nh). The lifetime cumulative lifting exposures for each participant was then calculated by summing the lifting exposure on the lumbar disk from all jobs.
The calculation method was expressed as the following equation:
Cumulative lifting exposures(Newton × hour) =
Σ [(F lift-up(Newton) × t lift-up(second) + F transporting(Newton) × t transporting(second) + F unload(Newton) × t unload(second)) × 1(minute)/60(second) × 1(hour)/60(second) × frequency of lifting/day × working days/year × working year]
where F represents the lifting load (Newton) on the lumbar disk and t represents time (seconds).
An example of the calculation of the sum of lifting exposures is given as below:
One male worker with 170 cm body height and 75 kg body weight reported that he had carried object 1 (weighting 10kg, lift up for 1 second, walking for 30 seconds, and unload for 1 seconds), 100 times a day; and object 2 (weighting 20kg, lift up for 1.5 second, walking for 10 seconds, and unload for 1.5 seconds), 50 times a day, working 220 days a year. The total work tenure had been 20 years.
Estimation of lumbar force by 3D SSPP
|
|
Estimated lumbar force (Newton)
|
Duration for one task
(second)
|
|
weight
|
lift-up
|
walking
|
unload
|
lift-up
|
walking
|
unload
|
Object 1
|
10 Kg
|
3713
|
1667
|
3713
|
1
|
30
|
1
|
Object 2
|
20 Kg
|
4858
|
2343
|
4858
|
1.5
|
10
|
1.5
|
Calculation of cumulative lifting exposure:
1. When the threshold is set at 0 N, the cumulative exposure is as following:
{[(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 calculation result indicated that this person is categorized into the intermediate lifting group (1.8 × 106~1.6 × 107 Newton-hours).
2. When the threshold is set at 3000 N, the cumulative exposure is as following:
{[(3713 × 1) + (3713 × 1)] × 100 +
[(4858 × 1.5)+ (4858 × 1.5)] × 50 } × 220 × 20 / 3600 =
1.79 × 106 (Newton-hours)
The calculation result indicated that this person is categorized into the intermediate lifting group (2.5 × 105~5.6 × 106 Newton-hours).
3. When the threshold is set at 3400 N, the cumulative exposure is as following:
{[(3713 × 1) + (3713 × 1)] × 100 +
[(4858 × 1.5) + (4858 × 1.5)] × 50 } × 220 × 20 / 3600 =
1.79 × 106 (Newton-hours)
The calculation result indicated that this person is categorized into the intermediate lifting group (0~4 × 106Newton-hours).
4. When the threshold is set at 4000 N, the cumulative exposure is as following:
[(4858 × 1.5) + (4858 × 1.5)] × 50 } × 220 × 20 / 3600 =
8.9 × 105 (Newton-hours)
The calculation result indicated that this person is categorized into the intermediate lifting group(0~4 × 106Newton-hours).
Regarding to the categorizing of the lifetime cumulative exposure, we try to divide the participants into low, intermediate and high cumulative lifting load group by tertile in male/female populations. However, only lift-up forces greater than proposed threshold value were included into lifetime calculation. Thus, higher the threshold developed, more participants were categorized in the low lifting group. Therefore, in male population, the cumulative exposure with 3000 N and 0 N thresholds above were categorized into low, intermediate, and high tertiles. For the cumulative exposure with 3400 N and 4000 N thresholds above, the low group was those with 0 Nh, and intermediate and high group were dichotomized among those with cumulative loads above 0 Nh. On the other hand, In female population, the cumulative exposure with 0 N threshold above was categorized into low, intermediate, and high tertiles. For the cumulative exposure with 2800 N, 3400 N, and 4000 N thresholds above, the low group was those with 0 Nh, and intermediate and high group were dichotomized among those with cumulative loads above 0 Nh.
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 reliability testing. These measurements included the working tenure, lifting weights, frequency of lifting per day, and lift-up time of the job. After observing and recording the fruit workers’ practices, we found 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. 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. MRI equipment and protocol, definition of disk condition, the evaluation of intrarater reliability regarding the presence or absence of protrusion were described previously [8]. Two radiologist were responsible for the image interpretation and were blinded for the lifting exposure status of the participants.
Data Analysis
The reproducibility of the calculation of the lifting load and lifting measurements was analyzed using SPSS version 16.0 for Windows (SPSS Inc, Chicago, Illinois) to compute intraclass correlation coefficients (ICCs). 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 applied to identify the association between lifetime cumulative lifting load and disk protrusion at either of the lower disk levels, namely, L4-L5 and L5-S1 disk. Each variable was examined to determine the influence on the disk protrusion and was considered to be a potential confounder as having a statistically significant association (p<0.05). Thus, the logistic regression was adjusted for age, BMI, and history of smoking. Odds ratio (OR) and 95 % confidence intervals were calculated by logistic regression analysis. To determine the best threshold of lifting load, four statistical values were used to compare outcome (L4-S1disk protrusion) to lifetime cumulative load while different threshold values was applied, namely, (1) 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 statistics were considered as the optimal model. The amount of cumulative lifting load explained by various threshold values in the model was evaluated based on the R2 statistic. AIC and BIC were obtained using SAS Version 9.1 (SAS Institute Inc.) AIC is closely related to BIC. Given a set of candidate models for the data, the preferred model is the one with the minimal AIC value, and the same applies to BIC.