Table 2 Exposure and results of each study included in the review that evaluated occupational load

 Battie et al. 1995

job code (1–4), total occupational lifting (day), mean time working twisted/bent, mean time sitting at work, occupation driving (hrs lifetime)

Disc degeneration (signal intensity)

There was an association between job code (0–4), occupational lifting and occupational sitting with disc degeneration. Greater occupational loading/lifting was associated with greater disc degeneration but associations were small (r = 0.18–0.31) Those with sitting had less disc degeneration.

 Biering-Sorensen et al. 1985

work is sedentary, light manual or heavy manual; worker undertakes heavy manual work, amount of physical activity at work

Relative disc degeneration (method was unclear) (for each level from L1 to S1)

RevMan: there was statistically significantly greater for L4 disc degeneration in daily manual workers compared to seldom manual workers (OR = 2.27; 95% CI 1.21 to 4.25), but no difference in disc degeneration at L5 (OR = 1.21; 95% CI 0.44 to 3.36) for physical activity at work. All other comparisons for disc degeneration were not significant but data was not presented.

 Elfering et al. 2002

Frequent lifting or carrying heavy objects, forward bending, vibration, sedentary activity, working night shifts

Disc degeneration (1–5 Pearce score)

(summary score for all levels together)

The association of working night shifts and disc degeneration did not reach statistical significance (OR = 9.58 95% CI 1.00 to 91.62)

 Hangai et al. 2008

Lifting more than 10 kg for more than one third of the working hours.

Disc degeneration (signal intensity with modified Pirfmann’s classification) (for each level from L1 to S1)

RevMan: Occupational lifting was not significantly associated with degeneration at any of the levels. L1 L2 (OR = 3.16 95% CI 0.37 to 26.75), L2 L3 (OR = 1.92 95% CI 0.20 to 18.61), L3 L4 (OR = 1.34 95% CI 0.05 to 38.91), L4 L5 (OR = 2.23 95% CI 0.21 to 23.84) and L5S1 (OR = 1.48 95% CI 0.09 to 23.88)

 Hartwig et al. 1997

Nurse, construction workers and controls

Unclear disc degeneration measure assessed as mono, bi, tri or multi-segmental.

Not enough data to calculate an odds ratio. Results suggest that 17% of patients with high workload had mono-segmental degeneration as opposed to 29% of those with no workload, suggesting that those with more workload had degeneration at more levels.

 Hung et al. 2014

Workers that carry heavy loads divided into low, intermediate and moderate lifting loads.

Disc dehydration (T2-weigthed signal intensity loss

There was a statistically significant difference in disc degeneration (dehydration) between lifting loads for L1 L2 (OR = 2.4 95% CI 1.4 to 4.0), L2 L3 (OR = 3.3 95% CI 1.3 to 3.2), L3 L4 (OR = 3.7 95% CI 2.4 to 3.5), L4 L5 (OR = 4.9 95% CI 3.0 to 8.0) and L5S1 (OR = 3.6 95% CI 2.3 to 5.7) when comparing the high load to the low load groups. There was also a significant difference between the intermediate and low load groups for L2 L3, L3 L4, L4 L5 and L5S1.

 Luoma et al. 1998

Drivers, carpenters and office workers

Disc signal intensity (L2 L3-L5S1)

RevMan: There were no differences between groups. L2 L3 (OR = 0.55 95% CI 0.16 to 1.96), L3 L4 (OR = 1.50 95% CI 0.66 to 3.42), L4 L5 (OR = 2.04 95% CI 1.35 to 3.08) and L5S1 (OR = 1.30 95% CI 0.70 to 2.43)

 Savage et al. 1997

ambulance workers, hospital porters, car production workers, brewery drayman and office workers

Disc degeneration (signal intensity) (all levels together)

RevMan: There was no difference between groups in relation to disc degeneration. Car production vs office workers (OR = 1.00 95% CI 0.34 to 2.94); hospital porters vs office workers (OR = 1.63 95% CI 0.45 to 5.91

 Schenk et al. 2006

nurses and office workers

Disc degeneration (signal intensity) (1–5)

RevMan: Disc degeneration was different between occupational groups for grade 2 with more degeneration in nurses (OR: 1.89; 95% CI 1.34 to 2.66, n = 544 all levels) and grade 4 with more degeneration in office workers (OR = 0.50 95% CI 0.29 to 0.86). There was no difference for grade 3 (OR = 1.14 95% CI 0.80 to 1.64) and grade 5 (OR = 0.75 95% CI 0.37 to 1.52),

 Videman et al. 2007

Job code (1–4)

History of lifting at work (1000 kg)

Disc signal intensity (L1-S1)

There was a significant association between history of lifting at work and signal intensity in the opposite direction (better signal with more load (Regression coefficient 0.001, p = 0.002), there was no association of occupational loading scoring and disc degeneration.

 Battie et al. 1995

job code (1–4), total occupational lifting (day), mean time working twisted/bent, mean time sitting at work, occupation driving (hr lifetime)

Disc height

There was an association between job code (0–4), occupational lifting and occupational sitting with disc height but the association was not strong r = −0.22)

 Biering-Sorensen et al. 1985

work is sedentary, light manual or heavy manual; worker undertakes heavy manual work, amount of physical activity at work

Disc height (for each level from L1 to S1)

There were no significant differences for disc height

 Brinckmann et al. 1998

Different occupations such as mining, steelworkers and normative data of unexposed individuals

Disc height (for each level from T12 to S1)

Occupational loading was associated with a smaller disc height at a few spinal levels, particularly in those working in underground mines.

 Hung et al. 2014

Workers that carry heavy loads divided into low, intermediate and moderate lifting loads.

Disc height narrowing (Farfan method. L4 L5 and L5S1

RevMan: There were significant differences between groups for disc height narrowing at L5S1 (OR = 5.8 (2.7 to 13.6)).

 Riihimaki et al. 1990

Concrete reinforcement workers and house painters

Disc space narrowing (0–5 for each level)

RevMan: Concrete workers had greater disc height narrowing overall (OR = 2.19; 95% CI 1.34 to 3.58), L3 L4 (OR = 5.34; 95% CI 1.17 to 24.39) and L4 L5 (OR = 2.54; 95% CI 1.26 to 5.11) than painters. There was no difference at L1 L2 (OR = 2.84 95% CI 0.57 to 14.25), L2 L3 (OR = 0.93 95% CI 0.13 to 6.66) and L5S1 (OR = 1.35 95% CI 0.73 to 2.48).

 Videman et al. 2006

Job code (1–4), occupational driving, maximum weight lifted at work (kg)

Disc height narrowing

There was an association between occupational lifting and changes in degeneration over 5 years. (0.1 points/disc decrease in disc height = 0.021)

 Videman et al. 2007

Job code (1–4)

Disc height T12-S1)

There was an association between lifetime occupational loading score and disc height (regression coefficient 0.038, p = 0.004) and no association between history of lifting at work and disc height.

 Munoz-Gomez et al. 1980

Work load calculated as above or below the general average

Degeneration (osteophytes, disc bulge and costotransversal arthrosis)

RevMan: Those with occupational load greater than the average had greater degeneration (OR = 1.63; 95% CI 1.03 to 2.57).

 Arevalo et al. 2014

Heavy physical work activities

Disc herniation

There was an association between heavy physical work and disc herniation (OR = 2.0; 95%CI 1.42 to 2.76)

 Battie et al. 1995

job code (1–4), total occupational lifting (day), mean time working twisted/bent, mean time sitting at work, occupation driving (hr lifetime)

Disc bulging

(summary score for upper T12-L4 and lower lumbar spine L4-S1)

There was no association between occupational loading and disc bulging

 Hung et al. 2014

Workers that carry heavy loads divided into low, intermediate and moderate lifting loads.

disc bulging, L4 L5 and L5S1

There was a statistically significant difference in disc bulging between lifting categories for L2 L3 (OR = 3.8 (2.3 to 6.3)), L3 L4 (OR = 3.6(2.4 to 5.6)), L4 L5 (OR = 3.1 (2.0 to 4.9) and (L5S1 (OR = 2.6 (1.7 to 4.0) when comparing the high load to the low load groups.

 Luoma et al. 1998

Drivers, carpenters and office workers

Disc bulging

RevMan: Carpenters were more likely to have posterior disc bulging at L3 L4 OR = 2.73; 95% CI 1.12 to 6.64) and anterior bulging at L45 (OR = 2.86; 95% CI 1.05 to 7.79) when compared to the sedentary group. There was no difference for anterior disc bulging at L2 L3 (OR = 2.60 95% CI 0.74 to 9.22), L3 L4 (OR = 3.0 95% CI 0.86 to 10.41) and L5S1 (OR = 1.39 95% CI 0.49 to 3.92) or posterior disc bulging at L2 L3 (OR = 2.53 95% CI 0.60 to 10.69), L4 L5 (OR = 1.92 95% CI 0.89 to 4.16) and L5S1 (OR = 1.33 95% CI 0.95 to 1.87)

 Schenk et al. 2006

nurses and office workers

Disc abnormalities (bulging, protusion, extrusion, etc)

RevMan: There was no significant difference in disc bulging between groups. (OR = 1.33 95% CI 0.95 to 1.97)

 Videman et al. 2006

Job code (1–4), occupational driving, maximum weight lifted at work (kg)

Disc bulging

There was an association between occupational lifting and disc bulging (0.07 points/disc increase in disc height = 0.065)

 Elfering et al. 2002

Summary score from 0 to 4 combining of lifting or carrying heavy objects, forward bending, vibration and sedentary work

Modic changes (summary score for all levels together)

Occupational loading was not associated Modic changes

 Han et al. 201

Work was self-reported and rated as light physical (mainly walking, moderate physical work (sitting/walking) and hard physical work (heavy working)

Modic changes assessed according to Modic et al. and graded into Type I, II or II.

There was a significant difference in the incidence of modic changes according to the level of physical work. In total 8 of 54 (15%) of those with light physical work had modic changes, 16 of 99 (26%) of those with moderate physical load and 23 of 57 (40%) of those with hard physical work had modic changes

 Schenk et al. 2006

nurses and office workers

Modic changes

RevMan: There was no difference in Modic changes between groups. (OR = 0.91 95% CI 0.52 to 1.58),

 Frymoyer et al. 1984

Lifting

Schmorl’s nodes

There was no association between lifting and the spine outcomes evaluated.

 Riihimaki et al. 1990

Concrete reinforcement workers and house painters

Endplate sclerosis

RevMan: There was no difference in endplate sclerosis between groups. Overall (OR = 1.97 95% CI 0.96 to 4.05), L1 L2 (OR = 4.70 95% CI 0.22 to 98.43), L2 L3 (OR = 2.84 95% CI 0.57 to 14.25), L3 L4 (OR = 0.93 95% CI 0.13 to 6.66), L4 L5 (OR = 2.84 95% CI 0.57 to 14.25) and L5S1 (OR = 0.83 95% CI 0.33 to 2.09)

 Videman et al. 2006

Job code (1–4), occupational driving, maximum weight lifted at work (kg)

Upper endplate irregularities

There was no association between occupational lifting and changes in endplate abnormalities over a 5-year follow-up.

 Frymoyer et al. 1984

Lifting

Osteophytes

There was no association between lifting and the spine outcomes evaluated.

 Riihimaki et al. 1990

Concrete reinforcement workers and house painters

Anterior and posterior spondylophytes

RevMan: Concrete workers had more spondylophytes at L1 L2 (OR = 3.68; 95% CI 1.20 to 11.27), L4 L5 spondylophytes (OR = 3.68; 95% CI 1.20 to 11.27) than painters. There was no difference for overall (OR = 1.68 95% CI 1.05 to 2.69), L2 L3 (OR = 1.84 95% CI 0.84 to 4.06), L3 L4 (OR = 1.97 95% CI 0.96 to 4.05) and L5S1 (OR = 1.43 95% CI 0.63 to 3.25).

 Videman et al. 2006

Job code (1–4), occupational driving, maximum weight lifted at work (kg)

Osteophytes

There was no association between occupational lifting and changes in osteophytes over time.

 Videman et al. 2006

Job code (1–4), occupational driving, maximum weight lifted at work (kg)

High intensity zones (signal intensity)

There was no association between occupational lifting and changes in high intensity zones over time.