A recent award winning Finnish study suggests a paradigm shift relative to the belief that physical loading of lumbar disc is associated with degenerative disc disorders. The investigation evaluated the L1-L4 discs of 44 paired monozygotic twins that had a moderate weight difference (minimum of 8 kg and an average weight difference of 13 kg). Statistically significant findings included:
- A higher lumbar spine bone density of L1-L4 (average 6.2%) among the heavier twins
- A higher (meaning better) disc signal variation (average 5.4%) among the heavier twins
No statistical difference was found between the twins in mean disc height measurements.
The authors concluded that long term, gradual physical loading of disc (as in body weight development) promotes tissue adaption and is not harmful to lumbar discs. In contrast, an infrequent heavy load, such as a sudden heavy lift, does not allow for this adaption.
The Bottom Line – How This Applies To Ergonomists
This study identifies a useful point – a significant weight difference between two similar individuals due to normal growth development most likely does not predispose the heavier person to lumbar disc disease.
However, to extend these findings to workplace tasks requires perspective. As mentioned by the authors, an industrial lifting task is usually infrequent and does not allow for disc tissue adaption. Also, much greater compressive forces are commonly generated across the low back during an industrial lifting task as opposed to upper body weight alone.
A useful future study would compare changes in the lumbar discs, via a similar analysis, within two groups of workers – those who performed heavy lifting tasks and those who did not.
The authors mentioned that this study looked at disc levels L1-L4 and not L5-S1, which may have produced some different findings. This is important, because the disc most commonly subject to disorder is L5-S1.
Also, a subset of 12 twins with the greatest weight difference (at least 15 kg) revealed a similar disc signal variation – meaning a loss of the beneficial effect.
Other Key Points
This study was named the 2009 Outstanding Paper: Medical and Interventional Science by The Spine Journal.
The mean age of all subjects was 48. The heavier sibling was taller (mean 1.9 cm), had a higher BMI (mean 3.6 kg/m2), and performed greater isokinetic lifting work (mean 70 joules).
Lumbar MRIs were obtained from 116 male monozygotic twins in Finland. From this group, 51 twin pairs had a weight difference of 8 kg. After controlling for several medical conditions, 44 pairs became the study subjects.
Information involving key environmental and behavioral determinants were collected (i.e., back pain history, physical activity history, smoking history) as well as anthropometric data (i.e., body weight, height, body mass index).
Custom software was used to assess disc areas and signal parameters of midsagittal spine images from L1 to L4. Signal variation measure was evaluated. Also, disc heights were measured.
This entire study can be freely acquired at: http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6W7P-4XR5W14-3-3&_cdi=6632&_user=10&_pii=S1529943009010225&_orig=browse&_coverDate=01%2F31%2F2010&_sk=999899998&view=c&wchp=dGLzVzz-zSkzk&md5=acc2020b1fd1ef217bcd05b63b2d505d&ie=/sdarticle.pdf
Article Title: Challenging the cumulative injury model: positive effects of greater body mass on disc degeneration
Publication: The Spine Journal, 10:26–31, 2010
Authors: T Videman, L E Gibbons, J Kaprio, and M C Battié
This article originally appeared in The Ergonomics Report™ on 2010-05-10.