Debunking one of the myths of the world of workers’ compensation—work causes injury.
Editor’s comment: We have heard the cry from hundreds of workers, “I have worked hard all my life and now I am injured from it.” As defense attorneys, we despise the claim “repetitive work” is a supposed “cause” of disc degeneration and back problems—the opposite conclusion may be more scientifically accurate. Workers who work regularly and lift properly may be healthier and have stronger discs than workers who lead more sedentary lifestyles. What could be a better comparison than birth twins?
To attack this truly non-scientific concept, researchers at the University of Alberta studied the effects of work on twins and reached some surprising conclusions. Michele Crites Battié, Ph.D. from the University of Alberta’s Faculty of Rehabilitation Medicine was the lead researcher in a long-term project they called the Twin Spine Study. Dr. Battié learned that when it comes to the world of disc degeneration, one should move slowly.
Dr. Battié: “In the last half-century the predominant model of disc degeneration was wear and tear from environmental influences, such as heavy lifting, smoking and exposure to whole-body vibration from motorized vehicles. Yet, there were conflicting findings about the effects of these environmental influences. In many epidemiological studies it is difficult to isolate the effect of a particular factor to determine whether it really contributes to an outcome of interest, in this case disc degeneration. With this in mind, we set out to conduct highly controlled studies.”
The Twin Spine Study, which began in 1991, was an international effort that grew to include researchers from the U.S., Finland, Canada, and the UK. Dr. Battié explains, “[w]e started off with a series of studies of identical twins who were exposure discordant, meaning that one twin was heavily exposed to the factor of interest, such as smoking or heavy lifting, and the other twin was not. Otherwise, the pairs of twins were the same gender, had the same genes, and were raised in the same childhood environment, so they shared many early exposures. It turned out that whether someone had a sedentary, light, or heavy physical job like logging actually had quite a modest effect on disc degeneration, even when exposed to these conditions over a lifetime. In one study we looked at 45 pairs of identical twins, with one twin of each pair having spent much more time driving (the issue being whole body vibration). Our results did not reflect any significant difference between twins where one drove and the other did not.”
As the researchers gathered around the MRIs from these early studies, they were particularly struck by one thing, says Dr. Battié. “We were quite surprised by the strong resemblance in disc degeneration between twin siblings, not only in the degree of degeneration, but also in the types of findings and locations involved. My colleagues and I then became very interested in exploring a genetic component to disc degeneration. To this end, we collected data on non-identical twins (same sex) who were raised in the same household. Thus most environmental exposures were similar as in the case of the identical twins. However, non-identical twins share only 50% of their genes on average. We found a very substantial genetic influence on disc degeneration in the lumbar spine. Also, the results of one of the recent studies indicate that the effect of factors such as body weight and muscle strength on disc degeneration, although modest, may be greater than that of physical exposures. Furthermore, regularly experienced physical loading may actually have a beneficial effect on disc degeneration.”
Dr. Battié continues, “Using both monozygotic and dizygotic twins we were able to conduct ‘classic’ twin studies on disc degeneration. Through these studies we were able to estimate the overall genetic contribution to disc degeneration and back pain history. We also looked at whether disc degeneration is a pathway through which genes influence back pain. Making this all more complex is that pain is subjective and difficult to measure. However, we did find evidence that disc degeneration is one pathway through which genes influence back pain, but it is only a small part of the back pain story.”
Getting to the nucleus of the issue, Dr. Battié states, “We looked at disc desiccation (drying), a primary aspect of disc degeneration, and found that routine loading is actually associated with less [drying]. So it may be that the disc isn’t totally unlike other musculoskeletal structures, but it is responding in a positive way to routine loading. It is sort of like a training effect. If you load a musculoskeletal structure gradually and routinely, it will strengthen, as is the case with muscles, tendons, and bones. Also, various environmental and behavioral exposures appear to have somewhat disparate effects on different signs of disc degeneration, such as disc desiccation and narrowing. In the past it was often thought that greater physical loading was bad for the disc across the board.”
In the future, says Dr. Battié, the field will have to expand its thinking beyond a biomechanical or simple wear and tear model of disc degeneration. “Going forward it will be important to understand that while we’ve been focused for decades on biomechanical factors as the cause of back problems, and the place to look for the answer, if we want to make headway we’ve got to broaden our approach to the problem. We also need to look in new ways at biomechanical factors; for example, not just looking at heavy or light physical demands, but at their interactions with other factors, such as genetics.”
As many things do, these new ideas about the origins of disc degeneration could involve a bit of a butterfly effect…one “small” change has implications for wider society. “There are definitely policy implications to this research,” states Dr. Battié. “Back pain was once attributed to degenerative findings and degenerative findings were attributed to work or leisure time physical demands. This implied an injury model―physical demands cause structural damage, which in turn leads to pain. Our data, however, suggests that the injury model may not be the most appropriate model for back pain and the degenerative changes seen on spine imaging. Some of the systems that operate on an injury model, such as North American workers’ compensation systems, will have to grapple with this issue. The bottom line is that if we don’t have a reasonable model or understanding of the problem we’ll never have a good solution.”
And with such a solid foundation, the team of North American and European researchers is marching forward. “We are embarking on research to identify the specific genes and biological mechanisms that influence disc degeneration and back pain problems, and we aim to clarify the role of environmental factors. It is also important to gain a sense of how degeneration occurs over time, and tease out what represents the natural progression and what can lead to back pain.”
Their research is continuing and we will continue to watch and report updated results. This is based on an article from the web at: http://ryortho.com/NEWSSHORTS/volume4/issue35/11-04-08-Double.html.
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