Does Physical Loading Damage the Disc?

Thursday, February 4, 2010 | 1

By Dr. Steven D. Feinberg

In a recently published award-winning study in The Spine Journal 10 (2010) 26–31,  Dr. Tapio Videman, PhD, and colleagues questioned one of the accepted principles in spine care and in workers’ compensation which is the belief that repetitive physical loading is inherently damaging to the intervertebral disc.

The authors studied male twin pairs where one of the siblings was substantially heavier than the other by an average of almost 30 pounds. Their assumption was that the lumbar discs in the larger twins were exposed to much heavier biomechanical loading than the discs of their brothers over a period of years.

The results were contrary to commonly held beliefs as the study found no evidence that the additional physical loading was harmful. In fact, it appeared to have a beneficial effect on lumbar discs.

Dr. Videman is quoted in the January issue of The BackLetter with stating that,  “Our findings provide evidence that more cumulative and repetitive loading appears to delay disc degeneration slightly. Repetitive loading is not necessarily bad for the disc—within limits,” he concluded. He did offer a qualifier, however. “Adaptation of the disc to heavier loading has to be a consideration,” he said. The BackLetter also noted that “The intervertebral disc is, of course, the largest avascular structure in the human body and has an extraordinarily slow metabolism. As a result, the disc may need to adapt gradually to increased loading. But this is true for nearly every structure in the musculoskeletal system.”

The major criticism of this hypothesis is that these findings may not be relevant to the workplace. The BackLetter articles notes that, “Gradual loading and strengthening of the disc that occurs with weight gain may not be relevant to workplace situations. The day-to-day physical demands of work might disrupt this adaptation process and lead to injury and ongoing chronic pain.”
 
It is true that studies over the last decade and a half - particularly Finnish twin studies - suggest that occupational physical loading plays a very modest role in disc degeneration.
 
The BackLetter noted that “On the other hand, one study reported that higher body mass, heavier physical work, and higher lifting strength are also associated with disc space narrowing. Disc space narrowing is a signal of the advanced stages of structural degeneration. One possible conclusion would be that heavier physical loading may have a negative effect on severely degenerated discs.”
 
So the overall message may be that physical loading in appropriate doses can strengthen healthy discs but may not have a similarly beneficial effect in those with severe degenerative changes. We know that exercise and strengthening in other joints and musculoskeletal structures in the body is physically beneficial. The same may well be true for the spine – exercise, physical loading and work are good! It may be though that once a joint or musculoskeletal structure suffers degenerative changes with age or substantial damage or injury, physical loading may have a more variable and negative effect. This makes sense clinically. An example would be someone who undergoes a partial knee meniscectomy and returns to sports without difficulty but then has trouble with advanced degenerative disease in that knee years later.
 
An explanation to tie what appears to be conflicting data is that in the workers’ compensation arena, it may be a combination of underlying non-industrial degenerative factors (which are a normal aging phenomenon) and work related factors that result in symptomatic spine conditions. In other words it may not be the work itself but rather the work in a susceptible person with “normal” underlying degenerative changes. Further for injured worker who has been doing physical labor for many years, it may be that genetic and aging factors along with microtrauma both at work and elsewhere over many years has contributed to the degenerative changes but only caused symptoms at a later point in time (“The straw that broke the camels back”).
 
While but for the industrial exposure injury there may well have been no disability, current law instructs us to apportion to the cause of the disability. That leaves the evaluating physician with the unenviable and at time herculean task of addressing apportionment and doing so via providing substantial evidence of an approximate percentage caused by work and non-industrial factors.

Dr. Steven D. Feinberg is an adjunct clinical professor at the Stanford University School of Medicine and principal of the Feinberg Group in Palo Alto, Calif.