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The effect of disc degeneration on anterior shear translation in the lumbar spine: Still more problems lie ahead

Letter to Dr. Linda Sandell: Low back pain is the single most common cause for disability in individuals aged 45 years or younger and as a result carries tremendous weight in socioeconomic considerations. Undoubtedly, numerous factors are contributed to low back pain; mechanical causes of low back pain include degenerative spinal pathologies such as spondylolisthesis, disc herniation, discogenic pain, and so on. Although the study by Melnyk et al. has dramatically improved our knowledge about the effect of disc degeneration on anterior translation of the lumbar spine under shear loading, we do not completely agree with the conclusion indicating that disc degeneration, as classified with the Pfirrmann scale, does not predict lumbar spinal motion in shear. First, recent biomechanical studies have revealed distinct behavior at the different lumbar levels; and the study of lumbar intervertebral disc morphology using MR images has shown segment-dependent geometric features. Moreover, different clinical outcomes following surgical treatment at the different lumbar levels have been reported. Various factors could influence segmentdependent function of the lumbar spine, such as the surrounding ligaments and muscles, vertebral geometries, the orientation of facet joints, and so on. Notably, the segment-dependent discrepancy may be an inherent factor on the effect of disc degeneration on anterior translation of the lumbar spine under shear loading, which specifically compared at different segment levels are necessary. Further, lumbar disc degeneration is found most often at the L5-S1 level. We believe that the study by Melnyk et al. has some important limitations. For example, the study only investigated the L3-4 and L4-5 levels that they did not take the other lumbar levels into account, especially the L5-S1 level. Finally, the in-vitro study applied shear loading conditions could be quite different from in-vivo shear loading conditions, which the surrounding ligaments and muscles are not considered. In addition, the in-vivo studies have reported that different abnormal vertebral motions are closely associated with different lumbar disc degenerations graded by Pfirrmann’s criteria. The conclusion of Melnyk et al. may mislead the surgeons on the operative treatment options of lumbar disc degeneration. In the end, we unanimously applaud Melnyk et al. for their significant work and the diligence with which they handled this controversial topic. Currently, it is still more problems waiting to be solved. Therefore, substantial work to determine the effect of disc degeneration on anterior translation of the lumbar spine under shear loading is essential in the near future.