‘NORMAL AGING’ VERSUS ‘SYMPTOMATIC DEGENERATION’ OF INTERVERTEBRAL DISCS: A STUDY OF THE ‘DARK DISC’ IN LOWER BACK PAIN.

Abstract

Aim of study: ‘Dark discs’ (intervertebral discs with normal shape and height but low signal intensity on T2-weighted MRI) are believed by some authors to represent an early stage in the degenerative process although other authors have suggested that they are simply an appearance of the normal aging process. This study addresses the above question by investigating the correlation between disc morphology on MRI of the lumbar spine (‘dark disc’ or obviously degenerative signs such as annular tear, loss of disc height and shape and end plate changes) and the distribution of lower back pain.

Background: It is generally accepted that spinal levels appearing degenerate on MRI may act as ‘pain generators’ either directly or indirectly (by affecting adjacent structures or levels). Furthermore, histopathologic and discographic studies have shown transmition of specific patterns of somatic pain from degenerate spinal levels.

Methods: 231 consecutive patients (mean age: 45 years, SD:15.9) presenting with mechanical lower back pain of more than six months duration completed topographic graphs of the distribution of their back pain (categorised for this study as upper-, mid-, low-lumbar and sacroiliac area pain) and underwent an MRI of the lumbar spine. Chi-square test was used to investigate the association between presence of abnormalities at any spinal level and pain at any of the considered body areas. Two sets of analyses were performed: one including ‘dark discs’ in the ‘pain generators’ group and the other excluding the ‘dark discs’ from this group.

Results: When the ‘dark discs’ were not considered as ‘pain generators’ the only association to reach statistical significance was that between L5/S1 abnormalities and mid-lumbar pain (P=0.02). However, inclusion of the ‘dark discs’ in the ‘pain generators’ group increased the strength of the association between L5/S1 abnormalities and mid-lumbar pain (P< 0.01) and also resulted in the associations between L3/4 and L4/5 level abnormalities and sacroiliac area pain reaching statistical significance (P=0.02 and P< 0.01 respectively).

Conclusion: The significant statistical effect of considering ‘dark discs’ as ‘pain generators’ on the association between specific spinal level abnormalities and presence of back pain in well-defined areas suggests that the MRI appearance of a ‘dark disc’ should be considered an early stage in the degenerative process and not simply an effect of normal aging.