SPINE Volume 35, Number 21, pp 1925–1929 ©2010, Lippincott Williams & Wilkins The Role of Back Injury or Trauma in Lumbar Disc Degeneration An Exposure-Discordant Twin Study Mark J. Hancock, PhD,* Michele C. Battie, PhD,† Tapio Videman, MD, PhD,† and Laura Gibbons, PhD‡ Study Design. Exposure discordant twin study. Objective. To investigate the effect of injury on lumbar disc degeneration in monozygotic twins with discordant exposures to recalled previous injury/trauma to the lum- bar spine. Summary of Background Data. Disc degeneration is considered a primary source of low back pain. Despite this, important determinants of disc degeneration other than genotype have not been identified. One possible important determinant of disc degeneration that has un- dergone limited investigation is previous back injury. Methods. We compared disc degeneration between 37 male monozygotic twin pairs with discordant exposures to recalled previous injury/trauma to the lumbar spine. Data on injury history were obtained through an exten- sive structured interview. Disc degeneration was as- sessed using quantitative measures of disc height and disc signal intensity. Results. Disc degeneration did not differ between twins who reported previous back injury and their unin- jured cotwins. This finding was consistent for both disc height and disc signal intensity regardless of whether mean scores or greatest difference at any one lumbar level was used in the analysis. Disc height averaged 0.3 mm higher in the injured twin (P = 0.302), and was on average 0.05% higher at the level of the greatest cotwin difference (P = 0.302). There was no evidence that greater period since injury resulted in greater twin differences in disc degeneration. Conclusion. The current study suggests that back in- jury based on patient report is not an important predictor of future disc degeneration. Key words: disc degeneration, magnetic resonance imaging, lumbar spine, injury. Spine 2010;35:1925–1929 During the last 15 years, beliefs regarding the determi- nants of disc degeneration have changed markedly. Pre- viously it was believed that the primary determinants included age, gender, cigarette smoking, exposure to ve- hicular vibration and, in particular, occupational physi- cal loading. 1 More recently it has been demonstrated that most of these factors have at most relatively minor influences on disc degeneration 2–4 and routine loading may actually have some benefits for the lumbar discs in terms of slowing desiccation. 5 Alternatively, genetic fac- tors have been shown to be strong determinants of disc degeneration. 6–8 Despite the significant role of genetic influences, there remains a large amount of disc degeneration, especially in the lower lumbar spine, which is not explained by either genetics or currently investigated environmental factors. 7 One possible important determinant of disc de- generation that has undergone limited investigation is previous back injury. Injury in this context and for the rest of this manuscript refers to a specific significant in- cident or accident, involving excessive loading or trauma likely to result in tissue damage. Injury may influence the disc structure and mechanics in a manner that accelerates the degenerative process. Animal studies have shown rapid disc degeneration after surgically induced periph- eral anular lesions. 9 Two previous studies investigating the relationship between vertebral fractures and disc de- generation, found conflicting results. 10,11 Kerttula et al 10 found young people with previous vertebral compression fractures had greater amounts of disc degeneration than healthy controls. This study has been criticized for failure to control for important confounders, including genetics, but provides preliminary evidence for an association between injury and disc degeneration. 12 Another piece of evidence suggesting that injury, in the form of a needle puncture, may influence disc degeneration comes from a recent study, which found discography resulted in increased disc degen- eration over the following 10 years. 13 Disc degeneration is a complex process and there is no universally accepted method for measuring it. Disc de- generation measured on magnetic resonance imaging (MRI) can include measures of signal intensity and disc narrowing among others. However, disc desiccation as indicated through signal loss on MRI is the finding most highly associated with ageing and may be most sensitive for the detection of early changes in the degenerative process. 14 Disc height narrowing represents substantial, end-stage disc degeneration, and has been found to be From the *Faculty of Health Sciences, University of Sydney, New South Wales, Australia; †Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, AB, Canada; and ‡University of Washington, Se- attle, WA. Acknowledgment date: September 21, 2009. Revision date: December 17, 2009. Acceptance date: January 21, 2010. Supported (data acquisition) by the NIH NIAMS grant AR-40857 and the Finnish Academy. Support was also received from the Alberta Her- itage Foundation for Medical Research and the Canada Research Chairs Program. The manuscript submitted does not contain information about medical device(s)/drug(s). Federal, Institutional, and Foundation funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. The Twin Spine study was approved by the Ethical Committee of the Department of Public Health at the University of Helsinki and the Human Subjects Committee of the University of Washington. Address correspondence and reprint requests to Mark Hancock, PhD, Faculty of Health Sciences, University of Sydney, PO Box 170, Lidcombe 1825, New South Wales, Australia; E-mail: M.Hancock@usyd.edu.au 1925