Study Design. Cadaveric motion segment experiment.
Objective. To show how two physical aspects of disc degeneration (dehydration and endplate disruption) contribute to spinal instability.
Summary of Background Data. The origins of spinal instability and its associations with back pain are uncertain.
Methods. Twenty-one cadaveric thoracolumbar motion segments aged 48 to 90 years were secured in cups of dental plaster and loaded simultaneously in bending and compression to simulate full flexion, extension, and lateral bending movements. Vertebral movements, recorded using a two-dimensional “MacReflex” motion analysis system, were analyzed to calculate neutral zone (NZ), range of motion (ROM), bending stiffness (BS), horizontal translational movements, and the location of the center of rotation (COR). Intradiscal “stresses” were measured by pulling a miniature pressure transducer through the disc along its midsagittal diameter. All experiments were repeated after each of two treatments, which simulated physical aspects of disc degeneration: creep loading to dehydrate the disc and compressive overload to disrupt the endplate. Results were analyzed using ANOVA and linear regression.
Results. Motion segment height was reduced by 1.0 (SD 0.3) mm during creep and by a further 1.7 (0.6) mm after endplate disruption. In flexion and lateral bending, the combined treatments increased NZ and ROM by 89% to 298%, and increased the “instability index” (NZ/ROM) by 43% to 61%. Translational movements increased by 58% to 86%, whereas BS decreased by 42% to 48%. In extension, ROM and NZ were little affected, although the COR moved closer to the apophyseal joints. Measures of instability increased most in lateral bending, and following endplate disruption. Stress concentrations in the posterior anulus fibrosus increased markedly after endplate disruption.
Conclusions. Two physical aspects of disc degeneration (dehydration and endplate disruption) cause markedsegmental instability. Back pain associated with instability may be attributable to stress concentrations in degenerated discs.
Physical effects of disc degeneration were simulated on cadaveric thoracolumbar motion segments by creep-induced water loss and endplate disruption. Both interventions markedly increased spinal &#x201C;instability&#x201D; as indicated by neutral zone, range of motion, bending stiffness, and translational movements. Endplate disruption contributed more to instability than did disc dehydration.
From the *Department of Orthopaedics, Sir Run Run Shaw Hospital, ZheJiang University, HangZhou City, ZheJiang, People’s Republic of China; and the †Department of Anatomy, University of Bristol, Bristol U.K.
Acknowledgment date: October 18, 2004. First revision date: April 19, 2005. Acceptance date: July 7, 2005.
Submitted for the ISSLS Awards, October 15, 2004. Revised version submitted for the ISSLS edition of Spine, April 19, 2005.
Supported in part in the United Kingdom by BBSRC. Fengdong Zhao is a Clinical Fellow supported by the Chinese Scholarship Council. Patricia Dolan is an ARC Research Fellow.
The manuscript submitted does not contain information about medical device(s)/drug(s).
Federal 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.
Address correspondence and reprint requests to Michael A. Adams, BSc, PhD, Department of Anatomy, University of Bristol, Southwell Street, Bristol BS2 8EJ, U.K.; E-mail: M.A.Adams@bris.ac.uk