SPINE Volume 28, Number 10, pp 973–981 ©2003, Lippincott Williams & Wilkins, Inc. Effects of Immobilization and Dynamic Compression on Intervertebral Disc Cell Gene Expression In Vivo Jeffery J. MacLean, BS,* Cynthia R. Lee, PhD,*† Sibylle Grad, PhD,† Keita Ito, MD ScD,† Mauro Alini, PhD,†‡ and James C. Iatridis, PhD* Study Design. An in vivo analysis of the intervertebral disc’s cellular response to dynamic compression and im- mobilization was performed using a rat-tail model. Objective. To assess the effects of immobilization and short-term dynamic compression on intervertebral disc cell expression of anabolic and catabolic genes. Summary of Background Data. Static compressive loads applied in vivo alter the composition of the disc matrix and cell viability in a dose-dependent manner. The effects of in vivo dynamic compression, which is a more physiologic load, and reported risk factor for low back pain have not been investigated. Methods. An Ilizarov-type device was implanted on the rat tail and used to determine the effects from 72 hours of immobilization (n = 6), 2 hours of dynamic com- pression (1 MPa/0.2 Hz) (n = 8), and the coupled effect of immobilization followed by compression (n = 8). Real- time reverse transcription-polymerase chain reaction was used to measure changes in anabolic and catabolic gene levels relative to both internal control subjects and a sham-operated group (n = 7). Results. Immobilization and dynamic compression af- fect anabolic and catabolic genes, with an overall down- regulation of types 1 and 2 collagen and upregulation of aggrecanase, collagenase, and stromelysin in the anulus. The effects of immobilization and compression appear to be additive for collagen types 1 and 2 in the anulus, but not in the nucleus, and not for catabolic genes. Conclusions. Short-duration dynamic compression and immobilization alter gene expression in the rat disc. In studying the response of the disc to loading, it is necessary to look at both anabolic and catabolic pathways, and to consider strain history. [Key words: aggrecan, anabolic and catabolic gene expression, collagen, intervertebral disc degeneration, matrix metalloproteinase, mechanobiology, spine biomechanics] Spine 2003;28:973–981 Despite significant advances in clinician’s understanding of the spine, low back pain remains the most common cause of disability among individuals 20 to 50 years of age. 1,2 Although there is no single root cause of low back pain, degeneration of the intervertebral disc (IVD) is one of the most highly investigated factors. 3–7 Links between degeneration of the disc and changes in material proper- ties, matrix composition, and morphology exist. 8 –16 There also is evidence that mechanical factors can con- tribute to these changes. 17–31 However, the precise con- ditions resulting in degradative or stimulatory alter- ations in the disc remain unclear. Clinical investigations have established a relation be- tween mechanical factors and low back pain (LBP). Spe- cifically, there is a decreased incidence of low back pain among individuals who are physically fit, whereas a sed- entary lifestyle, exposure to vibration, and hard physical work (i.e., frequent lifting) increase the risk for LBP and sciatica. 28,32–35 Furthermore, the development of lumbar disc rupture has been linked to activities such as frequent bending and twisting. 17,25,32 These findings support the hypothesis that mechanical factors influence the integrity of the disc. Several experiments on the intervertebral disc in vitro attempted to isolate the precise biosynthetic response to applied mechanical forces. Static compressive forces 36 and hydrostatic pressure 18,37 imposed on disc tissue stimulated sulfate and proline incorporation at low stresses and decreased incorporation at higher stresses. Dynamic stress applied to motion segments 22 and cells isolated in culture 26,38 demonstrated that the cellular re- sponse is dependent on the frequency of loading. Saamanen et al 29 performed one of the only in vivo studies investigating the biosynthetic response to dy- namic loading. They compared the discs of dogs that ran 20 km/day for 15 weeks to dogs that did not. Although they found a significant increase in collagen (hy- droxyproline) content, the abundance of unknown phys- iologic factors and loads make it difficult to attribute these findings to the dynamic loading alone. More controlled in vivo studies have investigated the effects of static compressive forces on the cellular activity and composition of the disc. Static compression caused a bimodal response in aggrecan similar to what has been observed in vitro. 19,23 However, the synthesis of collagen 2 was suppressed at all levels after 1 week of loading. 23 Hutton et al 20,39 reported that the amount of collagen (types 1 and 2) and aggrecan depended on both the static stress level and duration of loading (i.e., force-weeks). Most of the studies assessing the cellular response of the disc to loading have considered only the anabolic From the *Department of Mechanical Engineering, University of Ver- mont, Burlington, Vermont, the †AO Research Institute, Davos, Swit- zerland, and the ‡Orthopedic Research Lab, McGill University, Mon- treal, Canada. Supported by grants from the National Institutes of Health (1- K01AR02078) and the AO Foundation, Switzerland. Acknowledgment date: June 14, 2002. First revision date: October 30, 2002. Second revision date: November 26, 2002. Acceptance date: January 17, 2003. Device status/drug statement: The manuscript submitted does not con- tain information about medical device(s)/drug(s). Conflict of interest: Federal and Foundation funds were received to support this work. No benefits in any form have been or will be re- ceived from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to James C. Iatridis, PhD, 231B Votey Building, 33 Colchester Avenue, Department of Mechan- ical Engineering, University of Vermont, Burlington, VT 05405-0156; E-mail: james.iatridis@uvm.edu 973