Compression-induced changes in intervertebral disc properties in a rat tail model

Study Design. An Ilizarov-type apparatus was applied to the tails of rats t o assess the influence of immobilization, chronically applied compression, and sham intervention on intervertebral discs of mature rats. Objectives. To test the hypothesis that chronically applied compressive for ces and immobilization cause changes in the biomechanical behavior and bioc hemical composition of rat tail intervertebral discs. Summary of Background Data. Mechanical factors are associated with degenera tive disc disease and low back pain, yet there have been few controlled stu dies in which the effects of compressive forces on the structure and functi on of the disc have been isolated. Methods. The tails of 16 Sprague-Dawley rats were instrumented with an Iliz arov-type apparatus. Animals were separated into sham, immobilization, and compression groups based on the mechanical conditions imposed. In vivo biom echanical measurements of disc thickness, angular laxity, and axial and ang ular compliance were made at 14-day intervals during the course of the 56-d ay experiment, after which discs were harvested for measurement of water, p roteoglycan, and collagen contents. Results. Application of pins and rings alone (sham group) resulted in relat ively small changes of in vivo biomechanical behavior. Immobilization resul ted in decreased disc thickness, axial compliance, and angular laxity; Chro nically applied compression had effects similar to those of immobilization alone but induced those changes earlier and in larger magnitudes. Applicati on of external compressive forces also caused an increase in proteoglycan c ontent of the intervertebral discs. Conclusions. The well-controlled loading environment applied to the discs i n this model provides a means of isolating the influence of joint-loading c onditions on the response of the intervertebral disc. Results indicate that chronically applied compressive forces, in the absence of any disease proc ess, caused changes in mechanical properties and composition of tail discs. These changes have similarities and differences in comparison with human s pinal disc degeneration.