Effects of shear force on intervertebral disc: an in vivo rabbit study.

Purpose: A new in vivo rabbit model was developed to investigate the effects of shear force on intervertebral disc (IVD). Methods: Japanese white rabbits ( n = 38) were used for this study. The L4/5 discs in Group A ( n = 10) were subjected to a constant shear force (50 N) using a custom-made external loading device for 1 month; in Group B ( n = 10) for 2 months; whereas in Group C ( n = 10), loading device was attached to the spine but the discs remained unloaded. Group D ( n = 8) was a non-operated intact control group. After loading, the loading devices were taken out and the animals were given X-ray and MRI examination. After X-ray and MRI examination, the animals were euthanized for histological analysis. Results: After 1 and 2 months of loading, radiographic findings showed significant disc height narrowing in L4/5 discs of the animals in loading groups, and slight lumbar spondylolisthesis in some animals of Group B. MRI showed a significant decrease in nucleus pulposus (NP) area and signal intensity from T2-weighted images. Histologically, loss of normal NP cells and disorganization of the architecture of the annulus occurred, and proteoglycan stain decreased. Conclusions: The results of this study suggest that disc degeneration can be induced by hyper-physiological shear loading in the rabbit IVD. Long-term shear loading may result in structural disc failure inducing lumbar spondylolisthesis and progressive disc degeneration, which, however, has to be proven by further studies. [ABSTRACT FROM AUTHOR]