On the Extent and Nature of Nucleus-Annulus Integration

Study Design. Mechanical and microstructural assessment of nucleus-annulus integration. Objective. To investigate the existence of structural integration between the nucleus and the inner annulus. Summary of Background Data. The nucleus is often viewed as a hydrostatically functioning entity that is largely separate from its surroundings. The boundary between nucleus and annulus is acknowledged as difficult to define. Methods. Ten-millimeter-thick sagittal slabs were cut from the central region of ovine lumbar discs. The annulus-nucleus transition region was isolated and the resulting samples subjected to transverse tensile loading up to failure. Similar samples were stretched to about 4 to 5 times their original separation and then subjected to microstructural examination to investigate structural integration across the inner annulus-nucleus region. Results. The annulus-nucleus boundary could support an average load of 5.7 N (range, 2-11.5 N). Tensile loading causes the fibrous structure of the nucleus to be drawn into an approximate alignment in the transverse stretch direction with an associated reverse inpulling of the inner annular layers. At high magnification, the horizontally aligned nucleus fibers can be seen to branch and blend with the inner annular structure. Conclusion. The nucleus contains a convoluted but highly structured network of fibers of varying length, which appear to integrate with the inner annulus and confer a significant degree of transverse interconnectivity that can be demonstrated mechanically. This new experimental evidence, together with that from a previous study demonstrating nucleus-endplate connectivity, makes it clear that the nucleus cannot be considered as a separate entity in the disc. We propose that this structural integration provides the nucleus with a form of tethered mobility that supports physiological functions distinct from the primary strength requirements of the motion segment.