Neovascularization induced by anulus and its inhibition by cartilage endplate - Its role in disc absorption

Study Design. A study of neovascularization with respect to the anulus and the endplate. Objectives. To determine the ability of the anulus and endplate to induce neovascularization and to study the effect of the endplate on the neovascularizing activity of the anulus. Summary of Background Data. Previous studies showed that herniations contain varying amounts of nucleus pulposus, anulus fibrosus, and endplate. Results of recent studies have shown preferential neovascularization around around the herniated anulus rather than the endplate. Methods. Human L4-L5 discs were harvested from three specimens. In the left cornea of New Zealand White rabbits, an anulus (AF group) or an endplate (EP group) was implanted 4 mm from the superior corneoscleral limbus. In a third group (AF and EP group), an endplate was implanted between the anulus and the limbus. Twelve assays were done for each group. Sham surgeries were done on a fourth group as a control. Neovascularization was monitored by stereomicroscopy and scored from 0 (no neovascularization) to +5 (capillaries growing around the implant). Histologic studies were done at weekly intervals. Results. In the AF group, the anulus induced neovascularization in 11 of 12 corneas. By the third week, the anulus appeared thinner, and the newly formed vessels regressed. In the EP group, only two corneas showed neovascularization; 10 corneas showed no vascularization. In the AF and EP group, three corneas showed no neovascularization, seven had unsustained neovascularization, and two had vascularization. In the corneas containing the endplates, the implants remained unchanged. Control corneas did show vascularization. Histologic sections showed that neovascularization and cell infiltration were most abundant in the AF group, followed by the AF&tEP and the EP groups. Conclusion. The reactions to anulus fibrosus and to cartilage endplate differ. Anulus fibrosus may contain soluble substances that induce neovascularization, leading to its degradation and absorption. Cartilage endplate fails to induce neovascularization or diminishes the neovascular response, preventing its degradation. Disc herniations that contain a significant amount of endplate may not be degraded nor absorbed, leading to failure in spontaneous remission of symptoms.