Neural elements in the normal and experimentally injured porcine intervertebral disk.

There is increasing evidence that-back pain may originate from degenerated or damaged disks, even in the absence of disk herniation. For a study of the pattern of innervation in injured disks, the anterior part of the annulus fibrosus of a lumbar disk in 11 domestic pigs was incised with a scalpel through a retroperitoneal approach. The animals were killed 2 weeks, 1, 2, 3, and 5 months postoperatively, and the whole anterior annulus of each injured disk and corresponding tissue from intact animals were excised. Cryostat sections 20 microns thick were cut from the surface downward, fixed, and stained with different antisera. Antisera to neurofilament triplet protein (R39), protein gene product (PGP) 9.5 and synaptophysin were used as general neural markers. Antiserum to substance P (SP) and calcitonin gene-related peptide (CGRP) were used to localize nerves mainly of the sensory type, and C flanking peptide of neuropeptide Y (CPON) to visualize nerve fibers of the sympathetic type. It was observed that in the intact porcine disk, the outer and middle parts of the anterior annulus were innervated to a depth of 7 mm from the annular surface, but the innermost annular layers showed no immunoreactivity to any of the neural antibodies. Disk injury did not cause any major changes in the nerve topography of the wound area, even though there were granulation tissue and neovascularization in this area.