Annulus Fissures Encourage Blood Vessel and Nerve Ingrowth into Degenerated Intervertebral Disks

Introduction Discogenic back pain is closely associated with fissures in the annulus fibrosus,1,2 and with ingrowth of nerves and blood vessels.3 We test the hypothesis that annulus fissures encourage such ingrowth. Materials and Methods Three complementary studies were performed. Firstly, 15 cadaveric disks that contained an annulus fissure were subjected to 1 kN compression, while a miniature pressure transducer was pulled through the disk to obtain distributions of matrix compressive stress perpendicular to the fissure axis. Secondly, safranin O staining was used to evaluate focal loss of proteoglycans from within annulus fissures in 25 surgically removed disk samples. A novel image analysis method was used to quantify proteoglycan concentration along a line profile that was perpendicular to the fissure axis. Thirdly, in 21 elderly cadaveric disks, proteoglycan and water concentration were measured biochemically in disrupted regions of annulus containing one or more fissures, and in adjacent intact regions. The second experiment had good spatial resolution but was unable to quantify proteoglycan loss precisely, whereas the third experiment had poor spatial resolution but allowed precise quantification of proteoglycan loss. Results Reductions in compressive stress within annulus fissures averaged 36>46%, and were greater than average at the fissure axis. Stress reductions were greater in degenerated disks, and were inversely related to nucleus pressure (R2 = 0.47, p