Effect of aging and degeneration on the human intervertebral disc during the diurnal cycle: A finite element study

Alterations in the major biochemical constituents of intervertebral discs coincide with aging and degeneration, and can alter the disc's ability to support load. The most significant biochemical change that occurs in degeneration is the loss of proteoglycans in the nucleus pulposus. During a diurnal cycle, the disc experiences approximately 16 hours of functional loading, followed by 8 hours of recovery. An axisymmetric, poroelastic model was created using ABAQUS finite element software. Standard poroelastic theory is utilized, but a user-defined material was written to include the effects of osmotic swelling, which is directly related to proteoglycan content. Due to the high stresses in the nucleus, the annulus fibrosus must remodel itself to account for the change in properties of the nucleus. The stress experienced by the nucleus increases greatly in Grade 2 from Grade 1, but then decreases in Grade 3, and even Grade 4 experiences lower stresses than in Grade 2. The osmotic pressure in the central nucleus decreases approximately 75% with degeneration. This explains the increasing inability of Grades 3 through 5 to recover the fluid lost during loading, since the osmotic pressure gradient is the primary mechanism with which fluid flows back into the disc.