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Adjacent vertebrae articulate through zygapophyseal joints formed by the respective superior and inferior facets of the vertebral articular processes, as well as through interbody symphyses connecting the vertebral bodies. Zygapophyseal joints guide and limit the spine’s range of motion, whereas interbody symphyses facilitate mobility and provide the majority of the spine’s weight-bearing capacity. The inferior surface of a superior vertebral body articulates with the superior surface of the adjacent inferior vertebral body via intervertebral discs (IVDs). The adult spine contains 23 IVDs: 6 cervical, 12 thoracic, and 5 lumbar. No disc exists between the atlas (C1) and the axis (C2), and the most caudal disc resides at the lumbosacral junction (L5-S1). Collectively, these discs constitute approximately 25% to 33% of the spinal length. IVDs facilitate spinal flexibility while preserving structural integrity, dissipate axial compressive loads, and prevent direct osseous contact between adjacent vertebral bodies. Each disc consists of 3 principal components: the inner nucleus pulposus, the outer annulus fibrosus, and cartilaginous endplates (CEPs) anchoring the disc to adjacent vertebrae (see Image. Intervertebral Disc Anatomy). The nucleus pulposus is a gelatinous core eccentrically positioned slightly posterior to the geometric center of the IVD. This structure constitutes a critical component of spinal biomechanics by resisting compressive forces and facilitating segmental flexibility. Water comprises 66% to 86% of the nucleus pulposus, with the remainder composed primarily of type II collagen, accompanied by smaller amounts of type VI, IX, and XI collagen, and proteoglycans. The types of proteoglycans present include the larger aggrecan and versican, which bind to hyaluronic acid, as well as several small leucine-rich proteoglycans. Aggrecan is primarily responsible for water retention within the nucleus pulposus. Cellular density within this structure is low. Sparse cells produce extracellular matrix (ECM) products and maintain the structural integrity of the nucleus pulposus.

The nucleus pulposus is a gelatinous core eccentrically positioned slightly posterior to the geometric center of the IVD. This structure constitutes a critical component of spinal biomechanics by resisting compressive forces and facilitating segmental flexibility. Water comprises 66% to 86% of the nucleus pulposus, with the remainder composed primarily of type II collagen, accompanied by smaller amounts of type VI, IX, and XI collagen, and proteoglycans. The types of proteoglycans present include the larger aggrecan and versican, which bind to hyaluronic acid, as well as several small leucine-rich proteoglycans. Aggrecan is primarily responsible for water retention within the nucleus pulposus. Cellular density within this structure is low. Sparse cells produce extracellular matrix (ECM) products and maintain the structural integrity of the nucleus pulposus. The annulus fibrosus is a ring-shaped disc of fibrous connective tissue encircling the nucleus pulposus. This structure exhibits a highly organized architecture, consisting of 15 to 25 stacked sheets, or lamellae, composed predominantly of collagen, with interspersed proteoglycans, glycoproteins, elastic fibers, and connective tissue cells responsible for secreting these ECM products. Each lamella contains collagen fibers oriented uniformly within a plane that differs in alignment from the adjacent lamella by approximately 60°. This arrangement results in the parallel orientation of alternate lamellae. The resulting radial-ply formation confers exceptional tensile strength compared to an entirely longitudinal arrangement and has inspired analogous designs in engineered materials, such as car tires. Lamellae are interconnected through translamellar bridges. The density of translamellar bridges per unit area is optimized to balance mechanical strength and flexibility. A higher number of bridges enhances resistance to compressive forces but reduces flexibility, whereas fewer bridges increase flexibility at the expense of strength.

Lamellae are interconnected through translamellar bridges. The density of translamellar bridges per unit area is optimized to balance mechanical strength and flexibility. A higher number of bridges enhances resistance to compressive forces but reduces flexibility, whereas fewer bridges increase flexibility at the expense of strength. The annulus fibrosus contains an inner and an outer portion, differing primarily in collagen composition. Both regions are composed predominantly of collagen, but the outer annulus consists mostly of type I collagen, whereas the inner annulus contains predominantly type II collagen. Proteoglycan content is greater in the inner annulus than in the outer. The ratio of type I to type II collagen transitions gradually with increasing distance from the nucleus pulposus—type II collagen decreases while type I collagen increases. A further distinction between the 2 segments lies in the morphology of the connective tissue cells responsible for secreting ECM. Cells within the inner annulus are round and chondrocyte-like, whereas those of the outer annulus exhibit a more oblong, fibroblast-like appearance.[1]