Understanding spine anatomy helps patients make sense of their diagnosis and treatment options. This glossary defines the key terms clinicians use when discussing disc conditions — from basic spinal structures to biologic treatment concepts. Definitions apply broadly; individual anatomy and symptoms vary, and our clinical team evaluates each case separately.

Annulus Fibrosus

The annulus fibrosus is the tough, fibrous outer ring of an intervertebral disc. It is composed of concentric layers of collagen fibers arranged at alternating angles, which allows it to resist twisting, bending, and compression forces. When the annulus develops small cracks or fissures — known as annular tears — the disc’s structural integrity may be compromised, which in some patients contributes to discogenic pain.

Nucleus Pulposus

The nucleus pulposus is the gel-like inner core of the intervertebral disc. It is primarily made up of water and proteoglycans, giving it the ability to absorb compressive loads and distribute pressure evenly across the disc. As the disc ages or sustains injury, the nucleus pulposus may lose hydration and resilience, a process associated with degenerative disc disease.

Intervertebral Disc

An intervertebral disc sits between adjacent vertebrae and functions as a shock absorber and spacer. Each disc consists of the nucleus pulposus at its center and the annulus fibrosus surrounding it. Healthy discs allow for flexible movement while protecting the spine. Disc health depends on hydration, mechanical load, and the integrity of the annular wall.

Vertebra

A vertebra is an individual bony segment of the spine. The spine contains 33 vertebrae organized into cervical, thoracic, lumbar, sacral, and coccygeal regions. The lumbar vertebrae — labeled L1 through L5 — bear the greatest load and are the most common site of disc-related conditions. Each vertebra has a central canal that houses and protects the spinal cord and nerve roots.

Foramen

The foramen (plural: foramina) are openings in and around the vertebrae through which spinal nerve roots exit the spinal canal. The most clinically relevant is the neural foramen, located on either side of each vertebral level. When a disc herniates or degenerates, the resulting change in disc height or tissue displacement may narrow the foramen and compress a nerve root — a condition called foraminal stenosis.

Facet Joint

Facet joints are paired synovial joints located at the back of each vertebral segment. They guide spinal motion and limit excessive rotation. Facet joints are often affected alongside disc degeneration because changes in disc height alter the mechanical load placed on these joints. Facet-related pain is distinct from discogenic pain, though both may coexist in some patients.

Annular Tear

An annular tear is a crack, fissure, or disruption in the fibrous outer ring of the intervertebral disc. Annular tears range from minor inner-layer fissures to full-thickness radial tears that may allow nucleus pulposus material to migrate outward. In some patients, annular tears are a primary driver of chronic discogenic low back pain, particularly when inflammatory mediators reach sensitized nerve fibers within the outer annulus. Learn more about how annular tears contribute to pain in our overview of annular tears and chronic lower back pain.

Expert Take

Our clinical team evaluates annular tears with a focus on whether the structural disruption is the confirmed pain source — not just an incidental imaging finding. Not all annular tears on MRI produce symptoms, and not all symptomatic tears appear prominently on standard imaging. Provocation discography and clinical correlation are key steps before treatment decisions are made. Candidates are assessed individually to determine whether biologic approaches such as intra-annular fibrin injection may be appropriate.

Degenerative Disc Disease (DDD)

Degenerative disc disease is not a disease in the traditional sense but a term describing the age- and wear-related breakdown of intervertebral discs. As discs degenerate, they lose hydration, height, and elasticity. This process may contribute to discogenic pain, facet joint stress, and nerve root irritation. DDD is extremely common and does not produce symptoms in many people, while in others it may be a significant source of chronic pain. Outcomes and symptom progression vary by individual.

Herniated Disc

A herniated disc occurs when the inner nucleus pulposus protrudes through a tear in the annulus fibrosus and extends beyond the disc’s normal boundary. The displaced material may press on adjacent nerve roots, producing pain, numbness, or weakness that radiates into the arms or legs depending on the spinal level affected. Herniation severity, location, and individual nerve sensitivity all influence how symptoms present. For a detailed comparison, see our guide on bulging disc vs. herniated disc differences.

Bulging Disc

A bulging disc occurs when the disc wall extends outward beyond its normal perimeter without a complete annular tear. Unlike a herniation, the nucleus pulposus remains contained. Bulging discs are common findings on imaging studies, and many people have them without any symptoms. When a bulge does produce symptoms — particularly nerve compression — conservative and interventional treatments may provide relief in appropriate candidates. Our clinical team evaluates each case to determine the most suitable approach.

Sciatica

Sciatica describes pain that travels along the path of the sciatic nerve, which runs from the lower back through the buttocks and down each leg. It is a symptom, not a diagnosis — most commonly associated with compression or irritation of a lumbar nerve root from a herniated disc, bone spur, or spinal stenosis. Severity ranges from a mild ache to sharp, debilitating pain, and the underlying cause must be identified to guide treatment. Learn about common misconceptions in our article on 10 myths about sciatica and non-surgical relief.

Radiculopathy

Radiculopathy refers to symptoms — pain, numbness, tingling, or weakness — that result from compression, inflammation, or injury to a spinal nerve root. Lumbar radiculopathy most frequently affects the L4, L5, or S1 nerve roots and produces symptoms into the legs and feet. Cervical radiculopathy produces similar symptoms into the arms and hands. Radiculopathy follows a dermatomal pattern corresponding to the affected nerve root level and differs from non-specific referred pain.

Discogenic Pain

Discogenic pain is pain that originates from within the intervertebral disc itself — most often from annular tears, internal disc disruption, or disc degeneration. It is described as deep, axial low back pain that worsens with prolonged sitting, bending, or loading. Discogenic pain can be difficult to isolate from other spinal pain sources and in some patients requires provocation discography for confirmation. Biologic disc repair approaches may be appropriate for selected candidates with confirmed discogenic pain sources. See our overview of fibrin disc treatment for discogenic pain.

Fibrin

Fibrin is a naturally occurring protein in the body that plays a central role in wound healing and tissue repair. It forms a mesh-like scaffold that supports cellular repair processes at sites of injury. In the context of biologic spine care, fibrin may be used in intra-annular fibrin injection — a procedure designed to support repair of annular tears and promote stabilization of the disc wall in select candidates.

Intradiscal Injection

An intradiscal injection is a procedure in which a substance is delivered directly into the intervertebral disc using fluoroscopic or CT guidance. The specific substance — whether diagnostic contrast, therapeutic biologics, or other agents — depends on the clinical objective. Intra-annular fibrin injection is one form of intradiscal treatment in which fibrin is delivered to target annular tears. Candidacy depends on disc anatomy, pain source confirmation, and overall clinical picture.

Regenerative Medicine

Regenerative medicine encompasses treatments that aim to support the body’s own repair mechanisms rather than replacing or removing damaged tissue. In spine care, this includes biologic approaches such as fibrin disc treatment and other annular repair strategies. Patient selection, disc integrity, and confirmed pain source are key factors our clinical team considers when evaluating whether a regenerative approach may benefit a given patient. For a broader look at conditions that may respond to these approaches, see our page on biologic disc repair and the conditions it addresses.

This glossary is intended as an educational reference for patients navigating a spine diagnosis. For a deeper dive into how these terms relate to specific conditions and treatment pathways, visit our complete disc conditions glossary for patients.