The umbrella term for age-related wear-and-tear changes throughout the spine — the equivalent of grey hair on skin. It encompasses dehydrated discs, bone spur formation, and stiffening of spinal joints, accumulating gradually over decades. Most common in the cervical and lumbar regions.

A healthy intervertebral disc is roughly 80% water. Desiccation means the disc has lost water content and dried out — typically the first sign of disc aging, making the disc thinner and stiffer.

The disc pushes outward beyond its normal edge uniformly in all directions, like a slightly under-inflated tyre pressing against the road. The tough outer wall (annulus fibrosus) remains intact. A bulge involves more than 50% of the disc circumference — distinguishing it from a localised herniation.

A focused, localised outpouching of disc material where the base of the outpouching is wider than the protruding tip. The outer wall is mostly intact. This is the mildest form of disc herniation.

The inner disc material (nucleus pulposus) has pushed all the way through the outer fibrous ring. The displaced material is wider at its tip than at its base — like toothpaste squeezed out of a tube. The herniated portion remains connected to the disc.

A piece of disc material has broken off completely and is now free-floating in the spinal canal — entirely detached from the parent disc. The fragment can migrate above or below its original level and may press heavily on nerve roots.

The annulus fibrosus is the tough outer casing of the disc. An annular tear is a crack in this ring — ranging from superficial surface tears to deeper radial tears that extend from the inner to outer disc.

MRI signal alterations in the vertebral bone marrow immediately adjacent to a degenerated disc. They indicate disc disease has spread to the neighbouring bone.

Instead of herniating sideways into the canal, a small amount of disc material pushes vertically — upward or downward — into the cancellous bone of the adjacent vertebral body, creating a small crater-like depression in the endplate.

A bony projection that forms at the edge of a vertebra or facet joint as the body's attempt to redistribute stress across a degenerating disc. The body lays down extra bone to stabilise the segment — but these spurs can then project into nerve passageways.

Each vertebra has a cartilaginous endplate at its top and bottom — the interface between the disc and the bone. Endplate changes include irregularity, erosion, sclerosis (hardening), or defects in this surface, reflecting advanced disc degeneration that has affected adjacent bone.

The facet (zygapophyseal) joints are paired synovial joints at the back of each spinal level. Like any synovial joint (knee, hip), they develop osteoarthritis — with cartilage loss, joint space narrowing, osteophyte formation, and synovial inflammation.

The ligamentum flavum is a thick, elastic yellow ligament lining the back of the spinal canal. With age, the elastic fibres degenerate and are replaced by fibrosis, causing the ligament to stiffen and thicken. It buckles inward with extension, reducing canal space.

Narrowing of the bony canal protecting the spinal cord and nerve bundle. The narrowing can result from a combination of disc bulging, osteophytes, thickened ligamentum flavum, and facet joint enlargement — reducing the available space for neural structures.

A specific description of narrowing confined to the central portion of the spinal canal — as distinguished from lateral recess or foraminal narrowing. In the lumbar spine, severe central stenosis compresses the cauda equina (the bundle of lumbar and sacral nerve roots). In the cervical spine, it compresses the cord itself.

At every spinal level, paired openings (intervertebral foramina) on each side allow nerve roots to exit the canal and travel to the limbs. Foraminal stenosis means one or more of these exit holes is narrowed — most often by a disc herniation from in front or a facet osteophyte from behind.

The lateral recess is the narrow triangular channel inside the canal where the nerve root descends before it reaches the foramen exit. Stenosis here is caused by disc herniation combined with superior articular facet hypertrophy encroaching from behind.

The spinal cord is physically pressed by an encroaching structure — most commonly a disc herniation, osteophyte, thickened ligament, or tumour. Unlike nerve root compression (which affects one limb), cord compression affects all structures below the level.

Intrinsic cord damage — actual softening or destruction of spinal cord tissue resulting from prolonged or severe compression, ischaemia, or injury. Unlike cord compression (which is external), myelomalacia represents changes within the cord itself.

A nerve root is pinched as it exits the spinal canal — by a disc herniation, osteophyte, or stenosis. Because each nerve root supplies a defined territory (a dermatome for sensation, a myotome for motor function), compression produces predictable patterns of pain, numbness, or weakness.

The normal cervical and lumbar spine has a gentle inward (concave) curvature called lordosis. Loss of lordosis means this curve is reduced or entirely absent — the spine appears "straight" on a lateral (side-view) X-ray. Most commonly caused by paravertebral muscle spasm in response to acute pain.

An exaggerated forward-rounding curvature, most commonly in the thoracic region. A small degree of thoracic kyphosis is anatomically normal. Pathological kyphosis exceeds normal angles and may cause a visible "hump." Major causes include osteoporotic vertebral compression fractures, Scheuermann's disease, and severe degenerative disc disease.

A three-dimensional spinal deformity characterised by sideways curvature (either "C" or "S" shaped) with rotation of the vertebrae. In adolescents it is most commonly idiopathic (unknown cause). Degenerative scoliosis in adults develops from asymmetric disc and facet degeneration.

On X-ray, the intervertebral disc appears as a visible gap between two vertebral bodies. As a disc desiccates and collapses, this gap narrows — one of the oldest radiological signs of spondylosis, detectable on plain X-ray long before MRI changes.

In severely degenerated discs, internal fissures allow body gases (predominantly nitrogen) to accumulate in clefts within the disc. The result is a dark linear or crescent-shaped gas collection visible on imaging — the "vacuum" sign. It indicates end-stage disc degeneration where the disc has structurally failed.

DISH is a systemic condition characterised by calcification and ossification of spinal ligaments and entheses. On the spine, it produces flowing new bone formation along the anterolateral vertebral bodies across four or more consecutive levels — resembling dripping candle wax.

Two or more vertebrae have fused into a single immobile unit. This can occur spontaneously from disease (ankylosing spondylitis, DISH, severe degeneration) or be created intentionally through spinal fusion surgery. Once fused, the segment loses all motion — transferring mechanical stress to adjacent levels.

A tubular, fluid-filled cyst or cavity within the central grey matter of the spinal cord. Syringomyelia is the condition; a syrinx is the cyst itself. The fluid is usually cerebrospinal fluid (CSF) that has tracked into the cord under pressure, and it can slowly expand over years.

On T2-weighted MRI, normal spinal cord tissue appears grey. A "bright spot" (hyperintensity) within the cord indicates an abnormality — most commonly oedema or gliosis (scarring) from compression, ischaemia, demyelination, infection, or tumour.

One vertebra has shifted out of its normal position relative to the vertebra directly below it. Instead of sitting squarely stacked, the upper vertebra has slid forward, backward, or sideways.

The upper vertebra has slipped forward (anteriorly) relative to the one below. This is the most common direction of slippage in the lumbar spine. "Anterolisthesis" and "spondylolisthesis" are often used interchangeably in reports when the direction is forward.

The upper vertebra has shifted backward (posteriorly) relative to the one below. Less common than anterolisthesis, retrolisthesis tends to be associated with disc height loss. It is often seen at the L5–S1 level and in the cervical spine.

A stress fracture or defect in the pars interarticularis — the narrow bridge of bone at the back of a vertebra. The most common cause of low back pain in young athletes, occurring at L5 in approximately 85–95% of cases. Repetitive hyperextension loading is the mechanism.

The narrow waist of bone in the vertebral arch connecting the upper (superior) to the lower (inferior) facet joint on each side. It is the bridge between the front and back structures of the spine at each level and the most mechanically stressed part of the lumbar vertebral arch during extension loading.

The universally used system for classifying how far a vertebra has slipped. The degree of slip is measured as a percentage of the vertebral body width that has displaced forward.

The most common form in adults, caused by progressive facet joint degeneration and disc collapse. As the joints wear out and lose their ability to resist forward shear, the vertebra gradually slips forward — typically by a Grade I or II amount. The pars interarticularis remains intact, distinguishing it from isthmic spondylolisthesis.

When bilateral spondylolysis (pars defects on both sides) is present, the vertebral body is no longer anchored to its posterior arch. The body (along with the disc and everything above it) can then slide forward off the vertebra below. Most commonly L5 slipping on S1.

A spine segment is unstable when it cannot maintain its normal position under ordinary forces without excessive motion that risks neurological damage or deformity. Instability is a clinical and radiological diagnosis — it requires both imaging evidence of abnormal motion and clinical correlation.

The patient bends fully forward, then fully backward, with a lateral X-ray taken in each position. The surgeon measures how much the vertebrae shift relative to each other between the two positions — quantifying motion at each spinal segment.

When spinal levels are surgically fused, all motion is eliminated at those segments. The levels immediately above and below must then compensate — absorbing increased stress with every movement. Over years, this accelerated loading causes premature degeneration, disc herniation, stenosis, or new spondylolisthesis at adjacent unfused levels.

The transition zone between the base of the skull (occiput) and the first two cervical vertebrae — the atlas (C1) and axis (C2). It is anatomically unique: no intervertebral discs, responsible for approximately 50% of the neck's rotation and a significant portion of its flexion–extension.

The odontoid process (the upward peg of C2) has migrated upward — through the foramen magnum — into the posterior cranial fossa (the space containing the brainstem and cerebellum). This can compress the cervicomedullary junction.

The angle between the anterior and posterior cranial fossae (the basal angle of the skull) is excessively flat (obtuse). Rather than the normal gently curved skull base, platybasia produces an almost flat internal base of skull. It is a geometric measurement, frequently found alongside basilar invagination.

Abnormal, excessive motion between C1 and C2, placing the upper cervical cord at risk during neck movement. The primary stabilising structure is the transverse atlantal ligament (TAL), which holds the odontoid peg firmly against the anterior arch of C1. When the TAL fails, the dens can move backward and compress the cord.

The distance between the posterior surface of the anterior arch of C1 (the ring of the atlas) and the anterior surface of the odontoid process (dens). It is the single most important measurement for diagnosing atlanto-axial instability, routinely reported on cervical X-rays, CT, and MRI.

A tooth-shaped bony projection arising from the body of C2 (the axis), pointing upward through the ring of C1 (the atlas). It serves as the central pivot around which the atlas (and thus the skull) rotates — providing approximately 50% of total cervical rotation.

A structural abnormality in which part of the cerebellum — specifically the cerebellar tonsils — herniates downward through the foramen magnum into the upper spinal canal. This disrupts normal CSF flow, potentially causing symptoms and syringomyelia (see Term 28).

A congenital failure of segmentation in which the atlas (C1) is partially or completely fused to the base of the skull (occiput). Instead of being a separate mobile ring, C1 is incorporated into the skull base, eliminating the atlanto-occipital joint and normal occipito-cervical flexion–extension motion.

A well-corticated (smooth-edged), round or oval ossicle sitting in place of the normal odontoid process — separated from the body of C2 by a gap. Rather than one solid peg, there is a separate small bone that typically moves with C1, and a hypoplastic or absent odontoid stump at the top of C2.

The angle formed between the clivus (the bony slope at the base of the skull) and the posterior surface of the C2 vertebral body, measured on a midsagittal MRI. It quantifies how much the brainstem and cervicomedullary junction is being bent or kinked by CVJ pathology — such as basilar invagination or atlanto-axial instability.