Injury and Degeneration of the Disk

Definitions
Various authors have defined the terms herniation, protrusion, prolapse, and extrusion differently. The following definitions are used in this text.

• Herniation: a general term used when there is any change in the shape of the annulus that causes it to bulge beyond its normal perimeter.

• Protrusion: nuclear material is contained by the outer layers of the annulus and supporting ligamentous structures

• Prolapse: frank rupture of the nuclear material into the vertebral canal.

• Extrusion: extension of nuclear material beyond the confines of the posterior longitudinal ligament or above and below the disk space, as detected on magnetic resonance imagine (MRI), but still in contact with the disk.

• Free sequestration: the extruded nucleus has separated from the disk and moved away from the prolapsed area.

Fatigue Breakdown and Traumatic Rupture
A decrease in the continuity and integrity of structure of the annulus fibrosus may occur with (1) repeated stress over time causing fatigue breakdown or (2) traumatic rupture.Over time, the annulus breaks down as a result of repeated overloading of the spine in flexion with asymmetrical forward bending and torsional stresses.

• With torsional stresses, the annulus becomes distorted, most obviously at the posterolateral corner opposite the direction of rotation. The layers of the outer annulus fibrosus lose their cohesion and begin to separate from each other.

• Each layer then acts as a separate barrier to the nuclear material. Eventually, radial tears occur, and there is communication of the nuclear material between the layers.

• With repeated forward bending and lifting stresses, the layers of the annulus are strained; they become tightly packed together in the posterolateral corners, radial fissures develop, and the nuclear material migrates down the fissures. Outer layers of annular fibers can contain the nuclear material so long as they remain a continuous layer. After injury, there is a tendency for the nucleus to swell and distort the annulus. Distortion is more severe in the region where the annular fibers are stretched. If the outer layers rupture, nuclear material may herniate through the fissures.

• Healing is attempted, but there is poor circulation in the disk. There may be self-sealing of a defect with nuclear gel or proliferation of cells of the annulus. Any fibrous repair is weaker than normal and takes a long time because of the relative avascular status of the disk.
Traumatic rupture. Rupture of the annulus can occur as a one-time event, or it can be superimposed on a disk where there has been gradual breakdown of the annular rings. This is seen most commonly in traumatic hyperflexion injuries.

Axial Overload
Axial overload (compression) of the spine usually results in end-plate damage or vertebral body fracture before there is any damage to the annulus fibrosus.

Age
Individuals are most susceptible to symptomatic disk injuries between the ages of 30 and 45 years. During this time the nucleus is still capable of imbibing water, but the annulus weakens owing to fatigue loading over time and therefore is less able to withstand increased pressures when there are disproportionately high stresses. The nuclear material may protrude into the tears of fissures, which most commonly are posterolateral and, with increased pressures, may bulge against the outer annular fibers, causing annular distortion; or the nuclear material may extrude from the disk through complete fissures in the annulus.

Degenerative Changes
Any loss of integrity of the disk from infection, disease, herniation, or an end-plate defect becomes a stimulus for degenerative changes in the disk.

• Degeneration is characterized by progressive fibrous changes in the nucleus, loss of the organization of the rings of the annulus fibrosus, and loss of cartilaginous end-plates.

• As the nucleus becomes more fibrotic, it loses its capacity to imbibe fluid. Water content decreases, and there is an associated decrease in the size of the nucleus. Acute disk protrusions caused by a bulging nucleus pulposus against the annulus or extrusions of the nucleus through a torn annulus are rare in older people.

• It is possible to have protrusions of the annulus fibrosus without bulging from nuclear pressure. Myxomatous degeneration with annular protrusion has been demonstrated in disk lesions in older people.

Effect on Spinal Mechanics
Injury or degeneration of the disk affects spinal mechanics in general. During the early stages there is increased mobility of the segment with greater than normal flexion/extension and forward and backward translation of the vertebral body, leading to segmental instability. Force distribution through the entire segment is altered, causing abnormal forces in the facets and supporting structures.

Disk breakdown, showing breakdown and compression of fibrous layers of the annulus and displacement of disk material; radial fissures/tears with nuclear material bulging against the outer annulus; extrusion of nuclear material through the outer annulus but still in contact with the disk; sequestration of nuclear material beyond the annulus; and magnetic resonance imaging (MRI) scan of a 61-year-old patient with low back pain and symptoms radiating into the leg. The scan demonstrates moderate multilevel degenerative disk disease of T12/L1 through L4/5 with mild retrolisthesis of L2 on L3 and L3 on L4. At the L4/5 level, note a small diffuse disk bulge with large paracentral disk extrusion dissecting cranially.

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