and Introduction Abstract Study Design: Review and reinterpretation of existing literature. Objective: To suggest how intervertebral disc degeneration might be distinguished from the physiologic processes of growth, aging, healing, and adaptive remodeling. Summary of Background Data: The research literature concerning disc degeneration is particularly diverse, and there are no accepted definitions to guide biomedical research, or medicolegal practice. Definitions: The process of disc degeneration is an aberrant, cell-mediated response to progressive structural failure. A degenerate disc is one with structural failure combined with accelerated or advanced signs of aging. Early degenerative changes should refer to accelerated age-related changes in a structurally intact disc. Degenerative disc disease should be applied to a degenerate disc that is also painful. Justification: Structural defects such as endplate fracture, radial fissures, and herniation are easily detected, unambiguous markers of impaired disc function. They are not inevitable with age and are more closely related to pain than any other feature of aging discs. Structural failure is irreversible because adult discs have limited healing potential. It also progresses by physical and biologic mechanisms, and, therefore, is a suitable marker for a degenerative process. Biologic progression occurs because structural failure uncouples the local mechanical environment of disc cells from the overall loading of the disc, so that disc cell responses can be inappropriate or aberrant. Animal models confirm that cell-mediated changes always follow structural failure caused by trauma. This definition of disc degeneration simplifies the issue of causality: excessive mechanical loading disrupts a disc's structure and precipitates a cascade of cell-mediated responses, leading to further disruption. Underlying causes of disc degeneration include genetic inheritance, age, inadequate metabolite transport, and loading history, all of which can weaken discs to such an extent that structural failure occurs during the activities of daily living. The other closely related definitions help to distinguish between degenerate and injured discs, and between discs that are and are not painful.

What is intervertebral disc degeneration, and what causes it?

Study design A cross-sectional population study of magnetic resonance imaging (MRI) changes. OBJECTIVE.: To examine the pattern and prevalence of lumbar spine MRI changes within a southern Chinese population and their relationship with back pain. Summary of background data Previous studies on MRI changes and back pain have used populations of asymptomatic individuals or patients presenting with back pain and sciatica. Thus, the prevalence and pattern of intervertebral disc degeneration within the population is not known. Methods Lumbar spine MRIs were obtained in 1043 volunteers between 18 to 55 years of age. MRI changes including disc degeneration, herniation, anular tears (HIZ), and Schmorl's nodes were noted by 2 independent observers. Differences were settled by consensus. Disc degeneration was graded using Schneiderman's classification, and a total score (DDD score) was calculated by the summation of the Schneiderman's score for each lumbar level. A K-mean clustering program was used to group individuals into different patterns of degeneration. Results Forty percent of individuals under 30 years of age had lumbar intervertebral disc degeneration (LDD), the prevalence of LDD increasing progressively to over 90% by 50 to 55 years of age. There was a positive correlation between the DDD score and low back pain. L5-S1 and L4-L5 were the most commonly affected levels. Apart from the usual patterns of degeneration, some uncommon patterns of degeneration were identified, comprising of subjects with skip level lesions (intervening normal levels) and isolated upper or mid lumbar degeneration. Conclusion LDD is common, and its incidence increases with age. In a population setting, there is a significant association of LDD on MRI with back pain.

Prevalence and pattern of lumbar magnetic resonance imaging changes in a population study of one thousand forty-three individuals.

The intervertebral disc is a highly organized matrix laid down by relatively few cells in a specific manner. The central gelatinous nucleus pulposus is contained within the more collagenous anulus fibrosus laterally and the cartilage end plates inferiorly and superiorly. The anulus consists of concentric rings or lamellae, with fibers in the outer lamellae continuing into the longitudinal ligaments and vertebral bodies. This arrangement allows the discs to facilitate movement and flexibility within what would be an otherwise rigid spine. At birth, the human disc has some vascular supply within both the cartilage end plates and the anulus fibrosus, but these vessels soon recede, leaving the disc with little direct blood supply in the healthy adult. With increasing age, water is lost from the matrix, and the proteoglycan content also changes and diminishes. The disc—particularly the nucleus—becomes less gelatinous and more fibrous, and cracks and fissures eventually form. More blood vessels begin to grow into the disc from the outer areas of the anulus. There is an increase in cell proliferation and formation of cell clusters as well as an increase in cell death. The cartilage end plate undergoes thinning, altered cell density, formation of fissures, and sclerosis of the subchondral bone. These changes are similar to those seen in degenerative disc disease, causing discussion as to whether aging and degeneration are separate processes or the same process occurring over a different timescale. Additional disorders involving the intervertebral disc can demonstrate other changes in morphology. Discs from patients with spinal deformities such as scoliosis have ectopic calcification in the cartilage end plate and sometimes in the disc itself. Cells in these discs and cells from patients with spondylolisthesis have been found to have very long cell processes. Cells in herniated discs appear to have a higher degree of cellular senescence than cells in nonherniated discs and produce a greater abundance of matrix metalloproteinases. The role that abnormalities play in the etiopathogenesis of different disorders is not always clear. Disorders may be caused by a genetic predisposition or a tissue response to an insult or altered mechanical environment. Whatever the initial cause, a change in the morphology of the tissue is likely to alter the physiologic and mechanical functioning of the tissue.

Histology and pathology of the human intervertebral disc

Nomenclature and classification of lumbar disc pathology. Recommendations of the Combined task Forces of the North American Spine Society, American Society of Spine Radiology, and American Society of Neuroradiology

STUDY DESIGN A biomechanical and imaging study of human cadaveric spinal motion segments. OBJECTIVE To investigate the effect of both disc degeneration and facet joint osteoarthritis on lumbar segmental motion. SUMMARY OF BACKGROUND DATA Spinal degeneration includes the osteoarthritic changes of the facet joint as well as disc degeneration. Disc degeneration has been reported to be associated with spinal motion. The association of facet joint osteoarthritis with lumbar segmental motion characteristics and the combined influence of disc degeneration and facet osteoarthritis has not yet been investigated. METHODS A total of 110 lumbar motion segments (52 female, 58 male) from 44 human lumbar spines were studied (mean age = 69 years). Magnetic resonance images were used to assess the disc degeneration from Grade I (normal) to Grade V (advanced) and the osteoarthritic changes in the facet joints in terms of cartilage degeneration, subchondral sclerosis, and osteophytes. Disc height, endplate size, and facet joint orientation and width also were measured from the computed tomographic images. Rotational movements of the motion segment in response to the flexion, extension, lateral bending, and axial rotational moments were measured using a three-dimensional motion analysis system. RESULTS Female motion segments showed significantly greater motion (lateral bending: P < 0. 001, flexion: P < 0.01, extension: P < 0.05) and smaller endplate size (P < 0.001) than male ones. The segmental motion increased with increasing severity of disc degeneration up to Grade IV, but decreased in both genders when the disc degeneration advanced to Grade V. In male segments, the disc degeneration-related motion changes were significant in axial rotation (P < 0.001), lateral bending (P < 0.05), and flexion (P < 0.05), whereas female segments showed significant changes only in axial rotation (P < 0.001). With cartilage degeneration of the facet joints, the axial rotational motion increased, whereas the lateral bending and flexion motion decreased in female segments. In male segments, however, motion in all directions increased with Grade 3 cartilage degeneration and decreased with Grade 4 cartilage degeneration. Subchondral sclerosis significantly decreased the motion (female: axial rotation, P < 0. 05; extension, P < 0.05 vs.- male:flexion,P < 0.05). Severity of osteophytes had no significant association with the segmental motion. CONCLUSION Axial rotational motion was most affected by disc degeneration, and the effects of disc degeneration on the motion were similar between genders. Facet joint osteoarthritis also affected segmental motion, and the influence differed for male and female spines. Further studies are needed to clarify whether the degenerative process of facet joint osteoarthritis differs between genders and how facet joint osteoarthritis affects the stability of the spinal motion segment.

The effect of disc degeneration and facet joint osteoarthritis on the segmental flexibility of the lumbar spine.

A comparative study of cervical spines from 16 subjects who died of major trauma and 16 control subjects who died of natural causes, showed clefts in the cartilage plates of the intervertebral discs in 15 of 16 spines from the trauma victims. These were quite distinct from the uncovertebral clefts and central disc fissures that are a normal feature of aging in cervical discs. Posterior disc herniation through a damaged anulus fibrosus and hemarthrosis in facet joints were also observed. No directly comparable lesions were found in the control subjects, but two discs in this group showed "rim lesions," which may be old injuries. Disc lesions are common in injured cervical spines where translation is much greater than in the lumbar spine and these lesions are slow to heal. It is suggested that such injuries could cause the pain experienced by patients with neck sprain.

Acute injuries to cervical joints. An autopsy study of neck sprain

Measurements of disc thickness, shape and degeneration, using the criteria described by Rolander (1966), were recorded from 204 post-mortem lumbar spines. The "true average disc height" increased with age as the discs "sink" into the vertebrae. These results add information to previous studies which indicate that the loss of transverse trabeculae of lumbar vertebrae is primarily responsible for the change in shape of both vertebrae and discs in the elderly. While the incidence of disc degeneration does increase in old age, the majority of the discs examined did not show evidence of any such change.

https://www.tandfonline.com/doi/pdf/10.3109/17453678508993043

Age changes in lumbar intervertebral discs

This study reviews published observations of asymmetrical appearance of primary ossification centres in human fetal vertebral arches. It reports studies of vertebral asymmetry in 39 vertebral columns of infants and children including asymmetry in pedicle length and vertebral arch height, asymmetry in neurocentral fusion, and vertebral body flattening on its left anterior aspect. It relates these patterns of asymmetry to the commonly observed left thoracic scoliosis of infancy and right thoracic scoliosis of adolescents and adults. It discusses the implications of these observed asymmetries of normal vertebral growth in the aetiology of scoliosis, and the possible influences of handedness and aortic pressure in the production of these vertebral asymmetries in adolescence.

https://www.tandfonline.com/doi/pdf/10.3109/17453678308992896

Scoliosis and Growth: Patterns of Asymmetry in Normal Vertebral Growth

Thirty victims of blunt cervical trauma, who did not survive (acute death group), were studied at autopsy as were 11 patients who survived trauma for more than 1 month and ten controls without any known cervical trauma. All specimens were sagitally sectioned in 2-mm slices and carefully examined for injuries to the lateral parts of the atlanto-axial joints. In the acute death group, five cases showed a rupture of the joint capsule, three of these with a massive hemarthrosis. In 17 cases there was bruising around the second cervical spinal nerve. Bruising of the intra-articular synovial fold was present in 15 cases, the posterior synovial fold being most commonly affected. There were five intra-articular fractures of the lateral masses, in three cases combined with a dens fracture

The lateral atlanto-axial joints and their synovial folds: an in vitro study of soft tissue injuries and fractures.

Current research clearly indicates the importance of exercise and mobility in the treatment of low back pain and also that bed rest and inactivity should play a relatively small part in treatment. The use of intensive, physical exercise and "work hardening" routines have been shown to be necessary for treating chronic low back pain and returning individuals to work. Evidence derived from valid clinical studies of the use of manipulation in the treatment of low back pain shows a role for it in the acute treatment of "mechanical" low back disorders. These studies have demonstrated that manipulative procedures result in more rapid pain and functional relief compared with other conservative therapies. Over a longer time frame, this advantage disappears.

James R. Taylor

Papers

Acute injuries to cervical joints. An autopsy study of neck sprain

Age changes in lumbar intervertebral discs

Scoliosis and Growth: Patterns of Asymmetry in Normal Vertebral Growth

The lateral atlanto-axial joints and their synovial folds: an in vitro study of soft tissue injuries and fractures.

Exercise and spinal manipulation in the treatment of low back pain.