What is Intervertebral Disc Degeneration, and What Causes It?

Michael A. Adams, PhD; Peter J. Roughley, PhD

Disclosures

Spine. 2006;31(18):2151-2161. 

In This Article

Abstract and Introduction

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.

The problem of intervertebral disc degeneration has been approached from many sides, from orthopedic surgery to molecular biology, and the scientific literature on the subject is particularly diverse. Perhaps as a result of this, there is no consensus on what disc degeneration actually is or how it should be distinguished from the physiologic processes of growth, aging, healing, and adaptive remodeling. We suggest that a precise definition of disc degeneration is long overdue. It would focus attention on which degenerative features are most likely to influence patients' prognosis and which are the best targets for therapeutic interventions. It would help epidemiologists identify risk factors for the disease[1] and suggest improved strategies for prevention. In addition, medicolegal experts would be better able to distinguish between a disease process and normal constitutional changes.

Recently, the relevant research literature has been thoroughly reviewed and summarized,[2] although no definitions were suggested. At a subsequent symposium in Davos, Switzerland, in 2005, there was widespread agreement that a definition would be beneficial but no agreement on how it should be phrased.[3] We suggest that the time is right to introduce a working definition of disc degeneration, one which will stimulate further discussion and lead to a formulation that will satisfy most researchers working in the field.

The purpose of the present article is to propose and justify a working definition of intervertebral disc degeneration, and show how it facilitates interpretation of the diverse research literature. Initial sections review the evidence concerning intervertebral disc functional anatomy, metabolism, aging, structural failure, and pain. This review is followed by an account of disc degeneration as suggested by animal models and epidemiology. Finally, 2 interpretation sections consider what disc degeneration is and what causes it.

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