Until relatively recently disc degradation as opposed to degeneration was not a used term. Degeneration simply progressed to become symptomatic and dysfunctional. The model was that with aging (about 18 years) the nucleus became less hydrophilic and more compressible. This leads to a randomized separation of the anular lamellae in the form of circumferential tears as the anulus takes more and more load. Eventually, for one reason or another, these circumferential tears would link up and coalesce to form a radial tear extending from the inside of the disc to the outside providing a path for the nucleus to migrate through. A number problems makes this model difficult to live with. The most telling was the experimental difficulty in forcing the nucleus to herniate through the radial tear. It was too viscous and too well bound to the anulus to migrate.

Bogduk and Twomey (Clinical anatomy of the lumbar spine 2nd edition. Churchill Livingstone, Edinburgh 1991) suggest that one method of initiating degradation is via the nucleus becoming exposed to the vascular system in the body of the vertebra after an end plate fracture. The exposure of an avascular tissue to blood results in an auto-immune reaction. This leads to decreased hydrophilia, increased compressibility and altered loading of the anulus. The altered and increased loading of the anulus causes it to buckle and the disc looses height. If the anulus remains intact but the changes to the nucleus are severe there is no migration of the nuclear material and it is contained. If the nucleus is not severely affected disc height is retained and consequences are minimal or non-existent.

With the more severe nuclear degradation, the disc height loss results in alteration of the relationships of all the structures around the segment. This may lead to degeneration of the zygopophyseal joints, osteophytosis and sometimes pain and dysfunction if sensitive structures such as the joint synovial linings, anulus nerve endings or dural sheaths are compromised. In addition, if there is sufficient disc height loss or osteophytosis, there may be compression or irritation of the spinal nerve especially if there is concurrent spinal stenosis.

Progression consists of migration of the nuclear material within the anulus as it forces its way between the lamellae, along radial tears or even erodes the anulus via its inflammatory reactions. If the anulus looses its external integrity either by tearing of its outmost fibers or by internal erosion, the nucleus can escape its confines and become externalized in the form of a prolapse, extrusion or sequestration.

However, the main thrust of this theory is that the majority of discogenic pain is not due to migration of nuclear material compressing sensitive structures as these lesions are an uncommon cause of back pain. Rather the inflammation and/or the mechanical deformation of the anulus stimulates its nociceptive nerve endings causing pain without obvious objective evidence of bulging or prolapse, our normal clinical picture. Internal disruption is vulnerable to CT scanning after discography which itself gives an impression of the degree of symptomatology produced by the disc itself.

It is of course perfectly possibly that the earlier theory of radial tearing and nuclear migration may still be present as well. If the radial tear begins from the outside due to unmanageable mechanical stress on the anulus and this tear is invaded by granulation tissue, the erosion may occur from outside to inside, opposite to the theory above. The nucleus could then become exposed to vascular system, reactively change and become less viscous. It could then migrate through the radial tear to become external to the anulus.

A number of conflicting observations and theories question the origin of the externalized disc material. One theory holds that it is almost always anular, another that it is nuclear and yet another that it is immature collagen, a form of benign tumor akin to a keloid. For the practicing therapist, it really does not matter too much whether the material is anular or nuclear either is going to be extremely difficult to reduce. The idea of extension exercises pushing this back or traction sucking it back are not, at least to my mind, viable. Probably and usually, when we successfully treat a true uncontained nuclear migration we rest the patient and allow inflammation to subside rather than directly affect the disc. There is conflicting evidence that chiropractic, manual therapy and other forms of physical therapy have an effect on disc prolapse (Physical therapy outcomes for patients receiving worker's compensation following treatment for herniated lumbar disc and mechanical low back pain syndrome Di Fabio RP; Mackey G; Holte JB. J Orthop Sports Phys Ther, 23(3):180-7 1996 Mar and Magnetic resonance imaging and clinical follow-up: study of 27 patients receiving chiropractic care for cervical and lumbar disc herniations. BenEliyahu DJ. J Manipulative Physiol Ther, 19(9):597-606 1996).

It is important that we become capable of recognizing the uncontained disc lesion. Not so much that we avoid attempting to treat it, but rather that when treating it, we do not waste the patient's time and financial resources and the insurer's patience, nor do we frustrate ourselves by continuing non-productive treatment in a hopeless cause. Far better that we recognize a lesion that is unlikely to respond to our care, attempt treatment but understand that we are limited in what we can do for this patient and refer them on to a more productive treatment.

Addendum

The catalyst that made me write the columns of the subject of disc degeneration and degradation was a post I received from two students enrolled in the North American Institute of Orthopedic Manual Therapy (NAIOMT) 's certification program in manual therapy. They contacted me concerning disc degeneration and this lead to a discussion on degradation. It became apparent that they were not familiar with the distinction between degeneration and degradation and if two motivated and well read PTs such as these were unaware of the difference, then likely many others would also be in a similar situation. After reading the first part of the column on degeneration they wrote to me:

My reply (suitably edited for general consumption) was:

I would be interested in your opinons on this. Please email me if you have one and I will post it anonymously if you wish.