THE PHYSIOLOGY OF THE DISC AND ITS CLINICAL RELEVANCE

Abstract

Listen

Translate article

©2003 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.85B5.14403 $2.00 J Bone Joint Surg [Br] 2003;85-B:633-6. Disorders of the physiology of the intervertebral disc produce clinical distress of a high order and of such a frequency as to support a vast industry of clinicians offering a wide variety of skills, as well as the manufacture of an array of implants and associated instruments intended to ameliorate the suffering. Pain is the chief complaint, either local to the disordered disc or in the lower limb with associated paraesthesiae caused by root compression and spinal stenosis. Leaving aside the question as to whether the disordered disc is to blame for all the distress currently ascribed to it, the accumulated clinical and radiological evidence confirms that much of the spinal disorder starts as abnormal changes in the disc.1 Early laboratory work compared fresh discs harvested from young adults who were organ donors, with ‘degenerate’ (spondylotic) whole discs removed at operation during anterior fusion for back pain. The single most striking finding was the reduced water content of the degenerate discs in conjunction with the reduced proteoglycan content of the nucleus.2 MRI has since confirmed that the earliest change is a reduction in water content, even before there is any loss of disc height or change detectable on plain radiography3,4 (Fig. 1). Water is the hydraulic fluid of the disc. As with other mechanical systems, loss of hydraulic fluid can lead to difficulties ranging from irritation to catastrophe. Water is contained in the matrix of the disc and contributes approximately 65% to 80% of its total weight by virtue of the high density of negatively charged sulphate and carboxyl groups (cations) on the glycosaminoglycan chains of the main proteoglycan in disc, aggrecan. These cations attract neutralising anions. The resultant high osmotic pressure of 1 to 3 atmospheres in the nucleus is the force which enables the disc continually to absorb water.5 Aggrecan will swell until it is restricted by the tension which builds up in