The Intervertebral Disc:
Properties and Exercise Implications
The intervertebral discs act to transmit load, provide the spine with flexibility and stabilize the spine by anchoring adjacent vertebral bodies together.
The annulus fibrosus consists of 10-12 concentric layers of collagen fibres. These fibres run in opposite directions from an oblique angle of 300 in the outer annulus and becoming progressively more horizontal with each layer towards the centre. The outer annulus is mostly type I collagen to resist tension, while the inner annulus has a higher density of type II collagen to resist compressive loads.
The nucleus pulposus consists of collagen and proteoglycans. Proteoglycans have a prodigious ability to imbibe water, thus the disc will contain 70-90% water, allowing it greater ability to resist compression.
The vertebral end plates are an outer layer of cartilage - hyaline towards the vertebral body and fibrocartilage towards the disc. Due to extensive collagen networks, the end plates are strongly bound to the disc.
Only the peripheral aspects of the annulus have direct blood supply, therefore the disc depends on diffusion for nutrition and for waste removal. Proteoglycan concentrations determine the osmotic gradient of diffusion, while the lower pH of the proteoglycans create an ionic gradient. The diffusion march of nutrients such as oxygen and glucose are traced from the vertebral body, through the end plate and into the nucleus pulposus.
Age, calcification and degeneration disrupt normal diffusion and lead to the loss of proteoglycans. In the healthy disc, only the outer third of the annulus is innervated with pain fibres. Innervation of the degenerated disc is more extensive, extending up to 2/3 into the annulus.
In a pathologic disc with herniation, the extruded hydrophylic material may double in size within hours, creating a lesion that will compete for space with the nerve root. The herniated disc will also increase production of substances that suppress proteoglycan synthesis.
Stimulus for Disc Repair
The optimal stimuli for disc regeneration include modified tension with both compression and decompression.
Axial rotation will provide tension to the annulus fibres. A stronger annulus will help prevent tears and displacement of nucleus material. Annulus injury will allow the nucleus to move towards the path of least resistance, ie towards the damaged annulus fibres. Distraction or extension moments might move the nucleus away from the injured annulus, however these fibres will still require rotation at an early stage in order to promote the optimal stimulus for repair.
Chondrocytes within the nucleus pulposus require a change in intradiscal pressure to stimulate cellular metabolism. Compression and decompression will also assist in diffusing fluid in and out of the disc for improved nutrition.
Therapeutic Implications
If the nerve root is compressed then motions are only performed into the direction that will decompress the affected nerve.
Dosage of movements for discal rehabilitation must be numerous to stimulate metabolism of the relatively avascular structures. Thus frequent gentle exercises are prescribed as often as every hour to affect both chemical and mechanical variables.
Gentle, passive decompression may be beneficial in the acute phase. As weightbearing becomes better tolerated, walking programs can facilitate the compression diffusion of nutrients.
The disc must be able to withstand forces in all directions. Though these movements may be less tolerated in the early stages of disc injury or disease, they must be trained at some point in the rehabilitation plan to avoid reinjury during functional activities. The key again is appropriate tension.
References:
1. Grimsby O., Rivard J. Science, Theory and Clinical Application in Orthopaedic Manual Physical Therapy. Volume 1: Applied Science and Theory. Academy of Graduate Physical Therapy. Taylorsville 2008.
2. Karamouzian S et al. Frequency of lumbar intervertebral disc calcification and angiogenesis, and their correlation with clinical, surgical and magnetic resonance imaging findings. Spine, 2010 Apr 15;35(8):881-6.