INTERVERTEBRAL DISCS
Between each vertebral body is a cushion called an intervertebral disc. These discs contribute a significant proportion to the height of the spinal column around 25-30%. In addition to adding height, each disc plays a vital role absorbing the stress and shock the body incurs during movement and prevents the vertebrae from grinding against one another.
Despite the pivotal role intervertebral discs play in spinal function, their access to the blood supply of the body is nearly nonexistent. Each disc absorbs the nutrients they need by means of osmosis. It is this reduced blood flow that makes discs vulnerable to damage and slow to recover and heal from injury. This damage can occur from both acute trauma and chronic degeneration.
Each disc is made up of two parts:
- Nucleus pulposus – soft middle portion of the disc
- Annulus fibrosis – the outer ring that surrounds and holds in the nucleus pulposis
On either side of the disc is a cartilaginous end plate that acts as connection between the disc and the vertebral body.[1],[2]
Nucleus Pulposus
The center portion of each intervertebral disc is a structure known as the nucleus pulposus – a gel-like elastic substance. Together with the annulus fibrosis, the nucleus pulposus helps reduce and cushion stress and weight from vertebra to vertebra.
The structural components of the nucleus pulposus are similar to the annulus fibrosus. It is a collection of molecules combined to form matrix consisting of:
- Water
- Proteoglycans
- Type II collagen
- Hyaluronic acid
- Glycosaminoglycans
The difference between the nucleus and the annulus is the concentration of each of these substances. The nucleus contains more water than the annulus – the more water, the more cushioning or shock absorbing ability a disc will have and the more height each disc will maintain. This water will cause the discs to appear bright white on a T2 MRI image. Damage to a disc can cause water to leave or leak out of the disc and appear the disc to appear dark on MRI (see below –Pathology for causes of DDD). This lack of water will decrease the height of the disc and increase stress, as well as the amount of weight transmitted to the vertebra of the spine – also known as Degenerative Disc Disease (DDD)
Annulus Fibrosus
The annulus is a sturdy tire-like structure that encases a gel-like center, the nucleus pulposus. The annulus enhances the spine’s rotational stability and helps to resist compressive stress. The intervertebral discs are the largest structures in the body without a vascular supply. As mentioned above, the discs depend on osmosis to get the nutrients they need to survive and maintain their health.
The annulus contains water, although less than the nucleus. It also contains layers of sturdy elastic collagen fibers. These fibers are oriented at different angles horizontally, similar to the construction of a radial tire. The collagen within these fibers is comprised of fibrous bundles made of protein bound together by proteoglycan gel.
The annulus fibrosis consists of an outer dense circumferential fibrous band and an inner fibrocartilaginous layer. The outer layer fibers, also known as Sharpey’s fibers, insert into the ring apophyses.
The cartilaginous end plate is composed of hyaline cartilage that tightly adheres to the vertebral end plate.[3],[4]
A tear in the annulus fibrous is known as an annular tear. These are typically a consequence of trauma and can be exquisitely painful. Larger tears, perforations or weaknesses within the annulus can break containment of the nucleus, causing it to bulge or herniate beyond the confines of the annulus leading to a disc bulge or disc herniation.
Endplates
The top (superior) and bottom (inferior) of each vertebral body is coated with an endplate. Endplates are complex structures that blend into the intervertebral disc and help hold the disc in place.
Discogram
A discogram is a diagnostic test used to determine whether certain discs of the spine are the source of a patient’s neck or back pain. By injecting a small amount of contrast (dye) into the disc, your doctor can see whether it is damaged or abnormal, and further assess whether or not it is the disc causing pain.
Microdiscectomy
This is a highly advanced, minimally invasive procedure proven to reduce pain caused by disc herniations (sciatica) that are unresponsive to conservative, non-surgical therapy. By removing a small amount of excess disc material, your doctor can effectively “shrink” bulging discs and help return them to their normal state.
Sensation
The sensory component of intervertebral discs is complex and actually differs depending on their location within the spinal column.
In the cervical spine, some research has demonstrated the presence of both nociceptive nerve fibers as well as mechanoreceptors within the anulus fibrosus.[5] Sensory innervation of the discs is transmitted through the sinuvertebral nerve (a branch of the vertebral nerves) and branches of the ventral primary rami and rami communicantes.[6]
Not all individuals have complete sensory innervation of the discs, meaning for many, these discs are numb – lacking feeling. For those who have these sensory fibers within the annulus, any tears, perforations, or herniations will become painful causing discogenic pain.
Discogenic pain is diagnosed by performing a test called a Discogram. This allows the physician to locate which disc is causing pain and image the nucleus beyond what an MRI is capable of showing.
There are a number of treatments available for treating discogenic pain:
- IDET
- Nucleotome
- Coblation
- Biacuplasty
As a person ages, discs can loose height secondary to loss of water within the discs, primarily from the nucleus puplosis, leading to the formation of small tears and degeneration within the annulus fibrosis.
Disc hydration, normally decreases with age but there are several factors that can cause water loss with in the nucleus, leading to DDD[7]:
- Age
- Cigarette smoking
- Obesity
- Car Accidents
- Excessive lifting
- Contact sports
- Trauma
- Falls
- Excessing sitting, bending and twisting
Even with age-related water loss, healthy disc will appear brighter on MRI due to the presence of an expected amount of water. Disc desiccation, or water loss, is accelerated in DDD, which results in a more significant decrease in disc signal on MRI, causing the discs to appear darker on MRI.
Degenerated discs occasionally demonstrate an accumulation of intradiscal gas (nitrogen) that can be detected on plain film, CT, and MRI.[8]
Degenerative discs are considered fragile as compared to healthy, water-filled discs and more likely to suffer injury. This loss of water and degeneration can lead to:
Disc bulging- Herniation
- Protrusion
- Extrusion
- Disc collapse
- Disc rupture
- Annular tears
As mentioned above, loss of water within a disc can cause that disc to loss height. This will cause the end plates, and consequently the vertebral bodies, to move closer together. Disc height is interpreted relative to other intervertebral levels in the same patient. Individual disc heights can be categorized in the following ways:
- normal
- mildly diminished
- moderately diminished
- severely diminished
These categories are determined based on percentage loss of disc height compared to a normal level. In a study comparing the disc heights of young versus middle-aged men, it was found that young, healthy men had narrower disc heights compared with middle-aged men.[8] Taken alone, disc height is not used as an indicator of disc degeneration. An important consequence of decreased disc height is the decreased size of the intervertebral foramina at the same level – leading to foraminal stenosis. This can increase the likelihood for nerve root compression, irritation and radiculopathy.
Get Answers to Your Questions at the Ainsworth Institute
The key to finding the proper treatment for any type of pain is getting a proper diagnosis. Not all types of pain respond to the same treatments and the window for improvement can be limited. The experts at Ainsworth Institute offer the most advanced pain management treatments available today, including advanced clinical trials that aren’t yet available to the general public. Call us today for an appointment so we can get you started on your road to recovery.
REFERENCES
[1] Peacock A: Observations on the postnatal structure of the intervertebral disk in man. J Anat. 1952; 86:162-179.
[2] Ibrahim MA, Jesmanowicz A, Hyde JS, et al.: Contrast enhancement in normal intervertebral disks. time and dose dependence AJNR Am J Neuroradiol. 1994; 15:419-423
[3] Urban JPG, McMullen JF: Fifth international congress on biorheology symposium. some biorheological aspects of joint diseases Biorheology.
[4] Rabischong P,: The intervertebral disk.Anat Clin. 1978; 1:55-64.
[5] Bogduk N. The innervation of the cervical intervertebral discs. Spine (Phila Pa 1976). 1988; 13(1):2-8
[6] Bogduk N, The nerve supply to the human lumbar intervertebral discs. J Anat. Jan 1981;132:39-56
[7] Frymoyer JW. Lumbar disk disease: epidemiology. Instr Course Lect 1992; 41:217–23.
[8] Grenier N, Grossman RI, Schiebler ML, et al.: Degenerative lumbar disk disease. pitfalls and usefulness of MR imaging in detection of vacuum phenomenon Radiology. 1987; 164:861-865.
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