Disc Anatomy

Basic Disc / Lumbar Anatomy:

The lumbar spine of the human is made up of five lumbar vertebrae that are separated by five intervertebral discs (blue structures in fig. #1). The discs may be thought of as spinal shock absorbers, for they absorb the load (axial load) of the body. They also allow for movements such as flexion, extension, and rotation at the waist as they act as a pivot point for the trunk to twist upon.

There are 23 discs in the human spine: 6 in the neck (cervical region), 12 in the middle back (thoracic region), and 5 in the lower back (lumbar region). Although this page shall focus on the lumbar spine, the discs of the thoracic and cervical spine have generally the same design and anatomy.
The disc is made up of three basic structures: the nucleus pulposus, the annulus fibrosus and the vertebral end-plates, and with the exception of the nucleus pulposus, the foregoing structures are all innervated by pain-carrying nerve fiber. With regard to biochemical composition, all three discal structures are made of varying percentage compositions of proteoglycan (protein and carbohydrate), collagen (cartilage) and water, which is held by the proteoglycans molecules--the higher percent composition of proteoglycans, the higher the water content of the region.

Although their percent composition differs, the latter three structures are made of three basic components: proteoglycan (protein and carbohydrate), collagen (cartilage), and water. We will learn all about these structures below.

Figure #1 depicts a Front view (coronal view) of the lumbar spine. Here we can see how the discs (blue) lie in between every vertebrae. Spinal nerves (yellow) have emerged from between every two vertebrae and travel down the lower limbs to innervate (give life to) the skin and muscle. Note how the sciatica nerve is formed within the pelvis by branches from the last three lumbar spinal nerves. It is this giant nerve--i.e., the sciatic nerve--that causes so much trouble in many of us chronic pain sufferers. (more detail here)

Another important thing to understand about a normal disc is that it is a closed hydraulic system that is excellent at withstanding pressure. You can almost think of it as the tire of a car, except that instead of a air in the center, the disc has nucleus pulposus. And just like the tire of a car, punctures in the disc (i.e., annular tears) have dire consequences.

Figure #2 Shows a cut-away posterior view (PA view) of the lumbar spine. Now we can better visualize how the sciatic nerve is formed and see just how close the spinal nerve roots come to the back of the intervertebral discs. Note that the L5 root exits through the L5 IVF and the S1 root traverses downward and exits below the L5 disc through the sacral foramen. Any herniation of the posterior disc may compress and/or chemically irritate EITHER the exiting L5 root or traversing S1 spinal nerve root (this is more typical) and result in severe lower back pain and/or lower limb pain (i.e., sciatica). For more information on sciatica please visit my 'Sciatica Page'.

Axial (overhead) Disc Anatomy:

The human disc has two basic parts: an inner Jell-O like center called the Nucleus Pulposus and the Annulus Fibrosis. Let learn more about each:

The Nucleus Pulposus (#1 of fig. #9-pink) is the water-rich (proteoglycan-rich), gelatinous center of the disc, which is under very high pressure when the human is upright--especially in the seated or flexed position. It has two main functions: to bear or carry the downward weight (i.e., axial load) of the human body and to act as a 'pivot point' from which all movement of the lower trunk occurs. It's third function is to act as a ligament and bind the vertebrae together.

The Annulus Fibrosus (#2 of fig. #9-green) is much more fibrous (tougher) than the nucleus. It also has a much higher collagen content and lower water content (lower in proteoglycan) when compared to the nucleus. Its main job is to corral or hold-in-place the highly pressurized nucleus, which is constantly trying to escape its central prison. The annulus is made of 15 to 25 concentric sheets of collagen (a tough cartilage-like substance) that are called Lamellae (#9). The lamellae are arranged in a special configuration that makes them extremely strong; this arrangement assists with their job duties of containing that pressurized nucleus pulposus.

Neurology:

Warning: I'm going to try to keep this section as simple as possible, but it will end up becoming quite a complex notwithstanding my effort. So hang in there!

The first thing to understand is that the ultimate reason as to why humans feel pain is because of nerves. Nerves carry pain messages / signals from the periphery (i.e., anything outside of the brain and spinal cord) to the primary sensory cortex of the brain where they get interpreted into the feeling or perception of pain. in scientist speak, we say that pain travels from the peripheral nervous system (PNS) to the central nervous system (CNS).

There are two type of nerves pertinent to our discussion: motor nerves (aka efferent nerves) and sensory nerves (afferent nerves). Motor nerves carry messages away from the brain and spinal cord (i.e., the CNS) outward to the muscles of body, and sensory nerves carry messages (including proprioception (sense of postion), temperature, touch, pain, and pressure messages) from the periphery into the CNS.

Most folks have heard the term spinal cord before, which you can think of as a "superhighway" of sensory and motor neurons (nerves). The spinal cord is located vertically within a bony corridor running approximately in the center of all the vertebrae of the spine. This corridor has been given a special name: the central canal. The function of these neurons is to carry sensory Information to the brain (like the perception of touch, temperature, proprioception and pain) and motor information to the periphery (like commands that make you limbs move)--they are vital for the survival of our bodies. The spinal cord is made up of white and gray mater, which integrates sensory and motor axons into a protective tough gel like structure. For further protection, the spinal cord contains three protective layers of tough connective tissue called the meninges. More specifically, from outside to in, we have the dura, arachnoid and pia matter.

Something Strange: the real spinal cord stops somewhere between the L1 and L2 (L1/2) vertebra. This final tip of the spinal cord is called the conus medullaris. You might be interested to know that during early development, the spinal cord extended the entire length of the spine, but by the time of the birth, the longitudinal growth of the spinal cord could not keep up with the longitudinal growth of the spine – so it was left behind. Those observations lead to the obvious question, "if the spinal cord stops at L1/2, then how do peripheral nerves below this level enter the spine in order to get to the brain? The answer is this: incoming nerve roots below the level of L1/2 have a not-so-super highway of their own called the thecal sac (aka dural sac). Like the spinal cord, the thecal sac has a protective outer covering of dura; however, that is where the similarities end, for the thecal sac is not solid and contains only cerebrospinal fluid, which is not very protective of the free-hanging nerve roots in this region. The thecal sac terminates at the S2 level in most cases.

With regard to peripheral nerves entering the spine, the mechanism is the same at all regions of the spine: they enter (as well as leave) through a bony hole that is created by the vertebrae above and the vertebrae below. The hole is called the intervertebral foramen (IVF) [figure #9, red area entitled "IVF zone"]. The difference between cervical/thoracic versus lumbar is the distance the nerve roots have to travel before they enter the spinal cord. More specifically, nerve roots of the thoracic and cervical regions into the spinal cord immediately; whereas, nerve roots in the sacral and lumbar regions have to travel through the thecal sac (which is full of cerebrospinal fluid) all the way up to the L1/2 level before they enter the conus medullaris.

So what. Why do I tell you all this technical stuff? Because you need to understand why low back pain and radiculopathy occurs more frequently in the lumbar spine when compared to that of the thoracic and cervical spine. That is, the nerve roots of L2 thorugh S5 (which collectively hang in the thecal sac and make up what is called the cauda equina or horses tail) are vulnerable to compression and chemical irritation within the thecal sac because all that is protecting them is the thin, delicate, innermost layer of the meninges: the pia matter.

Figure#26) This is an advanced visualization is called a coronal view (front to back). The spinous processes, facets and half of the pedicles (i.e., the roof of the spinal canal) have been cut away so that we may visualize how the nerve roots of the cauda equina hang in the thecal sac (the thecal sac has also had the posterior half removed) and exit through the IVFs.

Note that the dorsal and ventral roots of L5 have pushed the meninges outward to form something called the dural sleeve, which courses all the way to the IVF and even contains the ultrasensitive dorsal root ganglia. As the two roots merged together, they form the spinal nerve, which immediately branches into the dorsal rami (this supplies the facet joints) and the ventral rami. The two branches supply all the muscles of the lower back, pelvis and lower limbs with sensory and motor function.

Not shown but noteworthy is the fact that the first two lumbar vertebrae (L1 and L2) also give rise to a white rami communicantes that carry sympathetic efferents to the chain ganglia.

Note that the nerve roots are only covered by the delicate and leaky pia mater (1), which is not nearly as tough as the dura mater (2) and arachnoid mater (3) that forms the cover of the thecal sac (aka: dural sac). The dura mater ultimately blends into and becomes the outer cover of the spinal nerve and all peripheral nerves--this changes names to the epineurium.

COMPRESSION OF THE ROOTS:

With regard to compression, the most common cause of it for folks under 60 is a herniated lumbar disc (see the Disc Herniation page). In folks over 60, a condition called "stenosis" is probably the most common cause of compression. With regard to chemical irritation, it is well known that a group of "evil biochemicals," which are collectively called cytokines, can leak from annular tears within the intervertebral discs and then soak the adjacent, naked nerve roots. This in turn results in an inflammatory reaction, which in turn results in radicular pain (sciatica), swelling, and the formation of granulation tissue (scar tissue) within the nerve roots. This intraneural granulation tissue may interfere with the circulation of the nerve root itself, which in turn may also lead to chronic pain. Where do these "evil biochemicals" come from? Mostly from degenerated nuclear material that can leak from a grade 5 annular tear. [53] Although, some research has indicated that degenerated facet joints can also leak cytokines. [51, 52]

The cervical and thoracic region do not have long ventral and dorsal roots. Once they emerge from the spinal cord, they almost immediately leave the spine through the IVF. So in these regions there are no dural sleeves or cauda equina.

Note that as the dorsal and ventral roots approach the actual spinal cord, they branch into what are called rootlets. It is these rootlets that actually connect to the gray matter of the spinal cord.

Even dorsal and ventral roots of the lumbar spine form rootlets as they approach the spinal cord at the L1/2 level.

The Epidural Space (#8 in fig. #9) is the space between the bony neural canal and the thecal sac; or the space outside of the thecal sac. In reality, this space is filled with blood vessels and fat and is grossly oversimplified in figure #9. Noteworthy is the fact that this is the region where 'epidural steroid injections' are placed. The Facet Joints (#5 in Fig. #9) (aka: zygapophyseal joints) of the spine are where the vertebrae articulate (join) with each other. Actually, the gap between the inferior and superior articular processes is the true facet joint (white region). Collectively the inferior and superior articular processes and the facet joint are called the Zygapophyseal Joints or articular pillars. These joints help carry the axial load of the body and limit the range of motion of the spine. They also make up the back border of the intervertebral foramen and may physically compress and trap the exiting nerves secondary to degenerative thickening (sclerosis); this condition is called lateral canal stenosis. The Ring Apophysis (#6) is the 'naked bone' of the outer periphery of the vertebral bodies. The very outer fibers of the disc (Sharpey's Fibers) anchor themselves into this region. Bone spurs (aka: Osteophytes) may arise from the ring apophysis as the result of the later stages of Degenerative Disc Disease (DDD) and/or Osteoarthritis (Spondylosis). Specifically, osteophytes arise from the prolonged 'pulling and tugging' of 'Sharpey's Fibers' at their anchor points. The Posterior Longitudinal Ligament (PLL) (#7) is a strong ligamentous tissue which courses down the anterior aspect of the vertebral canal and is attached to the outer fibers of the annulus fibrosus. This highly innervated (supplied with pain carrying nerve fiber) tissue is the last line-of-defense the posterior neural tissue has against the irritating and inflammatory effects of nucleus pulposus.

MRI AXIAL ANATOMY QUIZ:

Okay, lets see if you have learned anything: Figure #8 is a real over-head view (aka: Axial View) of an L4 vertebra. Name the numbered structures.

Here are the answers:

Remember, this is a T2-Weighted Axial View, which allows you to see the nerve roots (e.g. 3) hanging freely within the cauda equina. On the T1-Weighted or Proton Density view, you can't see these roots.

NERVE ANATOMY IN AND AROUND THE DISC:

In reality things don't look so 'nice and neat' within the human body. The below picture demonstrates what real nerve roots look like:

Figure. #15 is a back view (posterior to anterior) of a real human cadaver lumbar spine. The back part of the vertebrae (lamina & spinous processes) have been removed in order to see the dural sac (aka: thecal sac); the dural sac has been sliced open in order to see the dangling nerve roots of the cauda equina. Note: the cauda equina is only seen below the level of L2. Above L2, we have the more familiar spinal cord.

#1: This ball-like structure is the ultra-sensitive Dorsal Root Ganglion (DRG) that contains the sensory nerve cell bodies. (the motor nerve cell bodies live out of harms way and are found in the dorsal horn of the spinal cord.) The DRG is found within the protective bony intervertebral foramen (cut away in this photo) and can be pinched/irritated from 'far lateral disc herniations' and/or lateral canal stenosis.

#2) This is the famous 'spinal nerve root' and is the number one target of disc herniation. A good sized paracentral disc herniation often will compress this adjacent structure and 'might', if coupled with an inflammatory reaction, ignite the nerve root into 'anger' and sciatica.

#5) As the spinal nerves leave the spine and head-out into the body to do their respective jobs, they temporarily join into a mixed spinal nerve (#5). After this brief marriage, they spilt and become the smaller dorsal primary rami (#7) (which supplies the skin and muscle of the back) and the ventral primary rami (#6). The ventral primary rami (aka: anterior primary rami) of the bottom three nerve roots (L4, L5, and S1), merge within the pelvis to form the giant 'sciatic nerve', which not only causes so many of us grief when irritated but, importantly, gives life to the skin and muscle below the knees.

Inside the dural sac, you can see the free hanging motor nerve root (#4) and the sensory nerve root (#3). These nerve roots connect into the real spinal cord about at the L1 level. Pain signals travel along the sensory nerve root and register 'PAIN' within our brains in the sensory cortex, among other places.

The Sinuvertebral Nerve: A nerve of mystery

The Sinuvertebral Nerves (SN), is a mixed nerve as well as it carries both autonomic fiber (sympathetic) and sensory (afferent) fiber. [note: the Autonomic Nervous System (ANS) is beyond the scope of this site.] The sensory portion of the sinuvertebral nerve, which has the capability to carry the feeling of PAIN to the brain, arises from the outer 1/3 of the posterior annulus fibrosus (yellow balls) and PLL (#7). It then splits and attaches to both the dorsal ramus and the grey ramus communicans, although this nerves anatomy and course seems to be quite anomalous. Of importance is the fact that if irritated, the nerve ending within the disc have the potential to generate both back pain and/or lower limb pain (Discogenic Pain). This lower limb pain-referral has been greatly studied by Ohnmeiss et al. and is quite an interesting phenomenon. Discogenic Sciatica is the term I have given this referred discogenic pain. It is believed that the sinuvertebral nerve-endings are 'sensitive' to the irritating effects of degenerated nucleus pulposus, which may be introduced into the outer region from a grade three annular tear. (see may pages on Annular Tears for more information.) Amazingly, the sinuvertebral nerve also innervates (connects to) the disc above and below! So, the sinuvertebral nerve of the L4 disc also innervates the L5 and L3 disc. This may help explain why a L4 disc herniation/annular tear may clinically present with some signs of L5 and/or L3 involvement/overlap as well. It also carries autonomic nerve fiber to the blood vessels (not shown) of the epidural space. Sympathetic nerves control how the blood vessels function (vasomotor & vasosensory). Although rare, injury to these sympathetic nerves may cause RSD symptoms (now called CRPS) in the patients lower limbs; this usually would occur following surgery. (This may explain why I had a very slight case of RSD in my left foot following surgery - since my doc spent an hour cutting his way through a 'nest' of epidural vessels during my micro-discectomy.)

The exact pain-pathway (how pain travels from the disc to the spinal cord) of discogenic pain is another fascinating and controversial subject. It seems that the sensory pathway from the sinuvertebral nerves into the spinal cord, does NOT take the 'expected' route in every patient. Some research (101) has demonstrated that pain-signals travel from the disc, re-enter the IVF (via sinuvertebral nerve) and DRG at the SAME level. Other, more recent research has indicated that pain-signals travel from the disc, through the sinuvertebral nerve, through the Gray Rami Communicans, into the Sympathetic Trunk (ST), up the sympathetic chain to the L2 vertebral level (yes I said L2), through the gray rami communicans, into the L2 dorsal rami, into the L2 IVF, and into the L2 DRG (80, 81). The latter pain pathway is why some investigators believe that lower level disc herniations may present as L2 dermatomal pain (groin region) in some patients!

To drive-home my point that the pain path from the disc does NOT always re-enter at the same vertebral level, I present the 2004 randomized controlled investigation by Oh and shim (26). In an incredibly well designed outcome study, the latter investigators demonstrated that by 'cutting' (RF neurotomy) the Gray Ramus Communicans, the majority of chronic discogenic pain sufferers achieved substantial relief of their pain and avoid fusion surgery. (26) This proves that at least some of the incoming pain signals were traveling toward the sympathetic trunk (which is on the anterior side of the vertebra) and NOT re-entering the spinal nerves at the same level.

A Committee Error?

Another interesting oddity about the design of the nervous system is the fact that 'the committee' decided to put the delicate sensory nerve cell bodies within the IVF and NOT the within spinal cord, which is where the motor nerve cell bodies are located. The Dorsal Root Ganglion (DRG), which houses these sensory nerve cell bodies, is seen as a tiny bulging structure within the IVF. This structure is 'super-sensitive' to compression (because it houses all these sensory nerve cell bodies) and can cause extreme back and leg pain if compressed and irritated by discal material and/or bony outgrowth (stenosis). The placing of these sensory nerve cell bodies in such close proximity to the disc and within such a narrow bony tunnel (the IVF) was not 'the committees brightest idea! You see if you damage the axon (nerve fiber) of a nerve, the chances are quite good for recovery, but if you damage the 'brain of the nerve fiber' (nerve cell body) the nerve's chances of recovery are much less. This explains why patients often never recover completely from that tingling burning, and numbness following a major attack of sciatica (disc herniation-induced radicular pain and dysfunction).

The View from the Side: (the Sagittal view)

Fig. #2 Is a sagittal view (aka: lateral view) of the 'motion segment'; Two vertebrae which are 'sandwiching' the intervertebral disc.

The disc [which is made of a annulus fibrosis (blue) and the nucleus pulposus (green)] is made up of three distinct areas: 1) The nucleus pulposus (green), which is a water rich (due to proteoglycan aggrecan & aggrecate molecules which trap and hold water within the disc) gel in the center of the disc; 2) The annulus fibrosis (blue), which is the fibrous outer portions of the disc that is made up of type I collagen; and 3) The vertebral end-plates (yellow), which are cartilaginous plates that attach the discs to the vertebrae and supply food (nutrients) to the inner 2/3 of the annulus and entire nucleus pulposus.

To further increase the strength of the annulus fibrosus, individual sheets of collagen are layered throughout the annulus. There sheets of collagen are called lamellae (black curved lines within blue). The very outer lamellae (Sharpey's fibers), unlike the inner lamellae, are anchored into the solid bony periphery (Ring Apophysis) of each vertebral body. This is the region that 'osteophytes' or bone spurs typically like to form. (Click here to see a real axial view of a 'motion segment'.)

Disc Physiology 101:

The normal human intervertebral disc, which is considered the largest avascular structure in the human body, is made up of two main components, proteoglycan and collagen (type I and type II). The annulus is mostly made of collagen, which is a tough fibrous tissue similar to the cartilage that is found in the knee, and the nucleus is made mostly of proteoglycan. Proteoglycans, which are produced by disc cells that resemble chondrocytes, are extremely important for disc function (see next paragraph) and are what 'trap' and hold water molecules (H20) within the tissue of the disc. In fact both the disc and annulus are comprised mainly of water, i.e., the nucleus is 80% water, and the annulus is 65% water. Proteoglycans are the building blocks of the aggrecan molecule which is the true 'water trap' of the disc. Aggrecans combine within the disc on strands of hyaluronan acid to form huge structures called 'Aggregates'. These super water-filled proteoglycan aggregates are what give the healthy young disc its amazing strengths and pliability, in fact a well hydrated disc is often even stronger than the bony vertebral body. Fig. #3: Here we have the healthy disc of a teenager (cadaver). The water content is extremely high as you can even see by the 'glistening' appearance of the nucleus (which is the gray center of the white disc).

Disc Function:

In order for a disc to function properly, it MUST have high water content; this is especially true of the nucleus. A well hydrated (with water) disc is both strong and pliable. The nucleus pulposus needs to be strong and well hydrated to do its job (axial load), for it is this structure that supports or carries the lion's share of the axial load (downward weight of body) of the body. With an undamaged annulus, strongly corralling a fully hydrated nucleus, the disc can easily support even the heaviest of bodies! As the disc dehydrates (loses water) the disc loose ability to support the axial load of the body (loses hydrostatic pressure); this causes a 'weight bearing shift' from the nucleus, outward, onto the annulus, outer vertebral body, and zygapophyseal joints (facets). Now, we have an 'over-load' on the annulus (which may trigger other destructive biochemical reactions) which, if severe and/or is imposed upon a genetically inferior annulus, will result in pathological DDD. ( see below)

Hydration also is important with respect to disc nutrition. As we have already mentioned, nutrients (which all living tissue needs in order to survive) must diffuse (soak) through the discal tissue in order to reach the hungry disc cells. This diffusion process is much faster and easier IF the diffusing tissue has a high water content. We may use 'swimming' as an analogy: It's easier to swim through the water, than through the sands of a desert. The sands of the desert would be a dehydrated disc, and the water would be a hydrated disc. So, water and disc hydration are one of the key factors for a normally functioning spine and well feed disc.

So, we've learned WHY disc hydration is so important. Now it time to learn HOW this disc hydration is accomplished:

Water is held within the disc by tiny sponge-like molecules called proteoglycan aggrecans. These 'super sponges' have an amazing ability to attract and hold water molecules (324), and can in fact hold over 500 times their own weight in water; this gives the non-dehydrated disc the tremendous 'hydrostatic pressure' which is needed to bear the axial load of the body. Amazingly, the aggrecans water absorption is so powerful that over night (non-axial loading) the height of the disc and the body will actually measurably increase due to the discs engorgement with water. This phenomenon is called 'Diurnal Change' and is only present in non-degenerated discs.

Disc cells, particularly the chondrocyte-like cells of the nucleus and inner annulus, manufacture proteoglycan aggrecan molecules. Like little factories, they create, replace and rebuild aggrecan molecules. As long as the disc cells have food (glucose), building material (amino acids) and oxygen all is well in disc-land. It is also important for them of have a non-acidic working environment, which is taken care of, since wastes are diffused out of the disc the same way nutrients diffuse in. In the living disc up to 100 aggrecans combine on a long piece of 'hyaluronan acid' to form giant proteoglycan aggregate molecules. It's these aggregates that are found within the disc in the real world.

Disc Nutrition:

The intervertebral disc is the largest avascular structure in the human body. The reason for this is because it has no direct blood supply like most other body tissue. Nutrients (food) for the disc are found within tiny capillary beds (black arrows) that are in the subchondral bone, just above the vertebral end-plates . This subchondral vascular network 'feeds' the disc cells of the all important nucleus and inner annulus through the diffusion process. The Figure on the left shows the 'disc feeding setup' for disc. Note that the outer annulus has its oven blood supply that is embedded within the very outer annulus. This is a much more efficient system and nutrients don't have to diffuse very far to find their hungry disc cells. The 'more direct' blood supply of the outer annulus is why tears of the outer 1/3 of the annulus will heal/scar shut with the passage of time, which unfortunately is not true of the rest of the disc. Research has indicated that disc tears will not heal in the inner zones of the disc - probably because of the avascular nature of the inner two thirds of the disc. Note the nutrients (pink balls) diffuse directly into the tissue of the outer annulus, where as the nucleus and inner annulus has a much longer diffusion route that is block by the vertebral end-plates. Note how the nutrients (pink balls) are released from the blood vessels (red) in the subchondral bone just under the vertebral end-plates. These nutrients must 'diffuse' or soak their way through the vertebral end-plates and into the disc. This 'diffusion method' is how the cells of the disc get the nutrients oxygen, glucose, and amino acids which are required for normal disc function and repair. This poor blood/nutrient supply to the disc is one of the main reasons that the disc ages and degenerates so early in life. (Read my Disc Degeneration page for more information.)

The 'diffusion feeding process' is enhanced somewhat by a phenomena called 'Diurnal Change'. Our discs have the ability to expand and compress over the course of a day. As we start the day our discs, like squeezing out a sponge, will compress and dehydrate because of the gravity and physical activity which place axial loads upon the discs. In fact a healthy disc will shrink down some 20% (104), which in turn decreases our height by 15 to 25mm (194,441,815). As we sleep and decompress our spines, our discs swell with water plus nutrients and expand back to their fully hydrated state. This tide-like movement of fluids in and out of the disc will help with the movement of nutrients into the avascular center of the disc. (Click here to learn more on Diurnal Change).

Super Advanced Anatomy & Physiology:

The Nucleus Pulposus:

The nucleus pulposus is a hydrated gelatinous structure located in the center of each intervertebral disc that has the consistency of toothpaste. Its main make-up is water (80%). Its solid/dry component make-up are proteoglycan (65%), type II collagen fiber (17%) and a small amount of elastin fibers . Collectively the proteoglycans and the collagen are called the 'nuclear matrix'. The cells of the disc, which produce the water holding proteoglycan molecules are very similar to chondrocytes seen in articular cartilage and are also held within the matrix.

Proteoglycans are found in several structural forms within the disc but the most important 'arrangement' is called a proteoglycan aggrecan. These aggrecans main function is to trap and hold water, which is what gives the nucleus its strength and resiliency. Like a 'super sponge', aggrecans can trap and hold over 500 times their weight in water!

The nucleus has two functions. The first is to bear most of the tremendous axial load coming from the weight of the body above and second to 'stand-up' the lamellae of the annulus - so that the annulus can reach its full weight baring potential. In order for proper weight bearing the nucleus and the annulus MUST work hand in hand.

The Annulus Fibrosis:

The annulus is the outer portion of the disc that surrounds the nucleus. It is made up of 15 to 25 collagen sheets which are called the 'lamellae'. The lamellae are 'glued' together with a proteoglycans. These sheets encircle the disc and, in concert with the nucleus, give the disc tremendous axial load strength.

The posterior portion of the annulus if further strengthened by the 'posterior longitudinal ligament'. This structure is the final barrier between the disc and the delicate spinal cord, and nerve roots.

The biochemical make up is similar to that of the disc only in different proportions. The annulus is 65% water, with the collagen, both type I and II making up 55% of the dry weight, and proteoglycans (mostly the larger aggregate type - 60%) making up 20% of the dry weight. 10% of the annulus also contain 'elastic fiber' that are seen near where the annulus attaches into the vertebral end-plate.

The lamellae are made up of both Type I (very strong type) and Type II collagen fiber. The very outer lamellae are almost all Type I. As we move inward toward the nucleus the more Type II is seen and less Type I. The very inner layers are very hard to distinguish from the nucleus. There is not a clear boundary between the nucleus and the annulus.

A simply amazing fact about the lamellae design is that the collagen fibers that make-up each lamellae all run parallel at a 65 degree angle to the sagittal plane. Even more amazing is the fact that the each lamellae are flipped so that the 65 degree angle alternates between every lamellae, one to the right then one to the left. This design greatly increases the shear strength of the annulus and makes it had for cracks to develop through the layers of the annulus. This is just amazing if you think about it!! Brilliant design!

The function of the annulus is to help the nucleus support the axial weight from the body. The annulus does need some help from the nucleus in order to achieve its strongest configuration. It relies on the nucleus to push it outward which keeps the lamellae from collapsing inward. The nucleus must keep a very high hydrostatic pressure to achieve this. We saw what happens when the nucleus losses hydrostatic pressure under the 'Disc Degeneration' page. Bogduk used the analogy of a rolled up telephone book standing on end, to describe how strong the annulus could be when the nucleus holds the inner lamellae or phonebook in a rolled up position. If you unroll the phone book our 'on end phone book' it would not longer be able to support much axial loading.

The Vertebral End-Plates:

Both the top and the bottom of each vertebrae (spinal bones) are capped with a thin ¾ millimeter cartilaginous pad called the 'Vertebral End-Plate' (Figure #1). Despite their name, these end-plates are NOT attached to the subchondral bone of the vertebrae but are instead strongly interwoven into the annulus of the disc (156, 388). It is for this reason, as well as strong morphological similarities, that the vertebral end-plates are considered part of the disc and NOT part of the vertebral body.

The biochemical morphology of the end-plates is extremely similar to that of the disc: Water, proteoglycans, collagen and cartilage cells (chondrocytes). The concentration scheme of these components also mirrors that of the disc: The center of the end-plate is mostly water and proteoglycan. As we move outward toward the periphery, more and more collagen is seen with less and less proteoglycans. This similar biochemical makeup and distribution scheme helps the diffusion of nutrients between the subchondral bone of the vertebra and the depths of the disc.

The very outer rim of the vertebrae is NOT covered by the end-plate, which leaves a ring of exposed bone on the periphery of the top and bottom of each vertebra. This exposed peripheral area is called the 'Ring Apophysis' and is often a site for the development of spur formation associated with the degeneration process.

References:

12) Marchand F, et al. (1990) "Investigation of the laminate structure of lumbar disc anulus fibrosus." Spine - 1990; 15:402-410

80) Nakamura SI, Takahashi K, Takahashi Y, et al. "Origin of nerves supplying the posterior portion of lumbar Intervertebral discs in rats." Spine 1996; 21:917-924

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