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From Neurosurgical Focus

Pathophysiology of Lumbar Disc Degeneration: A Review of theLiteratureMichael D. Martin, MD, Christopher M. Boxell, MD, FACS, and David G. Malone, MD

Authors and Disclosures

Posted: 10/15/2002; Neurosurg Focus. 2002;13(2) © 2002 American Association of Neurological Surgeons

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• Abstract and Introduction

• Anatomy of the Intervertebral Disc

• Proteoglycan and Water Content

• Vascularity of the Disc

• Collagen Distribution

• Role of Apoptosis

• Mechanical Stress and Inflammatory Components

• Disc Herniation

• Conclusions

• References

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Abstract and Introduction

Abstract

Lumbar disc degeneration occurs because of a variety of factors and results in a multitude of conditions. Alterations in

the vertebral endplate cause loss of disc nutrition and disc degeneration. Aging, apoptosis, abnormalities in collagen,

vascular ingrowth, loads placed on the disc, and abnormal proteoglycan all contribute to disc degeneration. Some forms

of disc degeneration lead to loss of height of the motion segment with concomitant changes in biomechanics of the

segment. Disc herniation with radiculopathy and chronic discogenic pain are the result of this degenerative process.

Introduction

Lumbar disc degeneration occurs commonly in humans. There are a variety of factors that contribute to this condition.

The disc itself is active tissue that contains significant mechanisms for self-repair. [26]Reviewing the available literature

concerning the normal and abnormal physiology of the disc is useful in understanding why degeneration occurs, why

certain conditions are painful, and which mechanisms can be used to prevent further degeneration.

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Pathophysiology of Lumbar Disc Degeneration: Literature Review: Anatomy of the Intervertebral DiscAuthors and Disclosures


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Anatomy of the Intervertebral Disc

The intervertebral disc is composed of at least three elements. The central portion of the disc contains the nucleus

pulposus, which is composed of cells from the primitive notochord. The outer portion of the disc is the anulus fibrosis,

and it is composed of concentric layers of intertwined anular bands. These anular bands are arranged in a specific

pattern to resist forces placed on the lumbar spine. The anular bands are subdivided into inner fibers, which are

connected to the cartilaginous endplate, and outer Sharpy fibers, which are attached to the VB (Fig. 1). The ALL and

PLL further strengthen the disc space. The ALL attaches more strongly to the VB edges than to the anulus. It provides a

tension band to resist forces applied in extension and is a stronger ligament than the PLL. The PLL is not as strong as

the ALL, but it provides a tension band to resist flexion forces. The PLL strongly attaches to the anulus fibrosis, and

frequently is torn in cases of free fragment disc herniation.

(EnlargeImage)

Figure 1.

Artist's illustration showing the vascular supply to the disc space from the cartilaginousendplate. 1 = segmental radicular artery; 2 = interosseous artery; 3 = capillary tuft; and 4 = discanulus.

A meningeal branch of the spinal nerve, better known as the recurrent sinuvertebral nerve, innervates the area aroundthe disc space (Fig. 2). This nerve exits from the dorsal root ganglion and enters the foramen, where it then divides into

a major ascending and lesser descending branch.[6] It has been shown in animal studies that further afferent innervation

to the sinuvertebral nerve arises via the rami communicantes from multiple superior and inferior dorsal root ganglia. [43] In

both human and animal studies, the outer anular regions are innervated, but the inner regions and nucleus pulposus are

not innervated.[37,48] In addition studies have demonstrated that the ALL also receives afferent innervation from branches

that originate in the dorsal root ganglion.[7] The PLL is richly innervated by nociceptive fibers from the major ascending

branch of the sinuvertebral nerve. These nerves also innervate the adjacent outer layers of the anulus fibrosis.[7,37] Degenerated human lumbar discs have been shown to contain more nerve tissue and to be more vascular than

normal discs.[9,12,48]

(EnlargeImage)

Figure 2.

Artist's illustration showing innervation of the PLL and the disc anulus; 1 = ascending branch of

the sinuvertebral nerve; 2 = dorsal root ganglion; 3 = descending branch of the sinuvertebralnerve; 4 = disc anulus; 5 = PLL.

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Pathophysiology of Lumbar Disc Degeneration: Literature Review:Proteoglycan and Water ContentAuthors and Disclosures


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Vascularity of the Disc• Collagen Distribution



• Disc Herniation

• Conclusions

• References



Proteoglycan and Water Content

The strength of the lumbar disc is related to the fluid and proteoglycan content of the disc. Proteoglycan is a hydrophilic,

negatively charged, branched chain molecule composed of a protein attached to an oligosaccharide. Proteoglycans are

also known as glycosaminoglycans and include such structures as chondroitin and collagen. The negative charge on

the branched chains and the hydrophilic nature of proteoglycan internally pressurize the disc by drawing water via

osmosis into the nucleus pulposus. Unfortunately, proteoglycan and therefore water content in the disc tend to decrease

with age.[1-3,51,65] Additionally the amount of hydration within the disc is inversely proportional to applied stress, suggesting

that applied spinal loads lead to a loss of hydration and proteoglycan in the disc.[65] Interestingly, proteoglycan, and thus

water content, has also been shown to be low throughout the entire spine in patients with degenerated discs.[51] Studies

have shown that applied stress produces degenerative change in the disc; however, it remains unclear whether

degeneration in vivo is the result of stress applied to an already biochemically altered disc or a response to stress by an

otherwise normal disc. Disc degeneration may have a genetic basis; Sahlman, et al.,[59] have shown that mice that are

heterozygotes for the knockout gene, Col2a1, related to Type II collagen, had 4% shorter spines and a decreased

concentration of glycosaminoglycans in their endplates, vertebrae, and anuli fibrosis.

Degenerated discs have been demonstrated to vary specifically in the degree of sulfation of chondroitin.[27]Degenerated

discs lacked adequate sulfation of chondroitin when compared with controls, and the degree of degeneration was

greater in higher grade degeneration. The specific disaccharide involved was chondroitin sulfate,[6] which had been

reduced to unsulfated chondroitin in degenerated sections. This study suggested a possible mechanism for the

increased incidence of posterior failure of the disc, because the degree of undersulfation was accentuated in the

nucleus pulposus as well as the posterior central segment of the anulus fibrosis. Another specific anular proteoglycan,

fibromodulin, exhibits a structural change with increasing age that is characterized by a proportional increase in its

glycoprotein form, which lacks keratin sulfate.[61]

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Vascularity of the Disc

The disc itself has a low metabolic rate and receives most of its nutrition by diffusion.[26] Bulk transport contributes

somewhat to nutrition of the disc and this is facilitated by spinal motion.[25] The majority of disc nutrition is supplied via

the capillary beds of the cartilaginous VB endplate. [10] Blood flow to these capillary beds is under humoral control, which

increases with application of acetylcholine.[68] Early research suggests muscarinic receptors exist on the cartilaginous

endplate, implying a possible mechanism of the negative effects of nicotine on the end-plate. [68] These capillary beds

receive blood flow from the distal branches of the interosseous arteries supplying the VB. Disruption of the cartilaginous

endplate leads to formation of Schmoral nodes.[54]Vascular and lymphatic tissue is present in the anulus of patients who

are as old as age 20 years; however, lymphatics and blood vessels are not present in the nucleus pulposus at any age.[56] The concentration of oxygen has been found to be greatest at the disc edge, and the concentration of lactate is











































































greatest in the center, which is the portion of the disc farthest from the blood supply.[4]Although the concentrations of

oxygen and lactate vary widely from person to person, they do not seem to change in pathological states.[4]

Changes in the vascular supply appear in disc degeneration. Normal anastomotic arteries on the anterolateral surface

are obliterated and replaced by small, tortuous arteries in degenerated anuli.[33] The arterial changes occur before the

degenerative process and may be related to the ingrowth of vessels from osteophyte formation.[33] This implies a

relationship between nutrition to the disc and subsequent disease. Some authors believe that neovascularity occurs in

response to injury as an attempt to supply the damaged portion of the disc with nutrients.[66] Others suggest that

vascularization of the anulus occurs in response to trauma and that the new vessels are associated with endothelialcells, fibroblasts, and mononuclear cells.[53] Surgical specimens have been compared with those from normal cadaveric

disc and it has been found that the vasculature of normal discs was not associated with additional cells, but instead with

an inactive collagen matrix.[53] Both macrophages and blood vessels have been found to be prominent in herniated disc

fragments, but the presence or absence of these entities could not be correlated with the timing of radicular pain.[67]

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Pathophysiology of Lumbar Disc Degeneration: Literature Review: CollagenDistributionAuthors and Disclosures


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Anatomy of the Intervertebral Disc• Proteoglycan and Water Content





• Disc Herniation

• Conclusions

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Collagen Distribution

Collagen is a widely distributed proteoglycan in the body. Type I collagen is usually found in skin, bone, tendon, and dentin.

Types II and IX are normally found in hyaline cartilage and vitreous humor. Type III collagen can be found in skin and blood

vessels. Type VI is found in most interstitial tissues. Type I collagen is found in the nucleus pulposus and anulus fibrosis in

both normal and pathologically degenerated discs, whereas Type II and IX collagen are found to be increased in areas of

minor degeneration but absent in areas of more advanced disease. [44] In contrast, Lotz, et al.,[40]f ound that Type II collagen is

decreased at all levels of applied mechanical stress. In another study the total amount of collagen was found to be highest in

patients 5 to 15 years of age, and in this study the amount of denatured Type II was decreased in degenerated discs.[2,3] Types III and VI were increased in areas of degeneration regardless of the degree .[44] A marker of oxidative stress related to

modification of the protein structure of collagen, N -(carboxymethyl)lysine, has been shown to increase with advancing

degenerative disease.[45]

Additional studies of the collagen composition of the disc have revealed the importance of collagen crosslinks to the

mechanical stability of the intervertebral disc. A decrease in the presence of pyridinoline and an increase in pentosidine within

the disc are associated with aging.[52] The concentration of pyridinoline found in patients older than 65 years was

















































































approximately 50% of that found in younger people. Decreases in pyridinoline crosslinks lead to alterations in the collagen

matrix of the disc. Cells in the disc have the potential to produce a matrix that is inappropriate for the mechanical stresses

placed on it.[20]

It has also been suggested that matrix degeneration within the nucleus may result from the increase in certain proteinases

that do not appear in the normal disc. Leukocyte elastase, which is normally found in the VB, may be at least partially

responsible for the degeneration of the extracellular matrix in the anulus, nucleus, and endplate. [13] NEWS

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Pathophysiology of Lumbar Disc Degeneration: Literature Review: Role of ApoptosisAuthors and Disclosures


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Role of Apoptosis

Apoptosis is programmed cell death and occurs in both normal and pathological states. The role of apoptosis in lumbar

disc degeneration is not fully understood. It is apparent that an inappropriate disc matrix is produced in degeneratedlumbar discs.[20] This inappropriate matrix may isolate cells in the disc and lead to apoptosis. Further experimentation

has indicated that apoptosis occurs at a higher rate in cases of free disc fragments compared with contained disc

herniations.[50] This increased rate of apoptosis in free disc herniations may be a possible mechanism of resorption and

remodeling of the extruded material. In vitro evidence suggests that certain cytokines, namely insulin-like growth factor-

1 and platelet-derived growth factor, can inhibit apoptosis in cell cultures of disc material.[21] In vivo experiments indicate

that disc tissue responds in an autocrine fashion; that is, the cells release substances to which the cells themselves

respond.[31,36,49] This has been shown to be different in degenerated compared with normal disc tissue, and it has been

hypothesized that these substances can lead to disc degeneration.[36,49] These studies all indicate that the disc is much

more active biologically than previously thought, and further research may provide pharmacological means to change

the natural history of disc degeneration.

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Pathophysiology of Lumbar Disc Degeneration: Literature Review: MechanicalStress and Inflammatory ComponentsAuthors and Disclosures


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Mechanical Stress and Inflammatory Components

Authors of multiple studies have evaluated mechanical stress as applied to the intervertebral disc. Disc degeneration is

related to mechanical stresses. Degeneration may begin in early adulthood and may change the disc in such a way that

herniation is imminent.[23] The levels most commonly affected are L4-5 and L5-S1. Although degeneration at one of

these two levels is correlated with disease at the other, there is no correlation with disease in facet joints.[17] Disease in

either the facet joints or the disc is not reliably correlated with patient age. Stress in the vertebral column is transmitted

mostly to the endplates of the VB and less to the core of the body. [39] In healthy individuals the stress is transmitted from

the center of the endplate, whereas in a degenerative state, stress is transmitted more to the peripheral rather than the

central aspects of the VB. This is thought to be due to the loss of hydration of the nucleus pulposus that accompanies

aging.[39]

Other biomechanical studies have focused on the effect of applied stress. A study in which cadaveric disc was used and

with motion was applied in flexion-extension, axial rotation, and lateral bending showed that the laxity of a degenerated

disc is increased and the range of motion is decreased. [41] Spine flexibility and, thus range of motion, was shown to be

reduced in degeneration because of the increasing size of the endplate and the decreasing height of the intervertebraldisc space. The increased laxity of the motion segment was found to be due to decreased disc height of the nucleus

with resultant loss of tension of the surrounding ligaments.[41] When intradiscal pressure is increased, degenerated discs

herniate at lower pressures than normal discs.[28] It may be possible that a certain critical pressure exists above which

the stressed anulus is unable to counteract the force applied, resulting in rupture. [28] Umehara, et al.,[64] reported that the

anulus in degenerated discs is abnormal and that its elastic modulus is lowest in the posterolateral section of the disc.

The same group demonstrated that symmetry and regularity of the elastic modulus decrease as the grade of

degeneration increases. Others have shown that the degenerated disc had reduced ability to withstand applied stress

and that the section of anulus most responsible for this is the middle portion.[14]

Applied stress may trigger certain biochemical events within the disc. Degenerated discs have higher than normal

concentrations of fibronectin, which may be elevated as part of the response to injury.[47] This may translate to an

increase in proteolytic activity.[47] Mechanical stress has also been shown to increase the activity of MMPs 2 and 9 in

articular cartilage, indicating an increase in tissue turnover that has the potential to weaken the disc.[5]Increases in

MMPs, NO, PGE-2, and IL-6 are also seen in herniated discs.[32] Cathespin G, a proteinase with a wide variety of

biochemical actions, has been shown to be increased in the anulus of degenerated discs. [38] It has also been suggested








































































that applied stress may decrease the expression of Type II collagen and result in disorganization of the anulus fibrosus.[40]

The herniated disc also induces an inflammatory response. In a dog model with autologous implantation of both nucleus

and anulus, Hasegawa and colleagues[24] showed that nucleus fragments induce an inflammatory reaction. The number

of lymphocytes, macrophages, and fibroblasts was markedly increased in samples from older dogs.[24] Others have

examined human samples, finding that macrophages are the most commonly found cell type in both acute and chronic

herniation.[19,22,29] Despite this finding, evidence of macrophage infiltration does not correlate with clinical symptoms in

humans.[55] Interestingly, the presence of inflammation in patients does correlate with a better postoperative outcomewhen compared with those with herniated discs showing no inflammation.[69] A study in rats revealed an increase in theexpression of IL-1 , IL-6, and NO synthetase 1 week after implantation of nucleus fragments.[35] Further progression

of the inflammatory response may be caused by IL-1 ; the addition of this cytokine to normal disc tissue in vitro

causes an increase in MMPs, IL-6, PGE-2, and NO. [32] Degradation of the proteins in the extracellular matrix of the discmay also be caused by MMP's. Other inflammatory mediators, such as IL-1 and tumor necrosis factor- , are

also present in the herniated disc and may in crease the amount of PGE-2. [62] The painful disc may be the result of the

effect of PGE-2, and, in fact, a positive straight-leg raise result has been correlated with the amount of PGE-2.[46] Some

controversy exists as to whether levels of tumor necrosis factor- and phospholipase A2 are increased in the herniated

disc.

In addition to the inflammatory mediators already mentioned, authors of one study found that two thirds of patients with

lumbar disc disease and radicular symptoms had an increase in antiglycosphingolipid antibodies, suggesting an

autoimmune component to lumbar disc disease.[8] Another group of authors showed that there are antigen-antibody

complexes present in the herniated disc, although no conclusions were made regarding their clinical significance.[60] The

theory that there is an autoimmune component to lumbar disc disease is controversial; others authors have

demonstrated a lack of cells that were expected in an antigen-specific inflammatory response, although the number of

macrophages is known to be increased.[22,34]

Pathophysiology of Lumbar Disc Degeneration: Literature Review: Disc HerniationAuthors and Disclosures


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•Abstract and Introduction







• Disc Herniation

• Conclusions

• References


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Disc Herniation
































































The lumbar spine of the human is made up of five lumbar vertebrae that areseparated by five intervertebral discs (blue structures in fig. #1). The discsmay be thought of as spinal shock absorbers, for they absorb the load of thebody. They also allow for movement at the waist as they act as a pivot pointand allow the lumbar spine to bend, rotate, and twist.

There are 23 discs in the human spine: 6 in the neck (cervical region), 12 inthe middle back (thoracic region), and 5 in the lower back (lumbar region).This page shall focus on the lumbar spine; however, the thoracic andcervical spines are similar in make-up.

The disc is made up of three basic structures: the nucleus pulposus,the annulus fibrosus (aka annulus fibrosus) and thevertebral end-plates.Although their percent composition differs, the latter three structures aremade of three basic components: proteoglycan (protein), collagen (cartilage),and water. We will learn all about these structures below.

Figure #1 depicts a Front view (AP) of the lumbar spine. Here we can seehow 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 sciaticanerve 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 somuch trouble in many of us chronic pain sufferers. (more detail here)

Figure #2 Shows a cut-away

posterior view (PA view) of thelumbar spine. Now we can better visualize how the sciatic nerve isformed and see just how close thespinal nerve roots come to the back of the intervertebral discs. Note thatthe L5 root exits through the L5 IVFand the S1 root traverses downwardand exits below the L5 disc throughthe sacral foramen. Any herniation of the posterior disc may compressand/or chemically irritate EITHER the exiting L5 root re traversing S1spinal nerve root and result in severelower back pain and/or lower limb pain (i.e., sciatica). For moreinformation on sciatica please visitmy 'Sciatica Page'.



The Nucl

eus

Pulposus (#1 fig. #9-pink) is the water-rich, gelatinous center of the

disc, which is under very high pressure when the human is upright--especially in theseated 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 allmovement of the lower trunk occurs. It's third function is to act as a ligament and

bind the vertebr ae together. The Annulus Fibrosus (#2 fig. #9-

green) is much more fibrous (tougher) than the nucleus. It also has a much

highercollagen content and lower water content when compared to the nucleus. Itsmain job is to corral or hold in place or contain the highly pressurized nucleus (thenucleus is pressurized for hold up the weight of the body), which is constantly tryingto 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 strongand assists in their job of containing that pressurized nucleus pulposus.

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Neurology:

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

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The first thing to realize is that the ultimate reason why humans feel pain isbecause of nerves. Nerves carry pain messages / signals from the periphery(i.e., anything outside of the brain and spinal cord [CNS]) to the sensorycortex of the brain where they get interpreted into the feeling or perception of pain.

There are two type of nerves pertinent to our discussion: motor nerves (akaefferent nerves) and sensory nerves (afferent nerves). Motor nerves carrymessages away from the brain and spinal cord (i.e., the CNS) outward to themuscles of body, and sensory nerves carry messages (includingtemperature, touch, pain, and pressure messages) from the periphery intothe CNS.





These nerves are sensitive to compression and/or chemical irritation, whichcan occur if they get pinched and/or bathed from an adjacent disc that isripped open (see the Disc Herniation page). Such a syndrome can and doesoccur near the back (posterior) part of the disc wher e the nerve roots are invery close proximity (like the L5 roots in fig. #9). The traversing nerveroots (L5 roots in fig. #9 for example) in the lumbar spine are unusual in that

they are made up of individually pia-mater-wrapped sensory and motor rootsthat do not separate until they reach the actual spinal cord, which is locatedat about the level of the L2 disc (there is some variation of this). Above thelevel of L2, the motor and sensory roots immediately branch into a ventraland dorsal root and enter the cord. (see figure #26)

The nerve roots exit the spine through bony holes called the Intervertebral

Foramen (Red Zone in fig. #9). Once within the IVF the two nerve roots

merge into one mixed nerve call the Spinal Nerve. The Spinal nerves are called"mixed nerves" for they contains both sensory nerve fiber (aka: afferent) and motor nerve fiber (aka: efferent). After leaving the spine through the IVF, the nerve splitsinto a posterior division (Dorsal Ramus) and an anterior division (Ventral Ramus).

The Dorsal Ramus connects to the muscle and skin over the lower back, butt

and Facet Joint (#5 in fig.9). TheVentral Ramus combine in the pelvis and

form the giant Sciatic Nerve and Lateral Femoral Cutaneous Nerve that in turnconnect to all the skin and muscle of the lower limbs (See my 'Sciatica Page' for more information). As we will learn below, the ventral ramus has a "recurrent branch" that connects to the back of the disc, as well as laterally to the sympathetic

nervous system (Grey Ramus Communicans); this special nerve is

called the Sinuvertebral nerve (SN) (see below).As noted above, in the lumbar spine there is no spinal cord. Instead the nerve roots

hang like a "horses tail" in an enclosed, CSF-filled sac called the Thecal Sac

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(red stars in Fig.#9) . The thecal sac, is made of a mixture of pia-mater-

wrapped lumbar, sacral and coccygeal nerve roots [both motor and sensory] andserves to protects these all-important roots. The nerve roots of the thecal sac also

float freely in cerebral spinal fluid (CSF) that offers both protection and

nutrition for the roots. Its covering is made up of two distinct but tightly boundlayers called the dura mater and arachnoid mater. Note how the nerve roots withinthe thecal sac (fig.# 9 labeled S1, S2, S3, S4), which are collectively called

the Cauda Equina(#4), are often in a highly organized configuration.

Because of this arrangement, which usually puts the lower level traversing nerveroot in front, it is possible for a large disc herniation to irritate/compress more thanone root! This may explain why disc herniations do NOT always match their exactdermatomal distribution; i.e., a large disc herniation at L4 may clinically present asnerve root pain (aka: radicular pain, sciatica) and dysfunction of both the L4, L5 and

even S1 nerve roots!!! The Epidural Space (#8 in fig. #9) is the

space between the bony neural canal and the thecal sac; or the space outside of thethecal sac. In reality, this space is filled with blood vessels and fat and is grosslyoversimplified 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 ZygapophysealJoints or articular pillars. These joints help carry the axial load of the body andlimit the range of motion of the spine. They also make up the back border of theintervertebral foramen and may physically compress and trap the exiting nervessecondary 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 thisregion.Bonespurs (aka:Osteophytes) may arise from the ringapophysis as theresult of the later stagesof DegenerativeDiscDisease (DDD)and/or Osteoarthritis(Spondylosis).Specifically,osteophytes arisefrom the prolonged'pulling and tugging'of 'Sharpey's Fibers'at their anchor points.

The Posterior

Longitudinal

Ligament (PLL

) (#7) is a strong

ligamentous tissuewhich courses down



the anterior aspect of the vertebral canal and is attached to the outer fibers of theannulus 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 inflammatoryeffects of nucleus pulposus.

MRI AXIALANATOMYQUIZ:

Okay, lets see if you have learnedanything: Figure#8 is a real over-head view (aka:Axial View) of anL4 vertebra.Name thenumberedstructures.

Here are theanswers:

Remember, thisis a T2-Weighted AxialView, whichallows you to

see the nerveroots (e.g. 3)hanging freelywithin the caudaequina. On the

T1-Weighted or Proton Densityview, you can't seethese roots.

NERVE ANATOMY INAND AROUND THE

DISC:In reality things don'tlook so 'nice and

http://www.chirogeek.com/002_Ohnmeiss_IDDvsSciatica.htm

http://www.chirogeek.com/002_Ohnmeiss_IDDvsSciatica.htm

http://www.chirogeek.com/000_Radial_Tears_1.htm



neat' within the human body. The below picture demonstrates what realnerve roots look like:

Figure. #15 is a

back view (posterior to anterior) of a real

human cadaver lumbar spine. Theback part of thevertebrae (lamina &spinous processes)have been removedin order to see thedural sac (aka:thecal sac); the duralsac has been slicedopen in order to seethe dangling nerve

roots of the caudaequina. Note: thecauda equina is onlyseen below the levelof L2. Above L2, wehave the morefamiliar spinal cord.

#1: This ball-like

structure is the ultra-sensitive Dorsal RootGanglion (DRG) thatcontains the sensory

nerve cell bodies.(the motor nerve cellbodies live out of

harms way and are found in the dorsal horn of the spinal cord.) The DRG isfound within the protective bony intervertebral foramen (cut away in thisphoto) 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 compressthis 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 dotheir respective jobs, they temporarily join into a mixed spinal nerve (#5).After this brief marriage, they spilt and become the smaller dorsal primaryrami (#7) (which supplies the skin and muscle of the back) and the ventralprimary rami (#6). The ventral primary rami (aka: anterior primary rami) of the bottom three nerve roots (L4, L5, and S1), merge within the pelvis toform the giant 'sciatic nerve', which not only causes so many of us grief whenirritated 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 realspinal cord about at the L1 level. Pain signals travel along the sensory nerveroot 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: theAutonomic 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 quiteanomalous. Of importance is the fact that if irritated, the nerve ending within the dischave the potential to generate both back pain and/or lower limb pain (DiscogenicPain). 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 havegiven this referred discogenic pain. It is believed that the sinuvertebral nerve-endingsare 'sensitive' to the irritating effects of degenerated nucleus pulposus, which may beintroduced into the outer region from a grade three annular tear. (see may pageson Annular Tears for more information.) Amazingly, the sinuvertebral nerve alsoinnervates (connects to) the disc above and below! So, the sinuvertebral nerve of theL4 disc also innervates the L5 and L3 disc. This may help explain why a L4 discherniation/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 bloodvessels (not shown) of the epidural space. Sympathetic nerves control how the bloodvessels function (vasomotor & vasosensory). Although rare, injury to thesesympathetic nerves may cause RSD symptoms (now called CRPS) in the patientslower limbs; this usually would occur following surgery. (This may explain why Ihad a very slight case of RSD in my left foot following surgery - since my doc spentan 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 thesensory pathway from the sinuvertebral nerves into the spinal cord, does NOT takethe '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 DRGat the SAME level. Other, more recent research has indicated that pain-signals travelfrom 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 alwaysre-enter at the same vertebral level, I present the 2004 randomizedcontrolled investigation by Oh and shim (26). In an incredibly well designed

outcome study, the latter investigators demonstrated that by 'cutting' (RFneurotomy) the Gray Ramus Communicans, the majority of chronicdiscogenic pain sufferers achieved substantial relief of their pain and avoidfusion surgery. (26) This proves that at least some of the incoming painsignals were traveling toward the sympathetic trunk (which is on the anterior side of the vertebra) and NOT re-entering the spinal nerves at the samelevel.

A Committee Error?Another interesting oddity about thedesign of the nervous system is thefact that 'the committee' decided to

put the delicate sensory nerve cell bodies within the IVF and NOT thewithin spinal cord, which is wherethe 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 bydiscal material and/or bony outgrowth (stenosis). The placing of these sensory nervecell 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 damagethe 'brain of the nerve fiber' (nerve cell body) the nerve's chances of recovery aremuch less. This explains why patients often never recover completely from thattingling 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)] ismade up of three distinct areas: 1)The nucleus pulposus (green), which is awater 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 thefibrous outer portions of the disc that is madeup of type I collagen; and 3) The vertebral

end-plates (yellow), which are cartilaginous plates that attach the discs tothe vertebrae and supply food (nutrients) to the inner 2/3 of the annulus andentire nucleus pulposus.

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

Disc Physiology 101:

The normal human intervertebraldisc, which is considered the largest

avascular structure in the human body, is made up of two maincomponents, proteoglycan andcollagen (type I and type II). Theannulus is mostly made of collagen,which is a tough fibrous tissuesimilar to the cartilage that is foundin the knee, and the nucleus is mademostly of proteoglycan.Proteoglycans, which are produced by disc cells that resemblechondrocytes, are extremelyimportant 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 arecomprised 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

http://www.chirogeek.com/001_Degenerative-Disc-Disease.htm






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 strengthsand pliability, in fact a well hydrated disc is often even stronger than the bonyvertebral body. Fig. #3: Here we have the healthy disc of a teenager (cadaver). Thewater 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; thisis especially true of the nucleus. A well hydrated (with water) disc is bothstrong and pliable. The nucleus pulposus needs to be strong and wellhydrated 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 thebody. With an undamaged annulus, strongly corralling a fully hydratednucleus, the disc can easily support even the heaviest of bodies! As thedisc dehydrates (loses water) the disc loose ability to support the axial loadof the body (loses hydrostatic pressure); this causes a 'weight bearing shift'

from the nucleus, outward, onto the annulus, outer vertebral body, andzygapophyseal joints (facets). Now, we have an 'over-load' on the annulus(which may trigger other destructive biochemical reactions) which, if severeand/or is imposed upon a genetically inferior annulus, will result inpathological DDD. ( see below)

Hydration also is important with respect to disc nutrition. As we havealready mentioned, nutrients (which all living tissue needs in order to survive)must diffuse (soak) through the discal tissue in order to reach the hungrydisc cells. This diffusion process is much faster and easier IF the diffusingtissue has a high water content. We may use 'swimming' as an analogy: It'seasier to swim through the water, than through the sands of a desert. Thesands 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 anormally functioning spine and well feed disc.

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

Water is held within the disc by tiny sponge-like molecules calledproteoglycan aggrecans. These 'super sponges' have an amazing ability toattract and hold water molecules (324), and can in fact hold over 500 timestheir own weight in water; this gives the non-dehydrated disc thetremendous 'hydrostatic pressure' which is needed to bear the axial load of the body. Amazingly, the aggrecans water absorption is so powerful thatover night (non-axial loading) the height of the disc and the body will actually

measurably increase due to the discs engorgement with water. Thisphenomenon 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 littlefactories, they create, replace and rebuild aggrecan molecules. As long asthe disc cells have food (glucose), building material (amino acids) andoxygen 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 diffusedout of the disc the same way nutrients diffuse in. In the living disc up to 100aggrecans combine on a long piece of 'hyaluronan acid' to form giantproteoglycan aggregate molecules. It's these aggregates that are found

within the disc in the real world.

Disc Nutrition:



The intervertebral disc is thelargest avascular structure in thehuman body. The reason for thisis because it has no direct bloodsupply like most other bodytissue. Nutrients (food) for the

disc are found within tinycapillary beds (black arrows) thatare in the subchondral bone, justabove the vertebral end-plates .This subchondral vascular network 'feeds' the disc cells of the all important nucleus andinner annulus through thediffusion process. The Figure onthe left shows the 'disc feedingsetup' for disc. Note that the outer annulus has its oven blood supply

that is embedded within the veryouter annulus. This is a muchmore efficient system andnutrients 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 hasindicated 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 nucleusand inner annulus has a much longer diffusion route that is block by the vertebralend-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 nutrientsmust '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 anddegenerates so early in life. (Read my Disc Degeneration page for moreinformation.)

The 'diffusion feeding process' is enhanced somewhat by a phenomenacalled 'Diurnal Change'. Our discs have the ability to expand and compressover the course of a day. As we start the day our discs, like squeezing out asponge, will compress and dehydrate because of the gravity and physicalactivity 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 to25mm (194,441,815). As we sleep and decompress our spines, our discsswell with water plus nutrients and expand back to their fully hydratedstate. This tide-like movement of fluids in and out of the disc will help with themovement of nutrients into the avascular center of the disc. (Click here tolearn 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 mainmake-up is water (80%). Its solid/dry component make-up are proteoglycan

(65%), type II collagen fiber (17%) and a small amount of elastinfibers . 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 mostimportant 'arrangement' is called aproteoglycanaggrecan. These aggrecans main function is to trap and hold water, which iswhat gives the nucleus its strength and resiliency. Like a 'super sponge', aggrecanscan trap and hold over 500 times their weight in water!

The nucleus has two functions. The first is to bear most of the tremendousaxial 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 weightbaring potential. In order for proper weight bearing the nucleus and theannulus MUST work hand in hand.

The Annulus Fibrosis:

The annulus is the outer portion of the discthat 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 encirclethe 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 delicatespinal cord, and nerve roots.

The biochemical make up is similar to that of the disc only in differentproportions. The annulus is 65% water, with the collagen, both type I and IImaking 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 intothe vertebral end-plate.

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

A simply amazing fact about the lamellae design is that the collagen fibersthat make-up each lamellae all run parallel at a 65 degree angle to thesagittal plane. Even more amazing is the fact that the each lamellae areflipped so that the 65 degree angle alternates between every lamellae, oneto 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 thelayers of the annulus. This is just amazing if you think about it!! Brilliantdesign!

The function of the annulus is to help the nucleus support the axial weightfrom 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 itoutward which keeps the lamellae from collapsing inward. The nucleus mustkeep a very high hydrostatic pressure to achieve this. We saw what happenswhen the nucleus losses hydrostatic pressure under the 'Disc Degeneration'page. Bogduk used the analogy of a rolled up telephone book standing onend, 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 thephone book our 'on end phone book' it would not longer be able to supportmuch axial loading.

disc anatomy

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