Chapter 2 The intervertebral disc - ASNR



Chapter 2 The intervertebral disc.

Slide 1) This segment will define the normal anatomy and Pathophysiology of the intervertebral disc.

The motion segment 1

Slide 2 ) The spine is made up of a chain of vertebra with intervening discs. Two adjacent vertebra with their disc, ligaments and muscles is called a motion segment. This is diagramed by this fist set of images. The artist has depicted the intervertebral disc as a compression spring. This acts as a mobile cushion between the two vertebral bodies. Like the spring, he disc is compressible and deformable allowing the vertebra to bear weight and permit motion. The fulcrum of flexion is the center of the zygapophiseal joints. The posterior ligaments and muscles act to keep the body erect and prevent hyperflexion of the spine.

The motion segment 2

Slide 3) The intervertebral disc is depicted as being made up of two discrete structures, the nucleus pulposis and the annulus fibrosis. This is an oversimplification. In very young children the nucleus is definable as a structure differentiable from the inner annular fibers. This differentiation is all but lost in mature adults. The nucleus, although an avascular structure in adults, is a living organ made up of cells, fibers and Jelly-like matrix. The only real difference between the nucleus and the annulus is the relative amount of jelly to fiber. The nucleus is primarily jelly with a small amount of fiber and the annulus is primarily fiber with little jelly. The outer annular fibers are arranges as concentric crisscrossing bands similar in construction to the steal belts of a radial tire. The nucleus is seen like a jelly bean confined in the center of the fibrous annulus.

The motion segment 3

Slide 4) I like to describe the motion segment in terms of a jelly doughnut. The raspberry jelly is similar to the young nucleus and the crust represents the outer annular fibers which are dry and tough. The inner annular fibers are like the dough more fibers than soft material but not as tough as the crust. Just like the disc the jelly doughnut is deformable.

The very young motion segment

Slide 5) The anatomic specimen with its corresponding radiograph depicted here from Hadley’s wonderful book, is of a young child. The ossified portion of the vertebra, which is very obvious on the radiograph, is about half of the total volume of the developing bone. The endplates are not definable as discrete structures. The nucleus is seen as a discrete structure surrounded by the concentric annular fibers.

An older child

Slide 6) In this specimen from an older child, most of the vertebral body is ossified. The growing endplates are still cartilage. The square anterior white zones represent the outer apophyseal ring, which is the last portion of the maturing vertebra to ossify.

8 year old child

Slide 7) A close up micrograph of the anterior, superior edge of the vertebral body of an 8 year old child aloows one to differentiate the anterior longitudinal ligament, The ossified vertebra and the cartilaginous end plate and apophyseal ring.

Radiograph of 8 year old child

Slide 8) A radiograph of the previous specimen shows the rounded edges of the developing vertebral body.

Micrograph of the disc

Slide 9) This micrograph of the young developing disc, shows numerous blood vessels intermixed with the circumferential fibers. This rich vascular supply to the growing disc disappears with increasing age. In the mature adult disc oxygen and nutrients diffuse into the disc from the tissue adjacent to the outer annulus and the vertebral endplates. This diffusion is, for the most part a passive phenomenon. In the morning when we awake our discs are maximally hydrated. As we assume the upright position with the bodies weight born by the discs, the intradiscal pressure increases and water and the waste products of metabolism defuse out of the matrix into the vascular space at the borders of the disc. When we go to sleep at night the intradiscal pressure decreases and fluid flows into the disc bringing along with it oxygen and nutrients. This diurnal flow has been documented on MR experiments where the water flow in and out of the disc has been measured. We are actually taller in the morning than we are at night because of this process.

The adult disc

Slide 10) A micrograph from an adult disc shows that the vessels have disappeared.

The adult disc

Slide 11) Compare the two side by side..

Schmorl’s book

Slide 12) Georg Schmorl, one of the greatest spine reserchers ever, created a body of research in the first quarter of the 20th century which was translated from the original German into English by Besemann. This marvelous old book is required reading for anyone who truly desires to understand the spine. Some of the material may be dated, but the basic science is wonderful. I got my copy from the Barnes & Nobel dot com out of print book section. It is truly a treasure, which occasionally comes up for sale. It is still available in some good medical libraries.

Hadley’s book

Slide 13) Another wonderful old book that I consider required reading, is shown here. It is a pictorial marvel.

13 year old girl

Slide 14) A micrograph and corresponding specimen radiograph on a 13 year old child shows beginning ossification of the ring apophysis.

Cadaver disc

Slide 15) This is a photograph of a slice through the disc of a fresh frozen cadaver. Is it Normal or not?

Normal Disc Sagittal view

Slide 16) This is a cryomicrotome mid sagittal section of the lumbar spine showing a totally normal disc. There is no discoloration, no fissures or tears.

Normal sagittal view

Slide 17) This is the complete photo of the spine from which the normal disc was extracted. Note that the outer annular fibers of the disc insert several mm. above and below the vertebral endplates.

Normal disc axial plane

Slide 20) Anatomic section and CT scan through a normal disc. Note the avascular disc with smooth outer annulus. Note that this totally normal disc with no fissures, tears or discoloration appears to bulge slightly in the midline.

Slide 21 no text

Slide 22) All aging discs undergo a definable cascade of changes. The earliest changes are chemical. The matrix of the disc looses its affinity for water and the dehydration process begins. Little cracks and fissures occur within the aging disc. This process was elegantly described by Yu and his colleagues

Slide 23) This is a photo from Yu et al from Radiology showing the types of annular tears that occur in the normal aging disc.

Slide 24) Three sagittal MR scans herein depict the spectrum of aging .of the disc from normal to dehydration.

Slide 25) The normal aging process is said to end with the progression of small annular fissures into a fissure or tear that radiates from the center of the disc to the periphery. This is called a radial tear or fissure.

Slide 26) Again from Yu et al diagrams depicting the progression of annular fissures leading to a narrow collapsed disc.

Slide 27) MR and photograph of early degeneration of the disc. The homogeneously darkened dehydrated disc must have an annular fissure even if we don’t see it on MR.

Slide 28) The dehydrating disc has lost some of its volume. Since the annular fibers are not elastic, the weight of the body pressing down on the disc causes the annulus to bulge outward. Diffuse annular bulge is always due to normal aging and degeneration of the disc, it is never caused by a single cataclysmic injury.

Slide 29) These photographs from Yu et al show a somewhat later stage of degeneration. Note the discoloration of the center of the disc and the fissured annulus. Axial scan of the specimen shows the diffuse annular bulge.

Slide 30) This cryomicrotome section of a degenerated L5/S1 disc demonstrates a dark linear fissure within the disc extending to the outer annular fibers. This is the classic annular “tear”. Discogram performed by Schmorl, show the flow of opaque dye freely through the maze of intra discal channels in the degenerated disc. Note the high intensity zone in the posterior annulus on the MR scan on the right. HIZ indicates abnormal tissue within an annular fissure. Seeing an HIZ has no clinical value in the assessment of back pain. The disc bulges diffusely into the spinal canal.

Slide 31) Axial scan shows a broad high intensity zone in the posterior annulus. A cryomicrotome section from Yu’s work shows the pathological correlate. Note the central annular fissure and the discoloration in the central portion of the posterior annulus.

Slide 32) It was suggested in the early 1990’s, that visualization of a HIZ in the posterior annulus might correlate with a painful disc. Early studies showed that the incidence of HIZ on MR scans in patients with painful discograms was high. It was suggested therefore, that seeing an HIZ might be a predictor of the painful disc. Subsequent literature including the papers depicted here, proved that up to 50 % of asymptomatic people had HIZ’s on their MR scans.

Slide 33) Provocative discography is a very controversial procedure. It entails placing a small needle into the nucleus pulposis and injecting some contrast agent into the nucleus. The proponents of discography believe that if the patient experiences concordant severe pain from the injection, that suggests that the disc is a pain generator and the likely cause of back pain. Contrast injection is performed to assure that the fluid being injected is going into the nucleus and not the outer annulus. The pain response is totally invalid if the injection is not within the nucleus. On a normal Discogram the contrast material should be confined to the nucleus with no visible annular fissuring.

Slide 34) This is an abnormal Discogram. The contrast agent diffuses along the annular fissures and may actually flow out beyond the confines of the outer annulus. The existence of annular fissure on discogram means that the patient has degenerative disc disease. It does not indicate injury to the disc and does not mean that the patient is hurting from that disc. By definition since all discs which appear dark on T2 weighted MR scans are degenerated, they will have annular fissures whether we see them or not.

Slide 33) Diffuse bulging of the disc can be defined as a concentric expansion of the annulus with extension beyond the bony borders of the disc. It is always due to the aging process.

Slide 34) It is very important that one not measure the size of an annular bulge from an axial image. It is imperative that estimation of bulge size be made only on sagittal reformations of CT scans or on sagittal MR.

Slide 35) A ruler has been placed over the spinal canal and disc on the axial CT section. Note that the posterior edge of the vertebral bodies indicated by arrows is several mm’s anterior to the gray outer surface of the disc marked in red. It appears that the disc is bulging 2mm beyond the bony border of the vertebra. This is not actually the case.

Slide 36) Assume that the axial scan was performed along the plane of the red line superimposed on the normal disc in the sagittal specimen. You will note that the annulus does not insert into the edge of the vertebral endplate but several mm’s down from the edge. Because of this there is a small rim of normal disc material behind the bone. It is this crescent of normal disc which is seen on the axial view projecting behind the bone. Since one can never tell where the bone actually ends with respect to the annulus in this view, measurements of bulge size will be inaccurate.

Slide 37) Measurements must always be made on the sagittal views where it is clear where the bone ends with respect to the disc. It would be appropriate to measure the L4/5 disc as being 3mm. on the sagittal view.

Slide 38) As the disc dries out and as more fissures form, the weight of the body compressed the disc and the disc space height decreases. During this process, the outer sharpy’s fibers may stretch or tear at their insertion into the bone. The body reacts to this process by the build up of new bone.

Slide 39) These photos from Schmorl’s book show the effect of stretching or tearing of the outer annulus at the bony insertion. Note the upward projecting osteophyte.

Slide 40) Cryomicrotome section of a spine with end stage degenerative disc disease with bony ridging. Note the annular tear centrally. The residual outer annular fibers project slightly beyond the posterior edge of the osteophyte.

Slide 41) The medullary bone may also respond to the adjacent degenerating disc. Modic and the group from the Cleveland Clinic descried these anatomical change as seen on MR scans. Type I endplates are low in signal on T1 images and bright on T2 images. This represents Fibrovascular replacement of normal bone marrow.

Slide 42) Type II changes are due to increased fat deposition in the medullary bone. The bone adjacent to the disc is therefore bright on T1 weighted views.

Out of sinc

Slide 45 MODIC III

Slide 46) The presence, and position of annular fissures define the pattern of disc degeneration. The shape of the vertebral body determines the pattern of annular fissuring.

Slide 47) There are two basic variations of annular fissures of the posterior disc., central annular fissures and lateral annular fissures. The position of the fissure is dependent on the shape and size of the vertebral endplates.

Slide 48) Farfan in his wonderful book describes the anatomy of central annular fissures with and without nuclear migration as it relates to the shape of the posterior portion of the disc. The three discs depicted here show the three typical shapes; concave, flat and convex.

Slide 49) From chapter 7 in Farfan’s text are examples of a concave disc with lateral annular fissure and convex disc with central annular fissure.

Slide 50) Further in Farfan’s book The upper photo is of an L2 disc with central discoloration in a degenerated nucleus. There are concentric fissures but no radial tear.. The disc at L3 shows a central annular fissure with posterior migration of a nuclear fragment. The L4 disc contains a central annular fissure without internal migration of a nuclear fragment.

51) The vertebrae depicted here have posterior borders that are convex towards the spinal canal. The intervening discs must also have convex shape since they must attach from one convex vertebra to its next mate. The lower photo is of a normal convex disc.

52) The upper vertebra seen here has a relatively flat but convex posterior surface. The lower vertebra has a concave posterior border. The adjacent discs demonstrate the comparable shapes. The upper disc has a convex posterior edge and the lower disc a concave posterior edge.

53) The relative AP diameter of two adjacent vertebrae also plays a part in the shape of the intervening disc and the pattern of degeneration. When the AP diameters of two adjacent vertebrae are the same the disc space can be termed congruent. By that I mean that the anterior and posterior ends of the vertebral end plates align with one another. The MR scan depicted here shows a congruent disc. The length of the vertebral endplates is shown to be the same. Note also that the posterior edges of the two bones shown on the axial view are of the same shape.

Slide 54) Spines that have vertebral endplates of different lengths as seen here can be called incongruent.

Slide 55) Note that the intervening disc appears to protrude into the spinal canal. This must be the case since the disc has to connect the posterior ends of the vertebrae. According to Farfan’s anatomical dissections, motion segments with incongruent endplates develop central annular fissures and disc protrusions.

Slide 56) The patient in this set of MR scans has markedly incongruent endplates and prominent central disc protrusion.

Slide 57) This is a drawing in the axial plane demonstrating incongruent endplates due to different vertebral shapes. It is obvious that the disc which much attach the two bones, must appear to bulge with respect to the shorter one.

Close up disc

Slide 55) This is a specimen shown earlier. The disc is completely normal with no cracks, tears or protrusions. The disc forms a smooth sinusoidal curve continuing with the posterior edge of the L5 body.

Slide 56) If you look only at L5, you will note that there is no apparent disc bulge. All discs are round posteriorly as we see here. ( It is impossible to bake square Jelly doughnuts) The disc does not appear to protrude beyond the L5 body.

Normal disc sagittal view

Slide 57) I have now covered the L5 vertebral body. Observe the relationship of the disc to the short superior end plate of S1 It appears that there is 5 mm. of disc herniation This is an obvious paradox. We can’t have a disc be normal with respect to one adjacent vertebra and not its mate. As mentioned earlier this anatomic variant occurs in at least 10% of normal people.

Slide 58) Another look at what a disc might look like with a central annular tear in an incongruent disc. This is the pattern of degeneration and hence herniation seen with incongruent or convex motions segments.

Slide 59) The pattern of annular fissure and herniation is different in patients with concave posterior vertebral edges. As seen earlier from Farfan’s work, when the discs are concave, the annular fissures form laterally. There may be bilateral matching lateral tears. See here an example of a vertebra with a normal concave disc.

Slide 60) This sagittal cryomicrotome section was performed near the entrance to the neural foramen. It demonstrates a classical lateral annular fissure.

Slide 61) What is the most likely diagnosis here. What do you believe the posterior border of the vertebra will look like. Choose one of the two choices.

Slide 62) This patient had pain radiating across his lateral thigh above the knee in the distribution of the L3 nerve root. This is a foraminal L3 disc herniation. As previously mentioned lateral disc herniations occur in patient s with concave discs.

Slide 63) Here is a complete set of CT images including sagittal and curved coronal reformations. The lateral disc herniation fills the neural foramen. Note that just as we predicted, the disc is concave.

Slide 64) What do you think is the most likely diagnosis?

Slide 65) This patient has an extraforaminal disc herniation. The posterior vertebral edge is very slightly concave, almost flat. It is surely not convex. Consequently the annular tear has occurred laterally. In this case lateral to the neural foramen.

Slide 66) What do you think is the most likely diagnosis here? After you have thought about it for a while, click to see the actual report of this study. Do you agree with it?

Slide 67) The reading radiologist called this a soft tissue disc herniation. The CT scan clearly shows a large osteophytic ridge attached to the protruding disc. If a surgeon attempted a minimally invasive or percutaneous disc procedure to remove the disc his surgery surly would have failed. Remember that bony ridges may be hard to see on MR. Missing them can cause serious repercussions.

Slide 68) This is a very easy question. Only the answer is very difficult.

Slide 69) According to the position paper on definitions published by NASS and the ASSR, disc herniation is a focal extension of the border of the disc beyond its normal position regardless of how it got there or what is made up of.

Slide 70) Unfortunately this definition does not conform to the pathologic diagnosis of herniated nucleus pulposis which is; external migration of a fragment of nucleus pulposis along the path of a radial fissure with extension through the outer annulus. This creates a major problem for radiologist, surgeons as well as lawyers and insurance adjusters.

Slide 71) I believe it is more important to be anatomically correct when defining disc herniation. Surgeons need to know when bone rather than nuclear material is outside the disc. The presence of bone attached to a protruding disc nearly always indicates chronicity. This is important to insurance adjusters and lawyers.

Slide 72) It is important to distinguish acute from chronic disc herniation. Therefore we must define both processes.

Slide 73) A chronic disc herniation fits the NASS definition of disc herniation very well. It is the extension of nuclear material beyond the confines of the disc. The only way we know it is there is by seeing it on an imaging study. Since it is found in a large portion of the normal population one can never presume that just because we see it on our study, it is the cause of back pain. Similarly it is irrelevant if there is attached endplate or osteophyte.

Slide 74) What are the symptoms of acute disc herniation?

Slide 75) Rothman’s simpleminded clinical triad.

Slide 76) It certainly seems like it would be useful to differentiate an acute disc herniation which may well be associated with back pain from a chronic disc herniation which we can never say is the cause of back pain. Can it be done from our imaging studies?

Slide 77) An acute disc herniation is very much like the extension of the raspberry jelly out of this doughnut. If you bang on the doughnut out comes the jelly.

Slide 78) T2 weighted study would show high water content bright jelly sticking out of the hole in the doughnut.

Slide 79) This is the MR scan of a patient who developed the signs of acute disc herniation. Note that fragment is bright in signal.

Slide 80) The patient here has multilevel disease. There is a large disc herniation at L4/5 and a small protrusion at L3/4. Note that the diffuse disc bulge at L5/S1 is darker than the disc from which it comes. The disc protrusion at L3/4 is the same signal as the disc from which it comes. The Acute disc herniation at L4/5 is brighter than the disc frm which it comes.

Slide 81) Although it is rarely done, contrast enhancement is sometimes useful in distinguishing acute from chronic disc herniation. The center of the nuclear fragment will not enhance but the will often be an inflammatory pseudo capsule around the fragment which is hypervascular and contains an abundance of white cells. The pre enhancement scan is on the left and the post enhancement scan on the right.

Slide 82) This patient has modest degenerative disc disease. There is a transitional disc at L5/S1 with degenerative narrowing of the first mobile disc at the level above. Note that the protruding disc id dark on the MR scan. This is the hallmark of the chronically protruding disc. The nuclear material has decreased water content and is dried like the doughnut that was left standing for several years.

Slide 83) Chronic disc protrusion is extremely common in the population of asymptomatic people. In 1990 Boden et al. published this table indicating the incidence of bulging, protruding and herniated discs in asymptomatic people. You will note that the incidence of herniation is not less than 20% in any post teen age group, and the incidence of disc bulge at least 50%.

Slide 84) This series of photographs show the cascade of events leading to chronic disc protrusion. In the lower left image there is a normal nucleus at L5/S1. The nucleus is a gel. It therefore behaves like a liquid. Liquids can not be compressed but they may be deformed. As the body undergoes the full range of normal motion the nucleus deforms. As it dries the deformity becomes fixed and the dried fragment is free to wander along the plane of any annular fissures that have developed. These fragments may extend to or through the outer annular fibers must move very slowly not causing symptoms, the protrusions coming to light only when the person finds himself in a scanner of some sort.

Slide 85) What is the most common symptom of a chronic disc herniation?

Slide 86) The overwhelming majority of patients with chronic disc herniation are asymptomatic. 20-30% of asymptomatic adults will have chronic disc protrusion. Some patients with degenerative disc disease do have symptoms of back pain, leg pain or both.

Slide 87) Does this patient have a chronic or an acute disc protrusion?

Slide 88) This is a typical chronic disc protrusion. The disc is dark and the protruding disc material is dark.

Slide 89) On CT scans it is often not possible to determine the age of a disc herniation. When a bubble of gas is seen in the spinal canal, however it is very likely that a chronic process is ongoing. The gas extends into the epidural space and is contained within the outer annulus.

Slide 90) Calcification in a nuclear fragment as seen here at L5/S1 also strongly suggests chronicity.

Slide 91) The patient shown here claimed the onset of severe back pain immediately following a September accident. The MR scan was performed in October. There is an obvious central disc herniation. Was it likely caused by the accident or not?

Slide 92) It was certainly not caused by the accident. The central disc herniation is low in signal. In fact the same signal as the disc from which it comes. That is the hallmark of a chronic disc herniation.

Slide 93) This patient actually had a previous episode of back pain with an MR scan done the previous February. This case demonstrates the natural history of extruded disc herniation. Left alone 70% of herniations will shrink on their own within3-6 months. Note the dramatic signal change in the fragment with time.

Slide 94) Why does the L5/S1 disc look strange here?

Slide 95) This patient has had a previous discectomy. The scan was performed after gadolinium contrast enhancement. Note characteristic enhancement in the surgical bed.

Slide 96) What is the most likely diagnosis in this last quiz Case?

Slide 97) This patient clearly as a low signal disc protrusion. There is, however a new, bright signal extrusion superimposed upon the chronic herniation. Note that on both sequences that the acute disc fragment is brighter than the disc from which it comes.

Slide insert 1) Thus far, we have been using the lumbar discs as examples. The cervical and thoracic discs behave in an identical manner. This is a CT scan on a patient with the acute onset of left C6 radiculopathy. Note the large soft tissue disc herniation on both axial and sagittal views. It is not possible to determine if this is recent or old.

Slide insert 2) We can say that this disc herniation is old even on the CT scan because of the large attached osteophytes. Remember that osteophytes form because of bulging of the attached disc. They take years to occur unless the disc is torn apart by a fracture through the disc.

Slide insert 3) Is this an acute herniation or not?

Slide insert 4) The signal within the herniated fragment is the same as the disc from which it comes. The disc herniation is therefore old.

Slide insert 5) What about this one, acute or chronic?

Slide insert 6) The signal in the herniated disc is bright indicating it is of recent origin.

Slide insert 7) What is the most likely diagnosis here? What are the eyes peering at us from the vertebra?

Slide insert 8) These are intrabody disc herniations described Schmorl in the beginning of the last century. Schmorl’s nodes are found in 80% of normal people. A single or several scattered Schmorl’s nodes are unimportant. However, when there are many Schmorl’s in the endplates of multiple discs, this should be considered diffuse end plate disease. The nutritive and supportive functions of the endplate fail.

Slide insert 9) This photograph from Schmorl’s book demonstrates diffuse endplate disease. It is thought that the multiple herniations occur through cartilaginous defects in the endplates caused by failure of the fetal blood vessel channels to disappear in early childhood.

Slide insert 10) The most severe form of endplate disease is called Scheuermann’s juvenile kyphosis. The weakened vertebral endplates fail to support the anterior portion of the vertebra and wedge deformity occurs.

Slide insert 11) What have we here?

Slide insert 12) These photographs, also from Schmorl’s book, demonstrate a common variant of the typical Schmorl’s node. The intrabody disc herniation occurs far anteriorly before the apophyseal ring fuses to the vertebral body. This usually occurs in teenagers. It is called an anterior limbus vertebra.

Slide insert 13 What is the most likely diagnosis here?

Slide insert 14) This is another common variant of Schmorl’s node called the posterior limbus vertebra. These typically occur at L5/S1 while anterior limbus vertebra usually occurs at L4 or L3. It is very difficult to make the diagnosis on regular x rays or MR. It is very easy on good reformatted CT. This is an important entity because the displaced boney ridge and the disc that displaced it may compress the exiting nerves and cause radiculopathy.

Slide insert 15) This patient with intrabody disc herniation has displaced the entire apophysis nearly a centimeter. There is remarkable central and recess stenosis here.

Slide 43) There is an increase in the fat content of the marrow space of the vertebra. There is high signal in the bone adjacent to the vertebral endplates. The signal is isointense, or slightly hyperintense in the bone adjacent to the vertebral endplates.

Slide 44) Type three changes are due to increased bone around the endplates and correlates with increased bone density on regular x rays. The bone is low in signal on both T1 and T2 images.

Slide 57) A series of 100 lumbar MRI scans were reviewed to determine the incidence of congruent end plates at L5/S1. Asymmetry of the L5 / S1 endplates was found in 20% of the reviewed cases.

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