Chapter 20

1. Chapter 20 The Intervertebral Disk

2. Overview • The IVD forms a symphysis or amphiarthrosis between two adjacent vertebrae • Represents the largest avascular structure in the body • In the human spinal column, the combined heights of the IVD accounts for approximately 20-33% of the total length of the spinal column

3. Overview • Intervertebral discs are able to distribute compressive stress evenly between adjacent vertebrae because the NP and inner AF act like a pressurized fluid, in which the pressure does not vary with location or direction • The biomechanical studies of the IVD seem to indicate that the disc acts to provide flexibility at low loads, and stability at high loads

4. Anatomy • Lumbar Disk • The lumbar disc is approximately cylindrical, its shape being determined by the integrity of the annulus fibrosis (AF) • The AF consists of approximately 10-12 (often as many as 15-25) concentric sheets of predominantly type I collagen tissue bound together by proteoglycan gel • The number of annular layers decreases with age, but there is a gradual thickening of the remaining layers • The fibers of each successive sheet or lamella maintain the same inclination of 65º but in the opposite direction to the preceding lamella, resulting in every second sheet having the same orientation

5. Anatomy • Lumbar Disk • Although the posterior aspect of the IVD is thinner, the collagen is more tightly packed than it is anteriorly • Consequently, the posterior part of the annulus will have thinner but stronger fibers, and it is capable of withstanding tension applied to this area during flexion activities and postures which occur more frequently than with extension • However, due to the predominance of flexion activities in life, fatigue damage may occur in the posterior aspect of the disc, making it a common site of injury

6. Anatomy • Lumbar Disk • With the exception of early youth, there is no clear boundary between the NP and AF, and it resembles a transitional zone • The biomechanical make up of the NP is similar to that of the AF, except that the NP contains mostly type II collagen, as opposed to type I

7. Anatomy • Lumbar Disk • Each vertebral end plate consists of a layer of hyaline and fibrocartilage about 0.6 to 1 millimeter thick, which covers the top or bottom aspects of the disc, and separates the disc from the adjacent vertebral body • The two end plates of each disc, therefore, cover the NP in its entirety, but fail to cover the entire extent of the AF

8. Anatomy • Lumbar Disk • The outer half of the IVD, the posterior longitudinal ligament, and the dura are innervated by the sinuvertebral nerve, which is considered to arise from the ventral ramus and the sympathetic trunk

9. Biomechanics • Lumbar Disk • Although the lumbar IVD appears destined for tissue regression and destruction, it remains unclear why similar age-related changes remain asymptomatic in one individual, yet may cause severe low back pain in others • The basic changes that influence the responses of the disc to aging appear to be biochemical, and may concern the collagen content levels in the NP.

10. Pathology • Lumbar disk • Three main types of lumbar disc herniation are recognized: • Contained herniation (protrusion) • With this type, the nuclear material bulges outwards through the tear to strain, but not escape from, the outer AF and/or the posterior longitudinal ligament • Extrusion (prolapse) • The nuclear material remains attached to the disc, but escapes the AF and/or the posterior longitudinal ligament to bulge posterior-laterally into the intervertebral canal

11. Pathology • Lumbar disk • Sequestration • The migrating nuclear material escapes contact with the disc entirely, and becomes a free fragment in the intervertebral canal

12. Pathology • Nerve Compression • Mechanical compression of the nerve root alone does not explain sciatic pain and radiculopathy • Recent models of lumbar radiculopathy suggest that the underlying mechanisms are probably due, in part, to a local chemical irritant such as proteoglycans released from a disc creating an inflammatory reaction, an autoimmune reaction from exposure to disc tissues, or an increased concentration of lactic acid, and/or a lower pH around the nerve roots

13. Pathology • Specific Lumbar Disc Lesions • At the L 1 and L 2 levels, the nerves exit the intervertebral foramen above the disc. From L 2 downward, the nerves leave the dura slightly more proximally than the foramen through which they pass, and at a decreasing angle of obliquity, and an increasing length within the spinal canal

14. Pathology • High Lumbar Disc Lesions • The high lumbar radiculopathy does not typically radiate pain down the back of the leg, but instead causes an insidious onset of pain in the groin or anterior thigh, which is often relieved in a flexed position and worsens with standing • The superficial cremasteric reflex is also invariably present

15. Pathology • Third Lumbar Nerve Root • The L 3 nerve root travels behind the inferior aspect of the vertebral body and the L 3 disc • Clinical findings may include: • Symptoms in the mid lumbar, upper buttock, whole anterior thigh and knee, medial knee, and just above the ankle • Slight weakness of iliopsoas, grosser loss of quadriceps

16. Pathology • Fourth Lumbar Nerve Root • About 40% of IVD impairments affect this level, about an equal amount as those that affect the L 5 root. • Clinical findings may include: • Symptoms located in the lumbar area or iliac crest, inner buttock, outer thigh and leg, and over the foot to the great toe • Weak dorsi-flexion

17. Pathology • Fifth lumbar nerve root • Frequently compressed by the L 4-5 disc as well as the L5-S1 disc • Clinical findings may include: • Pain in the sacroiliac area, lower buttock, lateral thigh and leg, inner 3 toes and medial sole of the foot • Weakness of peroneal muscles, extensor hallucis and hip abductor muscles

18. Pathology • 1st, 2nd and 3rd sacral nerve roots • Can be compressed by a fifth lumbar disc protrusion • The clinical findings with a lesion at the S1 level may include: • Pain in the low back to buttocks to sole of foot and heel • Weakness of the calf muscles, peronei, and hamstrings

19. Pathology • 4th sacral nerve root • A lesion of this nerve root is always a concern as a permanent palsy may lead to incontinence and impotence • Clinical findings may include: • Pain in the lower sacral, peroneal and genital areas • Saddle paresthesia • Bladder, bowel and/or genital dysfunction

20. Pathology • Schmorl's node • A herniation of disc substance through the cartilaginous vertebral end plate of the IVD into the body of the adjacent vertebra

21. Examination • The conventional physical examination for a suspected disc herniation consists of tests for strength and range of motion, reflex, and sensory testing, and dural mobility tests such as the SLR test • It must be remembered that no single test in the physical examination has a high diagnostic accuracy alone for disc herniation

22. Anatomy • Cervical Disk • In the cervical spine, there are five discs, with the first disc located between C 2 and C 3 • The cervical discs are named after the vertebra above (the C 4 disc lies between C 4 and C 5) • The IVD height to body height ratio (2:5) is greatest in the cervical spine, and the intervertebral discs make up approximately 25% of the superior-to-inferior height of the cervical spine

23. Anatomy • Cervical Disk • Anteriorly, the cervical AF consists of interwoven, alar fibers, whereas posteriorly, the AF lacks any oblique fibers, and consists exclusively of vertically orientated fibers • In no region of the cervical AF, do successive lamellae exhibit alternating orientations • Protection against disc herniation is afforded by the uncovertebral joints

24. Anatomy • Cervical Disk • As in the lumbar spine, the cervical IVD functions as a closed but dynamic system, distributing the changes in pressure equally to all components of the container, i.e., the end plates and the AF, and across the surface of the vertebral body

25. Anatomy • Cervical Disk • It has been observed that in the first and second decades of life, before complete ossification occurs, lateral tears occur in the annulus fibrosus, most probably induced by motion of the cervical spine in the bipedal posture • The tears in the lateral part of the disc tend to enlarge toward the medial aspect of the intervertebral disc • The development of such tears through both sides may result in a complete transverse splitting of the disc • Such a process can be observed in the second and third decades of life in the lower cervical spine when the intervertebral disc is split in the middle into equal halves • With this aging process, the NP rapidly undergoes fibrosis such that, by the third decade, there is barely any nuclear material distinguishable

26. Pathology • Cervical disk • When considering cervical IVD, it is clear that the pathology affecting the cervical IVD is different from that affecting the lumbar disc

27. Pathology • Cervical disk • Almost everyone older than 40 years of age has evidence of cervical disc degeneration • According to Töndury and Theiler, the NP usually dries out in the fourth and fifth decades of life and acute extrusion is not expected then

28. Pathology • Cervical disk • Anteriorly, compression of the nerve roots is likely caused by protruding discs and osteophytes of the uncovertebral region, whereas the superior articular process, the ligamentum flavum, and the periradicular fibrous tissues often affect the nerve posteriorly

29. Pathology • Cervical disk • Considering the structure of the cervical AF, the possibilities that emerge for mechanisms of discogenic pain are strain or tears of the anterior AF, particularly after hyperextension trauma, and strain of the alar portions of the posterior longitudinal ligament when stretched by a bulging disc

30. Pathology • Cervical disk • In the lumbar disc, a prolapse is common. In the cervical spine, a straightforward prolapse is uncommon, and a cervical disc herniation should not be considered as a miniature version of lumbar disc herniation • Acute disc herniations may result in compression of nerve roots. Cervical discs may become painful as part of the degenerative cascade, from repetitive microtrauma, or from an excessive single load

31. Pathology • Cervical disk • The most common level of cervical nerve root involvement has been reported at the seventh (C7, 60%) and sixth (C6, 25%), followed by the C4‑C5 disc

32. Examination • As with the lumbar spine, the conventional physical examination for a suspected cervical disk herniation consists of tests for strength and range of motion, reflex, and sensory testing, and dural mobility tests

33. Anatomy • Thoracic disk • Thoracic disks have been poorly researched. They are narrower and flatter than those in the cervical and lumbar spine, and contribute approximately one-sixth of the length of the thoracic column • Disk size in the thoracic region gradually increases from superior to inferior • The disk height to body height ratio is 1:5, compared to 2:5 in the cervical spine, and 1:3 in the lumbar spine, making it the smallest ratio in the spine, and affording the least amount of motion

34. Anatomy • Thoracic disk • In the thoracic spine, the segmental nerve roots are situated mainly behind the inferior-posterior aspect of the upper vertebral body rather than behind the disk, which reduces the possibility of root compression in impairments of the thoracic disk

35. Anatomy • Thoracic disk • In contrast to the cervical and lumbar regions, where the spinal canal is triangular/oval in cross section and offers a large lateral excursion to the nerve roots, the mid thoracic spinal canal is small and circular, becoming triangular at the upper and lower levels • At the levels of T 4 through to T 9 the canal is at its narrowest

36. Pathology • Thoracic disk • Herniated disks have been found at every level of the thoracic spine, although they are more common in the lower thoracic spine • The intra-spinal course of the upper thoracic nerve root is almost horizontal (as in the cervical spine). Therefore, the nerve can only be compressed by its corresponding disk. More inferiorly in the spine though, the course of the nerve root becomes more oblique, and the lowest thoracic nerve roots can be compressed by disk impairments of two consecutive levels (T 12 root by 11th or 12th disk)

37. Examination • Thoracic disk • The clinical manifestations of thoracic disk herniation are extremely variable and vague. This often results in long delays between presentation and diagnosis