Intervertebral Discs

Loads on the AF and NP

- Anulus Fibrosis withstands tensile loads - Nucleus Pulposus withstands compressive loads

Shape, consistency, and location of the nucleus pulposus in the IVD?

- Changes shape and location during different positions - - Anteriorly with extension and posteriorly with flexion - Either fluid or solid depending on loading - Responsible for absorbing most of the fluid.

How to maintain a healthy disc?

- Frequent changes in posture and position - Any prolonged standing or sitting (or lying down) will be detrimental - This is due to osmotic nature of the disc

Causes of disc herniation

- Genetic predisposition - Improper lifting (hyperflexion OR lateral flexion + fast compression) - Overuse (possibly) - Posture (possibly)

Vertebral End Plate composition

- Hyaline cartilage on vertebral body side - Fibrocartilage on disc side

Functions of IVDs

- Maintain changeable space between bodies - Adds flexibility to spine - Helps disperse compressive loads

Recurrent Meningeal n. (sinuvertebral n.) innervates what structures?

- Posterior Longitudinal L. - Periosteum of posterior vertebral body - Epidural venous plexus - Anterior surface of dura mater - posterior aspect of IVDs

Injured discs cause what type of pain?

- Primary source of pain - Also radicular pain - Discs have direct nociceptive innervaiton

Composition of IVDs

- Water - Cells (primarily fibroblasts and chondrocytes) - Proteoglycans - Type I and II collagen fibers

Disc Healing

1) *Inflammation* With the herniation of the disc the body response to invading nuclear material with cytokines and chemokines 2) *Neovascularization* 3) *Break down protruded material* - by macrophages - scar tissue is laid down This can take 6 months to 1 year to heal completely

Purpose of Vertebral End Plate

1) *Nutrition of the disc* - VEP is very porous and allows transfer of fluid from the vessels in the vertebral bodies into the NP and AF 2) *Helps keep the AF and NP in their anatomical positions* - and prevents pressure atrophy

3 Types of Disc Herniation

1) *Protrusion* - The NP has protruded through the inner layers of the AF 2) *Extrusion* - The NP has protruded through all the layers of the AF, but is still attached to the disc of origin 3) * Sequestration* - there is a free segment of disc in the epidural space which is free to migrate

What is different about cervical IVDs?

1) There are no lamellae in the AF 2) C-shaped 3) Thick anteriorly and thin posterolaterally

IVDs make up what percentage of the spinal column height?

20-33%

How many IVDs are found in the body?

24: - 6 cervical - 12 thoracic - 5 lumbar - 1 between sacrum and coccyx

What is the functional classification of the IVD joint?

Amphiarthrodial

Where is the Anulus Fibrosus the strongest?

Anterior aspect

Three regions of IVD (and functions)

Anulus fibrosus - tensile load Nucleus pulposus - compressive load Vertebral (cartilagenous) endplate

Annulus Fibrosus attaches where?***

At the periphery: - directly to the vertebral bodies (via Sharpey's fibers) Inner lamellae: - attach to the vertebral endplates

Where do you find IVDs?

Between vertebrae EXCEPT: - between the occiput and atlas - between the atlas and axis There is a small disc between sacrum and coccyx

How might the Vertebral End Plate be fractured?

By compressive load - also may fracture adjacent subchondral bone of the vertebral bodies - Can cause Schmorl's Nodes

What is the structural classification of the IVD joint?

Cartilagenous symphysis joint

Pressure within the Nucleus Pulposus

Changes depending on body position - When lying down, the intradiscal pressure (IDP) on the nucleus pulposus is only 20% what it is while standing - Lower in relaxed sitting vs. relaxed standing - Slouching slightly decreases IDP, while standing or sitting straight slightly increases IDP - After 7 hours of sleep the IDP increase 240%

Creep

Deformation of soft tissues, which occurs slowly over an extended period of time - While standing/sitting compressive forces are placed on the IVD forcing out water - damaged tissue will go through creep much faster (and recover slower)

Hysteresis

Deformation of tissue to short duration loading - helps to protect the spine during rapid loadings - decreases with repetitive loading which can lead to repetitive use injury (Vertebrae, IVDs, and other tissues deform slightly to absorb shock)

What is the name of the joint formed by IVD?

Interbody Joint

Symptoms of disc herniation

Large % are asymptomatic - Severe pain - Radiating pain (into arms or legs) - Paresthesias and weakness - Usually symptoms are unilateral

Schmorl's Nodes

Lesions caused by the nucleus pulposus rupturing through the vertebral end plate - causes movement, increased pressure, and weakening of the vertebrae and misalignment of the zygapophyseal joints

What is the embryological origin of the Nucleus Pulposus?

Notochord - Notochordal cells are replaced by fibrocartilage by 11 years.

Disc Herniation

Nucleus pulposus has pushed through layers of the anulus fibrosus

Where does disc herniation usually occur?

On the posterolateral side of the disc - (due to inherent weakness, and no support from the PLL) - Can cause radicular pain - May cause inflammatory cytokines

Where is the blood supply to the IVD?

Only in the most superficial part of the AF

Anulus Fibrosus

Outer portion of the IVD - made of fibrocartilage

Radicular pain

Pain caused by pressing against a nerve root - or inflammation of nerve root ex. sciatica

Where is the anulus fibrosus the weakest?

Posteriolateral aspects - prone to herniation

Chordomas

Rare neoplasm resulting from remaining notochordal cells (rests) in the nucleus pulposus. - Most commonly occur at the base of the skull and lumbosacral region

Lamellae

Rings of fibrocartilage in the Anulus Fibrosus - Type 1 collagen - fibers run parallel to each other at ~ 65 degrees from verticle - fibers of adjacent lamella run in the opposite direction

AF innervation

The recurrence meningeal n. (sinuvertebral n.) - innervates posterior AF Ventral rami and sympathetic chain - innervate lateral and anterior sides

How are IVDs named?

Using the 2 vertebrae that surround the disc (T5-6) OR By referring to the vertebra directly above it (T5)

Disc Bulge

When there is expansion of disc material beyond its normal border

Imbibition

When water is drawn back into the IVD after compression - compression forces water out, but Na+ and K+ remain, thereby drawing water back into the disc.