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.