Understanding the Lumbar Spine Disc: Anatomy and Function

1. ADVANCED LUMBAR SPINE

2. Lumbar Disc • Function • Allow movement • Transmit load from one vertebral body to the next • Requirements • Must be strong enough to sustain weight without collapsing • Deformable to accommodate vertebral rocking motions • Strong enough to not get injured during movement

3. Nucleus Pulposus Semifluid, toothpaste consistency When subjected to force it attempts to deform and transmit forces in all directions 70-90% water Next major component: proteoglycans, which make up 65% of dry weight of nucleus and bind the water in the nucleus Collagen makes up for 20% of dry weight in nucleus

4. Anulus fibrosis Water is the main component, 60-70% Collagen makes up 50-60% of the dry weight The spaces between the lamellae are filled up with proteoglycan gel, which glues the lamellae together and prevents them from buckling and fraying Elastic fibers make up 10% in the anulus

5. Anulus Fibrosis The collagen fibers are arranged in 10-20 lamellae, thicker towards the center, thinner towards the back The orientation is appr 70 degrees from the vertical The direction of fiber inclination alternates with each lamella

6. Endplate Separate the disc form vertebral bodies The endplate is a layer of cartilage 0.6-1mm thick Covers the nucleus, but only part of the anulus Strongly bound to the disc, but only weakly attached to vertebral bodies Chemically they resemble the disc This resemblance means that it does not constitute an additional barrier to diffusion

7. Nutrition The disc receives no major arterial branches Small branches enter the disc but are restricted to outermost fibers of anulus Nutrition of disc therefore depends on diffusion, which comes from the vessels in outer anulus and the capillary plexus beneath the endplates Compression/distraction of the disc also helps with the diffusion process

8. Metabolism Cells in the disc are metabolically active but at a low level Because of the low blood supply, the O2 concentration in the nucleus is only 2-5% of that at the periphery The cells in the nucleus rely on anaerobic metabolism, which makes the environment of the disc acidic The disc metabolism is very sensitive to pH changes. If it gets to<6.3 the cellular activity drops to <15% Impaired nutrition, inflammatory mediators and changes in pH can lead to significant matrix status changes

9. Weightbearing mechanisms Independent The anulus is a relatively stiff body. When the lamellae are healthy, it will resist buckling and is able to bear weight simply on the basis of its bulk Under briefly applied loads, a disc without the nucleus has nearly the same weightbearing capacity as an intact disc By itself it is extremely vulnerable to creep though

10. Weightbearing Mechanisms Cooperation with the nucleus The nucleus provides an additional bracing mechanism It can be deformed, but not compressed Under compression, the nucleus decreases in height, and expands radially This in its turn stretches the anular fibers, which will resist deformation until equilibrium is reached In a healthy disc, this is attained with minimal radial expansion of the nucleus

11. Weightbearing Mechanisms The essence of this combined mechanism is the fluid property of the nucleus, which makes the disc a stiff body. Since the water content of the nucleus depends on the proteoglycan content, any change in this will alter the mechanical properties of the disc

12. Age Changes With aging, the concentration of proteoglycans decreases, which decreases the water binding capacity of the disc It drops to 65% by age 75 The disc becomes drier, more fibrous, less resilient and is more vulnerable to creep The distinction between nucleus and anulus becomes less apparent The nucleus is less able to exert fluid pressure, so a greater load is being carried by the anulus, subjecting it to greater stress Clefts and fissures develop in the anulus as a result of greater stress and repeated micro trauma

13. Age Changes Narrowing of the disc has always been considered to be a sign of aging, but that is not necessarily true. It occurs only in 1:5 segment Maintenance of disc height is the norm in aging. Loss of trunk height results from decreased vertebral body height Possible explanation: Consequence of nucleus degradation following endplate fracture

14. Facet joints Ovoid in shape 16 mm high, 145 mm wide Articular cartilage is thickest at the center of the joint, up to 2mm Joint surface is usually flat

15. Intra articular structures Meniscoid structures Connective tissue rim: wedge shaped thickening of internal surface of capsule Adipose tissue pads Fibroadipose meniscoids: projecting 5 mm into joint cavity

16. Intra articular structures Fatty tissue that fills any left over space underneath the joint capsule Located in sub capsular pockets at superior and inferior poles of the joint Communicates with outside of joint through foramina in joint capsule

17. Sacral Dysfunctions

18. Iliac DysfunctionsAnterior/Posterior Rotation • AI/PI dysfunction is defined as being present if at least 3 out of the 4 tests are positive • Palpation bony landmarks • Standing flexion test • Supine to long sitting test • Prone knee flexion test

19. Outflare • Cause • Fall on the lateral aspect of the right PSIS • Hip IR/adduction restriction • Signs • In supine, the involved ASIS will be more posterior than on the uninvolved side • In supine, the distance between involved ASIS and greater trochanter will be less than on the uninvolved side

20. Inflare • Cause • Fall on lateral aspect of ASIS • “Weakness” of involved SIJ ligaments • Hip restrictions • Signs • In supine, the involved ASIS is more anterior than the uninvolved one • In supine, the distance between the involved ASIS and the greater trochanter is greater than on the uninvolved side