Cornell University Ergonomics Web


Anatomy of the Spine

1. Spinal terminology - The spine consists of 24 movable bones, the vertebrae, bound with ligaments into an "S" shape. Vertebrae are separated by intervertebral discs. 10% of people have a greater or fewer number of vertebrae but this seems to have little functional effect. The spine connects to the hip bones at the sacro-iliac joints via a complex arrangement of ligaments. Vertebrae can be grouped into four groups:

a. Cervical - first 7 (neck) vertebrae. This area is very flexible and a common location of injuries. Here the spine is concave, curving toward the belly (lordotic).

b. Thoracic - next 12 (chest) vertebrae. Ribs attach to 10 of these vertebrae. This area is not very flexible as it is the structural part of the spine. Here the spine is convex, curving away from the belly (kyphotic).

c. Lumbar - next 5 (small of the back) vertebrae. This area a main load bearing region where the vertebrae are the largest. It is very flexible and a main site of injuries. Here the spine is concave, curving toward the belly, (lordotic). This curve of the spine can change in standing and sitting postures.
d. Sacrum and Coccyx - The 9 lowest vertebrae become fused into 2 bones. The five sacral bones are convex and the 4 coccygeal bones are concave.

2. Intervertebral discs - Acts a bit like a ball bearing, almost as if pivoting. The discs can be squeezed by gravity or jolts, causing vertebrae to come out of alignment. Discs are very flexible at low loads but increase resistance to movement at high loads. Discs do not have blood or nerve supplies. Nutrients are supplied and wastes are eliminated by the "pumping" action of daily activity and motion. The two mechanically distinct parts are the disc annulus and nucleus pulposus.

a. Nucleus Pulposus - incompressible watery gel contained within an elastic wall. Allows the disk to act as a shock absorber for the spine. Nuclei are located more centrally in the disks between vertebrae at the top of the body. Near the lumbar region nuclei are not located centrally. Herniation of the disc (when the nucleus extrudes) is more likely in lower regions of the spine.
b. Disc Annulus - Fibrous lamellae arranged differently from layer to layer. Surrounds nucleus and offers structural strength.
c. Hyaline Cartilage Endplates - protect the disc at each end.
d. Lumbar Motion Segment - two vertebrae either side of a disc and the disc.

3. Spinal forces-

a. Compressive -
vertebrae are squeezed together.
b. Tension - vertebrae are pulled apart.
c. Shear - vertebrae moved in different directions (side to side). This can happen in football and car accidents.
d. Torsion - vertebrae are twisted in opposite directions.

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