Cornell University Ergonomics Web
Cornell University Ergonomics Web
DEA 3250/6510 CLASS NOTES
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.