Basic Bone Structure

Bones are organs composed of hard living tissue providing structural support to the body – it’s scaffolding. It is a hard matrix of calcium salts deposited around protein fibers. Minerals make bone rigid and proteins (collagen) provide strength and elasticity.

In an adult, bone engages in a continuous cycle of breaking down and rebuilding. Bone absorbing cells called Osteoclasts break bone down and discard worn cells. After a few weeks the osteoclasts disappear and Osteoblasts come to repair the bone. During the cycle calcium is deposited and withdrawn from the blood.

The Periosteum, a fibrous membrane, covers the outside of bone. This membrane is rich with capillaries, which are responsible for nourishing bone.

The outer layer of bone is called Cortical bone. Eighty percent of skeletal bone mass is cortical bone. Cancellous bone (also called trabecular bone) is an inner spongy structure that resembles honeycomb, which accounts for 20% of bone mass. This spongy mesh–like bone is designed for strength similar to steel rods within a concrete structure. The inner bone cavities contain bone marrow where red blood cells are produced.

The shape of bone is described as long, short, flat, or irregular. They are further classified as Axial or Appendicular. Axial bones are protective. For example, spinal vertebrae act to protect the spinal cord. Appendicular bones are the limbs. Although there are many shapes and sizes of skeletal bone, the bones that make up the spinal column are unique.

Overhead View of a Lumbar Vertebra

(1) Vertebral Body (2) Spinous Process
(3) Articular Process (4) Transverse Process
(5) Foramen (6) Pedicle