Bone, despite its rigid nature, is not a permanent, immutable tissue. It is in fact a very dynamic and living tissue that maintains its structure via an equilibrium of opposing activities:
Bone Regeneration and
Bone Resorption. Both activities are constantly ongoing in living bone. This process of bone regeneration and bone resorption in equilibrium is also called
Bone Remodeling.
Bone consists of three distinct entities:
- The Organic Matrix (approximately 35% of the weight)
- The Inorganic Matrix (approximately 60-70% of the weight)
- Cells
The Organic Matrix is mainly composed (90%) of type I collagen. The remaining 10% is composed of non-collagenous components and sediment.
The Inorganic Matrix is often also called the "mineralized matrix" contains about 99% of the body's calcium, 85% of all the phosphorous and between 40 and 60% of the sodium and magnesium. The mineral crystals of the inorganic matrix can be classified as apatite, as it is mainly composed of carbonates and amorphous combinations of phosphate and calcium.
The Cells consist of Osteoblasts, Osteocytes and Osteoclasts. Osteoblasts secrete the bone matrix in form of an insoluble organic substrate, mainly composed of type I collagen. This substrate will then rapidly mineralize by means of the deposition of calcium phosphate crystal. So, in essence, these cells are the "bone builders". The Osteocytes are actually osteoblasts, which get entrapped in very matrix they secrete. They are relatively inactive cells, which are thought to have more of a regulatory function to other cells. Finally, the Osteoclasts are large macrophages, which resorb or destroy bone matrix. All of these cells are necessary for proper bone maintenance.
Bone maintains its structure and volume through mechanical stimuli. The skeletal bone is maintained through us walking in gravity. Our jaw bones are maintained by chewing on our teeth. Once this mechanical stimulus is gone (i.e. the teeth are gone), the body "thinks" that the calcium is no longer needed there and the resorption process starts. Dentures don't count in terms of delivering enough mechanical stimulus to the bone to prevent bone resorption. They are not "bone-anchored" devices. Dental implants are the only devices that can prevent further bone resorption.
This bone resorption can be rather dramatic as shown in the sequence of images below. These images show how a real mandible or lower jaw can resorb once the teeth are taken out.
