What is Bone Resorption?
Both skulls above are real The one on the right belonged to an elderly person who lost his teeth many years before he died. When he was young and he had teeth, his skull used to look like the one on the left. The first thing that jumps out at you is how thin the bone of his lower jaw is in comparison to the bone on the lower jaw of the skull on the left. But another thing that is not so apparent is the loss of the bone in the upper jaw.
Notice that both skulls are positioned with their lower jaws mounted so that the bone of the lower jaw is about parallel with the bone of the upper jaws. This tells you that the teeth are together. Even the skull on the right—if it had teeth. This gives you an idea of the amount of bone that that has been lost since this man had all his teeth extracted. This is the golden rule in dentistry:
Whenever a tooth is extracted, nature will remove the bone that once surround it.
When the body removes any tissue, we say that it has been resorbed. Think of resorption as the “melting” away of the bone after a tooth is extracted. The longer the tooth is missing, the less bone that remains behind. Thus, when a tooth is extracted from a young person, by the time that person is middle aged, a great deal of bone will be missing. If our friend in the image on the left above had all his teeth extracted when he was 30, by the time he reached the age of 75, his skull might look like the image below which I have Photoshopped. Note that without changing the relationship of the upper and lower jaws, he now looks just like the toothless image above, on the right:
Bone that surrounds a natural tooth is called alveolar bone. The job of the alveolar bone is to support the teeth. Once a tooth is extracted, the alveolar bone no longer has a purpose, and the body resorbs it. Eventually, the resorption slows down and stops. What is left behind is the cortical bone, a part of the skeleton which, like the rest of the skull, may change shape during life, but never entirely resorbs without being rebuilt. The cortical bone is like the main beam that supports the house.
The red ellipse highlights the symphysis of the lower jaw. The symphysis is made of the densest bone in the human body. Thus, it generally remains thicker than much of the other cortical bone in the jaw. The symphysis and its surrounding bone is very important to dentists who make dentures and who do implants. It is often the only bone in the lower jaw that remains high enough to present a ridge to support a denture.
The image below shows what the floor of the mouth may look like in a person who has been toothless for twenty or thirty years. The low “hill” in the form of an arch is called the residual ridge. The ridge is composed of firm gums overlying the bone of the lower jaw. The “gums” in the anterior part of the ridge (at the bottom of the picture) overlay the symphysis. When we build a denture, it must gain support and stability from the vertical height of the residual ridge. You can see that there isn’t much vertical ridge here to help stabilize the denture. And this is by no means the worst lower jaw we see on a regular basis.
The reason that I have circled the symphysis (chin) in the skull image above is to illustrate what happens even to dense, cortical bone if there are no teeth to maintain it. You are looking at illustrations of a cross section through the middle of the symphysis. The tongue would be to the left, and the tip of the chin would be at the lowest point of the gray outline of the bone. The prominent point on the left of each stage illustration is the genial tubercle, which you can feel with the tip of your tongue in the front of the floor of your mouth. The genial tubercle is a landmark which never resorbs, since it represents an important muscle attachment point. It also represents the level of the soft tissue floor of the mouth. For a full explanation of the way the lower jaw changes after the teeth are extracted, including lots of images, click the icon on the right.
The illustration above labeled stage 1 shows the general shape of the bone when teeth are present. Once the teeth are removed, the alveolar bone above the genial tubercle begins to resorb, and over the years, the shape of the symphysis progresses through the stages you see here. This process happens all over both jaws, but it is most pronounced in the lower jaw. This is the reason that so many people cannot wear their lower dentures.
Lest anyone think that the stage diagram above is some theoretical figment of an anatomist’s imagination, the two x-rays above show two of my own patient’s edentulous symphyses. This is what an x-ray looks like when you shoot broadside to a patient’s chin. The white stringy things that you see above each ridge are their dentures, which have been outlined with lead foil. The shape of the dentures give a fair idea of the extent of the bone that sticks up and is available as a ridge upon which to rest the denture. The one on the left has retained a minimal amount of its vertical height and represents about a stage II symphysis. The one on the right has lost virtually all of its alveolar bone and has suffered extensive cortical remodeling since the teeth were extracted. It represents a stage IV. The point of bone sticking up on the left side of the symphysis of the stage IV is the genial tubercle.