The structures that support the teeth
Healthy teeth are, of course, embedded in bone. The bone is covered with gums, and the gums attach not only to the bone, but also to the tooth itself.
The gingiva is that portion of the gums that surrounds the teeth and lies above the level of the bone. The diagram to the right is a detail which shows the microscopic structure of this vital attachment of the gums to the tooth. The soft tissue is covered by an epithelial layer (red) called the oral epithelium. This attaches to the surface of the tooth on the dentin between one and three millimeters below the level of the crest of the gingiva. The part of the gingiva below the crest but above the attachment is called the free gingival margin. The potential space between the free gingival margin and the tooth (collapsed in life) is called the gingival sulcus. Just below the epithelial attachment lies a large number of connective tissue fibers (blue) called the gingivo-dental fibers. Some, which are not visible here actually circle the entire tooth and are called circular fibers. These fibers are responsible for securely attaching the gingiva to the tooth.
This attachment is responsible for separating the dirty oral environment from the totally clean environment inside the body. The gingival attachment is doubly important because it protects the underlying bone (alveolar crest) from becoming infected. Nature is especially protective of the bone because it is not highly vascularized and an infection in bone, especially in pre modern man’s environment would have been a death-dealing event. An infection in bone is called osteomyelitis, and even today, with modern antibiotics, it is still quite a dangerous condition. Thus nature built in a simple mechanism to protect mammals from getting osteomyelitis as they aged and became more susceptible to oral infection. She programmed the bone to resorb (to be absorbed); to “get out of the way” before the infection reached it. This is the basis of periodontal disease—The loss of bone as a protective mechanism against a dangerous bone infection. Better the loss of the teeth than the premature loss of life!
The periodontal ligament is just visible in the diagram immediately above. It is the soft tissue that lies between the tooth and its bony socket. As you can see, it is really just a continuation of the connective tissue associated with the gingivo-dental fibers, and if you look at the large diagram at the top of the page, you can see that it continues around the entire tooth. In a healthy situation, there is never a direct attachment between the bone and the tooth itself. Such a direct attachment, when it occurs in pathological situations, is called ankylosis.
The PDL is composed of fibrous connective tissue in which the fibers run approximately perpendicularly from the tooth surface to the bony socket. In any given area, a cross section looks like a tangled mass of nearly parallel fibers that attach at one end into the cementum overlying the root of the tooth, and at the other end, into the alveolar bone inside the socket.
The bone that supports the teeth is called alveolar bone. It’s only purpose in life is to support the teeth, and if a tooth is extracted, the alveolar bone that originally supported it will eventually be resorbed by the body. The part of the alveolar bone that lines the socket is a thin layer of dense cortical bone called the lamina dura. The bone that underlies the lamina dura is cancellous bone (sometimes called medullar bone). Cancellous bone looks spongy and contains blood producing “organ” called bone marrow. In fact, all three of the features discussed in this section, the lamina dura, the periodontal ligament and the cancellous bone can be seen on any intraoral dental x-ray like the one above. In the x-ray seen above, follow the edge of any of the three teeth present from the top of the crown down into the bone. The dark line that separates the tooth from the bone represents the space where the periodontal ligament lives. The thin bright strip of bone directly beside the periodontal ligament space is the lamina dura. Under the lamina dura is the less bright cancellous bone. If you look carefully you can see the trabeculae –the tiny spicules of bone crisscrossing the cancellous bone that make it look spongy. These trabeculae separate the cancellous bone into tiny compartments which contain the blood producing marrow. These marrow spaces are seen in the colored image of the PDL above as bright “blobs”.