Orthodontics is not just about the teeth … it’s about the bone too!

Teeth are held in sockets in the bone. Specifically, the type of bone that holds teeth is called alveolar bone. The upper teeth are embedded in the maxilla, and the lower teeth are embedded in the mandible. Teeth are surrounded on top by gum tissue (also called Gingiva). Under the gum tissue, the Periodontal Ligament (also called the PDL) encases the bottom portion of the tooth and attaches to the adjacent bone.

When braces put pressure on your teeth, the periodontal membrane stretches on one side and is compressed on the other. This loosens the tooth. The bone then grows in to support the tooth in its new position.

Teeth and the surrounding bone in which they are embedded respond physiologically to the forces that are applied to them. The controlling biological process is known as bone remodeling. It is actually a biomechanical phenomenon. Loading bones with pressure – or applying force – actually has the effect of making the bone tissue stronger. Inversely, the absence of loading on bone tissue results in weaker bone. Bones are made of cells called osteocytes. Other cells also participate in the bone remodeling process: osteoclasts resorb or “tear down” existing bone, and osteoblasts deposit or “build” new bone.

The load increase causes a proliferation of osteoclasts, which break down bone in response to the load. When the load is removed, osteoblasts are created, which deposit new bony cells. This process is repeated through repetitive motion and eventually the bone density increases.

The PDL is an interesting structure with many unique characteristics not found in other ligaments throughout the body. In some sense, it serves as a means of communication between the teeth and surrounding alveolar bone. Pressure between the PDL and bone causes the bone to create osteoclasts and break down the bone tissue to restore the normal spacing between the teeth and bone. The corresponding tension on the PDL behind the movement causes the bone to create osteoblasts, effectively building new bone to fill in the vacated space left behind and restore the normal spacing between teeth and bone.

Clinicians know that the osteoclastic (breakdown or resorption) process takes about 72 hours to gain momentum, and the osteoblastic (rebuilding or deposition) process takes about 90 days. Stabilizing the result can take up to a year. This long duration is the reason it is so important to wear retainers following treatment to avoid a relapse, or in other words have the teeth return to their original, crooked positions.

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