Crown-to-root ratio

Crown-to-root-ratio is the ratio of the length of the part of a tooth that appears above the alveolar bone versus what lies below it.

Because of this fact, root length is considerably longer than crown length, and this helps allow for proper support of the teeth during normal function, not unlike a tree that has a root system hidden below ground which is more often than not considerably larger and more elaborate than the form taken by the growing branches.

If a towering tree doesn't have enough support provided by its root system, it will easily be knocked over in even a slight wind; similarly, a tooth requires a sturdy root system encased in bone to protect it from being knocked out of the mouth.

[3] The minimum crown-to-root ratio necessary is 1:1; any less support provided by the roots drastically reduces the prognosis of the tooth and its restoration.

[4] When the amount of root remaining in the bone is so short that the entire surface of bone adjacent to the root surface is constantly under compression or tension (with no middle section acting as a stabilizer for the fulcrum), the prognosis for the tooth is deemed highly unfavorable.

This X-ray film reveals a poor crown-to-root ratio for tooth #21 (right), the lower left first premolar . The tooth exhibits 50% bone loss, adding roughly 5-7 mm to the clinical crown of what is actually anatomical root. The fulcrum , existing somewhere immediately apical to the height of the bone, does not allow for any adjacent bone to avoid compression or tension, resulting in virtually complete widening of the PDL and a grim prognosis , due to secondary occlusal trauma .
This X-ray film displays horizontal bone loss of the mandible, in the lower right quadrant. Although the two premolars and the molar exhibit moderate to severe bone loss, there was no tooth mobility at the time this film was taken.
Models of human teeth as they exist within the alveolar bone . The roots, as they can be seen through the transparent "bone", exist naturally in greater inciso-apical lengths than the crowns.