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Crown-to-root-ratio is the
ratio In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
of the length of the part of a tooth that appears above the alveolar bone versus what lies below it. It is an important consideration in the diagnosis, treatment planning and restoration of
teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
, one that hopefully guides the plan of treatment to the proper end result.


Perspectives of different terminologies


Anatomical terms

All
teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
have two general sections, the crown and the root. Because the crown is covered with enamel and the root is covered with cementum, the line at which these two sections join is called the cementoenamel junction. In health, the roots of teeth are entirely surrounded by and submerged in the alveolar bone of either the
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
or the
mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
, depending on whether the tooth in question is an upper or a lower tooth, up until the cementoenamel junction. Overlying the bone is the gingival soft tissue, which is, on average, about 1 millimeter in thickness. Because of this gingiva, the crown, which exists entirely outside of the surrounding bone, is somewhat obscured at the apical millimeter or so. Thus, ''crown'' and ''root'' can be used as
anatomical Anatomy () is the branch of morphology concerned with the study of the internal structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old scien ...
terms, defining the actual parts of a tooth. Gingival recession is a related condition to decreased crown-to-root ratio, but referring to gingiva.


Clinical terms

These terms can also be used to refer to the clinical sense of the words, as ''relative crown'' and ''relative root'', by describing how much tooth remains surrounded by bone. As the inflammation associated with periodontal disease causes the bone to resorb and disappear, revealing more of the root structure, the ''effective crown height'' in relation to the ''effective root height'' is reduced and the
prognosis Prognosis ( Greek: πρόγνωσις "fore-knowing, foreseeing"; : prognoses) is a medical term for predicting the likelihood or expected development of a disease, including whether the signs and symptoms will improve or worsen (and how quickly) ...
shifts from favorable to unfavorable.


Clinical importance

Clinically, the anatomical definitions don't really matter; what is important in terms of support for the teeth within the bone is how much of the teeth remain embedded; this is where the crown-to-root ratio becomes important. Naturally, the cementoenamel junction exists much closer to the incisal or occlusal surface of a tooth than to the tip of the root or roots. 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. The natural crown-to-root ratio is thus termed a favorable crown-to-root ratio, because the root system existing within the surrounding bone is more than sufficient to support the tooth under normal physiologic stresses. After some bone loss is incurred, though, and more root structure is visible outside of the supporting bone, not only is there less effective root support, but there is an increased height of revealed tooth structure that this now diminished submerged root is responsible for supporting. In a way, then, each millimeter of lost bone contributes, for example, to a millimeter of less support ''and'' a millimeter of more structure ''to'' support. As can be extrapolated, this can easily become a very serious problem once three, four or five millimeters of bone have been lost due to periodontal disease. 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. Teeth are constantly subject to both horizontal and vertical occlusal forces. With the center of rotation of the tooth acting as a fulcrum, the surface of bone adjacent to the pressured side of the tooth will undergo resorption and disappear, while the surface of bone adjacent to the tensioned side of the tooth will undergo apposition and increase in volume.Trauma from Occlusion Handout, Dr. Michael Deasy, Department of Periodontics, NJDS 2007. page 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 is usually the outcome associated with untreated secondary occlusal trauma.


See also

* Crown lengthening *
Post and core A post and core crown is a type of dental restoration required where there is an inadequate amount of sound tooth tissue remaining to retain a conventional crown. A post is cemented into a prepared root canal, which retains a core restoration, w ...


References

{{Prosthodontology Prosthodontology