Ante’s Law / Crown to Root Ratio

Ante’s Law

In 1926, Irwin H. Ante wrote a thesis paper which resulted in what is commonly known as Ante's law regarding the crown-to-root ratio. Ante's law states the following, "the total periodontal membrane area of the abutment teeth must equal or exceed that of the teeth to be replaced."

In later claims, it was suggested that the length of the periodontal membrane attachment of the abutment tooth should be at least one half to two thirds of that of its normal root attachment. Based on these unsubstantiated, empirical concepts, many teeth were subsequently excluded from suitability as an abutment for fixed partial dentures. In addition, double abutments were a popular method used to comply with Ante's law. Ante's law has not been confirmed through the use of long-term, clinical trials.

Crown-to-root-ratio

The crown-to-root-ratio is the ratio of the length of the portion of a tooth which appears above the alveolar bone, compared to what lies below it. The crown-to-root-ratio is important in the diagnosis, treatment planning, and restoration of teeth. The goal of the ratio is to guide the treatment plan and achieve a good outcome.

Anatomical Terms

All teeth contain two general sections which includes the crown and root. The crown is covered by enamel, while the root is covered with cementum. The line where these two sections join is the cementoenamel junction. The teeth’s roots are completely surrounded by and submerged in the alveolar bone of either the maxilla or the mandible. This depends on if the particular tooth is an upper or a lower tooth, up until the cementoenamel junction. On top of the bone is the gingival soft tissue. On average, the gingival soft tissue is about 1 mm thick. Because of this gingiva, the crown of the tooth, which is completely outside the surrounding bone, is somewhat obscured at the apical millimeter. As a result, the crown and root can be used as anatomical terms, which are used to define actual parts of the tooth. Gingival recession is a condition where the crown-to-root ratio is decreased, but refers to the gingiva.

Clinical Terms

These terms can also be used to describe the clinical sense of the words, as it relates to the crown and relative root. The term is used to describe the amount of tooth which is surrounded by the bone. Over time as the inflammation from periodontal disease causes the bone to resorb and disappear, the root structure is more exposed. The effective crown height as it relates to the effective root height is reduced and the prognosis becomes unfavorable.

Clinical Importance

From a clinical perspective, the anatomical definitions are not critical. The important feature, in terms of supporting the teeth, is how much of the teeth are embedded in the bone. This is where the crown-to-root ratio becomes vitally important.

The cementoenamel junction is naturally closer to the incisal or occlusal surface of a tooth compared to the tip of the root or roots. Because of this, the root length is considerably longer than crown length. This aids in allowing for proper support of the teeth during their normal function. In a similar concept, a tree that has a complex root system which is below the ground. The root system is typically much larger and more advanced than the form of the branches which is visible above the ground. When a tree lacks fails to have a sufficient root system, it is easily destroyed and knocked over by wind. Similarly, the tooth requires a strong root system which is enclosed in bone in order to properly function and withstand regular forces.