Ante’s Law/Crown to Root Ratio

The crown to root ratio is the measurement of the length of the portion of a tooth that is visible above the alveolar bone compared to the portion that lies below this bone. The alveolar bone is the thickened portion of jaw bone that retains the sockets that hold the teeth in place. The crown to root ratio is an important measurement that is used in diagnosing and planning the restoration of teeth. In 1926, Irwin H. Ante, a Canadian dental practitioner, postulated in a paper that, “the total periodontal membrane area of the abutment teeth must equal or exceed that of the teeth to be replaced.” This led to later postulates about the length of root attachments for abutment teeth. Although these postulates were never confirmed in clinical trials, Ante’s Law led to modified treatment plans for many patients; it was determined that these patients were ineligible for fixed partial dentures and their treatments instead necessitated double abutments.

To understand the clinical significance of crown to root ratio, it is helpful to understand the anatomical terminology of the periodontium. Each tooth has two sections: the root, and the crown. The crown is covered with enamel, and the root is covered with cementum; the area where these two parts of the teeth meet is called the cementoenamel junction. When they are healthy, the entirety of the roots of a tooth, right up to the cementoenamel junction, are engulfed in the alveolar bone of either the upper or lower jaw. The upper jaw bone is called the maxilla, and the lower jaw bone is called the mandible; the alveolar bone is the thickened ridge of bone that is part of either the maxilla or the mandible. The soft tissue of the gums, or gingival tissue, is about one millimeter thick, covering the bone and also obscuring about one millimeter of the crown of the tooth, just coronal to the cementoenamel junction. The crown and root can also help describe the locations of different components in the oral cavity -- when something is toward the direction of the crown of a tooth, it is described as coronal, whereas something that is toward the root of the tooth is described as apical. This provides specificity and clarity in description so that the same terminology can be used when describing both the maxilla and mandible.

When describing the amount of tooth that remains in the bone, dentists use the clinical terms relative crown and relative root. As periodontal disease causes bone to resorb, more of the root of a tooth becomes exposed, and the crown to root ratio shifts, lessening the effective crown height as it relates to the effective root height. This has a dramatic effect on the potential for restorations. When they are being used to support dental restorations, such as dentures, there must be an ideal ratio of root that is still embedded in the bone in order for the tooth to provide sufficient support. In all teeth, root length is significantly longer than crown length. In a healthy mouth, this ratio is favorable, as there is a significant root system present to support the teeth. When bone loss occurs, and more of the root becomes visible, the roots must support a greater quantity of exposed tooth, and there is also less support for the roots themselves. Each millimeter of bone loss therefore leads to a millimeter less support for the teeth and a millimeter more tooth that must be supported, which can lead to serious problems when bone loss is significant. Whenever a crown to root ratio is not 1:1, there is an adverse effect on the potential for successful restorations.