Ante’s Law/Crown to Root Ratio

The ratio of the portion of the tooth that is visible above the bone versus the portion that is embedded in the bone is called the crown-to-root ratio. The crown-to-root ratio is an integral factor in diagnosing and treating teeth that are in need of dental restoration. In 1926, Canadian dentist Irwin H. Ante wrote a thesis paper that presented a postulate related to crown-to-root ratio that came to be known as Ante’s Law. Ante’s Law states that “the total periodontal membrane area of the abutment teeth must equal or exceed that of the teeth to be replaced.” Abutment teeth are the teeth that are used to support a dental bridge; the abutment teeth are on either side of the missing tooth or teeth and can support fixed bridges or partial removable dentures. While Ante’s Law has dictated the suitability of dental restorations in many cases, affecting the number of abutments used and occasionally excluded certain patients from restorative treatments entirely, it has not been confirmed in clinical trials. In reality, dental prostheses placed on abutment teeth that do not meet the requirements of Ante’s Law have been proven to be successful when additional means of securing the restoration, such as splints, are used when abutment teeth may be mobile, and when periodontal disease is effectively controlled and strictly managed.

It is helpful to understand the basic anatomy of the teeth when explaining crown-to-root ratio, and when discussing dental restorations in general. Every tooth is divided into two basic sections: the crown, and the root. The crown of each tooth is covered with a protective coating of enamel, and the root of each tooth is covered with a layer of cementum. The point where these sections join is known as the cementoenamel junction. When teeth are healthy, their roots are fully submerged in bone, called alveolar bone; in the upper teeth, the arch of jaw bone is called the maxilla, and in the lower teeth, this arch is called the mandible. The teeth are surrounded by the alveolar bone until the area of the cementoenamel junction. The soft tissue of the gingiva, or gums, lies atop the bone and is about 1 millimeter thick. This means that the base of the crown is obscured at its apical millimeter by this gingival tissue. The crown and root are therefore defined as two separate parts of the tooth. Gingival recession is manifested by a decreased crown-to-root ratio, meaning that more of the root of a tooth is visible when the gingival tissues recede, which usually occurs due to injury or disease.

Periodontal disease also affects the amount of root structure that is visible and the amount that remains surrounded by bone. When bone resorbs, due to periodontal disease, more of the root becomes visible. When discussing the clinical presentation of the crown and the root, the terms relative crown, relative root, effective crown height, and effective root height are used to describe the crown-to-root ratio. The roots of a tooth are considerably longer than the crown, allowing them to effectively support the tooth while in use. In a healthy mouth, the crown-to-root ratio is ideal. When bone loss occurs, however, more of the root structure is visible outside the bone, and the support for the bone decreases. Additionally, more of the crown is revealed, which creates a greater amount of tooth that must be supported by a smaller amount of bone. As bone resorption occurs, this can dramatically worsen the prognosis for the tooth, as more and more tooth area must be supported by less and less bone, leading to greater mobility of the teeth and a greater likelihood of tooth loss.