Читать книгу Fundamentals of Fixed Prosthodontics - James C. Kessler - Страница 113

The crown-root ratio is a measure of the length of tooth occlusal to the alveolar crest of bone compared with the length of root embedded in the bone. As the level of the alveolar bone moves apically, the lever arm of the portion out of bone increases, and the chance for harmful lateral forces increases. The optimum crown-root ratio for a tooth to be used as a fixed partial denture abutment is 2:3; a ratio of 1:1 is the maximum ratio that is acceptable for a prospective abutment under normal circumstances (Fig 7-5).

However, there are situations in which a crown-root ratio greater than 1:1 might be considered adequate. If the occlusion opposing a proposed fixed partial denture is composed of artificial teeth, occlusal force will be diminished, with less stress on the abutment teeth. The occlusal force exerted against prosthetic appliances has been shown to be considerably less than that against natural teeth: 26.0 lbs for removable partial dentures and 54.5 lbs for fixed partial dentures versus 150.0 lbs for natural teeth.¹

Fig 7-5 (a) The optimum crown-root ratio for a fixed partial denture abutment is 2:3. (b) A ratio of 1:1 is the maximum that is acceptable.

Fig 7-6 Although the root surface area of these teeth is similar, the root configuration of the maxillary premolar (a), with its greater faciolingual dimension, makes it a superior abutment to the maxillary central incisor (b), whose root is essentially circular in cross section.

Fig 7-7 The molar with divergent roots (a) will be a better abutment tooth than one whose roots are fused (b).

For the same reasons, an abutment tooth with a less-than- desirable crown-root ratio is more likely to successfully support a fixed partial denture if the opposing occlusion is composed of mobile, periodontally involved teeth than if the opposing teeth are periodontally sound. The crown-root ratio alone is not an adequate criterion for evaluating a prospective abutment tooth.²