Abutment Selection: Individual Resistance and Collective Support

Abutment An abutment is a tooth, portion of a tooth or portion of an implant used to support a fixed or removable prosthesis The location and condition of the abutment are among the most important factors controlling FPD design. ________________________________________ Factors Controlling Abutment Selection Crown Length • Refers to the clinical crown length. • Abutment height should preferably be at least 4 mm. • A taller crown produces a longer lever arm and greater torsional stress. Short Abutments • Less than 4 mm → core build-up or splinting to additional abutments may be required. • Less than 2 mm → post and core with crown lengthening may be considered. • Full-coverage restoration and crown-lengthening may increase retention. ________________________________________ Crown Form Tapered or poorly developed crowns may not provide adequate retention. with tall cusps and prominent morphology may transmit greater lateral loads. If the pontic occlusal table is larger than the abutment teeth, additional abutments may be required. ________________________________________ Degree of Crown Mutilation • Severely mutilated crowns may require a full-coverage retainer. • A post and core may be required for an endodontically treated tooth. ________________________________________ Root Length and Form • Longer roots provide greater periodontal attachment. • Broad buccolingual roots resist rotational forces better than round roots. • A molar with divergent roots is better than a molar with conical roots. • A single root with an irregular form or apical curvature is better than a perfectly tapered root. • Canines are excellent abutments because of their long roots. • Anterior triangular roots can resist lateral loads. • Posterior cylindrical roots are suited to vertical loads. ________________________________________ Crown–Root Ratio The crown–root ratio compares: • Length of tooth above the alveolar crest • Length of root embedded in bone Values • Ideal ratio → 2:3 • Minimum acceptable ratio → 1:1 • Ratio greater than 1 → generally unacceptable A 1:1 ratio may be acceptable when: • Opposing occlusion is a removable prosthesis. • Opposing teeth are periodontally weak. • Occlusal forces are reduced. ________________________________________ Root Support and Periodontal Ligament Area The supporting bone should show: • Healthy trabecular architecture • No active periodontal disease • No significant bone defect • Adequate periodontal ligament area ________________________________________ Ante’s Law The combined pericemental area of all abutment teeth should be equal to or greater than the pericemental area of the tooth or teeth being replaced. Clinical Application • One missing tooth can usually be supported by two adjacent abutments. • Two missing teeth supported by two abutments approach the limit. • Replacing three teeth with only two abutments is generally unacceptable. • If the primary abutments have insufficient periodontal area, an additional secondary abutment may be added. ________________________________________