This document discusses various types of failures that can occur in fixed partial dentures (FPDs). It classifies failures as either biologic, mechanical, or aesthetic. Biologic failures include issues like caries, pulpal degeneration, endodontic failure, periodontal failure, tooth perforations, sub-pontic inflammation, and occlusal problems. Mechanical failures involve loss of retention, connector failure, occlusal wear, and tooth fracture. Aesthetic failures can be immediate due to issues in design, materials or workmanship, or delayed due to gingival recession or sub-pontic tissue shrinkage over time. The document provides details on causes and treatments for each type of failure.