The fracture surfaces of sixteen clinically failed full-ceramic one- and two-piece zirconia implants with a time-to-failure range from 3 to 49 months were analyzed. Treatment planning, surgical protocol, fracture-origin location and characteristic fracture features were assessed. In-depth fractography revealed that the fracture-initiation sites differed for the one- and two-piece zirconia implants. All one-piece implants fractured at the constriction region between two threads in the endosseous implant part. Surface defects at the root area were connected to fracture origins. For two-piece implants, the abutment neck, internal abutment-implant connections and inner threads were possible fracture-initiation sites. Importantly, the clinical failure of the analyzed implants were often found to result from combined effects related to patient aspects (like heavy bone loss at the time of implantoplasty), treatment planning/protocols (like off-centered implant positioning), a high bending moment at the weakest link, implant-surface conditions and specific implant designs.