Introduction and Objectives: Implant-abutment connections (IAV) show in vivo loosenings (L) and fractures (F) probably as a result of preceding micro movements (MM). The failure modes and their causes shall be evaluated. Material and method: 9 IAV types ∅ = (4-5 mm, n=8) were exposed to 106 cyclic loadings each, (100 N, 90° to the implant axis, at a distance of 8 mm from the IAV). In addition to this, the actual force on the IAV per cycle was measured. A computer-assisted sound analysis was used for the online detection of MM, L and F. For the analysis of L and F X-ray, REM and light microscopic photos were used. At the IAVs which remained intact, the was measured as well. (the moment of solve of the abutment was intended.) Results: Two IAV types didn't show any failure modes, but wear traces at the connection areas as can be proved. By one IAV type, a MM was detected preceding a L. Fractures at the connection screws and the abutments appeared at six IAV types. Only at one IAV type implant fractures occured. At none conical IAV types the failure frequency correlates with decreasing implant-diameter. However, at conical IAVs there is no correlation between failure frequency and implant-diameter. Summary: The occurrence of wear traces or failure modes (MM, L, F) shows, that all of the IAVs, with regard to horizontal encumbrances, feature constructions-conditioned weaknesses, which, in case of a long -term osseointegration of implants, could lead to a clinical component-failure (constructional element-failure), particularly in case of single tooth implant restorations placed in the lateral region. Keywords: implant, abutment, failure mode, screw, fracture, loosening, Implantat, Aufbau, Pfosten, Versagensmodi, Schraube, Fraktur, Lockerung