Fatigue resistance of ITI implant-abutment connectors -- a comparison of the standard cone with a novel internally keyed design.

The Straumann Company has recently supplemented its standard 'morse-taper' configuration with an octagonal internal key. During the restorative phase of implant treatment, this additional feature was designed to ensure positional duplicability between the laboratory and the clinical environments. It was, however, unclear whether this keying mechanism would decrease the mechanical strength of the connection between the implant and the abutment. This applies to keyed male and female parts but also to combinations of the new and the standard designs. Specially constructed specimens analogs representing all three combinations were fitted with a T-shaped bar, preangled to 15 degrees and subjected to vertical force applications provided by a servohydrolic fatigue tester. The loading frequency was 2 Hz and the maximum cycle number was 106. The data were evaluated using the staircase technique. The specimens were also modeled and analyzed numerically using finite element procedures. The samples' failure locations were recorded and the displacement vs. cycle number plots were patterned in four groups. The fatigue tests and staircase analysis showed no difference in mechanical resistance between the standard and the internally keyed connectors. The finite element models revealed a stress concentration located at the apical edges of the octagonal connector. However, it appeared that this phenomenon was based on computational rather than mechanical grounds. The locations of the failure sites were distributed randomly across the structures, thereby indicating the absence of a locus of minor resistance. The patterns of the displacement vs. cycle number could not be attributed to specific combinations between the standard and the internally keyed designs. It was concluded that both connectors are equal in their mechanical resistance to bending and torquing forces.