Torsional test at the interface between implant and implant abutment

Objectives: Fracture in the implant-implant abutment interface restored by an implant-anchored bridge or single crown is sometimes encountered clinically. We speculated that this occurred because of bending moment, horizontal loading, rotational torque during chewing, and/or rotational torque at the installation of the abutment on the superstructure. The methods of abutment connection are classified as internal and external. The purpose of this research was to investigate fracture strength by torsional tests at the implant-abutment interface, and how this affects permanent transformation.

Materials & Methods: Autograph (Shimadzu, Tokyo) was used to prepare individual parts of the system for torsional testing. Three kinds of implants—including a GC screw implant with internal connection, a titanium screw implant with external connection (GC Tokyo), and an SPI implant with internal connection (Thomen Medical, Switzerland)—were used. Each implant was connected with an abutment by torque ratchet at 20 Ncm. Each implant specimen was mounted in the Autograph, and torsional force was applied. We measured the fracture strength by rotational speed at 3.6 degrees/min until the generation of permanent transformation.

The specimen was removed from the Autograph, and the abutment was also disconnected from the implant. The connection portion of the implant was observed by SEM.

Result: The titanium screw implant with external connection was 1.6 Nm, the titanium screw implant with internal connection was 1.0 Nmm, and the SPI implant was 2.0 Nm. SEM observation showed permanent transformation at the corner of the hexagonal joint in the titanium screw implant with external connection, and at the internal notch in the titanium screw implant with internal connection. A curve illustrated by a graph of torsional force to fracture showed two patterns.

Conclusion: The force leading to permanent transformation of the abutment connection was different in each implant.