Biomechanical study of implant/abutment connection in external hex, internal hex and morse taper dental implant systems

Abstract

The osseointegrated implants, introduced in dentistry in the 60´s, has been subjected to intense research and development in the last decades. In conventional systems, the implant is constituted of an intra-bone portion, in titanium, and a transmucosal abutment, also in titanium, on which the prosthetic work is made. These two components, in these conventional systems, are screwed to each other, with gold or titanium screws. The interface of these components is usually anti-rotacional, in a hexagon form, with parallel axial walls, that can be external or internal in relation to the intra-bone part of the implant. The reduced height of the hexagon doesn't guarantee any stability to the abutment, and the occlusal forces are concentrated in the fixation screw. This condition implies in a larger screw loosening or fractures risk. There is a direct correlation between the hexagon misfit and screw loosening; the abutment rotational movement around the implant hexagon can cause an unstable screwed joint. Oblique mechanical loading in an external hexagon implant seems to harm the preservation of the abutment installation torque. Even so, the external hexagon connection continues to be the most popular actually. Internal hexagon connections were developed to increase the surface contact between implant and abutment, increasing the abutment stability. It is demonstrated that the internal hexagon implants provide better force distribution, compared to the external hexagon implants. However, those implants also present some rotational movement of the abutment around of the implant, what can result in joint instability. Screw loosening occurrences seem to be similar in internal and external hexagon implants. The reduced thickness of the walls around the internal hexagon is a potential problem related to the implant fracture resistance. An alternative was proposed to the implant systems with external and internal hexagon connections, using a conical interface between implant and prosthetic abutment. The abutment used for cemented unitary prostheses in cone taper implant system can be solid, with a screwed apical portion, or a two components with a transfixing screw. For both abutments, the retention seems to be obtained mainly by his conical portion, and the screw threads seem to have little contribution for that retention. The good stability obtained by that system seems to provide larger resistance to the flexing forces in the abutment/implant interface, for both types of abutments. The objective of this study is to evaluate the biomechanical properties of external (HE), internal (HI) and cone taper (CM) abutment/implant connections. For this, the study will be divided in two different phases: the evaluation of the effects of mechanical loading over the torque loss of prosthetic abutments for the studied implant systems. This mechanical loading will be accomplished in an apparatus that simulates masticatory movements for 1-year period of function, using 80N load. This analysis will verify the effects of long term axial and oblique forces of masticatory movements over the retention and the stability of these systems; and the evaluation of fracture resistance of the abutment/implant connections when submitted to oblique compressive load. The test will be accomplished in an universal test machine, using a 500kgf load cell, and 1mm/min displacement, applied at 45º. (AU)