39
Clinical observations have indicated that the major causes of implant failure are
(a) deficient osseointegration, (b) complications of the neighboring soft tissues (peri-
mucositis and peri-implantitis) and (c) mechanical complications
2
. Among the
biomechanical problems, screw loosening, abutment rotation, and abutment fracture
are the major issues
3
. In a prospective multicenter investigation, Henry et al
4
treated 92
patients with 107 implants. The authors reported that the problems most frequently
experienced during the first year was related to loose screws. The two mechanisms
involved in screw loosening are: excessive bending ( plastic deformation that takes
place when a load larger than the yield strength of the screw is applied) and settling
(when external loads applied to the screw interface create micromotion between both
surfaces). As the mating surfaces wear, they “settle” closer together
5
. The factors that
contribute to screw instability are: misfit of the prosthesis, insufficient tightening force,
screw settling, mechanical overload, and mismatch in screw material and design
6
.
A number of studies have been conducted to compare the efficacy of different
connecting mechanisms securing the abutment to the implant head
7
. The design of the
implant-to-abutment mating surface and the retentive properties of the screw joints
affect the mechanical resistance of the implant-abutment complex
8-9
. The implant-
abutment connection is also influenced by factors such as component fit, machining
accuracy, saliva contamination and screw preload
10-11
.
Current designs are derived from two basic designs: the "butt-joint" (consisting
of 2 parallel flat surfaces contacting)
8
and the internal "cone-in-cone" design. The latter
was introduced in the ITI implant system (Institute Straumann AG, Waldenburg,
Switzerland) and offered an alternative sound, stable, and self-locking interface
12-13
.
Recent studies indicate a potential mechanical advantage of conical connectors over
butt-joint designs
9,14
. Indeed, the mechanics of the ITI cone-in-cone
9
resulted in lower
incidences of mechanical complications, specifically abutment screw loosening and
fracture, in comparison with those reported for butt-joint implants
15-16
. With few
exceptions, most of the long- term clinical data on implant performance involve external
hexagons. This is primarily the result of their extensive use, the broad number of
prescribed clinical applications, the level of complications reported, and the resultant
efforts to find solutions (specific torque application to abutment screws)
8
. Industry
surveys have shown that external hex implants still dominate the European market
17
.
Fatigue is a progressive, localized and permanent structural damage that
occurs in a material subjected to repeated or fluctuating strains. Experimentally, three
modes of loading may be used to duplicate fatigue failures: direct axial loading (the
specimen is submitted to a uniform stress through its cross-section), plane-bending
(the majority of the specimen is subjected to a uniform bending stress) and rotating-