During the physiological function of chewing, or the non-physiological function of
bruxism, compressive, bending, and torsional stresses are generated in teeth or prosthetics.
These stress will cause the abutment screw loosening or fracture, fracture of the
abutment, fracture of the implant, and the implant/abutment connection. The extent
of the damage is influenced by the design of the prosthesis, the fit of the implant
prosthesis, the implant inclination, and the loading force. In these circumstances,
tolerable enough clinical implant/abutment joint strength is required and ISO 14801
is used as their fatigue strength test method. However, torsional stresses generated
in the oral cavity should also be considered. For these reasons, this study was undertaken
in order to compare the torsional strength required to deform the implant-abutment
connection for various diameter implants. The mode of failure will be the future investigation
to be resolved by observing the fractured surfaces. In all specimens tested in this
study, the relationship between static torsional load and fracture occurrence followed
a parabolic curve. The primary curve was determined to be the torsional torque under
which plastic deformation occurred and subsequently proceeded, resulting in permanent
deformation. The curves illustrated in Fig. 2 showed two patterns in all specimens. Smaller diameter (3.3 and 3.8 mm) implants
fractured much easier and earlier at the implant-abutment interface. This load-displacement
curve is similar to the result of statistic loading test that Huang HM et al. 5] reported.
Examining all SEM images of the implant-abutment connections, we found that although
their anti-rotational notches had been destroyed, the corresponding internal grooves
remained intact. When torsion was applied, the grooves, which are composed of grade
4 commercially pure titanium (CP-Ti), were compressed; however, the abutment interlocks,
which are composed of a titanium alloy (Ti-6Al-4V), were completely sheared off. Although
the tensile strength of the Ti-6Al-4V alloy is at least 2Â % greater than that of CP-Ti,
much more titanium is adjacent to the grooves, which are compressed and can therefore
withstand the transmitted force. On the other hand, the interlocks contain less material
and therefore shear off if the applied force is too high. Nagel et al. studied the
implant-abutment connection of each of the Replace-Select and the CAMLOG implants
using FEM and reported that each design was very similar 15]. When comparing each system, they reported that the Replace-Select implant may fail
by fracture of the implant body at the thinnest part of the wall. This thin portion
represents the internal design that prevents the implant-abutment from rotating.
For comparison between the mechanical strength of CP-Ti screw implants with internal
tube-in-tube implant-abutment connections and that of external hexagonal-type connections,
all abutments and CP-Ti implants with external hexagonal-type connections were heavily
damaged or destroyed in all phases of loading. A typical fracture curve for CP-Ti
implants with external connections is shown. The proportional limit and a parabola-like
curve with eternal destruction were drawn. Torsion forces of 0.25, 0.50, 0.75, 1.00,
1.25, and 1.50 N?·?m were applied to the external CP-Ti implants. Deformation occurred
in both the implant and the abutment at each torsion force (Fig. 6). This might have been the result of the abutment connection design or the physical
properties of the implant materials. In addition, the deformation effect on the torsional
yield strength of the implants and abutments is worth noting, as deformation occurred
immediately before torsion fracture in all specimens.
Fig. 6. SEM picture of implant with external hexagonal connection
Balfour and O’Brien tested the following three kinds of implants for maximum anti-rotational
stability: external hexagon-type 0.7 mm-diameter CP-Ti implants, internal octagon-type
0.6 mm-diameter Ti-6Al-4V implants, and internal hexagon-type 1.7 mm-diameter Ti-6Al-4V
implants and abutments 16]. Testing comprised rigidly fixing a calibrated torque gauge to the abutment sleeve
and applying torque until failure of the components was apparent. The torques necessary
to separate the single-tooth abutments from the implants were 8.7 in.-lb (98.3 N?·?cm)
for the external hexagon-type, 3.3 in.-lb (37.3 N?·?cm) for the internal octagon-type,
and 10.0 in.-lb (192.1 N?·?cm) for the internal hexagon-type. In the internal octagon
and internal hexagon designs, failure was limited to the abutment connections. The
Balfour and O’Brien result differed from those reported in this study (4.3 and 3.8
mm diameters, 87 and 70 N?·?cm, respectively). The results from this study confirmed
that the torsional strengths were different depending on the connection dimensions
as reported by Balfour and O’Brien. CAMLOG implants (5 and 6 mm diameter) achieved
higher torsional strength than 4.3, 3.8, and 3.3 mm diameter. This resulted from a
combination of increased implant diameter and thickness of the implant walls.