Keywords
dental implant - cast-on abutment - fatigue resistance - mechanical failure
Introduction
Gold alloy has been accepted as the gold standard material for fabrication of dental
implant casting custom abutment.[1]
[2] Gold price has continuously increased over the past decades, and at the same time,
alternative alloys such as nickel–chromium and palladium alloys have been developed.
Nickel–chromium is one of the most commonly used alloys in prosthetic dentistry. This
alloy is considerably much cheaper than gold alloy and offers good biocompatibility,
corrosion resistance, and castability.[1]
[3] However, thick oxide layer from nickel–chromium casting procedure can cause surface
roughness that is difficult to smooth-up due to its high surface hardness, this results
in misfit of abutment-implant connection.[3]
[4] Palladium alloy offers good mechanical properties including flexural strength, stiffness,
and durability. It also poses excellent tarnish/corrosion resistance and good biocompatibility
in the oral environment.[5] For alternative cast-on abutment, there are many implant systems in the market in
which those cast-on abutment can be fabricated using nonprecious alloy to reduce the
overall cost. In a study by Kano et al,[6] the cast cobalt–chromium abutments showed greater rotational misfit compared with
stock titanium abutment. Similar conclusions were drawn by Barbosa et al,[1] who reported that one-piece cast frameworks made with cobalt–chromium alloy had
the worst result for passive fit compared with commercially pure titanium and nickel–chromium–titanium
alloys, but their vertical fit was comparable.
Despite long-term reliability of dental implant with survival rate of 95% after 5
years and 90% after 15 years,[7]
[8] both biological and mechanical complications still occur.[7]
[9] Fatigue is one of the factors that causes mechanical failure. Mechanical fatigue
test with a cyclic loading is a laboratory method used for evaluating clinical reliability
of dental implants.[10] This test aims to simulate the clinical intraoral conditions and masticatory function.[11] Fatigue tests have been used in many of dental implant studies, but the majority
had performed in stock titanium or zirconia abutment.[12]
[13]
[14]
Cast-on abutment for screw-retained prosthesis has the advantage of predictable retrievability.[15]
[16] It can be used in patients with minimal interocclusal space of less than 4 mm or
when the implant orientation is misaligned more than 30 degrees.[15]
[17] In patients with limited interocclusal space at edentulous area and economically
compromised, using nonprecious cast-on abutment could be a good solution. However,
there is still insufficient data regarding fatigue resistance of cast-on implant abutments.
The purpose of this study was, therefore, to evaluate and compare fatigue resistance
of cast-on abutments, cast with three different alloys as follows: gold, palladium,
and nickel–chromium alloys. Apart from the difference in mechanical properties of
the chosen alloys, these three types of alloy also represent different level of material
cost. The hypothesis of this study is that “the type of alloy used for cast-on abutments
does not affect fatigue resistance of the implant.”
Materials and Methods
Specimen Preparation
Forty specimens of fixture, with internal connection (Ø 4.5 mm, length 10 mm, TSIII;
Osstem, Seoul, Korea, Lot No. FTP5A348) and abutment, representing single implant-supported
crown ([Fig. 1]) were prepared and divided into four groups (n = 10) including Group TA (control): stock titanium abutment, Group GS: gold cast
abutment cast with semiprecious alloy, Group GP: gold cast abutment with palladium
alloy, and Group CN: nonprecious cast abutment cast with nickel–chromium alloy. All
materials and instruments listed by the manufacturer and composition of the abutments
are shown in [Table 1].
Fig. 1 (A) Implant components in Group TA. (B) Implant components in Groups GS, GP, and CN.
Table 1
Types of abutments and casting materials
|
Group
|
Type of prosthesis
|
Abutment
|
Materials
|
|
Alloys
|
Composition
|
Melting interval (°C)
|
|
Au
|
Pd
|
Ag
|
Cu
|
Ni
|
Cr
|
Mo
|
Sn
|
Etc.
|
|
TA (control)
|
Cement-retained crown
|
Stock titanium abutment (Osstem, Korea; Lot No. PGA15A219)
|
Titanium alloy grade 5
|
–
|
–
|
–
|
–
|
–
|
–
|
–
|
–
|
–
|
–
|
|
GS
|
Screw-retained crown
|
Gold cast abutment (Osstem, Korea; Lot No. PGG15B019)
|
Semiprecious (Minigold, Ivoclar Vivadent, United States; Lot No. V41451PM)
|
40
|
4
|
47
|
7.5
|
–
|
–
|
–
|
–
|
Zn, In, Ir
|
865–945
|
|
GP
|
Screw-retained crown
|
Gold cast abutment (Osstem, Korea; Lot No. PGG15B019)
|
Pd based (W-1, Ivoclar Vivadent, United States; Lot No. W03211QG)
|
–
|
53.3
|
37.7
|
13
|
–
|
–
|
–
|
8.5
|
In, Ru, Li
|
1,185–1,270
|
|
CN
|
Screw-retained crown
|
Nonprecious cast abutment (Osstem, Korea; Lot No. PTA15G220)
|
Ni-Cr (Argeloy N.P.(V), Argen, United States; Lot No. 10311446)
|
–
|
–
|
–
|
–
|
76
|
14
|
6
|
–
|
Al, Be, Fe, Si, C
|
1,230–1,290
|
The implant fixtures were mounted centrally and parallel to an implant-holding device
and embedded in self-cured resin (Chockfast orange, Shannon, Ireland) following ISO
14801:2016 specification. The resin was poured below the implant platform 3 ± 0.5 mm
to simulate the worst-case situation of crestal bone resorption. Artificial crowns
were fabricated from hemisphere wax pattern, with a dimension of 6 mm in diameter
and 6 mm in height.[18] The wax patterns were invested with phosphate-bonded investment (Bellavest SH; Bego,
Bremen, Germany) and cast with the selected alloy at suitable casting temperature
([Table 1]). After casting, the oxidized layer was removed by 100 µm aluminum oxide particles
(Cobra; Renfert GmbH, Hilzingen, Germany) using 60 psi pressure for full metal crown
in Group TA, whereas 50 µm glass beads (Rolloblast; Renfert GmbH, Hilzingen, Germany)
were used with 60 psi pressure for Groups GS, GP, and CN and followed by ultrasonic
cleansing. No further polishing and finishing were performed.
For Group TA (control), specimens were prepared as cement-retained crown to eliminate
any effect on abutment connection from the casting process. The stock titanium abutments
were torqued to the fixture (30 Ncm), according to manufacturer's recommendation.
Ten nickel–chromium crowns were luted and seated on their stock abutments with zinc
phosphate cement (HY-Bond Zinc Phosphate Cement; Shofu Dental Corporation, San Marco,
California, United States). Excess cement was then removed with microbrush.
For Groups GS, GP, and CN, screw-retained crowns of the cast-on abutments were torqued
to the fixture (30 Ncm), according to manufacturer's recommendation. The screw holes
were then filled with resin composite (Filtek Z350 XT; 3M ESPE, St. Paul, Minnesota,
United States) then light cured for 40 seconds.
Fatigue Testing
According to ISO 14801:2016, in Dentistry–Implants–Dynamic loading test for endosseous
dental implants, all specimens were positioned at 30 ± 2 degrees to the implant axis.[18] The universal testing machine (ElectroPuls E10000; Instron Corporation, Norwood,
Massachusetts, United States) was calibrated following the manufacturer's instructions.
Dynamic loading fatigue tests were run in dry conditions at room temperature (25 ± 2°C).
Cyclic load was programmed using dedicated software (BlueHill version 2.0; Instron
Corporation) at a frequency of 20 Hz, starting with 200 N load for 5,000 cycles (preconditioning
phase), followed by 400, 600, 800, 1,000,1,200, 1,400, 1,600, and 1,800 N at a maximum
of 30,000 cycles for each step. All specimens were loaded until catastrophic failure
occurred, or the specimen displaced at least 2 mm from the axis of the dental implant
or 245,000 cycles were reached.[19]
[20] The displacement was automatically measured and detected by the universal testing
machine software.
Catastrophic failure was defined as the fracture of any component of the sample. The
modes of failure of all samples were recorded and classified into three types as follows:
Type 1: fracture at abutment screw and abutment, Type 2: fracture at abutment screw
and fixture, and Type 3: fracture at abutment screw, abutment, and fixture. The fracture
surface was then examined with scanning electron microscopy (SEM) (JEOL, 6510LV, Japan).
Statistical Analysis
Group comparisons were analyzed using one-way analysis of variance (ANOVA) and Tukey's
post hoc test (α = 0.05). As a supplement to the ANOVA, Weibull survival analysis
was performed. The p-values < 0.05 were considered statistically significant.
Results
The mean value and standard deviation of total number of cycles are shown in [Table 2]. Group CN showed the highest total number of cycles, but no statistically significant
differences were found between the groups. Survival probability of Weibull models
is shown in [Fig. 2]. Group CN has higher Weibull characteristic strength but lower Weibull modulus,
which demonstrates greater durability but less structural reliability ([Table 2]).
Fig. 2 Results of survival probability using Weibull model.
Table 2
Results of the one-way ANOVA and Weibull test for total number of cycles between various
abutments
|
Group
|
Mean
|
SD
|
Weibull characteristic strength
|
Weibull modulus
|
|
TA
|
189,883[a]
|
22,734
|
200,081
|
8.86
|
|
GS
|
195,028[a]
|
22,371
|
205,759
|
8.73
|
|
GP
|
187,662[a]
|
22,555
|
197,614
|
8.97
|
|
CN
|
200,350[a]
|
30,851
|
213,991
|
6.77
|
Abbreviations: ANOVA, analysis of variance; SD, standard deviation.
a No significant difference (p-value = 0.673).
Modes of failures of CN group were all located at the fixtures and abutment screws,
whereas in the other groups, their failures predominantly located at abutment and
abutment screw level ([Fig. 3] and [Table 3]).
Fig. 3 Modes of failure of specimens. Top row: TA1, GS1, and GP1 showed fracture of abutment
and abutment screw in Groups TA, GS, and GP, respectively. Middle row: TA2, GS2, GP2,
and CN2 showed fracture of fixture and abutment screw in Groups TA, GS, GP, and CN,
respectively. Bottom row: TA3, GS3, and CN3 showed fracture of abutment, abutment
screw, and fixture in Groups TA, GS, and CN, respectively.
Table 3
Modes of failure of specimens
|
Group
|
Modes of failure
|
|
Fractured abutment screw and abutment
|
Fractured abutment screw and fixture
|
Fractured abutment screw, abutment, and fixture
|
|
TA
|
6
|
1
|
3
|
|
GS
|
7
|
2
|
1
|
|
GP
|
9
|
1
|
0
|
|
CN
|
0
|
5
|
5
|
Scanning electron micrographs of internal implant surfaces revealed only minimal damage
on implant-abutment connection in GS and GP groups, while the highest wear was observed
in CN group ([Fig. 4]).
Fig. 4 Scanning electron microscopy (×400) of internal implant surface, (B) before fatigue
test; (TA) moderate wear in TA group; (GS) minimal wear in GS group; (GP) minimal
wear in GP group; (CN) aggressive wear in CN group.
Low-magnification SEM micrographs of abutment screw surface show crack growth that
originated at the receiving force side. The fatigue crack growth exhibited the boundary
of crack progression known as progressive mark or beach mark. Crack progressed until
the material had become unstable and overloaded, then catastrophic failure occurred
([Fig. 5A]).
Fig. 5 Scanning electron microscopy of abutment screw surfaces, (A) Force direction and crack growth direction from b (fatigue zone) to c (catastrophic
zone). (B) High-magnification (×5,000) image of fatigue zone. (C) High-magnification (×5,000) image of catastrophic zone.
[Fig. 5A] displays a parallel pattern or fatigue striation, observed in the fatigue zone (b)
of high-magnification SEM micrographs of TA, GS, GP groups, which is a character of
fatigue failure. However, in CN group, fatigue striations could not be observed because
the crack surface was not parallel to SEM image plane ([Fig. 5B]).
High-magnification SEM micrographs of catastrophic zone (c) show characterized microvoids
or cup-like depressions or dimples ([Fig. 5C]).
Discussion
The purpose of this study was to evaluate and compare fatigue resistance of cast-on
implant abutment cast with different alloys. Although nonprecious cast abutment group
showed the highest total number of cycles, no statistically significant difference
was found between the groups. Weibull survival analysis showed that CN group had higher
Weibull characteristic strength and lower Weibull modulus which demonstrate greater
durability but less structural reliability compared with the other alloys. The primary
advantage of nonprecious alloys is their lower cost compare with gold and palladium
alloys. At the time of publishing, the cost of nonprecious alloy from Thai dental
laboratory is at least 90% lower than gold alloy. For a posterior restoration, the
cost difference could be up to 250 USD. In addition, nonprecious alloys demonstrate
good mechanical properties, good biocompatibility, and corrosion resistance.[3]
A stepwise loading protocol was applied in this study in which a fixed load was applied
to determine the total number of cycles, followed by incremental increases in load
step up to the upper limit of testing or failure of the specimen.[19]
[21] This protocol was proved to simulate a clinical situation and shorten the period
of testing time.[21] Several studies[19]
[21]
[22]
[23] also had selected the stepwise protocol to accelerate fatigue failure. Regarding
the ISO standard 14801:2016 (Dentistry–Implants–Dynamic fatigue test for endosseous
dental implants),[18] the test should be at the loading frequency not more than 15 Hz, but the test at
low frequency may increase testing time and cost. In a study by Fraga et al,[14] the fatigue strength was not significantly different between the groups using frequency
of 2, 10, and 20 Hz; therefore, using frequency of 20 Hz seems to be a good alternative
that has been applied in several studies.[19]
[24]
Bonfante and Coelho[25] reported mean maximum bite force of 418 to 690 N in females and 491 to 878 N in
males. The range of failure loads in this study were 1,200 to 1,800 N which demonstrated
that the specimens in all groups could tolerate a greater load than average occlusal
force in natural teeth. The results were similar in several studies.[12]
[19]
[20] In our research, the range of the total number of cycles were 180,000 to 230,000
cycles which were higher than previous studies.[19]
[21] This might be caused by the differences in implant diameter, type of implant abutment
connection, type of restorative materials, and the determination of the failure point.
A systematic review[26] reported that 5-year survival rate of screw-retained implant restoration was 96.8%.
The technical complications on implant-supported single crown include screw loosening
(4.7%), abutment/screw fractures (4.1%), fractures of the veneering material (2.4%),
and implant fractures (0.1%). Different failure patterns were observed. Lee et al[27] and Atieh[28] found that the mode of failures often located at implant fixture and screw abutment.
On the contrary, Shirazi et al[29] showed the fracture line located only at abutment screw. In this study, the failures
were located at fixtures and abutment screws in CN group, but predominantly involved
abutments and abutment screws fracture in other groups ([Table 3]). Such mode of failure can be explained by the higher modulus of elasticity of nonprecious
alloys (180–240 GPa) when compared with commercial pure titanium grade 4 (110–150 GPa).
The stress distribution then may be concentrated at the implant-abutment complex,
especially on the abutment screw.[30]
[31] Damages of implant-abutment complex can be found at different levels, which need
different treatment regimen for repairing. Damage at abutment screw or prosthesis
could be easily retrieved or repaired without complication, but the damage at fixture
level usually requires a complicated surgery or more extensive treatment.[27]
[32]
The SEM images revealed the highest wear in CN groups. This result is not surprising
because the fixtures used in this study made from commercial pure titanium grade 4
which has lower surface hardness and wear resistance than cobalt–chromium cast-on
abutment.[30]
[33]
Iwabuchi et al[34] found that both grade 5 titanium alloy and cobalt–chromium alloy have good wear
resistance. However, when sliding wear test was performed using aluminum oxide, more
surface damage was still found on grade 5 titanium alloy than cobalt–chromium alloy.
Besides mechanical properties of alloy, corrosion resistance and biocompatibility
are also important factors to be considered when selecting abutment material. Tuna
et al[35] reported that gold alloy and palladium alloy showed high corrosion resistance, in
agreement with several studies.[36]
[37] Palladium alloy, however, may cause allergic reactions in some population. Faurschou
et al[38] reported that palladium alloy allergy was found 7.4% in dental patients. While the
clinical concern of palladium allergy is still inconclusive,[38]
[39]
[40]
[41] nonprecious alloys have been reported of unstable galvanic corrosion.[42] Lee et al[43] reported the metal ion releasing was increased when nonprecious alloy was in contact
with titanium, and it can also cause tissue toxicity around the implant. A study showed
that base alloy coupled to CP titanium grade 2 had higher galvanic corrosion than
to noble alloy,[44] whereas other studies could not confirm the effect of galvanic corrosion when nonprecious
alloy was in contact with titanium.[44]
[45]
Based on fatigue resistance test results in this study, it was found that the use
of three types of alloys for cast-on abutment resulted in nonsignificant difference
in fatigue resistance. In terms of mechanics, using nonprecious or palladium alloy
instead of gold alloy which is the standard for casting custom abutment or screw-retained
crown is possible. Regarding the material properties, palladium alloy can be used
as a substitute for gold alloy due to the lower melting point than the abutment connection
of gold cast abutment.
The SEM images showed worn implant surface where in contact with the cast on connection
for the nonprecious abutment group. The worn surface could be a result of micromovement
since the thick oxide layer from casting process cannot be eliminated without causing
microgap at the implant-abutment connection.[46] In addition, the difference in elastic modulus and hardness may lead to higher wear
at the internal fixture surface.[47]
[48] This must be considered when choosing material because this kind of damage on the
implant surface is irreparable.
The fatigue resistance test in this study was tested in accelerated condition which
cannot be infer to a comparable fatigue life, but may be assumed as the worst-case
scenario that may happen. Therefore, clinically such wear may not occur throughout
the dental implant service life. In addition, this study was focused on fatigue resistance
of cast-on abutments cast with three different alloys, connected to 4.5 mm diameter
implant, without a complete oral environment simulation which could be related to
long-term complications. Therefore, further studies and evaluations using different
implant diameters, prosthetic designs, and other implant-abutment connection design
are needed and will fulfill the knowledge gap.
Conclusion
Based on the results in this study, no statistically significant difference between
the control and experimental groups was found. Although nonprecious cast abutment
showed promising fatigue resistance after cyclic loading, the lower Weibull modulus
and consistent failures at implant fixture level were also found in this group. These
factors should be considered when selecting abutment material.
