Alumina reinforced zirconia implants: Effects of cyclic loading and abutment modification on fracture resistance

Abstract

Objective

The aim of the study was to evaluate the thermomechanical behavior of alumina-toughened zirconia (ATZ) oral implants in the artificial mouth and the fracture resistance (fracture load and bending moment) in a subsequent static fracture load test. The effects of abutment modification and different cyclic loadings were evaluated.

Methods

A total of 48 implants were used. 24 implants were left as machined (Group A), and 24 implants were shape modified at the abutment (Group B). Groups were divided into three subgroups composed of 8 samples each (A1/B1: no cyclic loading; A2/B2: 1.2 million cycles; A3/B3: 5 million cycles). Subsequently, all implants were statically loaded to the point of fracture.

Results

The implants showed the following survival rates after the artificial mouth: A2 and B2 100%; A3 and B3 87.5%. The following average fracture resistance values were found (fracture load [N]/bending moment [N mm]): A1 (583/2907), B1 (516/2825), A2 (618/2737), B2 (550/3150), A3 (802/3784) and B3 (722/3809). After 5 million loading cycles a significant increase in fracture load and bending moment was found. Modification of the abutment significantly decreased the fracture load of implants without foregoing dynamic loading. However, the shape modification altered the lever arm. For that reason, a smaller load resulted in the same bending moment. Therefore, abutment modification had no significant influence on the fracture resistance of ATZ.

Significance

Neither thermomechanical cycling in an aqueous environment nor modification of the abutment had a negative effect on the fracture resistance of ATZ.

Introduction

Oral implants made of titanium have been shown to function well for many years . However, the esthetic and biological properties of titanium have presented challenges and raised questions. The presence of thin peri-implant mucosa or soft-tissue recessions may result in visibility of the opaque-gray titanium implant. For the patient, this is particularly undesirable in the esthetically demanding anterior tooth region . Furthermore, titanium residues have been detected in peri-implant soft-tissues and bone biopsies and it has been postulated that titanium may lead to hypersensitization .

Newly developed high-performance ceramics of zirconium dioxide (ZrO 2 ) may be an alternative to titanium as an implant material. These ceramics possess good initial mechanical strength , exhibit favorable tissue compatibility , and show osseointegration comparable to that of titanium . A further advantage of zirconia is the reduced plaque accumulation and greater resistance to mechanical processing . Moreover, the white-opaque ZrO 2 ceramic resembles the tooth in terms of color, and thus provides good esthetics even with a thin gingiva or with soft-tissue recessions . Zirconia has been successfully used in dentistry for restorations such as root posts , crowns and bridges , and implant abutments . The use of zirconia in oral implantology and fixed implant prosthodontics is still in its developmental stages and little research has been conducted so far for oral implants regarding the mechanical stability and ceramic aging . The aging of zirconia can result in a decrease in the initially high flexural strength which may lead to fatigue fractures under “normal” masticatory loading . Zirconia implants may undergo slow degradation during long term implantation in the human body . A zirconium dioxide ceramic reinforced with alumina [ATZ (alumina-toughened zirconia, with 20 wt% alumina); Fig. 1 and Table 1 ] may circumvent these limitations . Alumina-toughened zirconia has higher toughness values than yttria-stabilized zirconia. Additionally, increased alumina acts to constrain the zirconia particles, retaining the tetragonal zirconia in a metastable state, resulting in toughening the ceramic implant material. Moreover, the hardness of composites with increased alumina volume is greater, since alumina is harder than zirconia. This should lead to higher mechanical stability and lower aging of ATZ . Finally, ATZ was shown to have no negative influence on behavior or differentiation of surrounding cells .

Fig. 1
Structure of the ATZ (alumina-toughened zirconia) ceramic.

Table 1
Material properties according to the manufacturer (ATZ: alumina-toughened zirconia; TZP: tetragonal zirconiumdioxide polycrystal).
Characteristics Unit TZP ATZ
Components ZrO 2 /Y 2 O 3 ZrO 2 /Al 2 O 3 /Y 2 O 3
Composition wt% 95/5 76/20/4
Density g/cm 3 6.05 5.5
Grain size μm 0.4 0.4
Bending strength MPa 1.000 2.000
Compressive strength MPa 2.000 2.000
Young’s modulus GPa 200 220
Fracture toughness MPa m 1/2 8 8

There is limited data on the stability of oral implants fabricated of ATZ . The purpose of this investigation was to evaluate the thermomechanical stability of a commercially available ATZ implant system before and after artificial loading conditions in an aqueous environment over a simulated time period up to 20 years. Since one-piece implants often need intraoral final modification and given the fact that grinding is recognized to influence the strength of zirconia , an additional aim of this study was to investigate whether modification of the abutment would reduce the fracture resistance of this implant. The null hypothesis of this study was that neither thermomechanical cycling in an aqueous environment nor modification of the abutment will decrease the fracture resistance of the ATZ ceramic implant.

Materials and methods

A total of 48 one-piece alumina-toughened zirconia ceramic implants (4.4 mm diameter, 12 mm intraosseous length, 2.6 mm shoulder height, 6 mm length of the abutment; Metoxit, Thayngen, Switzerland) were used for the experiment ( Fig. 2 a ). Material properties, as indicated by the manufacturing company, are shown in Table 1 . The implants were divided into two groups ( Table 2 ). Group A consisted of twenty-four unmodified implants. The abutments of group B implants were shape modified according to the preparation guidelines of a central incisor ( Fig. 2 b). The two groups were further divided into three subgroups: subgroup 1 – eight implants that were not subjected to artificial loading in the chewing machine; subgroup 2 – eight implants that were subjected to 1.2 million loading cycles in the chewing machine with thermocycling; subgroup 3 – eight implants that were subjected to 5 million loading cycles with thermocycling. The preparation of group B implants was performed with a Gentlepower Lux handpiece (Gentlepower Lux 25 LP, KaVo, Biberach, Germany) at a maximum speed of 140,000 rpm and water cooling from a triple-port spray system with 50 ml/min. The primary preparation was executed with diamond-coated instruments with a grain size of 120 μm (Figure 6878.314.012; Brasseler Komet, Lemgo, Germany). The finishing of the preparation was performed with diamond instruments of a grain size of 40 μm (Figure 8878.314.012; Brasseler Komet). Since the implant-head consisted of a triangular rounded cone with a circumferential shoulder the modifications were limited to the preparation of a buccal bevel and a palatal flattening ( Fig. 2 b). A silicon index (Twinduo; Picodent, Wipperfuerth, Germany) of the master preparation served as control to ensure a standardized abutment shape for all modified implants.

Fig. 2
Implant without modification (a) and implant with modified abutment (b) according to the preparation guidelines of a central incisor.

Table 2
Grouping of test and control specimens.
48 Ziraldent ® -Implants
Group A
24 Implants without modification
Group B
24 Implants with modified abutment
A1
8 Implants
0 cycles
A2
8 Implants
1.2 × 10 6 cycles
A3
8 Implants
5 × 10 6 cycles
B1
8 Implants
0 cycles
B2
8 Implants
1.2 × 10 6 cycles
B3
8 Implants
5 × 10 6 cycles
Dynamic loading test (chewing simulator) Dynamic loading test (chewing simulator)
Static loading test

The implants were embedded in an autopolymerizing acrylic resin (Technovit® 4000, Heraeus Kulzer, Wehrheim, Germany) in special sample holders. To realize a standardized procedure, two master sample holders were filled with a silicon material (Twinduo; Picodent, Wipperfuerth, Germany; Shore A hardness: 90). The use of an adjusted drilling jig allowed to drill a hole at an angle of 45° to the vertical, replicating the position of upper central incisors . In order to represent a physiological clinical situation after one year (0.5–1 mm of bone remodeling ), two master implants (modified and unmodified) were screwed in the prepared hole up to the first implant thread. Silicon indices of these master models were used to embed 24 modified and 24 unmodified sample implants. The resin had a modulus of elasticity of approximately 12 GPa which approximates that of human bone (10–18 GPa) . In order to compensate deviations due to the embedding procedure, standardized photographs in front of an adjusted grid were used to determine the individual embedding angle and lever arm for each of the 48 samples (Fraunhofer Institute for Mechanics of Materials, Freiburg, Germany; Fig. 3 ).

Fig. 3
Standardized photographs of the embedded implants without (a) and with (b) abutment modification allowed the calculation of the lever arm ( y = sin α ·l).

Dynamic loading test

Thirty-two of the specimens were thermomechanically aged in a computer-controlled dual axis-chewing simulator in an aqueous environment (Willytec, Munich, Germany; Fig. 4 ) in order to simulate five years (1.2 million cycles; subgroups A2, B2) and almost twenty years (5 million cycles; subgroups A3, B3) of clinical service, assuming an annual masticatory performance of 240,000–250,000 occlusal contacts . The chewing simulator-environment consisted of eight identical sample chambers, two stepper motors controlling vertical and horizontal movements of the antagonists (Steatit ® ceramic balls, 6 mm in diameter, Hoechst Ceram Tec, Wunsiedel, Germany) against the implant samples, and a hot and cold water circulation system (Haake, Karlsruhe, Germany). The antagonist ball had a Vickers hardness similar to that of enamel . The applied load in the chewing simulator was 98 N (10 kg) and the point of load application on the implants was placed on the palatal upper edge. This resulted in an elongated lever arm for the modified samples. The load was applied onto the implants by combined vertical (6 mm) and horizontal (0.5 mm) movements, which – via computerized adaptation – represented an approximation to the physiological masticatory cycle of axial pressure and horizontal shear. The cyclic loading was set at 1.6 Hz. The thermocycling was from 5 °C to 55 °C for 60 s each with an intermediate pause of 12 s, maintained by the thermostatically-controlled liquid circulator (Haake, Karlsruhe, Germany). During the dynamic loading, all samples were examined twice a day. The chewing machine needed approximately 9 and 36 days to accomplish 1.2 million cycles (Group B) and 5 million cycles (Group C). Fractures of the implants were recorded as a failure. The details of the settings of the chewing simulator machine are listed in Table 3 .

Fig. 4
Schematic drawing of the chewing simulator (Willytec, Munich, Germany). The vertical guide rail and the sample holder weigh another 1 kg.

Table 3
Settings of the chewing simulator machine.
Chewing cycles 1,200,000/5,000,000
Cycle frequency 1.6 Hz
Vertical movement 6 mm
Horizontal movement 0.5 mm
Descending speed 60 mm/s
Rising speed 55 mm/s
Forward speed 60 mm/s
Backward speed 55 mm/s
Applied weight per sample 10 kg (98 N)
Hot dwell time 60 s
Hot bath temperature 55 °C
Cold dwell time 60 s
Cold bath temperature 5 °C
Intermediate pause 12 s

Static loading test

All samples that survived the exposure to the chewing simulator without fracture were statically loaded to fracture using a universal-testing machine (Zwick, Z010/TN2S, Ulm, Germany). All samples were loaded using the same sample holders and at the same contact point as used for the dynamic loading. A vertical compressive load was applied on the palatal side of the angulated implants under a crosshead speed of 10 mm/min. The loads required for fracturing the samples were recorded using the X-Y writer of the Zwick testXpert ® V 7.1 software, with failure recorded at the first sharp drop-down of the graphical curve (fracture of the ceramic, Fig. 5 ). A graph was drawn for each sample representing and was used to determine the load at time of fracture.

Fig. 5
Static loading test of an implant with modified abutment (a); site of fracture after loading (b).

Statistical analysis

Mean values of the acquired data (fracture load, lever arm and bending moment) of each subgroup (A1, A2, A3 and B1, B2, B3) were calculated to create graphs of the resulting confidence intervals. A two-way analysis of variance (ANOVA) was used. The continuous response variables (fracture load, lever arm and bending moment) were modeled as a function of abutment modification (w = with modification; w/o = without modification), cycles (0, 1.2 million cycles, 5 million cycles) and the corresponding interaction as explanatory variables. Pairwise differences of least-square means were calculated and p -values were adjusted by the method of Tukey. The level of significance was set at p < 0.05. The Wilcoxon-Test allowed multiple pairwise comparisons of different subgroups regarding their response variables. All computations were performed with the statistical software SAS (SAS system v9.1; SAS Institute Inc., Cary, NC).

Materials and methods

A total of 48 one-piece alumina-toughened zirconia ceramic implants (4.4 mm diameter, 12 mm intraosseous length, 2.6 mm shoulder height, 6 mm length of the abutment; Metoxit, Thayngen, Switzerland) were used for the experiment ( Fig. 2 a ). Material properties, as indicated by the manufacturing company, are shown in Table 1 . The implants were divided into two groups ( Table 2 ). Group A consisted of twenty-four unmodified implants. The abutments of group B implants were shape modified according to the preparation guidelines of a central incisor ( Fig. 2 b). The two groups were further divided into three subgroups: subgroup 1 – eight implants that were not subjected to artificial loading in the chewing machine; subgroup 2 – eight implants that were subjected to 1.2 million loading cycles in the chewing machine with thermocycling; subgroup 3 – eight implants that were subjected to 5 million loading cycles with thermocycling. The preparation of group B implants was performed with a Gentlepower Lux handpiece (Gentlepower Lux 25 LP, KaVo, Biberach, Germany) at a maximum speed of 140,000 rpm and water cooling from a triple-port spray system with 50 ml/min. The primary preparation was executed with diamond-coated instruments with a grain size of 120 μm (Figure 6878.314.012; Brasseler Komet, Lemgo, Germany). The finishing of the preparation was performed with diamond instruments of a grain size of 40 μm (Figure 8878.314.012; Brasseler Komet). Since the implant-head consisted of a triangular rounded cone with a circumferential shoulder the modifications were limited to the preparation of a buccal bevel and a palatal flattening ( Fig. 2 b). A silicon index (Twinduo; Picodent, Wipperfuerth, Germany) of the master preparation served as control to ensure a standardized abutment shape for all modified implants.

Fig. 2
Implant without modification (a) and implant with modified abutment (b) according to the preparation guidelines of a central incisor.

Table 2
Grouping of test and control specimens.
48 Ziraldent ® -Implants
Group A
24 Implants without modification
Group B
24 Implants with modified abutment
A1
8 Implants
0 cycles
A2
8 Implants
1.2 × 10 6 cycles
A3
8 Implants
5 × 10 6 cycles
B1
8 Implants
0 cycles
B2
8 Implants
1.2 × 10 6 cycles
B3
8 Implants
5 × 10 6 cycles
Dynamic loading test (chewing simulator) Dynamic loading test (chewing simulator)
Static loading test

The implants were embedded in an autopolymerizing acrylic resin (Technovit® 4000, Heraeus Kulzer, Wehrheim, Germany) in special sample holders. To realize a standardized procedure, two master sample holders were filled with a silicon material (Twinduo; Picodent, Wipperfuerth, Germany; Shore A hardness: 90). The use of an adjusted drilling jig allowed to drill a hole at an angle of 45° to the vertical, replicating the position of upper central incisors . In order to represent a physiological clinical situation after one year (0.5–1 mm of bone remodeling ), two master implants (modified and unmodified) were screwed in the prepared hole up to the first implant thread. Silicon indices of these master models were used to embed 24 modified and 24 unmodified sample implants. The resin had a modulus of elasticity of approximately 12 GPa which approximates that of human bone (10–18 GPa) . In order to compensate deviations due to the embedding procedure, standardized photographs in front of an adjusted grid were used to determine the individual embedding angle and lever arm for each of the 48 samples (Fraunhofer Institute for Mechanics of Materials, Freiburg, Germany; Fig. 3 ).

Fig. 3
Standardized photographs of the embedded implants without (a) and with (b) abutment modification allowed the calculation of the lever arm ( y = sin α ·l).

Dynamic loading test

Thirty-two of the specimens were thermomechanically aged in a computer-controlled dual axis-chewing simulator in an aqueous environment (Willytec, Munich, Germany; Fig. 4 ) in order to simulate five years (1.2 million cycles; subgroups A2, B2) and almost twenty years (5 million cycles; subgroups A3, B3) of clinical service, assuming an annual masticatory performance of 240,000–250,000 occlusal contacts . The chewing simulator-environment consisted of eight identical sample chambers, two stepper motors controlling vertical and horizontal movements of the antagonists (Steatit ® ceramic balls, 6 mm in diameter, Hoechst Ceram Tec, Wunsiedel, Germany) against the implant samples, and a hot and cold water circulation system (Haake, Karlsruhe, Germany). The antagonist ball had a Vickers hardness similar to that of enamel . The applied load in the chewing simulator was 98 N (10 kg) and the point of load application on the implants was placed on the palatal upper edge. This resulted in an elongated lever arm for the modified samples. The load was applied onto the implants by combined vertical (6 mm) and horizontal (0.5 mm) movements, which – via computerized adaptation – represented an approximation to the physiological masticatory cycle of axial pressure and horizontal shear. The cyclic loading was set at 1.6 Hz. The thermocycling was from 5 °C to 55 °C for 60 s each with an intermediate pause of 12 s, maintained by the thermostatically-controlled liquid circulator (Haake, Karlsruhe, Germany). During the dynamic loading, all samples were examined twice a day. The chewing machine needed approximately 9 and 36 days to accomplish 1.2 million cycles (Group B) and 5 million cycles (Group C). Fractures of the implants were recorded as a failure. The details of the settings of the chewing simulator machine are listed in Table 3 .

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Alumina reinforced zirconia implants: Effects of cyclic loading and abutment modification on fracture resistance
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