Influence of abutment-to-fixture design on reliability and failure mode of all-ceramic crown systems

Abstract

Objective

Evaluate the effect of implant connection designs on reliability and failure modes of screw-retained all-ceramic crowns.

Methods

Central incisor ceramic crowns in zirconia abutments were screwed and torqued down to external hexagon (EH), internal hexagon (IH) and Morse taper (MT) implant systems. Single-load-to-fracture (SLF) test ( n = 4 per group) determined three step-stress fatigue profiles with specimens assigned in the ratio of 3:2:1. Fatigue test was performed under water at 10 Hz. Use level probability Weibull curves and reliability for missions of 50,000 cycles at 400 N and 200 N were calculated (90% confidence bounds-CB). Weibull probability distribution (90% CB) was plotted (Weibull modulus vs characteristic strength) for comparison between the groups. Fractographic analyses were conducted under polarized-light microscopy and SEM.

Results

Use level Weibull probability calculation indicated that failure was not associated with fatigue in groups EH ( β = 0.63), IH ( β = 0.97) and MT ( β = 0.19). Reliability data for a mission of 50,000 cycles at 400 N revealed significant reliability differences between groups EH (97%), IH (46%) and MT (0.5%) but no significant difference at 200 N between EH (100%) and IH (98%), and IH and MT (89%). Weibull strength distribution (figure) revealed β = 13.1/ η = 561.8 for EH, β = 5.8/ η = 513.4 for IH and β = 5.3/ η = 333.2 for MT. Groups EH and IH exhibited veneer cohesive and adhesive failures. Group IH also presented adhesive failure at zirconia/titanium abutment insert while MT showed fracture at abutment neck.

Significance

Although group EH presented higher reliability and characteristic strength followed by IH and MT, all groups withstood reported mean anterior loads.

Introduction

Although the external hexagon connection presents anti-rotational mechanism and compatibility among different implant fixture systems, the short index and higher center of rotation lead to lower resistance for rotational and lateral movements . In contrast, several studies demonstrated that internal connections provide better biological sealing, joint stability, stress distribution and resistance to bending moments as a result of deeper and greater joining walls. However, the thinner lateral fixture wall at the connecting part may lead to high stress concentration and abutment/implant failure .

Within the restoration systems, ceramic implant abutments have been introduced to reduce the risk of gray or blue discolorations of the surrounding soft tissues, especially in patients with a thin gingival biotype . In this sense, zirconium oxide abutments have been commonly used because of its high flexural strength, fracture toughness, low thermal conductivity, low corrosion potential, biocompatibility, and favorable interaction with soft and hard tissues . However, zirconia may also exhibit failures as a result of low temperature degradation, mechanical damage from adjustment or particle abrasion and limited bond strength with the veneering ceramic . Clinical trials and in vitro studies reported that abutment fracture, cohesive failure of the veneering ceramic and adhesive failure at zirconia-veneer interface are common complications with ceramic restorations on zirconia framework .

Several authors have demonstrated the mechanical performance of zirconia abutments with different implant–abutment connections and veneering systems. However, the results are usually based upon either static loading until failure or fatigue tests that do not reproduce loading conditions to cause the system fatigue in a timely manner. To solve this limitation, few studies have applied the step-stress accelerated life testing (SSALT) for reliability evaluation of implants and ceramic crown systems . This fatigue approach consists tests the samples at stress levels higher than clinical conditions within a short time span so that failure is accelerated and specimens can be analyzed in a reasonable time period . The behavior of the samples at accelerated stress is used to reveal the failure behavior of the specimens at use stresses . However, comparison of screw-retained ceramic restorations on zirconia abutments with different implant–abutment connections using SSALT has not been explored to date.

The aim of this study was to evaluate the reliability and failure mode of screw-retained all-ceramic crowns fabricated with zirconia abutments attached to implants with different connections. The research hypothesis assumed that implant connection design influences reliability calculations and failure modes of screwed-retained zirconia abutment/crown system.

Materials and methods

Study design

Three implant–abutment connection designs were evaluated in this study ( n = 22 per group): EH (external hexagon system – Nobel Biocare, Goteborg, Sweden), IH (internal hexagon system – Biomet 3i, Palm Beach Gardens, FL, USA), and MT (Morse taper system – Ankylos – Friadent, Mannheim, Germany).

The dimensions and characteristics of components are shown in Table 1 and Fig. 1 . All abutments and implants were previously screened in stereomicroscopy (SteREO Discovery V20, Carl Zeiss MicroImaging GmbH, Jena, Germany) to check any deformation/crack on component surface and connecting area.

Table 1
Characteristics of implant–abutment connection, implant, abutment and retention screw.
Group Implant–abutment connection Implant Abutment Retention screw (torque N cm)
EH External hexagon Nobelspeedy Groovy RP
(4.0 mm × 13.0 mm)
(Nobel Biocare)
Y-TZP – Procera ® Esthetic Branemark System ® (RP # 6)
(Nobel Biocare)
Titanium alloy
(35 N cm)
IH Internal hexagon Osseotite ® Certain ® (4.0 mm × 13.0 mm)
(Biomet 3i)
Y-TZP + Ti insert – Certain ® ZiReal ® Post
(Biomet 3i)
Gold alloy
(20 N cm)
MT Morse taper Implant C/X B11
(4.5 mm × 11.0 mm)
(Ankylos-Friadent)
Y-TZP – Cercon balance C/large, 3.0 straight, neutral
(Ankylos-Friadent)
Titanium alloy
(15 N cm)

Fig. 1
Longitudinal section of crown/abutment-retention screw-implant systems of groups EH, IH and MT.

Samples preparation

All implants were embedded in modified polyester self-curing resin (Technovit 4000, Heraues Kulzer, Wehrheim, Germany) at 30° angulation to the horizontal plane according to the ISO 14801:2007 ( Dynamic fatigue test for endosseous dental implants ). According to the manufacturer and a previous study , the resin has a modulus of elasticity of approximately 12 GPa, which approximates that of human bone.

Maxillary central incisors crowns (11.0 mm in height, 8.0 mm proximally) were fabricated with the nano-fluorapatite ceramic IPS e.max Ceram (Ivoclar Vivadent, Liechtenstein, Swiss) layered on the prefabricated zirconia abutments according to the manufacturer’s instructions for firing temperature (750 °C) and cycles (furnace Programat EP500, Ivoclar Vivadent). The crowns dimensions were checked in a silicone matrix during layering process to standardize the final shape of all replicas. No additional preparation was conducted in the abutments unless when the height was reduced to provide enough veneering thickness in the incisal edge. Translucent ceramic was veneered for better visualization of flaws/cracks before and during testing events. After polishing and finishing, e.max Ceram glaze was applied according to the manufacturer’s instructions (Ivoclar Vivadent).

After sample manufacturing, the abutment/crown specimens were positioned on each respective implant design group. The retention screws were then placed and tightened using a torque gauge (BTG36CN-S, Tohnichi MFG Co. Ltd., Tokyo, Japan) using the torque level recommended by each manufacturer ( Table 1 ). After 3 min, the screws were retightened to avoid preload loss as demonstrated in previous studies . The screw access channel was restored with polytetrafluoroethylene and composite resin (Z-250, 3M ESPE, St. Louis, MN, USA).

Mechanical testing and reliability analysis

Four specimens of each group underwent single load-to-failure (SLF) testing at cross-head speed of 1 mm/min in a universal testing machine (INSTRON 5666, Canton, MA, USA). A stainless steel blade with rounded edges ( r = 1.0 mm) applied the load 3.0 mm below the incisal edge of the crown in the central region. Based upon the mean load-to-failure, three step-stress accelerated life-testing (SSALT) profiles were assigned to mild ( n = 9), moderate ( n = 6), and aggressive ( n = 3) fatigue profiles (ratio 3:2:1, respectively) . These profiles are named based on the load increase in which a specimen is fatigued to reach a certain level of load. It means that specimens assigned to a mild profile will be cycled longer to reach the same load as a specimen assigned to either moderate or aggressive profiles. The fatigue testing was conducted under water at 10 Hz using an electrodynamic fatigue testing machine (ELF 3300, EnduraTec Division, Bose Corporation, Minnetonka, MN, USA). The load was applied 3.0 mm below the incisal edge of the crown in the central region to standardize the lever arm in all groups.

Fatigue testing was performed until failure (fracture of the veneer, bending or fracture of the abutment) or survival (no failure at the end of the profile) of the specimens. Bending was determined when the displacement of the indenter surpassed the lower machine displacement limit (standardized at 1.5 mm).

Based upon the step-stress distribution of failures, use level probability Weibull curves (probability of failure vs cycles) were calculated (Alta Pro 7, ReliaSoft, Tucson, AZ, USA) using power law relationship for damage accumulation. Reliability for a mission of 50,000 cycles at 400 N and 200 N (90% two-sided confidence interval) was calculated for comparison between the groups. Weibull 2-parameter probability plots were generated yielding Weibull modulus and characteristic strength.

Failure mode analysis

All samples were inspected under polarized light microscopy (MZ-APO stereomicroscope, Carl Zeiss MicroImaging, Thornwood, NY, USA) for failure mode classification and comparison between the groups. The most representative failed samples of each group were gold sputtered (Emitech K650, Emitech Products Inc., Houston, TX, USA) and further observed using a scanning electron microscope (SEM) (Hitachi, Model 3500S, Osaka, Japan) for fractographic analysis.

Materials and methods

Study design

Three implant–abutment connection designs were evaluated in this study ( n = 22 per group): EH (external hexagon system – Nobel Biocare, Goteborg, Sweden), IH (internal hexagon system – Biomet 3i, Palm Beach Gardens, FL, USA), and MT (Morse taper system – Ankylos – Friadent, Mannheim, Germany).

The dimensions and characteristics of components are shown in Table 1 and Fig. 1 . All abutments and implants were previously screened in stereomicroscopy (SteREO Discovery V20, Carl Zeiss MicroImaging GmbH, Jena, Germany) to check any deformation/crack on component surface and connecting area.

Table 1
Characteristics of implant–abutment connection, implant, abutment and retention screw.
Group Implant–abutment connection Implant Abutment Retention screw (torque N cm)
EH External hexagon Nobelspeedy Groovy RP
(4.0 mm × 13.0 mm)
(Nobel Biocare)
Y-TZP – Procera ® Esthetic Branemark System ® (RP # 6)
(Nobel Biocare)
Titanium alloy
(35 N cm)
IH Internal hexagon Osseotite ® Certain ® (4.0 mm × 13.0 mm)
(Biomet 3i)
Y-TZP + Ti insert – Certain ® ZiReal ® Post
(Biomet 3i)
Gold alloy
(20 N cm)
MT Morse taper Implant C/X B11
(4.5 mm × 11.0 mm)
(Ankylos-Friadent)
Y-TZP – Cercon balance C/large, 3.0 straight, neutral
(Ankylos-Friadent)
Titanium alloy
(15 N cm)

Fig. 1
Longitudinal section of crown/abutment-retention screw-implant systems of groups EH, IH and MT.

Samples preparation

All implants were embedded in modified polyester self-curing resin (Technovit 4000, Heraues Kulzer, Wehrheim, Germany) at 30° angulation to the horizontal plane according to the ISO 14801:2007 ( Dynamic fatigue test for endosseous dental implants ). According to the manufacturer and a previous study , the resin has a modulus of elasticity of approximately 12 GPa, which approximates that of human bone.

Maxillary central incisors crowns (11.0 mm in height, 8.0 mm proximally) were fabricated with the nano-fluorapatite ceramic IPS e.max Ceram (Ivoclar Vivadent, Liechtenstein, Swiss) layered on the prefabricated zirconia abutments according to the manufacturer’s instructions for firing temperature (750 °C) and cycles (furnace Programat EP500, Ivoclar Vivadent). The crowns dimensions were checked in a silicone matrix during layering process to standardize the final shape of all replicas. No additional preparation was conducted in the abutments unless when the height was reduced to provide enough veneering thickness in the incisal edge. Translucent ceramic was veneered for better visualization of flaws/cracks before and during testing events. After polishing and finishing, e.max Ceram glaze was applied according to the manufacturer’s instructions (Ivoclar Vivadent).

After sample manufacturing, the abutment/crown specimens were positioned on each respective implant design group. The retention screws were then placed and tightened using a torque gauge (BTG36CN-S, Tohnichi MFG Co. Ltd., Tokyo, Japan) using the torque level recommended by each manufacturer ( Table 1 ). After 3 min, the screws were retightened to avoid preload loss as demonstrated in previous studies . The screw access channel was restored with polytetrafluoroethylene and composite resin (Z-250, 3M ESPE, St. Louis, MN, USA).

Mechanical testing and reliability analysis

Four specimens of each group underwent single load-to-failure (SLF) testing at cross-head speed of 1 mm/min in a universal testing machine (INSTRON 5666, Canton, MA, USA). A stainless steel blade with rounded edges ( r = 1.0 mm) applied the load 3.0 mm below the incisal edge of the crown in the central region. Based upon the mean load-to-failure, three step-stress accelerated life-testing (SSALT) profiles were assigned to mild ( n = 9), moderate ( n = 6), and aggressive ( n = 3) fatigue profiles (ratio 3:2:1, respectively) . These profiles are named based on the load increase in which a specimen is fatigued to reach a certain level of load. It means that specimens assigned to a mild profile will be cycled longer to reach the same load as a specimen assigned to either moderate or aggressive profiles. The fatigue testing was conducted under water at 10 Hz using an electrodynamic fatigue testing machine (ELF 3300, EnduraTec Division, Bose Corporation, Minnetonka, MN, USA). The load was applied 3.0 mm below the incisal edge of the crown in the central region to standardize the lever arm in all groups.

Fatigue testing was performed until failure (fracture of the veneer, bending or fracture of the abutment) or survival (no failure at the end of the profile) of the specimens. Bending was determined when the displacement of the indenter surpassed the lower machine displacement limit (standardized at 1.5 mm).

Based upon the step-stress distribution of failures, use level probability Weibull curves (probability of failure vs cycles) were calculated (Alta Pro 7, ReliaSoft, Tucson, AZ, USA) using power law relationship for damage accumulation. Reliability for a mission of 50,000 cycles at 400 N and 200 N (90% two-sided confidence interval) was calculated for comparison between the groups. Weibull 2-parameter probability plots were generated yielding Weibull modulus and characteristic strength.

Failure mode analysis

All samples were inspected under polarized light microscopy (MZ-APO stereomicroscope, Carl Zeiss MicroImaging, Thornwood, NY, USA) for failure mode classification and comparison between the groups. The most representative failed samples of each group were gold sputtered (Emitech K650, Emitech Products Inc., Houston, TX, USA) and further observed using a scanning electron microscope (SEM) (Hitachi, Model 3500S, Osaka, Japan) for fractographic analysis.

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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Influence of abutment-to-fixture design on reliability and failure mode of all-ceramic crown systems
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