Effect of Different Implant-Abutment Connection Materials on the Fracture Resistance of Zirconia Abutments

Placement of an implant in the esthetic zone for replacement of a single tooth has demonstrated survival rates of up to 100%.^1,2  However, oral rehabilitation of partial edentulism in the esthetic zone is challenging as numerous factors including surgical protocol, soft tissue status, implant dimensions, and abutment design influence the final emergence profile of the implant-supported prosthesis.^3–7  It is known that the implant abutment connection region is the frailest portion of an internal connection zirconia abutment, which makes it vulnerable to fracture.⁸  In this context, an ideal implant abutment should be mechanically strong and withstand functional loads without risk of deformation and fracture. A densely sintered alumina ceramic abutment exhibits reduced load to fracture; however, the fracture load of alumina abutment is weaker compared with zirconia.⁹  Due to the weakness of the mechanical properties of alumina, yttrium oxide–stabilized zirconia was introduced as an alternative material for implant abutment.

The abutment can be fixed into the implant either by external or internal connection, whereas the internal connection of zirconia abutments can be accomplished either by the 1- or 2-piece zirconia. In the 1- and 2-piece zirconia abutment, the implant connection is made of titanium, correspondingly (Figure 1). In an in vitro study, Alqahtania and Flinton¹⁰  investigated the influence of different levels of preparation of Zirconia-implant abutments on fracture load. The results showed a statistically significant difference between the unprepared zirconia abutment (567.3 ± 35.4 N), and prepared zirconia abutment with 1 and 1.5 mm step in the labial margin (445.4 ± 41.0 N and 430.5 ± 39.4 N, respectively).¹⁰  In this regard, zirconia implant abutments have gained popularity over titanium and alumina abutments due to their improved fracture load property.⁹ 

Studies¹¹  comparing the fracture load and mode of failures of 1- and 2- piece zirconia have shown controversial results. Kim et al¹¹  compared the fracture load and mode of failures of 1- and 2-piece Zirconia abutment under static load. The results reported no statistically significant difference in maximum load capacity for 1- and 2-piece zirconia.¹¹  On the contrary, in vitro results by Gehrke et al¹²  showed that the 1-piece zirconia abutment have a lower fracture load than 2piece zirconia abutments. Furthermore, experimental results by Chun et al⁸  showed that the 2-piece zirconia abutments have a higher fracture load compared to 1-piece zirconia abutments. The null hypothesis of the present study is that there is no difference of the post-fatigue load-to-fracture value of 1- and 2-piece zirconia abutments.

With this background, the aim of the present in vitro study was to assess the modes of failures under static load among titanium (Ti) and 1- and 2-piece zirconia abutments.

The 1- and 2-piece zirconia abutment specimens were fabricated via scanning the titanium abutment using a scanner (NobelProcera 2G Scanner, Zürich-Flughafen, Switzerland). This was done to standardize the design of the 1-piece abutment and make it as a blueprint of the titanium abutment (Figure 2).

Twenty-one implant abutments and 21 implant replicas (Noble Replace, Zürich-Flughafen, Switzerland) were equally divided into 3 groups as follows: Group 1 (titanium group); Group 2 (1-piece zirconia abutment group); and Group 3: 2-piece zirconia abutment group).

The implant replicas were mounted in a metal jig. The jig was machined out of stainless steel into 2 parts with engraved pins to receive one implant replica. A metal base was fabricated to hold the metal jig with secure screws to fix the position of specimen. The top of the base had a 135° inclination to the load application axis and the bottom attached to machine table. The load applicator was fabricated of stainless steel and shaped to simulate the lower central incisor edge. The abutment was screwed into the implant replica at 35 Ncm using an implant motor console (Digital Torque, W&H Implantmed, Bürmoos, Austria). The abutment screw was retightened again after 10 minutes as described elsewhere.¹³  The implant abutment (a) titanium; (b) 1-piece zirconia; and (c) 2-piece zirconia mounted in the metal jig are shown in Figure 3.

Statistical analysis was performed using a computer-based software package (SPSS, Version 20, Chicago, Ill). Sample distribution was assessed using the Kolmogorov and Shapiro tests. The homogeneity of variances was statistically assessed using the Levene's test. P values <.05 were considered statistically significant. Group comparisons were performed using the 1-way analysis of variance. For multiple comparisons, the Bonferroni post-hoc adjustment test was also performed. The sample-size calculation was based on the mean values and standard deviation as used by Gehrke et al.¹²  Based upon the results of a pilot investigation, it was estimated that with the use of 7 abutments per group, the study would attain a power of 80% power to detect differences among the mean with a .05 (α) level of significance. All statistical analysis was performed by a trained statistician (Zakaria Ullah, Department of Biostatistics, University of Panjab, Pakistan).

In all groups, the specimens survived under fatigue test after delivering 250,000 load cycles that were performed to simulate a 1-year human chewing function.

The mean load-to-fracture values were significantly higher in the titanium (1095.5 ± 290.5 N; P < .001) and 2-piece zirconia (1137.8 ± 20.5 N; P < .001) abutment groups compared with the 1-piece zirconia group. There was no statistically significant difference in the mean load-to-fracture values for the titanium (1095.5 ± 290.5 N) and 2-piece zirconia (1137.8 ± 20.5 N) abutment groups (Table). The lowest and highest abutment load-to-fracture values were recorded in the 1-piece zirconia (633.9 N) and titanium (1708.4 N) groups, respectively.

The null hypothesis of the present in vitro experiment was that there is no difference of the post-fatigue load-to-fracture value of 1- and 2-piece zirconia abutments. The current results are in contradiction to this hypothesis as a statistically significant difference in the load-to-fracture values was observed among the 1- and 2-piece zirconia abutments. In summary, the 1-piece zirconia abutments displayed significantly lower fracture-load values than the 2-piece zirconia abutments. One explanation for this could be associated by analyzing the failure occurring at the thinnest section of the abutment in the most apical part implant connection segment (ICS), which represents the weakest component. The present results also demonstrated that the 2-piece zirconia abutments had a significantly higher fracture-load compared with the 1-piece zirconia abutment. This could possibly be associated with the titanium adapt at the implant abutment connection. However, it is noteworthy that the mean load to fracture of 1-piece zirconia abutments was approximately 740 N (Table). From a clinical perspective, this suggests that these abutments are functional as the maximum anterior bite forces in healthy humans range from 108 to 370 N.

To recommend different types of all-ceramic abutments in clinical practice, a comparison of the present results with previous studies may be a useful strategy. Nevertheless, such comparisons are exigent to perform as a variety of parameters and factors including abutment dimensions, specimen aging, preceding fatigue loading, fatigue cycle number, a direction of load, and uncrowned abutments are associated with the overall functional effectiveness of the abutments.^12,14  According to the results reported in the study by Gehrke et al,¹²  1-piece zirconia abutment have lower load to fracture than 2-piece zirconia abutment. Similarly, Chun et al⁸  reported that 2-piece zirconia abutments have higher load-to-fracture values than 1-piece zirconia; and in their study,⁸  the maximum fracture load was in titanium abutments. Though the current results support the studies by Chun et al⁸  and Gehrke et al,¹²  there are some differences in the present results, which could be attributed to the different abutment system and dimensions, fatigue load, number of cycles, and direction of load applied. Our findings are in contradiction to those reported in the study by Kim et al,¹⁵  which compared the fracture load of 1- and 2-piece zirconia abutment under static load. The authors found no significant difference in maximum load capacity for 1- and 2-piece zirconia abutments.¹⁵  One explanation for this may be associated with the different abutment system, direction of the load, and preceding the load-to-fracture test without doing the fatigue test. Moreover, Alqahtani and Flinton¹⁰  tested the fracture-load of zirconia implant abutments with different levels of preparation and reported the mean load to fracture of unprepared 1- and 2-piece abutments.

In the present study, all the specimens were exposed to fatigue test by delivering 250,000 load cycles, which ranged between 10 N and 210 N. This was done to simulate a one-year human chewing function.¹⁶  Moreover, we tested the fatigue and load fractures on uncrowned abutments and all other variables associated with the assembly were removed from the experiment. This was primarily done since use of a full crown could have acted as a confounding variable; and the crown cement interface may withstand a higher force before failure due to its stress shielding mechanism. These factors could have considerably (if not significantly) changed the load-to-fracture values. Furthermore, stainless steel implant replicas (instead of titanium implants) were used in the present study. This was done because replica made from stainless steel accept a higher load to fracture before failure than titanium implants; and replicas fabricated from stainless steel eliminate any confounding variable that might affect the results as recommended in the study by Adatia et al.¹⁷ 

A limitation of the present study is that the outcomes were based on an in vitro study design. In this regard, the results presented in the present in vitro investigation should be interpreted with caution due to the study design and on a rather small sample size. From a clinical perspective, it is speculated that in patients that exert high occlusal forces during mastication or patients with habits such as nocturnal bruxism exhibit fracture modes around 1- and 2-piece zirconia abutments that are similar to the results presented in the present experimental study. Further studies are needed to test the aforementioned hypothesis. Moreover, 2 variables, the screw design and abutment thickness, were not standardized in 2-piece zirconia implants. This limitation is attributed to the software that did not assess the same Ti abutment scanned files. Such a limitation may be addressed via modifications at the access hole and adding zirconia to fill the junction gap between the Ti adapt and zirconia core. This warrants further studies.

In vitro, the 1-piece zirconia abutments are more fracture resistant than titanium and 2-piece zirconia abutments. From a clinical perspective, further studies are needed to determine the minimum static load value required to induce fracture of the 1- and 2-piece zirconia abutments.

Abbreviations

ICS:

implant connection segment

Ti:

titanium

The authors thank Abed Mahmud of Gina Medical Center, Lahore, Pakistan, for his kind assistance with statistical analysis and data interpretation.

The authors declare no conflicts of interest.