This in vitro study showed that the 3 different types of zirconia abutments had a comparable fracture resistance but different modes of failure under the static load for the platform-switched internal hexagonal implants. Therefore, the null hypothesis could not be rejected. In recent years, zirconia abutments have been increasingly used in clinical applications because of their excellent fracture resistance relative to other ceramic abutments, including abutments made of alumina. Among them, yttrium-stabilized tetragonal zirconia polycrystal (Y-TZP) received the most attention due to its excellent mechanical properties. However, the impact of the load applied on zirconia abutment is still inconclusive. Studies have determined that the type of implant-abutment connection, the physical properties of raw stock, and the manufacturing and experimental methods may significantly affect the abutment strength29. Xu et al.30 studied the effect of grinding parameters on the strength of Y-TZP. Its strength increased substantially only when 25 µm diamond wheel was used in fine machining, while coarse grinding resulted in a decrease in strength. The coarser the diamond wheel they use, the lower the measured strength.
Cyclic loading, which simulates fatigue loading, is the cause of most clinical failures. However, static load tests can simulate situations where the implant complex hits on a hard object and traumatized. In some cases, such as patients with parafunction habits, (e.g., bruxism, clenching, etc.), the incisive force could also be much higher than the physiological range31. Cyclic loading and static loading are two independent conditions, and both may affect the settling of the implant-abutment connection after occlusal load32. Gehrke et al.21 investigated fatigue resistance of one- and two-piece CAD/CAM zirconia abutments and reported that two-piece abutments with an internal-hex connection demonstrated greater resistance to fracture compared to one-piece zirconia abutments and might be clinically beneficial in high-load areas such as posterior tooth replacements.
The weak point of many all-ceramic abutments is located at the implant-abutment interface. The two-piece abutment design provides metal reinforcement at the implant-abutment connection for greater fracture resistance and meanwhile possesses the ideal aesthetic of all-ceramic abutment22. Stimmelmayr et al.33 reported that the zirconia implant abutments attached to the titanium core showed higher fracture strength than the one-piece zirconia abutments and indicated that they may be more suitable for clinical use. The authors also suggested that titanium implants exhibited higher interface wear under cyclic loading when attached to one-piece zirconia abutments than attached to titanium abutments34. In this regard, two-piece abutment with titanium-titanium connections may be beneficial for clinical applications.
It has been reported that implant abutments with internal hexagonal connections are more stable than external hexagonal designs because of their wider stress distribution along the interface35. However, Sailer et al.24 concluded that under oblique loading, internally connected one-piece zirconia abutments are less resistant to fracture than externally connected one-piece zirconia abutments. The mean fracture load of externally connected (NobelBiocare) zirconia abutments is 480.9 N (± 182.8), whereas that of internally connected (Straumann, Basel, Switzerland) zirconia abutments is 292.0 N (± 218.4), which is similar to the maximum load measured in present study. The fracture resistance of the 3 types of zirconia abutments is within the physiological shear range of the anterior zone (approximately 90 to 370 N). Whether the mechanical strength is sufficient for long-term use in the anterior region remains to be confirmed by more clinical studies.
Along with the introduction of CAD/CAM and aftermarket prosthetic components into the clinical use, non-original abutments receive more attention. Although aftermarket zirconia abutments showed similar fracture resistance with OEM zirconia abutments in current study, Gigandet et al.22 reported aftermarket abutments that changed the original design and materials showed higher rotational misfit. This mismatch may result in increased wear and micromotion between the titanium and zirconia interfaces, ultimately lead to surface flaws and zirconia cracks36. In comparing the torque loss of four types of abutments, Park et al.37 reported that the torque loss of original abutments was lower than those of copy abutments. Similar result was obtained by Alonso-Pérez et al.38 and concluded that internal precision at the implant–abutment connections is a crucial determinant to the mechanical properties of abutments, possibly explaining the superiority of original abutments over its non-original counterparts. However, few clinical studies have evaluated the effects of non-original abutments on implants. Therefore, more clinical studies should be conducted to test the incidence of failure and complications of implant complexes with the original and non-original connections.
In All-Zr group, the zirconia abutments showed an oblique fracture pattern. The fracture surface was higher at palatal side of the abutments and significantly lower at buccal side, showing an oblique fracture line starting from the buccal aspect at the region of the implant platform, which is similar to the results of the study reported by Nothdruft et al.27 In those zirconia abutments with a titanium insert, the fracture surface showed a relatively horizontal pattern, which demonstrated titanium inserts raises the buccal fracture surface away from the implant platform, the level of fracture surface at buccal side of zirconia abutments increased with the height of the titanium inserts, which may have the effect of protecting the implant-abutment connection. Kim et al.39 compared the modes of failure among 3 different zirconia abutments after static load. The abutment consisted entirely of zirconia showed fractures arisen from the connection area. The zirconia abutment with friction-fitted titanium insert showed fractures generated from the contact area between zirconia and the screw head. The zirconia abutment with bonded titanium insert showed the separation between the two parts. The results demonstrated that the mode of failure among three types of zirconia abutments was different. However, the present study demonstrated the zirconia abutment with titanium inserts had a similar mode of failure with different height of fracture surfaces, which depended on the height of titanium inserts.
Clinically, the deeper the abutment structure damaged may make the clinicians more difficult to manage, increasing the complexity of complication. The removal of fractured components may cause irreversible damage to the implant platform and internal structure, and if catastrophically damaged, the implant fixture may need to be surgically removed or replaced. Therefore, the titanium inserts might keep fracture surface away from the implant platform and is beneficial to the clinicians to replace implant prostheses. In this study, there was no catastrophic damage or fracture of the abutment screws or implants founded, which is consistent with the results of studies conducted by Mitsias et al.17. This mode of failure is quite different from titanium abutments. Yilmaz et al.40 investigated five different titanium abutments for load to failure, with four of which showing fracture of retentive screw. Only one abutment did not show any components to fracture, but eventually, bending of the screw beyond the plastic range did occur.
The limitations of this study require careful interpretation to correctly explain the clinical implications. The first is the use of static load rather than cyclic load in this study. However, this static load test can be regarded as a preliminary study, future fatigue load projects can be designed based upon the mean failure load in this study. Secondly, this study uses only internal hexagonal connections and standard diameter implants with one implant system, the results may not be applicable to other implant systems. Therefore, additional clinical studies are needed to determine the clinical performance of various zirconia abutments and to provide guidelines for clinical use.