According to the results of this study the null hypothesis shall be accepted. To the best of our knowledge, a thorough failure analysis of retrieved NDI fixtures has not been reported. However, fatigue failure and fracture analysis of prosthetic retaining screws for implant-supported and retained prostheses have been previously reported in the literature [23]. Al Jabbari et al. documented the classical three stages of fatigue failure in prosthetic retaining screws that failed in patients after long-term service. Their findings were comparable to the observations in this study regarding the presence of ratchet and beach marks and striations. The only difference were the features of fatigue failure in this study; beach marks were fainter and striations smaller in size. This is mainly due to the difference in the alloy type. In this study, the alloy was found to be Ti-14Zr, whereas in a previous study, the alloys of the prosthetic retaining screws were mainly 70% wt gold alloys. Gold-based alloys are more ductile with larger grain size, allowing for more distinct macro/microscopic fatigue fracture features. In this study, striations (Fig. 6d) were difficult to observe because of the small grain size (2–5 µm) of the Roxolid® alloy.
In this study, both metallography and energy-dispersive spectroscopy showed that the alloy was Roxolid®. Figures 8 shows that this alloy is extremely fine-grained, with the grain diameters of the irregularly shaped grains being in the range of 2–5 µm. This is in total agreement with the microstructure of Roxolid® reported in two previous studies [22, 24]. As shown in Fig. 6C and Table 2, the alloy had a composition of Ti-15 wt.% Zr, which is the composition of the Roxolid® alloy. In the current study, microhardness measurements showed that the implant had a Vickers hardness of HV288, which was slightly higher than that reported previously (HV250) by Correa et al[18].
In the current study, the authors further examined and characterized the outer SLA surface on NDI-specimen-2 by SEM, as shown in Figs. 2A. As shown in Fig. 6D, the outer surface of the implant fixture exhibited a bimodal distribution of coarse and fine features. These features are similar to those reported for Roxolid® by Bernard et al[24] and Medvedev et al[22]. As described by Medvedev et al[22], SLA treatment involves sand blasting with corundum, followed by acid etching with a solution of sulphuric and hydrochloric acid.
In this study, NDI-specimen-1 failed first after 12 months of in-service, whereas NDI-specimen-2 failed after 17 months. This difference may be because NDI-specimen-2 was splinted to another distal implant and was not supported by a cantilever pontic alone, which possibly increased the in-service time in the patient’s mouth. However, in both scenarios, a pontic cantilever connected to a crown supported and retained by an NDI in the posterior regions may not be advisable as it may lead to an NDI fixture fracture within the first 18 months of being in-service. In addition, this study suggests that the treating prosthodontists preferably limits the size of the occlusal table of a crown, retained and supported by an NDI in the posterior area, to minimize off-axis loading, which may initiate fatigue cracks and fractures.
The observations and treatment recommendations of this study are valuable and important for consideration, considering that a previous study reported favorable tensile and fatigue behavior of Straumann’s Roxolid® (Ti-15 wt.% Zr) alloy[24]. Bernhard et al[24] reported a tensile strength of 953 MPa of the alloy, which was 40% greater than that of cp-Ti. They also reported that the endurance levels for the Roxolid® was 13 to 42% greater than that for cold worked cp-Ti implants. Despite their improved mechanical properties, NDIs fail by a mechanical degradation mechanism of the fatigue mechanism, implying that they are not free of mechanical degradation phenomena during intra-oral off-axis loading. Therefore, further controlled studies with larger samples number and further laboratory testing are required to provide best guidelines for safer and more successful applications of NDI fixtures in posterior regions.