This study evaluated the internal accuracy and the fatigue mechanical integrity of three different implant abutment configurations. Original and non-original abutments showed good marginal fit between the implant and abutment. No microgap between the platform of the implant and the shoulder of the abutment could be detected before loading (Fig. 3). Intimate contact at this level is required to prevent or minimize ingress of bacterial contaminants. However, internal contact among both components was different. Original abutments presented enhanced fit between their components than non-original abutments. Meanwhile, the highest fatigue limit value and the lowest fatigue strength exponent was registered in the original configuration, thus first and second null hypothesis were rejected. The results of this research correlate with the study published by Binon [36] who stablish that a better fit and better contact between the abutment and implant surfaces lead to a better load distribution. TC between components was significantly higher when original components were used. The original abutments are milled for their specific implants with a tailor-made connection, which is supposed to improve the stability of the components, reducing the micromotion when the system is under cyclic loading. To the authors’ opinion, this specific characteristic of the original abutments produce more homogeneous stress distribution between the components, improving the fatigue behaviour of the system.
Mattheos et al [8, 9, 21] studied and compared the quality and quantity of contact between original and two compatible abutment brands in the cross section of the components. External and internal connection was evaluated finding significant better contact between original abutments and implants. The authors, however, indicated that the implant shoulder area was the surface where compatible abutments accuracy was closer to originals, which correlates with the findings observed in this research.
Therefore, internal accuracy determines the grade of friction between the surfaces, preventing micromotion and decreasing stress [37, 38] which improves the stability of the system. In the case of non-original abutments the tendency of stress to concentrate at the abutment screw increased the risk for microfracture, and therefore for microgap formation. This was confirmed by the results of a previous investigation [8, 34] where original abutments after in vitro fatigue testing showed the best accuracy within their components as well as the lowest values of screw loosening.
The aim of this investigation was to study the long-term stability of original and compatible “cast-to” gold abutments of different manufacturers with an internal hexagonal connection implants. All abutments were composed by the same noble alloy; however, fatigue behaviour of non-original abutments showed higher sensitivity to cyclic loading than originals. Non-originals configurations showed higher slopes in the S-N curves, which represents a higher risk of failure and microgap formation in a short period of time. As a result, the fatigue strength exponent was 1.5 times lower in the original configurations (-0.067) than in the non-original groups (-0.104 and − 0.103). In a fatigue loading regime (in vivo mastication), a combination of non-original abutment/implant may fail at 280 N, that is within the range of the occlusal forces applied in the premolar region during chewing and swallowing for humans in normal conditions [39] in a very short period of time (Non-original 1: ≈80,000 cycles and Non-original 2: ≈300,000 cycles). However, at this value (280 N) the original abutment/implant combination will not fail. It was found that the fatigue limit for original abutments was of 280 N (Table 3) and therefore the original abutment-implant connection cycling at stress levels below this limit will give infinite life. In practice, infinity may be regarded as the largest number of cycles that will be applied due to other limitations of the product life.
Failure mode was also different. Original abutments showed implant fracture in the area apical to the screw. Non-original abutments showed a failure mode where the fracture included implant and screw (Fig. 8). The SEM micrographs observation of the fractured surface of abutment screws of these non-original abutments group allowed the consistent identification of fractographic markings, catastrophic or ductile failure (Fig. 9A) and fatigue striations in the fatigue zone (Fig. 9B). Surface analysis examined by other studies showed similar results [40, 41]. The narrowest part of a component is usually its weakest part because it is the region where the maximum stresses occur [42, 43]. The study of Morgan et al [44] reported a similar failure mode: resistance to bending was reduced as the area changed from a solid cylinder to an annulus with no central screw. The idea of the abutment, screw and implant acting as a solid cylinder reinforces the authors’ theory that the original abutments present a design that allows an homogeneous distribution of the forces through the assembly optimizing its mechanical behaviour.
A different design of the internal connection or screw and differences in the machining process to fabricate the collar between the three companies of the abutment could explain these differences, since the composition of the components are equal. The internal area was the one where the biggest differences among original and non-original abutments were found. The original configuration presented the smallest gap and the highest percentage of tight contact between the two connecting components.
Some limitations must be taken into consideration before transferring the results into clinical situation. This in vitro test was not able to simulate dynamic occlusion and normal loading conditions in the mouth. The numerous biological parameters that influence mechanical outcomes in vivo are not taken into account. Moreover, internal accuracy was measured using the cross-section technique only at two defined areas. Further investigation using X-ray microtomography is being carried out to evaluate internal accuracy with a 3D non-destructive technique.
For the complete understanding of the fatigue phenomena, future developments in this matter should evaluate the effects that the use of non-original components could have on the mechanical integrity and service life of the implants.