The null hypothesis was confirmed in this study, as mechanical cycling altered the behavior of the indexed and non-indexed specimens, both for bacterial infiltration and for detorque values.
Studies comparing implant prosthetic connections show that the lack of adaptation between implants and abutment can result in higher tension around the implant, bone and implants connections, which can be aggravated by mechanical cycling, leading to decrease in detorque values [11–14], which can cause screw loosening, as demonstrated by some prospective clinical studies with 3 to 5 years of follow-up [15, 16].
Chandra et al.  simulated the masticatory load of a 6 month in function prosthesis, correlating it to 500.000 mechanical cycles. However, for other authors, 100.000 mechanical cycles correspond to five years of chewing [10, 22]. The methodology of this study was performed in such a way that the mechanical cycles and loads simulated 4.5 years of masticatory function, replicated and evaluated the performance of implants systems, using the ISO 14801:2007 standard [3, 5, 19, 23].
After 1.000.000 cycles of the mechanical cycling tests, which occurred without the fracture of the implants abutments screws, the group NIIAMC maintained values close to the initial torque value, when the abutment-implant sets were submitted to detorque. In contrast, the group IIAMC with a Morse taper implant and an indexed abutment reached a mean detorque value of 55.20 Ncm compared with the 20 Ncm initial torque value and was significant in relation to the other groups.
For the group NIIAMC, the influence of the mechanical cycling was significant because it directly interferes with the sealing capacity of the implant-abutment interface, which made bacterial infiltration impossible.
In this study, 1 million cycles were performed, with no loosening of the implants-abutments screws being identified. The detorque values of IIA, NIIA, and NIIAMC were similar, however for IIAMC, the values were higher.
The indexed Morse taper connection presents an internal conical design that promotes an intimate adaptation between the surfaces during the abutment installation into the implant, reaching a mechanical resistance similar to that of a single-piece implant, and improving the mechanical properties and the stability of the abutment, preventing its loosening and maintaining high resistance to the opening force, due to the maintenance of the friction coefficient between the components, thus ensuring excellent prosthetic stability, as reported in previous studies [3, 9, 10, 24–27].
Recent studies have shown that the Morse taper implant connection has a better sealing capacity concerning the space between implant-abutment to the loss of the bone crest [6, 9, 10, 28, 29]. The Morse taper implant appears to be more efficient regarding biological aspects, favoring less bacterial infiltration and bone loss in single implants, including aesthetic dental areas. It can also be successfully indicated for fixed partial dentures and overdentures .
Tuzzolo et al.  performed a comparative analysis of the mechanical resistance of implants of different diameters, showing that the implants of smaller diameter provided less fracture resistance, both in the tensile strength tests and in the maximum bending moment, mainly in relation to the single-piece extra-narrow and single-piece narrow implants that deformed in the implant body area, thus demonstrating that the implant diameter selected and the use of intermediate abutments is also a factor of the utmost clinical importance.
The deformation resistance and fracture on the oblique forces loads proves that the Morse taper implant connection presents significantly better results, due to the solid design of the prosthetic abutment and the friction locking mechanism .
In this study, the mechanical cycling process influenced the prevention of bacterial infiltration, as observed in the mechanically activated indexed and non-indexed groups (IIAMC and NIIAMC) and had greater statistical significance for the non-indexed and non-cycled group (NIIA) with higher rate of bacterial infiltration.
The infiltration of saliva through the implant-abutment interface in different types of implant connections (external hexagon, internal hexagon and morse taper implant), with and without load, was confirmed in some studies and found microorganisms present on the internal surface of all groups, however, the groups with the Morse taper connection showed results with lower infiltration values, both when submitted and not submitted to load [5, 28].
The implant’s sealing capacity to prevent bacterial infiltration is granted by the mechanical sealing and locking provided by macrogeometry and internal conical connection, which has been shown to be hermetic regarding bacterial infiltration, in vitro . In the comparison of bacterial sealing between the implant and the abutment both solid and pass-through-screw types, no statistical difference was found, neither concerning the number of bacteria colonies nor the percentage of bacterial infiltration . Implant-abutment interface contamination has been studied as to its influence on the adjacent tissues inflammation, which can manifest itself through mucositis and/or peri-implantitis, resulting in the loss of the dental implant [1, 34, 35].
To evaluate the bacterial infiltration in this study, a new testing technique was proposed from the implants’ external to the internal environment, representing a more accurate and financially viable situation in the clinical routine. Various techniques for the verification of implants contaminations and bacterial infiltration have been recommended in several studies [30, 36, 37].
Smut, in BHIA, was used for being an important microorganism that occurs in the oral cavity as its natural habitat, in healthy and good oral hygiene conditions. Calan et al.  described the presence of implant-abutment interface contamination for one or more periodontal odontogenic microorganisms that inhabit in the internal part of the implant, even though there is no difference between the colonization of an individual species of microorganisms, when compared to the different regions of implant placement, in vivo.
The presence and quantity of oral lubricant (saliva, peri-implant fluid and/or blood) between the implant components can affect the friction coefficient between the Morse taper connection components, decreasing this coefficient as the amount of lubricant increases .
Initially, the presence or absence of index was not relevant in the bacterial infiltration evaluation results, as both groups presented bacterial infiltration. After mechanical cycling simulating chewing, groups IIAMC and NIIAMC demonstrated an absence of bacteria in the bacterial infiltration test.
However, regarding the bacterial infiltration, the hermetic sealing of the specimens and cemented abutments provide low bacterial permeability in the conical types connections and a high incidence of infiltration in screwed connections .
An important and quite relevant factor for the evaluation of the stability of the implant-abutment connections is the detorque values, as per to the preload value remaining in the screw, after mechanical cycling [12, 39]. The crucial reason for the loss of the implant prosthetic abutment, in an external hexagon or internal hexagon connection, is the loss of the abutment screw preload and the resulting unscrewing or fatigue failure of the screw material .
Detorque values close to or higher than the initial torque values indicate a good prognosis for the connections in question . Mechanical cycling promotes movements that simulate human chewing movements and mimics the distortion and fatigue of the implant-abutment specimens .
The detorque values of prosthetic abutments applied in the Morse cone system tend to decrease as the number of cycles of insertion and removal of the abutment increases .
In order to improve the knowledge regarding the contamination process that can occur in the union space between the implant-abutment, in indexed Morse taper implants, with indexed and non-indexed abutments, this study was performed using the analysis technique of bacterial infiltration, described in the methodology, searching for greater approximation with the reality of the oral environment.
Although these results were obtained, in another in vitro study, with the same purpose, however, with contamination from the internal environment to the external environment of the implant, the mechanical behavior and the bacterial micro-infiltration in the implant-abutment conical interface, after mechanical cycling, showed no statistically significant difference between the removal force, reverse torque and contamination values when compared to implants of the same type. The abutment removal force or detorque was not affected by mechanical cycling, since the bacterial sealing of the implant-abutment conical interface was not effective, in any of the analyzed conditions. Inaccurate mechanical activation of implants and abutments does not allow a surface area that provides effective frictional sealing and bacterial infiltration. In clinical relevance, the microscopic space caused by the desadaptation of the implant-abutment, superficial irregularities and plastic deformation of all parts allowing bacterial contamination of oral implants .
The precise application of the technique used in this study, after mechanical cycling, within the parameters historically analyzed and validated in the literature, resulted in the finding of significant results on the investigation of the proposed hypothesis. The mechanical activation process favors the mechanical overlap of the implants and abutment specimens regarding the bacterial infiltration of the indexed and non-indexed Morse taper implants, biostatistically significant for the non-indexed prosthetic abutments, becoming of the utmost importance, as well as literature states that the Morse taper connections tend to have intimate contact between surfaces. Besides that, the initial torque recommended by the manufacturer must be maintained, because it was noted that the initial torque remained close to the detorque value of the specimens with indexed and non-indexed non-cycled abutments, and it was only higher for the sets with cycled indexed abutments. Bacterial infiltration, in vitro, was only observed in Morse taper implants with non-indexed and non-cycled prosthetic abutments.
From the prosthetic rehabilitation perspective, the use of the indexed components can facilitate prosthesis manufacturing, because there is the possibility of transferring the implant platform to the working cast, that can help choosing the best prosthetic abutment. However, the clinical disadvantage is to possibly reduce the implant’s frictional retention area, due to the presence of the index, influencing on the bacterial infiltration and on the detorque of the abutment screws.