INTRODUCTION

Oral rehabilitation with dental implants is an alternative treatment that benefits patients when well planned. Different interfaces between implants and prosthetic abutments were developed with the aim of obtaining better performance before functional occlusal loads1 that provide clinical longevity (^{Zielak et al., 2011}; ^{Aguirrebeitia et al., 2013}).

Different forms of connections between implants and abutments, such as external hexagon, internal hexagon, and Morse taper, are used nowadays (^{Mangano et al., 2009}). Morse taper implants provide better adaptation between the abutment and the implant and eliminate gaps between them, thus reducing levels of peri-implant bone resorption and minimizing micromovements. Loosening of the abutment/prosthesis set is less frequent in Morse taper connections than in other types of connections (Mangano et al.; Zielak et al.; ^{Aguirrebeitia et al., 2013}; ^{Zipprich et al., 2018}). Moreover, this system is reported to have a high success rate (^{Dibart et al., 2005}).

Morse connections can also be classified into the following two types: the first type is one where the fit between the abutment and the implant is made using a screw and the second type, called true Morse taper system, is where the fit is made only by the taper and its connection is exclusively frictional (^{Rack et al., 2010}). The true Morse connection, called frictional union or frictional Morse taper implant (FMTI), is found in some implant systems and does not use screws. It is placed on the long axis of the implant using beats to protect the frictional resistance and provide resistance to displacement (Dibart et al.; Rack et al.; ^{Aguirrebeitia et al., 2014}).

The FMTI has some advantages, such as the possibility of adequate aesthetics in the cervical region and a lower number of prosthetic components, with consequent reduction of costs, ease in clinical procedures, and increased resistance to fracture of the prosthetic abutment (^{Rabelo et al., 2015}). Stability of the abutment is fundamental to achieve aesthetics and longevity of the prostheses, which is fundamental to the success of and satisfaction with the rehabilitation (Rack et al.; ^{Aguirrebeitia et al., 2014}; Rabelo et al.). Although this system promotes interlock between the components, the professional may need to remove the abutment for some reason.

If this occurs, the question is whether this procedure would compromise the retention of the abutment to the implant. Moreover, deformation of the Morse taper could occur after several loosening events (^{Feitosa et al., 2013}; Rabelo et al.). Therefore, the present study aimed at evaluating the tensile strength of frictional Morse taper implants of a specific brand after a sequence of insertions and removals (SIR), as well as evaluating possible deformations in the head of the implants using scanning electron microscopy (SEM).

MATERIAL AND METHOD

Study population. This study was designed as an in vitro analysis of frictional force insertion and removal (SIR) upon abutments in morse taper implants. The sample size was calculated according to the PD (mean and standard deviation) and the level of significance was 5 %, with an effect of 0.80. Considering a statistical power of 95 %, the sample size was fixed in 10 morse taper implants.

The insertion and removal tests were based on the study of Zielak et al. The sample tests were conducted by a single Calibrated Researcher (C.R.1), who had previously experience in clinical and in vitro studies. The SIR testes results were analyzed by scanning electron microscopy (SEM) by another C.R.2 who had previously experience SEM analysis. All the images obtained from SEM were compared to newly control implants, the analysis occurred between August 2017 and December 2018.

Sample preparation. 10 titanium frictional morse taper implants of 11 mm in length and 3.3 mm in diameter (Kopp, Curitiba, Brazil), with Morse taper prosthetic interfaces, and 10 anti-rotational abutments of 13 mm in length 3.5 mm in diameter (Kopp, Curitiba, Brazil) were used (Fig. 1). The implants and the abutments were made of titanium alloys (ASTM F67 and ASTM F136, respectively), with hardness values of 20 HRC and 29 HRC. All implants and abutments surfaces were manufactured in a Swiss lathe using hard metal inserts. The internal calibration was verified by C.R.1 using standard methods cited by Zielak et al. After the internal calibration all abutments and implants were passively mounted together by C.R.2 prior to the application of force, with rounding up to a total of 10 implant- abutment mounts.

Fig. 1 implant and anti-rotational abutment. 

Base to be coupled in the universal test machine (BCUTM): its shape had a cylindrical body and surface, with the upper portion having a larger diameter than the body, and it had a drill hole in the center with thread pitches for coupling the fastener of the specimens. In the lower portion, there was a hole to lock the BCUTM in the test machine (Fig. 2).

Fig. 2 A - Representation of the Diagram of the Base to be coupled in the universal testing machine B - Attached base. 

Specimen fastener (SF): used to connect the implants. It had an external hexagonal shape and a cylindrical central hole on its surface, with thread pitches similar to the ones of the implant used. In the lower portion, the SF had a cylindrical screw to connect itself to the BCUTM through threading (Fig. 3).

Fig. 3 Representation of specimen fastener and the same one coupled to the BCUTM. 

The implants and the abutments were removed from their package one by one, and the implants were threaded into the SF. Then, they received the abutments, with a light digital pressure without force, always by the same operator, making a total of 10 specimens (implant/abutment). Next, the abutment was attached to the implant by means of an impact instrument, beat-connection (Kopp, Curitiba, Brazil), parallel to the long axis of the implant/abutment (IA), with three attachments (corresponding to the three beats for abutment fixation to the implant) in each SIR, as recommended by the manufacturer, and then subjected to the tensile strength tests (Fig. 4).

Fig. 4 Attachment of the abutment to the implant. 

Tensile strength test. The tensile strength test was performed at the University of the State of Pará (CESUPA), using a Kratos universal mechanical testing machine model KE 2.000 MP (Cotia, SP, Brazil). A handcrafted device to be coupled in the neck of the abutment was used for the verification of the removal force. This device was called a hitch (Fig. 5), which consisted of a hollow, cylindrical metal object with a lower portion having an aperture of the size of the cylinder radius and a width of 2 mm, sufficient to be hitched to the abutment neck. The upper portion had a ringto be connected to a hook that was attached to a load cell (model CKS - Kratos; Cotia, São Paulo, Brazil), with a capacity of 50 kgf.

Fig. 5 Hitch/ hook positioned in the abutment neck to perform the traction test. 

After the interlock in the abutment neck, the tensile tests were performed at a displacement speed of 0.50 mm/min until the abutment decoupling was achieved. Then, a new attachment (three beats), and a new tensile strength test were performed. This process was repeated nine times. Each of these repetitions was called SIR, referring to the sequence of insertion and removal. The measure of the decoupling force (separation of the abutment from the implant) was transmitted to a computer that provided the value in Newtons (N) from a specific software of the universal mechanical testing machine.

Scanning electron microscopy analysis. The ten implants were subjected to scanning electron microscopy (SEM) analysis and compared to two new implants microscopically. The implants were pre- metalized with gold for 1.5 minutes on an Emitech model K550X metalizer (Ashford, Kent, England). Secondary electron images were obtained from the Microanalysis Laboratory of the Institute of Geosciences (IG) of the Federal University of the State of Pará (UFPA). The equipment used was a Zeiss SEM model EVO-MA-10 (Jena, Thuringia, Germany).

The operating conditions were electron beam current = 100 mA, constant acceleration voltage = 10 kV, working distance = 11 to 12 mm. The images of the head of the implants used after the tensile test, as well as the control implants, were saved in JPEG format, analyzed, and compared for the search for possible deformations.

Statistical analysis. For the analysis of the values obtained for the tensile strength tests (in Newtons), the results were tabulated for mean values, standard deviation (SD), and percentage of decoupling force increase of the IA set (%DFIIA).

Given the non-normal distribution of the data evaluated by the Shapiro-Wilk test, the Kruskal-Wallis analysis with Dunn’s post-test were performed for the statistical treatment of the results, in which the difference between the SIRs was evaluated (a = 0.05). The significance level was set at 5 %.

As a complementary analysis, a calculation was made of the percentage (%) of decoupling force increase of the implant and abutment set (% DFIIA), from the formula:

SIRF = Final sequence of insertion and removal

SIR1 = Initial sequence of insertion and removal

RESULTS

The null hypothesis (Ho) was accepted, that is, there was no significant statistical difference between the SIR numbers and the final retention. Thus, the safety of retention of the prosthesis can be obtained from the first insertion of the abutment to the implant until the ninth one, as demonstrated in this study.

Table I shows the mean of the SIR 1 to SIR 9 and the mean of their standard deviations, respectively. Equal letters mean similarity between the groups that contain them. Therefore, these groups behave similarly and, thus, there is no difference between them.

An increase of 29.03 % in the ninth SIR (SIR9) was observed compared to the first SIR (SIR1) when analyzing the decoupling force increase between the implant/abutment set (%DFIIA). Thus, SIR1 was the basis for comparing the other eight SIRs (Fig. 6).

Figure 7 shows the SEM image of a new implant compared with an implant after the mechanical test. No significant deformations were evident, and only small marks of contact in the direction of the abutment attachment were observed.

Table I Mean values in (N) of the sequences and respective standard deviation (SD). 

Fig. 6 The graphic shows the percentage of force required to decouple the abutment in each SIR. 

Fig. 7 Scanning electron photomicroscopy of the control implant (left) with the implant subjected to the tests (Radiation source: monochromatic Al Ka; kV levels: 10 kV; Magnifications: 1000X; Filter: polycarbonate). 

DISCUSSION

In this study, we evaluated the possible changes in final retention of the abutment to the Kopp frictional Morse taper implant and the necessary force for dissociating the implant/abutment set (IA) between the SIRs. We also sought for possible deformations of the head of the implant. After the 9 SIRs of the abutment to the implant, there was no loss of retention. Therefore, if it is necessary to remove and replace the abutment during installation andmaintenance of the prosthesis, the final retention will not be impaired. As the SIRs were made, the decoupling force increased, which proves that in the screwless system the higher the number of attachments, the higher the tensile strength and, consequently, the greater the retention (^{Bozkaya & Müftü, 2003}; ^{Moon et al., 2009}; ^{Schmitt et al., 2014}).

Kopp friction system consists of a cylindrical abutment with less than 2° between the inner walls of the implant and the outer walls of the abutment, which when coupled to the implant requires a removal force greater than the insertion force (^{Steiner et al., 2009}; ^{Ricciardi Coppedê et al., 2009}; Rabelo et al.). This Morse taper interface is related to the phenomenon of cold welding, which occurs through the intimate contact between the surfaces of the prosthetic abutment and the implant, leading to an interlock between the parts and, consequently, greater friction retention and stability (Steiner et al.). The high frictional force comes from the high- pressure contact by sliding the two surfaces. Consequently, the oxide layers break, and the roughness melts as cold welding (Ricciardi Coppedê et al.).

Zielak et al. analyzed implants of the same length and different diameters in a five-fold sequence of insertions and removals, and their result was similar to that of the present study, showing a positive correlation between the sequence number and the removal forces. The mean value of removals was increased from T1 (111.4 N) to T5 (294.6 N), and the highest value was 53.2 %, between the first and the second measurements (Zielak et al.). In this study, 9 SIRs were performed, with the objective of extrapolating a possible number of abutment removals. In this proposal, the sequential increase of the removal force was obtained, reaching 29.03 % between the first and ninth sequences.

The present study showed an association between the number of insertions/removals and the decoupling force increase of the IA set. The mean removal values ??were sequentially increased from sequence two (SIR2), 176.40 N, with 1.61 % of DFIIA to sequence nine (SIR9), 222.54 N, with 29.03% of DFIIA.

According to Bozkaya & Müftü, during the insertion of the abutment, elastic deformation occurs with consequent plastic deformation. The authors state that a certain degree of plastic deformation increases the extraction force of the abutment due to the increase of the insertion depth and concluded that the mechanical characteristics that the tapered connections, such as the insertion and removal forces, besides the distribution of forces by the abutments depend on the taper angle, the length of the contact area, the internal and external diameters of the abutments, the depth of insertion of the abutment, the properties of the materials and the coefficient of friction of the contact surfaces (Zielak et al.).

Similarly, the greater the activation force over the long axis of the implant the greater will be frictional retention (Bozkaya & Müftü; Moon et al.; Steiner et al.; Ricciardi Coppedê et al.; Schmitt et al.). Therefore, the more force applied to the prosthetic abutment, the more likely it is to intrude into the implant, thereby having a more intimate contact between the implant and the abutment and causing them to act physically as if they were a single body, which can be, thus, clinically relevant during the distribution of masticatory loads.

Based on the literature, masticatory forces under physiological conditions in natural teeth can range from 10 N to 120 N, and the highest maximum forces vary from 190 N to 290 N in anterior teeth and from 200 N to 360 N in the region of the molars (Moon et al.). In tapered interface implants, the occlusal compression force acts in the direction of insertion of the abutment, favoring the auto-attachment in the implants (Steiner et al.). In this way, masticatory forces influence the retention of the prosthetic abutment to the implant, since the chewing movements cause intrusion forces more expressive than those of extrusion and laterality. A continuous attachment of the exclusively frictional abutments could still occur over time (Bozkaya & Müftü). Thus, a removal force greater than that observed in the present study may be required.

According to the manufacturer of this Kopp biological friction system, the correct attachment of the prosthetic abutment to the implant should be done at the long axis of the implant, that is, at 0º (Feitosa et al.). This study followed the manufacturer’s indication for angulation and no other angulation was performed for analysis. However, it is possible to find difficulties in the angulation indicated during the attachment in the mouth, due to the limitation of mouth opening or even the positioning of the teeth. The literature shows that the attachment of the prosthetic abutment at a 30º inclination shows a lower resistance to decoupling when compared to the attachment at the long axis of the implant/prosthetic abutment, which is 0º (Zielak et al.). Thus, it is observed that the retention is reduced for a 30º angulation, evidencing that a sub-attachment may cause a higher index of mechanical failure, such as the loosening of unitary prostheses (Zielak et al.).

Even though the retention was not statistically relevant in the present study, it should be emphasized that the force for removal increased with each SIR and, therefore, it can be clinically more difficult to manipulate the abutment. The abutment should receive a greater force made by the dental surgeon, who will have to look for angles and shapes to remove this abutment so as not to cause injuries to teeth or tissues adjacent to the area of the implant/abutment set, thus avoiding discomfort to the patient (Zielak et al.).

Samples evaluated by SEM at the end of SIRs showed small marks resulting from the friction where the implant contact the abutment, which may not interfere in the final retention of the IA set. No cracks or fractures were observed as in the study by Ricciardi Coppedê et al. In contrast, Dibart et al. analyzed the morphology of the surfaces, the internal surface of the implants and the external surface of the prosthetic abutments, where friction marks and scrapes oriented vertically in the direction of the long axis of the implant were identified. Significant grooves in the tapered portion of the abutments and deformations of irregularities with dent aspect were also observed in this study (Ricciardi Coppedê et al.). Therefore, further studies may standardize the amount of force applied to the implant/abutment without any damage to its structures and in the final retention of the component are suggested.

Lastly, further in situ studies confirming the clinical relevance of our findings are necessary, since in vitro studies may not faithfully represent this reality. Further studies should also be conducted to evaluate the influence of mastication on the removal force of the abutments considering its biomechanical complexity (Ricciardi Coppedê et al.).

No significant statistical difference was observed in change in final retention among the nine SIRs evaluated in the ten abutment/implant sets. The force required for the abutment decoupling increased as the sequences were performed on all ten abutment/implant sets. No significant deformations, fractures, or cracks in the heads of the implants were observed in the SEM analysis. Only small contact marks in the direction of the abutment attachment were seen.