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 Table of Contents  
Year : 2022  |  Volume : 22  |  Issue : 4  |  Page : 338-342

Stability of implant–abutment connection in three different systems after fatigue test

1 Dental Implants Research Center, Department of Prosthodontics, Dental Research Institute, Isfahan University of Medical Sciences, Isfahan, Iran
2 Dental Materials Research Center, Department of Prosthodontics, Dental Research Institute, Isfahan University of Medical Sciences, Isfahan, Iran
3 Department of Prosthodontics, School of Dentistry, Qom, Iran
4 Department of Prosthodontics, School of Dentistry, Shahrekord University of Medical Sciences, Shahrekord, Iran

Date of Submission24-May-2021
Date of Decision20-Oct-2021
Date of Acceptance20-Dec-2021
Date of Web Publication03-Oct-2022

Correspondence Address:
Mohmoud Sabouhi
Department of Prosthodontics, Dental Implants Research Center, School of Dentistry, Isfahan University of Medical Sciences, Hezarjarib St, Isfahan
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jips.jips_247_21

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Aim: Abutment screw loosening of implant-supported prosthesis causes a mismatch between the abutment and the implant. This screw loosening is influenced by the implant–abutment connection type, however, with contradictory results reported in different studies. The present study evaluates the stability of abutment–implant connections in three different systems before and after the fatigue test.
Settings and Design: Thirty implants (4.3 mm in diameter and 12 mm in length) were divided into three groups of 10: Implantium, Zimmer, and straight internal hexagonal connection (SIC) implants.
Materials and Methods: Two torques of 35 Ncm with an interval of 10 min were applied, followed by measuring removal torque value (RTV). The samples were re-torqued and then underwent a simulation of 1-year chewing clinical performance of dental implant under axial force of 400 N, with a frequency of 8 Hz (one million cycles). After fatigue test, the RTV was calculated and recorded.
Statistical Analysis: The mean RTVs obtained before and after cyclic load were analyzed by SPSS version 22 software using multivariate analysis.
Results: Significant differences in RTV and role of cyclic loading were found between SIC and Implantium groups (P = 0.006 and 0.021, respectively), as well as between Zimmer and SIC groups (P = 0.032 and 0.006, respectively), but not between Zimmer and Implantium groups (P = 0.771 and 0.248, respectively).
Conclusion: The type of connection could affect the screw loosening, the preload loss, and the implant component stability. SIC group revealed the highest RTVs before and after cyclic loading.

Keywords: Dental implant, fatigue, reverse torque, screw loosening

How to cite this article:
Bajoghli F, Sabouhi M, Pourali M, Davoudi A. Stability of implant–abutment connection in three different systems after fatigue test. J Indian Prosthodont Soc 2022;22:338-42

How to cite this URL:
Bajoghli F, Sabouhi M, Pourali M, Davoudi A. Stability of implant–abutment connection in three different systems after fatigue test. J Indian Prosthodont Soc [serial online] 2022 [cited 2023 Feb 8];22:338-42. Available from: https://www.j-ips.org/text.asp?2022/22/4/338/357794

  Introduction Top

Replacing lost or missing teeth with implant has become the first treatment plan in many situations.[1] It is important to pay special attention to technical and biomechanical parameters along with esthetic issues. Unfortunately, most implant manufacturers do not discuss the potential problems with their designed connection systems.[2] Therefore, many studies have been conducted to achieve a precise and consistent connection between implant components.[3],[4],[5],[6]

At present, the implant manufacturers fabricate two types of implant connections, including (1) butt joint or slip-fit joint with a completely passive connection and a small space between the implant and abutment and (2) conical interface connection designed based on friction fit. These two groups are divided into subcategories of internal hexagon, internal octagon, external hexagon, and other varieties.[7]

A screw is used to fix an abutment on an implant with the aid of a connection and helps to stabilize the abutment–implant system components. One of the greatest complications in cement/screw-retained prostheses is the screw loosening that causes the implant–abutment mismatch after occlusal loading of the prosthesis.[8],[9] Due to the screw loosening, the combination of horizontal and vertical misfits causes a gap formation between the components, thereby resulting in bacterial plaque accumulation, nonadherence to health, gingivitis,[10],[11] peri-implantitis, bone loss, and screw fracture.[12] One of the success factors in implants with a single prosthesis is the stability of their components. As oral function causes tendency for the abutment screw loosening, the torque plays an important role in the integrity of implant–abutment interface and may reduce the possibility of abutment screw loosening.[13] The screw torque value determines the preload level, which is distributed over the contact surfaces of the implant–abutment–screw threads, and some will be spent to overcome friction.[14] The stretching of implant and abutment screw threads creates a compressive force between the prosthetic components and holds them together.[15] In addition to preload, another major factor affecting the stability of implant and prosthetic components is the conical geometry between implant and prosthetic components in different abutment–implant connections.[16],[17]

There are few studies evaluating the removal torque value after fatigue test in different implant–abutment interface designs, most of which compare the internal against the external designs. Therefore, the purpose of this study was to determine the removal torque value (RTV) in geometrically different internal connections of three different implant systems before and after the cyclic load test. The first null hypothesis was that there is no difference in RTVs, before and after cyclic loading, between three studied systems. The second null hypothesis considered that the cyclic loading has no effect on RTV in the three studied systems.

  Materials and Methods Top

In this experimental study (approved ethical No.394843), the sample size calculation in each group was defined in accordance with d = 2.5 and α = 0.05. A total of 30 implants (4.3 mm in diameter and 12 mm in length) were categorized into three groups (n = 10): (a) Implantium with conical connection as 11° internal hexagon, (b) Zimmer (paragon) with conical connection as 8° internal hexagon, and (c) SIC with butt joint connection as internal hexagon with completely parallel walls. A computer-generated randomization was used in this study. The implants were mounted in the epoxy resin using the parallelometer in the mounting jig of chewing simulator CS (SD Mechatronik) to ensure the parallel placement of the implants and were standardized to perform subsequent cyclic loading.[18] This set has an elastic modulus of about 20 GPa, similar to the bone.[19] Prefabricated abutments were used to restore the coronal part. The antirotation standard abutments of corresponding system were applied for each of the three groups. To harmonize the conditions of applying force to the samples after mounting in the epoxy resin, the abutments were matched in height from the implant platform to the same length in the Milling Machine (Surveyor/Milling Machine Song Young). For torque application, the samples tightly closed in a clamp were placed under a force of 35 Ncm by a Cedar DID-4 digital torque meter (Sugisaki Meter Co., Ltd). After 10 min, the samples were re-torqued with a force of 35 Ncm according to the method specified by Khraisat et al.[20] in 2004 to achieve the maximum preload on the samples. After 2 min of the second torque, the RTV was measured and recorded based on the method described by Khraisat et al.[20]

The samples were then re-torqued according to the previous method. To create the moment arm for the loading process, identical hemispherical zirconia crowns were machined, sintered, and cemented over the implant–abutment assembly with polycarbonate cement. The models were then placed in the chewing simulator CS (SD Mechatronik) for the cyclic loading process. Jemt et al.[21] showed that most of abutment screw loosening occurs in the 1st year of function, and this then decreases over time, followed by an axial force of 400 N with a frequency of 8 Hz with 1-mm vertical and horizontal range of motion at a rate 1 mm/s to 1 million cycles. Artificial saliva was used to model oral conditions in the test environment. In the application of force to prevent damage to the device and to avoid the wear process in the head of the device and abutments, the simulator area was made of titanium grade 4. An axial load of 400 N with a frequency of 8 Hz was applied. Each of the samples was left in the device for about 2 weeks to reach a millionth fusion cycle. This was the simulation of 1 year of clinical implantation.[22] After the fatigue test, RTVs of the samples were calculated and recorded by the digital torque meter, as described above. The mean RTVs before and after cyclic load were computed, and the data were inserted into the SPSS version 22 (IBM, NY, USA) software and then analyzed by one-way ANOVA and repeated-measures ANOVA tests. The RTVs before and after cyclic loading were evaluated.

  Results Top

In all samples, the RTV was reduced relative to the initial removal torque, and this reduced RTV was higher after the cyclic loading process. The mean RTVs are presented in [Figure 1]. Based on one-way ANOVA, there is a significant difference in the mean RTVs of abutment screw between the three groups before the cyclic loading process (P = 0.006). Post hoc Tukey's HSD showed no significant difference between Zimmer and Implantium groups (P = 0.771). There was a significant difference between SIC and Implantium groups (P = 0.006), as well as between Zimmer and SIC groups (P = 0.032) [Table 1]. Considering the difference in the groups before cyclic loading, a two-way analysis of covariance was performed to examine the intergroup difference after each cyclic load on each of the groups separately. It should be noted that the two-way analysis of covariance showed that there is no significant difference between Zimmer and Implantium groups (P = 0.248). Moreover, there was a significant difference between SIC and Implantium groups (P = 0.021), as well as between Zimmer and SIC groups (P = 0.006). The best results belonged to the SIC group, which showed a preload loss less than the other two groups. Reductions in RTV were observed in all three groups after cyclic loading.
Figure 1: Mean removal torque values before and after cyclic loading in studied groups

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Table 1: Tukey's post hoc test results of removal torque values before and after cyclic loading in the studied groups

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To evaluate the effect of cyclic load on each of the groups separately, the repeated-measures ANOVA showed that there is a significant difference between the mean RTVs before and after the cyclic load process (P = 0.019). The interaction between implant type and cyclic load on RTV showed no significant difference (P = 0.836). The effect of system type on RTV revealed a significant difference between the three groups (P < 0.001) [Table 2]. Tukey's post hoc test showed no significant difference between Zimmer and Implantium groups (P = 0.303) but a significant difference between Zimmer and SIC groups (P = 0.004), as well as a significant difference between Implantium and SIC groups (P < 0.001) [Table 1].
Table 2: Results of repeated-measure ANOVA test to evaluate the effect of cyclic load and implant type on the removal torque values

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  Discussion Top

Studies on the types of implant–abutment connections have reported very different results, and each has done various tests on varied systems. In the present study, the presumed first null hypothesis was rejected as the RTV of abutment screws was lower in all groups than the primary RTV. Nevertheless, Ferreira et al.[23] showed that Morse Taper abutments have higher RTV than the initial torque due to cold soldering in the implant–abutment interface. The reason of different conclusion can be the implant system (Straumann ITI) or using one-piece abutments in their study. In agreement with the results of the present study, Sahin and Ayyildiz[14] examined the correlation between microleakage and screw loosening at implant–abutment connection. They reported a decrease in RTV relative to the initial torque in all specimens. In their study, the minimum and maximum RTVs were 9% and 14% in the Morse Taper samples. This value in the present study was estimated to be 10% in the SIC system with straight internal hexagonal connection (the lowest torque loss) using the equation of and the highest torque loss (about 20%) was in the Implantium group. The Zimmer group was positioned between the two groups.

The defined second null hypothesis was rejected as well. In the present study, the cyclic load process also reduced the RTV. Cho et al.[24] examined the effect of cyclic load on the screw loosening in internal hexagon and external hexagon systems. They reported that the RTV of abutment screw was less than the primary RTV in all samples, in line with the present study. In addition, they showed that the cyclic load process on all of their samples caused a significant decrease in the RTV of the abutment screw. The method used in their study was very similar to that in the present study, with the retightening process being considered with a time interval of 10 min from initial torque. This process led to an increase in RTV in both internal and external hexagon groups. The mean RTV in their study was 27 Ncm for the external group and 25 Ncm for the internal group. Compared with the present study, this value was 28 Ncm in the internal hexagon conical connections and 31 Ncm in the butt joint internal connection in the SIC group, which could be due to the design of the system. In fact, parallel walls with an appropriate space between the components will result in better assembly of parts, consequently spending less torque to overcome the friction between components, increasing preload values, and elevating RTV; the present results confirm this point. In another research, Kim et al.[25] investigated the RTV on five different connections and reported a reduction in RTV after cyclic loading in all samples, but this was not significant in some groups.[25] This amount of torque loss seems to be spent to overcome the friction between the components of the abutment–implant system. This issue has been investigated in the study of Haack et al.,[26] who reported that most torque values applied on the abutment screw are used to overcome the friction between components, and only 10% is spent to create preload. They stated that different material types could affect the torque loss. Therefore, this study used titanium screws.

In this study, there was a significant difference in the RTV between the SIC group and the other groups before and after cyclic loading. The SIC group showed a higher RTV than Implantium and Zimmer groups. This is because of the different connection types in these three groups. In the SIC group with parallel-wall internal hexagon connection, the component stability is obtained through the stretching of the abutment screw threads and the implant body, while the stability of the components in the other two groups is achieved through the friction between the abutment taper walls and the implant inner surface. Therefore, it can be concluded that the most torque values used in the SIC group are spent to create the preload, while the torque of the abutment screw in two other groups is distributed to create friction between the conical walls of abutment and implant and create preload in the abutment screw threads. This was also proved in the study of Cho et al.[24] This result merely indicates that in vitro screw loosening was less in the SIC group than in the other two groups. Other mechanical tests, such as joint opening, screw fracture, and marginal gap, as well as bacterial penetration tests, should be carried out for these connections to draw definite conclusion on the advantages and disadvantages of these connections.

Previous studies have shown that the implant–abutment connection gets degraded by the processes of wear and corrosion in the oral cavity, which contributes to loosening the connection during mastication. The glycoproteins in the oral fluids act as a lubricant and amplify these processes.[27] Therefore, we incorporated artificial saliva in the simulation to mimic the oral cavity environment better. The used chewing simulator had two moving axes that were controlled by programming software to simulate all the paths of masticatory movements. The axis's load and sliding motion was adjusted to best replicate the oral cavity conditions. Despite great efforts to ensure the highest study quality, there were also some limitations. It should be noted that statistical analyses are unable to express the exact clinical condition; therefore, the results should be interpreted with caution. The analyzed data regarding the RTV dispersion in the three groups studied before and after cyclic load showed that the distribution of data in the Implantium group is much greater than the other two groups. As statistical analyses use the mean of these data, they underestimate the abutment screw loosening which is one of the present study limitations. Regarding the results of this study and considering the limitations of this study, it can be said that the parallel-wall internal hexagon connection shows less screw loosening.

  Conclusion Top

Despite the limitations of the current study, the following conclusions were drawn:

  1. The type of implant–abutment connection affects the abutment screw loosening and the component stability
  2. The parallel-wall internal hexagon butt joint connection in the SIC system showed the least screw loosening
  3. The cyclic loading affected the removal torque value and the screw loosening and reduced the removal torque value.


We would like to express our gratitude to Dr. Bahram Soleymani (Assistant Professor of Epidemiology and Biostatistics, School of Health, Isfahan University of Medical Sciences, Isfahan, Iran) who cooperated with us as an independent statistician.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1]

  [Table 1], [Table 2]


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