Evaluation of Screw Loosening and Removal Force of Original and Compatible Implant-Abutment Connection (Tissue Level): In Vitro Study

Abstract

Objective

Screw loosening in dental implant prosthetics can lead to serious issues. Various factors contribute to screw loosening, including prosthesis design, misalignment between the prosthesis and abutment or between the abutment and implant, insufficient screw tightening force, excessive biomechanical loading, the material and design of the screw, chewing forces, and parafunctional habits.

Materials and Methods

This study evaluated three implant systems (SNUCone, Dentis, and Implant Swiss) along with their original and compatible abutments. The implants were mounted in acrylic blocks measuring 20 × 6 × 10 mm, and the abutments were connected to the fixtures with a torque of 30 N/cm. The samples were subjected to a chewing simulator with a load of 90 N and 10,000 cycles, and the amount of screw loosening was measured. Subsequently, the removal torque of the abutments was recorded using a universal testing machine. The data were analyzed using analysis of variance and Tukey's post hoc test.

Results

According to the study results, the screw loosening torque of abutments is significantly influenced by the type of abutment and implant. Among the groups with Implant Swiss fixtures, the Implant Swiss abutments had the highest loosening torque, with an average of 27.24 N/cm, whereas in SNUCone fixtures, Dentis abutments showed the lowest loosening torque. Similarly, in Implant Swiss fixtures, Dentis abutments recorded the lowest loosening torque, with an average of 23.86 N/cm. In SNUCone implants, Dentis abutments had the lowest loosening torque, averaging 23.86 N/cm. The removal torque required for abutments did not significantly differ across the different implant systems.

Conclusion

According to the study findings, the screw loosening torque of abutments varies depending on the type of abutment and implant. Implant Swiss abutments exhibited the highest loosening torque, while SNUCone abutments showed the lowest. The highest average abutment removal torque was observed in the Implant Swiss implant with the SNUCone abutment, and the lowest was in the Dentis implant with the Implant Swiss abutment. The abutment type has a limited effect on this parameter, which may be influenced by other factors such as the overall design of the implant systems.

Keywords

screw loosening - abutment - removal force - implant

Introduction

Efforts to replace missing teeth with implants date back to the ancient Egyptians. They changed the shape of the shells and placed them directly in the jaws. Dental implants as the selected treatment have been widely accepted by many patients with a survival rate of 89.4% after 10 years and a success rate of 90%.[1] [2] However, clinical complications have also been observed in some cases despite the high success of this treatment.[3] Screw loosening is one of the most common complications of dental implants with a rate of 3.4 to 10% during the first year, especially in single prostheses that restore molar teeth.[3] [4] Loosening screws can be costly and time-consuming. Studies have shown that between 6 and 20% of prosthetic implants experience screw loosening at least once in the first year of function. The prosthetic screw is known to be the weakest point of connection in the prosthetic chain. Any misalignment in occlusion, cast fitting, or forces can lead to screw loosening or failure during function. These issues can protect the implant body from more serious damage, but once these problems occur in a splinted restoration, other supporting implants may also be at risk of excessive loading and additional complications due to the increased unintended load on them. Various factors contribute to screw loosening, including the design and dimensions of the restoration, misalignment of the prosthesis with the abutment or the abutment with the implant, insufficient tightening force of the screw, excessive biomechanical loading, the material and design of the screw, the amount of chewing force, the presence or absence of parafunctional habits, and the accuracy of the connected components.[5] [6] Screw loosening may lead to mechanical or biological complications.[1] [4] Mechanical complications such as screw instability, screw fracture, abutment fracture, and occlusal overload may occur.[1] [3] Biological complications can include microleakage of bacteria, infection, granulation tissue and fistula formation, swelling, pain, peri-implant mucositis, crestal bone loss, and peri-implantitis.[1] [3] Applying torque causes elongation and tension, producing a force in the screw known as preload.[7] Preload is the axial force produced between the threads of the abutment screw and the internal parts of the implant alongside the longitudinal axis of the implant.[8] The preload force should be continued and reduced to the minimum level as much as possible to prevent disconnection of the joints.[9] Preload is positively associated with screw-tightening torque values.[9] The ideal preload is mostly 60 to 80% of the yield strength of the building material.[10] Only 10% of the torque is converted into preload, while the remaining 90% is used to overcome the friction between the mating surfaces of the components.[11] Loss of preload has been observed in the first 2 to 3 minutes[9] or 15 hours[12] after tightening, even without external force.

Screw loosening occurs when occlusal or lateral forces on the implant exceed preload.[13] This occurs due to loss of preload or insufficient initial preload.[1] [14] The initial torque value can vary from 10 to 35 N. However, the initial torque value depends on the different values of the friction coefficient, tightening rate, and the surface hardness of the threads. It may not produce the same value in the initial loading.[1] [13] Other factors including the improper position or direction of implant placement leading to adverse occlusal loading, screw design or material, screw deformation or wear, and improper components have been considered as possible causes of screw loosening.[15] [16] In the treatment of edentulous patients, dentists may use compatible abutments to reduce treatment costs or in case of lack of access to the original abutment.[1] [17] However, the nonoriginal components have many differences with the original components including the difference in designs, shapes, and dimensions so differences higher than 10 µm lead to nonfit and screw loosening.[18] [19] According to the contradictory results of previous studies and the lack of a study similar to this study, the present study was conducted to answer the question of whether there is a difference in the abutment screw loosing and the abutment removal force in the original and compatible abutments in tissue-level implants.

Materials and Methods

This study was conducted in the Faculty of Dentistry of Medical Sciences in Isfahan university of medical sciences, Isfahan, Iran. An experimental method was used to conduct this study. A convenience sampling method was used. The minimum sample was calculated to be 1. In this experimental study, three implant systems (SNUCone, Dentis, and Implant Swiss) were investigated along with original and compatible abutments. The SNUCone implant system was made in South Korea with dimensions of 4.8 mm in diameter and 10 mm in length, the Dentis implant system was made in South Korea with dimensions of 4.8 mm in diameter and 10 mm in length, and the Implant Swiss system was made in Switzerland with dimensions of 8.4 mm in diameter and 10 mm in length.

The implants were mounted in acrylic blocks with dimensions of 20 × 6 × 10 mm perpendicular to the surface ([Fig. 1]). Donated fixtures were used. They were fully evaluated for chemical corrosion, mechanical wear, and fracture before testing to ensure the validity of the research results.

Fig. 1 Fixture- and abutment-mounted samples.

First, the abutments were mounted on the corresponding fixtures. Then, a torque of 30 N/cm was applied to the abutments using a Cedar DID-4 digital torque driver (Japan-made).

The samples were placed in the chewing simulator. At this stage, a compressive force equal to 90 N was applied to the samples and entered into the center of each sample for 10,000 cycles with a frequency of 75 revolutions per minute (which is equivalent to the number of times a human chews in a year).

After the loading period, the screws were loosened again and the torque required to loosen them was measured. The loosening value was calculated. These steps were repeated for each abutment group (SNUCone, Dentis, and Implant Swiss). For example, for the Dentis fixture, the Dentis, SNUCone, and Implant Swiss abutments were mounted and tested. Then, a universal testing machine was used to measure the force required for abutment removal from the fixtures ([Fig. 2]).

Fig. 2 Samples mounted in Universal Testing Machine.

A hook-shaped device was connected to the universal testing machine and the abutment. The force required to separate the abutments from the fixtures was measured and the results were recorded as charts and numbers. This process was repeated for each type of abutment and fixture to collect all the data.

In this study, data were analyzed using SPSS-28 software. Shapiro–Wilk test was used to evaluate the normality of data distribution. Due to the normal distribution of data, a one-way analysis of variance (ANOVA) test was used to investigate the significant differences in the mean of abutment screw loosening force and abutment removal force in different types of implants. To analyze the differences more accurately, Tukey's post hoc test (Tukey honestly significant difference [HSD]) was used to examine pairwise differences between groups. Descriptive statistics including mean, standard deviation, minimum, and maximum values are presented and the results are displayed in tables and figures to provide the analysis results more clearly.

Results

[Table 1] and [Fig. 3] show the descriptive results of the abutment screw loosening force for Dentis, SNUCone, and Implant Swiss implants separately based on abutment type (Dentis, SNUCone, and Implant Swiss). The results show that Implant Swiss with Swiss abutment has the highest mean screw loosening force (27.24 N) and the lowest mean belonged to the SNUCone implant with the Dentis abutment and Swiss implant with Dentis abutment (23.86 N). Generally, Swiss abutments showed the highest resistance to screw loosening of all three implants, while Dentis abutments had the lowest ([Table 1]).

Table 1 Descriptive statistics of abutment screw loosening force (Newton) in different abutments separately based on implant type

Fixture	Abutment	N	Min	Max	Mean	SD
Dentis	Dentis	10	20.80	25.40	23.94	0.79
	SNUCone	10	25.10	26.60	25.88	0.51
	Implant Swiss	10	26.30	27.80	27.00	0.52
SNUCone	Dentis	10	23.10	24.30	23.86	0.48
	SNUCone	10	22.90	27.40	25.15	1.19
	Implant Swiss	10	26.20	27.90	26.86	0.57
Implant Swiss	Dentis	10	23.20	24.60	23.86	0.49
	SNUCone	10	23.20	25.20	24.40	0.59
	Implant Swiss	10	25.10	28.10	27.24	0.86

Abbreviation: SD, standard deviation.

Fig. 3 Bar of mean abutment screw loosening force (N) in different implants separately based on abutment type.

[Table 2] and [Fig. 4] show the descriptive results of the abutment removal force. The maximum mean abutment removal force is related to the Swiss implant with SNUCone abutment (25.87 N) and the minimum value is related to the Dentis implant with Swiss abutment (18.65 N).

Table 2 Descriptive statistics of abutment removal force (N) in different abutments separately based on implant type

Fixture	Abutment	N	Min	Max	Mean	SD
Dentis	Dentis	8	17.70	28.95	22.84	3.37
	SNUCone	6	13.13	24.41	20.23	4.94
	Implant Swiss	5	13.30	25.31	18.65	5.83
SNUCone	Dentis	6	20.21	28.72	25.34	3.42
	SNUCone	7	22.10	26.75	25.42	1.60
	Implant Swiss	6	18.94	26.40	22.68	3.20
Implant Swiss	Dentis	6	17.27	31.98	24.19	5.12
	SNUCone	5	23.66	28.49	25.87	1.77
	Implant Swiss	7	20.01	26.70	22.58	2.40

Abbreviation: SD, standard deviation.

Fig. 4 Bar chart of the mean abutment removal force (N) in different implants separately based on abutment type.

[Fig. 5] shows the abutment removal force (in N) in different types of abutments by implant type. These data were obtained using a universal testing machine. The results are graphically presented in [Fig. 5].

Fig. 5 The abutment removal force (in N) in different types of abutments by implant type (A) The abutment removal force (in N) in SNU cone implant. (B) The abutment removal force (in N) in Dentis implant. (C) The abutment removal force in implant swiss.

This section presents the results of inferential statistical analyses to investigate significant differences between different groups. First, the Shapiro–Wilk test was used to evaluate the normality of data distribution. Due to the normal distribution of data, a one-way ANOVA test was used to compare the mean abutment screw loosening force and abutment removal force in different implants. Tukey's post hoc test (Tukey HSD) was also used to examine the differences more closely. The results of the one-way ANOVA test showed significant differences in the abutment screw loosening force between different types of abutments in Dentis, SNUCone, and Swiss implants. The results of the follow-up analysis also indicated that the force required for abutment removal is not significantly different in different implants and probably other factors such as the type and overall design of the implants are not influential in this regard.

There was a significant difference between the Dentis and SNUCone abutments (p < 0.001). A significant difference was also observed between the Dentis and Swiss implant abutments (p < 0.001). Additionally, a significant difference was found between the SNUCone and Swiss implant abutments (p = 0.001).

A significant difference was observed between the Dentis and SNUCone abutments (p = 0.004). A significant difference was also reported between the Dentis and Swiss implant abutments (p < 0.001). Another significant difference was found between the SNUCone and Swiss implant abutments (p < 0.001).

There was a significant difference between the Dentis and Swiss implant abutments (p < 0.001). No significant difference was observed between the Dentis and SNUCone abutments (p = 0.186). A significant difference was found between the SNUCone and Swiss implant abutments (p < 0.001).

Discussion

This study investigated the abutment screw loosening and the force required to remove the abutment in original and compatible abutments in tissue-level implants. The present study evaluated the performance of different types of abutments and identified the factors that affect screw stability. The results of this study can help improve the design and selection of abutments and reduce the clinical problems related to screw loosening. The results revealed that there was a significant difference between the abutment screw loosening force in three types of abutments (Dentis, SNUCone, and Implant Swiss) in Dentis implants. Tukey's post hoc test revealed a significant difference in screw loosening force in Dentis and SNUCone, Dentis, Implant Swiss, and SNUCone abutments. Some abutments are able to withstand more force before loosening than others, and these differences are not due to chance. The order of screw stability is as follows: Implant Swiss > SNUCone > Dentis. In SNUCone implants, a significant difference was observed between three types of abutments regarding the screw loosening force.

Tukey's test indicated that the screw loosening force is different in Dentis and SNUCone, Dentis, Implant Swiss, and SNUCone abutments. The test shows that the type of abutment affects how much force is needed to loosen the screw, and these differences are statistically significant. The best order in screw stability is Implant Swiss > SNUCone > Dentis. In Implant Swiss, there was a significant difference between the abutment screw loosening force in three types of abutments (Dentis, SNUCone, and Implant Swiss). Tukey's post hoc test showed that the screw loosening force was not significantly different in Dentis and SNUCone abutments, but a significant difference was observed between Dentis, and Implant Swiss, and SNUCone and Implant Swiss. The best order in screw stability was Implant Swiss > SNUCone > Dentis.

The results of the present study showed that all three types of Swiss, Dentis, and SNUCone abutments performed well. However, the screw loosening force in Implant Swiss abutments was significantly higher than in Dentis and SNUCone abutments. These results indicate that Implant Swiss abutments perform better in maintaining screw loosening force compared to other types. The results of the statistical analysis confirmed these statistically significant differences. Implant Swiss abutments showed higher resistance to screw loosening due to unique design features, use of high-quality materials, optimal connection design, and accurate screw design. The advanced design of these abutments, which may include special geometries and advanced manufacturing technologies, helps distribute loads and stresses more evenly, leading to more force required to loosen the screw. Additionally, using materials with superior mechanical properties, such as high-quality titanium or special alloys, plays a vital role in increasing the strength and stability of abutments. These results are consistent with those of other studies such as Dincer et al[20] and Lee et al,[21] emphasizing the importance of the abutment design and materials in reducing screw loosening. These studies have referred to the key role of advanced design and high-quality materials in improving the stability of screws and reducing their loosening.

The results of the present study are in line with those of the study by Alsubaiy. The mentioned study investigated the effect of abutment design and materials on screw loosening force in several different implant systems. Straumann, Nobel Biocare, and Astra Tech brand abutments were compared. The results of this research showed that advanced designs and the use of high-quality materials have significantly helped to increase the resistance against loosening of screws. Similar to the results of the present study, this research also showed that brands with more optimized designs and materials with superior mechanical properties, such as Straumann and Nobel Biocare, require more force to loosen screws. The results revealed that advanced designs and the use of high-quality materials significantly increased the resistance against screw loosening. Consistent with the results of the present study; the mentioned study also showed that brands with more optimized designs and materials with superior mechanical properties, such as Straumann and Nobel Biocare, require more force to loosen screws. These results emphasize that advanced design and the use of high-quality materials play a vital role in increasing the stability of abutments. It is consistent with the results of the present study, which found Implant Swiss abutments are more successful in this regard.[22]

Shin et al also investigated the effect of implant-abutment connection design and diameter on screw stability. This study revealed that optimal design and use of high-quality materials could significantly improve the stability of screws. The results of this study are consistent with those of our study, emphasizing the effect of abutment design on screw loosening force and confirming the importance of design and material quality in reducing screw loosening.[23] In a study titled “The effect of abutment design and materials on resistance to loosening of screws in dental implants,” Yeo et al investigated the effect of abutment screw length on screw loosening. The results showed no significant difference between different groups of screws in screw loosening force and abutment design or material characteristics did not have a significant effect. These results are inconsistent with those of the present study, which showed that Implant Swiss abutments require more force for screw loosening. The reason for this difference may be that the study by Yeo et al focused on the effect of screw length and did not investigate the abutment design and material properties. However, in our study, differences in design and manufacturing technology (especially in the Implant Swiss) were identified as the primary factors in increasing the screw loosening force.[24]

The study by Paek et al focused on the comparison of screw loosening in prefabricated abutments and custom computer-aided design/computer-aided manufacturing abutments. This study revealed that the design and special material of abutments significantly reduces screw loosening. This study emphasizes the importance of high-quality materials and optimal design and shows that custom abutments can perform better. The results of our study are also consistent with these results as it confirmed the positive impact of high-quality materials and optimal design on reducing screw loosening in Implant Swiss abutments. Advanced design and materials with superior mechanical properties in both studies effectively increased the stability of screws and reduced their loosening.[25]

The results of this study revealed that the mean force required to remove abutments in different abutments (Dentis, SNUCone, and Implant Swiss) does not differ significantly. These results may be due to compatible design features used in implant systems. These systems may use standardized designs and compatible principles for connecting the abutment to the fixture, leading to reduced differences in the force required for abutment removal. Comparing the results of the present study with those of a study by Cosola et al, which investigated the difference in the force required for abutment removal in bone-level and tissue-level implants, indicated no significant difference between these results regarding the force to remove abutments. This similarity may be due to the compatible design or common technical characteristics of the implant systems.[26]

Additionally, the study by Kumar et al investigated the relationship between the force required to remove the abutment and the shape of the implant connection. They found that the force required to remove abutments was lower in bone-level implants than in tissue-level implants, although they did not observe a significant difference in this regard. The results of the present study are also consistent with these results since it showed that the force required to remove abutments is not affected by the type of abutment or specific designs and may depend on other factors such as the general characteristics of implant systems. In other words, the lack of a significant difference in the abutment force removal may be due to the general characteristics of the implants and the compatible design of the systems used in this regard.[27]

The study by Fathi et al investigated the effect of the shape and design of the internal connection in tissue-level and bone-level implants on the screw loosening force value and the force required to remove the abutment. The results revealed that the screw loosening force values were similar between different groups (p = 0.087). However, there were significant differences between the groups regarding the abutment removal force (p < 001). These results are inconsistent with those of the present study. The study by Fathi et al reported significant differences in abutment removal force, but the present study did not observe a significant difference in this regard.

This inconsistency may be related to differences in the design of implant systems, testing methods, or sample size.[28] According to the present study, it can be stated that the implant type does not affect the force required to remove abutment from the fixture. In other words, the results revealed that the torque force of the examined abutments primarily depended on the abutment type, factors such as the level of applied torque, and the dentist's performance. This issue indicates that factors such as the overall design of implants and the characteristics of implant systems may play a more significant role in determining the force required to remove abutments. However, the analysis of the results indicates that other factors such as the connection type and the design of the implants may play a role in the abutment removal force. A more detailed investigation of these factors can help better understand their impacts on the torque force and abutment removal, and bring different results.

Limitation

Due to the lack of cooperation from some manufacturers and the difficulty in accessing certain reputable tissue-level brands in our country, it was not possible to examine all brands. This issue makes it challenging to generalize the results to all tissue-level implant brands.

The high cost of acquiring fixtures and abutments, along with budget constraints, resulted in a sample size that was smaller than desirable. This could reduce the accuracy and generalizability of the results.

The chewing simulator was unable to fully replicate real oral conditions such as moisture, temperature, and saliva, which may have effects on the final results.

The accuracy of the equipment used for measuring torque force and abutment removal force had limitations, which might have led to minor deviations in the results.

Conducting in vivo research for the validation of laboratory results and evaluation of abutment performance in real clinical conditions is recommended.

Conclusion

Based on the results, the abutment screw loosening force is different depending on the implant and abutment type. Implant Swiss abutments have the highest loosening force, while Dentis abutments show the lowest loosening force. The highest abutment removal force is related to the Implant Swiss fixture with the SNUCone abutment and the lowest value is related to the Dentis fixture with the Implant Swiss abutment. However, the force required for abutment removal was not significantly different in different types of implants, indicating the limited effect of the abutment type on this parameter and may be related to other factors such as the overall design of the implant systems.

Conflict of Interest

The authors declare no known financial conflicts of interest or personal relationships that could have influenced the work reported in this article. The corresponding author, Dr. Amirhossein Fathi, serves as an Editorial Board Member for the Dental Research Journal.

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Publication History

Article published online:
03 June 2025