Evaluation of the Effect of Different Contaminants on the Reverse Torque Values at the Implant Abutment Junction: An In Vitro Study

Abstract

Background

Screw loosening at the implant-abutment junction is the commonest failure reason of implant prosthesis. This happens mainly due to improper insertion torque and friction between the clamping surfaces.

Objectives

This article evaluates the effect of three different contaminants on the screw loosening at the implant abutment junction. This effect is measured as reverse torque value, that is, the force required to loosen the screw after the contamination is done. Saliva, titanium nanoparticles, and sodium fluoride are used as contaminants which were prepared as 1000 ppm solutions with artificial saliva as a solvent.

Materials and Methods

Thirty Genesis Aktiv Internal hex parallel implants were divided into three groups and contaminated in their screw access channel with the respective contaminants and clamped with straight abutments at 35 Ncm. They were torqued again after 10 minutes to compensate for settling effect. After that, they are stored in artificial saliva for 14 days at 37°C. Reverse torque values were measured by a physiodispenser. The data collected was subjected to one-way analysis of variance test to determine the difference between groups. Tukey post hoc analysis was done to evaluate the intergroup mean differences.

Results

A statistically significant difference was noted in the mean reverse torque values between group 1 and groups 2 and 3. But, when intergroup comparisons were made, the difference between group 1 and group 2 only had statistical significance.

Conclusion

The reverse torque values increased significantly when contaminated with sodium fluoride and titanium nanoparticle solutions, which can be attributed the reduced friction between the clamping surfaces, thereby negating the settling effect.

Keywords

screw loosening - implant abutment junction - reverse torque values - titanium nanoparticles - artificial saliva

Introduction

Implant prosthodontists, whenever there is a screw loosening, usually check for high points on the prosthesis, the angulation of the abutment, and immediately change the abutment[1] without actually accounting for the etiology of the loosened screw. The result being repeated screw loosening in spite of numerous attempts to treat it with abutment screw replacement, abutment replacement, or prosthesis replacement.

Abutment screw loosening has been a major setback from achieving a long-term success in implant prosthodontics. This screw loosening has been encountered from single tooth replacements to multiple teeth replacements with implant-supported prosthesis.[2]

Apart from the masticatory forces and the angulation of the abutment, its force factors, the force required for accurate clamping of the surfaces within the implant-abutment junction (IAJ),[3] is usually overlooked or not given enough importance.

The clamping surfaces usually have some mechanical friction, which helps in the joint stability, but there is something called settling effect, which sets in at a later stage due to which the protocol is to tighten the screw again after 10 minutes.[4] [5] This does not help us in a longer run as we still see many patients coming back with a mobile prosthesis.

This study aims at attempting to reduce screw loosening, purely due to clamping surfaces behavior by altering the way the surfaces come into contact with each other while keeping the factors like mastication and other force factors away as these may interfere with how the surfaces may react with each other by putting them under load.

The objective of this study was to disperse an interspersing medium between the clamping surfaces, which are different solutions, with artificial saliva as the solvent and sodium fluoride and titanium nanoparticles as solutes.

This study helps us to understand whether the behavior of the clamping surfaces at the IAJ can be modified in a way to achieve positive outcomes, that is, reduced incidence of screw loosening.

In this study, screw loosening is measured in the terms of reverse torque values (RTVs) as the name obviously suggests it is the opposite of torque, which is usually used to clamp the joint together.

Materials and Methods

The aim of this study was to determine the effect of different contaminants, that is, saliva, fluoride, and titanium nanoparticles on the screw loosening using RTVs in an internal-hex titanium implant-abutment connection.

The null hypothesis is stated that there will be no effect of the contaminants used in this study on the screw loosening at the IAJ.

This in vitro study was conducted in the Department of Prosthodontics and Implantology, Government Dental College and Hospital, Hyderabad, Telangana, India.

The implants and abutments are of Genesis Company, Aktiv model, manufactured by Keystone Dental Inc., an American company, the implants were used in Hyderabad, Telangana, India for this study.

The minimum sample size to stay away from type 1 and type 2 errors is calculated at 95% confidence interval to be 30.

Thirty implants (3.75 mm × 13 mm, Tapered, Internal Hex connection, grade 4 titanium, spiral threads) and 30 regular platform straight abutments of diameter 3.75 mm and height 9 mm were used in this study. The implants were mounted in acrylic block of dimensions 20 × 20 × 20 mm³ with the help of a dental surveyor (Jintai Machines, Yuyao, Zhejiang, China). This acrylic block was fabricated with the help of polyvinyl siloxane impression of a stainless steel die of dimensions 20 × 20 × 20 mm³. The implant was attached to the analyzing rod of the dental surveyor with modeling wax ([Fig. 1]). Then, a glass plate was placed on the platform to ensure proper positioning of the putty mold. Once the perpendicular position of the implant to the center of the mold was confirmed, autopolymerizing acrylic resin (DPI Co.) was mixed and poured into the mold and the analyzing rod lowered into the resin, allowed to set. Care is exercised the implant's most coronal portion of 2mm is left outside the acrylic block (ISO 14801:2016 standard). This process is repeated for all the 30 samples ([Fig. 2]).

Fig. 1 Dental surveyor with implant.

Fig. 2 Mounted implant assemblies.

The samples were then mounted on a wooden plank with L clamps ([Fig. 3]). The screw holes were then contaminated with artificial saliva, artificial saliva and sodium fluoride—the solution was prepared as 1000 ppm (as those in dentifrices) with 0.01 mg of sodium fluoride, which is measured with a weighing machine with artificial saliva, artificial saliva, and titanium nanoparticles (Nanowings Pvt. Ltd., Khammam, Telangana, India).

Fig. 3 Samples mounted on a wooden plank with L-clamps.

The 30 implants were attached to 30 implant abutments with a 35-Ncm torque, which was recommended by the manufacturer using a motorized hex driver and physiodispenser (NSK Surgicpro) ([Fig. 4]). Now, after 10 minutes another torque of 35 Ncm was given to compensate for settling effect.

Fig. 4 Contamination and torquing at 35 Ncm.

The 30 samples were divided into three groups with 10 samples each as follows ([Table 1]).

Table 1 Grouping

Group	Criteria	Number of samples
I	Artificial saliva contamination	10
II	1000 ppm of solution of artificial saliva and titanium nanoparticles	10
III	1000 ppm of solution of artificial saliva and sodium fluoride powder	10

Procedure for Contamination

Group I

A 2-mL disposable syringe was used. Artificial saliva was loaded into the syringe and the contaminant is deposited into the implant fixture access channel until it is filled to its brim.

Group II

A solution was prepared which consisted of 0.01 mg of titanium nanoparticles in 100 mL of artificial saliva to get a 1000 ppm solution. This solution is then syringed into the fixture access channel to its brim.

Group III

A 1000 ppm solution was prepared with 0.01 mg of sodium fluoride and 100 mL of artificial saliva, which is then syringed into the access channel to its brim.

Procedure for Attaching the Abutments

All 30 samples were attached with their respective straight abutments with the help of hex driver attached to the physiodispenser at 35 Ncm torque and another torque after 10 minutes of the same magnitude to compensate for settling effect.

Hydrostress Aging

After torquing, all of the samples were stored in artificial saliva (Fusayama-Meyer composition) at 37°C for 14 days in a constant temperature water bath for hydrostress aging. The pH is maintained neutral with a commercial potassium citrate buffer solution at pH 7.

Measurement of Reverse Torque Values

After aging, the reverse torque of all the samples are measured with the hex driver and physiodispenser with a reverse torque setting ([Fig. 5]). The RTV was increased incrementally through 5, 10, 15, 20, 25… and so on, until all the abutment screws were loosened from the assembly. The greater the RTV, the greater the stability acquired at the IAJ and lesser the screw loosening.

Fig. 5 Reverse torque values measured.

Statistical Analysis

The collected data was entered into Microsoft Excel 2020 and subjected to statistical analysis using Statistical Package for Social Sciences (SPSS Version 20.0).

The mean of the data in each group was calculated and comparisons made with one-way analysis of variance (ANOVA). Intergroup comparisons of the means were made with Tukey post hoc test.

Results

The measured RTVs for all the samples in all groups were tabulated and subjected to one-way ANOVA test, which measures the difference in mean RTVs among the group. The p-value was set at 0.05 level of significance (p < 0.05), the confidence interval was set at 95% confidence ([Table 2]).

Table 2 Reverse torque values of all samples

Reverse torque values of the three groups
Group 1	Group 2	Group 3
S1 - 20 Ncm	T1 - 55 Ncm	F1 - 30 Ncm
S2 - 15 Ncm	T2 - 55 Ncm	F2 - 10 Ncm
S3 - 20 Ncm	T3 - 45 Ncm	F3 - 55 Ncm
S4 - 15 Ncm	T4 - 35 Ncm	F4 - 10 Ncm
S5 - 10 Ncm	T5 - 10 Ncm	F5 - 60 Ncm
S6 - 10 Ncm	T6 - 10 Ncm	F6 - 10 Ncm
S7 - 10 Ncm	T7 - 10 Ncm	F7 - 10 Ncm
S8 - 10 Ncm	T8 - 55 Ncm	F8 - 55 Ncm
S9 - 15 Ncm	T9 - 10 Ncm	F9- 30 Ncm
S10 - 10 Ncm	T10 - 55 Ncm	F10 - 55 Ncm

Statistically significant difference (p = 0.028) existed in the mean RTVs among the three groups ([Table 3] and [Graph 1]), the mean RTVs for group 1, group 2, and group 3 being 13.50, 34.00, and 32.50 Ncm, respectively.

Table 3 Mean comparison

Mean comparison among three groups
Groups	Min	Max	Mean	SD	p-Value (< 0.05)
Group 1	10.00	20.00	13.50	4.12	0.028 S
Group 2	10.00	55.00	34.00	21.58
Group 3	10.00	60.00	32.50	21.89

Abbreviations: ANOVA, analysis of variance; S, statistically significant at the 0.05 level of significance; SD, standard deviation.

Note: Statistical analysis: ANOVA one-way test.

Graph 1 Mean comparison of reverse torque values among the three groups.

After this, three intergroup comparisons are made, that is, group 1 and group 2, group 1 and group 3, and group 2 and group 3 ([Table 4] and [Graph 2]) and a statistically significant difference was found between group 1 and group 2 (p < 0.05), while other comparisons remained nonsignificant (p > 0.05).

Table 4 Intergroup comparison of mean difference

Groups	Mean	SD	Mean difference	p-Value
Group 1	13.50	4.12	20.50	0.042 S
Group 2	34.00	21.58
Group 1	13.50	4.12	19.00	0.063 NS
Group 3	32.50	21.89
Group 2	34.00	21.58	1.50	0.987 NS
Group 3	32.50	21.89

Abbreviations: NS, nonsignificant statistically; S, Statistically significant at the 0.05 level of significance; SD, standard deviation.

Note: Statistical analysis: Tukey post hoc test.

Graph 2 Intergroup comparisons of mean of reverse torque values.

Some of the samples in group 2 and group 3 showed drastic differences, which may be attributed to the difference in the machining of the implants, although they all belong to the same company; no two implants of the same company can be replicated exactly at a microscopic level which may be statistically insignificant when they are clamped without any interspersing medium. But, in this study as media are used between the clamping surfaces, it may have affected the screw tightening, which indicates a future study in this aspect.

Discussion

This in vitro study evaluated the effects of three different contaminants on the RTVs at the IAJ. The contaminants used here are the artificial saliva, titanium nanoparticles, and sodium fluoride. However, the sodium fluoride and titanium nanoparticles are used as solution with saliva as the solvent. The results suggested that the higher RTVs were obtained with the titanium nanoparticle contamination group and that of the saliva contamination group were the lowest. Hence, null hypothesis is rejected.

Screw loosening occurs when joint separating forces are greater than joint clamping forces, that is, preload, which depends on the internal surfaces of the IAJ in terms of their surface roughness and mechanical behavior. The tricky part still being that the preload should remain within the confines of the ultimate tensile strength of the abutment screw.[6] But, preload alone does not warrant a secure clamping. The abutment screws are retightened after 10 minutes at the original preload value to compensate for the embedment relaxation, consistent with the findings of Cardoso et al and Winkler et al.[7] [8]

Various studies have determined that repeated screw tightening have caused a loss of preload. As supposed by Burguete et al,[9] there are three types of tightening: torque control, angle control, and torque/angle control. Of these methods, only the first is used in prosthetic dentistry, and thus it was used in this study.

In the present study, autopolymerizing acrylic resin has been used as the base for the implants, which has a similar modulus of elasticity to that of bone (trabecular bone of the human mandible has a modulus of 0.14 × 10⁹ N/m², while autopolymerizing acrylic resin has a modulus of 0.21 × 10⁹ N/m²).[7] This is done to negate the effect of abutment screw elongation and subsequent additional preload as every 1.0-µm elongation of the screw was equivalent to a 47.9-N increase of the preload in the implant complex.[10]

Structural mechanical properties embody both the intrinsic material property (titanium and its alloys) and the geometric shape of the device (implant) being considered. These properties manifest ultimately when the implant is subjected to loads. In the present study, different RTVs are obtained of the samples even within the same contamination group. These differences in machining can be related to the study done by Al Rafee et al,[11] where they found that screws from different manufacturers, even though they looked alike, could withstand different maximum preload torque before fracture.

Embedment relaxation occurs due to friction within the IAJ. This happens because no two surfaces are absolutely smooth and have roughness at a microscopic level. Lang et al and Guda et al in their finite element model[12] suggested a reduction in coefficient of friction and increase in RTVs when lubricated screws were used. So, to form a lubricating interface, different contaminants were used.

Koosha et al[13] studied the effect of saliva, chlorhexidine, blood, and fluoride as contaminants on the RTVs at IAJ in grade 4 titanium implants and compared them with a no-contamination group. The results suggested that higher RTVs were found with chlorhexidine and lowest RTV in saliva contamination group consistent with the results of the present study; however, chlorhexidine is not used here.

In all the earlier studies,[4] [11] [13] [14] [15] [16] [17] [18] saliva was obtained from donors and human subjects which posed a wide range of standardization issues as the composition and consistency of saliva is different in different individuals at different times of the day. In this study, artificial saliva was prepared under controlled laboratory conditions to put all of the above factors which may vary the quantity and quality of saliva under check. Although laboratory saliva may also pose some disadvantages when trying to simulate intraoral conditions, due to lack of antibodies and enzymes, care has been exercised to include all the minerals under standardized concentrations to arrive at an acceptable quality.

Koosha et al[13] used 0.2% mouthwash for fluoride contamination. In this study, a 1000 ppm solution was prepared by weighing the 0.01 mg sodium fluoride in 100 mL of artificial saliva. Note that 1000 ppm is chosen because it is the most common concentration of sodium fluoride encountered in the dentifrices. Titanium was found to undergo corrosion and fatigue at a fluoride concentration of 1000 ppm at pH below 4.2. Nakagawa et al[19] also put forth that 1000 ppm of sodium fluoride at neutral pH has no corrosion effect. In the present study, care has been taken to maintain the pH at a neutral level and above such that the corrosion is ruled out of the picture as a confounding factor to achieve an organic outcome.

Metal debris as particles of nanometers in width, may originate from the titanium surface itself as the result of wearing and grooving of the surfaces. However, titanium nanoparticles are used in implant tool designing and manufacturing to improve the bone-implant contact, antimicrobial properties the nanoparticles hold. In a study done by Lee et al,[20] he used titanium nanoparticle (60–80 nm) suspension of 20 mg/mL with distilled water as a contaminant in which titanium implants were immersed and RTVs measured after 10 opening closing cycles of abutment screws. Polyoxyethylene sorbital ester is used as a surfactant to ensure even particle distribution, which is of lesser clinical possibility. In the present study, titanium nanoparticles (60–80 nm) suspension of 1000 ppm was made with artificial saliva, which is more clinically relevant.

Furthermore, all the contaminants in the present study are syringed into the screw access holes of grade 4 titanium, 4 mm diameter, internal hexagon connection implants in contrast to previous studies[10] [20] [21] in which the specimens are immersed in contaminants. After this, the prefabricated straight abutments are placed and torqued with a motorized hex driver connected with a physiodispenser at an insertion torque of 35 Ncm with 50 revolutions per minute.

Ageing can be of hydrostress ageing, hydrothermal ageing, thermocycling, etc. Mechanical cycling reduced the vertical misfit of all the groups, but there was no correlation between vertical misfit and torque loss. In the present study, hydrothermal ageing is performed in the presence of artificial saliva at 37°C for 14 days. This is done to remove mechanical cycling as a confounding factor for screw loosening.

When we go through the existing literature,[13] [20] [21] [22] [23] the RTVs of all the samples were below the initial torque value, but in the present study, few samples had RTVs (55–60 Ncm) above the initial torque value (35 Ncm). This suggests that preloads have been improved by the contaminants used in the study. However, clinical studies may be required to confirm this finding.

This study can further be improved by using mechanical cycling and hydrostress aging, which would yield a whole different set of results that can be more clinically applied. A custom-made buffer would have given a much lesser aberrations in the RTVs, which may or may not be statistically significant from the current results, as a commercially available buffer solution is used here.

However, this study redeemed its purpose by focusing on the change in behavior of the clamping surfaces under contamination suggesting that it is an area of practicality worth researching. Future studies can also focus on altering the concentrations of fluoride solution, titanium nanoparticle solution, and then evaluating the effect of change in concentrations on the surfaces. It can also include a Finite Element Analysis (FEA) and scanning electron microscopic studies to pinpoint the causative of the changes in RTVs.

Conclusion

Screw loosening is one of the most common prosthetic failures in implantology. There are a wide variety of reasons why it occurs and a good clinician should be able to know or establish the etiology before he treats the problem. This study focuses on screw loosening which occurs purely due to surface behavior of the IAJ and tries to manipulate that behavior to bring out positive outcomes. It is a topic of wide applications and has an extended scope from the researchers' point of view.

Conflict of Interest

None declared.

Acknowledgments

It is with supreme sincerity and great pleasure that I express my heartfelt gratitude to my guide Dr. B. Sreeramulu, Professor and H.O.D., Prosthodontics, Government Dental College & Hospital, Hyderabad for his constant encouragement and invaluable counsel throughout the period of the study.

I am thankful to the Academic Committee of Government Dental College and Hospital, Hyderabad for permitting me to utilize the facilities required for the study. My sincere gratitude to the teaching faculty of Department of Prosthodontics, GDC & H, for their valuable suggestions, encouragements and support for my study.

My special debt of appreciation is to my batchmates Dr. R. Janaki and Dr. P. Vineela.

Mr. Shaik Nazeer, Asst. Statistical Officer, Guntur who gave meaning to the numbers of my dissertation.

I would like to salute to the pillars of strength in my life with love and affection who stood with me in all the times. Love, guidance, and sacrifice of my parents, Mr. V. Chandra Sekhar and Mrs. V. Manikyamba without them I would not have joined M.D.S.

My sincere thanks to my elder brother Mr. V. Naga Sai Chakravarthy and my sister-in-law Mrs. Jyothi for encouraging all my thoughts to fruitarians and being there for me, not only in times of need, but at all times indeed. Finally, let me bow down my head for the creator of the life, who has always watched and guided and has given me so much. All that I am and have is because of him.

Ethics Approval

Institutional Ethics Committee of Government Dental College and Hospital, Hyderabad (GDCH-IEC/PG/1910 dated 14-11-2019).

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Publication History

Article published online:
03 February 2026