The purpose of this in vitro study was to evaluate the influence of taper angle in internal conical connections of implant systems and the application of chlorhexidine gel as an antibacterial agent or polyvinyl siloxane sealant on reverse torque values of abutment screws after dynamic loading. Four implant systems having different taper angles (5.4°, 12°, 45°, 60°) were tested in this study. Test specimens were divided into 3 groups: control (neither chlorhexidine gel filled nor silicone sealed), 2% chlorhexidine gel filled, and silicone sealed. The samples were subjected to a dynamic load of 50 N at 1 Hz for 500 000 cycles before reverse torque measurements. The taper angle of conical connections presented a quantitative positive correlation between the degree of the taper angle and the percentage of tightening torque loss. However, it was significant only between 60° angled connection and others except for the sealant applied groups (P = .013 for control groups and P = .007 for chlorhexidine groups). The percentages of decrease in torque values of silicon sealant–applied specimens were significantly higher than both the control and chlorhexidine groups (P values are .001, .002, .001, and .002, respectively, according to increasing taper angles), but the percentage of decrease in torque values due to chlorhexidine application was not statistically significant when compared with control groups. Gel form chlorhexidine application as an antibacterial agent does not significantly affect the implant-abutment connection stability under dynamic loads. Polyvinyl siloxane sealant may cause screw loosening under functional loads.

Most dental implant systems consist of 2 components: the endosteal part and a transmucosal abutment that connects to the implant to support the prosthetic restoration. The implant-abutment connection is exposed to a wide range of physiological forces during chewing and biting cycles, for a single implant in the molar area 120N in the axial direction and 50N in the horizontal direction at each chewing cycle may be given as an example.1,2  These recurrent forces may exceed either fatigue strength or the fracture resistance of the implant-abutment assembly and lead to various mechanical failures such as screw loosening, micro-movement at the abutment-implant interface, and even fracture of the screw, abutment, or the implant.37 

Preload is defined as the tension created in a screw, especially the threadings, when tightened. Preload is an engineering term used in dentistry, usually in implant dentistry, to describe the degree of tightness or clamping force of a screw.8  According to Bickford,9  screw loosening occurs in 2 steps. At first, the tightening torque value (TTV) decreases because of the functional loads acting on the screw joint followed by the backing off of the screw due to vibration and micro-motion. Thus, the effective preload is lost, and the screw cannot further maintain the joint stability. On the second stage, the preload drops below the critical level, leading to unscrewing movement of the threads, which eventually ends up in lost screw-joint function. Pjetursson et al6  calculated the cumulative incidence of implant-abutment connection complications involving screw loosening or fracture as 7.3% after 5 years of clinical service. Goodacre et al7  reported that the incidence of screw-related complications was 2% to 45% among single-unit and multiunit cement retained fixed prosthesis, with the highest rate in single-crown restorations. Unfortunately, implant manufacturers usually refrain from providing actual data related to mechanical failures of their specific connection type.10 

In addition to many other factors, a number of in vitro studies evaluated the influence of implant-abutment interface (IAI) geometry on the mechanical strength of the connection. Khraisat et al11  reported the fatigue resistance of an internal conical connection as superior to an external hexagon connection. Balfour and O'Brien12  compared the fatigue resistance and fracture strength of internal and external hexagon connections in which the internal connection revealed superior results over the external connection related to failures involving abutments, implant bodies, and the screws. Another study by Ugurel et al4  compared the mechanical resistance of 4 implant systems, 2 of which had 14° and 45° internal hexagon connections, 1 of which was a 3° Morse taper friction-grip connection and another with an 8° tapered internal octagon connection under simulated chewing, in which the latter was found to have higher fracture and bending resistance. On the other hand, numerous studies have compared the reverse torque values (RTVs) of abutment screws between internal and external connections. However, related research cannot yet provide a full reconciliation on this topic, as some studies have found internal connections to be superior to external connections,13,14  whereas others found the opposite.15,16 

Several manufacturers claim that their precise-fit internal conical connections are free of micro-motion, rotation, and micro-gaps under dynamic loads. However, according to the recent literature, none of the existing connection types can prevent abutment micro-movement, which is not only one of the reasons leading to mechanical failure but also the major cause of bacterial leakage.1719  Furthermore, several studies have investigated the efficiency of antibacterial agents and silicone sealing materials to prevent bacterial contamination through the abutment-implant interface under unloaded conditions, and a certain degree of benefit against bacterial colonization is reported for both chlorhexidine variants (gel and varnish forms) and silicone sealants.17,2023  However, the influence of neither chlorhexidine nor silicon sealant application to the IAI on the maintenance of screw preload under dynamic conditions has been investigated yet.

The purpose of this in vitro study was to evaluate the influence of taper angle in internal conical connections and the application of chlorhexidine gel as an antibacterial agent or polyvinyl siloxane (PVS) sealant on RTVs after dynamic loading. The null hypothesis is that there are no significant differences between various taper angles and the application of antibacterial agent or silicone sealant on RTVs.

Implant systems

Four implant systems with internal conical connections having different taper angles were investigated (Figure 1). Commercially packaged implants and abutments for cement-retained prosthetic restorations were used (Table 1). Eighty-four specimens, 21 from each implant system, were prepared. To examine the effects of agents and connection geometry on RTVs, each implant system was divided into 3 subgroups as follows: unsealed (control), 2% chlorhexidine gel filled, and silicone sealed (n = 7).

Figure 1.

Four implant systems with internal conical connections having different taper angles: (a) Ankylos 5.4°, (b) DTI 12°, (c) Bego 45°, (d) Trias 60°.

Figure 1.

Four implant systems with internal conical connections having different taper angles: (a) Ankylos 5.4°, (b) DTI 12°, (c) Bego 45°, (d) Trias 60°.

Close modal
Table 1

List of tested implant systems

List of tested implant systems
List of tested implant systems

Antibacterial and sealant agents

Gel form chlorhexidine at 2% concentration (Gluco-CHeX, Cerkamed PTT) and a silicone sealant (Kiero Seal, Kuss Dental) were used in this study. Gluco-CHeX gel is an antiseptic designed for intraoral use that contains 2% chlorhexidine digluconate as the antimicrobial agent. It interacts with the lipophilic cell membranes of bacteria causing osmotic imbalance, which subsequently results in cell death.24  Kiero Seal is a PVS-based material that was specifically developed for sealing the IAI. It has low viscosity during the application period, and the setting time is up to 3 minutes.

Experimental design

Implants were embedded in epoxy resin that had an elastic modulus similar to human bone (EpoFix) to resemble the implant bone assembly. The resin also fixed the specimens in the custom-made test chambers, which were designed to provide secure mounting of specimens on the chewing simulator and keep them immersed in 8 mL of artificial saliva (PL17000305 200 ml, Pickering Lab Inc) that aligned halfway up the crown restorations to imitate wet in vivo conditions (Figure 2).

Figure 2.

View of the chewing simulator in which the dynamic loadings were performed.

Figure 2.

View of the chewing simulator in which the dynamic loadings were performed.

Close modal

Full metal crown restorations with 30° cusp inclinations were prepared for all abutments and luted with a dual-cure resin cement (RelyX U-200, 3M Espe). Screw chambers of the implants were either filled with 1 of the 2 tested agents or left empty for the control group. Each abutment was screwed to the implant at the specific torque value according to the manufacturers' recommendations with the help of a digital torque meter, which has an accuracy of ±0.3% (Apac WS2-030-13000148, Mas Apac International Co. Ltd). To avoid the settling effect, the same operator applied the same torque values after 10 minutes.

Dynamic loading

The specimens were subsequently mounted on a dual-axis chewing simulator (CS-4.2, SD Mechatronik, Feldkirchen) that hosted 4 specimens at a time. A simulated chewing load of 50 N at 1 Hz for 500 000 cycles was applied to the specimens with a rounded stainless-steel stylus to represent 2 years of function, which is accepted to be within the physiological range.2,3  The stylus motion started 2 mm away from the center of the full metal restoration, moved 3 mm vertically, and slid 2 mm in the horizontal plane toward the center at each cycle (Figure 3). The housings kept the specimens immersed in artificial saliva during dynamic loading. No additional lubricant was applied on the sliding surfaces. After mechanical loading, specimens were removed from the simulator, and RTVs were measured by the same operator with the digital torque meter.

Figures 3. and 4.

Figure 3. Schematic drawing of dynamic loading test setup. Figure 4. Box plot of removable torque values of implant systems in test groups and the control group.

Figures 3. and 4.

Figure 3. Schematic drawing of dynamic loading test setup. Figure 4. Box plot of removable torque values of implant systems in test groups and the control group.

Close modal

The amount of decrease in torque values was calculated in percentages using the following formula: tightening torque − removal torque/tightening torque × 100 (%).

Statistical analysis

The methodology was reviewed by an independent statistician. Statistical analysis was performed using Statistical Package for Social Sciences (SPSS) for Windows software (IBM Corp. 2013, IBM SPSS Statistics for Windows, version 22.0). The Shapiro-Wilk test was used to determine if the measured parameters met the assumptions of normal distribution. The results indicated that the data were not normally distributed. Therefore, quantitative analysis between groups of parameters was made with the Kruskal-Wallis test, and the Mann-Whitney U test was used to determine the group responsible for the difference. The statistical significance was set to .05.

The percentage of torque loss with respect to connection geometry and applied chemical agents along with the control group is presented in Table 2. Removal torque values are presented in Figure 4.

Table 2

Influence of sealant and antibacterial agents and connection type on torque loss in percentages*

Influence of sealant and antibacterial agents and connection type on torque loss in percentages*
Influence of sealant and antibacterial agents and connection type on torque loss in percentages*

All RTVs free of any parameters, whether statistically significant or not, showed a certain degree of decrease after dynamic loading when compared with TTVs. The evaluation of the connection types revealed a quantitative positive correlation between the degree of taper angle and the percentage of TTV loss. However, it was significant only between 60° Trias and other systems, except for the sealant-applied groups. The comparative evaluation between other systems did not present any significant differences regarding the percentage of lost torque values.

The application of antibacterial or sealant agents also resulted in varying amounts of loss from TTVs. The percentages of decrease in torque values of silicon sealant–applied specimens were significantly higher than both the control and chlorhexidine groups, but the percentage of decrease in torque values due to chlorhexidine application was not statistically significant when compared with control groups.

This study investigated the influence of taper angle and application of antibacterial or sealant agents at the IAI under simulated chewing on RTVs of abutment screws.

Recent dynamic loading studies on implant-abutment connections have used different dynamic conditions that vary in magnitudes from 15N to 300N, at inclinations between 30° and 90°, and with the number of cycles ranging from 16 000 to 1 200 000.1,19,2529  Even though the intervals seem a little wide, they were all claimed to be within the physiological range.2,3  Steinebruner et al26  reported mechanical failure of an implant-abutment connection under 120N dynamic load at 172 800 cycles. Ugurel et al4  also reported that 3 of 4 implant systems (Biohorizons, Xive, and Octo) had a maximum median failure at 539 719 cycles under 120N simulated chewing. Considering the results of these studies, single crowns with 30° cusp inclinations were subjected to 50N and 500 000 cycles of dynamic loading to be able to investigate the RTVs before any fracture or bending failures occur.

The settling effect is a well-investigated mechanism that causes significant preload loss due to partially wasted initial torque to overcome the rough surface friction between the internal threads of the implants and screws.30,31  Because this is a major factor that eventually leads to screw loosening when avoided, applying a second torque is recommended once the settling is complete. In this study, 10 minutes of settling time was considered, as recommended in the related literature.32 

Wiskott et al33  mentioned that the stability of the screw joint depends on the relationships between the pretension of the abutment screw, mating surface preload, and friction between abutment and implant surfaces upon screw tightening. Dynamic loads may cause micro-movements between the abutment and the implant, which may lead to a possible enlargement of micro-gaps along the IAI that may eventually result in loosening of the screw joint stability.

Dandan et al27  investigated the relation between the amount of initial torque values and dynamic fatigue performance of implant-abutment assemblies in addition to RTVs of abutment screws. They concluded that dynamic loading would lead to both declined RTVs and significant preload loss.27  Tsumita et al28  evaluated the joint stability related to zirconium abutments by fatigue loading, and their results revealed that RTVs were lower than the TTVs, independent of the investigated parameters. Similarly, Mohammed et al29  reported decreased RTVs for all screw lengths and connection types they investigated after dynamic loading. In the current study, all samples from each implant system also showed lower RTVs compared with TTVs after dynamic loading, similar to previous studies.2729 

Because of friction between the abutment and the implant in the internal taper connections, the screw in between is claimed to have a relatively small effect in maintaining the connection stability.34  The results of the present study seem to support this argument. The quantitative evaluation of the data regarding the relation between the degree of taper angle of the implant–abutment connection and the percentage of torque loss in the TTVs showed a parallel relation; however, it was significant only for the 60° Trias system. There were no significant differences between Ankylos (5.4°), Bego (45°), and DTI (12°) systems in the control and chlorhexidine-sealed groups. Ugurel et al4  investigated 4 implant systems regarding their fracture and bending strengths under simulated chewing and static loading. Two of their systems had internal hexagon connections with 14° and 45° tapers. The 45° BioHorizons showed superior mechanical resistance over the 14° Frialit Xive; however, the 8° octagon Straumann was reported to reveal better results over the other two. The results of this recent study may seem to stand contradictory to the mentioned study, but actually, there was no rational objective of comparing RTVs and mechanical strengths related to degree of taper. Obviously, the degree of taper is one of the pieces of an engineering puzzle, and there is still a lot to investigate to be able to draw the principles of a secure implant-abutment connection.

There are limited data in the literature on the effect of silicone sealants on RTV under dynamic conditions. According to the obtained results, there were no significant differences between the percentages of loss in TTVs of the silicone-sealed groups from all implant systems. The amount of loss in TTVs increased quantitatively for all implant systems as compared with the control and chlorhexidine groups.

This diverse result may be related to the setting contraction behavior of silicone materials. Silicone materials have an initial dimensional change on the polymerization process. Although PVS shows the smallest dimensional change on setting as compared with the other elastomeric materials,35  this limited contraction might have caused instability in the screw joint and increased the amount of loss in TTVs.

In addition, the dimensional stability of silicone materials is also affected by changes in environmental temperature and humidity. Even though our study design evaluated the influence of a wet environment on the connection stability, the effects of thermal changes have not yet been investigated. Contraction and expansion of the sealant material because of wet conditions might have caused micro-gaps and micro-movements along the implant-screw interface, which might be one of the reasons for the significant torque loss in the silicone-sealed samples.35  Another possibility is that the thin layer of sealant residue at the IAI might have led to incomplete settling between the 2 mating surfaces of the assembly. When applying the desired torque for the second time after 10 minutes, the thin film residue of the sealant, which hardens before settling is complete, might have interfered with the settling process, resulting in incomplete settling. According to this scenario, the TTV could have been erroneously achieved without overcoming the settling effect. Under dynamic loads of simulated chewing, the sealant might have degraded easily, as the surfaces of the interface continued to settle, thus resulting in earlier and higher loss of preload, affecting the RTVs.

Further investigations may reveal a safer protocol for the use of sealant materials. Lee et al36  reported that 20% additional torque not only improves the critical bending moment of the implant but also delivers the minimum amount of micro-motion without any evident drawbacks. A similar study involving sealant agents may reveal a certain amount of higher torque value to be appropriate when sealant materials are to be used.

Removing the abutment and cleaning the residual sealant from the IAI after initial application of the recommended torque may also be helpful before starting the conventional tightening protocol. Another parameter to be considered should be the setting time of the sealant material, which is currently way below the time needed for the settling effect to complete. The short setting time of the material has no apparent objective and may be a cause of early preload loss.

Screw loosening has been a frequent problem since the beginning of implant dentistry.37  The stability of an implant-abutment assembly depends on the maintenance of the clamping force, in another term, the preload, which is created within the abutment screw when it is tightened. One well-known cause of screw loosening is embedment relaxation, which is more commonly known as the settling effect.38  After exerting the recommended amount of torque on the abutment screw, microscopic rough spots on screw threads start to flatten, leading to a loss in the clamping force. This happens not only under dynamic loads but also when the assembly is not loaded after initial tightening. The amount of loss in the preload progresses because of several peripheral factors, such as dynamic masticatory loads, the lubricating effect, the hydrodynamics of oral fluids, and thermal changes of within the oral cavity.39  This progressive decrease in the preload eventually results in screw loosening; however, there is no actual way to monitor the real-time variations in the preload of an abutment screw. An analog method of stress analysis is too rough for standard tiny abutment screws, and the digital modeling of any scene is vastly complicated; for calibration of such a model, a basic analogue physical model is still needed. On the other hand, as the preload is directly proportional to the amount of tightening torque, the RTVs will apparently deliver quantitative and comparable data when analyzing factors acting on the preload of the abutment screw. With this perspective, RTVs can be used to evaluate and compare the factors that may lead to screw loosening and help to improve tightening protocols for abutment screws.

Chlorhexidine variants, either in gel or rinse forms, are recommended for use as antibacterial fillers between implants and abutment screws against bacterial colonization due to micro-leakages.40,41 

However, the effect of chlorhexidine application between the screw and implant interface on RTVs has not yet been clarified. Gumus et al42  reported similar results for 0.1% aqueous solution of chlorhexidine application in contrast to findings of Micarelli et al,43  who used a gel form of chlorhexidine; however, the main difference of these studies from this recent research in common is that they have not used mechanical loading either statically or dynamically. Gumus et al did not apply a second torque to overcome the settling effect, and Micarelli et al evaluated the RTVs of freshly replaced screws. These differences between experimental methods require the independent evaluation of results.

Within the limitations of this study, our results revealed the following conclusions:

  • Higher degrees of taper angle in the internal conical abutment-implant connections may have adverse effects on maintaining the preload of the abutment screw.

  • Filling the screw chamber of an implant with a gel form chlorhexidine does not seem to jeopardize the implant-abutment connection stability under dynamic loads.

  • PVS-based sealant may lead to screw loosening under functional loads. The material needs further investigation/improvement for safe clinical use.

Abbreviations

Abbreviations
IAI:

implant-abutment interface

PVS:

polyvinyl siloxane

RTV:

reverse torque values

TTV:

tightening torque value

The authors would like to acknowledge independent statistician Hande Kartal and AC Dental Medikal Ltd, Istanbul, Turkey, and DTI Implants Ltd, Istanbul, Turkey, for donating and manufacturing the experimental implants and screws used in this study. This work was supported by a grant from Scientific Research Projects Coordination Unit of Istanbul University. ONAP; Process 2014, project No. 1509-42829.

The authors report no conflict of interest.

1. 
Steiner
M,
Mitsias
ME,
Ludwig
K,
Kern
M.
In vitro evaluation of a mechanical testing chewing simulator
.
Dent Mater
.
2009
;
25
:
494
499
.
2. 
Richter
EJ.
In vivo vertical forces on implants
.
Int J Oral Maxillofac Implants
.
1995
;
10
:
99
108
.
3. 
Steinebrunner
L,
Wolfart
S,
Ludwig
K,
Kern
M.
Implant-abutment interface design affects fatigue and fracture strength of implants
.
Clin Oral Implants Res
.
2008
;
19
:
1276
1284
.
4. 
Ugurel
CS,
Steiner
M,
Isik-Ozkol
G,
Kutay
O,
Kern
M.
Mechanical resistance of screwless morse taper and screw-retained implant-abutment connections
.
Clin Oral Implants Res
.
2015
;
26
:
137
142
.
5. 
Haack
JE,
Sakaguchi
RL,
Sun
T,
Coffey
JP.
Elongation and preload stress in dental implant abutment screws
.
Int J Oral Maxillofac Implants
.
1995
;
10
:
529
536
.
6. 
Pjetursson
BE,
Tan
K,
Lang
NP,
Brägger
U,
Egger
M,
Zwahlen
M.
A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years
.
Clin Oral Implants Res
.
2004
;
15
:
667
676
.
7. 
Goodacre
CJ,
Kan
JY,
Rungcharassaeng
K.
Clinical complications of osseointegrated implants
.
J Prosthet Dent
.
1999
;
81
:
537
552
.
8. 
The glossary of prosthodontic terms
.
J Prosthet Dent
.
2005
;
94
:
10
92
.
9. 
Bickford
JH.
Introduction to the Design and Behavior of Bolted Joints: Non-gasketed Joints. 4th ed
.
Boca Ration, FL
:
CRC Press;
2007
.
10. 
Eckert
SE,
Choi
YG,
Sánchez
AR,
Koka
S.
Comparison of dental implant systems: quality of clinical evidence and prediction of 5-year survival
.
Int J Oral Maxillofac Implants
.
2005
;
20
:
406
415
.
11. 
Khraisat
A,
Stegaroiu
R,
Nomura
S,
Miyakawa
O.
Fatigue resistance of two implant/abutment joint designs
.
J Prosthet Dent
.
2002
;
88
:
604
610
.
12. 
Balfour
A,
O'Brien
GR.
Comparative study of antirotational single tooth abutments
.
J Prosthet Dent
.
1995
;
73
:
36
43
.
13. 
Cardoso
M,
Torres
MF,
Lourenço
EJ,
de Moraes Telles
D,
Rodrigues
RC,
Ribeiro
RF.
Torque removal evaluation of prosthetic screws after tightening and loosening cycles: an in vitro study
.
Clin Oral Implants Res
.
2012
;
23
:
475
480
.
14. 
Lee
JH,
Kim
DG,
Park
CJ,
Cho
LR.
Axial displacements in external and internal implant-abutment connection
.
Clin Oral Implants Res
.
2014
;
25
:
e83
e89
.
15. 
Tzenakis
GK,
Nagy
WW,
Fournelle
RA,
Dhuru
VB.
The effect of repeated torque and salivary contamination on the preload of slotted gold implant prosthetic screws
.
J Prosthet Dent
.
2002
;
88
:
183
191
.
16. 
Kim
KS,
Lim
Y-J,
Kim
M-J,
et al
Variation in the total lengths of abutment/implant assemblies generated with a function of applied tightening torque in external and internal implant–abutment connection
.
Clin Oral Implants Res
.
2011
;
22
:
834
839
.
17. 
Koutouzis
T,
Gadalla
H,
Kettler
Z,
Elbarasi
A,
Nonhoff
J.
The role of chlorhexidine on endotoxin penetration to the implant-abutment interface (IAI)
.
Clin Implant Dent Relat Res
.
2015
;
17
:
476
482
.
18. 
Larrucea Verdugo
C,
Jaramillo Núñez
G,
Acevedo Avila
A,
Larrucea San Martín
C.
Microleakage of the prosthetic abutment/implant interface with internal and external connection: in vitro study
.
Clin Oral Implants Res
.
2014
;
25
:
1078
1083
.
19. 
do Nascimento
C,
Miani
PK,
Pedrazzi
V,
et al
Leakage of saliva through the implant-abutment interface: in vitro evaluation of three different implant connections under unloaded and loaded conditions
.
Int J Oral Maxillofac Implants
.
2012
;
27
:
551
560
.
20. 
Duarte
AR,
Rossetti
PH,
Rossetti
LM,
Torres
SA,
Bonachela
WC.
In vitro sealing ability of two materials at five different implant-abutment surfaces
.
J Periodontol
.
2006
;
77
:
1828
1832
.
21. 
Podhorsky
A,
Putzier
S,
Rehmann
P,
Streckbein
P,
Domann
E,
Wöstmann
B.
Transfer of bacteria into the internal cavity of dental implants after application of disinfectant or sealant agents in vitro
.
Int J Prosthodont
.
2016
;
29
:
493
495
.
22. 
Besimo
CE,
Guindy
JS,
Lewetag
D,
Meyer
J.
Prevention of bacterial leakage into and from prefabricated screw-retained crowns on implants in vitro
.
Int J Oral Maxillofac Implants
.
1999
;
14
:
654
660
.
23. 
Buzello
AM,
Schütt-Gerowitt
H,
Niedermeier
W.
Desinfizierende Spüllösungen zur Keimzahlreduktion im Interface zwischen Implantat und Aufbau [Antiseptic irrigations to reduce the bacterial growth at the implant/abutment interface]
.
Z Zahnärztl Impl
.
2005
;
21
:
216
223
.
24. 
Kanisavaran
ZM.
Chlorhexidine gluconate in endodontics: an update review
.
Int Dent J
.
2008
;
58
:
247
257
.
25. 
Tripodi
D,
D'Ercole
S,
Iaculli
F,
Piattelli
A,
Perrotti
V,
Iezzi
G.
Degree of bacterial microleakage at the implant-abutment junction in Cone Morse tapered implants under loaded and unloaded conditions
.
J Appl Biomater Funct Mater
.
2015
;
13
:
e367
e371
.
26. 
Steinebrunner
L,
Wolfart
S,
Bössmann
K,
Kern
M.
In vitro evaluation of bacterial leakage along the implant-abutment interface of different implant systems
.
Int J Oral Maxillofac Implants
.
2005
;
20
:
875
881
.
27. 
Xia
D,
Lin
H,
Yuan
S,
Bai
W,
Zheng
G.
Dynamic fatigue performance of implant-abutment assemblies with different tightening torque values
.
Biomed Mater Eng
.
2014
;
24
:
2143
2149
.
28. 
Tsumita
M,
Kokubo
Y,
Kano
T,
Sasaki
K.
Effect of fatigue loading on the screw joint stability of zirconium abutment
.
J Prosthodont Res
.
2013
;
57
:
219
223
.
29. 
Mohammed
HH,
Lee
JH,
Bae
JM,
Cho
HW.
Effect of abutment screw length and cyclic loading on removal torque in external and internal hex implants
.
J Adv Prosthodont
.
2016
;
8
:
62
69
.
30. 
Winkler
S,
Ring
K,
Ring
JD,
Boberick
KG.
Implant screw mechanics and the settling effect: an overview
.
J Oral Implantol
.
2003
;
29
:
242
245
.
31. 
Lee
HW,
Alkumru
H,
Ganss
B,
Lai
JY,
Ramp
LC,
Liu
PR.
The effect of contamination of implant screws on reverse torque
.
Int J Oral Maxillofac Implants
.
2015
;
30
:
1054
1060
.
32. 
Dixon
DL,
Breeding
LC,
Sadler
JP,
McKay
ML.
Comparison of screw loosening, rotation, and deflection among three implant designs
.
J Prosthet Dent
.
1995
;
74
:
270
278
.
33. 
Wiskott
HW,
Belser
UC,
Scherrer
SS.
The effect of film thickness and surface texture on the resistance of cemented extracoronal restorations to lateral fatigue loading
.
Int J Prosthodont
.
1999
;
12
:
255
262
.
34. 
Merz
BR,
Hunenbart
S,
Belser
UC.
Mechanics of the implant-abutment connection: an 8- degree taper compared to a butt joint connection
.
Int J Oral Maxillofac Implants
.
2000
;
15
:
519
526
.
35. 
Mandikos
MN.
Polyvinyl siloxane impression materials: an update on clinical use
.
Aust Dent J
.
1998
;
43
:
428
434
.
36. 
Lee
FK,
Tan
KB,
Nicholls
JI.
Critical bending moment of four implant-abutment interface designs
.
Int J Oral Maxillofac Implants
.
2010
;
25
:
744
751
.
37. 
Lee
KY,
Shin
KS,
Jung
JH,
et al
Clinical study on screw loosening in dental implant prostheses: a 6-year retrospective study
.
J Korean Assoc Oral Maxillofac Surg
.
2020
;
46
:
133
142
.
38. 
Varvara
G,
Sinjari
B,
Caputi
S,
Scarano
A,
Piattelli
M.
The relationship between time of retightening and preload loss of abutment screws for two different implant designs: an in vitro study
.
J Oral Implantol
.
2020
;
46
:
13
17
.
39. 
Huang
Y,
Wang
J.
Mechanism of and factors associated with the loosening of the implant abutment screw: a review
.
J Esthet Restor Dent
.
2019
;
31
:
338
345
.
40. 
Paolantonio
M,
Perinetti
G,
D'Ercole
S,
et al
Internal decontamination of dental implants: an in vivo randomized microbiologic 6- month trial on the effects of a chlorhexidine gel
.
J Periodontol
.
2008
;
79
:
1419
1425
.
41. 
D'Ercole
S,
Tetè
S,
Catamo
G,
et al
Microbiological and biochemical effectiveness of an antiseptic gel on the bacterial contamination of the inner space of dental implants: a 3-month human longitudinal study
.
Int J Immunopathol Pharmacol
.
2009
;
22
:
1019
1026
.
42. 
Gumus
HO,
Zortuk
M,
Albayrak
H,
Dincel
M,
Kocaagaoglu
HH,
Kilinc
HI.
Effect of fluid contamination on reverse torque values in bone-level implants
.
Implant Dent
.
2014
;
23
:
582
587
.
43. 
Micarelli
C,
Canullo
L,
Baldissara
P,
Clementini
M.
Implant abutment screw reverse torque values before and after plasma cleaning
.
Int J Prosthodont
.
2013
;
26
:
331
333
.