¹DDS, MS, Assistant Professor, Department of Prosthodontics, School of Dentistry, Hamadan University of Medical Sciences, Hamedan, Iran

²DDS, MS, Assistant Professor, Department of Prosthodontics, School of Dentistry, Shahid Beheshti University of Medical Sciences, Evin, Tehran, Iran

³DDS, MS, Assistant Professor, Department of Orthodontics, School of Dentistry, Hamadan University of Medical Sciences, Hamedan, Iran

⁴*DDS, MS, Assistant Professor, Department of Prosthodontics, School of Dentistry, Hamadan University of Medical Sciences, Hamedan, Iran

Corresponding author Email: sarakhazaei_83@yahoo.com

Article Publishing History

Introduction

Failure and complications with implant-supported prostheses still occur, despite the high clinical success rates in long-term. These complications include biological and mechanical problems. Screw loosening is the most commonly reported mechanical complication for single implant-supported prostheses. Different factors may contribute to loss or decrease in abutment screw torque such as fatigue, inappropriate tightening torque, failure in screw retightening after initial placement, settling effect, vibrating micro movements and excessive bending. The misfit of abutment or implant-supported crown, occlusal loading and structural design of implant connection are among other factors playing a role in this respect (Norton (1997, Jörnéus et al., 1992, Coelho et al., 2007 De Boever et al., 2006, Theoharidou et al., 2008).

The implant-abutment interface design can be roughly divided into two groups namely (I) butt-joints or slip fit joints with a passive fit and (II) conical interface with frictional fit between mating surfaces of abutment-implant complex; the latter type of interface also known as locking or Mores taper connection. In most previous studies, internal taper connections showed superior performance and were thought to minimize screw loosening and fracture, which commonly occur external hexagon butt-joint connections. An internal tapered abutment, based on the specific system used, may be attached to implant body by an external screw, orwith threads machined directly on the abutment body itself, which are calledone-piece (solid abutment) and two-piece abutments, respectively, (Finger et al., 2003 Budynas et al., 2008, Hansson 2000, Dittmer et al., 2011, Ricciardi et al., 2009, Cehreli et al., 2004 Aguirrebeitia et al., 2013 Rabelo et al., 2015).

It has been proven that both types are highly resistant to fatigue under dynamic loads and could function without any mechanical problems. One concern regarding the internal taper connection system is the possibility of cold welding of the abutment inside the implant. This was also mentioned by Sutter et al; whereas, other authors concluded that cold welding is presumably neutralized by a phenomenon called embedment relaxation. Clinically, it seems that One-piece abutments may be removed from the fixture more easily than two piece abutments, due to the nature of their design,which may causeless problems during removal, (Bozkaya et al., 1995, Weiss et al., 2000, Pintinha et al., 2013, Rabelo et al., 2015).

As mentioned earlier, screw loosening is the most common complication in single implant -supported restorations. This is important, particularly, incemented prosthesis, in which loosening or fracture of the abutment screw may lead to failure of prosthesis. Therefore, this study aimed to evaluate the effect of dynamic loading on removal torque value (RTV) of two types of internal taperabutments namely one-piece and two-piece abutments. The null hypothesis was that the RTV of one-piece and two-piece abutments would not decrease under dynamic loading.

Materials and Methods

In this in vitro experimental study, 32 analogs of fixtures (Ø 4.8 mm* 10 mm L, Simple line II, Dentium, Korea) with internal taper hexagon were chosen. In addition, 32 abutments (11-degree taper, Ø 4.5 mm* 5.5 mm L, Simple line II, Dentium, Korea) of two different types (one-piece and two-pieces) were used. Each analog was mounted in a mold containing auto-polymerizing acrylic resin.The customized molds were fabricated from brass and measured 20 mm in height and 25 mm indiameter, in addition, their upper surfaces were cut so that this surface had 30˚ angle relative to the horizontal plane. Then, on the upper ramp, ahole was drilled perpendicular to the surface for placement of implant analog (Figure 1). This design of the mold allowed for the fatigue tester to apply load to the abutment at a 30 ° angle relative to the long axis. A wooden jig was used for correct positioning of the molds on the surveyor. The jig was a ramp with a 60 ˚ angle relative to the horizontal plane. Therefore, by assembling the mold on the jig its upper surface was positioned parallel to the horizon. Next, analogs were placed inside the hole perpendicular to the ramp of mold using a surveyor. The hole was filled with auto-polymerizing acrylic resin in doughy stage right before analog insertion (Figure 2). Then, analogs were divided into two groups according to the type of abutment they would receive. The study groups were as follows:

OPC group: One-piece abutments that were not subjected to dynamic loading (control group, n=8).

TPC group: Two-piece abutments that were not subjected to dynamic loading (control group, n=8).

OPT group: One-piece abutments that were subjected to dynamic loading (test group, n=8).

TPT group: Two-piece abutments that were subjected to dynamic loading (test group, n=8).

Figure 1: Brass mold

 

Figure 2: Wooden jig for correct positioning of molds on the surveyor

To measure the tightening and removal torques, a digital torque meter (TQ-8800; Lutron electronic, Taiwan) with an accuracy of precision of 0.1 Ncm was used. Torque meter was positioned on top of a torque delivery device and the cylinder-analog-abutment assembly was fixed into a socket at the bottom of the device.

Before screw tightening, all abutments were lubricated with artificial saliva (Saliva Substitute; Roxane Laboratory Inc, USA). Then, all abutments in the control groups were tightened to 35 Ncm torque. After a 10-minute interval, abutments were retightened to the same torque to compensate for the loss of preload due to settling of surface at the interface. Ten minutes later, the RTV of abutments in the control groups was measured and recorded. For the test groups, 16 ceramic copings (e.max*Zir CAD, ivoclarvivodent) were fabricated with the same size and shape by computer aided design/computer aided manufacturing (CAD/CAM) technology (Sirona in Lab MCXL, Germany).

Since these abutments were cement-retained, for measurement of the RTV, the copings had to be removed from the abutment. For this reason, the coping were designed such that they had a hole in place of the abutment screw. Therefore, we hada direct accessto the coping hole, and there was no need to remove the crown after loading. Then, the adaptation of copings was verified and confirmed using light body silicone (Speedex, condensation polysiloxane, low consistency, Colten, Switzerland).

In the test groups, as well as the control groups, the abutments were torqued to 35 Ncmby digital torque meter in two cycles with 10-minutes intervals. Then, the crowns were placed on the abutments and cemented by a temporary cement (Temp bond, Kerr, Italy). During the experiment, the hole of crown was covered with composite. Afterward, each assembly of mold-analog-abutment-cap from the test groups was mounted and fixed to the electromechanical fatigue testing machine (CS-4, SDM echatronik, Germany) (Figure 3). The device has two lever arms that simultaneously apply force. The arm of device was so that the force was applied to the upper most part of the coping (Figure 4).

Figure 3: Dynamic fatigue tester

 

Figure 4: Ceramic coping and position of applying force

The fatigue tester was calibrated so that the lever arm soperated for 300,000 cycles (nearly corresponding toone year of chewing function) at a speed of 1Hz (60 rpm) [19].the position of load was Oblique load (withan angle of 30˚) of 100±5N was applied to each coping [19]. After each test, the specimens were transferred to the torque delivery device and the RTV was measured and recorded. The following formula was used to calculate the percentage of torque loss (Per tl):

SPSS software version 21 was used for statistical analysis. The Per tl were statistically analyzed by Two-way ANOVA and Tukey’s HSD post-hoc test. The significance threshold was set at 0.05.

Results And Discussion

None of the tested samples showed abutment or screw fracture; there was no sign of crown loosening either after loading. Two-way ANOVA indicated that abutment types, dynamic loadingand their interaction had significant effects (P<0.05) on Percentage of torque loss. Table 1 shows the mean and standard deviations of Pertl inall study groups. There moval torque of abutments decreased in both groups (control and test group). TheOPC group presented the lowest torque loss. The highest torque loss was observed in OPT group. Pair wise comparisons of Pertl were then performed with Tukey’s HSD post-hoc test (Table 2). The mean Pertl in abutments of OPC group was significantly lower than that in TPC and OPT groups (P<0.05). Also, the difference between OPT and TPT groups was significant (P<0.05), But there was no significant different in Per tl of abutments between TPC and TPT groups (P>0.05). In our study, all abutments in both control and test groups showed reduction in removal torque value compared to the insertion torque. Thisindicated that no cold welding occurred in any implant-abutment interface, which was consistent with the results of previous studies (Norton 1999, Ricciardi et al, 2009 Pintinha et al., 2013, Kim et al 2014).

Table 1: Mean and standard deviation of per tl for all groups (n=8)
Group	N	Mean±standard deviation	95% confidence interval for the mean Lower boundUpper bound		P value
OPC	8	8.93±4.76	4.9533	12.9167	.000
OPT	8	51.02 ±4.61	47.1711	54.8839
TPC	8	22.8±8.00	16.1642	29.5408
TPT	8	23.2± 8.97	15.8487	30.5538

 

Table 2: Results of Tukey’s HSD post-hoc test for pairwise comparisonsper tl
	Mean Difference (I-J)	95% Confidence Interval
		Lower Bound	Upper Bound	P value
OPC& OPT	-42.092*	-51.3843	-32.8007	.000
OPC&TPC	-13.917*	-23.2093	-4.6257	.002
OPC&TPT	-14.266*	-23.5581	-4.9744	.001
OPT&TPC	28.175*	18.8832	37.4668	.000
OPT&TPT	27.826*	18.5344	37.1181	.000
TPC&TPT	-0.34875	-8.9431	9.6406	1.000
*The mean difference is significant at the 0.05 level.

Norton showed that cold welding occurs only at the highest level of torque, right before the component failure and when plastic deformation is expected. In addition, cold welding does not occurin clinical levels of torque, and the removal torque is expected to be 10 to 20 % less than the initial torque. However, Sutter and colleagues stated that following torque application, the removal torque increases from 10 to 15 % compared to the initial torque in internal taper connections. They argued that probably, the effect of axial component of the simulated occlusal force surpasses other oblique and tensile forces that interact negatively with retention of abutments. But, other authors have reported different results, indicating that the cold welding, if occurs, is compensated by the settling effect, (Ricciardi et al., 2009 Cehreli et al., 2004).

The results from One-piece abutments showed that the mean torque loss Inthe test group was significantly higher than that in control group (8.93% in the control and 51.02 % in the test group). In One-piece systems, abutment serves asa screw; therefore, in the test group, with application of dynamic load, these forcesare directly transferred to the threads and decrease the removal torque. In addition, the bending and tensile stresses are produced at the interface andlead to greater reduction oftorque. The results of a previous study showed various ranges of torque loss in One-piece abutments. Pinhata et al. reported the mean torque loss values of 18.35% for the control group and 15% for the test group, Pintinha et al. (2013). This value was between 15 and 20% in the study by Norton et al. and between 10.5 and 5.4% in the study by Ricciardi et al. (2013).

A mean torque reduction of 8% by Cehreli et al. (2004) and 25% by Seol et al. (2015) has been reported in test groups. Different results of studies can partly be related to differences in the types of implant systems used. In addition, differences in experimental conditions should be considered. There are significant differences between our study and others in number of cycles, intensity, position, angle and rate of applied force. In two-piece abutments, the difference between the control (with 22.8% torque loss) and test group (with 23.2% torque loss) was not statistically significant.

Pinhata et al. (2013) reported a mean torque loss of 36% and 40.85% for the control and test groups, respectively. Similarly, Ricciardi et al. 2013 found the mean values of 32% forthe control group and 37.2% for the test group.Also, Seol et al. (2015) reported a mean torque loss of 48% for test group.Based on the results of our study, in the control groups, one-piece abutments showed significantly higher removal torque value than two-pieceabutments. When opening one-piece abutments, the retention caused by the tapered part of the abutment as well as retention caused by the threads should be overcome. But intwo-piece abutments, the removal torque recorded by torque meter is mainly spent to overcome the retention friction generated by the threads because in two-piece abutments, the abutment screw passes through the abutment andat the time of opening, this unit is removed from the abutment.Therefore, in these types of abutments, a large amount of torque required by the tapered part of the abutment is not registered by the torque meter (particularly inabutments that have anti-rotation feature). Thus, it seems logical that two-piece abutments show less removal torque than one-piece abutments.

After applying force, the torque loss of one-piece abutments was significantly higher than that of two-piece abutments. In two-piece abutments, since the screw and abutment are in two distinct parts (yet related), smaller amount of force exerted on the abutment is transferred to thescrew; whereas, in one-piece abutments, as mentioned earlier, the abutment serves as a screw and transfers dynamic forces directly to the threads and decreases the required removal torque.

In this study, we tried to establish conditions to simulate clinical masticatory conditions. Each sample under went 300,000 cycles of dynamic force, which corresponded to one year of normal chewing function. Inaddition, the abutments were lubricated by artificial saliva before applying torque because it has been suggested that greater initial preload can be achieved underwet conditions, Jaarda et al., (1993). Siamos et al., 2002 Lee et al., 2002, Winkler et al., 2003)

Applying proper torque recommended by the manufacturer is very important to prevent screw loosening and screw fracture. Jaarda et al. reported 15 to 48% error when closing the abutment screw by hand. Therefore, the abutments were tightened to 35 Ncm torque by a digital torque meter. Ten minutes later, the same torque was applied to compensate the loss of preload due to settling effect. Siamos et al. suggested that in order tominimize the loss of preload caused by the settling effect, the initial torque should beapplied again 10 minutes after initial tightening torque, (Siamos et al., 2002).

Considering the fact that biteforce actually acts on the super structure, it was appropriate to perform anexperiment in which dynamic forces are applied on the abutment after cementation of crown. Before cementation, precise and passive fit of caps was assessed by light-body silicon. Prostheses with active fit or improper adaptation can exertun desirable forces on the abutment, (Lee et al. 2002).

The limitations of this study included small sample size and short-term loading. In addition, our study had an in vitro design and had the limitation of in vitro studies in simulating the complex nature of mastication cycles. In the oral environment, forces are applied in different directions and angles to the axis of abutments; moreover, the intensity of these forces is variable in different situations. The maximum biting force has beenreported in the range of 200 to 3500 N, (Winkler et al. 2003). But in the present study, a force of 100 N was applied, which is at the low end of this range. Also, the rate of force in this study was one hit per second, which was continuously applied within 3 to 4 days, but in normal oral function, 300,000 cycles of force are applied in a much longer period (aboutone year). All these factors can affect the behavior of screw and its loosening.

Conclusion

Under mechanical loading, RTV of both groups (one-piece and two-piece) decreased and this reduction was significant for one-piece group. However, there was no significant difference in RTV between one-piece and two-piece abutments under dynamic loading.

Acknowledgements

I would like to express my sincere gratitude to my statistical advisor Dr. Saiid Mousavi for the continuous
support and immense knowledge. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of Interest

“None declared”

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