Biosci. Biotech. Res. Comm. 10(3): 551-556 (2017)
Effect of dynamic loading on removal torque value of
one-piece and two-piece abutments
Farnaz Firouz
, Bijan Heidari
, Sara Alijani
and Sara Khazaei
DDS, MS, Assistant Professor, Department of Prosthodontics, School of Dentistry, Hamadan University of
Medical Sciences, Hamedan, Iran
DDS, MS, Assistant Professor, Department of Prosthodontics, School of Dentistry, Shahid Beheshti
University of Medical Sciences, Evin, Tehran, Iran
DDS, MS, Assistant Professor, Department of Orthodontics, School of Dentistry, Hamadan University of
Medical Sciences, Hamedan, Iran
*DDS, MS, Assistant Professor, Department of Prosthodontics, School of Dentistry, Hamadan University of
Medical Sciences, Hamedan, Iran
The aim of this study was to assess the effect of dynamic loading and abutment type on removal torque value. Thirty-two analogs
of  xtures with internal taper connections were divided into two groups of 16. The one-piece (OP) group received solid (one-piece)
abutments and the two-piece (TP) group received two-piece abutments. Each group was further subdivided into subgroups C
(control) without mechanical loading and T (test) with mechanical loading. The screw of abutments in OPC and TPC groups, were
tighten and then removed to record the removal torque value (RTV). In OPT and TPT groups, abutments were tighten, mechani-
cally loaded (300,000 cycles), removed, and the RTV were recorded. Two-way ANOVA and Tukey’s HSD post-hoc testwere used
for data analysis.The signi cance threshold was set at 0.05. The mean torque loss of OPC group was signi cantly lower than both
TPC and OPT groups (P < 0.05). But there was not signi cant different in torque lossvalues between abutments in TPC and TPT
groups.Under mechanical loading, theremoval torque of both one-piece and two-piece groups decreased and this reduction was
only signi cant for one-piece group. Also, the abutment type has signi cant effect on removal torque value.
*Corresponding Author:
Received 29
June, 2017
Accepted after revision 19
Sep, 2017
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Online Contents Available at:
DOI: 10.21786/bbrc/10.3/32
Failure and complications with implant-supported pros-
theses 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 sin-
gle implant-supported prostheses. Different factors may
Farnaz Firouz et al.
FIGURE 1. Brass mold
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 bend-
ing. The mis t of abutment or implant-supported crown,
occlusal loading and structural design of implant con-
nection 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  t
joints with a passive  t and (II) conical interface with fric-
tional  t between mating surfaces of abutment-implant
complex; the latter type of interface also known as lock-
ing or Mores taper connection. In most previous studies,
internal taper connections showed superior performance
and were thought to minimize screw loosening and frac-
ture, which commonly occur external hexagon butt-joint
connections. An internal tapered abutment, based on the
speci c 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 Aguir-
rebeitia et al., 2013 Rabelo et al., 2015).
It has been proven that both types are highly resist-
ant 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  xture 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 com-
mon complication in single implant -supported restora-
tions. 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.
In this in vitro experimental study, 32 analogs of  xtures
(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 sur-
face 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  lled with auto-polym-
erizing 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 sub-
jected to dynamic loading (test group, n=8).
TPT group: Two-piece abutments that were sub-
jected to dynamic loading (test group, n=8).
To measure the tightening and removal torques, a
digital torque meter (TQ-8800; Lutron electronic, Tai-
wan) with an accuracy of precision of 0.1 Ncm was used.
Farnaz Firouz et al.
FIGURE 2. Wooden jig for correct positioning of
molds on the surveyor
Torque meter was positioned on top of a torque delivery
device and the cylinder-analog-abutment assembly was
xed into a socket at the bottom of the device.
Before screw tightening, all abutments were lubri-
cated with arti cial saliva (Saliva Substitute; Roxane
Laboratory Inc, USA). Then, all abutments in the con-
trol 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 meas-
ured and recorded. For the test groups, 16 ceramic cop-
ings (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 abut-
ment screw. Therefore, we hada direct accessto the cop-
ing hole, and there was no need to remove the crown
after loading. Then, the adaptation of copings was veri-
ed and con rmed using light body silicone (Speedex,
condensation polysiloxane, low consistency, Colten,
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 com-
posite. Afterward, each assembly of mold-analog-abut-
ment-cap from the test groups was mounted and  xed
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).
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].
FIGURE 3. Dynamic fatigue tester
FIGURE 4. Ceramic coping and position of apply-
ing force
Farnaz Firouz et al.
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 signi -
cance threshold was set at 0.05.
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 abut-
ment types, dynamic loadingand their interaction had
signi cant 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). The-
OPC group presented the lowest torque loss. The highest
torque loss was observed in OPT group. Pair wise com-
parisons of Pertl were then performed with Tukey’s HSD
post-hoc test (Table 2). The mean Pertl in abutments of
OPC group was signi cantly lower than that in TPC and
OPT groups (P<0.05). Also, the difference between OPT
and TPT groups was signi cant (P<0.05), But there was
no signi cant different in Per tl of abutments between
TPC and TPT groups (P>0.05). In our study, all abut-
ments in both control and test groups showed reduc-
tion 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, Ric-
ciardi et al, 2009 Pintinha et al., 2013, Kim et al 2014).
Norton showed that cold welding occurs only at the
highest level of torque, right before the component fail-
ure and when plastic deformation is expected. In addi-
tion, 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 signi cantly
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
Table 1. Mean and standard deviation of per tl for all groups (n=8)
Group N
95% con dence interval for the mean
Lower boundUpper bound P value
OPC 8 8.93±4.76 4.9533 12.9167
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% Con dence 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 signi cant at the 0.05 level.
Farnaz Firouz et al.
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 condi-
tions should be considered. There are signi cant differ-
ences 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 con-
trol (with 22.8% torque loss) and test group (with 23.2%
torque loss) was not statistically signi cant.
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 signi cantly 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 abut-
ment 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 inabut-
ments 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 abut-
ments was signi cantly 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 men-
tioned earlier, the abutment serves as a screw and trans-
fers dynamic forces directly to the threads and decreases
the required removal torque.
In this study, we tried to establish conditions to simu-
late clinical masticatory conditions. Each sample under
went 300,000 cycles of dynamic force, which corre-
sponded to one year of normal chewing function. Inad-
dition, the abutments were lubricated by arti cial 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 manu-
facturer 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 set-
tling effect. Siamos et al. suggested that in order tomini-
mize 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 anex-
periment in which dynamic forces are applied on the
abutment after cementation of crown. Before cemen-
tation, precise and passive  t of caps was assessed by
light-body silicon. Prostheses with active  t or improper
adaptation can exertun desirable forces on the abut-
ment, (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 dif-
ferent situations. The maximum biting force has been-
reported 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 nor-
mal 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.
Under mechanical loading, RTV of both groups (one-
piece and two-piece) decreased and this reduction was
signi cant for one-piece group. However, there was no
signi cant difference in RTV between one-piece and
two-piece abutments under dynamic loading.
I would like to express my sincere gratitude to my sta-
tistical advisor Dr. Saiid Mousavi for the continuous
Farnaz Firouz et al.
support and immense knowledge. This research received
no speci c grant from any funding agency in the public,
commercial, or not-for-pro t sectors.
Con ict of Interest “None declared”
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