Official Journal of the Society of the
Nippon Dental University
Correlation between resonance frequency,
insertion torque and bone-implant contact
in self-cutting threaded implants
Yahya Açil, Jan Sievers, Aydin Gülses,
Mustafa Ayna, Jörg Wiltfang & Hendrik
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Correlation between resonance frequency, insertion torque
and bone-implant contact in self-cutting threaded implants
Received: 7 April 2016 / Accepted: 19 July 2016
ÓThe Society of The Nippon Dental University 2016
Abstract The aim of this study was to evaluate the cor-
relation between resonance frequency analysis (RFA) val-
ues and the histomorphometric bone-implant contact (BIC)
immediately after insertion of the implant. Additionally, it
was examined to deﬁne which extent peak insertion torque
(PIT) was correlated with the latter measurements. 15
S plus root from dental implants were inserted in
fresh porcine frontal bones. The insertion torque was
measured with the Kavo Intrasurg 300 surgical unit. RFA
connector was coupled to the implants and the mean value
of 20 consecutive RFA measurements was calculated via
ISQ device. The implants were removed with the
adjacent bone with a band saw. The blocks were processed
for undecalciﬁed histology. Two perpendicular longitudi-
nal middle sections of the implant were made and stained
with toluidine blue and the BIC was assessed by histo-
morphometry. The correlation coefﬁcient (Spearman)
between RFA and BIC was R=0.579 (p=0.026, Ftest).
The correlation between PIT and BIC (0.33, p[0.05) and
PIT and RFA (0.153, p[0.05) was not statistically sig-
niﬁcant. The present data conﬁrmed a moderate and
statistically positive correlation between RFA and BIC. No
correlation between BIC and PIT and PIT and RFA was
observed. Further studies considering different bone qual-
ities would be beneﬁcial in understanding the relation
between RFA and BIC.
Keywords Bone-implant contact Resonance frequency
analysis Insertion torque Primary stability Correlation
Immediate loading Dental implants Histomorphometric
Resonance frequency analysis (RFA) basically measures
the acoustic behavior of the inserted implant body which in
turn is inﬂuenced by the bone contacts of the implant.
Despite the increasing use of RFA in the daily dental
practice, the reliability and validity of the method are still
controversial and, thus, it is an indirect measurement of the
implant stability which underlies a number of confounding
factors in the clinical situation. Because of the indirect
nature of the measurements, the clinical accuracy has to be
proven by correlations with either mechanical [peak
insertion torque (PIT)] or structural [bone-implant contact
(BIC)] variables. Similarly, Manresa et al.  have pro-
claimed that ISQ as determined by RFA is not able to
identify the relationship between RFA and histomorpho-
metrical data. It was also suggested that studies on the
accuracy and reliability of RFA are based mainly on
mechanical in vitro studies and are poorly supported by
histological measurements .
In the literature, several studies have demonstrated the
relation between PIT and the RFA [3–5]. PIT was con-
sidered to be clinically representative for primary implant
Department of Oral and Maxillofacial Surgery, Christian-
¨t zu Kiel, Kiel, Germany
¨rztliche Gemeinschaftspraxis Dres., Isabelle und Jan
Sievers, Waiblingen, Germany
¨lhane Military Medical Academy, Centre of Dental
Sciences, Etlik Kec¸io
¨ren, 06010 Ankara, Turkey
Center for Implantology, Private Practice, Duisburg,
Clinic for Oral and Maxillofacial Surgery, Rotes Kreuz
Krankenhaus Kassel, Kassel, Germany
Author's personal copy
stability. Although there is a strong correlation between
RFA and PIT, studies failed to ﬁnd a correlation between
RFA and BIC or PIT and BIC (Table 1).
In the literature, there are controversies regarding the
interdependence of RFA and BIC [6,7]. Nkenke et al. 
have found a signiﬁcant positive correlation of the existing
primary BIC with RFA in human fresh cadaver bone and
correlated RFA with PIT and BIC in conventional stepped
The threads of dental implants are representative of
macroscopic surface features that allow mechanical inter-
locking of implant within bone. The shape and depth of the
implant threads are determining factors in the aggressive-
ness of dental implants. The term ‘‘self-tapping’’ describes
an implant that cuts its own path into bone. Similarly, self-
cutting threaded implants are inserted in bone by a self-
cutting procedure by preparation of the implant bed
through cutting threads.
It is obvious that self-cutting threaded implants acquire
a higher primary stability in lower bone qualities through
the shape design of the relatively deep ﬂanks of the
threads. However, the relation between RFA, PIT and BIC
in self-cutting threaded implants has not been evaluated
yet. The aim of the present study was to evaluate the
possible presence of a positive correlation between RFA,
BIC and PIT immediately after installation of self-cutting
(Dentsply/Friadent Mannheim, Germany)
Materials and methods
The frontal bone of freshly slaughtered domestic pigs was
used as experimental subjects. The implant socket prepa-
rations were done by a single experienced implantologist.
15 sockets were created for 3.0 95.0 mm Xive
implants (Friadent GmbH Mannheim, Germany). The
quality of the bone was assessed by three different clini-
cians after performing osteotomies, close to the experi-
mental areas in a blinded mode.
The implants were placed according to the manufac-
turer’s guidelines. An independent observer, blinded to the
study, assessed the accuracy of placement. According to
the standard protocol of the manufacturer, in the ﬁrst 3 mm
from the entrance to the implant socket, the threads were
pre-tapped. Before the insertion of the implants, drill holes
were thoroughly rinsed with saline to remove all bone
chips to prevent false-positive measurements of BIC.
The PIT was recorded with the surgery unit Intrasurg 300
(KaVo Dental GmbH, Biberach/Riß, Germany). The value
was read from the display.
Osstell ISQ device (Integration Diagnostics AB, Go
Sweden) was used to perform the RFA. Smart pegs were
coupled to the implants. The insertion torque for the smart
pegs where screwed is limited through a plastic wrench. 20
consecutive RFA measurements were made per implant
immediately after seating of the implant. The data were
stored in the device and the mean value was used for fur-
The implants with at least 5 mm surrounding bone were
retrieved from the skull using the band saw. Then, the
blocks were dehydrated, embedded and processed accord-
ing to the sawing and grinding technology according to
Donath and Breuner .
The BIC was determined over the total length of the
implant. For this purpose, the implants were cut in half
longitudinally. Two longitudinal sections from the middle
of the implants were then made and examined from top to
tip. The remaining two halves of the implant were then
glued together again. Then, the implant was turned over
90°and the procedure was repeated so that another 2
longitudinal slices from the whole length of the implant
could be examined. With this method, we obtained 4 dif-
ferent longitudinal sections from different points of the
Table 1 Previous studies focusing on the comparison of RFA/BIC, IT/BIC and RFA/IT
Author Correlation RFA/BIC Value (p) Correlation IT/BIC Value (p) Correlation RFA/IT Value (p)
Nkenke et al.  Yes 0.024 – – No 0.193
Scarano et al.  Yes 0.016 – – – –
Degidi et al.  – – No 0.892 – –
Ito et al.  No 0.299 – – – –
Abrahamsson et al. No – – – – –
Kunnekel et al. Yes –– –– –
Author's personal copy
BIC was measured histomorphometrically as follows:
by diaphanoscopy using the cold light lamp Schott KL
1500 (Schott AG, Mainz, Germany) the middle axis of the
implant within the tissue blocks was identiﬁed and marked.
A cut with the diamond band saw followed through the
middle of the implant in the longitudinal direction. Both
sides of the middles section were ground and polished to
high gloss with a sandpaper of grain 4000. For this pro-
cedure, *200 lm section through the middle diameter of
the implant was required to gain a 30–40 lm section for
the histology. After that, the two remaining halves of the
implants were glued back together with Loctite
glued block was rotated 90°and cut again lengthwise
according to the above-mentioned protocol to gain another
30–40 lm section for the histology. In this way, 2 verti-
cally oriented and perpendicular longitudinal sections of
30–40 lm thickness through the middle of the implant
were gained and a more appropriate ﬁeld of observation for
BIC and the relative amounts of medullary vs. cortical
bone in hard tissue were achieved (Fig 1). The histological
slides were stained with toluidine blue.
The preparations were digitally photographed with the
Nikon photomicrography. With a magniﬁcation of 509,
the implant tissue interface and the bone interface of BIC
were assessed manually with the Leica
The correlation between two variables (BIC–RFA, BIC–
PIT and RFA–PIT) was analyzed using Pearson correlation
coefﬁcient, which is used to measure the strength of a
linear association between two variables, where the value
r=1 means a perfect positive correlation and the value
r=-1 means a perfect negative correlation. The coefﬁ-
was calculated to measure the validity of the cor-
relation estimates (0 Br
B1). An Ftest was calculated to
prove the signiﬁcance of r
. A signiﬁcance level of 0.05
The histological overview over the 15 implants showed
that a bone-implant contact in the cortical area mainly
occurred at the edges of the threads, while in the cancellous
bone BIC was partly located in the valleys of the threads
The correlation between RFA and BIC was statistically
signiﬁcant (p=0.26). Figure 4shows the values as a
diagram. A positive correlation with less reliability towards
higher BIC values was observed (Table 2).
There was no statistically signiﬁcant correlation between
PIT and BIC (p=0.398). The correlation was less than
that between the RFA and BIC (Fig. 5; Table 2).
There is a very weak correlation coefﬁcient of 0.1529
between PIT and RFA. The Ftest failed due to lack of
power (Fig. 6; Table 2).
RFA is applied clinically for the assessment of implant
stability, and the relevance of this application is widely
accepted. However, the relationship between RFA and
other parameters of implant stability, such as the histo-
morphometrical BIC parameter, has become controversial
in the last decade.
Kim and Lim  have indicated that the implant body
design without self-tapping blades has a good primary
stability compared with that with self-tapping blades in
Fig. 1 Arrangement of the 2 longitudinal sections perpendicular to
Author's personal copy
medium-density bone. Considering the RFA, a distinct
layer of cortical bone on marginal bone will yield implant
stability quotient values similar to those in medium-bone
density when implants have the same diameter. Similarly,
Kim et al.  have proclaimed that implants with an
aggressively deeper thread pattern without self-cutting
blades create a lateral compression with increased contact
surface area and, consequently, improve the primary sta-
bility in a simulated low-density bone model.
From a clinical point of view, Chowdary  have
stated that, implants without self-cutting thread design
showed more failures and bone loss than implants with
Fig. 2 Longitudinal section of the implant H section I (xive 3,
8913) after insertion in the OS frontale insertion torque: 45 NCM
(scale 12:1). The bone contact occurred mainly above the thread tips.
Measurement of BIC handed an enlargement of 950 out. The surface
of the implants could be assessed on the basis of four pictures per
Fig. 3 Implant L cut I—inserted in the OS frontale with mostly
cancellous bone, insertion torque: 36 Ncm. (scale: 12:1)-bone contact
over long distances especially in the cancellous bone area. Bone chips
by abrasion could be well distinguished from intact tissues (left side).
The Harversian systems and the osteocytes were easy to ﬁnd
Author's personal copy
self-cutting thread design and indicated that self-cutting
thread tapered implant can be used safely and effectively
under demanding conditions as an immediate post-extrac-
tion tooth replacement. In addition, histological analysis of
probes indicates that self-cutting implants are associated
with a generally higher bone-to-implant contact pro-
nounced at the crestal part, when compared to preparation
of the bony implantation bed [12,13].
Although an aggressive implant with increased thread
depth provides a close contact between the implant and the
bone and, in general, provides high PIT, there are also
various studies enouncing that an excessive strain or
compression on the press-ﬁt regions could exceed the
physiological limit and trigger bone resorption .
Bashutski et al.  have suggested that unusual implant
failures that likely occurred as a result of over compression
of the bone during placement and areas involving dense
bone seem to be at increased risk for compression necrosis.
In a canine study by Nevins et al. , several implant
designs and surgical technique modiﬁcations were inves-
tigated with the hypothesis of the design and site prepa-
ration changes would induce different compression states
on the native bone, by affecting the primary stability and
the rate and extent of osseointegration. They have tested
three compression scenarios, and have stated that the sce-
nario intended to induce a moderate degree of compression
provided the best overall results.
The controversies between these studies could be
attributed to the fact that during the use of a self-cutting
threaded implant, PIT undergoes several changes during
implant insertion . The cutting feature in the apical
region increases the torque as it cuts a thread in the cortical
region and the insertion torque increases further during the
gradual transition in implant diameter, which is generated
by friction and static strain. Finally, when the cut thread in
the bone matches the implant thread, and no more expan-
sion is required, the insertion torque values even out .
Dagher et al.  have compared RFA, PIT, and BIC of
four different implant surfaces and evaluate the correlation
between them. According to their results, irrespective of
the implant surface, there is no correlation between PIT
and BIC and between RFA and BIC. There are also several
histomorphometric studies with controversial results
focusing on the evaluation of the correlation between BIC
and the RFA values [6,19–21]. Similar to the results
obtained in the current study, Sacarano et al.  have
found a statistically signiﬁcant correlation between RFA
and BIC values. Correlation of RFA and BIC immediately
after insertion was also examined by Nkenke et al. .
Although mathematically the correlation was positive and
signiﬁcant in their study, looking upon data a great vari-
ability was obvious. They have stated that, clinically, the
RFA values still have to be used with caution.
Fig. 4 Relationship of resonance frequency values (ISQ) and bone-
implant contact (BIC): R-squared is 0.3349
Table 2 Correlation between resonance frequency analysis (RFA),
bone-implant contact (BIC) and peak insertion torque (PIT), the Ftest
to assess the signiﬁcance could not be calculated for the PIT–RFA
because of the lack of power
RFA–BIC 0.5787 0.3349 0.0263
PIT–BIC 0.3349 0.1122 0.3978
PIT–RFA 0.1529 0.0222 –
Fig. 5 Relationship between peak insertion torque (PIT) and bone-
implant contact (BIC), the R-squared is 0.1122
Fig. 6 Relationship between peak insertion torque (PIT) and reso-
nance frequency analysis (RFA): very weak correlation
Author's personal copy
Clinically, PIT value of an implant is still an easily
accessible parameter to estimate implant stability for the
immediate loading protocols [22–24]. In a review by
Esposito et al. , the importance of primary stability for
success of an immediately loaded loading of implants was
highlighted. In addition, for a reference period of up to
1 year, the initial PIT was considered to be representative
for primary stability [5,22,26,27]. The present experiment
showed only a weak and non-signiﬁcant correlation
between BIC and PIT. This suggests that PIT should be
considered with caution to assess the primary stability. The
same conclusion can be found also in the current literature
in a study of Degidi comparing BIC and torque values .
The correlation between PIT and RFA is still not clear in
There are a number of studies which found no statistically
signiﬁcant correlation between the RFA and PIT. Akca et al.
 could not found any relationship between RFA and torque
in an in vitro study of 16 implants. Nkenke et al.  examined
the relationship between PIT and RFA at 48 implants in vitro
and found no statistically signiﬁcant correlation. Also Degidi
et al. andCehrelietal. found no correlation between
the two values. Degidi et al.  suspected a larger inﬂuence
of bone structure to the torque as the RFA values. Also in the
current study, no statistically signiﬁcant correlation could be
found between PIT and RFA values.
Although there are differences in bone density and
quality between humans and the various animal models,
pigs represent a comparable osseous macro- and
microstructure behavior in comparison to humans [29–31].
As shown in Figs. 2and 3, the selected recipient site for
implant insertions could also present differences in bone
quality. Nevertheless, considering the limited depth and
thick compacta within the forehead of the domestic pig,
current model could be representative for the implant
recipient sites at the posterior mandible in humans. This
means that, immediately after insertion of the implant,
already relaxation will take place. This can affect the ISQ
measurements as well as bone contact measurements.
Besides that, it is well known that, ISQ measurements as
well as bone contact measurements could also be affected
by the visco-elastic behavior of the bone and possible
concomitant relaxation, which takes place immediately
after insertion of the implant. Additional studies with dif-
ferent bone types could also be beneﬁcial in understanding
the correlation between RFA, BIC and PIT.
The present data conﬁrmed a moderate and statistically
positive correlation between RFA and BIC. No correlation
between BIC and PIT and PIT and RFA was observed.
Further studies within the varying times of healing periods
representing commonly used protocols for early and delayed
implant loading protocols would be beneﬁcial for gaining
understanding of the bone remodeling characteristics.
Acknowledgments We gratefully acknowledge Grabriele Neßenius
for her assistance in the plastic embedding procedure and the pro-
duction of tissue slides. We also acknowledge Eylem Ugur Gu
conducting the statistical analysis of the manuscript. The manuscript
has been language edited by professional interpreter Ms. Serpil
Compliance with ethical standards
Conﬂict of interest The authors declare that they have no conﬂict of
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