Methods used to_assess_implant_stability

The International Journal of Oral & Maxillofacial Implants 743
Methods Used to Assess Implant Stability:
Current Status
Mihoko Atsumi, DDS, PhD1/Sang-Hoon Park, DDS, MS2/Hom-Lay Wang, DDS, MSD3
Successful osseointegration is a prerequisite for functional dental implants. Continuous monitoring in
an objective and quantitative manner is important to determine the status of implant stability. Histori-
cally, the gold standard method used to evaluate degree of osseointegration was microscopic or histo-
logic analysis. However, due to the invasiveness of this method and related ethical issues, various
other methods of analysis have been proposed: radiographs, cutting torque resistance, reverse torque,
modal analysis, and resonance frequency analysis. This review focuses on the methods currently avail-
able for the evaluation of implant stability. (More than 50 references.) INT J ORAL MAXILLOFAC IMPLANTS
2007;22:743–754
Key words: cutting resistance analysis, implant stability evaluation, radiographic assessment,
resonance frequency analysis, reverse torque test
Successful osseointegration has been viewed as a
direct structural and functional connection exist-
ing between ordered, living bone and the surface of
a load-carrying implant1,2 under a light microscope.
Histologic appearance resembled a functional anky-
losis with no intervention of fibrous or connective
tissue between bone and implant surface.3–7
Osseointegration is also a measure of implant sta-
bility, which can occur at 2 different stages: primary
and secondary.8 Primary stability of an implant
mostly comes from mechanical engagement with
cortical bone. Secondary stability, on the other hand,
offers biological stability through bone regeneration
and remodeling.5,9,10 The former is a requirement for
successful secondary stability.10 The latter, however,
dictates the time of functional loading.11 Degree of
implant stability may also depend on the condition
of the surrounding tissues. It is, therefore, of an
utmost importance to be able to quantify implant
stability at various time points and to project a long-
term prognosis based upon measured implant stabil-
ity. Presently, various diagnostic analyses have been
suggested to define implant stability: standardized
radiographs, cutting torque resistance analysis,
reverse torque test, modal analysis, and resonance
frequency analysis (RFA). Therefore, the purpose of
this paper was to review methods currently used to
evaluate implant stability.
An online search for studies in English and Japan-
ese was performed using MEDLINE, Pre-MEDLINE,
and the Cochrane Oral Health Group trials register.
Publications from January 1970 to March 2006 were
selected based on the following search terms:
“implant mobility,”“Periotest,”“resonance frequency
test,” “insertion torque,” “reverse torque,” “cutting
resistance,”“implant stability,” and “mobility.” All of
the search terms were combined with the term
“implant.” A hand search of International Journal of
Periodontics and Restorative Dentistry, Journal of Clini-
cal Periodontology, International Journal of Oral &
Maxillofacial Implants, Clinical Oral Implants Research,
Journal of Periodontology, implant-related textbooks,
and implant-related journals was also executed.
Papers were considered relevant if they included the
aforementioned key words and were published in
English or Japanese. Articles published in peer-
reviewed publications and current publications were
1Assistant Professor, Department of Oral and Maxillofacial Reha-
bilitation, Prosthetics and Geriatric Dentistry Division, Kanagawa
Dental College, Yokosuka, Japan.
2Assistant Professor, University of Maryland, Department of Peri-
odontology, Baltimore, Maryland.
3Professor and Director of Graduate Periodontics, Department of
Periodontics and Oral Medicine, School of Dentistry, University
of Michigan, Ann Arbor, Michigan.
Correspondence to: Dr Hom-Lay Wang, University of Michigan,
School of Dentistry, 1011 N. University Avenue, Ann Arbor, MI
48109-1078. Fax: +734 936 0374. E-mail: homlay@umich.edu
Atsumi.qxd 9/17/07 3:19 PM Page 743
COPYRIGHT © 2007 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART OF THIS
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744 Volume 22, Number 5, 2007
Atsumi et al
preferred to non-peer-reviewed and early publica-
tions. More than 200 papers matched the inclusion
criteria; however, only 114 most relevant
articles/chapters were selected and reviewed.
IMPLANT STABILITY
Implant stability, an indirect indication of osseointe-
gration, is a measure of the clinical immobility of an
implant.5,9 It is achieved at 2 levels: cortical bone (pri-
mary stability) and cancellous bone (secondary sta-
bility). Secure primary stability leads to predictable
secondary stability.12 Secondary stability has been
shown to begin to increase at 4 weeks after implant
placement.13 At this time point, the lowest implant
stability is expected. Therefore, the original Bråne-
mark protocol2 suggested a 3- to 6-month non-
loaded healing period to achieve adequate stability
before functional loading.
Osseointegration is, however, a patient-depen-
dent wound healing process affected by various fac-
tors (Table 1). Quantification of implant stability at
various time points may provide significant informa-
tion as to the individualized “optimal healing” time.
Raghavendra et al13 proposed that measurement of
osseointegration be approached in a quantitative
manner, as primary and secondary stability are in an
inverse relationship. However, in clinical practice,
experience-driven decision still dominates, as objec-
tive guidelines have not been established.
Table 2 summarizes currently available methods
for the objective assessment of implant stability at
pre-, intra-, and postsurgical time points. Histologic
or histomorphometric analysis, however, is not feasi-
ble for daily practice, as this may require unnecessary
biopsy.
RADIOGRAPHIC ANALYSIS
Radiographic evaluation is a noninvasive method that
can be performed at any stage of healing. Bitewing
view is used to measure crestal bone level, which has
been suggested as an important radiographic indica-
tor for implant success.14–16 It has been reported that
1.5 mm of radiographic crestal bone loss can be
expected in the first year of loading in a stable
implant, with 0.1 mm of subsequent annual bone
loss.17–20 However, several problems must be
addressed. First, 1.5 mm is a mean value. Second, due
to a low incidence of implant failure,changes in radio-
graphic bone level alone cannot precisely predict
implant stability. Third, it is impractical for a clinician
to detect changes in radiographic bone loss at 0.1
mm resolution. Fourth, crestal bone changes can only
be reliably measured without distortion when the
central ray of the x-ray source is perfectly parallel with
the structures of interest. This would necessitate a
series of standardized radiographs with a customized
template for reliable and repeatable measurements,
which is impractical. Lastly, conventional periapical or
panoramic views do not provide information on a
facial bone level, and bone loss at this level precedes
mesiodistal bone loss.21 Neither bone quality nor
density can be quantified with this method. Even
changes in bone mineral cannot be radiographically
detected until 40% of demineralization had
occurred.22 Numerous limitations exist with the use of
a conventional radiograph alone in making an accu-
rate, independent assessment of implant stability.
Computer-assisted measurements of crestal bone
level change may prove to be the most accurate way
to use radiographic information, as a standard devia-
tion within 0.1 mm (0.01 to 0.51 mm) has been
reported.23 However, this method is not convenient
for use in clinical practice.24
CUTTING TORQUE RESISTANCE ANALYSIS
In cutting resistance analysis (CRA), originally devel-
oped by Johansson and Strid25 and later improved
by Friberg et al26–29 in in vitro and in vivo human
models, the energy (J/mm3) required for a current-
fed electric motor in cutting off a unit volume of
bone during implant surgery is measured. This
energy was shown to be significantly correlated with
bone density, which has been suggested as one of
factors that significantly influences implant stabil-
ity.26,29 To minimize the interoperator variation, hand
pressure during drilling was controlled.27 CRA can be
used to identify any area of low-density bone (or
poor-quality bone) and to quantify bone hardness
Table 1 Factors that Influence Implant Stability
Factors Affecting Primary Stability
Bone quantity and quality
Surgical technique, including the skill of the surgeon
Implant (eg, geometry, length, diameter, surface characteristics)
Factors Affecting Secondary Stability
Primary stability
Bone modeling and remodeling
Implant surface conditions

Atsumi et al
during the low-speed threading of implant
osteotomy sites. A torque gauge incorporated within
the drilling unit (eg, Osseocare; Nobel Biocare, Göte-
borg, Sweden) can be used to measure implant inser-
tion torque in Ncm to indirectly represent J/mm3.
Insertion torque values have been used to measure
bone quality in various parts of the jaw during
implant placement.30
CRA gives a far more objective assessment of bone
density than clinician-dependent evaluation of bone
quality based on Lekholm and Zarb classification.31
Clinical relevance was demonstrated by studies that
showed the highest frequency of implant failures in
jaws with advanced resorption and poor bone qual-
ity, often seen in maxilla.17,18,32–34 Therefore, cutting
resistance value may provide useful information in
determining an optimal healing period in a given
arch location with a certain bone quality.26
The major limitation of CRA is that it does not give
any information on bone quality until the osteotomy
site is prepared. CRA also cannot identify the lower
“critical” limit of cutting torque value (ie, the value at
which an implant would be at risk).29 Furthermore,
longitudinal data cannot be collected to assess bone
quality changes after implant placement. Its primary
use, therefore, lies in estimating the primary stability
of an implant. For instance, in Misch’s 6 time-depen-
dent stages of implant failures—(1) surgical, (2)
osseous healing,(3) early loading, (4) intermediate, (5)
late, and (6) long-term35—CRA can only provide
information on the first 2 stages. Estimation of
implant primary stability alone from CRA is still of
value, as high implant failure rates are observed in
the first 3 phases.36,37 Nonetheless, long-term evalua-
tion of implant stability after implant placement,
phases 3 to 7, is desired and should not be over-
looked. This limitation has led to development of
other diagnostic tests.Table 3 summarizes CRA.
REVERSE TORQUE TEST
Unlike CRA, which measures the bone density and
the resistance to cutting torque, the reverse torque
test (RTT), proposed by Roberts et al38 and devel-
oped by Johansson and Albrektsson,39–41 measures
the “critical” torque threshold where bone-implant
contact (BIC) was destroyed. This indirectly provides
information on the degree of BIC in a given implant.
In the study conducted by Johansson and Albrekts-
son,a reverse torque was applied to remove implants
placed in the tibiae of rabbits 1, 3, 6, and 12 months
postsurgery. Reverse torque value and histologic
evaluation showed that greater BIC could be
achieved with a longer healing time. Similar observa-
tions at the histologic level have been made in other
animal studies.42–44 Removal torque value (RTV) as
an indirect measurement of BIC or clinical osseointe-
gration was later reported to range from 45 to 48
Ncm in 404 clinically osseointegrated implants in
humans.45 Sullivan et al further speculated that any
RTV greater than 20 Ncm may be acceptable as a cri-
terion for a successful osseointegration, since none
of the implants in their study45 could be removed
during abutment connection at 20 Ncm. It was fur-
ther suggested that RTT is, therefore, a reliable diag-
nostic method for verification of osseointegration.
Table 2 Currently Available Methods to Evaluate Implant Stability
and the Time of Use for Each Method
Pre Intra Post Noninvasiveness Objectivity
Histologic analysis + + + – +++
Percussion test – ++ ++ + +
Radiographs ++ ++ ++ ++ –
Reverse torque – – ++ – ++
Cutting resistance – +++ – + ++
Vibration analysis
Periotest – ++ ++ ++ ++?
RFA – +++ +++ +++ ++?
+++ = method with highest reliability; ++ = method with certain reliability; + = method
with doubtful reliability; – = application is impossible; ? = More information is needed.
Table 3 Advantages and Disadvantages of CRA
Advantages
1. Detect bone density
2. High correlation between cutting resistance and bone quality
3. Reliable method to assess bone quality
4. Identify bone density during surgery
5. Can be used in daily practice
Disadvantages
1. Can only be used during surgery

Atsumi et al
However, this method has been criticized as being
destructive.8 Brånemark et al2 cautioned about the
risk of irreversible plastic deformation within peri-
implant bone and of implant failure if unnecessary
load was applied to an implant that was still under-
going osseointegration. Furthermore, a 20-Ncm
threshold RTV for successful osseointegration has
not yet been supported by scientific data. The
threshold limit varies among patients depending on
the implant material and the bone quality and quan-
tity. A threshold RTV may be lower in type 4 bone
than in denser bone, for instance. Hence, subjecting
implants placed in this bone type to RTV may result
in a shearing of BIC interface and cause implant fail-
ure. Furthermore, RTV can only provide information
as to “all or none” outcome (osseointegrated or
failed); it cannot quantify degree of osseointegration.
Hence,RTT is mainly used in experiments.
MODAL ANALYSIS
Modal analysis measures the natural frequency or
displacement signal of a system in resonance, which
is initiated by external steady-state waves or a tran-
sient impulse force (Table 4). Modal analysis, in other
words, is a vibration analysis. It is widely used as an
effective test method for structural analysis in engi-
neering and the health-care field.46,47 Dental applica-
tions include the quantification of osseointegra-
tion.48–51 Modal analysis can be performed in 2
models:theoretical and experimental.52
Two or 3-dimensional finite element modeling
(FEM) is an example of computer-simulated theoreti-
cal modal analysis, which is mathematically con-
structed using known biomechanical properties (eg,
Young’s modulus [Pa], Poisson ratio, and density in
g/cm3) of structures of interest. Theoretical modal
analysis such as FEM may be useful in investigation
of the vibrational characteristics of objects that may
be difficult to excite because of a damping effect
from boundary conditions such as the periodontal
ligament (PDL) in an in vivo model.49 By altering
boundary conditions such as the bone level, FEM can
theoretically be used to calculate the anticipated
stress and strain in various simulated peri-implant
bone levels.50,51
Experimental or dynamic modal analysis, on the
other hand, measures structural changes and
dynamic characteristics (eg, natural characteristic fre-
quency, characteristic mode, and attenuation) of a
system that is excited in an in vitro model via vibra-
tion testing (eg, impactor or hammer). This in vitro
approach provides a more reliable assessment of an
object than a theoretical model. This analysis has
been applied in dentistry to quantify the degree of
osseointegration and implant stability.49 Frequency
analysis and mechanical impedance analysis can be
used for detecting response waves in modal
analysis.52 By combining the vibration and response
detecting methods, various kinds of vibration analy-
ses can be performed.53 Some techniques derived
from these theoretical concepts are being tested for
use in evaluating implant mobility.
Percussion Test
A percussion test is one of the simplest methods that
can be used to estimate the level of osseointe-
gration.8,54–56 This test is based upon vibrational-
acoustic science and impact-response theory. A clini-
cal judgment on osseointegration is made based on
the sound heard upon percussion with a metallic
instrument. A clearly ringing “crystal” sound indicates
successful osseointegration, whereas a “dull” sound
may indicate no osseointegration. However, this
method heavily relies on the clinician’s experience
level and subjective belief.Therefore,it cannot be used
experimentally as a standardized testing method.
Impact Hammer Method
Impact hammer method is another example of tran-
sient impact as a source of excitement force during
experimental modal analysis.53,57 It is an improved
version of the percussion test except that sound gen-
erated from a contact between a hammer and an
object is processed through fast Fourier transform
(FFT) for analysis of transfer characteristics. By
enhancing the response detection using various
devices, such as a microphone, an accelerometer, or a
strain gauge, and by processing the detected
response with FFT, it becomes possible to quantify
and qualify the response wave in the form of disloca-
tion, speed, acceleration, stress, distortion, sound, and
other physical properties. Periotest (Siemens, Ben-
sheim, Germany) and Dental Mobility Checker (DMC;
Table 4 Implant Stability Measurement Based on
Modal or Vibration Analysis
Theoretical Modal Analysis
1. Finite element method
Experimental Modal Analysis
1. Percussion test
2. Impact hammer method (Periotest, Siemens, Bensheim,
Germany; Dental Mobility Checker, J. Morita, Suita, Japan)
3. RFA (Osstell, Integration Diagnostics, Göteborg, Sweden;
Implomate, Bio Tech One, Taipei, Taiwan)
4. Others (pulsed oscillation waveform by Kaneko)

Atsumi et al
J. Morita, Suita, Japan) are currently available mobility
testers designed according to the impact hammer
method. The former has an electromagnetically dri-
ven and electronically controlled tapping head that
hammers an object at a rate of 4 times per second.
Contact time between the tapping head and the
object is also measured. DMC utilizes the same prin-
ciple of tapping a tooth or implant with a dental
hammer. A frequency response function is built-in to
detect bone-quality–dependent sound.
Pulsed Oscillation Waveform
Kaneko et al58,59 described the use of a pulsed oscil-
lation waveform (POWF) to analyze the mechanical
vibrational characteristics of the implant-bone inter-
face using forced excitation of a steady-state wave.
POWF is based on estimation of frequency and
amplitude of the vibration of the implant induced by
a small pulsed force. This system consists of acousto-
electric driver (AED), acoustoelectric receiver (AER),
pulse generator, and oscilloscope. Both the AED and
AER consist of a piezoelectric element and a punc-
ture needle. A multifrequency pulsed force of about
1 kHz is applied to an implant by lightly touching it
with 2 fine needles connected with piezoelectric ele-
ments. Resonance and vibration generated from
bone-implant interface of an excited implant are
picked up and displayed on an oscilloscope
screen.58,59 An in vitro study showed that the sensi-
tivity of the POWF test depended on load directions
and positions.58 Sensitivity was rather low for the
assessment of implant rigidity.
IMPLANT STABILITY
EVALUATION METHODS
DMC and Periotest are based on the impact hammer
method, in which impact force is used as the excita-
tion force. In this theory, “the width of the first peak
on the time axis of the spectrum generated by tran-
sient impulse is inversely proportional to the time
axis of the impulse.”57,60,61 Therefore, in the presence
of impact force, lower rigidity of the tested substance
results in a longer time axis.
Dental Mobility Checker
The DMC, which was originally developed by Aoki60
and Hirakawa,61 measures tooth mobility with an
impact hammer method using transient impact
force. Aoki and Hirakawa successfully detected the
level of tooth mobility by converting the integration
(ie, rigidity) of tooth and alveolar bone into acoustic
signals. A microphone was used as a receiver. The
response signal transferred from the microphone is
processed by FFT for conversion for analysis in the
time axis. Hence, the duration of the first wave gener-
ated by the impact was detected.62 DMC uses a small
impact hammer as an excitation device. It is easily
used even in molar regions. DMC may provide quite
stable measurement for osseointegrated implants.63
There are some problems, however, such as the diffi-
culties of double-tapping and difficulty in attaining
constant excitation. Furthermore, the application of a
small force to an implant immediately after placement
may jeopardize the process of osseointegration.2
Periotest
Periotest has been thoroughly studied and advo-
cated as a reliable method to determine implant sta-
bility.8,64–71 Unlike DMC, which applies impact force
with a hammer, Periotest uses an electromagnetically
driven and electronically controlled tapping metallic
rod in a handpiece. Response to a striking or “bark-
ing” is measured by a small accelerometer incorpo-
rated into the head. Like DMC, contact time between
the test object and tapping rod is measured on the
time axis as a signal for analysis. The signals are then
converted to a unique value called the Periotest
value (PTV), which depends on the damping charac-
teristics of tissues surrounding teeth or implants.72
Although they use different types of receivers for
impulse responses, DMC and Periotest are similar in
terms of their theoretical background.They both use
a transient impulse as an excitation force, and in both
cases analysis is conducted on the time axis. In addi-
tion, both were originally developed to measure the
mobility of a natural tooth.64,65
In the case of a natural tooth, the buffering capac-
ity of the PDL poses a problem in analyzing the dis-
tribution of impact force exerted on a tooth. When
dynamic characteristics are analyzed based upon an
assumption that the whole periodontal structure
functions as a mechanical unit, it is difficult to model
the attenuation from the PDL. The soft tissue, includ-
ing the periosteum, is considered a viscoelastic
medium; thus, Hooke’s law does not apply to the
behavior of the PDL under an applied load. Thus, vis-
coelasticity of the PDL has always posed a difficulty
in analysis of the physical characteristics of peri-
odontal tissue. By contrast, bone-implant interface
with no PDL is believed to be similar to the serial
spring model which follows Hooke’s law, and mobil-
ity measurement is considered easier.
Most reports of the use of a natural tooth mobility
detector such as Periotest to measure implant mobil-
ity have pointed out a lack of sensitivity in these
devices.55,68 Such devices permit a very wide
dynamic range (in case of Periotest, PTV is –8 to +50)
to permit the measurement of a wide variety of nat-

ural tooth mobility.68 However, the dynamic range
used for measuring implant mobility is very limited.
Thus, the sensitivity of these devices is insufficient to
measure implant mobility.
Although many similarities do exist between the
tissue structures around an implant and a natural
tooth, conclusions from periodontal studies may not
be directly applicable to implants.73 In the use of
mobility measurement to assess implant stability, the
presence or absence of a PDL makes a crucial differ-
ence. Similar to impact/vibration testing, values mea-
sured with Periotest are significantly influenced by
excitation conditions, such as position and direction.
The Periotest user’s manual contains clear instruc-
tions about striking point position and angle: “The
Periotest measurement must be made in a midbuc-
cal direction”and“During measurement the Periotest
handpiece must always be held perpendicular to the
tooth axes.”72 Considering the intraoral environment,
and the pen-grip–shaped handpiece of the Periotest,
it is clear that it can be used quite easily for the ante-
rior region. However, its use for the molar region is
extremely difficult because of the presence of buccal
mucosa.74 Derhami et al75 used a fixing device to
hold a handpiece at the correct angle. This fixing
device was used for an in vitro measurement using a
cranial bone model, and its clinical application seems
difficult. However, Periotest is believed to be an effec-
tive evaluation method once the difficulty of control-
ling impact force is solved.
Long-term data on Periotest have shown that it
can be an objective clinical measurement of the sta-
bility of bone-implant anchorage.70,71 Aparicio used
Periotest to measure implant stability and found a
direct correlation between PTV and the degree of ini-
tial osseointegration.69 It was further suggested that
PTV should be included in the current success crite-
ria. Another study with sample size of more than
2,900 implants showed a similar finding.70,71 How-
ever, differences with respect to implant design,
diameter, length, and bone quality and quantity were
not accounted for in that study; analysis in a pattern
of changes over time may be more reasonable. A
measured bone value only represents its condition at
the moment of measurement. Bone is subject to life-
long metabolism, which will in turn affect PTV over
time. Thus, average value is not a proper way to
determine a critical value for implant stability.
Even if it could be assumed that PTV precisely
reflects the condition of BIC as reported by previous
studies,76,77 an average PTV has no importance.
Johansson and Albrektsson observed that “implants
inserted in different people do not necessarily attain
the same degree of integration.”39 Despite a wide
variation in host factors such as bone density, normal
PTV of an osseointegrated implant falls in a relatively
narrow zone (–5 to +5) within a wide scale (–8 to
+50).64 Other studies have indicated that the PTVs of
clinically osseointegrated implants fall within an
even narrower zone (–4 to –2 or –4 to +2).76,78 There-
fore, the measured PTV may falsely be interpreted as
having a small standard deviation and therefore
viewed as having a good accuracy. PTV cannot be
used to identify a “borderline implant” or “implant in
the process of osseointegration” which may or may
not continue to a successful osseointegration.77 No
conclusion has been made with regard to this issue.
It has been suggested that these limitations of
Periotest measurement have been suggested to be
strongly related to the orientation of excitation source
or striking point. In vitro and in vivo experiments
demonstrated that the influence of striking point on
PTV is much greater than the effects from increased
implant length due to marginal bone resorption or
other excitation conditions such as the angle of the
handpiece or repercussion of a rod.55,75 Unfortunately,
controlling these influential factors is extremely diffi-
cult. Despite some positive claims for Periotest,68,69
the prognostic accuracy of PTV for implant stability
has been criticized for a lack of resolution, poor sensi-
tivity,and susceptibility to operator variables.8,79
RFA
RFA has recently gained popularity. It is a noninvasive
diagnostic method that measures implant stability and
bone density at various time points using vibration and
a principle of structural analysis.57 RFA utilizes a small L-
shaped transducer that is tightened to the implant or
abutment by a screw. The transducer comprises 2
piezoceramic elements, one of which is vibrated by a
sinusoidal signal (5 to 15 kHz). The other serves as a
receptor for the signal. Resonance peaks from the
received signal indicate the first flexural (bending) reso-
nance frequency of the measured object.In vitro and in
vivo studies have suggested that this resonance peak
may be used to assess implant stability in a quantitative
manner.
Currently, 2 RFA machines are in clinical use: Osstell
(Integration Diagnostics) and Implomates (Bio Tech
One). Osstell has combined the transducer, computer-
ized analysis and the excitation source into one
machine closely resembling the model used by
Meredith. In the early studies, the hertz was used as
the measurement unit.28,54,56,80–89 Later, Osstell cre-
ated the implant stability quotient (ISQ) as a measure-
ment unit in place of hertz.90–103 Resonance fre-
quency values ranging from 3,500 to 8,500 Hz are
translated into an ISQ of 0 to 100. A high value indi-
cates greater stability, whereas a low value implies
instability.The manufacturer’s guidelines suggest that
Atsumi et al

a successful implant typically has an ISQ greater than
65. An ISQ < 50 may indicate potential failure or
increased risk of failure.104
It is assumed that an implant and the surrounding
bone function as as a single unit; thus, a change in
stiffness is considered to represent the change of
osseointegration of an implant. A steady-state sinu-
soidal force in a form of sine wave is applied to the
implant-bone unit to measure the implant stability via
resonance. Frequency and amplitude are then picked
up as a response.90,91 An in vitro model showed that
resonance frequency of an implant placed in an alu-
minum block ranged from 8 to 9 kHz.8,54 An in vivo
human study also showed that, although amplitude
of the resonance peak was smaller than in vitro data,
the peak resonance frequency of clinically osseointe-
grated implants was also about 8 to 9 kHz.8 Moreover,
resonance frequency increased as polymerization of
the resin progressed.54
Effective implant length (EIL) was a value calcu-
lated by adding the amount of exposed implant
threads and the length of each abutment. EIL has
been shown to be inversely proportional to the level
of resonance frequency, with a correlation coefficient
of r = –0.94 in vitro and r = –0.78 in vivo.8,54 Several in
vivo animal and human clinical studies have con-
curred with this finding.56,80,102 No resonance peak
was observed in failed implants with clinical mobility.8
Longitudinal changes in resonance frequency have
also been evaluated. Implants placed in the rabbit
tibia were measured over 168 days from the time of
implant placement81; resonance frequency increased
over time. Other studies have evaluated longitudinal
changes in ISQ more in detail.90–92,94,95,98,105 ISQ was
found to decrease significantly after implant place-
ment for several weeks.However,a recovery to the ini-
tial ISQ level was found at the time of implant loading.
Furthermore, a greater increase of resonance fre-
quency over time was observed with implants placed
in softer bone.28,91,98 In the case of an implant placed
in grafted bone in an in vivo human study,106 very low
resonance frequency (4 to 5 kHz) was observed.
Based upon these findings, the following 3 con-
clusions have been suggested.106 First, “stiffness” of
an implant is a function of its geometry and material
composition (length, diameter, overall shape). Sec-
ond, the stiffness of the implant-tissue interface
depends on the bond between the surface of the
implant and the surrounding bone. Third, the stiff-
ness of the surrounding tissue is determined by the
ratio of cancellous to cortical bone and the density of
the bone with which an implant engages.8 Stiffness
found at the bone-implant interface (second point)
changes over time. The factors affecting stiffness
remain relatively stable, as the mechanical properties
of implant and bone are constant. The only factor
that could significantly influence the stiffness and
resonance frequency of the implant would be the
exposed implant length, as shown in several stud-
ies.8,56,80,102,107–109 Therefore, measurement of the
stiffness at the interface provides reliable informa-
tion as to the implant stability.
Stiffness of supporting structure may, however,
influence the stiffness of the interface of an area of
interest.80,81,109–111 In most in vitro studies,107,109,110
such as that of Meredith et al,54 an aluminum block
material with uniformity and linearity has been used
as a supporting structure. Therefore, in this model, an
implant behaves in a mathematically predictable
manner in which resonance frequency is inversely
proportional to the length of the cantilever beam.
Bone, on the other hand, is composed of calcium
phosphate (85%), calcium carbonate (10%), and fluo-
ride ions (~ 5%), the amounts of which continuously
change to maintain a dynamic equilibrium.112 There is
great interindividual variation. Furthermore, bone
does not behave like a uniform material under func-
tional loading.Hence,in modal analysis,the sharpness
and amplitude of the resonance peak of an implant
embedded in bone tend to be lower than those of an
implant in an aluminum block. In a nonlinear object
with a large attenuation (eg, PDL), a theoretical modal
analysis is a more feasible analysis than an experi-
mental modal analysis, as stress and strain do not
behave proportionally to one another. Many influenc-
ing factors render interpretation of implant stability
difficult from a single resonance value.
Like Osstell, Implomates, which was developed by
Huang et al,52,107–110 uses RFA. However, it utilizes an
impact force to excite the resonance of implant
instead of a sinusoidal wave. Impact force is provided
by a small electrically driven rod inside the trans-
ducer. The received response signal is then trans-
ferred to a computer for frequency spectrum analysis
(range, 2 to 20 kHz). The first biggest amplitude indi-
cates the resonance frequency of interest. Higher fre-
quency and sharp peak indicate a more stable
implant, whereas a wider and lower peak and lower
frequency indicate implant failure. Currently, few
studies have been reported regarding the efficacy of
this machine.
CLINICAL APPLICATION OF RFA
Presently, clinical application of RFA includes estab-
lishing (1) a relationship between exposed implant
length and resonance frequency or ISQ values; (2)
differential interarch and intra-arch ISQ values for
implants in various locations; 83,90–92,98,103,105 (3) prog-
Atsumi et al

nostic criteria for long-term implant success; and (4)
diagnostic criteria for implant stability.94,95,105
EIL has been shown to significantly influence ISQ
value.8,54,56,80,102,107–109 Although the stiffness of the
implant is generally constant, it can sometimes vary
in the presence of other contributing stiffnesses
(Table 5). Classification of ISQs based upon various
conditions may be a grand task. However, if these
variables are ignored, the reliability of the measure-
ment will be low.101,102 Therefore, only series of intra-
patient RFA values over various time points may pro-
vide useful information as to the stability of an
implant under investigation. Furthermore, these
series of values may not indicate the success or fail-
ure of the implants.105
This concurs the research of Friberg et al with
respect to cutting torque resistance measure-
ment.28,29,83 Insertion torque was also highly associ-
ated with resonance frequency of implants.30 Lower
resistance and lower resonance frequency values
were associated with poor bone quality. This may be
related to the finding that implant success and sur-
vival rates are greater in the mandible than in the
maxilla.101,113,114 Prolonged healing time is required
in cases with poor bone quality. Therefore, even
though an implant placement in softer bone shows
low stability, it seems to “catch up” to dense bone
sites over time.28
The prognostic value of RFA machines such as
Osstell and Implomates has, therefore, been investi-
gated. The most challenging factors to overcome are
the dynamic characteristics (eg, damping effect, total
mass, and stiffness) of various factors surrounding
the object of interest,111 bone-implant interface.
Without controlling these factors, information
gained from RFA is no better than guessing value. To
improve its prognostic value of RFA, longitudinal
studies and comparison of RFA values with histologic
studies are essential. Development of simulation
models on various EILs associated with various
defect types may further assist in the assessment of
implant stability.
The shape of the transducer (an L shape) restricts
its orientation, which adds a significant length to the
exposed implant length, potentially masking a small
amount of bone resorption.54 Osstell Mentor (Inte-
gration Diagnostics) eliminates the use of an
attached L-shape transducer by generating “pulse
trains” from a contact-free probe. Impact signals are
then picked up by a receptor called a “smart-peg.”
Hence, the measurement is believed to be more
accurate than the original Osstell machine. Moreover,
in cases of Kennedy III partial edentulism, this con-
tact-free smart-peg allows assessment of implant
stability from any direction. However, due to the dif-
ference in EIL and various bending forces from the
different design of the transducer, data collected
with the original Ostell machine and that collected
with the new contact-free Osstell should be com-
pared with caution.
The establishment of diagnostic criteria for suc-
cess, survival, and/or failure is another clinical appli-
cation with RFA. However, RFA can only give informa-
tion regarding success; it cannot provide information
with respect to survival or failure. ISQ can be fairly
reliable when an implant has achieved osseointegra-
tion and the bone-implant interface is rigid.98 In
cases where rigid integration is doubtful, however,
the ISQ tends to fluctuate. Some doubtful implants
result in failure, whereas some implants showing low
ISQ later stabilize and achieve a satisfactory out-
come.83 Hence, clinicians will continue to test the
implant stability until they get a reasonable value.
When unacceptable values are displayed, however,
these values are often rejected. If the repeated mea-
surements still indicate an unfavorable result, these
values are unwillingly accepted. Hence, small stan-
dard deviation is often reported with high ISQ.
The evaluation of implant stability using RFA
machines such as Osstell and Implomates still has
some uncertain issues. It is clinically being used with-
out much conclusive data on the bone-implant inter-
face and resonance frequency values.79,91 Further
research is needed to establish higher reliability of
these diagnostic devices.
Atsumi et al
Table 5 Factors that Influence RFA
Constants
Implant length
Implant diameter
Implant geometry (implant system)
Implant surface characteristics
Placement position
Abutment length
Variables
Bone quality
Bone quantity
Damping effect of marginal mucosa
BIC (3-dimensional)
EIL
Connection of transducer
Primary stability
Secondary stability

CONCLUSION
To date, no definite method to evaluate implant sta-
bility has been established. Although the theory
behind RFA is sound, the technology cannot provide
a critical value that can determine the success, fail-
ure, or long-term prognosis of an implant. Hence,
present position from this review is that information
should be assembled from many diagnostic aids to
assure long-term implant stability. More research in
this field is certainly needed.
ACKNOWLEDGMENT
This work was partially supported by the University of Michigan
Periodontal Graduate Student Research Fund.
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