Implant stability the password

Journal of Dental Implants | Jul - Dec 2012 | Vol 2 | Issue 2 103
REVIEW ARTICLE
Primary stability: The password of implant integration
Polsani Laxman Rao, Amreena Gill
ABSTRACT
The dental implant therapy has a phenomenal rise and has occupied the summit in today’s dentistry.
The scientific societies and researchers are in constant effort toward improvement, excellence, and
simplification of implant therapy. Today, the practitioners are enthusiastic to imbibe the simplified
implant procedures and techniques and would like to carry out the same in their private practice. Implant
design, bone biology, primary stability, osseointegration, prosthetic options etc., are the several issues
taken into account for successful implant therapy. The factors which influence integration are design of
implant, material composition of implant, variance in the bone quality (soft or hard), and type of surgical
procedure employed (simple or complex). The primary stability is the initial engagement between
the bone and implant and one has to ensure that it should be strong and paramount. Implantologists
completely monitor the primary stability by synchronizing the above factors. The primary stability is
unique and having singular expression is considered as the “password of Implant Integration Account.”
If the primary stability is good, implant can be loaded quickly and to the maximum.
KEY WORDS: Factors, implant, primary stability
INTRODUCTION
Osseointegration is a prerequisite for successful implant
treatment. The term was defined by Brånemark (1985)
as “a direct structural and functional connection
between ordered, living bone and the surface of a
load-carrying implant.” Primary implant stability has
been acknowledged as an essential criterion for later
achievement of such osseointegration. Dental implant
stability is a measure of the anchorage quality of an
implant in the alveolar bone and is considered to be
the consequential parameter in implant dentistry.
Implant stability can occur at two different stages:
primary and secondary.[1]
It has been established to
affect the process of osseointegration, the pattern of
implant loading, and, finally, the success of an implant.[2]
Primary stability of an implant mostly comes from
mechanical engagement with cortical bone. It thus
prevents the formation of a connective tissue layer
between implant and bone, consequently ensuring
bone healing. Therefore, primary stability of an implant
is a prerequisite to undisturbed peri-implant bone
healing. Secondary stability, on the other hand, offers
biological stability through bone regeneration and
remodeling. Secondary stability, which is seen after
the healing period, is primary stability with a further
gain in stability because of bone formation around the
implant[3]
[Figure 1]. 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 stability.[1]
A secure primary stability leads to a
predictable secondary stability.[1]
Secondary stability has
been shown to begin to increase at 4 weeks after implant
placement. At this time point, the lowest implant stability
is expected. Therefore, the original Brånemark protocol
suggested a 3- to 6-month non-loaded healing period to
achieve adequate stability before functional loading.[4]
BACKGROUND
Mihoko Atsumi et al.[1]
proposed the following factors
that affect the primary stability:
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Department of Prosthodontics, Army College of Dental Sciences,
Chennapur, Secunderabad, Andhra Pradesh, India
Address for correspondence: Dr. Amreena Gill,
Senior Lecturer , Army College of Dental Sciences, C/o Maj Gen Amarjit Singh,
Deputy Commandant and CI, MCEME, Trimulgherry, Secunderabad - 500015,
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Rao and Gill: Primary stability and implants
104 Journal of Dental Implants | Jul - Dec 2012 | Vol 2 | Issue 2
BONE QUANTITY AND QUALITY AFFECTING
THE PRIMARY STABILITY
Typically, implant stability is anticipated to decrease
during the early weeks of healing; this is followed by
an increase in stability.[9]
This is related to the biologic
reaction of the bone to surgical trauma. During the initial
bone remodeling phase, bone and necrotic material are
resorbed by osteoclastic activity, which is reflected by a
reduction in the ISQ (implant stability quotient) value.
This process is followed by new bone apposition initiated
by osteoblastic activity, ie, adaptive bone remodeling
around the implant.[10]
An accelerated formation of
bone-to-implant contact contributes to a faster increase in
secondary stability. This biologic process eliminates the
decrease in primary stability and ensures consistency of
stability over time (without the drop during the healing
period).[11]
The long-term success of dental implants in
various clinical situations depends to a large extent on
the quality of the implant and bone bond (Adell et al.
1981; Albrektsson et al. 1981; Zarb and Schmitt 1990).
Because of a higher ratio of compact to trabecular bone
in the mandible, implants inserted into the anterior
mandible have higher survival rates than implants
placed in the posterior maxilla (Adell et al. 1981; Lazzara
et al. 1996; Alsaadi et al. 2007). Initial implant stability
is mainly determined by the bone quantity and quality
(trabecular/cancellous to cortical bone ratio). A positive
correlation was found between primary stability and
cortical thickness of the artificial bone. In case of a poor
supportive capacity of bone (reduced bone density),
compared with the implant diameter, a smaller drill
diameter should be chosen, as from the findings of the
present study it can be assumed that the undersized
drilling technique locally optimizes the bone density
and consequently improves the primary stability.[12]
However, the thicker the cortical layer, the less the
effect of both an undersized surgical approach, as also
the presence of a roughened (etched) implant surface.[12]
Clinical studies have revealed a higher survival rate
for dental implants in the mandible. However, a lower
survival rate of the implants placed in the maxilla,
particularly in the posterior region, has been reported
in the literature, which can be explained by the bone
around the implant having poorer volume and quality
in maxilla.[5]
The results suggested that using thinner
drills for implant placement in the maxillary posterior
region where bone density is relatively low may be a
viable option to increase primary implant stability, which
may result in better implant survival rates.[5]
K. Wang
et al. in their study demonstrated that axial NF (natural
frequency) value which is a parameter to evaluate
implant stability in the vertical direction increased
with the buccal bi-cortical anchorages. In the study,
regardless of bone quality and implant diameter, buccal
1. Bone quantity and quality
2. Surgical technique, including the skill of the surgeon
3. Implant (eg, geometry, length, diameter, surface
characteristics).
Ilser Turkyilmaz et al.[5]
in a study mentioned that the
factors affecting the primary implant stability can be
divided into:
1. patient-related (ie, bone volume and quality)
2. procedure-dependent parameters
i) type of implant (drill size–implant size, pre-
tapped or self-tapped implant)
ii) type of surgical procedure
An insufficient primary stability causes poor healing
related to the early loss of the implant.[6]
The main
two factors affecting implant stability are the location
and the stiffness of the implant in the surrounding
tissue. The stiffness can be considered in three ways:
(1) the stiffness of the implant components themselves
associated with the geometry and material composition;
(2) the stiffness of the implant–bone interface; and
(3) the stiffness of the bone itself associated with
the trabecular/cortical bone ratio and bone density.
Maintenance of low implant micromovement,
especially in early healing periods, presents importance
in promotion of direct bone ingrowth to implant
surface.[7]
Thus, when the implant is stable in the bony
bed during placement and during healing, new bone
will predictably fill the bone-to implant interface and
most of the implant surface will become in direct
contact with living bone.[8]
Primary stability at implant installation is achieved by
the physical congruence between the surgically created
bone bed and the implant, which is dependent from the
macroscopic implant design, the surgical technique and
the bone density.
Figure 1: Comparison of natural socket periphery and implant
periphery

bi-cortical anchorages resulted in higher NF values than
the uni-cortical anchorage. Bi-cortical anchorage was put
forth from a clinical aspect, that is, the buccal (lingual)
modality, in which a wide-diameter implant is suggested
to be placed in a slightly buccal (lingual) position to
engage buccal (lingual) compact bone. It is speculated
that this pattern of anchorage has beneficial effects on
stabilizing the immediately loaded implants against
deteriorative micro-motion at bone-implant interface in
the initial phase of bone adaptation.[13]
In another study
by Nabeel H. M. Alsabeeha et al.[14]
it was observed that
for the midline area of the edentulous mandible, host-site
variables such as age, gender, bone volume, and quality
appear not to influence the primary stability of single
implants placed to support overdentures.[14]
Huang HL
and coworkers in their study concluded that the initial
stability at the time of implant placement is influenced
by both the cortical bone thickness and the elastic
modulus of trabecular bone; however, these parameters
are not totally linearly correlated with ITV (insertion
torque value), PTV (periotest value), and ISQ (implant
stability quotient). The placement of an immediately
loaded implant in cases with thin cortical bone and/
or weak trabecular bone can induce extreme bone
strains and may increase the risk of implant failure.[15]
In a study by Julie Roze et al.[16]
it was observed that the
ISQ values were significantly higher for the implants
placed in mandibular than in maxillary sites. Boronat-
Lopez et al. (2006) observed that ISQ values were higher
in anterior than in posterior regions, but recordings in
the mandible were systematically higher than in the
maxilla. The results showed showed thicker cortical bone
in mandibular than in maxillar sites in accordance with
the literature data (Miyamoto et al. 2005), thus explaining
the greater ISQ values observed in these sites. Cortical
bone thickness is thus of considerable importance for
primary implant stability. Trabecular bone structure
seems to play a minimal role in primary fixation but is
certainly of considerable importance for peri-implant
bone healing.[16]
IMPLANT CHARACTERISTICS INFLUENCING
PRIMARY IMPLANT STABILITY
Joe Merheb et al.[17]
in their study performed resonance
frequency analysis (RFA) test at implant placement,
and RFA and PTV were scored at loading. Bone density
[Hounsfield (HU) scores] and coronal cortical thickness
at osteotomy sites were measured from pre-operative
computerized tomography scans. They concluded
that the implant length or diameter did not seem to
influence primary stability when considered as single
parameters. However, in a stepwise multiple regression
analysis, both parameters became significant probably
due to the elimination of the confounding influence of
the cortical thickness and/or the impact of bi-cortical
anchorage. The implant diameter, however, was shown
to affect the RFA scores significantly at loading and
confirms therefore the tendency to use wider implants
in zones of poor bone quality or poor anchorage to
improve success by increasing the possible bone to
implant contact (Winkler et al. 2000). In the literature,
the correlation between primary stability and implant
design was subject to controversy. Comparisons
have been carried out using implants with different
diameters, lengths or designs. Some authors (Balleri
et al. 2002; Miyamoto et al. 2005; Boronat-Lopez et al.
2006) have determined a negative correlation between
implant length and primary stability measured by RFA
(resonance frequency analysis). However, other authors
have not established any significant correlation between
ISQ value and length, diameter or position of the implant
(Bischof et al. 2004; Akkocaoglu et al. 2005; Akca et al.
2006). In a study by Julie Roze et al. a significant increase
in ISQ values between custom-made plain, and threaded
dental, implants inserted into the same sites was
observed. It is clear that ISQ values are affected by the
primary stability provided by the threads of the dental
implants. [16]
Primary stability can also be improved
by adapting the surgical technique and by implant
selection. For instance, the use of thinner drills and
wider and tapered implant designs will result in a high
primary stability. This improvement is due to lateral
compression of the bone trabeculae and an increase of the
interfacial bone stiffness.[18]
Surface texturing of implants
does directly contribute to initial implant stability. It
may reduce the risk of stability loss and consequently
facilitate wound healing (secondary osseointegration).[18]
Greater implant length and diameter increase the contact
surface area at the bone–implant interface and have
been found to result in better (more negative) PTVs
(periotest values) (Cranin et al. 1998; Engel et al. 2001;
Morris et al. 2003). Francisco Mesa et al.[19]
in a study
concluded that primary DIS (dental implant stability)
failure is more likely in females than males, at sites other
than the anterior mandible because of the differences
in bone masses. The texture of an implant’s surface can
influence the rate and extent of bone-implant fixation,
which is expressed by the amount of bone-to-implant
contact (BIC). For example, in poor bone quality sites,
implants with an acid-etched surface can achieve a
significantly higher BIC as compared with implants
with a machined surface (Weng et al. 2003; Veis et al.
2007). Surface-roughened implants have a failure rate
(3.2%) five times lower than machined surface implants
(15.2%) (Khang et al. 2001). Rough surfaces have to be
considered as “osteophilic,” because the rate and degree
of osseointegeration was found to be superior for the
rough surface as compared with the machined-surface
implants (Abrahamsson et al. 2004). A comparison of
different implant designs has been examined in the
past. O’Sullivan et al. compared the primary stability

characteristics of five different implant designs. The
study demonstrated higher RF (resonance frequency)
analysis and insertion torque values for tapered
implants than for nontapered implants, suggesting
increased stability in tapered implants.[20]
A study by
Linus Chong et al. concluded that fully inserted implants
without self-tapping blades have higher initial stability
than implants with self-tapping blades. The non–self-
tapping implant has a higher number of threads than
the self-tapping implant, thus increasing the surface area
in contact with adjacent block/bone walls. In the self-
tapping implant design, the self-tapping blades present
in the apical third of the implant minimize the contact
surface area of the implant thus leading to decreased
primary stability. Lateral compression of material/
bone associated with self-tapping thread design did
not enhance the primary stability of the self-tapping
design used in the study.[21]
A study by O’Sullivan and
colleagues[22]
was conducted to analyze the mechanical
performance and the primary and secondary stability
characteristics of dental implants with 1° and 2° of taper
when compared with the standard Brånemark design.
Their results showed that an implant designed with 1°
of taper results in a better primary stability compared
with the standard Brånemark design.
SURGICAL TECHNIQUE AFFECTING PRIMARY
IMPLANT STABILITY
To achieve better primary stability and expand the range
of indications with inferior bone quality, a procedure
known as the osteotome technique for bone condensing
was developed by Summer in 1994. The objective of this
procedure is to retain the bone that would otherwise be
removedbycompressingitlaterallyandaxiallytocreatea
precisely formed implant site.[23]
The use of the osteotome
technique for implant placement in normal ridge cases
in a study by Padmanabhan and Gupta[23]
revealed
an average crestal bone loss of 1.19 mm compared to
0.99 mm bone loss with conventional procedure. It was
concluded that the indications for the use of osteotome
technique should be limited to those for which it was
introduced, that is, knife-edge ridges and for bone with
less density. It should not be considered a substitute or
replacement for the conventional procedure of implant
placement.[23]
The drilling procedure used at the implant
site, the implant’s macrogeometry (tapered shape), and
the bone quality at the implant site seem to influence
the achievement of primary stability. Underpreparation
of the implant bone bed makes it possible to increase
the moment of force needed to screw the implant into
position [Figure 2]. This moment of force is referred to
as insertion torque. By increasing the insertion torque it
is possible to improve an implant’s primary stability.[24]
A study by Paolo Trisi et al. showed that implants
from the HT (high torque) group showed significantly
higher bone apposition than implants from the LT (low
torque) group at all examined healing times.[24]
In recent
years, titanium mini-implants have been used as stable
orthodontic anchorages, allowing predictable tooth
movement to be achieved successfully without patient
cooperation. In their study, Mizuki Inaba and coworkers
found that the inclination of the implant in the direction
opposite to the tractional force was thought to facilitate
anchorage. Slanting a mini-implant at an angle of 60°
and 120° other than 90° increases the apparent cortical
bone thickness, and might enhance stability.[25]
A study
by Ramakrishna et al. evaluated the effect of immediate
loading on the primary stability of endosseous implants
placed in the anterior incisor region by mapping the
stability over a period of time, using resonance frequency
analysis. It was concluded that immediate loading of
implants placed in the maxillary and mandibular incisor
region did not seem to affect the osseointegration of the
implants which showed a high primary stability.[18]
DISCUSSION
Primary implant stability is a prerequisite for implant
survival, thereby preventing the formation of a
connective tissue layer between implant and bone, thus
ensuring bone healing (Branemark et al. 1977; Adell et al.
1981; Albrektsson et al. 1981; Meredith 1998; Lioubavina-
Hack et al. 2006). Not only do the quantity and quality
of bone dictate primary stability, but the surgical
technique (relation between drill size and implant size)
and the combination of the microscopic and macroscopic
morphology of the implant itself are also decisive
parameters (Adell et al. 1981; Zarb and Schmitt 1990;
Sennerby and Roos 1998; Buchter et al. 2003; O’Sullivan
et al. 2004; Saadoun et al. 2004; Sevimay et al. 2005).
Optimal implant stability is especially essential in bone
of low density (Listgarten 1997; Martinez et al. 2001).
Several modifications of surgical technique have been
Figure 2: Comparison of natural extraction socket and implant
osteotome

described to increase the primary stability of implant in
bone of low density. Some authors suggest the use of a
final drill diameter that is smaller than the diameter of
the implant (Friberg et al. 2001, 2002), others propose
the technique of bone condensing, where, after using
the pilot drill, the bone is pushed aside with awl-shaped
instruments, called “condensers,” thereby increasing the
density of the surrounding bone (Summers 1994a–1994c).
In case of insufficient bone density, it is advisable to
seek anchorage of an implant in at least two cortices
(Sennerby et al. 1992). Besides the surgical technique,
implant surface parameters have a significant influence
on primary implant stability. Many studies have already
demonstrated that rough surfaces, by enlarging the
implant area in contact with the host bone, favor primary
stability (Vercaigne et al. 1998; Hansson 1999). It was
determined that there were significant correlations
between bone density and insertion torque values, bone
density and ISQ values, and insertion torque and ISQ
values.[6]
The survival rate of self-tapping implants was
found to be higher as compared to that of pre-tapped
standard implants, which indicates that self-tapping
leads to better implant stability.[26]
A precise drilling
technique to avoid overpreparation of implant sites has
been cited as important, especially in soft-quality bone.[27]
Venturelli[28]
described a modified surgical protocol for
soft-quality bone aimed at optimal primary stability
by avoiding overpreparation and striving for bicortical
support. Experimental studies have suggested that
bicortical anchorage may be one way to improve implant
stability.[26]
Cylindric and grooved hydroxyapatite
implants had lower failure rates than hydroxyapatite
screws, titanium alloy screws, commercially pure
titanium screws, and titanium alloy baskets.[26]
From
the literature on machined commercially pure titanium
screws, it is clear that short implants fail more often than
longer implants.[29-32]
It was confirmed by many studies
that the primary stability is influenced mainly by implant
diameter and not by implant length.[33-37]
Placement of
implants in the posterior maxilla can be compromised
because of the proximity of the floor of the maxillary
sinus or the presence of soft bone. SLActive (sandblasted,
large-grit/acid-etched active surface) is a new chemically
modified implant surface that is conditioned in nitrogen
and immediately preserved in an isotonic saline solution.
This process maintains its chemical purity and high
surface activity. Hydrophilic properties increase early
cellular activity and bone apposition, thus promoting
faster osseointegration and increased bone-to-implant
contact and further increase in the stability over time.
It has been reported that implants with a sandblasted
large-grit acid-etched active surface, when placed with
the osteotome sinus floor elevation technique, can be
subjected to an early loading protocol, provided their
stability is confirmed by resonance frequency analysis.[38]
Initial implant stability is mainly determined by the bone
quantity and quality (trabecular/cancellous to cortical
bone ratio). A positive correlation was found between
primary stability and cortical thickness of the artificial
bone. However, the thicker the cortical layer, the less the
effect of both an undersized surgical approach, as also
the presence of a roughened (etched) implant surface.[12]
It was established by Ilser Turkyilmaz[5]
et al. that using
thinner drills for implant placement in the maxillary
posterior region where bone density is relatively low may
be a viable option to increase primary implant stability,
which may result in better implant survival rates. It has
been claimed that using Summer’s technique of bone
expansion and simultaneous implant placement results
in less chance of heat generation and increases initial
stability because of lateral condensation of bone. It is also
claimed that this approach gives better primary stability,
results in less chance of crestal bone loss around the
implant, and so leads to less fear and anxiety related to
implant failure.[39-42]
Although alveolar ridge expansion
can be achieved by the osteotome technique, this kind of
preparation seemed to put pressure on the crestal cortical
bone layer, causing a significant peri-implant marginal
bone loss.[43]
The indications for the use of osteotome
technique should be limited to those for which it was
introduced, that is, knife-edge ridges and for bone with
less density. It should not be considered a substitute or
replacement for the conventional procedure of implant
placement.[23]
Underpreparation of an implant bone bed
makes it possible to increase the moment of force needed
to screw the implant into position. This moment of force is
referred to as insertion torque. By increasing the insertion
torque it is possible to improve an implant’s primary
stability.[24]
High implant insertion torque produces
compression and distortion on the peri-implant bone.
This has been claimed to induce deleterious effects on the
local microcirculation, which may lead to bone necrosis
and possibly to failure of the implant. To achieve good
primary stability without creating excessive compression
in the peri-implant bone, it has been suggested that
implants be inserted with a torque of at least 30 Ncm for
immediately loaded full-arch prostheses in the mandible
or partial prostheses in either arch.[44]
Maintenance of
low implant micromovement, especially in early healing
periods, presents importance in promotion of direct bone
in growth to implant surface, therefore, achievement of
optimum primary implant stability.[6]
Primary stability
can be improved by adapting the surgical technique and
by implant selection. For instance, the use of thinner
drills and wider and tapered implant designs will result
in a high primary stability. This improvement is due
to lateral compression of the bone trabeculae and an
increase of the interfacial bone stiffness.[18]
To enhance
the stability of the non–self-tapping implants at initial
surgery, many surgeons placed these implants without
using a surgical tap to prepare a threaded channel in
the bone. This technique allows the placement of the

implant in slight compression within the bone. In theory
this compression enhances implant primary stability by
developing circumferential or hoop stresses within the
bone at the zone of the bone-implant interface.[3]
CONCLUSION
The following conclusions can be drawn from the above
review [Table 1]:
• Surgical technique and surface roughness have a
huge impact on the primary stability of implants in
low-density bone.
• The outcome of implant stability assessment depends
on environmental factors such as bone quality and
implant geometry.
• Bone density certainly influences implant stability at
placement. A thick cortical bone is associated with
high implant stability quotients and thus high loading
capacity.
• The bone quality and implant stability is lower in the
posterior area; for this reason the posterior implant
success rate is less than the anterior. In the anterior
area, the thick cortical and the dense trabecular bone
will increase primary stability, hence ISQ values
would be higher in this area than the posterior region.
• In case of a poor supportive capacity of bone (reduced
bone density), an undersized drilling technique
locally optimizes the bone density and consequently
improves the primary stability.
• Bi-cortical engagements are more effective than
increasing implant diameter in improving implant
stability.
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Table 1: Primary stability and factors affecting it
Factors Influence on
primary stability
Bone quality and quantity
Cortical bone > trabecular bone Favorable
Increased cortical bone thickness Favorable
Bone in posterior mandible > bone in
posterior Maxilla
Favorable
Cortical bone in males More favorable
Cortical bone in females Less favorable
Implant characteristics
Wide diameter implants Favorable
Threaded implants > custom made
implants
Favorable
Tapered wide implants > non tapered
implants
Favorable
Long implants More favorable
Short implants Less favorable
Acid etched implants > machined
surfaceimplants
Favorable
Non–self-tapping implant > self-
tapping implant
Favorable
Surgical techniques
Osteotome technique Less favorable
Conventional technique More favorable
Bicortical anchorage Favorable

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How to cite this article: Rao PL, Gill A. Primary stability: The password
of implant integration. J Dent Implant 2012;2:103-9.
Source of Support: Nil, Conflict of Interest: None.

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