https://doi.org/10.53453/ms.2026.5.5
Mechanical stability thresholds and their influence on
osseointegration in posterior maxillary implants: a systematic
review
Ralfas Stanevicius
1
, Inesa Stonkute
1
, Zygimantas Petronis
1
1
Faculty of Odontology, Medical Academy, Lithuanian University of Health Sciences, Kaunas, Lithuania
Abstract
Background. Successful implant therapy depends on osseointegration, which is strongly influenced by primary
implant stability. Stability is commonly assessed using insertion torque and resonance frequency analysis.
However, implant placement in the posterior maxilla remains challenging due to low bone density and anatomical
limitations, and no clear consensus exists regarding optimal mechanical stability thresholds in this region.
Aim. To evaluate the relationship between primary implant stability measured by insertion torque and implant
stability quotient and osseointegration outcomes in implants placed in the posterior maxilla.
Methods. Searches were conducted in PubMed/MEDLINE, ScienceDirect, and Springer Nature Link for studies
published between 2016 and 2026. Eligible studies included randomized controlled trials and cohort studies
reporting quantitative stability measures and osseointegration-related outcomes. Study selection, data extraction,
and risk-of-bias assessment were performed independently by two reviewers using RoB 2 and ROBINS-I.
Results. Of 782 identified records, five studies met the inclusion criteria. These included two randomized
controlled trials, two prospective clinical studies, and one retrospective cohort study with sample sizes ranging
from 46 to 122 implants and follow-up periods from 5 months to 10 years. Moderate stability levels, typically
insertion torque values of 20-35 N·cm and ISQ values of 55–65, were consistently associated with favorable
outcomes, including high implant survival and minimal marginal bone loss. Surgical technique influenced primary
stability but differences diminished during healing.
Conclusions. Moderate primary mechanical stability appears sufficient for predictable osseointegration in
posterior maxillary implants, while long-term outcomes are also influenced by biological and anatomical factors.
Keywords: dental implants, posterior maxilla, primary implant stability, osseointegration, insertion torque.
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Medical Sciences 2026 Vol. 14 (3), p. 34-48, https://doi.org/10.53453/ms.2026.5.5
34
1. Introduction
Dental implants represent a predictable
treatment modality for the rehabilitation of
partially and completely edentulous patients,
with long-term survival rates frequently
exceeding 90% in clinical studies [1]. The
success of implant therapy relies on
osseointegration, defined as a direct structural
and functional connection between living bone
and the implant surface. This process depends on
both mechanical and biological factors during
early healing, particularly the achievement of
sufficient primary implant stability at the time of
placement [1,2].
Primary stability reflects the mechanical
engagement between the implant and
surrounding bone immediately after placement
and is influenced by bone density, implant
macrodesign, surgical technique, and osteotomy
preparation [1]. Insufficient stability may allow
excessive micromotion at the bone–implant
interface, which can disrupt the formation of
new bone and lead to fibrous encapsulation
rather than osseointegration. Experimental and
clinical evidence suggests that micromotion
exceeding approximately 50–150 µm may
compromise implant integration [3].
Clinically, primary implant stability is most
commonly assessed using insertion torque (IT)
and resonance frequency analysis (RFA),
expressed as the implant stability quotient (ISQ).
Insertion torque reflects rotational resistance
encountered during implant placement, whereas
RFA evaluates the stiffness of the implant-bone
interface and allows longitudinal monitoring of
stability changes over time [4]. Several studies
report that torque values around 30-45 N·cm are
frequently used as thresholds for immediate or
early loading, while ISQ values above
approximately 60-65 are often considered
indicative of adequate stability [4,5].
The posterior maxilla presents particular
challenges for implant therapy due to lower bone
density, reduced cortical thickness, and frequent
sinus pneumatization. These anatomical
characteristics may compromise primary
stability and increase the risk of early implant
failure. Although quantitative measures such as
insertion torque and ISQ are widely used to
guide clinical decision-making, there is no
universal consensus on the stability thresholds
required to ensure predictable osseointegration,
especially in low-density maxillary bone [6].
Variability in surgical protocols, implant
designs, stability measurement techniques, and
outcome reporting has limited the ability to
derive consistent clinical recommendations.
Consequently, a focused synthesis of available
clinical evidence is needed to clarify the
relationship between mechanical stability
thresholds and osseointegration outcomes in this
anatomically challenging region.
Therefore, the aim of this systematic review was
to evaluate the current evidence regarding
mechanical stability thresholds and their
influence on osseointegration outcomes in
posterior maxillary implants, with particular
emphasis on quantitative stability measures and
their association with implant survival, marginal
bone changes, and stability progression during
healing.
2. Methods
2.1 Protocol and Registration
This systematic review was conducted in
accordance with the Preferred Reporting Items
for Systematic Reviews and Meta-Analyses
(PRISMA) 2020 guidelines [7]. The protocol
was prospectively registered with the
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35
International Prospective Register of Systematic
Reviews (PROSPERO; registration ID:
CRD420261333955). The research question was
formulated using the PICO framework presented
in Table 1.
Table 1. PICO Framework
Component
Description
P (population)
Adult patients receiving dental implants in the posterior maxilla (native bone or
grafted bone).
I (intervention)
Implants placed under defined mechanical stability thresholds (e.g., insertion
torque, RFA/ISQ, micromotion limits).
C (comparison)
Implants placed with lower or higher stability values, or without reported
quantitative thresholds.
O (outcome)
Primary: Osseointegration success (clinical/radiographic stability, survival).
Secondary: MBL, implant survival/failure, ISQ progression, peri-implant
complications.
RFA – Marginal bone loss; ISQ – Implant Stability Quotient; MBL – Marginal bone loss
2.2 Eligibility Criteria
2.2.1 Inclusion Criteria
• Human studies evaluating implants placed
in the posterior maxilla
• Randomized controlled trials, controlled
clinical trials, and prospective or retrospective
cohort studies
• Quantitative mechanical stability
measurement (insertion torque [N·cm], ISQ
values, or micromotion thresholds)
• Reporting at least one osseointegration-
related outcome
• Minimum follow-up of 3 months
• Articles published in English
2.2.2 Exclusion Criteria
• Case reports or case series (<10 implants)
• In vitro or animal studies
• Studies lacking quantitative stability
measurements
• Studies evaluating implants exclusively in
the anterior maxilla or mandible
• Reviews, editorials, or letters
2.3 Information Sources and Search Strategy
A comprehensive electronic search was
conducted from January 1, 2025 to February 1,
2026. The following databases were searched:
PubMed/MEDLINE, ScienceDirect and
Springer Nature Link. The search was limited to
human studies published in English between
January 2016 and January 2026. The Boolean
search strategy combined the following key
terms: (Posterior maxilla) AND (dental implant)
AND (osseointegration OR implant survival)
AND (primary stability OR mechanical
stability). All records were imported into
EndNote reference management software, and
duplicates were removed prior to screening.
Reference lists of included studies were
manually screened for additional eligible
articles. Grey literature and trial registries were
not systematically searched.
2.4 Study Selection
The study selection process was conducted
independently by two reviewers (R.S. and I.S.)
under the supervision of a senior investigator
(Ž.P.). The selection proceeded in three phases:
initial title screening to assess topic relevance,
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abstract screening to evaluate compliance with
the predefined eligibility criteria, and full-text
review to determine final inclusion. Any
disagreements between reviewers were resolved
through discussion and consensus, with a third
reviewer consulted when necessary. The entire
selection process will be summarized and
illustrated using a PRISMA flow diagram,
detailing the number of records identified,
screened, excluded, and included in the final
analysis.
2.5 Data Extraction
Two reviewers (R.S. and I.S.) independently
extracted data using a standardized data
extraction form developed specifically for this
review. The form was piloted on three eligible
studies to ensure clarity, consistency, and
completeness prior to full data extraction.
Extracted variables included study design and
characteristics, patient demographics, implant-
related parameters, quantitative mechanical
stability measures, osseointegration-related
outcomes, follow-up duration, and reported
study limitations.
Discrepancies between reviewers were resolved
through discussion and consensus. If agreement
could not be reached, a senior reviewer (Ž.P.)
was consulted to adjudicate the final decision.
2.6 Data Items
Data extracted from the included studies were
organized into predefined categories as follows:
• Study identification: Author and year of
publication
• Study design: Randomized controlled trial,
controlled clinical trial, prospective or
retrospective cohort study
• Sample size: Number of patients and/or
implants evaluated
• Patient characteristics: Age, sex
distribution, bone quality (if reported), and sinus
augmentation status (native or grafted posterior
maxilla)
• Implant characteristics: Implant system,
surface characteristics, macrodesign, diameter,
length, and loading protocol (immediate, early,
or delayed)
• Mechanical stability parameters: Insertion
torque (N·cm), resonance frequency analysis
(RFA), implant stability quotient (ISQ) at
placement and follow-up, and reported
micromotion thresholds (µm)
• Comparator or threshold definition:
Predefined stability cut-offs or comparison
groups (e.g., high vs. low torque, ISQ ≥70 vs.
<70)
• Osseointegration-related outcomes:
Implant survival/failure rate, marginal bone
level changes (mm), secondary stability
progression (ISQ evolution), and biological or
mechanical complications
• Key findings: Principal results concerning
the association between mechanical stability
thresholds and osseointegration outcomes
2.7 Risk of Bias Assessment
The methodological quality of included studies
was assessed using design-specific validated
instruments. Randomized controlled trials were
evaluated using the Revised Cochrane Risk of
Bias tool (RoB 2) [8], which assesses internal
validity across five domains:
(1) bias arising from the randomization process;
(2) bias due to deviations from intended
interventions;
(3) bias due to missing outcome data;
(4) bias in measurement of the outcome; and
(5) bias in selection of the reported result.
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Each domain was evaluated using the signaling
questions and decision algorithms provided in
the RoB 2 framework and rated as “low risk of
bias,” “some concerns,” or “high risk of bias.”
An overall judgment was assigned according to
Cochrane guidance, based on the highest level of
risk identified across domains.
Non-randomized studies were assessed using the
Risk Of Bias In Non-randomized Studies – of
Interventions (ROBINS-I) tool [9], which
evaluates seven domains: confounding; selection
of participants; classification of interventions;
deviations from intended interventions; missing
data; measurement of outcomes; and selection of
the reported result.
Each domain was rated as “low,” “moderate,”
“serious,” or “critical” risk of bias, and an
overall study-level judgment was derived
according to ROBINS-I guidance.
Risk-of-bias assessments were conducted
independently by two reviewers (R.S. and I.S.).
Disagreements were resolved through discussion
and consensus, with arbitration by a third
reviewer (Ž.P.) when necessary.
2.8 Synthesis Methods
Due to substantial clinical, methodological, and
outcome heterogeneity among the included
studies, a quantitative meta-analysis was not
performed. Heterogeneity was observed in study
design (randomized controlled trials,
prospective clinical studies, retrospective
cohorts), surgical protocols (immediate
placement, lateral sinus floor elevation, short
implants), implant macrodesign, bone quality,
and loading strategies.
Mechanical stability was assessed using
heterogeneous metrics, including insertion
torque (reported as means ± standard deviations,
ranges, or minimum threshold criteria) and
resonance frequency analysis (ISQ values
reported as means or medians with interquartile
ranges). Follow-up duration varied widely (5
months to 10 years), and osseointegration-
related outcomes were inconsistently defined
and reported (implant survival, marginal bone
level changes, secondary stability progression,
late implant failure).
Because comparable effect estimates with
associated measures of variance at standardized
time points were not consistently available,
statistical pooling and formal heterogeneity
assessment (e.g., Q statistic, I²) were not
considered methodologically appropriate.
Accordingly, a structured narrative synthesis
was conducted. Studies were organized into
thematic categories based on stability-related
variables:
• Surgical technique and primary stability
• Implant macrodesign and stability–bone
relationships
• Low mechanical stability thresholds in
atrophic posterior maxilla
• Stability progression during healing
The synthesis summarizes study characteristics,
quantitative stability measures (torque and ISQ
ranges), osseointegration outcomes, statistical
associations, and sources of variability.
Numerical findings are presented descriptively
in structured summary tables to facilitate
transparent comparison while avoiding
inappropriate quantitative aggregation.
2.9 Reporting Bias Assessment
Because no meta-analysis was conducted and the
number of included studies was limited, formal
statistical assessment of publication bias (e.g.,
funnel plot asymmetry or Egger’s regression
test) was not considered appropriate.
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Potential reporting bias was therefore evaluated
qualitatively. This involved:
• Comparing outcomes specified in the
methods sections with those reported in the
results
• Assessing completeness of reporting of
mechanical stability parameters (insertion torque
and ISQ values)
• Evaluating whether predefined
osseointegration outcomes were selectively
omitted
• Reviewing transparency in reporting
follow-up duration and attrition
Selective outcome reporting was additionally
assessed within the “selection of the reported
result” domain of the RoB 2 tool for randomized
studies and the corresponding domain of the
ROBINS-I framework for non-randomized
studies.
Any suspected reporting bias was documented
narratively in the Results section.
3. Results
3.1 Study Selection
The electronic search identified 782 records.
After removal of 98 duplicate citations, 684
studies underwent title and abstract screening.
Following this process, 43 full-text articles were
assessed for eligibility. Based on the predefined
inclusion and exclusion criteria, five studies
fulfilled all eligibility requirements and were
included in the qualitative synthesis [10–14].
These studies investigated quantitative
mechanical stability parameters – specifically
insertion torque and/or ISQ – in implants placed
in the posterior maxilla, and reported at least one
osseointegration-related outcome, including
marginal bone loss, implant survival, late
implant failure, or secondary stability
progression.
The included studies comprised randomized
controlled trials, prospective case–control
studies, and retrospective cohort analyses
evaluating the influence of mechanical stability
on osseointegration outcomes in posterior
maxillary implants.
The PRISMA flow diagram illustrating the study
selection process is presented in Figure 1.
3.2 Study Characteristics
Key characteristics of the included studies are
summarised in Table 2. The five included
investigations were published between 2021 and
2025 and consisted of two randomized
controlled trials, two prospective clinical studies,
and one retrospective cohort study. All studies
evaluated implants placed in the posterior
maxilla and reported quantitative primary
mechanical stability measures in relation to
osseointegration-related outcomes.
Sample sizes ranged from 46 implants [10] to
122 implants [12]. Follow-up duration varied
considerably, from 5 months [13] to a mean of
10 years [12].
Clinical targets across studies included:
• Primary and secondary implant stability
assessed by ISQ [10,13];
• Peri-implant marginal bone remodeling
(PBR/MBL) [11,12,14];
• Implant survival and late implant failure
(LIF) [12,14];
• Influence of surgical technique on
mechanical stability [13];
• Immediate implant placement accuracy and
its clinical impact [14].
Primary mechanical stability was assessed using:
• Insertion torque (N·cm) [12,14];
• Resonance frequency analysis (ISQ)
[10,11,13];
• Or a combination of both [11].
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Comparators included different surgical
techniques (osteotome vs conventional vs
guided) [13], implant macrodesigns (tissue-level
vs bone-level) [11], navigation-assisted vs
freehand placement [14], and evaluation of long-
term risk factors affecting marginal bone loss
and implant failure [12].
Figure 1. PRISMA flow diagram
Table 2. Key Characteristics of Included Studies
Author
(Year)
Study
Design
Number
of
Patients
/
Implants
Surgical
Procedure
Interventio
n
Comparator
Mechanical
Stability
Measure(s)
Osseointegra
tion
Outcome(s)
Timing of
Assessmen
t
Al-Hity
et al.,
2025 [10]
Prospecti
ve clinical
study
14 / 46
Posterior
maxillary
implant
placement
Standard
implant
placement
protocol
Maxilla vs
mandible
subgroup
comparison
ISQ
(AnyCheck
RFA)
Secondary
stability
progression
Baseline; 3
months; 6
months
Lombardi
et al.,
2025 [11]
Controlle
d clinical
study
58 / 71
Short
implants in
posterior
maxilla
Tissue-
level
implants
Bone-level
implants
Insertion
torque; ISQ
Marginal
bone
remodeling;
survival
Implant
placement;
12 months
Dung et
al., 2025
[12]
Retrospec
tive
cohort
study
60 / 122
One-step
lateral sinus
floor
elevation
(LSFESI)
Implants
placed with
torque >15
N·cm
No direct
comparator
(risk-factor
analysis)
Insertion
torque
(minimum
threshold)
Marginal
bone loss; late
implant
failure;
survival
Placement;
long-term
follow-up
(mean 10
years)
Planinić
et al.,
2021 [13]
Randomi
zed
clinical
trial
150 / 150
(posterio
r maxilla
subgroup
)
Posterior
maxillary
implant
placement
Osteotome
technique
Conventional
drilling;
flapless
guided
placement
ISQ (Ostell)
Secondary
stability
Baseline; 5
months
Yang &
Geng,
2024 [14]
Randomi
zed
controlled
trial
60 / 96
Immediate
implant
placement in
posterior
maxilla
Dynamic
navigation-
assisted
placement
Freehand
placement
Insertion
torque
Marginal
bone
resorption;
implant
survival
Placement;
mean 27.8
months
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3.3 Risk of Bias in Included Studies
The risk of bias of the included studies was
assessed using the Revised Cochrane Risk of
Bias tool for randomized trials (RoB 2) [8] and
the Risk Of Bias In Non-randomized Studies –
of Interventions (ROBINS-I) tool [9], according
to study design. The results are summarized in
Tables 3 and 4 and visually presented in Figures
2 and 3.
Overall, the two randomized controlled trials
were judged to have low to some concerns of risk
of bias across assessed domains. One trial was
rated as low risk of bias, while the second was
assessed as having some concerns, primarily
related to outcome measurement procedures and
limited reporting of blinding. No substantial
issues were identified regarding randomization,
missing outcome data, or selective reporting.
Among the three non-randomized studies,
overall risk of bias ranged from moderate to
serious. Two studies were judged to have
moderate risk of bias, mainly due to potential
confounding factors such as bone quality,
implant design, and surgical technique, as well
as participant selection in single-centre settings.
One long-term retrospective cohort study was
assessed as having serious risk of bias due to
unadjusted confounding and variability in
baseline clinical characteristics that may have
influenced both mechanical stability and
osseointegration outcomes.
Across all studies, selective reporting bias was
considered low. However, methodological
limitations inherent to observational designs
should be considered when interpreting the
reported associations between mechanical
stability measures and osseointegration in
posterior maxillary implants.
Table 3. Risk of Bias Assessment of Non-Randomized Studies (ROBINS-I)
Study
Confounding
Selection of
Participants
Classification
of
Interventions
Deviations
from
Intended
Interventions
Missing
Data
Measurement
of Outcomes
Selection
of
Reported
Results
Overall
Risk
Al-Hity et
al., 2025
[10]
Moderate
Moderate
Low
Low
Moderate
Low
Low
Moderate
Lombardi
et al., 2025
[11]
Moderate
Moderate
Moderate
Low
Low
Low
Low
Moderate
Dung et
al., 2025
[12]
Serious
Moderate
Low
Low
Moderate
Moderate
Low
Serious
Figure 2. ROBINS-I risk-of-bias traffic-light plot
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Table 4. Risk of Bias Assessment of Randomized Controlled Trials (RoB 2)
Study
Randomization
Process
Deviations from
Intended
Interventions
Missing
Outcome
Data
Measurement of
Outcome
Selection of
Reported
Result
Overall
Risk
Planinić et
al., 2021
[13]
Low
Low
Low
Some concerns
Low
Some
concerns
Yang &
Geng, 2024
[14]
Low
Low
Low
Low
Low
Low
Figure 3. ROB-2 risk-of-bias traffic-light plot
3.4 Results of Individual Studies
3.4.1 Surgical Technique and Primary
Stability
Two randomized clinical trials [13,14] evaluated
the influence of surgical technique on primary
mechanical stability in posterior maxillary
implants.
In the randomized study by Planinic et al. [13],
three preparation techniques (osteotome,
conventional drilling, and flapless guided
placement) were compared. The osteotome
technique demonstrated significantly higher
primary ISQ values compared with the other
techniques (p < 0.01). However, at 5 months,
secondary ISQ values converged across groups,
and no statistically significant intergroup
differences were observed (p = 0.660). These
findings suggest that although surgical under-
preparation may enhance early mechanical
engagement, biological osseointegration reduces
these differences during healing.
Similarly, Yang and Geng [14] compared
navigation-assisted and freehand immediate
implant placement in the posterior maxilla. The
mean insertion torque was 24.38 ± 1.84 N·cm,
with no statistically significant difference
between groups. At a mean follow-up of 27.8 ±
8.4 months, implant survival was 100% in both
groups. However, marginal bone resorption was
significantly lower in the navigation group.
Mesial bone loss was 0.19 ± 0.88 mm
(navigation) versus 0.25 ± 1.46 mm (freehand),
and distal bone loss was 0.31 ± 0.66 mm versus
0.54 ± 1.08 mm, respectively (p < 0.05). These
findings indicate that moderate torque values
around 24 N·cm were sufficient for predictable
osseointegration, while surgical accuracy
influenced crestal bone preservation.
3.4.2 Implant Macrodesign and Stability–
Bone Relationship
One controlled clinical study [11] evaluated the
interaction between implant macrodesign,
insertion torque, ISQ, and marginal bone
remodeling in posterior maxillary implants.
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In Lombardi et al. [11], insertion torque values
ranged between approximately 25 and 40 N·cm,
and a statistically significant positive correlation
was identified between insertion torque and ISQ
(p < 0.05). At 12 months, implant survival was
100%.
Radiographic marginal bone level changes,
reflecting peri-implant bone remodeling,
differed significantly between implant designs.
Tissue-level implants demonstrated mean bone
remodeling of 0.11 ± 0.27 mm at T1 and 0.30 ±
0.23 mm at T2, whereas bone-level implants
showed 0.34 ± 0.35 mm at T1 and 0.55 ± 0.42
mm at T2. The difference between designs was
statistically significant (p = 0.003).
These findings suggest that primary stability
within a moderate torque range (25–40 N·cm)
was compatible with successful
osseointegration, but implant macrodesign
influenced peri-implant crestal bone remodeling.
3.4.3 Stability in Severely Atrophic Posterior
Maxilla
The long-term retrospective cohort study by
Dung et al. [12] evaluated implants placed in
severely atrophic posterior maxillae using a one-
step lateral sinus floor elevation technique.
Implant placement required a minimum insertion
torque threshold of >15 N·cm. Over a mean
follow-up period of approximately 10 years,
overall implant survival was 87.7%, with a late
implant failure rate of 12.3%. Marginal bone loss
greater than 1 mm was observed in 16.8% of
implants.
Importantly, multivariate analysis identified
reduced keratinized tissue width (<2 mm), sinus
membrane perforation, and poor oral hygiene as
significant predictors of late implant failure,
whereas the initial mechanical stability threshold
itself was not independently associated with
failure.
These findings suggest that even relatively low
torque thresholds (>15 N·cm) may be sufficient
to achieve osseointegration in augmented
posterior maxilla, although long-term success
depends on peri-implant biological and
anatomical factors.
3.4.4 Stability Progression During Healing
Two studies [10,13] specifically evaluated
stability progression over time using resonance
frequency analysis.
In Al-Hity et al. [10], posterior maxillary
implants demonstrated a baseline ISQ median of
55 (IQR 51–61). ISQ values increased
significantly at 3 and 6 months, with a
statistically significant improvement at 6 months
(p = 0.0006), indicating progressive biological
stabilization.
Similarly, Planinić et al. [13] demonstrated that
although primary ISQ differed significantly
between surgical techniques (p < 0.01),
secondary stability values at 5 months were not
significantly different (p = 0.660), reflecting
convergence of mechanical stability during
osseointegration.
Across studies, primary ISQ values in posterior
maxilla typically ranged between 55 and 65, and
torque values ranged from >15 N·cm to
approximately 40 N·cm, suggesting that
moderate mechanical stability levels were
generally sufficient for predictable
osseointegration.
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Table 5. Summary of Mechanical Stability Thresholds and Osseointegration Outcomes
Stability
Category
Included
Studies
(n)
Stability Range
Reported
Osseointegration
Outcome
Overall Interpretation
Surgical
technique
influence on
primary
stability
2 [13,14]
ISQ differences at
baseline; torque
≈24 N·cm
Secondary stability
equalized by 5 months;
survival 100%
Early mechanical differences
diminish during biological healing
Implant
macrodesign
(tissue vs bone
level)
1 [11]
Torque ≈25–40
N·cm
100% survival at 12
months; lower marginal
bone loss in tissue-level
implants
Moderate torque sufficient; crestal
bone remodeling influenced by
design
Low torque
threshold in
atrophic
maxilla
1 [12]
>15 N·cm
(minimum
required)
87.7% survival at 10
years; late failure 12.3%
Long-term success influenced more
by biological and anatomical factors
than torque alone
Moderate ISQ
range in
posterior
maxilla
2 [10,13]
ISQ ≈55–65
Significant ISQ increase
over 3–6 months; no early
failures
Moderate initial stability adequate;
secondary stability compensatory
Moderate
torque range
(general
finding)
3
[11,12,14]
≈20–35 N·cm
100% short- to medium-
term survival; MBL
generally <0.6 mm
No universal cut-off identified;
moderate thresholds consistently
successful
3.5 Synthesis of Results
Across the five included studies [10–14],
mechanical stability thresholds demonstrated
variable numerical ranges but consistently
supported predictable osseointegration in
posterior maxillary implants. Due to substantial
heterogeneity in study design, surgical protocols,
stability assessment methods (insertion torque
vs. ISQ), outcome definitions (marginal bone
loss, secondary stability, survival), and follow-
up duration (5 months to 10 years), results were
synthesized narratively rather than
quantitatively.
Moderate primary stability levels were the most
consistently associated with favorable outcomes.
Insertion torque values typically ranged from
approximately 20–35 N·cm [11,14], while
baseline ISQ values were generally reported
within the 55–65 range [10,13]. Across
randomized and prospective studies, these
stability levels were associated with 100% short-
to medium-term implant survival and limited
marginal bone remodeling (<0.6 mm at 12–28
months) [11,14].
Differences in surgical technique influenced
primary stability [13], however, these
differences diminished during healing, and
secondary stability values converged over time
[10,13].
In severely atrophic posterior maxillae, a lower
torque threshold of >15 N·cm was used as the
minimum criterion for implant placement [12].
Although long-term survival remained relatively
high (87.7% at 10 years), late implant failure was
more strongly associated with peri-implant soft
tissue conditions and sinus-related
complications than with the initial mechanical
stability threshold itself [12].
Implant macrodesign and surgical accuracy
influenced peri-implant bone remodeling but did
not alter short-term survival when moderate
mechanical stability was achieved [11,14].
Overall, the synthesis suggests that no single
universal mechanical stability cut-off
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guaranteeing osseointegration can be defined.
Rather, moderate primary stability (torque ≥20
N·cm or ISQ ≥55) appears sufficient for
predictable osseointegration in posterior
maxillary implants [10,11,13,14], while long-
term outcomes are influenced by anatomical,
biological, and surgical variables beyond the
initial mechanical threshold [12].
Publication bias could not be formally assessed
because fewer than 10 studies were included,
making funnel plot–based methods unreliable.
The certainty of evidence was not evaluated
using the GRADE approach due to the small
number and heterogeneity of the included
studies.
4. Discussion
Primary implant stability remains an important
prerequisite for osseointegration because
excessive micromotion at the bone-implant
interface may impair bone healing. Experimental
evidence indicates that micromovements
exceeding approximately 50-150 μm may
promote fibrous tissue formation rather than
bone integration [10–14]. These findings
indicate that extremely high insertion torque
values may not be necessary to achieve
predictable osseointegration in posterior
maxillary bone, which is typically characterized
by lower density and anatomical constraints
related to the maxillary sinus [15].
Primary implant stability remains an important
prerequisite for osseointegration because
excessive micromotion at the bone–implant
interface may interfere with bone healing.
Experimental studies have shown that
micromovements exceeding approximately 50-
150 μm may promote fibrous tissue formation
instead of bone integration [16]. Consequently,
insertion torque and resonance frequency
analysis (RFA) are widely used to evaluate
implant stability and monitor stability changes
during healing [16,17]. Several clinical
investigations have demonstrated a positive
relationship between insertion torque and ISQ
values, indicating that both parameters reflect
the mechanical engagement between the implant
and surrounding bone. For example, Sarfaraz et
al. reported a significant correlation between
insertion torque and RFA measurements,
supporting their use as complementary
indicators of implant stability during early
healing [18].
The influence of surgical technique on primary
stability has also been reported in the literature.
Planinić et al. [13] observed higher initial ISQ
values with the osteotome technique compared
with conventional drilling and flapless
placement; however, these differences
disappeared during healing, suggesting that
biological bone remodeling reduces early
mechanical differences. Similar stability patterns
have been described in studies evaluating
different osteotomy techniques, where implants
placed using various surgical approaches
demonstrate comparable stability trajectories
over time [19]. Yang and Geng [14] likewise
reported comparable insertion torque values
between navigation-assisted and freehand
implant placement, with both approaches
achieving 100% survival after approximately 28
months. Nevertheless, navigation-assisted
placement resulted in lower marginal bone loss,
indicating that surgical accuracy may contribute
to improved peri-implant bone preservation even
when primary stability values are similar.
Implant macrodesign may also influence peri-
implant bone responses. Lombardi et al. [11]
demonstrated a significant correlation between
insertion torque and initial ISQ values and
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45
reported lower marginal bone remodeling in
tissue-level implants compared with bone-level
designs. These findings suggest that implant
design may influence peri-implant bone
behavior even when implants are placed within
similar mechanical stability ranges.
Evidence from severely atrophic posterior
maxillae further indicates that relatively low
stability thresholds may still be compatible with
long-term implant success. Dung et al. [12]
reported that implants placed with insertion
torque values above 15 N·cm achieved a 10-year
survival rate of 87.7%, despite challenging
anatomical conditions. Importantly, implant
failure was more strongly associated with
biological factors such as insufficient keratinized
tissue width, sinus membrane perforation, and
poor oral hygiene than with the magnitude of the
initial stability threshold. Similarly, clinical
observations summarized in a study by Elian and
Salem [20], reported successful osseointegration
in implants placed with insertion torque values
below 10 N·cm when implants were protected
from functional loading during healing,
suggesting that biological conditions may
partially compensate for low mechanical
stability.
Finally, several studies demonstrate that implant
stability evolves during healing. Both Al-Hity et
al. [10] and Planinić et al. [13] reported increases
in ISQ values over time, reflecting the transition
from primary mechanical stability to
biologically mediated secondary stability.
Similar stability progression patterns have been
described in clinical investigations where ISQ
values gradually increase during healing and
reach a plateau within several weeks to months
after implant placement [17,19].
Overall, the available evidence suggests that
while adequate primary stability is necessary,
moderate mechanical stability levels appear
sufficient for predictable osseointegration in
posterior maxillary implants. Long-term success
in this region appears to depend not only on
mechanical stability but also on biological
healing processes, implant design, and surgical
accuracy. Future research should focus on
establishing clearer stability thresholds and
evaluating their interaction with bone quality,
implant design, and surgical technique in the
posterior maxilla.
5. Conclusions
Within the limitations of the available evidence,
moderate primary mechanical stability appears
sufficient to achieve predictable
osseointegration in implants placed in the
posterior maxilla. Across the included studies,
insertion torque values ranging from >15 to
approximately 40 N·cm and initial ISQ values
around 55-65 were consistently associated with
successful implant integration and high survival
rates.
While surgical technique and implant
macrodesign may influence early stability and
peri-implant bone remodeling, long-term
implant success appears to depend more strongly
on biological factors and peri-implant conditions
than on achieving extremely high primary
stability values.
Overall, the findings suggest that adequate,
rather than maximal, mechanical stability is
sufficient for successful osseointegration in the
posterior maxilla, provided that appropriate
surgical planning and biological conditions are
maintained.
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