Keywords dental implants - complications - implant survival - bone loss
Introduction
Edentulism is a debilitating condition that affects not only oral health, but also
the impact on general health and quality of life.[1 ] The treatment of missing teeth is very important to overcome functional, esthetic,
and social challenges and to improve the quality of life.[2 ]
The first scientifically documented concept for dental implant treatment, the Brånemark
System, included a two-stage surgical technique with intermediate healing periods.[3 ] Because of the discomfort, inconvenience, and anxiety that waiting periods impose
to both patients and clinicians,[4 ] the immediate function rehabilitation technique has been widely used in implant
dentistry. This technique involves placing the implant(s), abutment(s), and crown(s)
in the same surgical procedure, which means that the implant immediately fulfills
the requirements of masticatory function and esthetics,[5 ] with demonstrated high implant survival rates under well-defined circumstances.[6 ]
[7 ]
Mechanical risk factors are determinants of complications or failure of prefabricated
components caused by mechanical forces, while technical factors are complications
or failures of a structure fabricated in the laboratory or its materials. Both risks
play an important role in implant dentistry, as a source of time and financial resources
loss.[8 ] Considering that the periodontal ligament is absent from the peri-implant structure,
excess occlusal, functional, or parafunctional forces may lead to mechanical complications
and adverse effects on the implants’ structural integrity.[9 ] This occlusal overloading can also lead to marginal bone loss due to the inability
of the host tissues to accommodate excessive forces.[10 ]
Biological and mechanical complications in implant-supported fixed dental prostheses
are frequent over a 5-year observation period (33.6%), with fractures of the veneering
material (13.5%), peri-implant pathology or soft tissue complications (8.5%), loss
of access hole restoration (5.4%), abutment or screw loosening (5.3%), and loss of
retention of cemented fixed dental prostheses (4.7%) as the most recurrent.[11 ]
[12 ] Implant failure is secondary to mechanical complications, including implant fracture,
abutment screw fracture, and abutment fracture,[13 ] with implant fracture as the most serious and rare complication with a prevalence
of 1.6%.[14 ] It implies implant removal and replacement by a larger diameter implant, with consequent
delay of the prosthetic rehabilitation due to the necessity of overcoming a new osseointegration
period.[12 ]
Based on the existing literature on implant-supported restorations, there is a relative
gap in the understanding of the effect of mechanical complications on long-term outcomes.
Mechanical complications could potentially be a primary factor for implant failure
through a direct effect due to implant fracture or marginal bone resorption, or through
an indirect effect due to biological complications occurring secondary to mechanical
complications. It is important to understand the effect of mechanical complications
on implant survival, marginal bone resorption, and incidence of biological complications.
The aim of this study was to fill this gap by analyzing the effect of mechanical complications
on the outcome of implant-supported restorations.
Materials and Methods
This case–control study was performed from 2016 to 2019 at a private rehabilitation
center, (Maló Clinic, Lisbon, Portugal). The study was approved by a local ethics
committee (Ethical Committee for Health, authorization no. 011-2012). Informed consent
was obtained from all participants. The study population consisted of patients rehabilitated
with dental implants (age range = 18–80 years) and selected from a database with registered
mechanical complications. Some conditions that could compromise treatment success
such as smoking and systemic conditions were considered and not excluded. There were
282 patients identified with mechanical complications occurring in fixed prosthetic
rehabilitation supported by immediate function implants (cases). The date of implant
surgery ranged from September 1997 to December 2006.
The sample size (n = 282 in each group) enabled the detection of factors associated with survival, with
an odds ratio (ψ) of 1.545 or greater in exposed relative to unexposed (controls)
groups according to calculations (PS power and sample size calculation software, version
2.1.30, February 2003).[15 ] The remaining assumptions used in the calculation were the following:
Probability of treatment failure among controls of 30%[16 ]
[17 ]
[18 ]
[19 ]
Correlation coefficient for exposure between matched exposed and unexposed subjects
of 20%[16 ]
[17 ]
[18 ]
[19 ]
Ratio of matching controls per exposed subjects of 1:1
Statistical significance level of 95% (type I error or α = 0.05)
Statistical power of 80% (type II error or β = 20%)
A total of 282 patients with mechanical complications (cases) were matched for sex,
age (within a range of 3 years), and follow-up time (within a range of 11 months)
with the patients without mechanical complications (controls).
Surgical Protocol
The patients’ medical history was examined, and clinical observations complemented
with orthopantomography and computerized tomography scans were performed. Teeth were
extracted when needed at the time of surgery before implant placement. A mucoperiosteal
flap was raised at the ridge crest, relieving incisions on the buccal aspect of the
molar area. The insertion of implants (external connection, Mk II, Mk III, Mk IV,
NobelSpeedy; Nobel Biocare AB) followed standard procedures, except that underpreparation
was used to achieve an insertion torque of at least 35 N∙cm before the final seating
of implants. The preparation was typically performed by using a full drill depth with
a 2-mm twist drill, followed by twist/step drills according to the manufacturer’s
protocol.[20 ]
[21 ] The implant neck (Mk II, Mk III, or Mk IV implant) or the implant head (NobelSpeedy
implant) was positioned at the bone level, and bicortical anchorage was established
whenever possible. The implant diameter ranged from 3.3 to 4 mm, and the implant length
ranged from 10 to 18 mm. The abutments were used according to the type of restoration
(Cera-one, STR, multiunit straight and angulated abutments, Nobel Biocare AB). After
closing and suturing the flap with 3–0 nonabsorbable sutures, the abutments were assessed
by using a punch if needed, and impression copings were placed.
Immediate Provisional Prosthetic Protocol
High-density acrylic resin (Heraeus Kulzer GmbH, Hanau, Germany) and titanium cylinder
(Nobel Biocare AB) prostheses (complete or partial edentulous) or crowns (single tooth)
were manufactured at the dental laboratory and inserted on the same day of surgery
(n = 465) to achieve immediate function.
Final Prosthetic Protocol
Considering the patient’s preference, a metal-ceramic implant-supported fixed prosthesis
with a titanium framework and all-ceramic crowns (Procera titanium framework, Procera
crowns, Nobel Rondo ceramics, Nobel Biocare AB), or a metal-acrylic resin implant-supported
fixed prosthesis with a titanium framework (Procera titanium framework; Nobel Biocare
AB) and acrylic resin prosthetic teeth (Heraeus Kulzer GmbH) was used to replace the
provisional prosthesis for the complete edentulous. For partial and single tooth rehabilitation,
ceramic crown/prosthesis (Procera crowns, Nobel Rondo ceramics, Nobel Biocare AB)
was inserted. In the final prosthesis, occlusion mimicked natural dentition.
Outcome Measures
Outcome measures were evaluated yearly between implant insertion and the last clinical
appointment for follow-up. The primary outcome measure was implant survival, which
was evaluated based on the function of the implant as part of a prosthetic rehabilitation
unit. The secondary outcome measures were marginal bone loss, which was measured at
5 years of follow-up, and complications. Considering marginal bone loss, periapical
radiographs were obtained by using the parallel technique with a film holder (Super-Bite,
Hawe-Neos, Switzerland) and an aiming device. Each periapical radiograph was scanned
at 300 DPI by using a scanner (HP Scanjet 4890, HP Portugal, Paço de Arcos, Portugal),
and the marginal bone level was assessed by using an image analysis software (Image
J version 1.40g for Windows, National Institutes of Health, United States). The reference
point for reading was the implant platform, which is the horizontal interface between
the implant and the abutment. Marginal bone loss was defined as the difference in
marginal bone level relative to the bone level at the time of surgery. Radiographs
were accepted or rejected for evaluation based on the clarity of implant threads;
a clear thread guaranteed both sharpness and an orthogonal direction of the radiographic
beam toward the implant axis.
The following biological complication parameters were assessed: peri-implant pathology
(probing pocket depth >4 mm, with concurrent presence of marginal bone loss and bleeding
on probing/suppuration), soft tissue inflammation, fistula formation, and abscess
formation. This evaluation was performed after surgical healing, every 6 months, and
over at least 5 years of follow-up.
Statistical Analysis
Descriptive statistics were performed on the variables of interest, and frequencies
of mechanical complications, including prosthesis fracture, prosthetic decementation,
abutment screw milling or loosening, and prosthetic screw milling or loosening, and
biological complications, including inflammation, infection, and peri-implant pathology,
were estimated. Implant survival was recorded as survival or failure and was estimated
through life tables, mechanical, and biological complications were recorded as present
or absent, and marginal bone loss was recorded through the average and 95% confidence
interval (CI) using the patient as a unit of analysis. Inferentially, smoking status,
systemic compromising status, implant survival, and the incidence of biological complications
were compared between groups using the Chi-square test, with complementary analysis
(Chi-square test and odds ratio) to assess the difference in the distribution of biological
complications according to the timing of occurrence (less than 6 months or between
6 months and 1 year after the incidence of mechanical complications), and marginal
bone loss at 5 years was compared between groups using the Mann–Whitney U test. Statistical
analyses were performed by using the SPSS software (version 17.0; IBM, New York, United
States). The significance level was set at 5%.
Results
A total of 564 patients (330 women and 234 men) were included, with an average age
of 52.6 years (range = 18–80 years) and with 175 patients who were smokers and 151
patients with a systemic condition. There were 167 single-tooth rehabilitations, 106
partial rehabilitations, and 291 complete edentulous rehabilitations. Regarding the
opposing dentitions, 16 patients had a removable prosthesis, 144 patients had natural
teeth, 108 patients had fixed prosthetics over natural teeth, 250 patients had a combination
of natural teeth and implant-supported fixed prostheses, and 46 patients had a removable
prosthesis. Mechanical complications included prosthetic fracture (n = 159), abutment loosening (n = 89), prosthetic screw loosening (n = 20), milled abutment (n = 12), milled prosthetic screw (n = 1), and decemented crown (n = 1). The distribution of mechanical complications in these cases is presented in
[Table 1 ].
Table 1
Distribution of mechanical complications in the cases
Type of Mechanical Problems
Number of occurrences per patient
Number of occurrences per restoration
Number of recurring complications
Number of recurrences per restoration
Note: Total number of nine patients with more than one incidence: n = 4 patients with fracture prosthesis and abutment screw loosening; n = 3 patients with fractured prosthesis and abutment screw milling; n = 1 patient with fractured prosthesis and prosthetic screw milling; n = 1 patient with fractured prosthesis and prosthetic screw loosening.
Veneer fracture
159
Full-arch: 110
Partial: 12
Single 37
264
Full-arch: 240 Partial: 4
Single: 20
Prosthetic screw loosening
20
Full-arch: 13
Partial: 5
Single 2
33
Full-arch: 33
Milled prosthetic screw
1
Partial: 1
0
Abutment screw loosening
89
Full-arch: 64
Partial: 6
Single 19
78
Full-arch: 62 Partial: 1
Single: 15
Milled abutment screw
12
Full-arch: 11
Partial: 1
12
Full-arch: 11 Partial: 1
Decemented crown
1
Partial: 1
3
Partial: 3
Total
282
390
The average follow-up period of the sample was 8.5 years. The distribution of patients
with smoking habits was 88 (50.3%) and 87 (49.7%) individuals for cases and controls,
respectively (p = 0.500, Chi-square test); for systemic conditions, there were 71 (51.1%) and 68
(48.9%) patients for cases and controls, respectively (p = 0.423, Chi-square test); and for history of periodontal disease, there were 161
(52.3%) and 147 (47.7%) patients for cases and controls, respectively (p = 0.430, Chi-square test). No significant differences between cases and controls
were observed in the distribution of smoking habits, systemic condition, or history
of periodontitis.
Implant failure occurred in one patient from the control group after 70 months of
follow-up, with survival rates of 100 and 99.6% for cases and controls, respectively
([Table 2 ]). The difference in survival outcomes between the groups was not significant (p = 0.317, Chi-square test).
Table 2
Life tables evaluating the cumulative survival rate (global sample, patients exposed
to mechanical complications, and patients unexposed to mechanical complications)
Global sample
Time
Total number of patients
Failures
Lost to follow-up
Follow-up not completed
Survival rate (%)
Cumulative survival rate (%)
Placement–1 y
564
0
0
0
100
100.0
1–2 y
564
0
0
0
100
100.0
2–3 y
564
0
0
0
100
100.0
3–4 y
564
0
0
0
100.0
100.0
4–5 y
564
0
0
0
100.0
100.0
5–6 y
564
1
17
55
99.8
99.8
6–7 y
491
0
11
146
100.0
99.8
7–8 y
334
0
10
124
100.0
99.8
8–9 y
200
0
7
113
100.0
99.8
Patients with mechanical complications (cases)
Time
Total number of patients
Failures
Lost to follow-up
Follow-up not completed
Survival rate (%)
Cumulative survival rate (%)
Placement–1 y
282
0
0
0
100
100.0
1–2 y
282
0
0
0
100
100.0
2–3 y
282
0
0
0
100
100.0
3–4 y
282
0
0
0
100.0
100.0
4–5 y
282
0
0
0
100.0
100.0
5–6 y
282
0
7
29
100.0
100.0
6–7 y
246
0
1
79
100.0
100.0
7–8 y
166
0
2
66
100.0
100.0
8–9 y
98
0
0
59
100.0
100.0
Patients without mechanical complications (controls)
Time
Total number of patients
Failures
Lost to follow-up
Follow-up not completed
Survival rate (%)
Cumulative survival rate (%)
Placement–1 y
282
0
0
0
100
100.0
1–2 y
282
0
0
0
100
100.0
2–3 y
282
0
0
0
100
100.0
3–4 y
282
0
0
0
100.0
100.0
4–5 y
282
0
0
0
100.0
100.0
5–6 y
282
1
10
26
99.6
99.6
6–7 y
245
0
10
67
100.0
99.6
7 y
168
0
8
58
100.0
99.6
8–9 y
102
0
7
54
100.0
99.6
The average (95% CI) marginal bone loss registered for cases and controls was 1.72
(95% CI: 1.60–1.84) and 1.55 (95% CI: 1.45–1.65). The frequencies of marginal bone
loss are shown in [Table 3 ] and [Fig. 1 ]. The difference in marginal bone loss between the cases and controls was not significant
(p = 0.068; Mann–Whitney U test).
Table 3
Marginal bone level for cases and controls at 5 years of follow-up
Cases
Controls
Mean (mm)
1.72
1.54
Standard deviation (mm)
0.90
0.76
Number
230
229
Frequencies
n
%
n
%
0 mm
0
0.0
1
0.0
0.1–1.0 mm
47
20.4
62
27.1
1.1–2.0 mm
122
53.0
123
53.7
2.1–3.0 mm
38
16.5
34
14.8
>3.0 mm
23
10.0
9
3.9
Fig. 1 Boxplot of marginal bone loss at 5 years for cases (with mechanical complications)
and controls (without mechanical complications). Box edges represent the first and
third quartiles of data (25 and 75%, respectively, of all data collected); black line
represents the median marginal bone loss registered for cases (1.48 mm) and controls
(1.45 mm); whiskers represent all data not suspected to be outliers; dots represent
data suspected of being outliers.
Biological complications were observed in 90 patients (54 from the cases and 36 from
the controls; [Tables 4 ] and [5 ]). The biological complications recorded were peri-implant pathologies (n = 78 patients; 46 cases, 32 controls), fistula formation (n = 2 patients; 1 case, 1 control), and abscess formation (n = 10 patients; 7 cases, 3 controls), and the distribution of biological complications
according to the type of mechanical complication and timing of occurrence (<6 months
or >6 months after the incidence of a mechanical complication) are shown in [Table 4 ]. A significant difference was observed in the incidence of biological complications
between cases and controls (p = 0.038, Chi-square test), with an odds ratio of 1.63 ([Table 5 ]). Abscess and fistula formation were managed nonsurgically through prophylaxis (using
chlorhexidine) and antibiotic therapy. Peri-implant pathology was managed nonsurgically
(n = 47 patients; 27 cases, 20 controls) through mechanical debridement and pocket irrigation
with 0.2% chlorhexidine gel, or surgically (n = 4 patients; 3 cases, 1 control) through an open flap surgical intervention to mechanically
clean the implant surface, disinfecting the implant surface with 0.2% chlorhexidine,
suturing, and medicating the patient with antibiotics. In 27 patients (16 cases, 11
controls), the interventions were unsuccessful through nonsurgical interventions (n = 23 patients; 13 cases, 10 controls) or surgical interventions (n = 4 patients; 3 cases, 1 control).
Table 4
Distribution of biological complications according to the timing of occurrence after
the mechanical complication
Biological complications <6 mo
Biological complications >6 mo
Mechanical complications
Abscess
Fistula
Peri-implant pathology
Total
(per row)
Abscess
Fistula
Peri-implant pathology
Total
(per row)
Prosthetic screw loosening
1
0
1
2
0
0
1
1
Abutment screw loosening
1
1
5
7
0
0
15
15
Prosthesis fracture
3
0
7
10
0
0
19
19
Total (per column)
5
1
13
19
0
0
35
35
Table 5
Frequencies of biological complications in the exposed and unexposed groups
Mechanical complications
Biological complications
Absent
Present
Total
a An odds ratio of 1.63 was estimated based on the figures presented.
Absent
(unexposed)
Number
246
36
282
Within mechanical complications (%)
87.2
12.8
100.0
Within biological complications (%)
52.0
40.0
50.1
Total (%)
43.7
6.4
50.1
Present
(exposed)
Number
227
54a
281
Within mechanical complications (%)
80.8
19.2
100.0
Within biological complications (%)
48.0
60.0
49.9
Total (%)
40.3
9.6
49.9
Total
Number
473
90
563
Total (%)
84.0
17.4
100.0
Discussion
The results of this case–control study indicate that the exposure to mechanical complications
in immediate loading protocols did not significantly impact implant survival after
an average of 8.5 years of follow-up.
The effect of mechanical complications on successful outcomes of implant-supported
rehabilitation is unclear. Salvi and Brägger,[8 ] in a systematic review of 35 publications with the purpose of understanding which
mechanical/technical risk factors impacted implant-supported reconstructions, identified
10 mechanical/technical risk factors, including the history of mechanical/technical
complications. The study concluded that none of the mechanical/technical risk factors
for overloading had an impact on implant survival and success rates.[8 ] The present study supports this finding as mechanical/technical complications such
as loosening or fracture of prosthetic components did not had a significant effect
on survival, based on the nonsignificant difference in implant survival. On the other
hand, previous studies have indicated an association between occlusal overloading,
which is the primary cause of mechanical complications,[22 ] and possible late implant failure through marginal bone resorption.[10 ] The present study demonstrated that while there was no impact on implant survival,
there was a potential deleterious influence on the long-term as observed when analyzing
the clinical implications of both marginal bone resorption and incidence of biological
complications.
The marginal bone loss after 5 years of follow-up revealed an increased bone loss
in cases compared to controls; however, the difference was not statistically significant.
Occlusal overloading, manifested through signs of mechanical complications, is the
primary cause of biomechanical implant complications and may also disrupt the intricate
bond between the implant surface and bone, leading to peri-implant bone loss and eventual
implant failure.[10 ] According to Fu et al,[10 ] although the exact mechanism of peri-implant bone loss occurrence caused by occlusal
overloading remains debatable due to confounding factors, it is obvious that a positive
correlation between occlusal overloading and peri-implant marginal bone loss exists.
Nevertheless, other researchers acknowledged that the effect of traumatic forces in
peri-implant bone loss is poorly reported and provides limited evidence to support
a cause-effect relationship considering the strength of a clinically relevant traumatic
occlusal force.[23 ] In the present study, the marginal bone loss trend in cases (with 23 patients in
cases compared to 9 patients in controls who had marginal bone loss of >3 mm at 5
years) might have a significant clinical impact. This marginal bone loss can be considered
pathologic because “physiological” bone remodeling around implants was previously
described to be approximately 1 mm during the first year of function and <0.2 mm per
year subsequently.[24 ] Furthermore, the difference of 0.18 mm in marginal bone loss at 5 years of follow-up
between the two groups clinically translates to roughly half an implant’s thread.
Based on this difference, it may be hypothesized that the bone loss pattern in cases
could have a significant impact on long-term follow-up considering functional and
esthetic outcomes, with a potential visible abutment/crown transition that implies
prosthetic rehabilitation failure. This hypothesis needs to be confirmed through studies
with longer follow-up periods.
Given that prosthetic treatment takes place at the implant/abutment level, it became
evident that the implant restoration process contributes significantly to the prognosis
and peri-implant disease experience.[24 ] The present study reported a significant effect of mechanical complications on the
incidence of biological complications (with a 63% increased odds ratio), a result
that is supported by the current understanding of peri-implant disease etiopathogenesis.
Epidemiologically, peri-implant pathology is considered a multifactorial disease with
several nonsufficient and non-necessary causes, containing factors of biological and
biomechanical origin that act independently or in association.[25 ] Nevertheless, other factors may contribute to the susceptibility or progression
of peri-implant disease, such as patient’s health, smoking habits, the presence or
preexistence of periodontal disease, the type of implant and its surface, and the
quality of the existing bone.[24 ]
[26 ] Recent meta-analyses registered a higher incidence of peri-implant pathology in
periodontitis-susceptible patients,[27 ] even under regular supportive postimplant treatment,[28 ] and smokers with significant effect on the incidence of postoperative infections,
marginal bone loss, and implant failure rate.[29 ]
[30 ]
[31 ] Lin et al,[28 ] in a systematic review and meta-analysis of 13 studies to investigate if periodontal
disease could still be a risk indicator for peri-implant health under supportive postsurgical
treatment, estimated an increased marginal bone loss irrespective of the implant surface,
and lower survival rates, increased pocket depth, and bleeding on probing in rough
surface implants for patients with history of periodontitis. Chrcanovic et al,[31 ] in a systematic review and meta-analysis of 107 studies to estimate the effect of
smoking on dental implants, reported a significant effect of smoking on implant failure
(risk ratio of 2.23) and marginal bone loss (mean increase of 0.32 mm), which are
two of the outcomes evaluated in the present study. Furthermore, certain systemic
diseases, medications, radiotherapy, and behavioral factors, such as inefficient oral
hygiene and lack of compliance with periodontal maintenance therapy, appear to significantly
increase the risk of peri-implant pathology.[32 ] In the present study, all three conditions that could impact the outcome negatively
were prevalent, with 54.6% of patients with history of periodontitis, 31% of patients
who smoke, and 24.7% of patients with systemic conditions. However, because the distribution
of patients with a history of periodontitis, smoking habits, and systemic condition
between groups was not significantly different, it was suggested that mechanical complications
may represent a risk indicator for the incidence of biological complications, rather
than a confounder. A similar result was reported in a large case–control study to
evaluate risk indicators for peri-implant disease, and the presence of mechanical
complications such as prosthetic screw loosening, abutment screw loosening, or prosthetic
passive misfit implied a 5.9-fold increase in the odds for peri-implant pathology[25 ] and consequent inclusion in a risk score to predict this disease.[33 ]
[34 ] Nevertheless, the fact that the results of the present study were not controlled
for the presence of bacterial plaque implies both interpreting this result with caution
and performing studies with stronger design to establish causality.
While there is no clear and long-term evidence of the impact of mechanical complications
on implant treatment, clinicians must be aware of all patient-related conditions,
providing the best treatment possible and preventing complications. An appropriate
and individualized treatment plan, combined with regular routine appointments to identify
warning signs, provides greater treatment success. Mechanical complications not only
affect the clinical outcome, but also negatively impact the patients’ quality of life,
translating into a source of frustration for both clinicians and patients, with the
necessity of significant investment in terms of service, maintenance, costs, and time.[35 ]
The limitations of the present study include the lack of randomization and its retrospective
design. Another limitation is the fact that potential risk indicators or confounding
factors, such as bacterial plaque and frequency of maintenance appointments, were
not analyzed. Long-term longitudinal studies with multivariable analysis that include
competing risk indicators and confounders should be performed to investigate both
the effect and impact of the occurrence of mechanical complications on the outcome
of implant-supported rehabilitations.
In conclusion, within the limitations of the present study, the occurrence of mechanical
complications did not significantly impact implant survival or marginal bone loss
at 5 years of follow-up, but did impact the incidence of biological complications,
with a greater incidence in cases. Additional studies with longer follow-up periods
are necessary to assess the implication of the incidence of biological complications
and marginal bone loss pattern exhibited by patients with mechanical complications
on the outcome of implant-supported rehabilitations.
