The outcome of oral implants placed in bone with limited bucco-oral dimensions: a 3-year follow-up
study
For figures, tables and references we refer the reader to the original paper.
Whenever bone of sufficient quantity and quality is available, oral implants are considered to be a
valuable treatment option to replace missing teeth. Consequently, implants became part of mainstream
dentistry (Adell et al. 1981, Albrektsson et al. 1981). Survival rates of over 95% in non-compromised
patients have been reported (den Hartog et al. 2008). Unfortunately, an oral implant does not always
osseointegrate.
The clinical success of an implant partially depends on several external factors such as: primary implant
stability, bone quality, time of loading, infection control, surgical technique and height and width of the
alveolar bone. Primary implant stability is determined by the properties of the bone that contribute to its
strength. Implant micro-movements within a range of 50–150 μm seem to be acceptable for optimal
bone healing (Szmukler-Moncler et al. 1998). The defining factors for bone quality are trabecular
thickness, mineral density and micro-architecture (Ribeiro-Rotta et al. 2007). In relation to time of
loading, a recent review showed no differences in marginal bone level changes between different
loading protocols (Suarez et al. 2013). However, the length of the implant seems of interest for the
survival rate of most implant systems. Since implant survival decreases when the implant is shorter than
7 mm (Pommer et al. 2011).
To determine the health of the peri-implant tissues a radiological assessment of the marginal bone level
around the shoulder of an implant is a reliable option (Grusovin et al. 2008). Several factors might
influence the marginal bone level. These factors include trauma during implant placement (Sakka et al.
2012), trauma during abutment surgery (Vela-Nebot et al. 2006), the loading protocol (Kim et al. 2005),
abutment type (Annibali et al. 2012a) and establishment of the biological width (Hermann et al. 2000).
However, these factors are still not proven and the debate is still going on. To consider an implant as
successful, it has to meet criteria with respect to tissue physiology (osseointegration), function
(chewing), absence of pain and user satisfaction (Tonetti & Palmer 2012). In 1986, Albrektsson et al.
stated that a mean marginal bone loss of 1.5 mm after the first year of loading is acceptable (Albrektsson
et al. 1986). More recent studies show a mean marginal bone loss of 1.0 mm after the first year of
loading and in the following years an annual 0.1 mm additional bone loss (Cecchinato et al. 2008). More
recently the criteria for success have become more stringent with three domains that are important for
identifying success. These domains are: patient-reported outcome measures (health-related quality of
live and general satisfaction), peri-implant health (marginal bone level, bleeding on probing and probing
depth) and implant-supported restorations (longevity of the restoration, function/occlusion related
outcomes and technical complications) (Tonetti & Palmer 2012).
Periodontal disease, tooth extraction or trauma can lead to alveolar ridge atrophy. This eventually results
in a ridge with deficient width and height for optimal implant therapy (Schropp et al. 2003). To provide
adequate bone volume and to assure an adequate aesthetic result bone augmentation procedures are
often required. Some papers indicate that the bone thickness around an implant should be at least 1 or 2
mm to assure long-term success and bone coverage (Esposito et al. 2007, Grunder et al. 2005). A recent
systematic review on this issue, however, failed to confirm this hypothesis (Teughels et al. 2009).
To increase bone volume, different techniques can be used such as guided bone regeneration (GBR),
block grafts, ridge splitting, distraction osteogenesis, osteotomy of the ridge or combinations of the
above (Aghaloo & Moy 2007, Milinkovic & Cordaro 2014). In addition, there are a lot of different
materials such as autografts, allografts, xenografts, alloplasts, barrier membranes, osteosynthesis
materials or combinations of the above that can be used (Jensen & Terheyden 2009). The results of these
treatments are, however, not always predictable. For large onlay grafts a survival rate of 87% was
reported (Aghaloo & Moy 2007, Chiapasco & Zaniboni 2009, Jensen & Terheyden 2009). For the less
invasive grafting techniques the survival rate was 95.5% (Aghaloo & Moy 2007, Chiapasco & Zaniboni
2009, Jensen & Terheyden 2009). Complications occurred in 3.8–29.8 per cent of the patients (Aghaloo &
Moy 2007, Chiapasco & Zaniboni 2009, Jensen & Terheyden 2009).
To date, there is insufficient evidence to set a threshold for the minimal amount of bone necessary in
bucco-oral dimensions at implant placement (Merheb et al. 2014, Teughels et al. 2009). This prospective
follow-up study aims to evaluate radiological the interproximal bone changes of oral implants placed in
sites with ≤ 4.5 mm of bucco-oral bone width.
Material and Methods
This prospective, single-centre study was carried out at the Department of Periodontology of the
University Hospitals Leuven. All oral implants (Astra Tech®, DentsplyImplants, Mölndal, Sweden) were
placed in the period between 2009 and 2010. The following inclusion criteria had to be fulfilled:
Implant sites with ≤4.5 mm bone in bucco-oral dimensions, as measured on CBCT cross-sectional images
(2-mm subcrestally) Only those patients, with a very limited bucco-oral dimension over the crest were
included.
Patients expectations were pure functional, without any severe aesthetical demand.
This study was approved by the ethical committee of the University Hospitals of Leuven. All patients
included fulfilled all of the inclusion criteria. After oral and written information, all patients signed the
Informed Consent Form.
All implant sites were analysed on a multi-slice CT (Somatom Plus S®, Siemens, Erlangen, Germany) or
cone-beam CT (Scanora 3D®, Soredex, Tuusula, Finland). The implants were placed according to the
surgical protocol provided by the implant company (Instruction manual, Astra Tech®, DentsplyImplants,
Mölndal, Sweden). However, in most cases, the surgical protocol was changed accordingly to the
situation. Precision drills were used as a pilot drill. Bone condensators or piezosurgery were used as an
extra tool to widen the osteotomy without the need of rotatory burs. By using these tools a selective
preparation of osteotomy can be performed. The amount of buccal bone on the implant was <1 mm at
each implant site. Whenever a fenestration or dehiscence occurred, a GBR procedure was performed. A
thin layer of “deproteinized bovine bone matrix” (Bio-Oss™, Geistlich®, Switzerland) was used and
covered with a resorbable collagen membrane (Bio-Gide™, Geistlich®, Switzerland). The digital intra-oral
radiographs (follow-up) were obtained using the parallel technique, with position holders and a longcone radiographic unit (Digora® photostimulable phosphor plates and the MinRay® intra-oral
radiographic system, Soredex, Tuusula, Finland). The University Hospitals Leuven use a standardized
protocol in which the setting of the radiographic system is depended on the region of the mouth where
the photo is taken.
Marginal bone level changes
The intra-oral, long-cone radiographs were taken at: implant placement, functional loading, 1, 2 and 3year(s) follow-up (Fig. 1). The distances, in millimetres, between the shoulder of the implant and the first
clear bone-to-implant contact were recorded both mesially and distally. The thread pitch distance and
the full length of the implant were also recorded and were, if needed, used for conversion. These
measurements (accuracy of 0.01 mm) were performed with a software program (image j®, NIH,
Bethesda, Maryland, USA). The measurements were initially made in a pixel format. Linear distance
measurement (mm) could be retrieved after calibration of the images according to the respective thread
pitch distance, provided by the manufacturer. The full length of the implant was used for the conversion.
The analysis of peri-implant bone level alterations was performed by two calibrated, independent
periodontologists (JK and AT). Results were re-evaluated when there was a ≥ 1 mm inter-examiner
difference. The following parameters were also recorded: age, medical history, implant length, timing of
placement and position in the mouth.
Width of the alveolar process
The width of the alveolar process on the future implant osteotomy sites was measured, pre-operatively,
on cross-sectional images of the cone-beam CT or multi-slice CT, using the software tool to the nearest
0.1 mm (PACS lightbox, IMPAX pacs, Agfa). Measurements were made at the top of the crest and at
every 2 mm (2, 4, 6, 8, 10 and 12 mm below the first measurement).
Statistics
Data are represented by their mean, standard deviation, median and range, and were calculated based
on means per patient. Empirical cumulative distribution functions were used to represent them
graphically.
Results
Patient and implant data
A total of 28 patients (three males and 25 females) were included. All patients belonged to the Caucasian
race with an average age of 63.4 (range 42–76 years, SD 9.1 years). A total of 100 implants were placed
(13 in males, 87 in females). Two in one patient with diabetes and seven in two patients with a history of
radiotherapy outside the head and neck region (Table 1).
Eighty-eight per cent of the implants were placed in the upper jaw and 12% in the lower jaw, primarily in
the region between the 2nd incisor and 2nd premolar (Table 2). All implants had a diameter of 3.5 mm
and their length ranged from 8 mm to 15 mm (Table 3). The majority of the implants had a length of 13
mm (45%) or 11 mm (33%). The mean width of the future osteotomy at the top of the crest was 2.8 mm
(SD: 0.8; range [1.1; 4.4]). Others measurements are shown in Table 4.
The mean insertion torque was 35 N/cm, with a mean “implant stability quotient (ISQ)” of 68.9 ± 8.3 at
insertion, and of 67.7 ± 9.8 at final abutment placement (Osstell AB®, Göteborg, Sweden). None of the
implants were placed in extraction sockets. The mean submerged healing period was 3.6 ± 0.9 months.
(Table 5). Only for seven implants a simultaneous grafting, according to the GBR principle, was required.
The implants were loaded with single crowns, partial or full fixed bridges and mostly with overdentures
(Department of Prosthetic Dentistry). One hundred implants were followed during the first year. Over
the time course of the study, two patients did not show up for the follow-up appointment so that the
number of followed implants dropped to 98 during the second year and to 95 during the third year.
None of the implants failed, resulting in an overall implant survival rate of 100% after 3 years.
Marginal bone level changes (Table 6)
The implants were primarily placed subcrestally (mesially: 0.94 mm ± 0.87, and distally: 0.68 mm ± 0.97
below bone level). At abutment connection the marginal bone was located 0.65 ± 0.6 mm apically of the
implant shoulder (mesially 0.63 ± 0.7 mm, distally 0.67 ± 0.6). One, 2 and 3 years after final abutment
connection, this “bone level” was slightly more apically (0.80, 0.84 and 0.79 mm apically to the implant
shoulder, respectively).
The overall mean marginal “bone loss” during first year of functional loading was 0.17 mm ± 0.40,
between the first and second year 0.05 mm ± 0.37. Between the second and third year there was a slight
bone gain of 0.06 mm ± 0.14.
The cumulative percentage distribution of the implants according to their marginal bone level at 1, 2 and
3 years is presented in Table 6 & Fig. 2. After 1 year, 80% of the implants had their bone level between
0.25 mm and 1.12 mm apically to the shoulder After the 2nd year this was between 0.20 mm and 1.19
mm, and after the third year this was between 0.18 mm and 1.03 mm.
Discussion
The results of this prospective clinical follow-up study demonstrated that the interproximal bone
changes for implants placed in sites with ≤ 4.5 mm of bucco-oral bone width were stable during the 3
years of functional loading. Furthermore, the implant survival rate was 100%.
The marginal bone changes around implants are most common, between abutment connection and the
first year of functional loading. The present study showed a mean marginal bone loss of 0.17 mm ± 0.40
in the first year of functional loading. After the first year the marginal bone loss varied a bit with after 2
years a marginal bone loss increase with another 0.05 mm ± 0.37 and after 3 years a marginal bone loss
decrease with 0.06 mm ± 0.14. In 2012 the success criteria for implants were adjusted and divided into
three domains (Tonetti & Palmer 2012). The domains, patient-reported outcome measures and implantsupported restorations were not evaluated in this study. Only for the domain peri-implant health, the
marginal bone level was measured. These data for the marginal bone level are in agreement with the
present success criteria that allow a marginal bone loss during the first year of 1–1.5 mm and further
annual loss of 0.1 mm (Albrektsson et al. 1981, Cecchinato et al. 2008). Wennström et al. reported for
the same implant a mean marginal bone loss, between implant insertion and functional loading, of 0.02
mm ± 0.65 during the first year, 0.08 mm ± 1.02 in the second year, 0.03 mm ± 0.81 in the third year
(Albrektsson et al. 1981, Wennstrom et al. 2005). Another study showed a mean marginal bone loss after
5 years of 0.39 mm ± 0.28 for the upper jaw and 0.12 mm ± 0.16 for the lower jaw (Astrand et al. 2004).
After 12 years the mean marginal bone loss was 0.32 mm ± 0.78 for the upper jaw and 0.29 mm ± 0.77
for the lower jaw (Ravald et al. 2013). The latest meta-analysis for AstraTech® implants showed that the
weighted mean marginal bone loss over a period of 5 years was 0.27 mm. All these implants were placed
in sites were the bucco-oral bone width was 5 mm or more, and for all studies functional loading was
taken as baseline. These observations are similar to our findings, however, in sites with ≤ 4.5 mm of
bucco-oral bone width. However, a remark on the 2-dimensional evaluation of the radiographs has to be
made, as marginal bone level measurements are done only at the mesial and distal aspect. As all
implants are placed in narrow ridges, the assumption can be made that marginal bone level changes are
primarily to be expected on the vestibular or oral side.
These favourable outcomes might be explained by the presence of the micro-threads in the marginal
part of the implant. Bratu et al. (2009) compared implants with a 1-mm machined neck to implants with
a roughened surface with micro-threads (Bratu et al. 2009). The results after 1 year showed that the
implants with a machined neck had a marginal bone loss of 1.47 mm ± 0.40 while the implants with a
roughened surface with micro-threads had a loss of 0.69 mm ± 0.25. These micro-threads and the
combination of the roughened surface in the marginal part of the implant might be of importance for the
maintenance of the marginal bone level (Amid et al. 2013, Lee et al. 2007, Ozkir & Terzioglu 2012, Rocci
et al. 2008).
The bucco-oral bone width is considered to be crucial for osseointegration and even more important for
an aesthetic outcome. In the literature there are some guidelines available which suggested a zone of
1.5–2 mm of bone around the implant (Esposito et al. 2007, Grunder et al. 2005). In the current study,
implants with a diameter of 3.5 mm were placed in sites with ≤4.5 mm vestibule-oral bone width. This
means that <1 mm of bone is surrounding the most coronal part of the implant. The marginal bone loss
observed around these implants was, however, comparable to the latest long-term results of Astra Tech®
implants (Astrand et al. 2004, Ravald et al. 2013, Wennstrom et al. 2005). Small-diameter implants have
become a popular treatment option to avoid bone augmentation procedures and more complex surgery.
In some studies, implants with a width of 3.5 mm are considered small-diameter implants (Sohrabi et al.
2012). Other studies consider small-diameter implants as those with a width ≤3.3 mm (Ortega-Oller et al.
2014). The question remains, whether implants with a diameter of 3.5 mm, which were used in our
study, can be considered as small-diameter implants. A recent meta-analysis has analysed the influence
of implant diameter on the survival. They concluded that narrower implants (<3.3 mm) had a
significantly lower survival rate compared to wider implants (>3.3 mm). On the other hand, recent
(systematic/narrative) reviews concluded that the survival rates small-diameter implants are similar to
those reported for standard width implants (Hasan et al. 2014, Sohrabi et al. 2012). However, it seems
logical that these implants may be at an increased risk of functional overload due to occlusal forces and
therefore implants with higher fatigue strength can be beneficial to overcome this problem (Al-Nawas et
al. 2014). In our study, no problems that could be reduced to fatigue strength, occurred after 3 years of
functional loading.
Factors that can interfere with preserving the marginal bone thickness around an implant, besides
bacteria, may probably be bone compression and blood supply. Bone compression can be reduced to the
minimum by using a pre-tap or cortical drill. In the current study a minimal undersized drilling sequence
was used. The discrepancy between the osteotomy and the implant was only 0.3 mm. Moreover, the
manufacturer recommends a cortical drill for sites where more cortical bone is present. This cortical drill
reduces the stress forces exhibited on the cortical bony walls while inserting the implant. This might give
less surgical trauma and finally it may lead to a less pronounced remodelling of the cortical bone around
the shoulder of the implant. Lowering the stress forces on the bony wall could jeopardize the primary
stability but at the end it might lead to the preservation of the marginal bone level.
In the literature different invasive surgical procedures are reported to obtain an ideal three-dimensional
bony structure to place an implant in the prosthetic most ideal position. The drawbacks of these
procedures are more complications, longer treatment time, more costs, higher morbidity and postoperative discomfort for the patient (Milinkovic & Cordaro 2014). The question could rise whether in
cases without aesthetic demands these invasive treatments are justified when less invasive options are
available such as tilted implants (Menini et al. 2012), short implants (Annibali et al. 2012b), cantilever
extensions on implants (Aglietta et al. 2009) and the, rarely used, cantilever extensions on natural teeth
(Pjetursson et al. 2004, Tan et al. 2004). The present study indicates that bone grafting procedures can
be avoided when sites with ≤4.5 mm of bucco-oral bone width are available. Of course, it has to be taken
into account that all included cases were case without aesthetical demands.
Today, the aesthetic outcome of an implant positioned in the anterior area is of high priority for the
patient. Factors that could influence the aesthetical outcome are: optimal three-dimensional implant
planning, adequate bone volume around an implant, ideal soft-tissue dimensions and a stable bone level
(Belser et al., 2004) In the literature several different clinical guidelines are available for the implant
positioning in bucco-oral dimension. The idea for a perfect aesthetical outcome is a need of 1 or 2 mm of
bone thickness around an implant (Esposito et al. 2007, Grunder et al. 2005). Most of the patients in this
study have been treated with an overdenture. Here, the aesthetics are not very relevant because most of
the time it can be compensated with the dental prosthesis. Therefore, the implants were often placed
bone driven instead of prosthetic driven. In this study the aesthetical outcome of the implants were not
reported, but so far no gingival recession nor exposure of the implant has been encountered. An
important issue is the way an implant osteotomy is prepared. Today various tools can be used to
perform a very selective osteotomy (de Oliveira et al. 2007, Markovic et al. 2011, Nobrega et al. 2012,
Schlegel et al. 2003, Canullo et al. 2014, Kfouri et al. 2014). In this way it was possible to limit the amount
of implants, needing a simultaneous GBR procedure, to only 7. GBR procedures were only performed
when there was a buccal dehiscence or fenestration. At every implant site were the implant was
surrounded by a very thin layer of bone, no GBR procedures were performed. Bone condensation is a
very suitable technique for ridge expansion when used in type 3–4 bone. A study by Strietzel and
coworkers revealed an 0.8 mm difference between the alveolar ridge level and the implant shoulder at
the end of the unloaded healing period and a marginal bone loss of 1 mm after an average period of 6
months of functional loading, which was comparable to that of other techniques (Strietzel et al. 2002).
As most implants in this study were placed in the maxilla, bone condensation can performed rather
easily, but with care. In the mandible selective preparation using piezosurgery seems to be more
efficient.
All patients included in the study, were following a very strict maintenance protocol and as a result had a
high level of oral hygiene. This might not be the best reflection of an average patient in the daily
practice. Further studies are necessary to analyse the long-term stability of the bone level, the aesthetic
outcome and the bucco-oral bone changes around the implant. One of the drawbacks of the present
study is the lack of clinical data and the lack of a control group. However, this last one was overcome by
comparing the results of the study with the results of other studies using the same implant type in less
challenging types of patients.
Within the limits of this study, it can be concluded that implants placed in sites with limited dimensions
(≤4.5 mm vestibule-oral) can be successful for a period of 3 years, comparable to implants placed in
wider alveolar crests. This effect could be explained by the micro-threads in the conical and marginal
part of the implant system used as well as by the special drilling sequence avoiding bone compression.
These results showed that if patients are not ready for bone grafting procedures this treatment option
could be a good alternative.
Clinical Relevance
As the dimensions of the alveolar bone reduces after tooth loss, surgeons are often encountered with
limited bony dimensions in order to place oral implants. Definately in bucco-oral sense, 1-2mm
buccal/lingual bone is considered mandatory in literature. This article provides information on the
survival and marginal bone loss of oral implants placed in limited bucco-oral dimensions.