Buccal Bone Resorption Around Posterior Implants
After Surgery: A 1-Year Prospective Study
Toshio Takuma, DDS1,2/Keiji Oishi, DDS, PhD3/Tomofusa Manabe, DT2/
Satoshi Yoneda, DDS, PhD4/Toshihiko Nagata, DDS, PhD5
Purpose: This prospective study aimed to examine postoperative dimensional changes in the buccal boneand mucosa around single-stage implants placed in the posterior region. Materials and Methods: The
dimensions of peri-implant tissue around screw-type implants placed in the posterior region were examinedat surgery (baseline) and 6 months and 1 year after surgery. The lateral contour of the buccal bone andmucosa was horizontally measured at five vertical heights at 1-mm intervals (+1 to –3 mm from the implant
platform) using custom-designed instruments. Bone resorption on the proximal sides was assessed onradiographs. Mucosal recession was measured on plaster casts of the dentition. Results: Sixty-six implants
placed in 30 patients were examined. All implants were clinically osseointegrated and stable throughout the
study period. The buccal bone exhibited horizontal resorption throughout the study period, even at the mostapical height measured. Assessed at each height, thicker bone (> 2 mm thick) tended to exhibit horizontal
resorption during the first 6 months after surgery. However, the bone resorbed horizontally by approximately
0.4 mm during the final 6 months, irrespective of its contour. Vertical resorption of the buccal marginal
bone was approximately 1 mm during the period from 6 months to 1 year. The bone-retaining group at the
1-year time point was found to have thicker bone walls at baseline compared with the bone-loss group. The
thickness of the buccal mucosa showed little change. There was no obvious correlation between buccal
bone resorption and mucosal recession. Conclusions: The buccal bone exhibited both horizontal and vertical
resorption over the year after surgery. The initial contour of the bone was significantly associated with boneretention or loss at 1 year. However, mucosal recession was not directly affected by buccal bone resorption.
Int J Oral Maxillofac Implants 2014;29:634–641. doi: 10.11607/jomi.3018
Key words: buccal bone resorption, dental implants, mucosal recession, prospective study

M

aintenance of peri-implant tissue has been an
important issue in the field of implant dentistry1,2
ever since it was anticipated that osseointegration was
1Graduate student, Department of Periodontology andEndodontology, Institute of Health Biosciences,
The University of Tokushima Graduate School, Japan.
2Private Practice, Kagawa, Japan.
3Associate Professor, Department of Periodontology andEndodontology, Institute of Health Biosciences,
The University of Tokushima Graduate School, Japan.
4 Assistant Professor, Department of Periodontology andEndodontology, Institute of Health Biosciences,
The University of Tokushima Graduate School, Japan.
5Professor and Head, Department of Periodontology andEndodontology, Institute of Health Biosciences,
The University of Tokushima Graduate School, Japan.
Correspondence to: Dr Keiji Oishi, Department of
Periodontology and Endodontology, Institute of Health
Biosciences, The University of Tokushima Graduate School,
3-18-15 Kuramoto-cho, Tokushima 770-8504, Japan.
Fax: +81-88-633-7345. Email: ohishik@tokushima-u.ac.jp©2014 by Quintessence Publishing Co Inc.

highly likely. Maintenance of the level and shape of
peri-implant tissue facilitates easy cleaning of the fixed
prostheses, contributing to maintenance of marginal
tissue health. The health and stability of peri-implanttissue are key factors affecting the longevity of implants.3,4 Proper shape is also related to the esthetics
of implant-based prostheses. However, alveolar bonetends to exhibit both vertical and horizontal resorption after tooth extraction.5–7 In addition, it is wellknown that marginal bone loss around implants isradiographically observed soon after abutment placement.1,8,9 These trends may affect the level and shapeof peri-implant tissue.
The shape of peri-implant tissue may be closely
associated with the morphology of the underlying
alveolar bone.10 Marginal bone loss that has been reported on the proximal side11,12 was also anticipatedon the buccal side during the first 6 months after implant treatment. However, the shape and thickness ofthe buccal bone cannot be examined on a conventional radiograph. Although some researchers used
computed tomography (CT) to assess the buccal bone

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Takuma et al

thickness around implants,13,14 the accuracy of thistechnique was limited because of artifacts caused by
the implants.15,16 In another study,17 the critical thickness required to maintain the buccal bone wall was
proposed on the basis of a comparison between the
thickness in placement surgery and that in uncovering surgery. The study, however, did not examine theprogress of changes after uncovering surgery. Therefore, dimensional changes in the buccal bone around
implants have not been examined in detail because of
the lack of suitable measurement techniques.
To predict dimensional changes in peri-implanttissue after surgery, it is important to understand thegeneral progress of dimensional changes after treatment. This includes evaluation of the extent of horizontal and vertical dimensional changes in the buccal
bone after surgery as well as the thickness of buccal
bone required around implants to prevent its resorption after surgery. To address these clinical questions,the authors created a set of instruments to precisely
measure the lateral contour of the buccal bone andsoft tissue and planned a 1-year prospective cohort
study to assess dimensional changes in the buccal
peri-implant tissue in patients who received singlestage implants.

MATERIALS AND METHODSPatient Selection

The present study was conducted in the Departmentof Periodontology at Tokushima University Hospital(Tokushima, Japan) and Takuma Dental Clinic (Marugame, Japan) between April 2009 and June 2011. The
research protocol was approved by the Ethics Committee of Tokushima University Hospital (Approval No.
819) in accordance with the 1975 Helsinki Declaration
as revised in 2000. Consecutive patients visiting theclinics for implant treatment were recruited for thisstudy. Inclusion criteria were as follows: (1) age over 20
years, (2) planned treatment using posterior implants
without ridge augmentation, and (3) implant sites allowed to heal for more than 3 months after tooth extraction. Exclusion criteria were as follows: (1) poor
oral hygiene and uncontrolled periodontal disease;(2) present or past history of smoking18; (3) presenceof systemic diseases such as diabetes and osteoporosis; (4) pregnancy; and (5) history of chemotherapy, irradiation therapy, or long-term steroid treatment. All
participants provided written informed consent prior
to entry into this study.

Treatment Protocol

All patients received oral hygiene instructions and adequate periodontal treatment for the remaining teeth

before implant treatment. The precise position andangulation of implants were designed on CT imagesof jawbones using image analysis software (SimPlant,
Materialize). The vertical axis of the implant fixture wasapproximately in the center of the alveolar bone andfinal superstructures. The interimplant distance andimplant-tooth distance were greater than 3 and 2 mm,
respectively.19
All surgical procedures were performed by a single
surgeon (TT). Under local anesthesia, full-thickness
mucosal flaps were elevated, and implant holes were
drilled as planned using prefabricated surgical templates. Implants measuring 4.0 mm in diameter (dualacid-etched Osseotite NT, Biomet/3i)12 were placedin the planned positions such that their platforms
were approximately level with the bone surface. Healing abutments were screwed onto the implants, andwounds were sutured. This point represented thebaseline of this study.
The surgical sites in the maxilla and mandible wereallowed to heal for 4 and 3 months, respectively, without loading. Implants were loaded with provisional
restorations after the implant stability quotient (ISQ)
values were checked using Osstell Mentor (Osstell AB).
All patients received screw-retained definitive restorations 6 months after surgery. As part of the maintenance program, the patients were recalled 1 year after
surgery to check and clean the implants. The modified
gingival index (mGI) and modified plaque index (mPI)
were recorded at 6 months and 1 year after surgery. The
morphology of the buccal peri-implant tissue was evaluated at baseline and 6 months and 1 year after surgery.

Morphologic Evaluation of the Alveolar BoneThe lateral contour of the buccal bone was measured
at the five vertical heights defined with reference tothe implant platform (PF): PF + 1 mm, PF, PF − 1 mm,
PF − 2 mm, and PF − 3 mm (Fig 1a). These heights were
1 mm coronal to the platform, at the platform level,and 1 mm, 2 mm, and 3 mm apical to the platform, respectively. The height coronal to the platform (PF + 1mm) was measured to check the growth of the buccal
bone. At baseline, ie, just after implant placement, thelateral contour of the buccal bone was directly measured using the authors’ original handmade device(Figs 1b and 1c). The base of the instrument was fixed
on the implant with a screw to direct the guide holes
in the buccal direction. Next, a horizontal ruler component was inserted into the guide holes from the buccal side until resistance was encountered against the
buccal bone (Fig 1d). These rulers were graduated in
0.5-mm increments on the upper horizontal bar (seeFig 1b). The 0 mark appeared on the guide hole ofthe base of the horizontal bar when the apical tip wasplaced on the lateral surface of the implants. ThereThe International Journal of Oral & Maxillofacial Implants 635© 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY.
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Takuma et al

5

PF + 1 mm

Platform

(mm)
0

PF

PF + 1 mm

PF

PF – 1 mm

PF – 1 mma

PF – 2 mm
PF – 3 mmaBone

Mucosa

d

b

c

e

f

PF – 2 mm

PF – 3 mm

Figs 1a to 1f   Evaluation of bone morphology during surgery. (a) Measurements were obtained at five heights. (b) Device for measuring the lateral contour of the buccal bone: (left) base component, (right) ruler component. (c) Ruler components for the different
heights. (d) The base was fixed onto an implant with a screw. The ruler component was then inserted into guide holes on the base tomeasure the contour of the buccal bone. (e) Devices to measure the vertical height of bone around the implants, left to right: basecomponent, probe for measuring buccal walls, probe for measuring mesial/distal walls. (f) A probe was vertically inserted into a fixedbase to measure the vertical height of the buccal, mesial, and distal bone walls.

fore, the device could measure the distance between
the buccal bone surface and lateral implant surface ina horizontal direction. To measure the bone contour at
the five different heights, the authors used five ruler
components of different lengths (see Fig 1b). The scaleon the ruler could be accurately read under a surgicalmicroscope (Universa 300, Mölar-Wedel International)
at 8× to 16× magnification. Similarly, the vertical distance between the bone around the implants and theplatform was measured using the same base and vertical ruler components (Fig 1e). The ruler was inserted
from the top of the base to probe the mesial, distal,and buccal walls of the bone around the implants (Fig
1f ), and the vertical distance between the bone andplatform was measured on the scale on the ruler using
a surgical microscope. If the bone level was apical tothe platform, it was expressed as a negative value.
At the 6-month and 1-year time points, the contour of the buccal bone was measured using an individual guide fixed onto each prosthesis (Fig 2a). To
prefabricate the acrylic guide, the definitive prostheses were fixed with a screw on an implant analog at a4-mm height from the base before actual placementon the implant. The acrylic guides had a buccal platethat was parallel to the implant axis and contained
five 0.45-mm-diameter holes. Starting 1 mm fromthe bottom, the five holes were aligned vertically at
1-mm intervals such that each hole was at one of thefive defined heights from PF + 1 mm to PF − 3 mm
(Fig 2b). Self-curing resin was inserted between theacrylic guide and the prosthesis so that the guide wasfirmly fixed on the occlusal and buccal surfaces of theprosthesis (see Fig 2a). The distance between the implant surface and the buccal plate of the guide wasmeasured using a precise caliper and recorded. Afterthe prostheses were fixed onto the implants, sterilized
individual guides were seated on their crowns. Under
local anesthesia, a 27-gauge sterile needle (0.40-mm
diameter) with a rubber stopper was inserted into
each hole to sound the bone surface at each verticalheight (Fig 2c). The distance between the needle tip
and the rubber stopper was measured using a scalethat was read under a microscope. The distance between the implant surface and buccal bone surface ateach height was then calculated from these measured
values. A calculated value of more than 0.1 mm indicated the presence of buccal bone around the implant.
Vertical heights of the most coronal part of the buccal bone at 6 months and 1 year after surgery were
estimated from the buccal bone contour. For example,
when bone contours were 0 and 0.5 mm at PF and PF–1 mm, respectively, the height was estimated to be
between 0 and –1 mm.
All measurements were performed by a single surgeon (TT). Consistency between repeated measurements was within 0.2 mm in a preliminary study.

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bTakuma et al

Fig 2  Measurement of buccal bone andsoft tissue contour after crown placement.
(a) An acrylic guide with five holes was
prefabricated to fit the crown at a parallelorientation to the implant axis. Arrowheads
indicate the location of the platform. (b)Five holes were created on the guide at thesame heights from PF + 1 mm to PF − 3
mm. (c) Bone sounding was performed under local anesthesia using a 27-gauge needle inserted through a hole at each height.
(d) The contour of soft tissue was probed
on a plaster cast using the same guide.

a

b

c

d

Measurement of Horizontal Thickness andVertical Recession of the Buccal Mucosa
In order to measure the horizontal thickness of the
buccal mucosa at 6 months and 1 year after surgery,
impressions of the prostheses with alveolar ridges
were obtained to create plaster casts. The same guides
used to measure bone contour were seated on the cast
tooth crowns, and the cast surface was probed with a
27-gauge needle (Fig 2d). The distance between theimplant surface and alveolar surface at each verticalheight was calculated from the measured values. The
bone contour was subtracted from the distance, andthe resulting values were estimated to represent thehorizontal thickness of the buccal mucosa.
To assess vertical recession of the buccal mucosa after prosthetic treatment, the vertical distance from the
top of the crown to the margin of the buccal mucosa
was measured on plaster casts prepared 6 months and1 year after surgery. The extent of recession was calculated from these measurements.

Radiographic Examination

To examine marginal bone resorption on the proximalsides, periapical radiographs were obtained using theparalleling technique with a Rinn film holder (Dentsply-Rinn) and personalized bite blocks. All standardized
periapical radiographs were scanned and digitizedusing an image scanner (ES-2200, Epson) at 720 dpi
resolution. On these images, the distance between theimplant-abutment junction (IAJ) and the most coronalpoint of the bone-to-implant contact was measured
on the mesial and distal sides using Micro-Measuresoftware (Scalar). Measurements were calibrated on

PF + 1 mm
PF
PF – 1 mm
PF – 2 mm
PF – 3 mm

each image using the distance between two consecutive threads of the implant as a reference (0.9 mm).
These distances were used as an estimate of the extent
of bone resorption.

Sample Size Calculation

Sample size was calculated to ensure adequate power
for the comparison of baseline buccal bone contour
between the groups with (bone-retaining group) andwithout (bone-loss group) bone wall at 1 year. In the authors’ preliminary studies, standard deviations of buccal
bone contour were found to be 0.6 to 0.8 mm. Differences between groups were estimated to be 0.6 mm.17
The sample size was calculated to provide an 80% power with an α of 0.05, and it was found to be 17 to 29
subjects per group. On the basis of these calculations,more than 60 participants were recruited in this study.

Statistical Analysis

Linear regression analyses were used to examine differences between data obtained at two time points.
Spearman correlation coefficient (two-tailed) was used
to examine the relationship between changes in thevertical height of the buccal bone and recession of the
buccal mucosa. To identify any association between
initial contour and the fate of the buccal bone, subjects
were divided into two groups at each height with respect to the presence of buccal bone at 1 year. Buccal
bone contour at baseline was compared between thetwo groups using the Mann–Whitney U test. A P valueof < .05 was considered statistically significant. All statistical analyses were performed using PASW Statistics
software version 18.0 (IBM).
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Takuma et al

Fig 3  Lateral contour of the
buccal bone at each height
and time point. Distributions ofdata are shown using box plots.
The median data are indicatedby horizontal bars, the verticalbars indicate the range, thehorizontal boundaries of theboxes represent the first andthird quartiles, and the asterisks (*) represent the outliers.

Buccal bone contour (mm)

4.0–

3.0–

2.0–

1.0–

0.0–
Time (mo) 0 6 12HeightPF + 1 mm

0

6 12
PF

0 6 12PF – 1 mm

RESULTSA total of 66 dental implants were placed in 30 patients (26 women and 4 men; mean age, 47.3 ± 10.1
years; range, 30 to 63 years). All participants were
nonsmokers and were systemically healthy. Implants
were placed in molar/premolar regions in the maxilla(n = 16) or mandible (n = 50). Twenty-eight implants(42.4%) were placed in a region that had remainededentulous for more than a year after tooth extraction.
Of the remaining implants, 18 (27.3%) and 11 (16.7%)
were placed after waiting periods of 6 to 12 months
and less than 6 months, respectively, after extraction.
Just after implant insertion, the vertical height ofthe buccal bone was −0.14 ± 0.86 mm (n = 66; range,+1.6 to −3.5 mm). The mesial and distal bone heights
were 0.82 ± 1.07 mm and 0.62 ± 0.87 mm, respectively.
After a healing period of 3 to 5 months, all implants
showed an ISQ that was greater than 60, suggesting
osseointegration of the implants. These were utilized
for definitive restorations, and the superstructures
were placed 6 months after surgery.
All patients completed the follow-up. There wereno cases of implant rejection or severe complications.
Implants fulfilled the criteria of success proposed by
Albrektsson et al in 1986.11 The implants exhibited a
probing depth of 2.7 ± 0.6 mm (n = 66) on the buccal side. mPI and mGI scores were 0 for all implants,
implying that the patients maintained a good level ofmarginal tissue hygiene throughout the study.18,20

Lateral Contour and Vertical Heightof Buccal Bone

Figure 3 shows the distribution of data pertaining to
the lateral contour of the buccal bone at each height
and time point. At the platform (0 mm) level, buccal bone was present in 37.9% of the participants at

0 6 12PF – 2 mm

0 6 12PF – 3 mmbaseline, and its contour decreased during the first 6
months after surgery. At 1 year, only 6.1% participants
exhibited buccal bone walls. More apically, the median
contour values during the study period decreased to0.75, 0.60, and 0.60 mm at PF − 1 mm, PF − 2 mm, andPF − 3 mm, respectively. Figure 4 shows the differencesin buccal bone contour at the PF − 2 mm level between
the different time points. During the first 6 months after surgery (see Fig 4a), any buccal bone wall less than
1.5 mm thick maintained its contour; however, bonewalls more than 2 mm thick tended to resorb slightly.
In contrast, during the period from 6 months to 1 year
(see Fig 4b), the buccal bone resorbed horizontallyby approximately 0.4 mm, irrespective of its contour.
There were no significant differences in the amount ofhorizontal bone resorption between the maxilla andmandible (Table 1). Similar tendencies were observedin data for the PF − 1 mm and PF − 3 mm levels (datanot shown). Table 2 shows the number of participants
with each category of vertical buccal bone height during the study period. The distribution of participants
at baseline was almost unchanged at 6 months butchanged noticeably at 1 year. Changes in heights
ranged from 0 mm to −4 mm, with a median change of−1 mm during the final 6 months.
Table 3 shows a comparison of the buccal bone
contour at baseline between the groups with (boneretaining group) and without (bone-loss group) bonewalls at each height at 1 year. The bone walls at baseline were thicker in the bone-retaining group than in
the bone-loss group, and the difference was significant
for the PF − 1 mm and PF − 2 mm levels (P = .002). At
1 year, the median bone contour in the bone-retaining
group was 0.55 mm and 1.10 mm at the PF − 1 mm
and PF − 2 mm levels, respectively. This analysis was
not performed for the PF + 1 mm or PF − 3 mm levels
because of the lack of data in one group.

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3.0

3.0
Buccal bone contour at 1 year (mm)Buccal bone contour at 6 months (mm)Takuma et al

2.0

1.0

0.0
0.0

a

1.0

2.0

2.0

1.0

0.0

3.0

Buccal bone contour at baseline (mm)

0.0
b

1.0

2.0

3.0

Buccal bone contour at 6 months (mm)Fig 4   Differences in buccal bone contour at the PF − 2 mm level (a) between the baseline and 6-month after surgery time points
and (b) between the 6-month and 1-year after surgery time points. Regression lines and 95% confidence intervals are shown. Dottedlines indicate the y = x lines.

Table 1   Comparison of Horizontal Bone
Resorption Between the Maxilla
and Mandible
Bone resorption at thePF − 2 mm level (mm)Implant
position

Number of
implants

Baselineto 6 months*

Table 2   Vertical Height of the Buccal Bone:Distribution Comparison (n = 66)*
Number of subjects

Verticalheight (mm)Baseline

6 Months

1 Year

2

0

0

≥1

6 Months
to 1 year*≥0, <123

26

4≥−1, < 0

35

32

20
32

Maxilla16−0.27 ± 0.53

−0.42 ± 0.32

Mandible50−0.16 ± 0.47

−0.33 ± 0.24≥−2, <−14

8*No significant difference between maxillary and mandibular values.

≥−3, <−2

1

0

9

<−3

1

0

1

Horizontal Thickness and Vertical
Recession of Buccal Mucosa
Figure 5 shows the differences in horizontal thicknessof the buccal mucosa at the PF – 2 mm level between
the 6-month and 1-year time points. The horizontal
thickness of the mucosa remained unchanged during
the 1 year after surgery, independent of its initial thickness. The same results were obtained for the PF, PF – 1
mm, and PF – 3 mm levels (data not shown).
Vertical recession of the buccal mucosa during theperiod from 6 months to 1 year was 0.39 ± 0.14 mm (n= 66; range, 0.2–0.8 mm). There was no correlation between the change in buccal bone height and mucosal
recession (Spearman ρ, 0.191; n = 66; P = .124).

Radiographic Evaluation

At baseline, mesial and distal heights of bone-to-implant contact from IAJ were −0.3 ± 0.5 mm and −0.3 ±0.4 mm, respectively (n = 66). These heights decreasedby an average of 0.9 mm at 6 months and decreased
by a further 0.3 mm at 1 year. Mesial and distal boneheights at 1 year were −1.5 ± 0.4 mm and −1.5 ± 0.4
mm (n = 66), respectively.

*Median values are in bold.

Table 3   Comparison of Buccal Bone Contour
at Baseline Between Groups, Divided
According to the Presence or Absenceof Bone at Each Height at 1 Year
Bone contourat baseline (mm)Height

Bone
walls

PF

Present

4

Lost

21

Present

22

Lost

38

0.65

0.2 to 2.1

Present

53

1.50

0.5 to 3.2

Lost

11

0.80

0.3 to 2.4

PF − 1 mm
PF − 2 mm

Range

P
value

1.10

0.5 to 1.9

.18

0.60

0.2 to 1.5

1.30

0.2 to 2.8

Number Median

.002*
.002*

*Significant difference (P < .05).

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Takuma et al

6.0Soft tissue thickness at 1 year (mm)

5.0
4.0
3.0
2.0
1.0
0.0
0.0

1.0

2.0

3.0

4.0

5.0

6.0

Soft tissue thickness at 6 months (mm)Fig 5   Differences in buccal soft tissue thickness at the PF − 2
mm level between the 6-month and 1-year time points. Regression line and 95% confidence intervals are shown. Dotted line
indicates the y = x line.

DISCUSSION
In this study, the authors measured the buccal contour of peri-implant bone and mucosa at prescribedtime points during 1 year after implant surgery using
measuring instruments that they designed. To thebest of the authors’ knowledge, this is the first study
to directly measure dimensional changes in the buccal bone and mucosa after surgery. The following
findings were noted: the buccal bone exhibited horizontal resorption during the study period at all vertical heights measured; the most coronal point of the
buccal marginal bone exhibited vertical resorption of
approximately 1 mm during the final 6 months; thickbuccal bone at baseline tended to persist, albeit with adecreased thickness, at 1 year; and the buccal mucosa
exhibited recession of approximately 0.4 mm during
the period from 6 months to 1 year.
The buccal bone at vertical heights 0 to 3 mm apicalto the platform resorbed horizontally for 1 year. In addition, when assessed at each height, resorption wasobserved regardless of the initial contour of the buccal
bone after surgery. These findings indicate that bone
resorption occurred on the buccal surface and was not
limited to the marginal area around the implants. The
authors’ findings suggest that the reformation of biologic width8,21 is not the only cause of the observedbone resorption. Effects of flap surgery on bone homeostasis have been suggested in the literature.22–24
This could be a cause of the buccal bone resorption
observed in our study, while it is uncertain whether theeffects of surgery persisted even after 6 months. Another possible interpretation may be the physiologic
remodeling process of the edentulous alveolar ridge.

Buccal marginal bone resorbed vertically by approximately 1 mm during the period from 6 months
to 1 year. However, marginal bone loss on the proximalsides was primarily observed during the 6 months after surgery, as reported in the literature.9,11,12 This discrepancy in timing was probably due to differences inmeasuring points, bone-to-implant contacts on radiographs, and the most coronal point of the bone wallon direct measurements. In fact, proximal bone levels on baseline radiographs were 0.9 to 1.1 mm lower
than those measured during surgery. If marginal boneloss could be associated with the platform level,21,25
it is reasonable to assume that marginal bone would
be evenly resorbed at the buccal side of the implants.
These findings suggest that marginal bone resorption may have started on the implant surface withoutchanges in the most coronal point of the buccal bone(baseline to 6 months) and that the point started toresorb later (6 months to 1 year). Initial contour andhorizontal resorption of the buccal bone also played apotential role in this process.
The thick buccal bone present at baseline tended topersist at 1 year, albeit with a decreased thickness. Between the bone-retaining group and bone-loss group,
the ranges of the initial contour almost overlapped, implying that there was no clear threshold of initial contour for determining the results. However, there was asignificant difference between the median initial contour values between the two groups, suggesting thatthick bone has the advantage of retaining the bonewalls after treatment. If the median initial contour value
in the bone-retaining group was used as a reference, abuccal bone contour of more than 1.5 mm at surgery
would be preferable for retaining the bone wall after
1 year. Spray et al reported the influence of facial bone
thickness at the first surgery of a two-stage protocol on
the marginal bone response observed at the second
surgery and proposed a critical thickness of 1.8 to 2.0mm for maintaining bone thickness.17 They did not assess the facial bone after surgery; however, the resultsof the current study suggest that bone with a thickness
greater than the critical thickness may remain present
with a slightly decreased thickness after treatment.
Dimensional changes in peri-implant tissue on the
buccal side are clinically observed as vertical recession
or growth of the marginal mucosa. The buccal bone
resorbed vertically by approximately 1 mm during the
6 months after prosthetic treatment; however, recession of the marginal mucosa was approximately 0.4
mm over the same period; these measurements were
not correlated. Benic et al13 reported a similar discrepancy between the levels of marginal bone and mucosa
around implants after 7 years of function. These findings suggest that other unknown factors affect thelevel of the marginal mucosa. However, buccal bone

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Takuma et alloss may be a risk factor for recession in the long term
because mucosa does not firmly integrate with the implant surface.
This study has two limitations. First, the participants were not evenly distributed with regard to age,sex, and implant location. These factors could haveaffected dimensional changes in the peri-implant tissue after surgery26; however, the authors were unable
to include enough participants to assess the effectsof these factors. Second, the study was a prospective
cohort study, not a randomized controlled trial. The
authors’ results showed an association between theinitial buccal bone contour and the fate of the bone;
however, the association of unknown confounding
factors could not be ruled out.

CONCLUSIONS
Within the limits of the present prospective study, theresults highlight the following:
1. Buccal bone resorbed on its surface by approximately 0.6 mm in the horizontal direction over 1
year after surgery.
2. Vertical height of the buccal bone decreased by approximately 1 mm during the period of 6 months
to 1 year after surgery.
3. Thick buccal bone at baseline tended to persist at 1
year, albeit with a decreased thickness.
4. The buccal mucosa receded by approximately 0.4
mm over 6 months after prosthetic treatment,
and it exhibited no correlation with buccal bone
resorption.

ACKNOWLEDGMENTs
The authors reported no conflicts of interest related to thisstudy.

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