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Dimensional changes in alveolar bone following extraction of maxillary molars in humans: a retrospective CBCT analysis

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Dimensional changes in alveolar bone following extraction of maxillary molars in humans: a retrospective CBCT analysis

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Content i
UNIVERSITY OF SOUTHERN CALIFORNIA
OSTROW SCHOOL OF DENTISTRY

DIMENSIONAL CHANGES IN ALVEOLAR BONE FOLLOWING EXTRACTION
OF MAXILLARY MOLARS IN HUMANS: A RETROSPECTIVE CBCT ANALYSIS

BY
MATTHEW R. SOMERVILLE

A thesis submitted in partial fulfillment of the degree of
Master of Science in CRANIOFACIAL BIOLOGY

Degree awarded: May 2017

ii
Authors and affiliations
Matthew R. Somerville DDS
Graduate student, candidate for Master of Science in CBY
Division of Periodontology, Diagnostic Sciences and Dental Hygiene
Ostrow School of Dentistry, University of Southern California
925 W 34
th
St Room 4278 Los Angeles CA 90089
Neema Bakhshalian DDS PhD
Research Associate
Division of Periodontology, Diagnostic Sciences and Dental Hygiene
Ostrow School of Dentistry, University of Southern California
925 W 34
th
St Room 4278 Los Angeles CA 90089
Seiko Min DDS PhD
Research Associate
Division of Periodontology, Diagnostic Sciences and Dental Hygiene
Ostrow School of Dentistry, University of Southern California
925 W 34
th
St Room 4278 Los Angeles CA 90089
Homayoun H. Zadeh DDS PhD
Associate Professor
Division of Periodontology, Diagnostic Sciences and Dental Hygiene
Ostrow School of Dentistry, University of Southern California
925 W 34
th
St Room 4278 Los Angeles CA 90089
Correspondence
somervim@usc.edu

iii
TABLE OF CONTENTS
Authors and Affiliations.………………………………………………………………….ii
Table Legend…………..…………………………………………………………………iv
Figure Legend………………..………………………………..…………………………..v
Abstract………………………………………………………………………………….vii
1. Introduction….…………………………………………………………………………1
2. Methods…………………………………………………………………………..…….4
3. Results…………..………..……………………………………………………..……...8
4. Discussion………………………………………………………………………..…...10
5. Conclusion……………………………………………………………………………14
6. Tables…………………………………………………………………………………15
7. Figures……………………..…………………………………………………………16
8. References……………………………………………………………………………21

iv
Table legend
Table 1. Clinical characteristics of study patients and sites.

v
Figure legend
Fig 1: Landmarks used for linear measurements of alveolar bone width and height
identified in pre-and post-extraction CBCT images. Representative CBCT cross-sectional
views generated from pre-operative (A) and post-operative (B) imaging. Measurements
with the following parameters are annotated: 1) Horizontal green lines demarcating levels
(1, 2, 3 and 5 mm from the osseous crest in an apical direction), at which horizontal
alveolar ridge widths were measured; 2) vertical blue lines indicating locations (buccal,
palatal and midcrestal) at which alveolar ridge heights were measured; and 3) horizontal
red line references stable landmarks used in pre- and post-op scans for vertical height
measurements at the two time points.
Fig 2: Frequency of included molar teeth with CEJ to alveolar bone crest distance
measuring <2mm or >2mm.
Fig 3: Alterations in the horizontal dimensions of the crestal 5mm of the alveolar bone
following extraction of maxillary molars. Mean change in horizontal width of the
alveolar bone from pre- to post-extraction (A). Percentage of the initial horizontal bone
lost following tooth extraction (B). The measurement points (Y-axis) represent positions
relative to the pre-extraction alveolar crest.
Fig 4: Changes in vertical dimensions of the alveolar bone following extraction of
maxillary molars. Mean change in vertical height of the alveolar bone from pre- to post-
extraction (A). Percentage of the initial vertical height of the alveolar bone lost following
tooth extraction (B). The measurement points (X-axis) represent positions relative to the
pre-extraction alveolar crest.
Fig 5: Total alveolar bone height measured from crest to maxillary sinus at pre- and post-
extraction time points. Measurements made at three locations, namely, buccal, mid-
crestal, and palatal.
Fig 6: Frequency of available alveolar ridge width following post-extraction healing. The
ridge width dimensions represent mean of the measurements made at the crestal 5mm of
the alveolar bone.
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Figure 7:
Frequency of available alveolar ridge height following post-extract healing. Mean bone
in mm from crest to sinus was found to be 8.65 + 4.19mm.
Figure 8:
Frequency of available alveolar ridge height following post-extract healing, representing
likelihood of bone being within 6-8 mm width range.
Figure 9:
Scatterplot representing crest-to-sinus distance pre- and post-extract at buccal, mid-
crestal and palatal. Horizontal black lines denote the mean of each data series.
Figure 10:
Correlations between alveolar plate thickness and dimensional changes of the ridge
following extraction of maxillary molars. The thickness of the buccal (blue) and palatal
(red) bone measured at 1mm apical to the crest was correlated to post-extraction
dimensional changes in linear vertical height (A), percentage of vertical height loss (B)
and percentage of ridge width resorption (C). Correlations between alveolar plate
thickness and linear vertical height loss in buccal (R=-0.111, p=0.596) and palatal
(R=0.009, p=0.967), percentage of vertical loss in buccal (R=-0.254, p=0.231) and palatal
(R=0.208, p=0.329), as well as percent horizontal bone loss in buccal (R=-0.254,
p=0.231) and palatal (R=-0.251, p=0.237).

vii
Abstract
Objective: To investigate the dimensional changes in the alveolar crest following
extraction of maxillary molars and unassisted healing by performing linear measurements
on cone beam computerized tomography (CBCT) images.
Materials and Methods: The population of patients who had presented to the Ostrow
School of Dentistry of USC for extraction of maxillary first or second molars between
March of 2009 and June of 2015 were quarried. Inclusion criteria consisted of the
availability of CBCT taken at both before and after extraction. Exclusion criteria
consisted of ridge preservation grafting. Twenty-two patients were identified that had
required a total of twenty-four teeth to be extracted. Linear measurements were
performed on CBCT images using SIMPLANT PRO 6.0 software. Fixed anatomic
locations were used as reference points to compare pre- and post-extraction dimensions
of the alveolar bone. The buccal and palatal plate thickness, horizontal width at 1, 2, 3
and 5 mm apical to the crest, as well as, ridge height at buccal middle of crest, were
measured on pre-operative images. The distance from bone crest to CEJ on buccal and
palatal aspects of roots wear measured. The pre-operative landmarks were transferred to
post-operative images, using standardized reference points in order to repeat the
measurements on the ridge width and height at same positions.
Results: Quantitative analysis of CBCT images demonstrated that the mean horizontal
thickness of the alveolar bone in the maxillary molar area was 9.8 + 4.9, 12.2 + 4.2, 13.0
+ 3.4, 13.5 + 1.7 mm at 1, 2, 3 and 5mm apical to the alveolar crest, retrospectively.
Following a healing period of 10.9 + 11.5 months, post op ridge remodeling was
measured as being 87.8% + 26.3, 71.9% + 30.4, 56.0% + 31.7 and 28.7% + 33.2% of the
original width of the alveolar bone at 1, 2, 3 and 5mm apical to the alveolar crest,
retrospectively. The alveolar crest also underwent 2.4 + 1.4mm of vertical bone loss.
Statistical analysis demonstrated the following correlations to be statistically significant:
bone to CEJ distance on the palate and vertical palatal resorption (P=0.01); mean
horizontal resorption and palatal plate thickness (P=0.01); mean horizontal resorption
(P=0.0001); and mid-crestal vertical resorption (P=0.0001).
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Conclusions: Extraction of maxillary molar teeth without additional intervention led to
extensive horizontal and vertical bone atrophy that extended at least up to 5mm apical to
the original crest. The extensive loss of alveolar bone can potentially compromise
implant therapy, requiring additional augmentation procedures.

1
Introduction
The earlier events in bone healing are well established in the literature (Chen et al 2004,
Cardaropoli et al 2006 and Covani et al 2011). The beginning sequence of events
consists of a blood clot that is replaced by granulation tissue, 1 week later, with
subsequent conversion to osteoid at the bottom of the socket. It was determined in these
investigations that a blood clot is replaced by granulation tissue over a few days, then a
week later there is a cell-rich provisional matrix. At two weeks, the socket has been
filled with woven bone and, at two months, mature bone is present. However, while
these represent gross observations of the early healing stages, primarily from canine
studies, there is still great variability in the rate of bone healing in the socket.
One of the challenges for the clinician is whether the healing events following tooth
extraction can yield adequate bone volume, suitable for implant placement. The decision
tree includes unassisted healing of post-extraction socket, alveolar ridge preservation
(ARP) immediately following extraction or assessment of the post-extraction healing to
perform alveolar ridge augmentation (ARA) procedures in case the healed bone volume
is not suitable for implant therapy.
It has been demonstrated in multiple animal models and clinical investigations that the
alveolar ridge undergoes marked dimensional changes following tooth extraction (Araujo
and Lindhe 2005, Cardaropoli et al 2005, Berendsen et al 2009, Vignoletti et al 2011,
Ryu et al 2015, Min et al 2016, Zadeh et al 2016). While many clinical studies have
investigated post-extraction dimensional alterations of the alveolar bone (Barone et al
2008, Engler Hamm et al 2008, Januario et al 2011, Barone et al 2012, Chappuis et al
2013, Abdelhamid et al 2015, Misawa et al 2015, and Zadeh et al 2015). These clinical
investigations that have either not isolated molars as their own group, not reported on
maxillary molars, or excluded molars. To address the paucity of data in this area, the
present investigation sought to answer questions about what will happen to the crest of
the maxillary molar if healing proceeds without such additional intervention.
For a wide variety of reasons, ensuring the presence of ample bone following tooth
extraction is a relevant topic. Since extraction of teeth is one of the most commonly
performed dental procedures, increasing the predictability of implant placement is

2
important. Lekholm et al 1986 put forth that successful implant therapy depends upon
adequate volume of bone at the site of implant placement because the long-term
prognosis of dental implants is adversely affected when bone volume is inadequate.
Because bone quantity is highly relevant in implant dentistry, it is important to consider
what the available literature says about bone healing.
A recent consensus report determined the expected post-extraction resorption to be 3.8
mm in the horizontal dimension and 1.24 mm in the vertical dimension (Hämmerle et al
2011). The expected time frame for ridge alterations is also important. Although the
prospective clinical study Schropp et al 2003 reported that alterations mostly occur in the
first 3 months post-extraction, those changes can still continue throughout one entire
year. Moreover, according to Tan et al 2011, there is a predictable loss of horizontal
dimension (weighted mean 3.79 mm) and vertical dimension (weighted mean 1.24) after
6 months of unassisted healing. Therefore, while the evidence for healing time frames
for human teeth in general is strong, unassisted healing in maxillary molars has not yet
been investigated.
A number of specific strategies to manage extraction sockets have been reported. These
types of interventions have included immediate implant placement (Paolantonio et al.
2001), ridge preservation, (Cardaropoli et al. 2005; Araújo et al. 2006; Fickl et al. 2008a;
Fickl et al. 2008b; Fickl et al. 2008c; Araújo et al. 2010; Vignoletti et al. 2012; Barone et
al. 2013; Abdelhamid et al. 2016; Zadeh et al. 2016, Scheyer et al. 2016), root retention
(Garver and Fenster 1980) and socket-shield technique (Hürzeler et al. 2010).
A very recent cohort study (Koutouzis et al 2017) compared frequency of implant
exposure between bone-grafted sites that had been intact versus bone-grafted sites that
had either partial or complete facial plate compromised during extraction. This group
found no statistically significant correlation between compromise site and non-
compromised sites. However, they did note that 26.3% of premolar and 28.5% of
anterior sites had exposed implant surfaces. This finding, combined with their comment
that some dimensional alterations are to be expected even after preserving the ridge post-
extract in a non-compromised socket, reinforces the idea that ARP should be considered.

3
Ridge preservation therapies have been reported to be effective (Wang and Lang 2012,
MacBeth et al. 2016, Iocca et al. 2016). Vignoletti et al 2011 reported the achievable
reduction in resorption to be 1.84 mm in width and 1.47 mm in height. Several recent
studies also compared unassisted healing with ridge preservation using anorganic bovine
bone minerals (ABBM). Specifically, several investigations reported a volumetric ridge
contour change of over 90% and alveolar bone loss of over 67% in the crestal 3 mm of
the alveolar ridge (Abdelhamid et al. 2016; Zadeh et al. 2016). Ridge preservation
according to those studies using ABBM maintained over 50% of ridge contour volume
and 75% of the alveolar bone volume 6 month following ridge preservation. Although
controversies still exist about whether ridge preservation is effective, recent systematic
reviews (Avila Ortiz et al 2014, Mardas et al 2015, De Risi et al 2015) have commented
that ridge preservation reduces the magnitude of dimensional change following
extraction.
One very recent investigation (Walker et al 2017) reported on post-extract dimensional
changes following molar extraction, but it did not specifically report on maxillary molars.
This group reported mid-socket horizontal dimension change ranging from 0.53 + 0.92
mm to 3.11 + 3.83 mm, with a mean buccal height change ranging from 2.33 + 1.72 mm
to 3.01 + 2.24 mm.
Although a number of recent clinical investigations assessed alveolar bone dimensional
change in both anterior teeth and posterior teeth (Barone et al 2012, Engler-Hamm et al
2008; Barone et al 2008, Chappuis et al 2013; Januario et al 2011, Misawa et al 2015,
Zadeh et al 2015), no investigation has reported specifically on dimensional changes
associated with maxillary molar extraction. This retrospective CBCT analysis therefore
aimed primarily to address this gap in the literature by describing the dimensional
alterations that occur in the alveolar crest following extraction of maxillary molars in
humans. Specifically, this investigation sought to determine whether any relationship
exists between several key variables: 1) alveolar plate thickness 2) distance from CEJ to
alveolar crest pre-extraction and 3) crestal dimensional change. The null hypothesis of
this investigation was that the alveolar ridge remains unchanged following extraction of
maxillary molars in human.

4
Methods
The methodology of this investigation was designed to reproducibly measure dimensions
of the alveolar crest pre- and post-extract by using a three-point technique.
IRB
All steps were performed in accordance with an IRB protocol approved exempted by the
University of Southern California that aimed to collect clinical notes on patient care at the
Ostrow School of Dentistry and access CBCT scans for those patients.
Screening and inclusion/exclusion criteria
A dental school patient population was screened for those patients with a CBCT taken
both before and after extraction of a maxillary molar where healing had been unassisted
(no bone grafting performed). Approximately five thousand records of patients of the
Ostrow School of Dentistry were found with CBCTs available for examination. That
pool of patients was screened further for those with a record of a maxillary molar
extraction between two CBCT scans at the School. After inclusion and exclusion criteria
were applied, twenty-two patients met final criteria as appropriate for measurements.
Inclusion criteria consisted of 1) adult patients ages 18 to 75 years old and 2) CBCT
taken both before and after extraction of a maxillary first or second molars. Exclusion
criteria consisted of 1) third molars, 2) adjacent extractions, 3) situations rendering
reliable measurements impossible (ie no possibilities for reference points in same
quadrant and 4) history of surgery at that site.
Time frame for CBCTs taken spanned from March of 2009 to September of 2016. With
the exception of three teeth that had CBCT only 2 months post extraction, they were
taken at least 3 months (10.9 + 11.5 months) after extraction was performed. As
represented in Table 1, twenty-two patients (10 M/ 12F, mean age 71 + 10.9 years)
contributed twenty-four maxillary molars. Two patients contributed two teeth. Table 1
demonstrates the breakdown by teeth number.

5
Creating landmarks
As shown in Figure 1, linear measurements were performed using SIMPLANT PRO 6.0
software (Dentsply Implants, Waldham, MA, USA). DICOM files were imported into
the software for measurements. CBCTs were utilized with a slice thickness of 0.296 mm.
Two point linear measurements with fixed anatomic locations were used to ensure
reproducibility of measurements.
DICOM files were identified which included maxillary molars of patients meeting the
inclusion and exclusion criteria. Those files were extracted from the USC database and
exported into SimPlant. At that point, files were screened. Following creation of an
appropriate panoramic curve (panoramic curve points carefully defined in central fossae
of all teeth), axial views were adjusted so that the long axis of the tooth was vertically
oriented. Such reference lines were created using three points that were made constant
between measurements by using carefully determined landmarks as follows: the CEJ or
cusp tip of an adjacent tooth, the CEJ or cusp tip of a second adjacent tooth, and the
mesiodistal location of the subject tooth. Generally when possible, the cups tips of
adjacent teeth were used as a way to minimize error most effectively. However, what is
key is that whatever landmarks were determined at pre-op as most appropriate were
repeated exactly at post-op.
Measurements
As demonstrated in Figure 1, two aspects were examined for alveolar dimensional
alteration: the horizontal dimension and the vertical dimension. Each will be discussed
separately.
Horizontal dimensional change
Pre-extraction horizontal alveolar bone thickness was measured at 1 mm, 2 mm, 3 mm
and 5 mm apically from the crest. The first step in performing a horizontal measurement
was to identify the reference point in CBCT cross-section, both distally and apically from
the landmark. In this way, it was possible to reproduce where the alveolar crests were in
the post-extract CBCT. Measurements were taken to assess the thickness of the alveolar

6
plate on the buccal and on the palatal side. This is demonstrated by the blue lines in
Figure 1. Reference increments were used as a ruler to ensure that measurements were
made with very low error at the 1, 2, 3 and 5 mm marks.
Vertical dimensional change
Reference points were created in the same way as they were for horizontal resorption.
Once the pre-extraction landmarks were identified, vertical measurements were taken at
three points. Vertical resorption measurements were performed using the location of the
panoramic curve as a mid-crestal landmark, the buccal cortical plate as a buccal
landmark, and the palatal cortical plate as palatal landmark.
Bone to CEJ distance
CEJs were identified by verifying in both the cross section and panoramic views using
the reference line in SimPlant. Wherever buccal bone posed a measurement challenge by
being particularly thin, those alveolar crest levels were identified by examining the
CBCT both in the cross section view and in the three dimensional view.
Thickness of buccal and palatal plates
Locations of the alveolar crest were verified similarly as for bone to CEJ distance.
Measurements were performed at intervals corresponding to locations for the horizontal
thickness.
Vertical dimension of the crest
In an effort to provide results particularly relevant to implant dentistry, distance from pre-
extract crest to maxillary sinus floor and from post-extract crest to maxillary sinus floor
was determined.
Statistical analysis
Means and standard deviations were calculated for all described measurements.
Descriptive statistics using SPSS Version 24 were performed for all variables. Results
were reported as mean+SD. The paired sample t-test was used to compare the horizontal

7
and vertical dimensions of the ridge pre- and post-extraction. Bivariate correlations were
sought to identifying any statistically significant correlation between bone to CEJ
distance (both mean and specific location) and mean bone loss in either the horizontal or
the vertical dimension.

8
Results
Clinical characteristics of patients
As shown in Table 1, a total of 24 teeth from 22 patients were identified that had on
record at USC a CBCT taken both pre- and post-extraction. Dates of CBCTs ranged
from April of 2009 to September of 2016. Two patients had two non-adjacent teeth
extracted, with the remainder of patients being single tooth cases. Mean healing time
was 10.4 + 11.5 months, with a range of 2 to 48 months. 56% of patients were female,
44% male. Mean age was 71.0 + 10.9 years, with a range of 48 years to 87 years. Only
one patient was ASA 1 with all others being ASA 2. Nineteen patients were never
smokers and three had previously smoked but were not smoking at the time of treatment.
Pre-op CEJ to alveolar crest
As described in Figure 2, most teeth had a mean bone to CEJ distance of less than 2 mm,
therefore showing no radiographic evidence of periodontal attachment loss.
Horizontal resorption
The mean horizontal resorption at all four intervals (1, 2, 3 and 5 mm from the crest) was
found to be 6.6 + 3.7 mm. Mean horizontal resorption at each interval was measured as
8.5 + 5.2 mm, 8.5 + 4.9 mm, 7.2 + 4.7 mm, and 3.8 + 4.6 mm (at 1, 2, 3, and 5 mm from
the crest, respectively). Thus, as shown in Figure 3, the greatest degree of alveolar bone
resorption was observed at the alveolar crest and at 1 mm, while the magnitude decreased
as the measurements were made in more apical direction. This yielded a final mean
horizontal resorption of 6.6 + 3.7 mm and a percent mean horizontal resorption of 57.5 +
26.1%.
Vertical resorption
As demonstrated in Figure 4, site-specific measurements include a mean resorption at the
buccal of 2.9 + 1.8 mm, at the mid-crestal of 2.0 + 1.8 mm, and at the palatal of 2.8 + 1.7
mm. Thus, it was demonstrated that the crest underwent the greatest resorption at the
buccal component, with the mid-crest being the least. The mean (buccal, mid-crest and

9
palatal) vertical resorption was 2.4 + 1.4 mm, with a range of 27.3% to 35.6%. Percent
of remaining bone post-extraction was 28.55% on the buccal crest, 27.3% on the mid-
crest, and 35.58% on the palatal, yielding a mean percent resorption of 30.48%. Figure 4
demonstrates the comparative breakdown of crestal resorption by percent.
Resultant amount of bone available vertically following extraction
As outlined in Figure 5, there was mean of 8.65 + 4.19 mm of bone present vertically
following extraction (buccal mean 9.42 + 4.4 mm, mid crestal mean 8.45 + 4.7 mm,
palatal mean 8.1 + 4.2 mm), with an average vertical resorption of 30.5%. Mean
remaining bone from crest to sinus was 9.39 + 4.46 mm on buccal, 8.46 + 4.66 mm mid-
crest, and 8.05 + 4.13 mm at the palate. Remaining bone from crest to sinus (mean of
buccal, mid-crest and palate) ranged from 3.30 mm to 20.40 mm. Seven teeth (29.2%)
showed less than 6 mm of mean remaining bone from crest to sinus.
Resultant crestal widths following extraction
As shown in Figure 6, most crest widths ended up being 4-6 mm, with 6-8 mm and 0-2
mm being the next most frequent.
Relationship between alveolar plate thickness and mean resorptions (% vertical, vert in
mm, and % horizontal)
Correlations
A regression model was run to look at relationships among all parameters. An
observation that was made was that the bone to CEJ distance on the palate correlated with
amount of vertical palatal resorption (P=0.01). It was also observed that the mean
vertical resorption (2.4 mm + 1.4 mm) did correlate (P=0.05) with the mean horizontal
resorption (7.0 mm + 3.6 mm). In addition, the mean horizontal resorption correlated
with thickness of the palatal plate (P=0.01). Interestingly, however, the distance between
bone and CEJ on the buccal did not correlate with buccal vertical resorption (P=0.929).

10
Discussion
In this retrospective investigation, three point references were used in CBCTs of patients
pre- and post-extraction to describe dimensional changes that had occurred in the alveolar
processes of maxillary molars. It was determined that in cases of unassisted healing, vast
dimensional changes in the ridge occur in both the horizontal and the vertical directions.
This finding is in agreement with the few studies that have assessed crestal dimensional
changes that included maxillary molars in their control groups (Zadeh et al 2015;
Abdelhamid et al 2015). This restrospective investigation of unassisted healing in
twenty-four maxillary molars found the following: mean (resorption at 1, 2, 3 and 5 mm
points) horizontal resorption of 7.0 + 3.6 mm, mean (buccal, mid-crestal and palatal)
vertical resorption of 2.4 + 1.4 mm, and a mean percent vertical resorption of 30.5% post-
extraction. At the mid-crest, the mean vertical remaining bone in mm was found to be
8.45 + 4.7 mm. In most cases, the width of the crest post-extract was found to be
between 4 and 5.9 mm.
With these findings in mind, it is important to compare with previous work. Several
clinical investigations evaluating dimensional change (Zadeh et el 2015, Abdelhamid et
al 2015, Barone et al 2012) included maxillary molars in their control groups, but they
did not separately report maxillary molars.
In a report of a canine model (Ryu et al 2015), the alveolar bone width at 1 mm
completely resorbed by 12 weeks and saw resorption (baseline 6.2 + 0.9) to 4.7 + 0.3 by
week 12 at the 5 mm point. Over the same time frame, the buccal vertical dimension
decreased from 4.9 + 0.9 to 2.6 + 0.9, the mid crestal from 4.8 + 1.0 to 3.3 + 0.7, and the
lingual 4.9 + 1.0 to 3.1 + 0.6. Interestingly, this study saw greater resorptions in the
horizontal dimension of the crest but not as much resorption in the vertical dimension.
Another recent investigation in a non-human primate model (Min et al 2014) also
provided evidence that tremendous resorption occurs post-op. This paper represented
percent of remaining bone post-extract, measuring both the horizontal resorption as well
as vertical loss, at both 6 weeks and 12 weeks. In the control group in Min at al 2014,
percent remaining bone at 6 weeks ranged from 18.9 + 23.3% at 1 mm to 91.5 + 7.2% at

11
the 5 mm point from the crest. At 12 weeks however, it ranged from 33.5 + 30% at 1
mm, to 64.4 + 13.7% at 2 mm, to 77.7 + 6.3 at 3 mm, and finally 89.8 + 5.1% at 5 mm.
At 6 weeks, vertical bone height ranged from 52.8 + 17.9 at the buccal to 85.6 + 10.7% at
the lingual part of the ridge. At 12 weeks, the percent resorption from baseline ranged
from 49.2 + 18.9% at the buccal, 84.2 + 9.2 at palatal, to 86.9 + 9.1% at the middle.
Although this study did not group based on time since extraction, they can still be
compared. At 1 mm from the crest, if comparing with 12 weeks of resorption in the NHP
model, this study found 87.84 + 26.32% resorption. At 2mm, this investigation found
71.94 + 30.44% resorption. At 3 mm, resorption was 56.03 + 31.71%, and at 5 mm was
28.70 + 33.15%. The mean (of all four measurements) in this study was therefore
61.13%. It can be concluded that, with the exception of the most crestal portion (84.57%
vs 33.5%), this investigation found less horizontal resorption than Min et al 2014. This
investigation’s mean buccal remaining percent was 71.45 + 16.81%, 72.69 + 20.15% at
mid-crest, and 64.42 + 18.65 for palatal.
How dimensional changes compare between the studies therefore depends on the site.
This investigation saw greater remaining bone on the buccal, with less at the mid-crest,
and also less at the palatal. Therefore, this study saw varying results when compared
with Min et al 2014.
These findings are meaningful in a number of different ways. First, the resultant
dimensions of alveolar bone are pertinent to treatment planning for dental implant
therapy since the dimensions of the alveolar housing directly limit the length and width of
the implant that can be placed. Carefully determining the widest implant that can be
placed is important because stress is reduced more effectively by increasing implant
width than by increasing length (Himmlova et al 2004). For example, if maintaining at
least 1 mm of alveolar bone thickness on both the buccal and the palatal is prudent for
avoiding dehiscence, this would restrict the clinician to a 4 mm implant platform.
These findings can directly translate to clinical practice in terms of determining any
needs for augmentation procedures. Firstly, considering the importance of at least 1 mm
of bone both buccal and palatal to an implant, a ridge would need to be 6 mm post-op for

12
successful placement of a 4 mm wide implant. As Figure 6 shows, the frequency at
which post-op ridge widths are 6 mm or greater is only 38%. Secondly, as demonstrated
in Figure 8, if a practitioner prefers to place an implant of 10 mm length and it is assumed
that mean residual bone from crest to sinus floor must be at least 10 mm in order to avoid
any need for sinus augmentation, then only 33% of sites in this investigation would be
able to forego sinus augmentation.
It is also important to note that crestal resorption can impact the implant restoration.
According to Mailoa et al 2015, there is increased marginal bone loss when an implant
has been placed with its platform further than 3 mm from the CEJ of adjacent teeth.
Several hypotheses could be formulated to explain the statistically significant
relationships that were identified:
Bone to CEJ distance on the palate vs palatal vertical resorption (P=0.01): a hypothesis
for this finding could be that having a more apical alveolar crest level makes the bone
more vulnerable to vertical resorption, although it would still be left to question why this
relationship does not exist on the buccal. Since this investigation is purely radiographic,
conclusions cannot be made regarding whether increased CEJ-bone distance is attributed
to periodontitis or just local dehiscence. Nevertheless, one recent key investigation
(Zadeh et al 2015) did find a higher degree of resorption in sites with dehiscence. That
study’s findings therefore are in agreement with the current investigation.
Mean horizontal resorption and mid-crestal vertical resorption (P=0.0001): it might be
said that it is fairly intuitive that the more horizontal resorption that occurs, the more
vertical resorption would be observed, since bone resorption does not occur in a linear or
two dimensional direction.
Palatal bone thickness and mean horizontal resorption (P=0.01): similarly to the first
hypothesis, it could be surmised that having thicker palatal plate protects the crest from
extensive resorption. In the current study no correlation was observed between buccal
plate thickness and horizontal resorption. This finding differs from Chappuis et al 2013,
which reported an inverse relationship between plate thickness and crestal resorption.

13
This difference in observation could be related to the differences in methodology of our
observations or differences in sample size and study population.
Comparison with systematic reviews
When looked at alongside the findings of Tan et al 2011 and Hammerle et al 2011, there
was a markedly greater resorption in the crestal dimensions of the present investigation.
This might be explained by the fact that neither the systematic review nor the consensus
report focused solely on molars. The conjecture might thus be made that resorption in
molars is greater than resorption in premolars and anteriors when unassisted healing is
compared in both cases.
Limitations of the current investigation
This investigation has several shortcomings. Firstly, there is measurement error
associated with CBCT analysis. To determine the magnitude of the measurement error,
repeated measurements were made on 25% of the samples, which found the discrepancy
between the two measurements to be approximately 15%, Secondly, two teeth in this
study were from patients that had their second CBCT scan only two months following
extraction. However, Ryu et al 2015 demonstrated that during the initial 12 weeks, the
timeline of dimensional resorption in the canine model is 0.2mm per week. Thirdly, this
investigation did not take into consideration a number of additional factors that could
have had an impact on how the crest healed: reason for extraction; presence of periapical
infection; technique used, such as atraumatic extraction or sectioning individual roots;
complications such as fracture of the buccal plate; ankylosis of the tooth; furcation
involvement, etc.

14
Conclusion
Within the limitations of this investigation, both horizontal (6.6 + 3.7 mm and 57.5 +
26.1%) and vertical (2.4 + 1.4 mm and 30.48 + 16.64%) dimensional changes were
observed after 2-48 months (10.4 + 11.5) post-extraction of maxillary molars. The
influence of dimensional changes on limitations in implant therapy, the potential need for
additional interventions, and the efficacy of preservation techniques to limit dimensional
changes of maxillary molars still need to be investigated.

15
Tables
Table 1. Clinical characteristics of study patients and sites.

Parameter Value
Population
Subjects N=22

Teeth N=24

Mean age 71+10.88

Age range 48-87

Male N=10

Female N=12

Healing period
10.4 + 11.5 (range 2 to 48)
months
Medical history
Smoking at time of
treatment
0
Never-smoker 19
History of smoking 3
Hypothyroidism 4
Hypertension 10
Penicillin allergy 2
Osteoarthritis 2
Diabetes mellitus 3
Oral bisphosphonate 1
Sites Max right second molar 5
Max right first molar 10
Max left first molar 7
Max left second molar 2

16
Figures
Figure 1

Figure 2

17
Figure 3

Figure 4

18
Figure 5

Figure 6

19
Figure 7

Figure 8

20
Figure 9

Figure 10

21
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Creator Somerville, Matthew R. (author)

Core Title Dimensional changes in alveolar bone following extraction of maxillary molars in humans: a retrospective CBCT analysis

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 04/19/2017

Defense Date 03/21/2017

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag alveolar ridge preservation,buccal plate,cbct,DICOM,guided bone regeneration,OAI-PMH Harvest,palatal plate

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zadeh, Homayoun H. (committee chair), Kar, Kian (committee member), Paine, Michael L. (committee member)

Creator Email MattRSomerville@gmail.com,somervim@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-360087

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