![The lower border of the mental foramen (M, M'), the sigmoid notch (S, S'), and the gonion (G, G') were used to construct the triangles M-S-G and M'S'-G', with centers N and N', respectively. Boundary lines were constructed as follows: M-G and M'-G', A-L and A'-L' (crest of the residual ridge to the lower border of the mandible perpendicular to M-G and M'-G'), M-N and M'-N', and G-P and G'-P' (G-N and G'-N' extended to the crest of the residual ridge at P and P'). The lines C 1-B 1 and C 1 '-B 1 ' (line from marginal bone level [point C 1 , C 1 '] to first bone-to-implant contact [point B 1 , B 1 ']) and B 1-I 1 and B 1 '-I 1 ' (line from point B 1 , B 1 ' to implant shoulder [point I 1 , I 1 ']) were measured at the distal aspect of the implants. The lines C 2-B 2 , C 2 '-B 2 ', B 2-I 2 , and B 2 '-I 2 ' were measured at the mesial aspect of the implants.](https://www.researchgate.net/profile/Ahmed-Habib-21/publication/51455534/figure/fig4/AS:483737159901187@1492343334131/The-lower-border-of-the-mental-foramen-M-M-the-sigmoid-notch-S-S-and-the-gonion_Q320.jpg)
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306 The International Journal of Prosthodontics
The duality of support that characterizes distal ex-
tension removable partial dentures (RPDs) is often
characterized by time-dependent adverse changes
in both the abutment teeth and edentulous areas.1
Consequently, adjunctive implant support has been
proposed for mandibular Class I and II removable
partial denture designs.2,3 The premise is that this
will minimize the risk of potential problems of patient
discomfort associated with prosthesis retention and
stability resulting from residual ridge resorption.4,5
The aim of this preliminary study was to retrospec-
tively compare the possible influence that implant
placement under distal extension RPDs might have
on the residual ridges in treated Class I mandibles at
the end of a 5-year observation period.
Materials and Methods
Thirty-four healthy men who regularly attended the
Department of Removable Prosthodontics, Faculty of
Dentistr y, Mansoura University, Egypt, for dent al follow-
up treatment were recruited for this study. They were
enrolled following their acceptance of the faculty com-
mittee’s duly approved and explained research proto-
col, and a signed informed consent form was obtained.
Each patient had been partially edentulous in the
mandible for 3 to 8 years and either already wore or
was a candidate for wearing an RPD because of the
presence of only eight anterior teeth (first premolar to
first premolar) opposing a complete maxillary denture.
The study’s sample size of 34 men (age range: 44 to 61
years) was calculated to yield a power of 80% (two-
tailed α = .05) using a computer program (Power and
Precision version 3, Biostat). Calculations were based
on results from previous studies6–8 that demonstrated
that a .06 change in Posterior Area Index (PAI) be-
tween treatment groups is regarded as significant.
Single bilateral implants (Dyna Dental Engineering)
were placed in the edentulous first molar area of the
distal extension ridges using a standardized two-stage
submerged surgical protocol. Table 1 shows the distri-
bution of implant lengths and diameters for all patients.
aLecturer of Removable Prosthodontics, Faculty of Dentistry, Man-
soura University, Eldakahlia, Egypt.
bProfessor of Removable Prosthodontics, Faculty of Dentistry, Man-
soura University, Eldakahlia, Egypt.
Correspondence to: Moustafa Abdou ELsyad, Department of
Removable Prosthodontics, Faculty of Dentistry, Mansoura Univer-
sity, PO Box 35516, #68 ElGomhoria Street, ElMansoura, Egypt.
Fax: +502260173. Email: M_syad@mans.edu.eg
Implant-Supported Versus Implant-Retained Distal
Extension Mandibular Partial Overdentures and
Residual Ridge Resorption: A 5-Year Retrospective
Radiographic Study in Men
Moustafa Abdou ELsyad, BDS, MSc, PhDa/Ahmed Ali Habib, BDS, MSc, PhDb
Purpose: This retrospective study sought to examine posterior mandibular ridge
resorption under implant-supported and implant-retained distal extension partial
overdentures in men at the end of a 5-year observation period. Materials and Methods:
Class I mandibular partial edentulism was managed in 34 patients with removable
partial overdentures that were adjunctively supported (n = 18) or retained (n = 16) via
resilient attachments placed bilaterally on single implants (n = 68) in the first molar
areas. Posterior Area Indices (PAI) were calculated for each patient by digitizing the
traced rotational tomograms taken immediately before and after 5 years of treatment.
Proportional rather than actual measurements were used in an effort to minimize errors
related to magnification and distortion. Results: Residual ridge resorption associated
with the implant-supported partial overdentures was recorded as PAI =0.012 ± 0.022;
it was PAI = 0.073 ± 0.044 for the implant-retained group. Estimated average reductions
in ridge heights were 0.15 and 1.03 mm for implant-supported and implant-retained
partial overdentures, respectively. Multiple linear regression models demonstrated
that prosthesis type, initial mandibular ridge height, and relining frequency were
significantly correlated with PAI. Conclusion: Implant-supported partial overdentures
appear to be associated with reduced posterior mandibular alveolar ridge resorption
when compared to implant-retained ones. Int J Prosthodont 2011;24:306– 313.
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Volume 24, Number 4, 2011 307
ELsyad/Habib
The partial overdenture design prescribed for all
patients relied on lingual bar major connectors, bi-
cuspid abutments with RPA (mesial occlusal rest,
distal proximal plate, Aker arm) clasp assemblies for
retention support, and indirect retention from canine
cingulum rests.
After construction of the mandibular cobalt-
chromium alloy frameworks, an impression was re-
corded for the distal extension ridges using a mixture
of equal parts medium- and light-bodied polyether
material (Impregum F and Permadyne LV, 3M ESPE),
and an altered cast impression technique was em-
ployed.9 Semianatomical acrylic resin teeth (Vitapan,
Vita Zahnfabrik) were arranged to ensure balanced
occlusal contact.
Patients were then divided randomly into two
groups according to the overdenture design con-
cepts employed using a computer program. Group 1
included 18 patients treated with implant-supported
partial overdentures with direct contact of the met-
al framework to the top of each healing abutment
(Fig 1). Disclosing wax (Kerr) was used intraorally to
eliminate extraneous contact other than that on the
top of each healing abutment to reduce lateral forces
on the implants3 and permit axial loading. Group 2
included 16 patients treated with implant-retained
partial overdentures via a resilient attachment (Ball
Abutment and Gold Smart Matrix, Dyna Dental
Engineering). Positioning rings were placed over the
ball abutments to create space between the matrices
Table 1 Dimensions of Implants Used
Length
8.0 mm 10.0 mm 11. 5 mm 13.0 mm Total no.
of implantsDiameter Group 1 Group 2 Group 1 Group 2 Group 1 Group 2 Group 1 Group 2
3.6 mm 0 0 0 1 1 1 2 3 8
4.2 mm 1 1 4 2 3 2 2 2 17
5.0 mm 1 0 2 0 1 2 1 2 9
Total 2 1 6 3 5 5 5 7 34
Group 1 = imp lant-sup ported partial overdenture; gr oup 2 = impla nt-retained partial overdenture.
Fig 1a Healing abutment on the cast.
Fig 1b Metal framework contact on the fitting surface of the partial overdenture.
Fig 1c Healing abutment in place.
Fig 1 Implant-supported partial overdenture.
a
c
b
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308 The International Journal of Prosthodontics
Implant-Supported vs Implant-Retained Distal Extension Mandibular RPDs and Residual Ridge Resorption
and balls. The matrices were functionally related to
the denture-fitting surface by direct pickup using
autopolymerizing acrylic resin. The positioning rings
were removed to allow vertical play of the denture on
loading (Fig 2).
Patients were recalled every 6 months to check
the top contact in group 1 and the space between
the components of the resilient attachment in group
2 using disclosing wax. If contact existed between
the matrices and balls in group 2, the matrices were
separated from the denture base and a “pickup” pro-
cedure was repeated with positioning rings in place.
Two prosthodontists who were blinded to the
treatment groups determined the need for relin-
ing by checking the occlusion and evaluating the
tissue fit of the denture base using a thin mixture
of irreversible hydrocolloid impression material
(Alginate CA 37 Superior Pink, Cavex Holland).10
Data Collection from Tomographic Images
Two rotational tomograms for each patient (taken im-
mediately before [baseline] and 5 years after overden-
ture insertion) were obtained from available patient
records during routine examination. To standardize all
tomographic images, the panoramic unit (Orthophos
Plus, Siemens) was operated at 69 kV with a constant
current of 16 mA/s and an exposure time of 16 sec-
onds while each patient bit down on a custom acrylic
occlusal stent connected to the chin stabilizer of the
unit. The films were processed in an automatic pro-
cessor. All radiographs were examined carefully to
select only those clearly showing all the main points
to be traced. The mandibular ridge heights at the
region of the mental foramen and the ridge lengths
were measured from rotational tomograms taken at
baseline. Relining frequency for both groups was also
recorded.
Evaluation of Posterior Mandibular Alveolar
Bone Changes
Bilateral posterior areas of the residual ridges were
measured on rotational tomograms using a method
of proportional measurement that was similar to that
described by Wilding et al.11 Boundaries for the pos-
terior area were identified by drawing a line joining
the gonion to the lower border of the mental fora-
men and the crest of the residual ridge. The area
was expressed as a proportion of a further area of
bone, which was independent of the crest of the re-
sidual ridge (a posterior triangle formed on each side
Fig 2a Ball abutment on the cast.
Fig 2b Gold smart matrix on the fitting surface of the partial overdenture.
Fig 2c Ball abutment in place.
Fig 2 Implant-retained partial overdenture.
a
c
b
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Volume 24, Number 4, 2011 309
ELsyad/Habib
connecting the gonion, the lower border of the mental
foramen, and a point that was the center of triangle
gonion–mental foramen–sigmoid notch). In this study,
a modification was introduced to this method to sub-
tract peri-implant crestal bone loss from the posterior
mandibular areas (Figs 3 and 4). Therefore, patients
who had excessive peri-implant bone loss were ex-
cluded to avoid misleading values of PAI.
The rotational tomogram films were scanned us-
ing a black and white translucent scanner. The land-
marks were traced on the images and digitized, and
the necessary calculations were performed using the
assisted drawing program AutoCAD 2008 (Autodesk)
(Fig 5).
The mean differences for right and left PAIs were
calculated for each patient. The area difference,
which represents bone resorption along the entire
ridge length, was estimated by multiplying the aver-
age initial area with the value of the change in PAI.
Then, approximate changes in height could be cal-
culated by dividing the change in bone area by the
average length of the posterior residual ridge.8
Fig 3 The lower border of the mental foramen (M,
M’), the sigmoid notch (S, S’), and the gonion (G, G’)
were used to construct the triangles M -S-G and M’-
S’-G’, with centers N and N’, respectively. Boundary
lines were constructed as follows: M- G and M’-G’,
A-L and A’-L’ (crest of the residual ridge to the lower
border of the mandible perpendicular to M-G and
M’-G’), M- N and M’-N’, and G-P and G’-P’ (G-N and
G’-N’ extended to the crest of the residual ridge at P
and P’). The lines C1-B1 and C1’-B1’ (line from mar-
ginal bone level [point C1, C1’] to first bone-to-implant
contact [point B1, B1’]) and B1-I1 and B1’-I1’ (line from
point B1, B1’ to implant shoulder [point I1, I1’]) were
measured at the distal aspect of the implants. The
lines C2-B2, C2’-B2’, B 2-I2, and B2’-I2’ were measured
at the mesial aspect of the implants.
Fig 4 The areas were defined as follows: X and
X’ were defined by the crest of the residual ridge
P-C1-B1-I1-I2-B2-C2-A and P’-C1’-B1’-I1’-I2’-B2’- C2’-
A’ and the boundary lines A- M and A’-M’, M-G and
M’-G’, and G-P and G’-P’, respectively; Y and Y’
were defined by the triangles M- G-N and M’-G’-N’,
respectively. PAI was calculated as (X/Y + X’/Y’)/2.
Fig 5 Traced rotational tomography with reference
points and lines.
S
G
N
M
L
A
B1B2
C1I1I2C2
G'
N'
S'
P'
A'
L'
M'
B1'
B2'
C2'I2'I1'C1'
S
G
N
Y
M
L
A
G'
N'
X'
S'
P'
A'
L'
M'
B1'
B2'
C2'I2'I1'C1'
S
G
N
Y
M
L
A
G'
N'
X'
S'
P'
A'
L'
M'
B1'
B2'
C2'I2'I1'C1'
S
G
N
M
L
A
B1B2
C1I1I2C2
G'
N'
S'
P'
P
A'
L'
M'
B1'
B2'
C2'I2'I1'C1'
X
Y
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310 The International Journal of Prosthodontics
Implant-Supported vs Implant-Retained Distal Extension Mandibular RPDs and Residual Ridge Resorption
Data Analysis
All data were analyzed using SPSS version 10 (IBM).
Descriptive statistics were taken for all patients at the
beginning of the study. Radiographic measurements
were completed by a single operator blinded to the
treatment groups. The mean differences in PAIs were
compared within the same group using paired sam-
ple t tests and between groups using independent
sample t tests. A multiple regression analysis using
a stepwise procedure was also performed to test if
there was a relationship between PAI and potential
confounding factors: type of prosthesis, age, ridge
length, years of edentulism, number of worn den-
tures, initial ridge height of the mandible, and relining
frequency. A P value of ≤ .05 at a confidence interval
of 95% was considered significant.
Results
The total number of subjects at baseline was 34. Two pa-
tients were omitted from further investigation (1 in each
group) because of unidentified mental foramina on their
rotational tomograms. Another 2 patients in group 1
were excluded because they had excessive peri-implant
crestal bone loss that accompanied late implant failures.
Therefore, 30 patients (30 pairs of radiographs) were
suitable for the study (15 pairs in each group).
The descriptive statistics of the study population
are shown in Table 2. An independent sample t test
showed that patients in group 1 were significantly
older in comparison to patients in group 2 (P = .00).
There was also a significant difference between
groups in both initial height of the mandible and relin-
ing frequency (P = .001 and P = .033, respectively).
However, time of edentulism, number of worn den-
tures, and ridge length demonstrated no significant
difference between the two groups at baseline.
PAI results for both groups are shown in Fig 6.
There was no significant difference in PAI between
baseline and the 5-year follow-up in group 1, while
in group 2, PAI at 5 years was significantly less than
PAI at baseline (paired samples t test, P = .00). The
change in PAI in group 2 was significantly higher than
that in group 1 at the 5-year follow-up (independent
samples t test, P = .00) (Table 3). The threshold for
bone resorption was previously established at a .04
change in PAI.6, 11
Overall, change in bone areas was approximately
6.6 and 43.8 mm2 in groups 1 and 2, respectively.
When averaged over the ridge length (44 mm in group
1 and 42.5 mm in group 2), the change in bone ar-
eas resulted in an approximate 0.15-mm loss of ridge
height (0.03 mm per year) in group 1 and a 1.03-mm
loss of ridge height (0.21 mm per year) in group 2 over
a mean period of 5 years.
Table 2 Clinical and Radiographic Characteristics of the Study Population
Age
Ridge length
(mm) Years edentulous
No. of dentures
worn
Initial height of
mandible (mm)
Relining
frequency
Mean Range Mean Range Mean Range Mean Range Mean Range Mean Range
Group 1 55.0 48–61 44.0 39–47 6.2 4–8 1.6 0–3 16 .3 14–19 0.06 0–1
Group 2 49.2 44–55 42.5 39–46 5.5 3–8 1.1 0–3 18.5 15 –21 0.46 0 –2
t test P = .00 P = .10 P = .14 P = .14 P = .001 P = .033
Baseline
5-year follow-up
Difference
1.7431.731
0.012 0.073
1.740
1.813
Group 1 Group 2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Mean PAI
Fig 6 PAI at baseline and the 5-year follow-up for both groups.
Table 3 Mean Difference in PAI Between Groups
Mean change in
PAI ± SD
Maximum
PAI
Minimum
PAI
Group 1 –0.012 ± 0.022 +0.03 –0.05
Group 2 –0.073 ± 0.044 –0.16 –0.02
Independant t test P = .00
SD = standard deviation.
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Volume 24, Number 4, 2011 311
ELsyad/Habib
In the multiple regression analysis, only the type of
prosthesis, the initial height of the mandible, and relin-
ing frequency were significantly correlated with change
in PAI (P = .002, P = .033, and P = .011, respectively;
Table 4).
The final model, therefore, contained three factors:
type of prosthesis, initial height of the mandible, and
relining frequency (Table 5). The effect of group 2 in
comparison to group 1 (effect of prosthesis type) was
to reduce PAI by 5.7% per year (P = .00). The effect of
the initial height of the mandible was such that for ev-
ery 1-mm increase in the height of the mandible, a re-
duction of PAI by an extra 0.9% per year was observed
(P = .014). For each relining incident recorded, a cor-
responding reduction in PAI by 3.7% was observed (P
= .002). The effects on PAI each year were different
in the two groups ( Table 6). For group 1, every 1-mm
increase in height of the mandible resulted in a 0.2%
reduction in PAI per year (P = .627); in group 2, every
1-mm increase in height of the mandible led to a re-
duction of PAI by an extra 1.3% per year (P = .04).
Discussion
The results of this study should be interpreted with
caution since only men were included. Women were
conveniently excluded since it appears that the risk
of elevated bone resorption resulting from the influ-
ence of hormonal factors6,7 would require a far larger
group of patients than this preliminary design per-
mitted. Proportional area measurement in terms of
area index (PAI) was used in the present study since
it reduces the problems associated with magnifica-
tion inherent in rotational tomograms in the posterior
mandibular region. Such a method is more accurate
and comprehensive in determining mandibular alveo-
lar bone resorption than the conventional method on
cephalometric radiographs, which was described by
Tallgren12 and modified by Uçta¸sli et al.13 Tallgren’s
technique12 measured bone resorption at four select-
ed points only (not the entire area of the ridge) and
did not consider the variability in the amount of bone
resorption between different sites of the ridge.
Table 4 Multiple Linear Regression Analysis of All Factors over 5 Years
Variable Coefficient Standard error t P 95% confidence interval
Prosthesis type –0.062 0.018 –3.445 .002 –0.100 to –0.025
Age 0.000 0.002 0.119 .906 –0.003 to 0.003
Ridge length –0.001 0.003 –0.246 .808 –0.006 to 0.005
Years edentulous –0.004 0.004 –1.094 .286 –0.014 to 0.004
No. of dentures –0.002 0.006 –0.336 .740 –0.013 to 0.010
Initial height of
mandible
–0.008 0.004 –2.279 .033 –0.015 to –0.001
Relining frequency 0.036 0.013 2.787 .011 0.009 to 0.063
Table 5 Multiple Regression Including Type of Prosthesis, Initial Height of the Mandible,
and Relining Frequency Only
Variable Coefficient Standard error t P 95% confidence interval
Prosthesis type –0.057 0.013 –4.323 .000 –0.085 to –0.030
Initial height of
mandible
–0.009 0.003 –2.624 .014 –0.015 to –0.002
Relining frequency 0.037 0.011 3.428 .002 0.015 to 0.059
Table 6 Effect of Initial Height of Residual Ridge by Type of Prosthesis
Variable Coefficient Standard error t P 95% confidence interval
Group 1 –0.002 0.004 –0.498 .627 –0.012 to –0.007
Group 2 –0.013 0.006 –2.265 .041 –0.026 to –0.001
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312 The International Journal of Prosthodontics
Implant-Supported vs Implant-Retained Distal Extension Mandibular RPDs and Residual Ridge Resorption
After 5 years, the difference in PAI was significant
in group 2 when compared with group 1. This finding
may be related to the presence of space between the
components of the resilient ball attachment, which
may permit free vertical rotation of the overdenture
during function with concentration of diverse forces
on the residual ridge areas. This unrestricted vertical
and presumably horizontal movement could result in
most of the masticatory load being transferred di-
rectly to the posterior edentulous ridge, with mini-
mal stress transmitted to the implants.14,15 Increased
bone loading as a result of this resilient support could
then contribute to increased ridge resorption since it
interferes with blood circulation in the mucosa and
alveolar bone, as well as loading bone unfavorably.
The significant association between bone resorp-
tion and relining frequency in group 2 concurred with
the findings of Naert et al.16 They reported that ball
anchors were associated with an increased frequen-
cy of relining events of the denture base compared
to other types of overdenture attachments. The few-
er partial overdenture relining frequencies found in
group 2 may also be a result of the masking of the
posterior mandibular resorption by an increased an-
terior resorption in the maxilla without provoking oc-
clusal instability of the dentures.6
It is tempting to presume that the reduced resorp-
tion rates in group 1 could be attributed to the direct
metal frame contact with the healing abutments, which
provides effective support and prevents denture base
rotation. As a consequence of this direct support, the
posterior ridge may be protected from excessive load-
ing, with most of the load being transmitted vertically
to the implants. The slight bone reduction in group 1
after 5 years may be attributed to the peri-implant al-
veolar bone loss, which was subtracted from the PAI.
Most follow-up studies on distal extension RPDs
have not included measurement of bone resorption
beneath the distal extension bases.17,18 Uçta¸sli et
al13 reported a mean 1.15-mm ridge reduction in the
posterior mandible after 5 years. A similar amount of
bone resorption was reported in this study in group 2
(1.03 mm). However, the study by Uçta¸sli and associ-
ates13 was conducted on patients wearing conven-
tional distal extension RPDs.
Finally, evaluation of ridge resorption alone is only
part of a prosthodontic patient’s clinical outcome.
Therefore, additional studies on larger and mixed-
gender patient groups that include survival rate of the
implants, condition of the terminal abutment teeth,
prosthetic maintenance, and patient-mediated out-
come concerns are necessary to evaluate the long-
term merits of modified treatment modalities such as
the one employed in this study.
Conclusions
Within the limitations of this preliminary study’s re-
search design, the following conclusions can be
drawn with caution, since only men were included:
•Implant-supported partial overdentures appear to
be associated with reduced posterior mandibu-
lar ridge resorption when compared to implant-
retained partial overdentures, since mean ridge
height reductions at the end of a 5-year observa-
tion period were 0.15 and 1.03 mm, respectively.
•The type of prosthesis design, the mandible’s initial
height, and relining frequency showed an associa-
tion with posterior mandibular ridge resorption.
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The Greater New York
Academy of Prosthodontics
2011 Student Grant Program
The Greater New York Academy of
Prosthodontics (GNYAP) offers grants
to support students enrolled in accredited advanced
specialty education programs in prosthodontics.
The goal is to provide the student with an organized
and meaningful research experience, under the
guidance of an experienced faculty mentor, to better
prepare the student for the rigors of clinical practice
and academia. A wide range of research topics may
be considered, but funded grants will offer contri-
butions to the body of knowledge that encompass
prosthodontics. Funding is up to $2,000.00 and mul-
tiple grants (up to 6) will be awarded.
Applications must be received by August 15, 2011
For more information and application materials, contact:
Dr. Vicki C Petropoulos, Chair,
Scientific Investigation Committee
The Greater New York Academy of Prosthodontics
(G N YAP )
University of Pennsylvania School of Dental Medicine,
4001 Spruce Street, Philadelphia, PA 19104
E-mail: vp718@comcast.net
© 2011 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY..
NO PART OF MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
