Mechanics Analysis For Implant Soft Tissue Retained Overdenture ©2004
Mechanics Analysis For Implant Soft Tissue Retained Overdenture ©2004
Mechanics Analysis For Implant Soft Tissue Retained Overdenture ©2004
To cite this article: Chwei‐Goong Tseng & Yuan‐Show Jiang (2004) Mechanics analysis for
implant‐soft tissue retained overdenture, Journal of the Chinese Institute of Engineers, 27:3,
405-415, DOI: 10.1080/02533839.2004.9670887
Article views: 20
ABSTRACT
This paper primarily compares retention among three types of stud attachments
of dentures and analyzes the stress associated with polarized photoelasticity to eluci-
date the stress distribution of various implants near the ambient surface of an implant
under different conditions of loading on the prosthetics of a tooth. The experimental
results yield many significant data. Using the two-force principle to measure the
retention of attachments yields two important areas of results. Under dry conditions,
the O-ring attachment has the largest retention force. While under wet conditions, the
Zest attachment has the largest retention force.
The photoelastic method is applied to analyze the stress distribution on the arch
mandible. When the implants are loaded, the magnetic attachment will generate the
largest stresses on the implant. The stresses associated with the Zest attachment are
the smallest and can be neglected. Concerning the materials of the experimental arch
model, PL-2 photoelastic birefringent material is used to make the experimental arch
model, which is similar to the true arch mandible. PSM-1 photoelastic birefringent
material is used to make a semicircular model, which is easier to manufacture and is
also used to simulate the human mandible. The experimental results indicate that the
stresses in the PL-2 arch model are concentrated on the bottom of the implant, and
those in the PSM-1 are on the circumference of the implant approximately one-third
of the height from the bottom. The stress on the PL-2 arch model clearly exceeds that
on the PSM-1 arch model. The PL-2 arch model more closely captures the practical
clinical situation because its implant is imbedded in the alveolar bone with no clearance.
Therefore, the PL-2 arch model is strongly recommended for future study.
Key Words: i m p l a n t - s o f t t i s s u e o v e r d e n t u r e , i m p l a n t , a t t a c h m e n t , s t r e s s ,
photoelasticity, retention, arch mandible.
Overdenture
Athachment
Soft tissue
Implant
Arch model
Rentention
Rentention
Vertical loading
Vertical loading
Tensile
Tensile
Time
Time
Fig. 3 The relationship between the loading and duration of ex- Fig. 4 The relationship between the loading and duration of ex-
tension (O-ring attachment) tension (magnetic attachment)
advantages of easy operation, maintenance and repair. into the denture such that only the head of the stud is
Accordingly, in clinical practice, the O-ring attach- visible. The other part is the concave metal part
ment is the most popular and preferred. (female porion), made of stainless steel, whose sur-
Kenny and Wrichurd (1998) analyzed the me- face is coated with a compound of titanium to increase
chanics of photoelastic stress in dentures and implants its hardness and decrease its abrasion (James, 1998a).
which were attached by bartype attachment and an The fitting of the Zest attachment involves inserting
O-ring attachment respectively. Their experimental the male portion into the female portion, allowing
results showed that the O-ring attachment was the best universal free rotation (James, 1998b), and allowing
choice. The commercial standard attachment called easy operation, maintenance and repair (Merill and
the Branemark system was used in the experiment, in Mehsor, 1990). So, in clinical practice, the Zest at-
which the metal ball’s diameter is 3.5 mm, and the tachment is very popular.
height is 4 mm.
II. MEASURING OF RETENTION
2. Magnetic Attachment
Retention is defined as the maximum force that
Magnetic attraction has been used to support the can connect a denture and a mandible. If the reten-
retention of dentures. Behrman (Sasaki, et al., 1984) tion force is insufficient or in the incorrect direction,
presented the magnetic attachment in 1952. Behrman the denture will be easily loosened when the patient
embedded magnetic Tefloncoated Co-Pt (cobalt- opens his mouth or masticates.
platinum) attachments into the interior of the man- However, if the retention is too strong, the den-
dible and denture about 5-10 mm deep by hand, and ture cannot be easily removed, and will excessively
such that they attracted each other, ensuring the re- concentrate in the abutment of the denture. In this
tention of the denture. However, in 1964 and 1967, study, the tensile testing method is used to measure
Behrman and Winklev used an Alnico bar magnet, and compare the retention of different types of
which has been extensively researched (Barrie, 1981). attachments.
The volume of the Alnico bar magnet is too large and
its magnetic attraction is too weak for it to be con- 1. Measuring Retention
sidered optimally effective. In 1969, the CO5Sm was
discovered which had the advantages of double The experiment on retention can be divided into
strength magnet and smaller volume, and has been two parts. The first part directly extends dry attach-
used to serve as an attachment widely. The magnetic ments by adding vertical loading and measures their
attachment provides the advantages of easy operation, retention. However, in a mouth, the attachment is
maintenance and repair. Every magnetic attachment usually embedded in saliva (wet conditions), and the
consists of a magnet and a Keeper, as shown in Fig. wet conditions will reduce retention. The wet condi-
2(b). The magnet is a hard permanent magnet and tion of the mouth is simulated by adding normal
the Keeper an impermanent magnet made of soft saline. Hence, the second part of the experiments is
material. They are attached to the root and the to measure the retention under such conditions.
implant, respectively. The O-ring attachment is extended by vertical
loading. Fig. 3 plots the relationship between the
3. Zest Attachment loading and duration of extension. After a particular
period of time, loading does not increase but decreases
Figure 2(c) shows another ring attachment called slowly to zero. As shown in the figure, the retention
the Zest attachment (Robert, 1997). In 1972, Zest is defined as the maximum load in the curve.
Anchor designed the ZAAG (Zest Anchor Advanced A fastening system is designed to apply the two-
Generation), which has two parts. One is the stud- force principle when the attachments undergo tensile
type part made of plastic (male portion) and inserted testing.
408 Journal of the Chinese Institute of Engineers, Vol. 27, No. 3 (2004)
Fast axis
Light
Slow axis
source
σ2
σ1
Slow Polarizer
axis
Fast axis
Quarter-wave
plate
A Model
Quarter-wave
plate
Z
Analyzer
(Dally and Riley, 1991), photoelastic material has Second, formed silica plastic material is used to make
light refractive indices of n 1 and n 2 in the directions the arch model by casting. Finally, the PL-2 liquid
of the maximum principal stress and the minimum photoelastic resin material is shot into the interior of
principal stress, respectively, after loading. They are the formed silica plastic model, following the opera-
related by, tional procedures supported by the manufacturer
(Measurements group Company).
n 1−n 2 =C( σ 1− σ 2) (1) After hardening, the arch model must be checked
for defects including air holes, impurity, irregularity
where C is called the relative stressoptical coefficient. of scale, or bending deformation. If any of these de-
When light passes through the circularly polarized fects are present, the model cannot be used in an
light system and model, it can obtain following for- experiment. If it has no defects, the arch model is
mulation placed in the field of the circularly polarized light to
determine its transmitance and whether it has residual
Nf σ stresses inside. If so, the model is placed in a hot
(σ 1 – σ 2 ) = (2)
h furnace for tempering. The arch model is then placed
again in the field of the circular polarized light to
where N is called the fringe order, and fσ is called the check whether it still has the residual stresses. If so,
material fringe value. Accordingly, the stress is pro- the model is discarded. Otherwise, it serves as an
portional to the fringe order. experimental model. The transmittance of the arch
Circularly polarized light can be divided into the model can be decided by the compound proportions
arrangement of dark field and of the bright field, and of base plastic and hardener. The shooting model
the arrangement of dark field is chosen. Fig. 6 shows must reserve suitable clearance.
the circularly polarized light through the combined
system. 2. Inserting the Implant
1. Manufacturing an Artificial Mandible This paper considers two types of arch model.
The first is made of material PSM-1 (hard plate),
An arch model is fabricated according to the which is formed into an arch model by CNC manufac-
normal occlusion condition of the natural mandible, turing. The second involves shooting PL-2 liquid
and considering the statistical data on arches pre- photoelastic resin material into a silica plastic model.
sented in (Lavelle, 1975; Robert, 1999). First, the These two types of models are manufactured using
statistical data are input into a CNC (computer nu- different processes, so the methods by which their
merical control) machine and the arch model is made. implants are inserted are also different.
410 Journal of the Chinese Institute of Engineers, Vol. 27, No. 3 (2004)
3. Soft Tissue
b a a b
d c c d
e e
Nf σ
Id = (σ 1 – σ 2) = (6)
(b) h
Fig. 9 (a) The fringe doubling method; (b) the fringe doubling The homemade plate model can be used to cali-
plus the middle filter method
brate the material fringe value of the PL-2 photoelas-
tic model, f σ = 2.015 Kpa-m/fringe and the material
fringe value of the PSM-1 photoelastic model, f σ =
decreases the environmental noise. The absolute 6.62 Kpa-m/fringe. The experiment results also yield
value Eq. (3) minus Eq. (4) is used. the fringe order of selecttive points of every model at
positions a, b, c, d and e, as shown in Fig. 10. Now,
I r = I d – I b = A COS(2φ) (5) these fringe orders can be substituted into Eq. (6),
and the stress concentration index at every point can
It is worthwhile to observe that the frequency is be derived. Furthermore, these data are obtained ex-
doubled, since the argument in the periodic function perimentally to calculate and compare the stresses of
is 2 φ instead of φ. Fig. 9(a) displays the photoelastic each model.
pattern of fringe doubling, governed by Eq. (5). The
doubling and subtracting treatment result in a 1. Experimental Results
photoelastic pattern that may still include some noise,
so the middle value filter method is applied again to The bite positions or the loading positions are
reduce the noise (Miu, 1999). If the photoelastic pat- shown in Fig. 11. Points 46 and 36 indicate the posi-
tern includes very strong noise, the difference be- tions of premolars, and points 43 and 33 indicate the
tween the gray levels of neighboring pixels will be positions of the cuspids.
relatively large. While the middle value filter method Tseng et al. (2000) used practical statistical arch
can be used to force the gray levels to be similar data and hard material to create an arch model. The
among neighboring pixels, thus the noise is reduced. experiment performed here on the PL-2 arch model
412 Journal of the Chinese Institute of Engineers, Vol. 27, No. 3 (2004)
36
46
43
33
(a)
Table 3 The comparison of maximum stress among the different models and attachments
Attachment Magnet O-ring Zest
Model attachment attachment attachment
PL-2 arch model 19.38Kpa 16.15Kpa 12.92Kpa
PSM-1 arch model 10.61Kpa 7.63Kpa None
PSM-1 semi-circle model 10.90Kpa 8.49Kpa 6.37Kpa
(1)Magnet attachment
When the loading reaches 294.3N, the fringe
order is N=5, and point d has the most fringes and so
is the point of stress concentration. If the loading is
1N, then the stress concentration index is Id=
10.61Kpa, as shown in Fig. 13(a).
(2)O-ring attachment
When the loading reaches 294.3N, the fringe (a)
order is N=4, and points c and d have the most fringes,
and so are the points of stress concentration. If the
loading is 1N, the stress concentration index is Id=
8.49Kpa, as shown in Fig. 13(b).
(3)Zest attachment
When the loading reaches 294.3N, the fringe
order is N=3, and point c and d have the most fringes
and so are the points of stress concentration. If the
loading is 1N, the stress concentration index is Id=
6.37Kpa, as shown in Fig. 13(c). (b)
The above results indicate that the stress dis-
tribution of the Zest attachment near the implant is
the smallest, that of the O-ring attachment is next
smallest, and that of the magnetic attachment is the
largest. Therefore, the Zest attachment exhibits the
best buffer effect under vertical loading.
Table 3 shows that the PL-2 arch model has
about twice the stress as the PSM-1 model has. The
implant of the PL-2 arch model is inserted into the
arch by casting and there is no clearance between the (c)
implant and the arch. However, the implant of the Fig. 13 The stress fringe patterns of PSM-1 arch Model in point
PSM-1 arch model is inserted into the arch by 43 by vertical loading; (a) Magnetic attachment (b) O-ring
threading, and, that leaves clearance between the attachment; (c) Zest attachment
implant and arch. Therefore a vertical loading ap-
plied to point 43 is directly delivered through the In Table 3 the maximum stresses among the dif-
implant to the PL-2 arch model, and the stress is con- ferent models and attachments are compared. Al-
centrated at the bottom of the implant. In the PSM-1 though different shapes are used, the stress values are
arch model, under the same loading conditions, the similar and the points of stress concentration are the
loading will be taken by the circumference of the same. Accordingly, the experimental results show
implant because the bottom of the implant is not in that the semi-circular model can be used to replace
contact with the arch model. Therefore, the majority the arch model.
of the loading cannot be delivered to the bottom of
the implant or the PSM-1 arch model. Therefore, un- (iii) Non-Direct Loading in Implant
der the same conditions, the stress on the PSM-1 arch
model will be much less than that on the PL-2 arch Loading on point 46 and inspecting the fringe
model. pattern at point 43 of the implant reveal that the
414 Journal of the Chinese Institute of Engineers, Vol. 27, No. 3 (2004)
ACKNOWLEDGMENTS
non-direct loading produces very low stresses on the
implant, which can be neglected. This situation is The writers are thankful to the reviewers for
shown in Fig. 14. their valuable suggestions and comments.
The above analysis and discussion yields the C the relative stress-optical coefficient
following conclusions. fσ the material fringe value
1. Under dry conditions, the orders of retention of h thickness of model
the three attachments is, 0-ring attachment > Zest Ib gray level of the bright field image
attachment > magnetic attachment. Id gray level of the dark field image
2. Under wet conditions, the orders of retention of Ir gray level of the bright field image minus the
the three attachments is, Zest attachment > 0-ring dark field image
attachment > magnetic attachment. Id the stress concentration index
3. After normal saline is added, the retention of each n1 the light refractive index in the direction of the
attachment is compared under dry and wet maximum principal stress
conditions. Finally, the drop in retention under wet n2 the light refractive index in the direction of the
conditions is greatest for the O-ring attachment at minimum principal stress
about 17.83%, next largest for the magnetic N the fringe order of a photoelastic pattern
attachment, at about 11.19%, and the least for the σ1 the maximum principal stress
Zest attachment, at about 4.26%. σ2 the minimum principal stress
4. Experimental results indicate that the method of
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