Load-related implant reaction of mini-implants used for orthodontic anchorage.

Load-related implant reaction of mini-
implants used for orthodontic anchorage
Andre´ Bu¨chter
Dirk Wiechmann
Stefan Koerdt
Hans Peter Wiesmann
Josef Piffko
Ulrich Meyer
Authors’ affiliations:
Andre´ Bu¨chter, Stefan Koerdt, Hans Peter
Wiesmann, Josef Piffko, Ulrich Meyer, Department
of Cranio-Maxillofacial Surgery, University of
Mu¨nster, Mu¨nster, Germany
Dirk Wiechmann, Orthodontic Department,
Medical School Hanover, Germany
Correspondence to:
Dr Andre´ Bu¨chter
Department of Cranio-Maxillofacial Surgery
University of Mu¨nster
Waldeyerstra�e 30
D-48129 Mu¨nster
Germany
Tel.: þ49 251 834 7013
Fax: þ49 251 834 7020
e-mail: Buchtea@uni-muenster.de
Key words: anchorage, implant stability, orthodontic tooth movement, osseointegration
Abstract: The purpose of this study was to determine the clinical and biomechanical
outcome of two different titanium mini-implant systems activated with different load
regimens. A total of 200 mini-implants (102 Abso Anchors and 98 Dual Tops) were placed
in the mandible of eight Go¨ttinger minipigs. Two implants each were immediately loaded
in opposite direction by various forces (100, 300 or 500 cN) through tension coils.
Additionally, three different distances between the neck of the implant and the bone rim
(1, 2 and 3 mm) were used. The different load protocols were chosen to evaluate the load-
related implant performance. The load was provided by superelastic tension coils, which are
known to develop a virtually constant force. Non-loaded implants were used as a reference.
Following an experimental loading period of 22 and 70 days half of the minipigs were
sacrificed, and implant containing bone specimens evaluated for clinical performance and
implant stability. Implant loosing was found to be statistically dependent on the tip
moment (TM) at the bone rim. Clinical implant loosing were only present when load
exceeded 900 cN mm. No movement of implants through the bone was found in the
experimental groups, for any applied loads. Over the two experimental periods the non-
loaded implants of one type of implant had a higher stability than those of the loaded
implants. Dual Tops implants revealed a slightly higher removal torque compared with
Abso Anchors implants. Based on the results of this study, immediate loading of mini-
implants can be performed without loss of stability when the load-related biomechanics do
not exceed an upper limit of TM at the bone rim.
The number of adult patients requiring
orthodontic therapy has undergone a
marked increase in recent decades (Bauer
& Diedrich 1990). However adults fre-
quently present pathologic findings like
periodontal and endodontic diseases, dys-
function of the mandibular joint, and early
loss of teeth (Diedrich 2000). As the tem-
porary use of the native dentition for ortho-
dontic anchorage is frequently limited, and
extraoral appliances are rejected for aes-
thetic reasons, an alternative approach
based on stable and cooperation-indepen-
dent anchorage is needed (Fritz et al. 2003).
Numerous studies have shown that con-
ventional osseointegrated implants are sui-
table for orthodontic loading and offer
stable anchorage for orthodontic purposes
(Gray et al. 1983; Roberts et al. 1994;
O¨dmann et al. 1988; Smalley et al. 1988;
Wehrbein & Diedrich 1993). Wehrbein &
Diedrich (1993) moreover reported that the
application of orthodontic forces positively
affects the peri-implant bone situation of
osseointegrated implants. In additional stu-
dies, Wehrbein (1994) and Wehrbein et al.
(1999) evaluated the quantity of minera-
lized bone in the vicinity of the implantCopyright r Blackwell Munksgaard 2005
Date:
Accepted 29 September 2004
To cite this article:
Bu¨chter A, Wiechmann D, Koerdt S, Wiesmann HP,
Piffko J, Meyer U. Load-related implant reaction of
mini-implants used for orthodontic anchorage.
Clin. Oral Impl. Res. 16, 2005; 473–479
doi: 10.1111/j.1600-0501.2005.01149.x
473

surface and found that osseous adaptation
mechanisms during orthodontic loading
resulted in increased implant stability.
Mini-implants with a small diameter
were developed to ease the surgical inser-
tion procedure and to allow anchorage at
different positions of the alveolar bone.
Both clinical and experimental studies
have demonstrated that these implants
were basically able to provide sufficient
and stable anchorage for tooth movement
during the entire time period of orthodontic
therapy. Whereas dental implants have
high long-term success rates at about 90–
95% (Adell et al. 1981; Albrektsson et al.
1981), mini-implants failed to reach these
high success rates despite the shorter usage
period. Besides infection-dependent im-
plant failure, factors that decrease implant
success are related to interrelated biome-
chanical parameters such as implant de-
sign, bone quality and extent and time of
loading (Orenstein et al. 1998; Miyawaki
et al. 2003). Whereas titanium screws have
mainly been evaluated as orthodontic an-
chorage systems in clinical trials (Park
2001), less is known about the factors
affecting implant stability in respect to
load-related biomechanics. Recent reports
on mini-implant anchors used in clinical
studies (Park 2001; Sawa et al. 2001)
demonstrated a high number of failures.
Thus, standard clinical protocols for a bio-
mechanical by related insertion and loading
scheme are not present.
The objective of the present study was
therefore to investigate the stability of two
mini-implant systems under immediately
applied continuous orthodontic loading.
Different loads as well as various insertion
situations, mimicking the clinical situa-
tion, were used to gain insight into an
optimised application of these anchor
systems.
Material and methods
Implant designs
Screw-shaped titanium mini-implants
with a length of 10 mm and a diameter
of 1.1 mm (Abso Anchors, Dentos. Inc.,
Taegu, Seoul, Korea) and 1.6 mm (Dual
Tops, Jeil Medical Corporation, Korea)
were used in this study (Fig. 1).
Experimental animals and mini-implants
Eight male Go¨ttinger minipigs, 14–16
months of age and with an average body
weight of 35 kg, were used. A total of 200
implants (102 Abso Anchors and 98 Dual
Tops) were placed in the mandible of
Go¨ttinger minipigs (Fig. 2). In accordance
with the experimental design, two treat-
ment groups were tested in each animal
(Table 1). The study was approved by the
Animal Ethics Committee of the Univer-
sity of Mu¨nster under the reference num-
ber G4/2004.
Surgical procedure
All surgery was performed under sterile
conditions in a veterinary operating thea-
tre. The animals were sedated with an
intramuscular injection of ketamine
(10 mg/kg), atropine (0.06 mg/kg) and
stresnil (0.03 mg/kg). In the areas exposed
to surgery 4 ml of local anaesthesia (2%
lidocaine with 12.5mg/ml epinephrine,
xylocain/adrenalines, Astra, Wedel, Ger-
many) was injected. The left and right
mandible was exposed by skin incisions
via fascial-periosteal flaps (Fig. 3). There-
after, the implants were placed in the
caudal part of the mandible, using spiral
drills according to the standard protocol of
the manufacturer.
Two corresponding implants were in-
serted by prefabricated distance holders in
a standard distance of 17 mm (Fig. 3) at
various insertion depths (Fig. 4), leading to
force applications at different (1, 2 and
3 mm) distances from the crestal bone
(Table 1). After insertion the screws were
immediately loaded with transverse forces.
The load was provided by Sentalloys
(GAC, Gra¨felfing, Germany) superelastic
tension coil springs (Fig. 5), which develop
a virtually constant force of 100, 300 or
500 cN (Table 1). The tension coil springs
were protected against soft tissue ingrowth
Fig. 1. Screw-shaped titanium mini-implants with a
length of 10 mm and a diameter of 1.1 mm (Abso
Anchors) left and 1.6 mm (Dual Tops) right.
Fig. 2. Minipig with placed mini-implants in the
mandible.
Table 1. Experimental design
Minipigs 8
Implants 192þ 8 fractured
Sacrifice (days) 22 70
Implant type Abso Anchors/Dual Tops Dual Tops/Abso Anchors
Implants 96 96
0 cN, 1 mm lever arm Group: LT 0 8 � 8 � 8 � 8 �
100 cN, 1 mm lever arm Group: LT 1 8 � 8 � 8 � 8 �
300 cN, 1 mm lever arm Group: LT 2 8 � 8 � 8 � 8 �
500 cN, 1 mm lever arm Group: LT 3 8 � 8 � 8 � 8 �
300 cN, 2 mm lever arm Group: LT 4 8 � 8 � 8 � 8 �
300 cN, 3 mm lever arm Group: LT 5 8 � 8 � 8 � 8 �
LT, loading type.
Bu¨chter et al . Use of mini-implants in orthodontic anchorage
474 | Clin. Oral Impl. Res. 16, 2005 / 473–479

by small latex tubes. Screws without load-
ing served as control. Six different groups
(loading types¼LT, Table 1) were evalu-
ated: 0 cN, 1 mm lever arm (LT 0); 100 cN,
1 mm lever arm (LT 1); 300 cN, 1 mm
lever arm (LT 2); 500 cN, 1 mm lever arm
(LT 3); 300 cN, 2 mm lever arm (LT 4) and
300 cN, 3 mm lever arm (LT 5). The tip
moment (TM) at the bone rim is calculated
by the formula: TM¼ force � lever arm.
After coil application the skin and the
fascia periosteum were closed in separate
layers with single resorbable sutures (Vi-
cryls4-0, Ethicon, Norderstedt, Ger-
many). Perioperatively, an antibiotic was
administered subcutaneously (2.5 ml benzyl-
penicillin/dihydrostreptomycin, Tardomy-
cels, BayerVital, Leverkusen, Germany),
every 48 h for 7 days.
Clinical follow-up
The animals were inspected after the first
few postoperative days for signs of wound
dehiscence or infection and weekly there-
after to assess general health. Loading per-
iods of 22 and 70 days were used for half of
the implants. At days 22 and 70 animals
were sacrificed with an overdose of T61
given intravenously. Following euthanasia,
mandibular block specimens containing
the two corresponding implants and sur-
rounding tissues were dissected from all of
the animals. The samples were sectioned
by a saw to remove unnecessary portions of
bone and soft tissue. Implants were con-
trolled for signs of material failure, clinical
mobility, peri-implant infection and im-
plant distance was measured.
Removal torque testing
The removal torque test was performed by
applying a counter-clockwise rotation of
the implant about its axis at a rate of
0.11/s according to the experimental set
up of Li et al. (2002) (Fig. 6). For each
implant the torque rotation curve was
recorded. The removal torque was defined
as the maximum torque (N mm) on the
curve.
Statistical analysis
Mean values and standard deviations were
calculated for removal torque testing. Be-
tween both groups descriptive statistic was
used, because of the different implant geo-
metry. Multiple comparisons between
groups were performed using two-way ana-
lysis of variance and t-tests. Difference was
considered significant when Po0.05. All
calculations were performed through the
use of SPSS for Windows (SPSS Inc.,
Chicago, IL, USA).
Results
Clinical observation
During the insertion process six Abso An-
chors and two Dual Tops implants frac-
tured the caudal of the head and were not
included in the study; new implants were
inserted. All other implants were clinically
immobile after insertion. Implants were
monocortically inserted in the mandibular
bone. The animals recovered well after
surgery and no signs of infection were
noted at any time during the observation
period. All together five tension coils were
lost (two Abso Anchors LT 4 (one after 22
and one after 70 days); two Abso Anchors
LT 5 (22 days); one LT 3 (22 days) (Tables 2
and 3) and one Dual Tops LT 5 (22 days).
Three of the Abso Anchors implants
showed a little tip in the direction of the
applied force, so that the tension coils
slipped from the implants head. The reason
for the loss of the two tension coils is
unknown. All other implants, without
the five loosening implants, showed a
good stability at a clinical level. None of
the implants was found to have moved
through the bone. One Dual Tops, one
Abso Anchors LT 5 (22 days) and three
Abso Anchors LT 5 (70 days) showed an
implant bending in the upper part of the
screws accompanied by peri-implant
bone loss and slight signs of inflammation
(Table 4).
Removal torque
During the removal torque test one Abso
Anchors and one Dual Tops implants (22
days) fractured the caudal of the head. Over
the experimental periods the Dual Tops
(22 (without LT 3) and 70 days) and Abso
Anchors (22 days) reference implants of
both groups revealed a significantly higher
implant/bone stability than those of the
loaded implants (Tables 5 and 6). Dual
Tops implants revealed a higher mean
removal torque value compared with
Fig. 3. Prefabricated distance holder for two corre-
sponding implants.
Fig. 4. Various insertion depths by prefabricated
distance holder.
Fig. 5. Loaded implants with transverse force by
superelastic tension coil springs.
Fig. 6. Removal torque test.
Bu¨chter et al . Use of mini-implants in orthodontic anchorage
475 | Clin. Oral Impl. Res. 16, 2005 / 473–479

Abso Anchors implants. As the implant
geometry differed between both implants,
removal torque testing was not statistically
compared between these two groups. The
results of the torque testing increased for
Abso Anchors implants stability over
time (significantly at LT 3). Whereas
Dual Tops implants showed no increase
in implant stability during the experiment.
Implant stability in terms of resistance
towards torsion forces were not statistically
dependent from the applied TMs.
Discussion
Whereas conventional or modified oral
implants have been shown to successfully
serve as anchorage for orthodontic appli-
ances (Wehrbein et al. 1996) mini-implants
failed to reach these high success rates.
When the high failure rates of mini-
implants are under evaluation two main
factors have to be considered. The biome-
chanical loading of peri-implant bone as
well as the time schedule of loading, have
been shown to have a major impact on the
peri-implant bone healing (Meyer et al.
2004) and can be assumed to determine
the clinical fate of mini-implants. A mono-
cortical mandibular anchorage system as
well as a load application in magnitudes of
orthodontic practice was therefore used in
the present study to gain insight into an
optimised clinical loading protocol.
Two main findings can be demonstrated
through our experimental approach. First,
implant failure is directly related to the
tipping moment (TM) at the bone rim.
Second, by reducing the main TM under
a threshold of 900 cN mm (300 cN and
3 mm lever arm) mini-implants can be
loaded immediately without impairment
of implant stability and implant success
rates. In most of the long-term clinical
studies implant failures have been attribu-
ted to overloading or excessive loading
when no peri-implantitis phenomena
were present. Most of the implants losses
were considered to be the result of exces-
sive strains and stresses at the bone/
implant interface (Adell et al. 1981). The
present study shows that all the implants
installed into mandibular bone were suc-
cessfully loaded and remained stable
throughout the entire duration of the study,
when tipping forces were not higher then
900 cN mm. This finding is in agreement
with earlier studies (Roberts et al. 1989,
1994) demonstrating that implants re-
mained stable when subjected to tip forces
ranging from 100 to 300 cN. Studies by
Isidor (1996, 1997) revealed that excessive
lateral loading of conventional osseointe-
grated implants, a loading regimen that
was also used in our experimental set up
(900 cN mm), resulted in a high risk for the
loss of osseointegration. The fact that the
degree of loading has a direct influence on
the stability of implants confirms the
superior influence of loads generated by
function. A number of in vitro studies,
including finite-element analysis (FEA),
have reported stress concentrations to oc-
cur in the marginal peri-implant bone after
lateral or oblique load application (Borchers
& Reichart 1983; Solte´sz & Siegele 1984;
Mihalko et al. 1992). FEA clearly demon-
strate that the local strain distribution had
a significant impact on the biological activ-
ity of the adjacent bone tissue (Meyer et al.
2001a). FEA calculations of Isidor (1997)
revealed that not all forces may be tolerated
by the crestal bone on a long-term basis.
High strain values above 6700 mstrain re-
sulted in peri-implant bone resorption and
a negative balance between bone apposition
Table 2. Clinical results after 22 days
Implants types Dual Tops Abso Anchors
Tension
coil lost
Implant
loosening
Implant fracture/
removal torque
Tension
coil lost
Implant
loosening
Implant fracture/
removal torque
0 cN, 1 mm neck/bone distance
100 cN, 1 mm distance to the bone
300 cN, 1 mm neck/bone distance
500 cN, 1 mm neck/bone distance 1
300 cN, 2 mm neck/bone distance 1 1
300 cN, 3 mm neck/bone distance 1 1 2 1 1
Table 3. Clinical results after 70 days
Implants types Dual Tops Abso Anchors
Tension
coil lost
Implant
loosening
Implant fracture/
removal torque
Tension
coil lost
Implant
loosening
Implant fracture/
removal torque
0 cN, 1 mm neck/bone distance
100 cN, 1 mm distance to the bone
300 cN, 1 mm neck/bone distance
500 cN, 1 mm neck/bone distance
300 cN, 2 mm neck/bone distance 1
300 cN, 3 mm neck/bone distance 3
Table 4. Implants loosening in relation to
tip moment (TM) [TM (N mm)¼ force
(N) � lever arm (mm)]
0
1
2
3
4
Im
pl
an
t l
oo
se
ni
n
g
(n
)
100
cNmm
500
cNmm
900
cNmm
Abso Anchor
Dual Top
Bu¨chter et al . Use of mini-implants in orthodontic anchorage
476 | Clin. Oral Impl. Res. 16, 2005 / 473–479

and bone resorption (Isidor 1997), confirm-
ing the hypothesis posted by Frost (1998) as
well as the findings of several authors, who
reported adaptation mechanisms to a win-
dow of physiological loading (Meyer et al.
1999a). This is also in agreement with
observations made by Hoshaw et al.
(1994), who reported that high stress con-
centration may cause marginal bone re-
sorption around implants in a load model
(Meyer et al. 2001b; Bu¨chter et al. 2005).
Some studies using static load models con-
firm our results that low mechanical load is
not accompanied by marginal bone defects
and impaired bone mineralization (Meyer
et al. 2001a). This, in turn, means that
mini-implants could serve as anchorage for
orthodontic force systems when loads do
not exceed a tolerable strain level. It is
important to note that the amount of
stresses and strains are dependent on the
geometry of the screw as well as on the
mechanical properties of the implant and
bone. As we have not performed an FEA
analysis in our investigation, strain and
stress values in peri-implant bone cannot
directly be correlated to implant instabil-
ities and loss rate in our study. The finding
of differences in implant stability between
implants systems may be based in the
differences in implant geometry influen-
cing the strain and stress distribution in
peri-implant bone (Bu¨chter et al. 2004).
The results of the present study confirm
that implants can be immediately loaded
by continuous forces. While it has been
stated in some studies that immediate or
early loading of oral implants impedes
successful osseointegration (Sagara et al.
1993), recent studies suggest that immedi-
ate loading can be performed under defined
conditions (Meyer et al. 1999a, 1999b,
2004). Different investigations have indi-
cated that although premature loading has
been interpreted as inducing fibrous tissue
interposition, immediate loading per se is
not responsible for fibrous encapsulation. It
is the excess of micromotions during the
healing phase that interferes with bone
repair (Szmukler-Moncler et al. 1998). Ex-
perimental observations suggest that mi-
cromotions do not systematically lead to
fibrous tissue interposition and that toler-
ance to micromotion is design and load
dependent. The effect of a continuous lat-
eral or oblique load on the peri-implant
bone has been described in in vitro experi-
ments, also using FEA (Solte´sz & Siegele
1984; Mihalko et al. 1992). As these mod-
els reveal that under conditions of a direct
bone/implant contact and moderate load
applications, marginal strains are lower
than the upper limit of tolerable bone
strains, immediate loading seems to be
possible without impairment of implant
stability. Furthermore, experimental stu-
dies have not generally been able to de-
monstrate implant loosening induced by
orthodontic load (Wehrbein et al. 1997;
Akin-Nergiz et al. 1998) even when these
loads were applied immediately. Addition-
ally, a number of clinical studies have
confirmed a positive effect of orthodontic
loading on the stability of titanium screw
implants, as well as the effects on peri-
implant bone (Roberts et al. 1994; Wehr-
bein et al. 1993, 1997; Hurzeler et al.
1998). The clinical view that loaded mini-
implants showed no movement through
the bone is confirmed by the present study.
Whereas this study was of short duration
and, hence, possible long-term effects on
implant stability were not elucidated, it
must be stressed that most of the load
Table 5. Removal torque values (N mm), Abso Anchors
Treatment group Loading
type (LT)
Days 22 Abso
Anchors (N mm)
Days 70 Abso
Anchors (N mm)
Change
within group
Change days
22 vs. 70
Reference, 1 mm distance neck/bone 0 31 � 1.41 29.85 � 1.48 � 1.15 � 1.53 P¼ 0.482
100 cN, 1 mm distance neck/bone 1 11.15 � 1.33 38.25 � 11.99 27.1 � 11.99 P¼ 0.109
300 cN, 1 mm distance neck/bone 2 15.35 � 6.54 41.75 � 6.53 26.4 � 6.83 P¼ 0.23
500 cN, 1 mm distance neck/bone 3 13.95 � 4.08 33 � 141 19.05 � 4.1 P¼ 0.018
300 cN, 2 mm distance neck/bone 4 15.5 � 4.18 30.15 � 10.86 14.65 � 11.06 P¼ 0.233
300 cN, 3 mm distance neck/bone 5 15.05 � 4.24 15.25 � 1.25 0.2 � 2.46 P¼ 0.938
Change reference vs. 100 cN, 1 mm distance P¼ 0.001 P¼ 0.535
Change reference vs. 300 cN, 1 mm distance P¼ 0.003 P¼ 0.165
Change reference vs. 500 cN, 1 mm distance P¼ 0.024 P¼ 0.12
Change reference vs. 300 cN, 2 mm distance P¼ 0.004 P¼ 0.98
Change reference vs. 300 cN, 3 mm distance P¼ 0.0043 P¼ 0.001
Table 6. Removal torque values (N mm), Dual Tops
Treatment group Loading
type (LT)
Days 22 Dual
Tops (N mm)
Days 70 Dual
Tops (N mm)
Change within
group
Change days
22 vs. 70
Reference, 1 mm distance neck/bone 0 109 � 2.08 111.05 � 3.05 2.05 � 2.08 P¼ 0.363
100 cN, 1 mm distance neck/bone 1 76.25 � 3.8 73.9 � 6.49 � 2.35 � 7.53 P¼ 0.765
300 cN, 1 mm distance neck/bone 2 72.75 � 5.28 55.6 � 5.32 � 17.15 � 7.5 P¼ 0.062
500 cN, 1 mm distance neck/bone 3 82.3 � 15.72 52.05 � 6.89 � 30.25 � 17.17 P¼ 1.29
300 cN, 2 mm distance neck/bone 4 42.95 � 5.09 55.15 � 7.41 12.2 � 6.3 P¼ 0.101
300 cN, 3 mm distance neck/bone 5 60.15 � 5.87 59.95 � 18.1 � 0.2 � 19.59 P¼ 0.992
Change reference vs. 100 cN, 1 mm distance P¼ 0.001 P¼ 0.002
Change reference vs. 300 cN, 1 mm distance P¼ 0.003 P¼ 0.004
Change reference vs. 500 cN, 1 mm distance P¼ 0.143 P¼ 0.01
Change reference vs. 300 cN, 2 mm distance P¼ 0.001 P¼ 0.001
Change reference vs. 300 cN, 3 mm distance P¼ 0.002 P¼ 0.0797
Bu¨chter et al . Use of mini-implants in orthodontic anchorage
477 | Clin. Oral Impl. Res. 16, 2005 / 473–479