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Stability of Miniscrews at Different Time Points, Measuring From Cone-Beam CT
Prasanpong Pongsamart1,*, Suwannee Luppanapornlarp2,#, Supatchai Boonpratham2,#,
Suchaya Pornprasertsuk-Damrongsri3,#
1
Department of Orthodontics, Faculty of Dentistry, Mahidol University, Master of Sciences
(M.Sc.) in Orthodontics (International Program), Thailand
2
Department of Orthodontics, Faculty of Dentistry, Mahidol University, Thailand
3
Department of Oral and Maxillofacial Radiology, Faculty of Dentistry, Mahidol University,
Thailand
*p_pongsamart@hotmail,com, #[email protected], #[email protected],
#[email protected]
Abstract
Introduction: Nowadays, the miniscrew is reported to be an effective device to
reinforce anchorage in orthodontics. Many studies proved that miniscrews can provide
absolute anchorage. However, some studies reported that miniscrews can be displaced by
loading. Materials and methods: 24-miniscrews were placed between upper second premolars
and first molars using a guide loop device in 12 patients. The miniscrew at each side was
used to retract upper right and left canines. NiTi closed-coil springs of 50 or 150 g-force was
randomly applied from upper canine to miniscrew on each side. Cone Beam Computed
Tomography (CBCT) was taken at 2 and 3 months after loading to evaluate miniscrew
position and exported as Digital Imaging and Communications in Medicine (DICOM) files.
Head and tail of miniscrews together with anterior nasal spine as reference point were
recorded as X,Y,Z coordination points and were calculated for the displacement in
millimeters by Invivo5 imaging anatomy software. Results: The miniscrews were
significantly displaced after both forces were applied at 2 and 3 months (P<0.001). When
comparing miniscrew head and tail displacements using 50 and 150 g-forces, there were no
statistical significances at 2 months (P = 0.295, 0.059) but 150 g-force was found
significantly greater displacement than 50 g-force at 3 months (P = 0.032, 0.030).
Conclusions: A miniscrew can be used as an anchorage reinforcement on the maxilla.
However, it can be slightly displaced after force application, especially in heavier force.
Therefore, a miniscrew actually was not an absolute anchorage in clinical use.
Keywords: miniscrews, cone-beam ct, stability, absolute anchorage, displacement
Introduction
Each patient has various types and levels of difficulty regarding their problems for
which the orthodontist has to determine the best way to effectively treat each case, such as to
extract or not. In extraction cases, the anchorage situation must be included in the treatment
plan. Sources of anchorage are from the teeth, oral mucosa and underlying bone, implants
and extra-oral anchorage (1). However, anchorage loss might occur in the case of traditional
anchorage control.
Nowadays, to overcome the limitation of traditional anchorage control, the miniscrew
is reported to be an effective device to reinforce anchorage in orthodontics. Many studies
already proved that miniscrews can provided absolute anchorage (2-4). However, the position
of miniscrews after insertion is an important factor to determine absolute anchorage. To
evaluate miniscrew position, a radiograph is the easiest way to evaluate angulation and
position. In the past, 2-Dimensional radiographs were used in medical and dental treatment
with this limitation, and now 3-Dimensional radiograph or Cone Beam Computed
Tomography (CBCT) has been invented to overcome the problem.
Methodology
The studied group was diagnosed as upper dental protrusion or severe crowding which
need miniscrews as anchorage reinforcement in orthodontic treatment. There were totally 12
female patients in this study. Mean age of the patients was 22.55±4.8 years old. Each patient
was randomly placed with 2 miniscrews at both right and left sides. This study determined to
compare miniscrew displacement before (T0) and after loading at 2 months (T1), at 3 months
(T2) and to compare the displacements of the miniscrew differences in 50 and 150 g-forces.
The protocol was approved by the committee on human right related to human
experimentation of Mahidol University to study in the human (COA. No. MU-IRB
2010/103.0804) before the study had begun.
All patients had met the criteria as followed: 1) the age of 18 to 30 years;
2) good oral and gingival health; 3) no systemic or bone diseases; 4) orthodontic treatment
plan of extraction upper first premolars with or without lower premolars; 5) maximum
anchorage with miniscrews in the upper arch for canine retraction; 6) completed orthodontic
record. They were informed and signed consent forms.
Each subject was placed with brackets (0.022 inch slot, Ormco Corp., Orange,
California, USA) on the left and right upper canines and second premolars. Molar bands with
transpalatal arch were inserted at upper first molars. Segmented archwires (0.018 inch
stainless steel wire) were used on both sides. 24 miniscrews (the AbsoAnchor® SH 1413-07,
Dentos, Daegu, Korea) were inserted between second premolars and first molars. A
miniscrew-placement guide were customized with 0.036 inch stainless steel loop components
on left and right sides to controlled the position and angle of miniscrews (Figure 1).
Miniscrew position was 6-8 mm below the CEJ between second premolars and first molars
where the interradicular distance was more than 3-4 mm study (5). Angle of insertion of
miniscrew was controlled by the loops with adjusted angle of insertion of 60°-70°(6). Bone
density was recorded during the miniscrew insertion as D1 to D4 based on a tactile sense (7)
by one operator (Pongsamart P). Prior to the miniscrew placement, operator had been
calibrated. All subjects were were given same oral hygiene instruction and same brushing
technique to prevent peri-miniscrew inflammation. Two weeks after miniscrew placement,
upper left and right canines of each subject were randomly retracted by 50 and 150 g of
nickel-titanium (NiTi) closed coil springs (Tomy®, Tokyo, Japan) loading from miniscrews
(Figure 2). The force magnitude of each coil spring was measured and recorded with a
calibrated orthodontic force gauge (Gram Gauges, Mecmesin Asia Co. Ltd., Bangkok,
Thailand).
Figure 1. A miniscrew surgical guide was customized to control the miniscrew’s position and angulation.
Figure 2. Miniscrews with nickel-titanium coil springs of 50 and 150 g to randomly retract upper left and
right canines.
Cone-beam computed tomography (CBCT; 3D Accuitomo FPD, J Morita MFG Corp.
Kyoto, Japan) using 6 x 6 cm field of volume (FOV) with the exposure factors of 75 - 80 kV,
4-5 mA and 17.5 sec was taken to evaluate the miniscrew position at before loading force as
baseline (T0) and after force application at 2 months (T1) and 3 months (T2). The safety of
subject was followed the ALARA principle (As Low As Reasonably Achievable) (8). The
CBCT data were exported in Digital Imaging and Communications in Medicine (DICOM)
multi-file format and exported to 3D imaging software (Invivo5, Anatomage Co., San Jose,
California, USA; Figure 3). Anterior nasal spine (as a reference point), head and tail of
miniscrews were manually digitized and recorded on each volume to set the X, Y, Z
coordination point as origin (0, 0, 0). The head of miniscrew was at the middle point of the
top surface of miniscrew. The tail and the anterior nasal spine were the sharpest point of the
thread part and the spine respectively. Displacement distances of miniscrew heads and tails
were performed using a 3-dimensional equation formula (9). The 3-dimensional distance
equation formula definition between two points was the length of the path connecting them.
The path distance was a straight line. The distance between points (X1, Y1, Z1) and (X2, Y2,
Z2) was given by d =
. Displacement distances of
miniscrew heads and tails from 50 and 150 g loading were then calculated from T1-T0, T2T0, and T2-T1.
Figure 3. Head and tail of the miniscrew, and anterior nasal spine as a reference point were recorded
to X,Y,Z coordination points at T0, T1, and T2 using Invivo5 anatomy imaging software.
Displacement distances of miniscrew head and tail from T0-T1, T0-T2, and T1-T2 were calculated in
millimeters.
All data of miniscrew displacements were computed for means and standard
deviations using the Statistical Package for Social Sciences version 17.0 (SPSS Inc., Chicago,
Illinois, USA). The Shapiro-Wilk test was used to investigate the normal distribution.
Miniscrew displacements during T0–T1, T0–T2 and T1-T2 from 50 and 150 g were
estimated by one sample t-test and compared by pair t-test statistics. The statistical significant
values were set at P < 0.05.
Measurements of all miniscrew displacements were repeated twice by the same
operator (Pongsamart P) one month interval. A paired t - test was used to calculate the
measurement error. Cronbach’s alpha was calculated to test the reliability of the
measurement.
Results
The data of this study showed normal distribution when determined with the ShapiroWilk normality test. Cronbach’s coefficient alpha of all values showed an acceptable higher
number of reliability (Table 1). Bone density levels of the upper right and left alveolar bones
at miniscrew placement areas of all female subjects were between D2 and D3 which was
interpreted as normal bone quality.
Table 1. Reliability test of miniscrew displacement (Cronbach’s alpha)
Time
Miniscrew
Head
2 months (T1)
Tail
Head
3 months (T2)
Tail
Force (g)
50 g
150 g
50 g
150 g
50 g
150 g
50 g
150 g
Cronbach's alpla values
0.952*
0.933*
0.855*
0.883*
0.953*
0.889*
0.956*
0.902*
*Cronbach’s alpla > 0.7
Table 2 and 3 show means and standard deviations at 2 (T1) and 3 months (T2) of
miniscrew-displacement heads and tails loading with 50 and 150 g, and show the
comparisons within groups at two time points (T1 and T0, T2 and T0). At T1, the
displacements of miniscrew heads and tails for the force of 50 g were 0.331 ± 0.066 and
0.257 ± 0.075 mm respectively. For the 150 g, displacements of miniscrew heads and tails
were 0.352 ± 0.046 and 0.306 ± 0.066 mm. Both forces showed statistically significant head
and tail displacements when compared to their baselines (T1-T0, P < 0.001). At T2, the
displacements of miniscrew heads and tails for the 50 g were 0.385 ± 0.090 and 0.316 ±
0.115 mm respectively. Head and tail displacements for the 150 g were 0.453 ± 0.082 and
0.398 ± 0.089 mm respectively. Statistically significant differences of miniscrew head and
tail displacements of both 50 and 150 g were also found when compared to their baselines
(T2-T0, P < 0.001). In addition, each force showed comparisons of means miniscrew
displacements of heads and tails between 2 and 3 months with significant differences at table
3 (T2-T1, P < 0.05). Interestingly when compared miniscrew displacements between forces
of 50 and 150 g at T1, table 4 show that no statistically significant differences (P = 0.295 and
0.059, Respectively). But at T2, statistically significant head and tail displacements were
found with P value of 0.032 and 0.030 respectively (P < 0.05).
Table 2. Mean and standard deviation comparison of miniscrew head and tail displacements before and
after loading at 2 (T1) and 3 (T2) months.
Time
2 months
(T1)
3 months
(T2)
Force (g)
50
150
50
150
Miniscrew
Head
Tail
Head
Tail
Head
Tail
Head
Tail
N
12
12
12
12
12
12
12
12
Mean±SD
0.331±0.066
0.257±0.075
0.352±0.046
0.306±0.066
0.385±0.090
0.315±0.115
0.453±0.082
0.398±0.089
P-value
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
95%CI
(0.290, 0.373)
(0.210, 0.305)
(0.324, 0.380)
(0.266, 0.346)
(0.324, 0.446)
(0.238, 0.392)
(0.400, 0.505)
(0.341, 0.454)
*P < 0.001
Table 3. Mean and standard deviation comparison of miniscrew head and tail displacements at 2 (T1) and 3
(T2) months.
Force (g)
50 g
150 g
Miniscrew
Head
Tail
Head
Tail
Mean±SD (Time)
2 months (T1)
3 months (T2)
0.331±0.066
0.385±0.090
0.257±0.075
0.315±0.115
0.352±0.046
0.453±0.082
0.306±0.066
0.398±0.089
P-value
0.013*
0.005*
<0.001*
0.001*
*P < 0.05
Table 4. Comparing of mean and standard deviations of miniscrew displacements between 50 and 150 g
Time
2 months (T1)
3 months (T2)
Miniscrew
Head
Tail
Head
Tail
Mean±SD (Force)
50 g
150 g
0.331±0.066
0.352±0.046
0.257±0.075
0.306±0.066
0.385±0.090
0.453±0.082
0.315±0.115
0.398±0.089
P-value
0.295
0.059
0.032*
0.030*
*P < 0.05
Discussion and Conclusion
Duration and magnitude
Niti closed coil springs were used in this study because they provided a constant force
(10). Shpack et al. found that the duration of canine retraction has been reported to take
102±106 days with tipping and uprighting movement and 99±80 days with bodily movement,
achieved by 50 and 75 g NiTi-coil spring (11). Deguchi et al. also found that canine retraction
duration with 50, 100, 150 g-force loading took approximately 3 months on average
(minimum 2 months and maximum 5 months) (12). According to the studies of Shpack et al.
and Deguchi et al., the canine retraction duration always took 2-5 months. Therefore, this
study was designed to evaluate miniscrew displacement during 3 months by loading force
application from upper canine to miniscrews.
Additionally, the force requirement of each type of orthodontic tooth movement varies
depending on bone density, tooth morphology and friction along the archwire. It was found
that both of magnitudes of 50 and 150 g-forces from NiTi-coil spring can distalized the
canine along the archwire by sliding mechanics (13). Thus, using 50 and 150 g-forces for
canine retraction was suitable and achievable in this study.
Miniscrew selection
Liou et al. studied about stability of miniscrew (diameter 2 mm, length 17 mm), they
reported that miniscrews can be displaced within 1.5 mm at 9 months, thus they require 2mm for safety clearance to the roots (14). In our study, miniscrews used had safety for the
patients because there were smaller in diameter, shorter length and less displacement.
Miniscrew with diameter less than 1.2 mm will increase the risk of fracture (15, 16).
However, Wu et al. found that a miniscrew diameter equal to or less than 1.4 mm was
recommended in maxilla (17). Therefore, 1.4 mm in diameter and 7 mm in length miniscrews
used for this study for the interradicullar space at the area between the upper second premolar
and first molar were suitable (5).
Insertion angle of miniscrew
Insertion angle provides more cortical bone contact than perpendicular insertion and
also can be avoided root damage (18). In our study, even though insertion angle did not affect
success rate but miniscrews were angled 60-70 degree insertion to ensure that there were
more cortical bone contact (6) and not to affect other confounding factors.
Bone density of the patient
Bone density is one of the major factors of miniscrew stability. Turkyilmaz et al found that
the mean bone density values were 708±277 HU in anterior maxilla and 505±274 HU in
posterior maxilla (19). Similar to the study of Norton et al. (20), they found that the mean
bone density values of the anterior maxilla and posterior maxilla were 696 and 417 HU,
respectively. Furthermore, Shapurian et al. also reported that the bone density values of
anterior and posterior maxilla were 517 and 333 HU, respectively. In our study, only female
samples were recruited had maxillary bone density values of D2 and D3 ranging from 350 to
850 HU (7). Therefore, our samples were normal bone density.
The area of buccal cortical bone between the first and second upper premolar was the
most suitable bone density for miniscrew placement (21). In our study, first premolars were
removed as treatment plan in all patients that might affect bone density of the area between
upper first and second premolars. Moreover, the area above CEJ 5-8 mm between the second
upper premolar and first upper molar was the safest zone and also showed similar bone
density to the area between the first and second premolars (5) explaining why this area was
chosen for miniscrew placement.
Delayed VS immediate loading
Park et al. reported that the success rate of miniscrews between delayed loading and
immediate loading showed no significant difference in success rate (22). Miyawaki et al. also
reported the factors associated with success rates of miniscrew that success rates of the
waiting period had no statistical significance (23).
Moreover, osteointegration of a dental implant prosthesis requires a healing period of
several months, but it is not necessary for the miniscrews in orthodontic treatment (24). They
are used for only the temporary anchorage device. Therefore, a waiting period of miniscrews
before loading might not be necessary.
Although, many studies reported that delayed or immediate loading was not different
to stability of miniscrews. However, delayed loading might be better for soft tissue healing
because soft tissue irritation resulted from twisting miniscrew can occurred in some patients.
Some studies suggested that a 2-week soft tissue healing period was required for miniscrew
placement because 2 weeks was sufficient for soft tissue healing but insufficient for
osteointegration (24, 25). Therefore, the waiting period for loading of this study was 2 weeks
after miniscrew placement.
Cone Beam Computed Tomography (CBCT)
According to the principle of cephalometric superimposition of maxilla, the
anatomical landmarks which can be used for superimposition included the anterior nasal
spine, pterygomaxillary fissure, infraorbital foramen and posterior nasal spine (26). In this
study, the anterior nasal spine was the best anatomical landmark in this study which presents
an obviously sharp point structures the easiest structure to be localized.
For dose consideration, the effective dose of 3D Accutomo FPD in our study was 1177 μSv for one scan (27). It was reported that the one scan of maxilla caused approximately
29-44 μSv (28). In our study, each patient was taken 6 images from 3 times of CBCT (T0T2). Therefore, the maximum exposure of the radiation in each patient was 264 μSv (T0-T2,
44 μSv per image) or 0.264 mSv. This was less than exposure limits from ICRP publication
103 (41). In consideration, the study design was safety for all patients and operators.
Gold standard of measurement
Generally, skull can be clinically and radiographically measured. Many studies
reported that accuracy of CBCT were not statistical difference from caliper to measured the
skull (9, 29, 30). Therefore, CBCT and caliper measurement of skull can be used as a gold
standard of measurement.
Hassan et al. found that the gold standard measurement accuracy of the human skull
by CBCT was 0.5 mm (31). The study of Stratemann et al. also confirmed that error of CBCT
on skull measurement was small compared with physical caliper measurement. The gold
standard of CBCT measurement was 0.07 to 0.41 mm (32). From the Table 1, the mean
difference between raw data and error measurement was less than 0.1 mm. Therefore, the
measurement of this study was quite accurate, compared with the gold standard of
measurement.
Stability of miniscrews
Absolute anchorage has been defined as the situations where the anchorage units are
completely stationary in response to reaction forces applied to move teeth (33). In the
previous studies, many studies about miniscrew stability in 2 dimensions had reported that
miniscrew can be displaced after loading (14, 34, 35). However, rare studies of 3 dimensions
were available. El-Beialy et al. studied miniscrew displacement by CBCT. They found that
the miniscrew head and tail displaced 1.08 and 0.83 mm, respectively at 6 months after
loading (36). Alves et al also studied displacement of 30 miniscrews (diameter 1.4 mm,
length 8 mm) by CBCT. They reported that miniscrew head and tail were displaced
approximately 0.3-0.8 mm after loading 5 months (37).
The results of our study showed significant displacement of miniscrews after loading
at 2 and 3 months on 50 g-force and 150 g-force. The average displacements was
approximately 0.3-0.4 mm. However, miniscrew displacement from the study of El-Beialy et
al. was greater than in our study. This might be because they used higher loading (150 to 250
g), longer time (at 6 months after loading), and smaller in diameter (1.2 mm). In the study of
Alves et al., they used 100 g force and the same miniscrew diameter. Therefore, their result
was shown similar displacement to our study. This might be because the magnitude of
miniscrew displacement was not much different from our study. Moreover, displacement of
miniscrew in our study showed slightly greater with 150 g-force than 50 g-force at both 2 and
3 months. At 2 months, the mean difference between 50 and 150 g-force was not significant
but at 3 months showed significance. From the result of our study, it can be apply for clinical
practice that using lighter force for canine retraction in suitable duration could be more
success in preventing miniscrew failure.
Finally, the displacement of miniscrew in our study was statistically significant but it
seems clinically shown little displacement. This could be the reason that miniscrews can be
effectively used to reinforce orthodontic anchorage. The mean difference of displacement
was just 0.1-0.2 mm. that was not clinically significance in our practice.
Conclusion
Loading had a direct effect on miniscrew stability. Even though miniscrew was
widely used as anchorage reinforcement, they can be displaced by loading. From the result of
this study, we can conclude that:
1. Miniscrews were slightly displaced after loading with continuous forces of 50 and 150 g
for canine retraction during 3 months. The longer duration of miniscrew was used, the
greater miniscrew displacement was observed.
2. During early 2 months, the heavier force resulted in the same displacement as in the
lighter force. But at 3 months, a significant miniscrew displacement was observed
between 50 and 150 g (150 g > 50 g), and at head more than at tail.
3. Clinically, the displacement in this study might be considered as very small (0.3 – 0.4
mm) compared to other studies. However, lighter force for canine retraction seems to be
more success in preventing miniscrew failure. It is suggested to use proper magnitude of
force to miniscrews with care to overcome the failure.
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