healthmedicinet

Various kinds of mini-implants have been used for orthodontic anchorage reinforcement
ever since Kanomi et al. 13] suggested titanium mini-implants as intraoral anchorage devices. Wu et al.14] studied the success rate of mini-implants, concluded that careful diameter selection
for different locations is essential, and recommended an implant diameter equal to
or less than 1.4 mm in the maxilla, and diameter larger than 1.4 mm in the mandible
was suggested for better orthodontic anchorage. An assortment of geometric designs
based on length, diameter, composition of alloy, thread pitch, taper, and shapes of
head are available and are being tried clinically, and usually, the insertion angle
of mini-implants varies most often according to the clinician’s preference. Therefore,
it is necessary to compare the efficacy in terms of stress induced in the metal and
bone among the mini-implants of various geometric designs and insertion angles, when
they are subjected to force application and directions, according to the clinical
requisite (e.g., retraction force, intrusion and retraction, extrusive force).

The finite element method is an effective tool to identify optimal design parameters
and allow for improved mini-implant designs. The comparative analysis of numerical
and experimental data of orthodontic mini-implants by Chatzigianni et al.15] revealed a tendency that the finite element analysis offers a promising alternative
to experimental procedures. Hence, this study aimed to evaluate stress distribution
pattern among varying mini-implant dimensions of length, diameter, and insertion angulation,
when subjected to orthodontic loads directed to simulate clinical situations of anterior
segment retraction, anterior intrusion and retraction, and molar intrusion in a mathematical
model using the FE method.

To simulate orthodontic force levels, a force of 2 N was used in this numerical analysis
since previous studies used a load application of 2 N; but the study by Chatzigianni
et al.12] showed that differences in the results can also be explained by the applied force
level and a difference was found between the mini-implant groups in their study when
a high force of 2.5 N was applied. Further analysis of their data revealed that the
level of 1 N could be defined as the threshold for differentiation; but even they
agree that with the majority of clinical studies cited, load application was 2 N or
less and therefore no clear discrimination between force levels could be observed.

It has long been recognized that both the implant and bone should be stressed within
a certain range for physiological homeostasis. This mechanical stress in turn causes
strain in the bone tissue which is defined as a relative change in length, whether
lengthening or shortening. The degree of the strain correlates with stress and the
bone’s mechanical characteristics. According to Frost 16] (2003), the amount of strain can be divided into various ranges, permitting us to
predict the effects on the bone. The lower limit of the bone’s equilibrium (i.e.,
of the load range within which, due to continuous bone remodeling processes, as much
bone tissue is formed as is resorbed) is roughly 50–100 ?Strain (1-2 MPa). Below this
limit, (due to underuse), the result is bone resorption. The upper limit of this range
is roughly 1000–1500 ?Strain (20 MPa). Bone formation is the initial response above
this limit. Additional strain, however, leads to micro-fissures and micro-fractures
in the bone tissue, which, at roughly 3000 ?Strain (60 MPa), surpasses ongoing repair
processes leading to bone resorption. Therefore, if the mini-implant displacement
exceeds the specified physiologic limit, it is likely to cause a micro-fracture of
the bone trabecula and result in absorption, and necrosis of the osseous tissue in
implant-bone interface ultimately leads to the failure of the mini-implant.

Stress analysis on mini-implant metal

In our study, stress values observed on the mini-implant have shown that for dimensions
1.3?×?6 mm and 1.3?×?8 mm, insertion angles at 30° and 60° had a minimum value of
19.85 MPa (Table 5) and a maximum value of 43.34 MPa (Table 4), which were well within the acceptable fatigue limit of titanium of 193 MPa 17]. FEM studies by Zhang et al. 18] have shown similar results with 30° insertion angulation of mini-implants producing
a decreased stress value of 22 MPa. They also concluded that when the mini-implant
was embedded with a tilted angle of 30°, the length would be doubled correspondingly
to penetrate the cortical bone. Therefore, while the tilted angle is decreased, the
contact area of the micro-implant and cortical bone is increased to enhance the stability
of micro-implants accordingly.

The stress values on mini-implant dimensions 1?×?6 mm and 1?×?8 mm of 30° insertion
angulation and 1?×?8 mm of 60° insertion angulation, however, showed a higher range
above the acceptable fatigue limit (210–270 MPa) (Tables 3 and 4). The other parameters (3b of Table 3, 3b, 4a and 4b of Table 4) showed a higher range but within acceptable fatigue limits of titanium (125–159 MPa). However, Table 4 depicting molar intrusion simulation did show a lower range between 75 and 111 MPa which was also within acceptable fatigue limits of titanium.
Miyawaki et al. 4] (2003) reported a higher success rate for mini-implants of diameters 1.2 and 1.3 mm,
than for the 1.6-mm diameter. He also reported 0 % success rate when 1-mm-diameter
mini-implants were used, stating a reason of higher chance of fracture when used and
therefore advocated that it was not suitable for clinical use. It was found in studies
by Melo Pithon et al. 9] that the torsional strength values increased as their diameters also increased. However,
such a reduced size also decreases the mechanical strength, thus reducing the maximum
torsional strength and resulting in deformation and fracture.

According to Lemieux et al. 19], during mini-implant length selection, the clinician should consider the important
trade-off between anchorage and risk of placement complications or damage to the tissues.
Longer mini-implants enable more anchorage; however, they are associated with a higher
risk of damage to neighboring structures. Placement depth and bone density at the
site of mini-implant placement are the best predictors of primary stability.

Stress analysis on the cortical bone

The stress distribution patterns in the cortical bone showed that, on inserting the
mini-implant of dimension 1.3 mm (inclusive of 6- or 8-mm length) at a 30° angulation,
the stress distribution in the cortical bone was only marginally decreased, as compared
to the 60° insertion angulation. The minimum stress distribution values obtained in
the cortical bone for 30° insertion angulation were 22.66 MPa (Table 3), 17.22 MPa (Table 4), and 14.15 MPa (Table 5), for the three directions of force application studied. These values were in accordance
with results obtained from studies by Motoyoshi et al. 8]. The highest stress values obtained were for the 60° insertion angulation—32.23 MPa
(Table 3), 29.33 MPa (Table 4), and 28.92 MPa (Table 5), in all three directions of force application. However, it is pertinent to note
that the difference in the minimal and maximal values was only marginal and well within
Frost’s 16] mechanostat values.

For the 1-mm-diameter mini-implant (Tables 3, 4, and 5), however, the stress values observed in the cortical bone for both 30° and 60° insertion
angles ranged between 47.25 and 89.89 MPa, except for group 3a of Table 3, which showed a maximum value of 106.36 MPa, which was also within Frost’s mechanostat
values.

Kyung et al. 20] advocate mini-implant insertion at 30°–40° to increase the surface contact between
the implant and bone and allow the insertion of a longer screw in the available bone
depth. Also, Deguchi et al. 21] believed that angling the implant at approximately 30° would increase contact with
as much as 1.5 times more to the cortical bone. Pickard et al. 22] studied the effect of mini-implant orientation on stability, and they found that
the more closely the long axis of the mini-implant approximates the line of applied
force, the greater the stability of the implant and the greater its resistance to
failure.

The effect of diameter on mini-implant stability has been compared by many authors.
Miyawaki et al. 4] (2003) and Seon et al. 7] (2003) reported that the diameter of the mini-implant affected the success rate the
most, as compared to the other dimensional parameters. The diameter also affects the
placement and removal of the mini-implant, which in turn affects the stability as
well. Barros et al. 23] showed that an increase in mini-implant diameters significantly influenced the increases
of placement torque and fracture torque on quantities that progressively reduced the
fracture risk. Lee et al. 24] in their study showed that mini-implants with larger diameters and tapered shapes
caused greater microdamage to the cortical bone. This they believe in turn might affect
bone remodeling and the stability of the mini-implants. Lui et al. 25] believe that the screw diameter was the dominant factor for mini-implant mechanical
responses. They showed both that bone stress and screw displacement decreased with
increasing screw diameter and cortex thickness and decreasing exposed length of the
screw, force magnitude, and oblique loading direction. Differences in implant diameter
could also influence other aspects of implant integration, such as induction of remodeling,
and could interact with other factors of mini-implants (e.g., when the implant is
loaded) to influence microdamage 26].

Melsen 27] believes that the length of a mini-implant should be determined by depth and quality
of the bone, screw angulation, transmucosal thickness, and adjacent vital structures.
Short screws in regions with thick soft tissues, such as the palatal mucosa, can easily
become dislodged and therefore these authors advocate use of lengths greater than
6 mm. Baek et al. 3] advocate the use of longer mini-implants in areas of thicker cortical bone, for increased
primary stability. Seon et al. 7] (2003) reported that the maintenance of the mini-implant is more reliable on the
length and since the cortical surfaces of the maxillary buccal area are thinner and
less compact than those of the mandible and therefore require longer mini-implants.
The study by Motoyoshi et al. 28] showed that screws of 1.2-mm diameter and at least 8-mm length are preferable, because
they are stable and minimize the risk of root damage; and Upadhaya et al. 29] have shown that when using a mini-implant with a length of 8 mm for molar intrusion,
vertical dimension control is maintained.

Stress analysis on the cancellous bone

Stress distribution in the cancellous bone when analyzed between Tables 3, 4, and 5 showed values ranging between 0.06 and 0.59 MPa, which could be considered as least
stress induced in the cancellous bone during simulated orthodontic tooth movement.
Studies by Zang et al. 18] have shown similar results where stress values in the cancellous bone ranged between
0.63 and 0.56 MPa. Based on their findings, they concluded that the cortical bone
would receive larger stress while forces were conducted from micro-implant to the
implant-bone interface owing to the higher elastic modulus of the cortical bone compared
with that of the spongy bone.

The stress patterns obtained from Table 3, 4, and 5 showed that the values in the cortical bone and cancellous bone were well within
the normal limit for all dimensions of mini-implants considered in the present study
but not in the metal. The high values of stress perceived in the metal particularly
of 1-mm mini-implant maybe unfavorable for orthodontic use. This could be implying
a possibility for a fracture at the neck during orthodontic loading and hence not
recommended for clinical use. Also, results from Jiang et al.’s 30] study showed that the increases of the diameter and length reduced the maximum equivalent
stresses in cortical and cancellous bones and mini-implant.

Surface area of mini-implant-bone interface

The surface area was calculated for the amount of alveolar bone surrounding the various
dimensions of mini-implants used in this study. (Table 6). This calculation was done in relation to two aspects of the bone surrounding the
mini-implant, i.e., the surface area of cortical bone alone around the mini-implant
and the surface area of whole bone (cortical and cancellous bones) around the mini-implant.

Table 6. Comparison of Surface area (mm
²
) of cortical bone and whole bone surrounding the models

On considering the whole bone-implant surface area, Table 6 revealed that 1.3?×?8 mm at both insertion angles had the greatest implant-bone interface
surface area of 29.45 and 23.98 mm
²
, respectively. Kanomi 13] however, believed that, from an orthodontic point of view, when mini-implants are
used for skeletal anchorage, it is the cortical bone which provides this. Also, Muhsin
et al. 31] (2011) believe that to obtain a balanced intrusion, root surface area should be considered
when determining the appropriate forces. Therefore, it is important to take into account
the surface area of the cortical bone surrounding the mini-implant rather than the
whole bone. Also, Lin et al. 32] have shown that the exposure length of the mini-implants significantly influenced
bone stress; increased exposure lengths resulted in greater bone stresses adjacent
to the mini-implant.

On considering the cortical bone-implant surface area, it was evident that the surface
area increased when the mini-implant was inserted at a 30° angulation only, rather
than when it was used at a 60° angulation in each combination of 1- and 1.3-mm mini-implants,
(more so in the 1.3-mm combination than in the 1-mm combination of mini-implants).
Between 1.3- and 1-mm mini-implants, the mini-implant dimension of 1.3 mm (inclusive
of 6- and 8-mm length) at a 30° insertion angulation showed the highest surface area
of the cortical bone at 7.76 and 6.84 mm
²
, respectively. The other mini-implant dimensions, i.e., 1.3 mm at 60° insertion angulation
and all combinations of 1-mm-diameter mini-implants at both 30° and 60° insertion
angulations, ranged between 3 and 5 mm
²
only (Table 6).