Page 16 - MDJ 2022 Jan-Jun, Volume 45 Number 1

Husun / Md Dasor

the increased diameter, presumably there will be an refers to the whole length including the MI head. The
increase of primary stability. According to a meta-analysis, selection of the length should be based on the thickness of
MIs with a diameter less than 1.3 mm have better success the cortical bone and the soft tissue at the area of
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rates than a MI with diameter 1.4–1.6 mm and 1.7–2.0 mm, insertion.
but the difference in the numbers of MIs being compared Table 2: Classification of MI based on diameter and length by
is significantly different; 450 MIs of 1.3 mm and below Cunha et al. (2017)
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compared to 1586 medium diameter MIs and 391 MIs with
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large diameter. Large MIs of 2.0 mm diameter have a Classification Diameter (mm)
higher risk of failure because the larger diameter results in Small 1.0–1.3
a closer proximity of the MI to the root. While aiming for Diameter Medium 1.4–1.9
high primary stability with large diameter, consideration Large 2.0–2.5
of space in between the roots is a key determinant of Length Short 5.0–8.0
success. MIs are typically placed in the buccal region of Long 8.5–15.0
the maxilla and mandible, at the interradicular region
between the second premolar and first molar. The HEAD DESIGN
proximity of the MI to the roots and the shape of the roots Clinicians loathe unexpected turns mid-course of
determine the diameter of MI to be used to avoid contact orthodontic treatment, therefore one parameter that is
with the periodontal ligament. The effect of root contact worth discussion in this paper is the versatility of the screw
will primarily be on the MI, not on the root, as the root is head design. Specific head designs may seem
able to self-repair following minimal damage. This is advantageous, but the versatility of each design must be
supported by moderate quality evidence from a meta- considered in light of treatment course or mechanic
analysis. However, recent findings show that changes. Clinicians must choose the head design while
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manipulation of the angle at insertion can mitigate root keeping in mind possible changes during the treatment,
contact, the most suitable angle varies across the such as whether it can only be used for elastic chains or
literature. 18–20 For some manufacturers, producing a can be used with steel ligatures as well. As a result, it
smaller range of diameters helps in terms of inventory and avoids any issues with changing the MI to fit the
in reducing confusion of the end user; after all, only a few application due to incompatibility mid-treatment. Afterall,
ranges of diameters are suitable and needed for most in orthodontics, keeping it simple works very nicely. An
clinical applications. On the other hand, some adverse effect of any head design is the constant
manufacturers prefer to produce an extensive range of occurrence of soft tissue ulceration caused by the friction
diameters. ORLUS® classifies their diameter based on of the MI head, affecting patient quality of life; therefore,
three sizes: mini, regular, and wide (Table 3). For Dual clinicians have invented several ways to mitigate the issue
Top and AbsoAnchor, the diameters range from 1.3mm to by placing a physical barrier such as soft flow or composite
2.0mm and 1.2 to 2.0mm respectively. For AbsoAnchor, on the MI head. Most manufacturers offer a number of
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they even manufactured a large MI with 2.7 mm diameter designs in the system that are specific to each clinical
(Table 3).
application and perhaps a derivative of existing MIs.
LENGTH AbsoAnchor offers 10 types of head, ORLUS® has 2 head
designs, and Dual Top with 8 designs (Table 3).
It is inherent that increasing the length of the MI
will increase its stability, but in actuality, the manipulation THREAD DESIGN
of MI length is based on the thickness of the cortical plate Thread designs are frequently overlooked,
at the site of placement. The thickness of the cortical plate despite the fact that threads are fundamentally designed to
affects the primary stability, 8,21 whereas the length of the maximise mechanical retention. Pre-drilling or self-
MI is a subordinate factor, which explains why certain tapping MIs require a pilot hole to be made using a bone
lengths are recommended for different anatomical sites. drill, while self-drilling MIs are those placed without any
Cortical bone thickness varies at different aspects of previous drilling and as they are inserted, space is created
placement. 21,22 Another point to consider when selecting and bone is displaced by the thread. Self-drilling MIs have
the MI is the varying type and thickness of the soft tissue a higher risk of failure because the bone is heated when
at the site. It is important to remember that the increase creating the pilot drill, leading to higher failure rates. 17,26
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in length does not increase the mechanical strength of the Another aspect of thread geometry is the inner diameter
MI; it only efficiently augments the stability with the (shaft) and the outer diameter (including the thread) as this
increase of contact with the cortical bone. The lengths of determines the volumes of cortical bone that gets in
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MIs are classified in the Table 2 below by Cunha et al. between it. When the MI fully rotates upon insertion or
ORLUS® MI length is divided into two types depending removal, the amount of the descent or ascent is determined
on the soft tissue or hard tissue at the site of insertion. As by the distance between each thread to another. It is
for Dual Top and AbsoAnchor, it is manufactured in the interesting to note that the distance between the threads
range of 5–12 mm (Table 3). The measurement of the requires a specific length and the smaller the distance, the
length refers to the length of the threaded shaft, but it really
depends on the manufacturer’s description. Some length

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