© Springer-Verlag Berlin Heidelberg 2014
Ki Beom Kim (ed.)Temporary Skeletal Anchorage Devices10.1007/978-3-642-55052-2_3
3. Contributing Factors to Success Rate of Orthodontic Mini-implants: Important but Ignored Results from Basic Researches
Department of Dentistry, Korea University Guro Hospital, Gurodong-Ro #148, Guro-Gu, Seoul, 152-703, South Korea
Department of Orthodontics, School of Dentistry, Seoul National University Dental Hospital, Seoul National University, Daehak-Ro #101, Chongno-Gu, Seoul, 110-768, South Korea
Among the numerous factors that can affect success or failure rates of orthodontic mini-implants (OMIs), some factors are well studied but others are neglected or ignored in spite of basic and clinical importance. The purpose of this chapter was to describe important but ignored results from basic researches such as shape and microstructure of OMIs, surface treatment of OMIs, predrilling before OMI installation, microdamage in the cortical bone, wobbling of OMIs during manual installation, root contact with OMIs, and fracture properties of OMIs. If more concern about these issues would be given, better clinical results might be obtained.
Orthodontists and other dentists have paid a lot of attention to installation and usage of orthodontic mini-implants (OMIs) in diverse clinical situations. The success rates of OMIs are known as 84–92 % [1–8].
There have been numerous studies about success or failure rates of OMIs including host factors (age, gender, skeletal pattern, oral hygiene, and inflammation), bone quality (thickness and stiffness), design of OMI (shape and diameter), and insertion and loading modality of OMI (implantation location, type of placement surgery, and immediate loading) [5, 6, 9–16]. Among these factors, some factors are well known but others are neglected or ignored despite of basic and clinical importance.
Therefore, the authors would like to describe important but ignored results from basic researches such as shape and microstructure of OMIs, surface treatment of OMIs, predrilling before installation of OMIs, microdamage in the cortical bone during installation of OMIs, wobbling of OMIs during manual installation, root contact with OMIs, and fracture properties of OMIs.
3.2 Shape and Microstructure of OMIs
The conical or tapered OMIs are known to provide tighter contact between the OMI and bone and better initial stability than the cylindrical ones [13, 17, 18]. However, the conical or tapered OMIs are reported to produce higher stress in the cortical bone compared to the cylindrical ones with the same diameters [13, 19].
In the mechanical and histomorphometric analysis of beagle dog, Kim et al.  reported that the conical OMIs exhibited significantly higher maximum insertion and removal torque values than the cylindrical ones. However, there was no significant difference in success rate, resonance frequency analysis, and bone-to-implant contact (BIC) ratio between conical and cylindrical OMIs . Therefore, Kim et al.  suggested that the conical OMIs might need modification of the thread structure and insertion technique to reduce the excessive insertion torque while maintaining the amounts of removal torque.
The results from several studies about the dental prosthetic implants suggested that the microgroove on the implant surface could play an important role in the proliferation and migration of fibroblasts, formation and adaptation of the connective tissue, and maintenance of the soft tissue around the implant [20–22].
According to the results of histomorphometric analysis of beagle dog by Kim et al. , the microgroove OMI exhibited higher success rate and BIC ratio on the pressure side compared to the non-microgroove OMI. In addition, the microgroove OMI revealed no significant differences in BIC between pressure and tension sides . Since the microgroove and non-microgroove OMIs exhibited different alignment of the gingival connective tissue fiber to OMIs (perpendicular or circular in microgroove OMI vs. parallel in non-microgroove OMI), they concluded that the microstructure such as microgrooves might have positive effects on the arrangement of gingival connective tissue fibers and the adaptation of the soft tissue and bony tissue around the OMI .
3.3 Surface Treatment of OMIs
The purposes of surface treatment for the dental prosthetic titanium implants are to increase the surface area, to provide a better BIC environment, and to increase the degree of osseointegration between the TiO2 layer of the implant and the surrounding bony tissue [16, 23–28].
As one of the typical surface treatment methods, sandblasted, large-grit, and acid etching (SLA) method has been commonly used. It involves sandblasting of the implant with 0.25–0.50 μm aluminum particles and acid treatment with HCl and H2SO4 . Buser et al.  reported that titanium alloy material treated with the SLA method had enhanced BIC ratio and exhibited a higher removal torque than the non-treated material.
Anodic oxidation method is another method that involves the use of 0.1 % phosphoric acid solution at 150–200 V for several minutes [30, 31]. Yamagami et al.  reported that titanium alloy treated with a combination of sandblasting and anodic oxidation (SLAO) enhanced active bone formation, resulting in stable fixation in the diaphysis of New Zealand white rabbit femurs.
Similar studies were carried out in the OMIs [16, 32, 33]. Kim et al.  reported that the SLA-treated OMIs provided lower maximum insertion torque and higher total removal energy than the machined-surface OMIs. However, the OMIs used in their study were not self-drilling type, and they needed a predrilling procedure for installation. In addition, Karmarker et al.  reported that the OMIs with anodic oxidation treatment only (without sandblasting) exhibited the same insertion torque value with the machined-surface OMIs and a higher removal torque value than the machined-surface OMIs.
Comparison of the mechanical properties of SLA- and SLAO-treated OMIs in the tibias of Beagles, Cho et al.  reported that both SLA- and SLAO-treated OMIs showed lower maximum insertion torque and total insertion energy than the machined-surface OMIs. They also showed that SLAO-treated OMIs exhibited a higher maximum removal torque than the machined-surface OMIs and SLA-treated OMIs . These results suggested that SLAO treatment might be an effective tool in reducing insertion damage to surrounding tissue and improving the mechanical stability of OMIs .
In summary, these studies showed a possibility that the surface treatment of OMI could provide less bone damage and higher mechanical stability.
3.4 Predrilling Before Installation of OMIs
In the area where the cortical bone thickness is thick such as the mandibular posterior area, direct installation using a self-drilling OMI can produce excessive insertion torque, resulting in failure of the OMI . Therefore, it is necessary to predrill the cortical bone and then to install the self-drilling or self-tapping OMI at the mandibular posterior area.
If the entire thickness of the cortical bone is predrilled in the mandibular posterior area, root damage of the posterior teeth and resultant failure of the OMIs can occasionally occur by the predetermined direction of OMI installation . In order to weaken the cortical bone without predetermining the direction of OMI installation, a partial predrilling technique can be used .
Cho and Baek  performed an in vitro study to investigate the effects of OMI shape (cylindrical and tapered type) and predrilling depth (non-predrilling [control], 1.5 mm predrilling, and 3.0 mm predrilling; predrilled with a drill bit [1 mm in diameter]) on the mechanical properties of OMIs during the insertion procedure. The results were as follows : Within the same shape group, although predrilling groups exhibited shorter total insertion time than control groups, there was no difference in total insertion time between 1.5 and 3.0 mm predrilling groups. Maximum insertion torque and total insertion energy decreased in the order of control, 1.5 mm predrilling, and 3.0 mm predrilling. The maximum insertion torque of the cylindrical shape/non-predrilling group was smaller than the tapered shape/non-predrilling group. In the same predrilling depth, no differences were observed in maximum insertion torque and total insertion energy between cylindrical and tapered groups. Therefore, they concluded that predrilling might be an effective tool for reducing maximum insertion torque, total insertion energy, and microdamage without compromising OMI stability in cases of thick cortical bone .
3.5 Microdamage in the Cortical Bone During Installation of OMIs
The self-drilling OMIs with a large diameter and a tapered or conical shape might cause overcompression of the cortical bone by excessive placement torque [13, 35]. This can lead to microdamage, a permanent deformation of the microstructure in loaded cortical bone in the form of fatigue, creep, and eventual cracking [35–37]. Therefore, the accumulation of microdamage can produce local ischemia, bone necrosis, bone remodeling, and premature loss of the OMI [35–38].
Lee and Baek  performed the animal study (New Zealand white rabbits) to investigate the effects of the diameter (1.5 and 2.0 mm) and shape of OMIs (cylindrical and tapered) on microdamage to the cortical bone during OMI placement. Experiment methods are as follows : Maximum insertion torque was measured during OMI installation. Immediately after placement of the OMIs, the block of bone with the OMI was harvested. Cortical bone thickness was measured by using microcomputed tomography, and histomorphometric analyses of the number of cracks, accumulated crack length, maximum radius of the crack, and longest crack were performed. Large diameter and tapered shape resulted in increased values of maximum insertion torque, number of cracks, and longest crack . Therefore, they concluded that OMIs with larger diameter and tapered shape might cause greater microdamage to the cortical bone than OMIs with smaller diameter and cylindrical shape . Further studies are needed to investigate how excessive insertion torque and microdamage can affect bone remodeling and the stability of the OMIs.
3.6 Wobbling of OMIs During Manual Installation
There are two methods for OMI installation: the manual insertion method using a hand driver and the motor insertion method using a handpiece.
The motor insertion method can maintain a constant drilling speed and force, prevent excessive insertion torque by the auto-stop or over-limit mechanism during drilling procedure, and approach easily to the palate or the most posterior area of the buccal-attached gingiva in the mouth [39–42]. However, this method requires space for equipment and expensive instruments and is difficult to obtain the proper tactile sensation and to monitor the insertion angle during the insertion procedure [41, 42].
The manual insertion method can provide better tactile sensation when the OMI tip contacts the alveolar bone and allow for confirmation of the insertion orientation [41, 42]. However, it is difficult to obtain the appropriate insertion torque, maintain the proper rotational speed, or access the palate or the most posterior area of the buccal-attached gingiva in the mouth with a hand driver [41, 42].
Lack of experience and improper technique in the manual insertion method may cause the OMI to wobble at the insertion site during the insertion procedure . Wobbling can increase the amount of reduction of the cortical bone, cause microcrack or damage of the cortical bone, and decrease the mechanical retention between the OMI and bone, which might compromise the primary stability of the OMIs .
Cho et al.  performed an in vitro experiment to investigate the effects of wobbling angle on the stability measures of OMIs during insertion and removal procedures in artificial bone blocks using a driving torque tester with a uniform speed of 28 rpm and wobbling analogs of 2° and 4°. Although the 4° wobbling group showed a 14.5 % (2.9 Ncm) increase in maximum insertion torque compared with the control (0° wobbling) group, there was only a 6 % (1.3 Ncm) decrease in maximum removal torque from the control group to the 4° wobbling group . They suggested that slight wobbling during the OMI insertion procedure might be acceptable in terms of the stability measures of OMIs during insertion and removal procedures . However, it will be necessary to perform an in vivo study to confirm that the same findings will occur in the real bone .