The use of Orthodontic mini implant (OMI) anchorage is rapidly growing. In recent years it has become a part of the orthodontic armamentarium.
Ding Y and Xia CS and Kafle D
The use of Orthodontic mini implant (OMI) anchorage is rapidly growing. In recent years it has become a part of the orthodontic armamentarium. With our improved understanding of the biomechanics, an array of tooth movements is possible with mini implants. Orthopedic and orthognathic-like effects can be produced with judicious application of this device. We present two cases treated with the help of OMI. The first case needed maximum anchorage, due to the degree of crowding and protrusive facial profile. Conventional orthodontic treatment could have produced good results, but a more ideal result was possible only with the absolute anchorage using mini implants. In the second case there was supra-eruption of a left maxillary second premolar and first molar. The OMI was used to intrude the supra-erupted teeth. Creating intrusive force vectors, using conventional orthodontic treatment, would be extremely difficult. By inserting three mini implants buccally and palatally, we intruded these teeth within a period of six months and without any side effects.
During the last two decades, the orthodontic mini implant (OMI) has progressed to the point where it is now a part of the everyday armamentarium of an orthodontist. A thorough review of the literature finds that implants for orthodontic purposes were tried even before Branemark, who introduced the dental implant. [1,2] The screws used for the internal fixation of the fracture sites were used for orthodontic purposes. Initially, stainless steel, then vitallium and now titanium screws have been used. However, except for a few case reports, the regular and advanced use of orthodontic mini implants started around the beginning of the new millennium. In 1970 Linkow described the prosthetic implant for orthodontic purposes in a case report.  He presented six cases treated with the help of blade implants. After that, regular prosthetic implants were used for orthodontic anchorage. It undoubtedly served the orthodontic speciality for many years and are still used in some multi-disciplinary cases. Those implants were tried even in the retromolar and tuberosity areas, for the sake of orthodontic anchorage. However, the biggest disadvantage of these implants was their size, which confined their placement to only edentulous ridges or retromolar areas.
In the search for a small dimensional implant, using the idea of surgical screws, the present day orthodontic mini implants were developed. The first purely orthodontic implant dates back to 1983. Creekmore, et al inserted a surgical screw just below the anterior nasal spine to reduce the anterior deep bite.  The implant was made of vitallium and served the orthodontic purpose by reducing the deep bite by 6 mm in one year. Roberts, et al did several animal experiments in the 1980’s. They had inserted 2mm diameter implants in the femur of dogs and rabbits with a high success rate, but similar results, were not found in the oral cavity.  Block and Hoffman in 1995 discovered onplant and used the palate as an anchorage source.  In 1997 Kanomi used the 1.2mm diameter implant for orthodontic treatment.  After that, there was a flood of case reports and clinical researches on mini implant anchorage. Now, there are different implant designs on the market. There is an ongoing modification process of the implant design. Due to its small dimensions(< 2mm in diameter), it can be placed virtually in all areas like an interradicular space, mid palatal suture, para-median area of the palate, retro-molar areas, etc. It can be used for multiple purposes like uncrowding, orthodontic camouflage, molar distalization, mesialization, intrusion, segmental intrusion, vertical control and the treatment of the canted occlusal plane. Another important advantage of the orthodontic implant is its non-invasive placement procedure. Raising a flap is unnecessary and most of the implants are drill-free (self-drilling) which eliminates the extensive surgical procedures and expensive implant kits. They are inserted using topical anesthesia. The local anesthesia of adjacent teeth is best avoided in order to allow the sensation of pain, if the implant touches the tooth roots or periodontal ligaments, instead of the interradicular alveolar bone. This phenomenon is described as a bio-feedback mechanism in a recent article by Dr. Sebastian Brumgartel. 
Note, that prosthetic implants are invariably osseo-integrated. Due to this property, their retention is very stable, but one must wait at least 4-6 months before using them for occlusal restoration or orthodontic loading. The orthodontic mini implants used today are not osseo-integrated. Rather, they are attached to the bone using mechanical retention. Using the property of mechanical stability, the orthodontic implant can be loaded immediately for orthodontic anchorage, unlike prosthetic implants in which we have to wait 4-6 months. Initially, light forces (50-100 grams) are used. Later, this force can be increased up to 200-250 grams, which is more than enough for orthodontic tooth movement. The success rate of these implants is very high, but of course, proper selection of the anatomic site is very important. The success rate of more than 90% is reported in many clinical trials. [9-12] However, overnight success of this frequency should not be expected. The learning and experience curve is always very steep. We see more versatile applications of orthodontic mini implants being reported in the literature. They can be used to move teeth with absolute preservation of anchorage. However, implant assisted orthopedic and orthognathic-like treatment is now possible. [13,14] Molar intrusion, once considered to be an almost impossible tooth movement by conventional orthodontic means, is now easily possible with orthodontic mini implants. Finally, orthodontic implants are far cheaper than dental implants, non-invasive, predictable and reliable. They can be easily removed, even without any kind of anesthesia. We are going to present
two cases which were treated in the department of orthodontics, second dental centre of Peking University School of Stomatology.
The patient is a 23 year old female who came to the department of orthodontics with the chief complaints of irregularities of her teeth and a protrusive profile.(Fig 1) Her cephalometric analysis (Fig 2) reveals her ANB angle to be 5 degrees with a slightly high mandibular plane angle. She was diagnosed as a Class II skeletal malocclusion with crowding due to maxillary excess. Her detailed pre-treatment cephalometric analysis is shown in the table. Considering her facial profile and the degree of crowding, the treatment plan was to extract four first premolars. Her anchorage plan was either to put two micro implants in the maxilla or use a TPA and headgear. After discussing the treatment plan with the patient, she agreed with the first option. The treatment was started with a pre-adjusted edgewise appliance, 0.022 × 0.028 inch MBT brackets. Two OMIs were inserted between the upper first molar and second premolar, one on each side. The anterior teeth were retracted by active tie-back from the crimpable hooks to the implants (Fig 3). 200grams of force was applied on each side. The final treatment shows an improved facial profile and an ideal occlusion with a Class I molar and canine relationship, ideal overjet and overbite (Fig 3). Her post-treatment cephalometric analysis is shown in table 2. Total treatment time was 20 months.
Figure 1. Case 1: Patient with crowding and protrusive profile before treatment.
Figure 2. Case 1: Pretreatmnet OPG, cephalogram and cephalometric tracing.
Figure 3. Case 1: During treatment, upper en-masse retraction with orthodontic mini implant.
Table Cephalometric Data
Figure 4. Case 1: Post-treatment facial and intra-oral photographs.
Figure 5. Case 1 Post treatment OPG, Cephalogram and Cephalometric tracing.
This patient is a 36 year old female who was referred to the orthodontic department by her prosthodontist. Her lower left second premolar and molars were missing causing the supra-eruption of the upper second premolar and first molar (Fig 6). Due to this supra- eruption, occlusal restoration of the lower edentulous area was virtually impossible. The treatment plan was OMI assisted orthodontic intrusion of the upper left second premolar and first molar so that the lower edentulous area could be restored with dental implants. Two mini implants were inserted buccally and another mini implant was inserted palatally. The second premolar was bonded, while the first molar was banded. The teeth were then intruded with elastic chains and Ni-Ti closed coiled springs (Fig 7). The total treatment time was 6 months and the post-treatment photograph shows well intruded teeth and the lower edentulous area was restored with dental implants (Fig 7 and 8).
Figure 6. Case 2: Pretreatment facial and intra-oral photograph.
Figure 7. Photograph at the start of treatment, during treatment and at the completion of treatment.
Figure 8. Extra-oral and intra-oral photographs after occlusal restoration with dental implant.
In our first case, we decided to place two mini-implants in the maxilla because of the need for maximum anchorage. Maximum anchorage can be obtained by the use of extra-oral appliances, e.g. headgear,TPA and OMI. But, patient compliance is the major factor. Adult patients have shown poor compliance when asked to wear extra-oral appliances. Trans-palatal arches (TPA) are commonly used to preserve the anchorage. But, in cases of maximum anchorage, a TPA is not a reliable option. To retract the teeth bodily, the line of applied force should pass through the centre of resistance. [15,16] In our case, the crimpable hook is not long enough to reach up to the centre of resistance of the anterior teeth. This produces an intrusive component, in addition to a retractive component which is beneficial in a deep bite case. Orthodontic mini implants reduce the overall treatment time.  Implant anchorage can not increase the rate of tooth movement, but it can definitely reduce undesirable tooth movements. Nanda described the duration of orthodontic treatment still approaches 2 years; arguably because of the time it takes to correct the unintended side-effects (undesirable tooth movements) that occur during orthodontic treatment.  Mini implant anchorage has predictable outcomes which eliminates side-effects. This may be the reason for decreased treatment time. Another reason for decreased treatment time is the anterior en-masse retraction. Canines do not need to be retracted individually, which is the usual procedure to preserve anchorage in conventional orthodontic treatment. Mini implant anchorage can be used to retract all the anterior teeth together.
In our second case of molar intrusion we placed two mini implants, one on the palatal side and another on the buccal side. Different methods of molar intrusion are described in the literature. [18, 19] To obtain the pure intrusion of molars and premolars, the force should again pass through the centre of resistance, buccolingually as well as mesiodistally. Otherwise, unwanted tipping of the tooth will result. If we can control the palatal force, then the tooth can be intruded with the implant only on the buccal area. To obtain these mechanics, a heavy wire trans-palatal arch can be placed which prevents buccal tipping of the tooth. We can add palatal crown torque in the TPA to add the intrusive effect of the buccal mini implant. If the line of force is applied through the centre of resistance, the molar intrusion can be obtained in a relatively short period of time. Usually, the result can be seen within months of treatment. In our case, it took us six months to intrude the molar so that sufficient space is available to put implant supported teeth in the mandibular edentulous space.
Mini implant anchorage can produce highly predictable results with decreased treatment time and decreased side-effects. Mini implant anchorage has expanded the horizon of orthodontic treatment towards orthognathic-like effects remaining within the boundary of the orthodontic domain.
5. Roberts WE:Endosseous miniscrews:Historical,vascular,and integration perspectives. In Nanda R and Uribe FA editors:Temporary Anchorage Devices in ORTHODONTICS St.Louis ,2009,Mosby-Elsevier,pp 3-13.
14.Liou EJW:Appliances,mechanics,and treatment strategies toward orthognathic-like treatment results. In Nanda R and Uribe FA Editors:Temporary Anchorage Devices in ORTHODONTICS, St. Louis , 2009,Mosby-Elsevier,pp 167-197.
16. Yoshida et al. Experimental evaluation of initial tooth displacement, centre of resistance and centre of rotation under the influence of an orthodontic force. Am J Orthod Dentofacial Orthop 2001;120:190-197.
Associate Professor ,Department of Orthodontics, Second Dental centre, Peking University School of Stomatology,Beijing,China.
Professor and Head, Department of Orthodontics, Zhengzhou University, Henan, China.
Resident, Department of Orthodontics, Zhengzhou University, Henan, China.