Abstract
Anchorage control during orthodontic treatment has been an Achilles hill for the orthodontists. Various devices have been used for anchorage control, but both extraoral and intraoral methods of anchorage control fall short of expectations as they fail to produce absolute anchorage and many of them depend upon patient compliance. Mini-implant–enhanced anchorage has become a popular concept in the orthodontic treatment over the past few years, and a paradigm shift has occurred in the overall perspective towards orthodontic treatment. This article is an attempt to highlight the concept of mini-implant anchorage.

Introduction
Orthodontic anchorage is defined as resistance to undesired tooth movement. This undesired tooth movement (Anchorage loss) can be in any plane of space i.e. sagittal, vertical and transverse. Throughout the history of orthodontics anchorage control has been a perplexing problem for orthodontist.  Angle stated that: “According to the well-known law of physics, action and reaction are equal and opposite, hence it must follow that the resistance of anchorage unit must be greater than that offered by the tooth to be moved, otherwise there will be displacement of the anchorage and failure in the movement of the teeth to the extent, or, possibly, in the direction desired”.

Understanding each patient’s anchorage requirements and subsequent anchorage planning is of paramount importance and ensures high-quality result. High-anchorage situations require excellent patient compliance with traditionally used extraoral traction devices (Headgear). This dependency on patient compliance greatly increases the risk for failure of extraoral appliances, whereas, intraoral appliance system failed to provide an ideal anchorage control.  Absolute anchorage, one where the anchorage units remain completely stationary, is sometimes desirable but is usually unattainable with traditional orthodontic mechanics. The exception is the presence of ankylosed teeth in the anchorage unit. The ankylosed teeth are devoid of periodontal ligament. Hence they do not move under the influence of orthodontic force. This situation is sometimes called skeletal anchorage and, by the above definition, could also be called absolute anchorage.

History
Obtaining absolute anchorage from an ankylosed teeth is not always possible in routine orthodontic treatment. Therefore, over the past 60 years, methods have been developed to create absolute skeletal anchorage and thus widen the scope of orthodontics.
In 1945, Gainsforth and Higley used vitallium screws in mongrel dogs to create absolute anchorage for tooth movement. A doctor in Sweden accidentally discovered that titanium can bond irreversibly with living bone tissue. Per Ingvar Branemark 1952 was interested in studying bone healing and regeneration. He screwed a Titanium Metal Cylinder into a rabbit’s thigh bone as part of an experiment. He attempted to retrieve these expensive chambers from the rabbits and found that he was unable to remove them. Bone had grown into such close proximity with the titanium that it effectively adhered to the metal. He named this phenomenon “osseointegration”. Linkow, described the use of an endosseous blade implant for retraction of anterior teeth in 1969. In 1983, Creekmore and Eklund performed maxillary incisor intrusion with the help of a titanium osteosynthesis screw. In 1984, Roberts et al applied these principles for molar movement in an adult patient. With the invention of the onplant in 1995, Block and Hoffman introduced the palate as an anchorage device location, and, in 1996, Wehrbein et al used the palate as an implant site. Kanomi used a 1.2-mm diameter mini-implant in 1997. After that, many reports have been published on orthodontic absolute anchorage systems, reflecting their increasing popularity and importance1.

Classification
Skeletal anchorage system (SAS) includes all the devices fixed to the bone with the goal of increasing anchor­age for orthodontic purposes. Term ‘Temporary anchorage device’ (TAD) refers to all variations of implants, screws, pins, and onplants placed specifically to provide orth­odontic anchorage and removed on completion of bio-mechanical therapy. The terms used in the literature include mini-implant, miniscrew, microscrew, micro-implant, anchor screw, mini-implant screw, mini-dental screw, mini-dental implant, micro-anchorage system, micro-anchor implant, ortho-anchor pin, orthoscrew, pin, and transi­tional implant1.

Skeletal anchorage systems can be classified into two categories according to their origin       (fig. 1). One group has been developed from dental implants and is char­acterized by an intraosseous part that is surface-treated to enhance the osseoiniegration. This category includes palatal and retromolar implants. A special variant in this cate­gory is the onplant introduced by Block and Hoffman.The other category of skeletal anchorage originates from surgical screws and is characterized by a polished intraosseous part with a surgical screw attached. It is loaded immediately after insertion. They are held by mechanical retention. The two main groups under this category are (1) miniplates with various trans-mucosal extensions  and (2) single screws or mini-implants1,7.

Fig. 1: Classification of Skeletal Anchorage System.

Depending on the configuration of the head, mini-implants can be used as direct or indirect anchorage devices. The head of the mini-implant may be formed as a button around which a wire or elastic can be tied. Some of these also have a hole in the neck which is used for ligation. Bracket head type implants can hold archwires in the bracket slots in their heads which can exert force directly on the teeth to be moved2 (fig. 2).

Biological Consideration:
The implant material must be nontoxic, biocompatible, possess excellent mechanical properties, and provide resistance to stress, strain, and corrosion. Commonly used materials can be divided into 3 categories: biotolerant (stainless steel, chromium- cobalt alloy), bioinert (titanium, carbon), and bioactive (hydroxylapatite, ceramic oxidized aluminum). Because of titanium’s characteristics (no allergic and immunological reactions and no neoplasm formation), it is considered an ideal material and is widely used. Bone grows along the titanium oxide surface, which is formed after contact with air or tissue fluid. However, pure titanium has less fatigue strength than titanium alloys. A titanium alloy—titanium-6 aluminum-4 vanadium is used to overcome this disadvantage11.
We will discuss orthodontic implants under two sub groups, (1) those that are osseointegrated and (2) those that have mechanical retention, as the biological processes are quite different for the two groups1,7,10.

Osseointegrated Devices

Several sequences of events occur after insertion of implant as stated below3.

1. The insertion of an orthodontic implant into bone initiates a series of biological processes, including formation of a blood clot, alteration in the nuclear morphology of the osteocytes surrounding the site of implantation, and formation of new bone. After placement of an orthodontic implant, the surface comes in contact with blood and is covered by a biofilm containing fibrinogen and serine proteases of the complement and coagulation system.

1. After 1 day, RBCs and inflammatory cells (mainly neutrophils) are present between the bone and implant.

1. From 3 to 7 days after implantation, inflamma­tory cell infiltration gradually disappears, and spindle-shaped or flattened cells start to appear in the interface between preexisting bone and orthodontic implant and new collagen fibers run circumferentially around the implant cavity, whereas the course of the fibers of existing bone are oriented parallel to the long axes of the bone. Numerous bone-remodeling units containing mul­tinucleated osteoclasts and blood vessels also appear in the cortical bone surrounding the device.

1. Six weeks after implant placement, active bone remodel­ing appears to decrease, and a region of empty osteo­cytic lacunae is still seen adjacent to the newly deposited bone. A minimum period of six weeks is required for osseointegration of implant.

After the nonloading healing period and osseous integration, application of orthodontic loading to the implant causes increased bone tissue turnover and increased density of the adjacent alveolar bone compared with the unloaded controls. Despite increased bone tissue turn­over, however, the implant maintains osseointegration even after 32 weeks of orthodontic loading. Originally, however, based on Brånemark’s work it was thought that all implants should undergo a 4- to 6-month healing period before functional loading7,10.
Mechanically Retained Devices

In screw-shaped, mechanically retained orthodontic TADs, areas of the screw in direct contact with the bone are responsible for primary mechanical stability of the device. Other areas show gaps hundreds of microns in size between the device surface and the bone. In the areas where small gaps exist between the screw surface and bone, the biological process is similar to that previ­ously described. However, the biological response is different in areas of direct contact with the screw as stated below.

1. No invasion of inflammatory cells occurs in the first week. Instead one day after insertion, not only are mineralized bone tissue contacts present between the surface of the implant and bone, but the osteoblasts are also attached firmly to the Titanium implant surface.
2. After 1 to 2 weeks, in the areas in direct contact with the bone, the bone is resorbed and replaced with newly formed, viable bone. Despite this temporary loss of hard-bone contact, the implants remain clinically stable. This process does not seem to be affected if the screw is immediately loaded or if there is a healing period before external loading begins. Therefore, these types of implants can be loaded immediately after insertion.

Stability Of Anchorage Devices

As with the biological processes, the factors that pre­dict stability also differ between osseointegrated and mechanically retained implants1,8.

Mechanically retained devices

Stability of mini-implants immediately after insertion, called primary stability, is critical in determining success in the early loading phase, particularly if it is immediately loaded.
Primary stability depends on various factors as stated below3.

1. The geometric design of the implant - conical shape, greater outer diameter, increased length, and use of abutments (attachments to screw for applying orthodontic force) increase the primary stability.
2. Bone quality- Placement of implant in areas of higher bone mineral density (BMD) increases the primary stability.
3. Insertion technique- placement of screw-type implants either requires or does not require the drilling of a pilot hole. A recent study indicated that drill-free screws have increased bone-metal contact, greater bone area, and less mobility than screws that require the drilling of a pilot hole.
4. Tip moment- increased tip moments (mag­nitude of orthodontic force x length of lever arm) at the bone rim caused decreased stability
5. Excessive loading of mechanically retained mini-implants has been associated with increased failure rates.

Fig-3 –Features in mini-implant for stability and clinical efficiency

Osseointegrated Devices
Two major factors affecting osseointegration are primary stability and a no-loading healing period. Primary sta­bility is critical to the processes of osseointegration of the implants. Immediate or early loading of these implants caused decreased stability and spontaneous fracture of the bone in animal models1,7.

Indications
Operator should critically evaluate the indication of orthodontic implants before implementing them in the treatment plan. This step is advantageous as using mini-implants appropriately will lead to improved treatment results, whereas not using them when traditional mechanics could lead to equally satisfying results prevents overtreatment. Since many orthodontic treatment planning decisions are based on decades of dogma, a clinician who is interested in using mini implants needs to adopt a new treatment-planning paradigm. The following treatment objectives might benefit from mini-implants5.

Corrections in the anteroposterior dimension

1. Because anchorage considerations are of no concern with use of implants, the choice between first or second premolars can be made by solely considering tooth size and arch discrepancy and periodontal and restorative status of teeth.
2. Adults with full-step Class II malocclusion and severe overjet requiring maximum retraction of the maxillary anterior teeth could benefit. Absolute anchorage might be indicated because anchorage loss is unfavorable in this situation, and treatment time will be reduced because of en-masse retraction.
3. Patients with severe bimaxillary protrusion with chief complaint of unpleasant protrusive profile or lip incompetence and unwillingness to wear headgear could use mini-implants after 4 premolar extractions because they allow for maximum retraction of teeth with maximum retraction of lips improving the facial profile.
4. Mini-implants could be used for protraction of posterior segments, in general, for extraction space closure, or for closing space resulting from tooth agenesis or tooth loss if prosthetic replacement is not desired. This is also possible in extraction sites with collapsed alveolar ridges when the patient can benefit from the osteogenic potential of orthodontic tooth movement.
5. Patients who need molar distalization for correction of Angle’s Class II malocclusion. Here relief of crowding would also occur.

Corrections in the vertical dimension
1. Anterior open bites can be corrected with intrusion of the maxillary posterior segments in patients with posterior maxillary excess.
2. Mini-implants can be used for vertical control of mandibular posterior segments in patients with vertical growth pattern.
3.  Maxillary incisors can be intruded in patients with deep bite and excessive gingival display.
4.  Mandibular incisors can be intruded in patients with deep bite and deep curve of Spee.
5.  Canted occlusal planes can be corrected.
Preprosthetic orthodontics, single tooth movement, and mutilated dentition

1. Mini-implants can be used for molar uprighting to create space for pontic.
2. Single-tooth intrusion of tooth extruded obliterate space in opposing arch can be done.
3. Desirable anchorage situations can be predictably achieved in patients with mutilated dentition.

Implant Site Selection

Various sites in maxilla and mandible for placement of orthodontic implants are shown in figure 3. Selecting the proper implant site can be an important factor in the overall success of this treatment approach. Five factors are important in determining an adequate site for implantation5,9.

1. Indication, system used, and required mechanics.

When placing an orthodontic mini-implant, the treatment objective and how long the implant will remain in situ are of paramount importance. Mechanics should be as simple and fail-safe as possible, but the future tooth movement must be anticipated to avoid any interference with the implant.

1. Placement in attached gingiva, clear of the frenulum.

The implant site should ideally provide sufficient attached gingiva for placement of the mini-implant. This prevents patient discomfort, tissue overgrowth, and micro jiggling due to mobile soft tissue, that can lead to long-term implant failure.

1. Sufficient interradicular distance.

The implant must be placed where roots are wide enough apart so that no damage is inflicted to them. Periapical radiographs or 3-dimensional cone-beam computed tomography are essential tools for evaluating potential implant sites.

1. Avoiding other anatomical structures.

Anatomical structures can interfere with the placement of an orthodontic mini-implant eg, inferior alveolar nerve, artery, vein, mental foramen, maxillary sinus, and nasal cavity etc. Again, 3-dimensional digital imaging can help evaluate the anatomical relationships.

1. 1. Anesthesia
   1. Screw can fracture if it is too narrow or the neck area is not strong enough to withstand the stress of insertion or removal.
   1. Infection can develop around the screw if the transmucosal portion is not entirely smooth.
   1. Application of excessive pressure during insertion of a self drilling screw can fracture the tip of the screw.
   2. Over tightening can cause it to loosen. It is crucial to stop turning the screw as soon as the smooth part of the neck has reached periosteum.
   3. It is important not to wiggle the screw driver when removing it from screw head.
   1. The prognosis for primary stability of a mini implant is poor in cases where the cortex is thinner than 0.5mm and the density of trabecular bone is low.
   2. In patients with thick mucosa.
   3. Inflammation around the implant site.
   4. Mini implants are contra indicated in patients with systemic alteration in the bone metabolism due to disease, medication , or heavy smoking.
   1. Independency from the number or position of the present teeth.
   2. Optimal use of the retraction forces.
   3. Independency from patient cooperation.
   4. Patient comfort.
   5. Easy and fast screw insertion.
   6. Possible application even in interceptive therapy.
   1. Severe systemic disorder eg. Osteoporosis.
   2. Alcoholics.
   3. Drug abusers.
   1. Insufficient volume of bone.
   2. Poor bone quality.
   3. Patients undergoing radiation therapy.
   4. Insulin dependent diabetes.
   5. Heavy smokers.
   1. • Nanda, R. and Uribe, F.A.: Temporary Anchorage Devices in Orthodontics, Mosby, St. Louis, 2008.
      • Kyung HM, Park HS, Bae SM, et al: Development of orthodontic micro-implant for introoral  anchorage: J Clinc Orthod 37;321-328,2003.
      • Nygren H, Tengvall P, Lundstrom I: The initial reactions of TiO2 with blood, J Biomed Mater Res 34(4):487-492, 1997.
      • Miyawaki S et al: Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage: Am J Orthod Dentofacial Orthop 124(4):373-378. 2003.
      • Baumgaertel S., Razavi Mohammad R., and Hans Mark G.: Mini-implant anchorage for the orthodontic practitioner. Am J Orthod Dentofacial Orthop ;133:621-7, 2008.
      • Applications of orthodontic Mini implants. : Jong Suk Lee, Jung Kook Kim, Young-Chel Park, Robert L. Vanarsdall jr: Quintessence Publishing. June-07 .
      • Jason B. cope. Temporary Anchorage Devices in Orthodontics: A Paradigm Shift. Semin Orthod 2005;11:3-9.
      • Factors affecting the clinical success of screw implants used as orthodontic anchorage.Am J Orthod Dentofacial Orthop. 2006 Jul;130(1):18-25.
      • Paola Maria Poggio; Cristina Incorvati; Stefano Velo; Aldo Carano ‘‘Safe Zones’’: A Guide for Miniscrew Positioning in the Maxillary and Mandibular Arch: Angle 2006;76(2): 191-7
      • Newman M.G, Takei H.H, Carranza A. Fermin: Clinical Periodontology-9th Edition, W.B. Saunders Company, 2002.
      • Dental Implants for orthodontic anchorage.Am J Orthod Dentofacial Orthop 2005; 127:713-22.
   • Absolute contraindications
   • Relative contraindications
Adequate cortical bone thickness.



Fig. 3: Different sites of orthodontic implants in Maxilla and Mandible.(Implants are shown in yellow colour)





Placement Protocol

Placement protocols can differ, depending on the various systems. The most common steps involved in the placement of mini-implants are described. Generally, topical anesthetic is sufficient for painless placement of mini-implants. The superficial layer—the gingival tissue—is strongly innervated, but topical anesthetic is effective for desensitizing the neural input from this tissue. The second layer is the periosteum, which also is highly innervated. Topical anesthetic can reduce painful stimuli originating in this tissue if sufficient time is allowed for diffusion of the medication to the periosteal layer. The third layer is the cortical plate of either the maxilla or the mandible. This is not highly innervated and thus does not require anesthetic. The fourth layer is the cancellous bone of the jaw. Bone is not well innervated and does not require anesthetic.

Because all other innervated structures inside the bone have not been blocked by anesthetic allows the patient to provide the clinician important feedback. If the clinician comes close to sensitive structures, such as the alveolar socket of a tooth, the nerve canal, or the maxillary sinus, the patient senses pain and can alert the doctor before the implant penetrates these sensitive structures, thus preventing potentially irreversible damage.

For an apprehensive patient, a small quantity of local anesthetic may be infiltrated submucosally. This anesthetic should not contain vasoconstricting drugs like epinephrine to prevent local ischaemia5.

2) Marking stage The insertion site should be cleaned with povidone iodine. Periodontal probe is used to mark the horizontal and vertical reference lines on gingiva(fig. 4a). Soft tissue thickness is measured with probe. Placing micro implants through loose, mobile oral mucosa requires 3 mm long stab incision (Fig. 4b). The incision can be avoided while placing the implant in attached gingiva6.



Fig. 4a: Marking of implant site.	Fig. 4b: stab incision to place implant in mobile mucosa




3) Perforating stage

This stage is important because cortical bone is the component most resistant to implant insertion. Two ways to perforate are with use of Orlus surgical drill and with use of an implant itself. The screws can be of 2 types- self tapping and self drilling-

Self tapping screws

It has a non cutting tip. It require pilot hole (fig. 5) of same length as implant. It is more invasive  but once pilot hole is drilled, implant can be placed  without difficulty (fig. 5) and with minimal tissue damage. The drilling should be done at low speed (800 rpm) under irrigation of sterile saline or distilled water. Speed reduction, high torque handpiece is used for this purpose  

Self drilling screws.

They have a cutting tip so pilot hole not required but high pressure is required to drill through cortical bone. Hence it can cause compression of bone, patient discomfort and resorption. Loss of tactile sensitivity occurs.  Hence the ideal combination appears to be a self drilling mini-implant system(fig-6) with perforation of only the cortical bone but without a true pilot hole extending into the bone the entire length of the implant.

Fig. 5: Drilling of pilot hole and insertion of selftapping implant in the pilot hole.	Fig. 6: Insertion of selfdrilling implant




Placement Angle
In the Upper jaw, relative to long axis of teeth, implant is placed at 30-40˚. In the Lower jaw, relative to long axis of teeth, it is placed at 20-60˚(fig. 7). The implants placed at lesser insertion angles are likely to approach root .
Advantages:

It increases surface area of cortical bone contact with micro implant. There is less chance of root contact and damage. Longer micro implants can be placed which enhance stability.



Fig. 7: Placement angles for implants. A- Maxillary, B- Mandibular.




4) Guiding stage
During this stage, the screw should be engaged with the bone and inserted at a planned angle. Implant should be inserted through rotation of the screw with minimal vertical force6.
5)  Finishing stage
Implant should be inserted to the planned depth, and the implant head should be exposed to an adequate extent. Finishing solely with rotational force is crucial to maximize contact with the cortical bone6.
After correct identification of the implant site and topical anesthesia, a self-drilling or a self-tapping implant must be placed into the bone by clockwise rotation with the system-specific driver or a torque wrench if torque control is desired. Only rarely is a soft-tissue punch or perforation of the cortical plate necessary. Some self-tapping systems require a pilot hole. After correct identification of the implant site and topical anesthesia, the soft tissues covering the bone (gingiva and periosteum) at the implant site should be excised with a soft-tissue biopsy punch. This ensures a clean soft-tissue margin surrounding the implant. An initial perforation of the cortical plate is made with a round bur, the pilot drill is used to create a channel though the bone with a smaller diameter than the implant. The drill should be either an implant hand piece or a slow-speed hand piece with torque reduction to allow for drilling at approximately 800 rpm. All steps that include drilling require constant irrigation with sterile saline solution. The implant can then be rotated manually in a clockwise direction with an applicator and a torque wrench or a driver.
Removal generally does not require anesthesia. The manual applicator or the driver is used to derotate the implant in a counterclockwise direction5.

Common Problems Associated With Mini Implants1
Screw related problems

Operator related problems

Patient related problems

Advantages Of Implants1

Contraindication For Implant  Therapy2

Conclusion
The concept of temporary anchorage device is a relatively new application of more established clinical methodologies. The future development of temporary anchorage devices for orthodontic anchorage will establish a more complete understanding of biology and biomechanics associated with both Osseo integrated and Non Osseo integrated devices. The presently available implant systems are bound to change and evolve into more patient friendly and operator convenient designs. Long-term clinical trials are awaited to establish clinical guidelines in using implants for both orthodontic and orthopedic anchorage. Implants for the purpose of conserving anchorage are welcome additions to the armamentarium of a clinical Orthodontist. They help the Orthodontist to overcome the challenge of unwanted reciprocal tooth movement.


References