INTRODUCTION:

Most orthodontic treatment plans need some form of anchorage to control the reciprocal forces oftooth movement. Orthodontic mini implants (OMIs) have been hailed for having revolutionized orthodontics, because they provide anchorage without depending on the collaboration of patients, they have a favourable effectiveness compared with conventional anchorage devices, and they can be used for a wide scale of treatment objectives.

The incorporation of mini implants into orthodontic treatmentplanning has allowed for predictable anchorage control and hasincreased the ability to correct severe skeletal and dentaldiscrepancies.There are two different types of anchorage: direct and indirect.

Determining the type of anchorage that is more favourable depends on the following clinical or radiological factors: local bone quality, available space in particular for inter radicular insertion and mucosal thickness. Orthodontic mini implants have been designed to circumvent the limitations posed by restorative dental implants. These smaller bone implants are significantly less expensive, are easily placed and removed, and can be placed in almost any intra-oral region, including between the roots of the teeth.^{1, 2}

COMPOSTITION OF MINI IMPLANTS:

Mini-implants are manufactured from a biocompatible titanium alloy composition, “Wrought Titanium-6 Aluminium-4 Vanadium ELI (Extra Low Interstitial)”, according to the ASTM F136-02 standards specification. In the medical field of CranioMaxillo-Facial surgery, this titanium alloy is the material of choice and has been used for several decades by all established manufacturers of titanium implants. Furthermore, this specific titanium alloy composition covered has been used for human implants, in contact with bone and soft tissues, for more than 3 decades. The Orthodontic Mini Implant (OMI) is made of implant steel 1.4441.

The alloy exhibits a well documented level of biological inertness being characterized as a) Corrosion free b) Non Toxic c) Strong d) Having a low module of elasticity e) Anti-magnetic.^{9, 10}

DESIGN OF A MINI IMPLANT:

Orthodontic mini implant can be described as having four distinct yet contiguous segments or zones: BODY, COLLAR, NECK, and HEAD.

The screw BODY is comprised of asolid, cylindrical central shaft (core) that terminates in a finely definedsharp tip. Encircling and spiralling around the full length of the shaftare the threads, formed to a specificconfiguration and cut at a proscribeddepth and angle to the shaft.

The COLLAR is a smooth, polished, and thread less transitionalzone immediately extending from the screw body. It is the portionof the screw that resides in soft tissue. It is also cylindrical inform with a slightly larger diameter than the body so as toprovide a definite “stop” when placing screws in the host bone.It terminates in a broad, flat plate.

The NECK area of the screw is another transitional area that joins the collar with the head and extends into the oral cavity. It,too, is cylindrical and smooth with a diameter that is narrowerthan both collar and head. Functionally, it serves as an “under tie area” for the application of ligatures, elastomeric threadand chain, or NiTi spring eyelets.

The HEAD is the terminal segment of the screw and is the“working “end for the orthodontist. The Aarhus system has threedistinct head designs that are able to receive force modules ofvarying types. These three head designs provide the orthodontist a complete range of options when planning and selecting thebiomechanics for each specific case.^11-13

TECHNIQUE EMPLOYED:

The anatomical site should be chosen so that the risk of root contact is minimized and the clinician should avoid inserting screws in areas where nerves or vessels are located, e.g. the posterior lateral part of the palate. Mini-implants, although generally well accepted, require that the unique biological environment of the individual patient has to be thoroughly evaluated and understood. The bone turn-over of the patient may be influenced by factors that will interfere with the normal tissue reaction surrounding the screw. A thorough exam and Medical History is required to rule out any contra-indications to treatment. If no contraindications are found, the patient must be informed about the advantages, oral hygiene requirements, and the possible risk-factors before the start of treatment.

When starting the insertion, the perforation of the periosteum and the initial introduction into the external layer of the cortex should be made perpendicular to or, as close to, perpendicular to the bone; once the screw feels stable, the direction can be changed to a more apically directed angle.

Thread length selection-In patients with a cortical thickness of 1mm or more, a 6mm screw may provide sufficient primary stability. When the cortical bone is thin and the primary stability is primarily dependent on trabecular bone, a Mini-implant with longer thread length (8mm or more) may be indicated. In edentulous areas where the cortex may be very thin, bi-cortical anchorage may be the only alternative and should be considered.

Once the placement site has been determined the patient is asked to rinse thoroughly (2 min.) with a.2% clorhexidine mouth wash. Local anaesthesia of the mucosa at the insertion site is administered either by an injection of .5ml local anaesthetic or by an alternative anaesthesia of the mucosa. Following the administration of anaesthesia, the doctor has to prepare for the screw insertion procedure according to established sterile field standards established for intraoral surgery. A sterile surface has to be available and prepared to receive all of the surgical armamentarium determined necessary for placement of the screws. The doctor and assistants prep according to established protocols for oral or periodontal surgery like the surgical scrub, surgical head cover, mouth mask, and sterile gloves. The screw is picked up with the screwdriver, the grip on the screw tightened, and the insertion performed.^14-17

INDICATIONS OF MINI IMPLANTS:

Min implants are indicated as an alternative to other anchorage methods—for example, in extraction cases using implants instead of headgear.^{3, 4}Mini-implants have replaced other types of fixed appliances for the delivery of differentiated force systems for posterior tooth movement13 or extrusion of impacted canines.

It is indicated in a) Patients with insufficient teeth for the application of conventional anchorage b) Cases where the forces on the reactive unit would generate adverse side effects c) Patients with a need for asymmetrical tooth movements in all planes of space d) In some cases, as an alternative to orthognathic surgery.^{5, 6}

TREATMENT CONSIDERATIONS:

Patient selection:

Mini implants are approved by the U.S. Food and Drug Administration for use in patients 12 years and older.Juvenile patients who have not completed skeletal growth, as determined by a hand-wrist radiograph, should notundergo mini implant placement directly into the maxillary palatal midline suture.¹⁸ Ossification of the palatal suture will continue through the late 20s.¹⁹Mini implants are contraindicated in heavy smokers and patients with bone metabolic disorders.²⁰

Proper location for insertion of mini implants:

Mini implants should be inserted into a region with high bone density and thin keratinized tissue. The location chosen should be the optimal one in terms of both the patient’s safety and biomechanical tooth movement. Bone density and soft-tissue health are the key determinants that affect stationary anchorage and mini implants success.²¹

Bone density:

Stationary anchorage failure often occurs because the mini implant was placed in a region of low bone densitywith inadequate cortical thickness.²² Misch classified bone density into four groups—D1, D2, D3 and D4—based on the number of Hounsfield units (HU)—units of measurement used in computed tomographic scanning to characterize tissue density. D1 (> 1,250 HU) is dense cortical bone primarily found in the anterior mandible, buccal shelf and midpalatal region. D2 (850-1,250 HU) is porouscortical bone with coarse trabeculae found primarily in the anterior maxilla, the midpalatal region and the posterior mandible. D3 (350-850 HU) is thin (1 mm), porous cortical bone with fine trabeculae, found primarily in the posterior maxilla and mandible. D4 (150-350 HU) is fine trabecular bone, found primarily in the tuberosity region.

Soft-tissue health:

Inflammation of the surrounding soft tissue is directly associated with stationary anchorage failure. Mini implants placed in nonkeratinized alveolar tissue have a greater failure rate than those inserted into attached tissue. The loose alveolar tissue is irritated easily, leading to gingival inflammation and overgrowthof the mini implants head. In the buccal posterior region where the mucogingival junction is shorter, the clinician may choose to place the mini implant in alveolar mucosa to avoid root proximity.²³

Bone availability:

In the maxillary posterior dentoalveolus, the greatest amount of interradicular bone is located between the second premolar and first molar, 5 to 8 mm from the alveolar crest. In the mandibular posterior dentoalveolus, the greatest amount of interradicular bone is on either side of the first molar, approximately11 mm from the alveolar crest. In the anterior region of the maxilla and mandible, the greatest amount of interradicular bone is located between the canine and lateral incisor. If inadequate interradicular bone is available, the clinician can place the mini implant palatally or diverge the roots before inserting it.^24,25

PLACEMENT SITE:

Placement site in maxilla:

The diameter of the mini implants will depend on the site and space available. In the maxilla, a narrower implant can be selected if it is to be placed between the roots. If stability depends on insertion into trabecular bone, a longer screw is needed, but if cortical bone will provide enough stability, a shorter screw can be chosen. The length of the transmucosal part of the neck should be selected after assessing the mucosal thickness of the implant site.

Possible insertion site in the maxilla: the area below the nasal spine, the palate, the alveolar process, the infra zygomatic crest, and the retro molar area.

In the maxilla, the insertion should be at an oblique angle, in an apical direction.

In case of anterior region it is between the central and lateral incisors in the maxilla.

In the maxilla, the highest buccolingual thickness existed between first and second molars; the highest mesiodistal buccal/palatal distances were between the second premolar and the first molar. The highest buccal cortical thickness was between the first and second premolars. The highest palatal cortical thickness was between central and lateral incisors.Higher the bone density higher the success rate.

Placement site in mandible:

In case of mandible the reliable sites are the alveolar process, the retro molar area, and the symphysis.

In the mandible, the screw should be inserted as parallel to the roots as possible.

The mini-implants should be inserted through attached gingiva. If this is impossible, the screw can be buried beneath the mucosa so that only a wire, a coil spring, or a ligature passes through the mucosa.

In case of anterior region the placement site is between lateral incisor and the canine at 6-mm level from the CEJ.

In the mandible, the highest buccolingual and buccal cortical thicknesses were between the first and second molars. The highest mesiodistal buccal distance was between the second premolar and the first molar. The highest mesiodistal lingual distance was between the first and second premolars. The highest lingual cortical thickness was between the canine and the first premolar.

Placement site in posteriors aspect of both the arches:

At the buccal aspect of the posterior region of both jaws, the optimal sites are between the second premolar and the first molar and between the first and second molars. Palatally, the optimal site is between the first and second premolars as it has the advantage of the highest cortical thickness. The more apical the site, the safer the placement. present. A transcortical screw can be used for added stability in edentulous areas, where trabecular bone is usually scarce.^{7, 8}

CONCLUSION:

The introduction of mini-implants has improved the practice of orthodontics. Treatment approaches have become available that can be an alternative to orthognathic surgery and the use of Mini-Implants (Temporary Anchorage Devices) in Resolving Orthodontic Problems has given acceptable results. Duration of treatment becomes shorter significantly and simpler. The envelope of tooth movement has increased to an extent that more versatile movements in three planes of space can be carried out with more success. Mini implants have allowed the orthodontist to overcome anchorage limitations and perform difficult tooth movements predictably and with minimal patient compliance. Mini implants can provide absoluteanchorage for the movement ofwhole dentition, as well as the movement of one or two teeth forprosthetic needs. It simplifies treatment mechanics by eliminatingreactive mechanics.

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Received on 24.07.2016 Modified on 12.09.2016

Research J. Pharm. and Tech 2018; 11(6): 2621-2624.

DOI: 10.5958/0974-360X.2018.00486.9