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Role of magnets in orthodontics and dentofacial orthopedics: a comprehensive review.

Introduction

Orthodontics and dentofacial orthopaedics are therapeutic approaches that modify the occlusion, facial form and function through the application of, mechanical forces. Traditional force delivery systems in orthodontics include the use of screw, archwires, springs and elastics. Alternately, magnetic forces can be used to generate the force for tooth movement and orthopaedic treatment (1). Advantages of magnetic force delivery include good force control at short distances, no friction, and no material fatigue.

Magnetic forces have been used in orthodontics for both tooth movement (2-5) and orthopaedic correction (6-10) with varying degrees of success. Magnetic systems permit precise control of the force levels that are applied, as the force generated can be calculated from specific force-distance diagrams (1). The magnets initially used were bulky and there were concerns raised about possible toxic effects. However, the current available literature evaluating magnetic fields shows no evidence of any direct or acute toxic effects (11,12). Improved safety with better coating and the introduction of rare earth magnets, which led to a dramatic reduction in magnet size, stimulated further interest in the field of orthodontics (11, 12).

Magnetism and Magnetic Fields

Magnetism is the physical form of energy that can be either static or time varying, and originate from the electromagnetic interaction of particles (13-14). All magnets have a magnetic field that exists in the space around them. The magnetic field is a vector which has both magnitude and direction. Magnetic fields are detected by the force they exert on other magnetic materials and moving electric charges. The Oersted is the unit of the magnetic field strength in the CGS system and it is measured in amperes per meter (A/m) in SI units (13). Magnetic flux is a measure of quantity of magnetism, i.e. the strength and extent of the magnetic field. The SI unit of the flux density is the tesla (T). The flux density is proportional to the magnetic field strength. The force produced by any two magnets is inversely proportional to the square of the distance between them (Fa 1/[d.sup.2]) (13).

Magnetic properties of matter

There are three different types of magnetic substances: diamagnetic, paramagnetic and ferromagnetic substances (13).

a) Diamagnetism causes lines of magnetic flux to curve away from the material and creates a magnetic field in opposition to the externally applied magnetic field, a repulsive effect. A diamagnetic substance is weakly repelled and exhibits no permanent magnetism.

b) Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field. A paramagnetic substance is weakly attracted to magnets. Iron and rare-earth salts are paramagnetic substances.

c) Ferromagnetic substance is one that is strongly attracted to magnets. Ferromagnetism comes from the early association of this behavior with ferrous or iron containing materials. The magnetic domains are parallel in a ferromagnetic material. Common ferromagnetic materials are iron, nickel, cobalt, chromium dioxide and alnico.

Permanent magnets

Permanent magnets create their own persistent magnetic field. All permanent magnets are made from ferromagnetic materials. The magnetic properties of materials depend mainly on the chemical composition and on the heat treatment they receive after fabrication. The behavior of magnetic material is highly sensitive to small amounts of impurities and temperature. The Curie temperature is an important characteristic of a permanent magnet. The temperature at which any ferromagnetic material loses its magnetism is known as the Curie temperature (Tc). Above this temperature, thermal agitation destroys the magnetic alignment and the magnet become demagnetized (13,15).

Alnico Magnets

Alnico magnets were the first type of permanent magnets to be used for biomedical purposes. Alnico magnets are alloys based on cobalt, aluminium, nickel and iron (15,16).

Cobalt-platinum Magnets

Cobalt-platinum magnets were available at the same time as Alnico magnets. They consist of equal percentages of cobalt and platinum which forms a continuous solid solution to produce an isotropic magnet. They had improved properties and corrosion resistance compared with the Alnicos. These types of magnets have no widespread use in medical or dental applications because of their high cost (15,16).

Ferrite Magnets

Ferrite or ceramic magnets are the most widely used permanent magnetic material and play an important role in bulk magnet applications. They are more resistant to demagnetisation than the Alnico materials which make them suitable for use in complex shaped magnets. They produce a low magnetic field but are very cheap to produce which makes them ideal for their current application (15).

Rare earth magnets

Rare earth magnets are capable of producing high forces relative to their size due to the property of magnetocrystaline anisotrophy and their very high coercivity. The development and availability of rare earth magnetic alloys have led to the increase use of magnets in orthodontics. There are several types of rare earth magnets--samarium cobalt, neodymium iron boron and samarium iron nitride (15,17).

Samarium-Cobalt Magnets: Samarium-cobalt (SmCo) magnets were developed in the 1960s and 1970s. These magnets are characterised by high saturation magnetisation and Curie temperature. They are more costly than other rare earth magnets but are chosen in preference to those with a lower Curie temperature.

Neodymium-iron-boron magnets: This type of rare earth magnet has an extremely high magnetic saturation, good resistance to demagnetisation and the highest value of energy production They are less costly to produce than Sm-Co alloys and hence are now the main rare earth permanent magnet in use today. The main limitation of the neodymium magnet is that it had a low Curie temperature, as low as 300[degrees]C.

Samarium-iron-nitride magnets: Samarium iron nitride permanent magnets are a promising candidate for future applications. It has high resistance to demagnetisation, high magnetism and better resistance to temperature and corrosion.

Biological Considerations

Materials designated for clinical use need to be evaluated for any potential side-effects at a local and systemic level. Biological safety tests of magnetic materials have been performed to investigate the effects of static magnetic fields and possible toxic effects of the materials or their corrosion products (11,12).

Surface Oxidation and coating

Rare earth magnets, especially those containing neodymium, are known to be susceptible to corrosion (18,19). As the corrosive tendency of magnets in the oral environment it is recommended they be hermetically sealed for dental use. Coating the magnets is advised due to the possible risk of negative biological effects of the corrosion products. A range of coating materials has been used, for example biocompatible epoxy resin, stainless steel or a thin layer of parylene (2). Bondemark and coworkers compared the in vitro cytotoxic effects of uncoated and parylene-coated rare earth magnets used in orthodontics (20). In a recent study it was found that, PTFE (polytetrafluoroethylene) was a better coating material than parylene (21). The use of coating materials is also advocated to preserve the magnetic properties and clinical usefulness of intra-oral magnets.

Static magnetic fields

The effects of magnetic fields on the growth of cell cultures, both animal and human, have been evaluated (1). In vitro tests have demonstrated that static magnetic fields can affect certain biological parameters, such as stimulate enzymes, cell proliferation and attachment and osteogenesis (22,23). The reported effects of magnetic fields on the growth of human cells are inconsistent. Most of the studies show no significant effects with regard to DNA synthesis, DNA content, cell shape, structure and number or glycolytic activity (24). The evidence available from tests of the safety and biological properties of magnets suggest that the risks of biological harm are negligible.

Application of Magnetic Forces in Orthodontics

Magnets were first used in dentistry to improve the retention of dentures (25) and maxillofacial prosthesis (26). Magnetic forces have been used in orthodontics for both tooth movement (2-5) and orthopaedic correction (6-10) with varying degrees of success.

Advantages of magnetic force delivery reported in the literature include

* Predictable force levels

* Better directional force

* No force decay over time

* Can exert their force through mucosa and bone

* Frictionless mechanism

* Less patient discomfort and more patient cooperation

* Good force control over short distances

Magnetic Forces for Tooth Movement

In orthodontics teeth move in response to the application of light continuous forces. Magnets have been used in a variety of configurations for tooth movement.

Magnetic brackets

The first magnetic bracket was designed by Kawata et al. in 1977 (27). These brackets were made from iron-cobalt and chrome but were later replaced by rare earth magnets as they did not generate sufficient forces. A new magnetic edgewise bracket was introduced by Kawata et al. in 1987 (28). The magnetic brackets were chromium and nickel plated SmCo magnets soldered to the base of an edgewise bracket. The brackets allowed mesial and distal movement of teeth only if the inter-bracket distance was less than 3mm and therefore required conventional retraction prior to this. Dai X et al found that Magnets made into orthodontic brackets to some extent could replace the mechanical orthodontic force produced by orthodontic wires and elastics (29).

Tooth movement

In an experimental study Tomizuka et al found that gradually increasing force generated by permanent rare earth magnets induced effective tooth movement in rats without inducing any pathological changes (30). Darendeliler et al suggested that the pulsed electromagnetic fields induced vibration may enhance the effect of magnetic forces on tooth movement (31). Phelan et al suggested effective use of magnetic attachment with clear aligner for effective tooth movement (32).

Space closure

Blechman and Smiley demonstrated the use of Alnico magnets for canine distalisation in two cats (17). Later in a pilot study, Blechman reported the successful use of SmCo magnets attached to edgewise appliances for the application of intra and inter-maxillary forces (3). Muller used small rectangular magnets directly bonded to the labial aspect of the teeth to close diastema without archwires (33).

Molar distalisation

Several authors have reported on the use of magnets to move molars distally (4,5,34,35). In 1988 Gianelly et al described a new intra-arch method, whereby distalisation of maxillary first molars was achieved with repelling magnets in combination with a modified Nance appliance (5). The molars were distalised at a rate of 0.75-1mm per month, without significant anchorage loss. Bondemark and Kurol using an analogous system to generate a repelling force of 116 grams at 1mm separation (34). Bondemark et al. compared the effectiveness of repelling magnets versus superelastic nickel titanium coils in maxillary molar distalisation (35). The advantages of this appliance include no need for patient cooperation, ease of insertion and well tolerated by patients (4). Some disadvantages reported were minor tissue irrational under the acrylic of the Nance, cost of the magnets, bulky appearance and requirement for weekly activation under certain protocols (4,35).

Extrusion

Attractive magnets have been used for orthodontic extrusion. The use of magnets to extrude a traumatised incisor and enhance root eruption was reported by McCord and Harvie (36). Bondemark et al. reported a similar protocol with NdFeB magnets for the extrusion of crown-root fractured teeth (1).

Intrusion

Different types of magnetic appliances have been used for intrusion of posterior teeth. Hwang and Lee intruded over-erupted posterior teeth by corticotomy and magnets (37). Uribe and Nanda used the Rare-earth magnets embedded in acrylic bite-blocks to intrude the supraerupted maxillary molars (38).

Impacted teeth

Therapeutic options for the management of impacted teeth include extraction, transplantation, and surgical exposure alone or with the application of orthodontic traction (39). With traditional orthodontic traction force levels can be difficult to control and also technique sensitive (11). An alternate option that has been presented in the literature involves the use of magnetic traction. The technique involves surgical exposure of the impacted tooth, after which a magnet is bonded to the tooth surface. Guided eruption is achieved by means of a second magnet embedded in an appliance and placed in such a way as to attract the sub-mucosal magnet into the ideal place (11). The technique was first reported by Sandler et al. in 1989 for the eruption of a vertically impacted canine (40). Darendeliler and Friedli combined the use of removable and fixed attraction systems for an impacted canine (41). Guided eruption is one of the most well accepted and promising applications of magnets in orthodontics. The reported advantages of this technique include: operator and patient ease as there is no need to attach hooks or elastics, reduction in adjustments, continuous forces over a long period of time, friction-free system, healthy periodontium (as the eruptive process is through normal, closed mucoperiosteum) and reduced risk of infection (11,42,43).

Retention

Micro-magnetic retainers were introduced by Springate and Sandler in 1991 (44). Directly bonded magnets have advantages over conventional fixed retainers. Oral hygiene can be maintained as flossing is not prevented and there are no wires close to the gingival margins. The teeth are not splinted which allows normal physiological movement (11). Hahn et al (45) and Yadav et al (46) also successfully used the rare-earth magnets for positioning and bonding the lingual retainers.

Magnetic Force for Dentofacial Orthopaedic Treatment

Maxillary Expansion

Vardimon et al were the first to investigate the use of magnets to provide the force for maxillary expansion (9). Darendeliler et al examined the effect of magnetic forces for maxillary expansion in human patients of different ages (47). The authors concluded that 250-500g of continuous magnetic forces can produce dental and skeletal movement in a light force expansion. Theoretically, magnetic expansion appliances may be useful because of the predictable, constant low force they deliver. However, the appliances are likely to be quite bulky as they must be adequately stabilised and contain guide rods to prevent the magnet coming out of alignment and causing unwanted rotational movements (11). Darendeliler commented that neodymium magnets which are more powerful than SmCo magnets could generate the same amount of force with a smaller and less bulky appliance (47).

Openbite

Magnetic forces have been used for the management of openbite cases. Removable or fixed appliances with acrylic bite blocks incorporating magnets to intrude the molars have been used (11). Dellinger introduced the Active Vertical Corrector (AVC) in 1986 (48). This appliance used four pairs of repelling samarium-cobalt magnets to produce a posterior intrusive force of 700 grams per magnetic unit. The current generation of the AVC uses four NdFeB magnets that produce 675 grams of force in opposition and are only 0.151 inches high (49). The MAD IV (magnetic activator device IV), designed in 1989, uses anterior attracting NdFeB magnets as well as posterior repelling magnets (50). The posterior repelling magnets generate an intrusive force of 300 grams each. Three types of MAD IV have been described for different openbite cases. Other magnetic appliances for openbite correction have been documented in the literature (51,52,53). Theoretically the bite blocks with repelling magnets transfer continuous forces to the posterior teeth, although the level varies according to the amount of separation between the magnets (11). Conversely, conventional bite-block appliances transfer intermittent forces to the teeth only when they are in contact.

Class II Magnetic Functional Appliances

A range of magnetic functional appliances have been developed for the correction of class II malocclusion (7,8,51). The mandible is kept in a more forward position with the help of magnetic forces. The patients rest position is altered by the presence of magnetic forces to a "magnetic rest position" which is dictated directly by the placement of the magnets (54). It has been suggested by Darendeliler (54) and Vardimon et al (8) that by using magnetic forces a full time influence on mandibular position and function can be achieved. Vardimon et al developed the functional orthopaedic magnetic appliance (FOMA) II, a functional appliance that uses anteriorly positioned attractive magnetic means to constrain the lower jaw in an advanced sagittal posture (8). Vardimon et al. also conducted a retrospective clinical study to determine the skeletal and dental response to the functional magnetic system (FMS) (55). Kalra and coworkers reported on the use of a fixed magnetic appliance with repelling magnets for Class II division I cases with mandibular retrusion and increased lower face height (51).

Another functional magnetic appliance, called the Magnetic Activator Device (MAD), was introduced by Darendeliler and Joho (7,56). Several types have been designed to manage different clinical problems e.g. MAD 1--lateral displacement (7), MAD II--class II malocclusions (7,56,57), MAD III--class III malocclusions (10) and MAD IV--open bite (50). The MAD can be worn full time, except during meals since phonation and deglutination are not as limited. It has also been suggested by Darendeliler that bonded magnetic appliances could be used as fixed functional appliances (54). Moss et al. incorporated magnets in the twin block appliance in the treatment of Class II division I malocclusions (58). Chate described the use of the propellant unilateral magnetic appliance (PUMA) in the treatment of hemifacial microsomia. Samarium-cobalt magnets embedded in unilateral blocks of acrylic were used to stimulate growth following an autogenous costochondral graft (59). Recently Phelan et al evaluated the effect of a new magnetic functional appliance, the Sydney Magnoglide and found that it is an effective functional appliance for Class II correction (60).

Class III Magnetic Functional Appliances

The Functional Orthopaedic Magnetic Appliance (FOMA) III was developed by Vardimon and co-workers for the treatment of Class III malocclusions with midface sagittal deficiency with or without mandibular excess (6). The FOMA III consists of upper and lower plates with two disc shaped neodymium-iron-boron magnets (6mm diameter x 3mm) in an attractive configuration. Over a 4 month treatment period midface protraction occurred and significant forward movement of the maxillary incisors and molars. Clinical application of a magnetic functional appliance for Class III treatment has been demonstrated by Darendeliler et al. and Luthy-Burhop et al (10,12). Both case reports document successful treatment with the MAD III, one in combination with a magnetic expansion device and the other with a Delaire facemask (10,12). The magnetic activator device (MAD) III consists of an upper and lower plate with two buccal pairs of attracting samarium cobalt magnets (6mm x 4mm x 5mm) placed eccentrically in the sagittal direction (10). Tuncer and Uner also found that magnetic appliance can be used effectively for correction functional Class III malocclusion.

Magnetic Sensor for Tooth Displacement

A system was developed for measuring tooth displacement by orthodontic force. Eight small magnetic sensors and a magnet are combined to measure three-dimensional displacement. Sensors, arranged cubically in the three planes of space, are placed in the mouth and fixed to the posterior teeth by a splint. A magnet is placed in the center of the 8 sensors and attached to a front tooth that is subjected to orthodontic force. Sensors detect the magnet's movement as target tooth displacement. The advantage of this system is the ability to detect tooth trajectories by decomposing displacement into translation and rotation and to determine the position of the center of rotation (62). Zhang et al used novel method of magnetic bead-based salivary peptidome profiling analysis to check the alterations in salivary proteins due to different orthodontic treatment durations (63).

Recycling of Magnets

The recycling does not affect the biocompatibility and force stability of the magnets even though the recycling process involved autoclaving. It is also recommend that new, partially encased SmCos magnets be stored in water for 24 hours before clinical use. Most of the cytotoxic, water soluble components present on the magnet will thereby be released, and the oral exposure of cytotoxic components be reduced (64).

Conclusion

Magnets can be used for various types of tooth movement and dentofacial orthopedic correction to give predictable forces in either attraction or repulsion. Magnets can be made small enough to suit most dental and orthodontic applications. Their use in orthodontics, however, is limited due to a number of factors. The force between two magnets drops significantly with distance and even at small distances apart the forces can be too low. When magnets were coated with acrylic or hot liquids, there was considerable loss of magnetic flux and therefore, force. For proper application of magnetic force, the orientation of one magnet to another is of the utmost importance and when not in perfect alignment the force between them drops significantly.

Orthodontics, as part of modern sciences, is also influenced by the rapid and constant development in technology, most particularly, in regards to science of dental materials. Therefore, a good comprehension about biomechanical concepts is very important for the development of innovative orthodontic materials and such innovations may result in new biomechanical principles. Even though the magnets give promising result in various therapeutic modalities, studies are required to evaluate long term stability. Further research is required to investigate into the biological effects of magnets.

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Ram Sukh *, Pradeep Tandon, Alka Singh, Gyan P. Singh

Dept of Othodontics and Dentofacial Orthopedics, King George's Medical University, Lucknow (UP), India

* Corresponding author, Dr. Ram Sukh, ramsukhorthokgmc@gmail.com

Received 27 January 2013; Accepted 16 June 2013; Available online 21 July 2013
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