From Volume 7, Issue 4, October 2014 | Pages 122-128
Magnets have been used in dentistry for many years. The force they deliver can be directed, and they can exert their force through mucosa and bone. There are various types of magnets used in the field of orthodontics and dentofacial orthopaedics, all with their advantages and disadvantages. The biological effect, different materials used, and the recycling of magnets is discussed in this article.
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 Magnetic forces have been used in orthodontics for both tooth movement2,3,4,5 and orthopaedic correction,6,7,8,9,10 with varying degrees of success. The earlier use of magnets was limited owing to the unavailability of small size magnets, and there were concerns raised about possible toxic effects. However, current 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, has stimulated further interest in the field of orthodontics.11,12 This article discusses the history, magnetism and magnetic force, the types of magnets, the role of magnets in orthodontics, and the recycling of magnets, together with advantages and disadvantages.
In dentistry, magnets (Table 1) were used first as implants for denture retention by Behrman and Egan in 1953.13
| Year of Development | Types of magnet |
|---|---|
| 1910 | Tungsten–steel |
| 1925 | Cobalt–steel |
| 1935 | Aluminum–nickel |
| 1945 | Aluminum–nickel–cobalt |
| 1950 | Aluminum–cobalt |
| 1955 | Ferrite |
| 1965 | Samarium–cobalt |
| 1985 | Neodymium–iron–boron |
In the field of orthodontics, elastics, springs and screws have been the main source of force with, however, some drawbacks, such as lack of patient co-operation, degradation of force or material, and irregular activation intervals. The use of magnets for orthodontic tooth movement was first described by Blechman3 who bonded earth magnets made up of aluminum–nickel–cobalt to the teeth of adolescent cats to produce tooth movement.14 They proved that magnets have sufficient duration and intensity while generating force in intermaxillary and intramaxillary mechanics. They are operator controlled and have a three-plane vector. Since then, many orthodontists have investigated the various clinical uses of magnets. Other rare earth magnets, like samarium cobalt, introduced by Becker in 1970 (SmCo5 and Sm2Co17 and NdFeB), have been of special interest as these alloys have properties superior to previously used magnetic alloys like Al–Ni–Co and ferrite magnets, which have their limitations, particularly in relation to their size, high cost, and risk of demagnetization (Table 1).
Magnetism is the physical form of energy and can be either static or time varying, and originates from the electromagnetic interaction of particles.15,16,17 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.15 Magnetic flux is a measure of quantity of magnetism, ie 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])15
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A. Based on alloys used
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B. Based on ability to retain magnetic properties (intrinsic coercivity or hardness)
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C. Based on surface coating (materials may be stainless steel, titanium or palladium)
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D. Based on the type of magnetism
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E. Based on type of magnetic field
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F. Based on number of magnets in the system
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G. Based on the arrangement of the poles
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Magnetic materials can be divided broadly into two groups, ‘hard’ and ‘soft’, based on their magnetic properties. Hard magnetic materials possess a large remanence and coercivity and are difficult to magnetize and demagnetize. The hard magnetic materials are, therefore, used for permanent magnets in devices such as motors, loudspeakers, and in a variety of household and industrial devices. The soft magnetic materials have low permeability and low coercivity and are easily magnetized and demagnetized.
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 behaviour of magnetic material is highly sensitive to small amounts of impurities and temperature. The temperature at which any ferromagnetic material loses its magnetism is known as the Curie temperature (Tc) and is an important characteristic. Above this temperature, thermal agitation destroys the magnetic alignment and the magnet becomes demagnetized.15,19 Commonly used permanent magnets include alnico, platinum-cobalt, chromium-cobalt-iron, cobalt-samarium and neodymium-iron-boron (Table 3).
| Alnico magnets | |
| Cobalt-platinum magnets |
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| Chromium cobalt iron |
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| Ferrite magnets |
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| Rare earth magnets |
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| Samarium-cobalt magnets |
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| Neodymium-iron-boron magnets |
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| Samarium-iron-nitride magnets |
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It is important to ensure that magnets used intra-orally for clinical use should not produce any side-effects at a local or systemic level. Biological safety tests of magnetic materials have been performed to investigate the effects of static magnetic fields and the possible toxic effects of the materials or their corrosion products.11,12
As there is a corrosive tendency of magnets in the oral environment, it is recommended they be hermetically sealed for dental use. Coating the magnets is advised to decrease the possible risks related to the corrosion products. Rare earth magnets, especially those containing neodymium, are known to be susceptible to corrosion.24,25 There are several coating materials used, for example, biocompatible epoxy resin, stainless steel or a thin layer of parylene.2 Bondemark and coworkers studied and compared the in vitro cytotoxic effects of uncoated and parylene-coated rare earth magnets used in orthodontics26 and found a range of effects from ‘no cytotoxic effects’ to ‘mild cytotoxic effects’. It is of paramount importance to prevent corrosion from occurring. Coating the magnets with parylene (polytetraxylene) in ultra thin sections will produce an effective barrier to corrosion. In a recent study, it was found that, PTFE (polytetrafluoroethylene) was a better coating material than parylene.27 The use of coating materials increases the use of magnets and preserves the magnetic properties and clinical usefulness of intra-oral magnets.
The effects of magnetic fields on the growth of cell cultures, both animal and human, have been studied.1In vitro tests have demonstrated that static magnetic fields can affect certain biological parameters, such as the stimulation of enzymes, cell proliferation and attachment and osteogenesis,28,29 but 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.30 However, tests on the safety and biological properties of magnets suggest that the risks of biological harm are negligible.
Magnets were first used in dentistry in prosthodontics to improve the retention of dentures32 and maxillofacial prosthesis.33 Magnetic forces have been used in orthodontics for both tooth movement2,3,4,5 and orthopaedic correction6,7,8,9,10 with varying degrees of success.
In 1977, Kawata and Takeda first used magnetic force to move teeth by using magnetic brackets of Co-Cr-Fe alloy.21 Initially, brackets were made from iron-cobalt and chrome but were later replaced by rare earth magnets as they did not generate sufficient force. A new magnetic edgewise bracket was introduced by Kawata et al in 1987.34 As a result of earlier work, claims were made that magnetic forces to move teeth were less stressful than the conventional use of springs, coils and elastics. The magnetic brackets were chromium-plated, samarium-cobalt magnets soldered to the base of an edgewise bracket which were directly bonded to the teeth and were designed to form an ideal arch shape in the maxilla and mandible at the completion of treatment. Force levels delivered to the teeth were estimated at 250 g. Bracket placement allowed mesial and distal movement of teeth only if the inter-bracket distance was less than 3 mm. Darendeliler and Joho7 described a similar system called the autonomous fixed appliance, which has no brackets or archwires, but uses individual samarium-cobalt magnets bonded to each tooth exerting a continuous force to create an ideal arch form.
Muller suggested that small magnets (approximately 5 x 3 x 1 mm) could be used to deliver light continuous forces to close diastemas without archwires.35 Muller suggested that rotations and angulation problems could also be corrected with this technique. The author noted that the magnets produced a light continuous force that increased as the teeth got nearer and was the reason the teeth moved quickly. Blechman and Smiley demonstrated the use of Alnico magnets for canine distalization in two cats.36 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
The use of magnets to extrude a traumatized incisor and enhance root eruption was reported by McCord and Harvie.37 Magnets that attract have been used for orthodontic extrusion. Bondemark et al reported a similar protocol with NdFeB magnets for the extrusion of crown-root fractured teeth.1
Hwang and Lee intruded over-erupted posterior teeth by corticotomy and magnets.38 There are many different types of magnetic appliances and functional magnetic appliances have been used for intrusion of posterior teeth. Uribe and Nanda used rare-earth magnets embedded in acrylic bite-blocks to intrude the supra-erupted maxillary molars.39 Recently, removable and fixed appliances with acrylic bite blocks incorporating magnets to intrude the molars have been used. Dellinger12 reported on the Active Vertical Corrector (AVC). This appliance uses samarium-cobalt magnets, orientated to repulse, hence producing a posterior intrusive force of 600–700 g per magnetic unit.
Orthodontic movement of the impacted canine after surgical exposure is the most common treatment. However, problems such as infection, gingival inflammation, apical migration of the epithelial attachment, bony recession, and exposure of the cement-enamel junction have been associated with traditional orthodontic methods. An alternate option that has been presented in the literature involves the use of magnetic traction. Sandler and colleagues40 reported the application of magnets in the eruption of an impacted tooth. These authors used an intra-oral magnet on a removable plate to attract a smaller magnet bonded to the surgically exposed impacted canine. Periodic grinding of the acrylic plate and repositioning of the larger magnet were required. 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. Advantages of the use of magnets includes:
A recent laboratory study16 looking at the effects of magnets used in this application has shown that the attractive force levels generated between neodymium-iron-boron magnets set in attraction are sufficient to induce the cellular and biochemical changes that are required to produce orthodontic tooth movement over a reasonable clinical range. When the angle of the pole face of the superior magnet relative to the base magnet is changed, the rate of decline of the force is very severe and care must be taken to ensure adequate forces are being generated between the tooth and base magnet.
Class II malocclusions can be resolved by several procedures. One involves the distal movement of the maxillary molars to establish Class I molar relationships. The premolars and canines are then sequentially moved posteriorly to Class I positions, and finally the incisors are aligned and/or retracted. Gianelly et al described a new intra-arch method, whereby distalization of maxillary first molars was achieved with repelling magnets in combination with a modified Nance appliance.5 The molars were distalized at a rate of 0.75–1 mm per month, without significant anchorage loss. The Molar Distalization System (MDS) uses two opposing magnets for each maxillary quadrant. The mesial magnet of each pair is mounted so that it can move freely along a sectional wire. A sliding yoke, with ligation hooks mesial to the mesial magnet, brings the repelling magnets together to activate the magnetic force. Bondemark and Kurol used an analogous system to generate a repelling force of 116 grams at 1 mm separation.43 Bondemark et al compared the effectiveness of repelling magnets versus superelastic nickel titanium coils in maxillary molar distalization.44 The advantages of this appliance include: no need for patient co-operation; ease of insertion; and being well tolerated by patients.4
Micro-magnetic retainers have been suggested by Springate and Sandler45 to retain central incisors that have been brought together to close a median diastema. Hahn et al46 and Yadav et al47 also successfully used rare-earth magnets for positioning and bonding lingual retainers.
For Class II correction, a wide range of functional appliances have been developed.7,48 Magnetic functional appliances can be utilized to keep the mandible in a more forward position with the help of magnetic forces. It has been suggested by Darendeliler49 and Vardimon et al8 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 position the lower jaw in an advanced sagittal posture.8 Kalra et al used a fixed magnetic appliance with repelling magnets for Class II division I cases with mandibular retrusion and increased lower face height.50 The Magnetic Activator Device (MAD) was introduced by Darendeliler and Joho.7,51 Several types have been designed to deal with differing clinical problems, eg lateral displacement (MAD 1), Class II malocclusions (MAD II), Class Ills (MAD III), and open bite cases (MAD IV). The MAD IV has been described by Darendeliler et al.52 A propellant unilateral magnetic appliance (PUMA) has been used in the treatment of hemifacial microsomia by Chate. Samarium-cobalt magnets embedded in unilateral blocks of acrylic were used to stimulate growth following an autogenous costochondral graft.53 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.54
Vardimon et al developed the Functional Orthopaedic Magnetic Appliance (FOMA) III for the treatment of Class III malocclusions with midface sagittal deficiency with or without mandibular excess.6 A clinical application of a magnetic functional appliance for Class III treatment has been demonstrated by Darendeliler et al.10,12 The magnetic activator device (MAD) III is another functional appliance for Class III correction. It consists of an upper and lower plate with two buccal pairs of attracting samarium cobalt magnets (6 mm x 4 mm x 5 mm) placed eccentrically in the sagittal direction.10
Orthopaedic movement of the palatal shelves with magnets has been undertaken by Vardimon et al9 in a study that looked into the effects of using samarium-cobalt magnets to provide the expansion force on monkeys. This study demonstrated that magnetic expansion does produce controlled forces over a predicted range and time. Darendeliler et al examined the effect of magnetic forces for maxillary expansion in human patients of different ages55 and concluded that neodymium magnets, which are more powerful than SmCo magnets, could generate the same amount of force with a smaller and less bulky appliance. Magnetic expansion appliances may be useful because of the predictable, constant low force that they deliver. The expansion force is slow, however, compared with rapid maxillary expansion techniques (RME) and, consequently, there is less tendency for the mid-palatal suture to fracture.
Dellinger introduced the Active Vertical Corrector (AVC) in 1986 for vertical control.56 This appliance used four pairs of repelling samarium-cobalt magnets to produce a posterior intrusive force of 700 grams per magnetic to intrude the posterior teeth, causing the mandible to rotate upward and forward. Currently, AVC is used as a non-surgical alternative treatment for skeletal open bite. The MAD IV (magnetic activator device IV), designed in 1989, uses anterior attracting NdFeB magnets as well as posterior repelling magnets.52
The sensor system was developed for measuring tooth displacement due to 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 centre of the eight sensors and attached to a front tooth that is subjected to orthodontic force. Sensors detect the magnet's movement as target tooth displacement. The system was designed to achieve displacement resolution of 1 μm.57 Zhang et al used a novel method of magnetic bead-based salivary peptidome profiling analysis to check the alterations in salivary proteins due to different orthodontic treatment durations.58
The recycling does not affect the biocompatibility and force stability of the magnets, even though the recycling process involves autoclaving.59 It is also recommended that new partially encased SmCo magnets be stored in water for 24 hours before use so that any cytotoxic, water soluble components present on the magnet will thereby be released, and the oral exposure of cytotoxic components be reduced.59 It has been reported that there is a high cytotoxicity for uncoated SmCo5 magnets and low for NdFeB magnets.
Magnets can be used to give predictable forces in either attraction or repulsion; they can be made small enough to suit most dental, orthodontic and orthopaedic applications. Their use in orthodontics, however, is limited owing to a number of factors. The force between two magnets drops dramatically with distance and, even at small distances apart, the forces can be very low. 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, with regard to the science of dental materials.
Currently, magnets can be used for orthodontic tooth movement as well as for orthopaedic corrections. Therefore, a good understanding of biomechanical concepts is important for the development of innovative orthodontic materials and such innovations may result in new biomechanical principles, even though the current magnets in various modalities give a promising result. Research studies are required to evaluate long-term stability, especially in open bite and Class III cases. Further investigation is also required to research the biological effects of magnets.
