Magnet as a dental material--an overview.
Magnetic force systems are mentioned in the dental literature. Initially they were used to aid retention of dental prostheses when used as jaw implants, as described by Behrman and Egan in 1953 (1). The widespread use of magnets in prosthetic dentistry was prevented because of cost of magnets and inadequate magnetic properties. Introduction of small, powerful and permanent rare earth magnets that are also reasonably priced has lead to magnets becoming more popular. The use of magnets for orthodontic tooth "movement was first described by Blechman and Smiley in 1978 (2). In recent years, magnets and magnetic force has been suggested as an alternative to traditional orthodontic devices such as elastics, springs and wires. Magnets can be made to attract or repel, and therefore to pull or push the teeth (3), The advantages of magnetic force systems are (4)--
--predictable force levels
--better directional force
--no force decay overtime
--can exert their force through mucosa and bone
--less patient discomfort and less patient co-operation.
Latest studies have shown that magnetic devices offer an optimum and biologically safe force generating system not only for denture retention but also for orthodontic tooth movement.
Although magnets are potentially very helpful, there are a number of problems that severely affects their performance. One of the common problems is dramatic reduction in force if magnets are not ideally aligned to one another (3). In addition, magnets corrode badly in the mouth and a robust coating is required to protect them. Overall magnetic force system is a total force system with a favorable benefit to risk ratio.
Thus the purpose of discussion is to give a general idea about the different types of magnets used in dentistry, their physical properties, their biological safety and gross reviews of various reported works by different authors so far.
MAGNETS AND THEIR PHYSICAL PROPERTIES (5)
A magnet is a material, which has an ability to attract iron or iron containing alloys and to lie in a north-south direction when freely suspended. All magnets have magnetic field around them. The field emerges from one pole of the magnet known as North Pole and returns to the other pole known as South Pole. Magnetic field induces changes in the medium surrounding the magnet and this is called as flux density. The unit of flux density is Tesla. Attraction of iron and attraction or repulsion between two magnets is because of their pole orientation to each other. The force either attraction or repulsion by any two magnets is inversely proportional to square of the distance between them and the flux density is proportional to the size of the magnet itself.
Since 1953 aluminium-nickel-cobalt (Alnico), platinum-cobalt and ferrite magnets were used in dentistry, These magnets offer high field strength at a reasonable cost. They have the highest working temperature of any commercially available magnets, can be electroplated for protection and readily magnetise after assembly. They are physically stronger than any other magnetic materials. These magnets are available in both isotropic and anisotropic form. But the risk of demagnetisation, less powerful and high cost were the problems with these conventional magnets. The clinical applications of these conventional magnets are highly restricted because of their size, which is almost exclusively in the range of centimeter.
In 1965, samarium-cobalt (SmCo) alloy magnet developed with superior magnetic properties was compared with conventional magnets. They allow significant size reduction and performance improvement for redesigned product, as well as having almost complete resistance to demagnetization. Thus it is a permanent magnet. This is available in isotropic form and also as sintered material having good thermal stability. Recently developed neodymium-iron-boron (NdFeB) magnet offers the highest magnetic energy per unit volume. Bonded material is available in isotropic form and the sintered material in anisotropic form. Although it is less brittle than sintered samarium-cobalt magnet but it has a lower working temperature and corrosion resistance is very poor thus protecting coatings are necessary to protect from corrosion.
The rare earth magnets are capable of producing high forces relative to their size due to the property of magneto crystalline anisotropy. This property allows single crystals to be preferentially aligned in one direction, thus increasing the magnetism. Therefore, the magnetic force necessary for dental application may be obtained with very small magnets. Thus these magnets are Innocuous magnet. The high coercivity property or resistance to demagnetisation of rare earth magnets is because of their intrinsic property and the manufacturing process. In dentistry, so called hard magnetic materials, which generate static or constant fields, are used. When magnets are heated to even modest temperature they suffer irreversible magnetic loss. In many applications magnets are embedded in acrylic appliances and during curing of methylmethacrylate reaches a temperature of between 80 and 90 degree centigrade could cause significant amount of flux loss due to the exothermic setting reaction of the acrylic.
BIOLOGICAL SAFETY OF MAGNETS
It is important to ensure as far as it is reasonably possible that any new material destined for clinical use should not produce any side effects at a local or systemic level. Biological safety testing of magnets has evaluated the effects of both the static magnetic field and possible toxic effects of the material or their corrosion products. Despite being of primary interest, Information on the biological effects of magnets In human is currently somewhat limited. However a number of biological investigations have been conducted in various animal species and in cell cultures
One of the first studies by Tustsui et al in 1979(6) found that the corrosion resistance of the samarium-cobalt magnet was similar to that of usual dental casting alloys but the add resistance was relatively low. The magnets had virtually no toxic or other negative effects on the tissues. Thus samarium-cobalt magnets could be safely used as a dental material if plated or coated. According to Vardimon and Muller in 1985 (7) the electrochemical properties, corrosion tendencies and reactivity of samarium-cobalt and neodymium-iron-boron magnet to oral environment revealed the necessity to improve surface coating. In a detailed study, Kitsugi et al in 1992 (8) concluded that although the corrosive activity of the neodymium, iron-boron magnet was higher than that of samarium-cobalt magnet, it was necessary to seal both the magnets for dental use. In a retrospective study, Drago in 1991 (9) reported that the edges of all magnetic implants used in various clinical prosthodontic procedures showed evidence of tarnish and somewhat corroded, thus significantly affecting the useful lifespan of intraoral magnets.
Cell culture studies
Short term exposure of N human lymphocytes, WI-18 human embryonic fibroblasts and LM mouse embryo fibroblasts to a magnetic field of 60 millitesia intensity produced by samarium-cobalt magnets revealed no significant effect on growth rate or type of cell response (Esformes et al., 1981)(10). Similarly neodymium-iron-boron magnets did not appear to have cytotoxic effects on fibroblast iota cells (Sandler et al, 1989) (3) and no effect on cell activity in either attractive or repulsive magnetic field (Papadopulous et al, 1992) (11). Mc Donald in 1893 (12) found that In presence of static magnetic fields generated by SmCo magnets there is increased proliferation and systemic activity of fibroblasts. The effects of magnetic fields on the growth of human cultured cells showed no significant consequences with respect to DNA synthesis, DNA content, cell shape, surface structure or cell number (Sato et al, 1992) (13) or glycolytic activity (Yamaguchi et al, 1993) (14). However, orthodontic magnetic brackets producing a field strength of 130 gauss have been shown to influence the oral microbial flora, significantly stimulating the growth of candida albicans (Stafollani at al, 1991) (15). The cytotoxic effect of a new magnet is highest where as less marked with the clinically used one and least with the recycled magnets, thus biocompatibility of magnets can be maintained upon recycling (Bondemark at al 1994) (16). The outcome of these studies thus demonstrated a range of effects from no cytotoxic effects to mild cytotoxic effect.
Implantation of platinum-cobalt alloy magnets in dog mandible for a period of six months showed a normal sequence of repair in and about the bone in the presence of the magnetised implants (Toto et al, 1962) (17). One of the first animal studies Investigating the effects of samarium-cobalt magnets implanted within the tissues reported no adverse effects on blood cells (Cerny, 1979) (18), no abnormalities of tissues around magnetic implants (Cerny, 1980) (4) and no changes in the dental pulp, periodontal and gingival tissue, buccal mucosa or alveolar bone in presence of magnetic exposure of up to 95 millitesia (Cerny, 1980) (4). Even after implantation of titanium coated samarium-cobalt magnets in dog mandible far a period of six months showed no abnormal healing or osteoblastic activity and no notable difference in cell size, shape or content (Altay et al, 1991) (19). The pulsed electromagnetic field and static magnetic fields enhances the amount of bone formation and hard tissue density at the asteotomy site in guinea pig mandibles (Darendeliler et al, 1997) (20). Also bath static and pulsed magnetic field significantly enhances tooth movement and reduces serum calcium level, probably due to Increased rate of osteogenesis (Darendeliler et al, 1995) (21). There is an increased white blood cell count in blood which is possibly as a response to corrosion products of magnets (Darendeliler et al, 1995) (21). In contrast an Investigation on the effects of static magnetic field on bone surface and skin reported by Under-Aronson and Lindskog in 1991 (22), showed a reversible reduction in the number of epithelial cells in the area where the magnets had been applied and a significant increase in bone resorbing areas after 3 and 4 weeks. Bruce et al in 1987 (23) demonstrated that fracture bone units when exposed to static magnetic fields showed no histological change but a stronger callus formed between bone units.
As previously stated, there were very few human studies on the biological effects of magnetic ,field: Among important human studies, implantation of platinum-cobalt alloy magnets into the molar regions of edentulous mandible for thirteen months period showed that bone had grown around the periphery of magnets (24). Blechman in 1985 (25) found no effect on urinary cobalt level measured at B months Intervals, whilst Kawata et al in 1987 (26) observed no significant changes in ascorbic acid, calcium or citric acid concentration. However, Bondemark et al in 1995 (27) found that magnetic fields have no effect on maxillary buccal mucosal blood flow and pulpal tissues. Similarly, Saygili et al in 1992 (28) concluded that magnetic field has no detrimental effect on blood flow of maxillary buccal mucosa. A study by Bondemark et al in 1998 (29) found no adverse long term effects on human buccal mucosa which had been in contact with an acrylic coated neodymium-iron-boron magnets and subject to the static magnetic field.
Thus evidence currently available from biological safety testing would suggest that the conceivable risk of harmful biological effects are negligible.
CLINICAL APPLICATION OF MAGNET
Magnets have been used in dentistry for a variety of purposes most commonly to aid retention of dentures, maxillofacial prostheses and In orthodontics.
Retention is fundamental to the adequate functioning of dental prostheses. Retention is usually obtained by one or a combination of friction, mechanical locking, atmospheric pressure and neuromuscular control. It Is possible that permanent, dynamic and positive retention can be obtained by placing the magnet in natural tooth structure or a bony fault in the jaw and a corresponding magnet in a dental prostheses (30). This is possible be applicable to a full denture (implant or overdenture), a partial denture, a crown or bridge or a surgical or periodontal splint (30). Some advantages of using magnetic retention are lateral stresses on anchor teeth would be minimized as magnet slide relatively freely across abutting surfaces, technical procedures involved are simple and quick, material required are relatively Inexpensive and a constant retention force (30). But disadvantages of using magnetic retention are creation of space for magnets may involve devitalizing and decrowning teeth, the bulk of magnets must increase with the retention requirement and difficulty in proper alignment of magnets to each other (30).
The treatment and rehabilitation of patients with cancer of head and neck and the importance of maxillofacial prostheses has increased several times. But still there are several problems in every phase of construction as well as in the retention of maxillofacial prostheses. A number of methods including mechanical devices and anatomical methods are available. Magnets have been effectively used for the retention, maintenance and stabilization of combined maxillofacial prostheses. Magnets In the coin form have more advantages in maxillofacial prostheses than the other form. The size of magnet depends on size of defect (31).
For the treatment of anterior open bite many treatment regimens have been recommended. Recently removable or fixed Orthodontic appliances with acrylic bite blocks incorporating magnets to intrude the molars have been used. Dellinger in 1986 (32) introduced Active Vertical Corrector with samarium-cobalt magnets oriented in repulsion to produce intrusive force 600-700 gms on posterior teeth. Intrusive force can also be provided by placing small powerful rare earth magnets on occlusal surface of posterior-teeth by direct bonding (33). Use of magnets for intrusion of teeth produce continuous force but the magnets should align properly for desired force direction.
In spite of success with fixed retainers to stabilize anterior spacing, there are number of undesirable characteristics. Small magnets can be used to retain central incisors that have been brought together to close a median diastema (34). After closure of median diastema small neodymium-iron-boron magnets can be bonded with light cured composites on the mesiopalatal aspect of the teeth. During bonding, incisors must be separated by an: acetate-finishing stripe so that magnets are not fixed together. The advantages with such type of retention are easy to maintain oral hygiene or flossing can be done, no wires or ledges close to the gingival . margins and teeth can move completely physiologically as they are not splinted together. Unfortunately, there has riot been any long term following up of this technique and therefore it cannot be considered routine clinical practice at present.
Maxillary expansion and orthopedic movement of palatal shelves has been used for many years.' Magnetic appliances for expansion of maxilla has been tried in experimental animals (35) and found that magnetic expansion produce controlled force over a predictable range and time, The expansion with magnetic appliances is slow thus there is less tendency for the midpalatal suture to fracture. Maxillary expansion in humans has not been tried as it is very difficult to attain proper fine of force in proper direction and bulkiness of the appliances.
Many methods for dealing with an unerupted or impacted teeth have been described. In many cases exposure alone, or exposure and applying an attachment to the tooth is used. A small high energy magnet can be used to provide the traction force to aid the eruption of impacted tooth (36). Bonding a small neodymium-iron-boron magnet on to the unerupted tooth and incorporating a large magnet into the removable appliance can be used to bring an impacted tooth into the arch. It is easy for the operator, the patient does not have to attach elastics or hooks to the chain, few adjustments are needed, the attachment is less likely to be knocked and dislodged from the teeth. However, there are number of limitations with this approach. if the tooth is far from the oral cavity e the, forces may be very small between the magnets and theme is chance of corrosion of magnets if their coating is damaged.
Small magnets can be used to deliver light continuous forces to close diastemas without arch wires (37). Bonding magnets on labial or palatal aspect of the teeth, diastema as well as rotations and angulation problems can be corrected. But the difficulty in correctly positioning the magnets and the risk of inhalation If one dislodged are main disadvantages. Magnets can also be used to edgewise appliances to move teeth along the arch wires, to provide class-II traction and to extrude or intrude an individual teeth: Using repelling magnets maxillary molars can be moved distally In conjunction with a modified Nance appliance (38). Static magnetic field and its bioeffects are may be possible mechanisms of action of repelling molar distalizing magnets.
Magnets can be incorporated to an edgewise bracket to deliver force (26). The magnetic brackets are chromium plated samarium-cobalt magnet soldered to the base of an edgewise bracket which can be directly bonded to tooth. If the distance between the brackets is less than 3 mm then forces are delivered in mesiodistal direction. Magnetic brackets can be best used for treatment of class-I malocclusion.
Magnets can be used in functional appliances for correction of skeletal jaw discrepancies. Functional orthopedic magnetic appliances (FOMA)-II (39) & III (40) has shown positive treatment effect in experimental animals. The first clinical experience with Magnetic Activator Device (MAD) for correction of class-II (41) and class-III (42) cases has recently been described. It has e been found that incorporating magnets into twin block decreases the time taken to produce the sagittal changes and increases the soft tissue changes compared to those appliances without magnets (43). Magnets are also used to stimulate autogenous costochondral grafts for the treatment of hemifacial microsomia and the appliance is known as Propellant Unilateral Magnetic Appliance, PUMA (44).
In dentistry, rare earth magnets have been used successfully far fixation of dentures and in force systems for tooth movement. However, magnets have not yet been routinely used. Magnets and the magnetic force systems are better device for theoretic and academic purposes. Not easily one can practice in day today life. Needs to be very thorough in magnetic physics. Thus the main and only idea of all these discussions were to review the works of various authors, to team from them, enjoy them and to think differently for better force generating systems.
(1) Behrman SJ and Egan G. Implantation of magnets in law and denture retention. New York State Dental Journal, 19,353-371, 1953.
(2) Blechman AM, Smiley H. Magnetic force in orthodontics. Am J Orthod., 74, 435-443, 1978.
(3) Sandler PJ, Meghil S, Murray AM, Springate 813. Sandy JR Crow V and Reed RT. Magnets and orthodontics. Br J Orthod., 16,243-249, 1989.
(4) Cerny R. The reaction of dental tissue to magnetic fields. Australian Dental Journal, 25, 264-268,1980.
(5) The complete magnet collection. Magnet Developments Limited, Unit-17.
(6) Highworth Industrial Park, Highworth, Swindon, Wiltshir SN67NA, England. Tsutsui H, Kinouchi Y. Sasaki H, Shiota M, Ushita T. Studies on the Sm-Co magnet as dental material. J Dent Res., 58, 1597-1606, 1979.
(7) Vardimon AD and Mueller HJ. In vivo and In vitro corrosion of permanent magnets in orthodontic therapy. J Dent Res., 64, 185 (Abstract). 1985.
(8) Kitsugi A, Okuno O, Nakano T, Harmonica H and Kurada T. The corrosion behavior of NeFeB and Sm-Co magnets. Dental Materials Journal, 11, 119-129, 1992.
(9) Drago CJ. Tarnish and corrosion with the use of intraoral magnets. Journal of Prosthetic Dentistry, 66, 536-640, 1991.
(10) Esformes I, Kummer FJ and Livelli TJ. Biological effects of magnetic fields generated with Sm-Co magnets. Bulletin of the Hospital for Joint Diseases Orthopedic Institute, 41, 81-87, 1981.
(11) Papadopulos MA, Horler I, Gerber H, Rahn BA and Rakosi Th. EinfluB Statischer Magnetischer Felder auf die Aktivitat Osteoblasten:eine in vitro Underschung. Fortschritte der Kieferothopa dia., 53, 218-222, 1992.
(12) Mc Donald F. Effect of static magnetic fields on osteoblasts and fibroblasts in vitro. Bioelectromagnetics, 14, 187-196, 1993.
(13) Sato K, Yamaguchi H, Miyatoma Hand Kinouchi Y. growth or human cultured calls exposed to a non-homogenous static magnetic field generated by Sm-Co magnets. Biochemica et Biophysica Acta., 1136, 231-238, 1992.
(14) Yamaguchi H, Hosokawa K,' Soda A, Myatoma H and Kinouchi Y. Effects of seven months exposure to a static 0.2T magnetic field on growth and glycolytic activity of human gingival fibroblasts. Biochemica et Biophysica Acta., 1158, 302-306, 1993.
(15) Staffollani Net al. Influenza dei magneti Ortodontici sulla flora microbica orale. Minerva Stomatologica, 40, 483-488, 1991.
(16) Bondemark L, Kurol J and Wennberg A. Biocompatibility of new, clinically used and recycled orthodontic samarium-cobalt magnets. Am J Orthod Dentofac Orthop., 105, 568-74, 1994.
(17) Toto PD, Choukas NC and Sanders DD. Reaction of bone and mucosa to implanted magnets. J Dent Res., 41, 1438-1449, 1962.
(18) Cerny R. The biological effects of implanted magnetic fields. Part-I: Mammalian blood cells. Australian Orthodontic Journal., 6, 64-70, 1979.
(19) Ahay OT, Kutkam T, Koseoglu 0 and Tanyeri S. The biological effects of implanted magnetic fields on the bone tissue of dogs. International Journal of Oral Maxillofacial Implants, 6, 345-349, 1991.
(20) Darendeliler MA, Darendeliler A and Mandurino M. Clinical application of magnets in orthodontics and biological implications: a review. European Journal of Orthodontics, 18, 431-442, 1997.
(21) Darendeliler MA, Sinclair PM and Kusy RP. Effects of static and pulsed electromagnetic fields on orthodontic tooth movement. Am J Orthod and Denlofac Orthop., 107,578-588, 1996.
(22) Linder- Aronson A and Lindskog S. A morphological study of bone surfaces and skin reactions after stimulation with static magnetic fields in rats. Am J Orthod and Dentofac Orthop., 99, 44-48, 1991.
(23) Bruce GK. Howlett CR and Huckstep RL. Effect of a static magnetic field on fracture healing in a rabbit radius. Clinical Orthopedics and Related Research. 222, 300-306, 1997.
(24) Behnnan S. The implantation of magnets in the jaw to aid denture retention. J Prosth Dent., 10,807-841, 1980.
(25) Blechman AM. Magnetic force systems in orthodontics, clinical results of pilot study. AmJ Orthod., 87,201-210, 1985.
(26) Kawata T, Hirota K, Sumitani K, Yano K, Tzang HJ and Tabuchi T. A new orthodontic force system of magnetic brackets. Am J Orthod Dentofac Orthop., 92, 241-248, 1967.
(27) Bondemark L, Kurol J and Larsson A. Human dental pulp and gingival tissue after static magnetic field exposure. Eur J Orthod., 17, 85-91, 1985.
(28) Saygill G, Aydinlik E, Ercan MT. Naldokn S and Ulutunci N. Investigation of the effects of magnetic retention systems used in prostheses on buccal mucosal blood flow. International Journal of Prosthodontics, 5, 328-332, 1992.
(29) Bondemark L, Kurol J and Larson A. Long term effects of orthodontic magnets on human buccal mucoss- a clinical, histological and immunohistochemical study. Eur J Orthod., 20, 211-218, 1998.
(30) Cerny R. Magnetodontics. The use of magnetic forces in dentistry. Australian Dental Journal, 23, 392-394, 1987.
(31) Javid N, The use of magnets in a maxillofacial prostheses. J Prosth dent., 25, 334-341, 1971.
(32) Dellinger EL. A clinical assessment of the Active Vertical Corrector -A non surgical alternative for skeletal open bite treatment. Am J Orthod Dentofac Orthop., 89, 426-436, 1986.
(33) Noar JH, Shell N and Hunt NP. The performance of bonded magnets used in the treatment of anterior open bite. Am J Orthod Dentofac Orthop., log, 549-56, 1996.
(34) Springate SD and Sandler PI. Micro-magnetic retainers: An attractive solution to fixed retention. Br J Ortho- 18" 139-141., 1991.
(35) Vardimon AD, Graber TM, Voss LR and Verrsio E. Magnetic verses mechanical expansion with different force thresholds and point of application. Am J Orthod Dentofac Orthop., 92, 455-488, 1087.
(36) Sandler JP. An attractive solution to unerupted teeth. Am J Orthod Dentofac Orthop., 100,489-93, 1991.
(37) Mutler M. The use of magnets in orthodontics: an alternative means to produce--tooth movement. Eur J Orthod., 6, 247-253, 1984.
(38) Glanelly AA. Vaitas AS and Thomas WM. The use of magnets to move molars distally. Am J Orthod Dentofac Orthop., 96,161-167, 1989.
(39) Vardimon AD" Stutzman_JJ" Graber TM" Voss LR and Petrovic AG. Functional orthopedic magnetic appliance (FOMA) II--Modus Operandi. Am J Orthod Dentofac Orthop., 95, 371-87, 1989.
(40) Vardimon AD, Graber TM, Voss LR and Muller TP. Functional Orthopedic Magnetic Appliance (FOMA) 111-Modus Operandi. Am J Orthod Dentofac Orthop., 97, 135-48, 1990.
(41) Darendeliler MA and Joho JP. Magnetic Activator Device II (MAD II) for correction of class-II, Division 1 malocclusion. Am J Orthod Dentofac Orthop., 103, 223-39, 1993.
(42) Darendeliler MA, Chlarini M and Joho JP. Early class-III treatment with magnetic appliances. J Clin Orthod., 27,563-576, 1993.
(43) Moss JP, Linney AD, Goodwin P and Shaw IA. Three dimensional study of treatment with magnet and non magnetic twin block. Eur J Orthod., 15, 342, 1993.
(44) Chate RAC. The propellant unilateral magnetic appliance (PUMA) a new technique for hemifacial microsomia. Eur J Orthod., 17,263-271, 1995.
Ashok Kumar Jena (#), Ritu Duggal * Punest Batra ($)
(#) PG Student, * Associate Professor, ($) Senior Resident Division of Orthodontics, Department of Dental Surgery, All India Institute of Medical Sciences, New DOW 110 029
Table 1: Typical magnetic and physical properties of different types of magnets used in dentistry (5) Magnets Energy Coercive *-Anisotropic Product Remanence Force ***-Isotropic ([KJM.sup.3]) (Tesla) (Oersted) Alcomax-III * 42 1.27 650 Mycomax-III * 45 0.90 1600 ALNICO-N ** 14 0.77 560 Sintered Sm-Co * 145 0.85 8250 Polymer bonded Sm-Co * 56 0.58 4750 Sintered NdFeB * 250 1.18 10000 Resin bonded NdFeB ** 64 0.59 5500 Magnets Density Max working *-Anisotropic (gm/[cm.sup.3]) Temp ([degrees]C) ***-Isotropic Alcomax-III * 7.3 550 Mycomax-III * 7.3 550 ALNICO-N ** 7.2 550 Sintered Sm-Co * 8.3 250 Polymer bonded Sm-Co * 5.3 60 Sintered NdFeB * 7.4 50 Resin bonded NdFeB ** 5.9 20
|Printer friendly Cite/link Email Feedback|
|Author:||Jena, Ashok Kumar; Duggal, Ritu; Batra, Puneet|
|Publication:||Trends in Biomaterials and Artificial Organs|
|Date:||Jan 1, 2003|
|Previous Article:||Microstructure development in machinable mica based dental glass ceramics.|
|Next Article:||A method of fabrication of an extensive facial prosthesis.|