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Published on January 30, 2009

Author: mahajan


Slide 1:  Journal club Presented by Dr. Vivek mahajan P.G. student Narayana Dental College Slide 2: Magnetic strength and corrosion of rare earth magnets Khalid A. Ahmad, James L. Drummond, Thomas Graber, and Ellen BeGolec AJO2006 vol130,P 275-279 Authors Slide 3:  PURPOSE OF THE STUDY The purpose of the study was to evaluate the several magnet coatings and their effects on magnetic flux density Slide 4:  Abstract: A total of 60 neodymium-iron-boron magnets divided into 6 equal groups—polytetrafluoroethylene- coated (PTFE), parylene-coated, and noncoated—were subjected to 4 weeks of aging in saline solution, ball milling, and corrosion testing.Throughout the processes of coating, soaking, ball milling, and corrosion testing, PTFE was a better coating material than parylene for preserving magnet flux density. Slide 5:  INTRODUCTION What are magnets What are the Properties of magnets What are the types of magnetic materials used in dentistry What are the Clinical applications of magnets in orthodontics Advantages of the magnets Disadvantages of magnets Recycling of magnets Slide 6: Magnets : are alloys that can attract certain metals such as iron, nickel, cobalt, and other alloys, and exert attractive or repulsive force on other magnets. Modern permanent magnets are made of special alloys, leading to an increase in magnetic flux density and the potential to minimise loss of magnetism over time. INTRODUCTION Slide 7: Magnetic fields can be either Natural or artificial. These fields were tested and proven to have no adverse effects on humans. These test results have made possible the use of magnets in the field of dentistry. Slide 8: The first use of magnets in Dentistry was by Behran and Egan in the year 1953. They used it as implants for denture retention. First use of magnets for tooth movement was described by Blechman and Smiley by experimenting on cats. Becker in 1970 introduced rare earth magnets having properties superior to previously used magnetic alloys. Slide 9: Properties of magnets Magnetic field static Time varying Direct current Alternating current Slide 10:  Nd2 Fe14B SmCO5 Ferrit AlNiCO5 d d d d 1.High force to volume ratio Slide 11: High Energy product = B X H Ability to attract or repulse Magnetic Field strength Magnetic induction Slide 13: Shortcomings of high energy product Slide 14: Coulombs Law F = 1 d2 Hooke’s law F = Ed 2.Maximum force at shorter distance Slide 16:  Hook’s law Coulomb’s law F=Ed F= 1/d2 Slide 17: 3.Three dimensional centripetal orientation of attractive magnetic force: if two magnets are displaced from each other in more than one plane , they attract to full overlap. Slide 18: 4.No interuption of magnetic force lines by intermittent media Magnet attached to impacted tooth Soft tissue Intra –oral magnet Slide 19:  Muller Prong M+ M+ 5.No Friction in the attractive force configuration Slide 20: When elastics are exposed to saliva Deterioration of force in short time Fluid adsorption occurs Viscoelastic properties are prone to relaxation 6.No energy loss Slide 21: Types of Magnetic materials: In various dental applications the following materials have been used. 1. Platinum-cobalt 2. Aluminium-Nickel-Cobalt 3. Ferrite 4. Chromium-Cobalt-Iron 5. Samarium-Cobalt 6. Neodymium-iron-Boron. Slide 22: Clinical applications of magnets in orthodontics 1. Distal movement of canines and molars Blechman &Smiley AJO 1978 Slide 23: Gianely et al presented so called molar distalizing system(MDS) Gianely et al AJO 1989 Slide 24: Gianely et al AJO 1989 Slide 25: Gianely et al AJO 1989 0.75-1mm / month where second molars have also erupted 3mm/ month when second molars are not present Force 200-225gm Slide 26: 2.Space closure Muller et al EJO 1984 Muller in 1984 bonded rectangular magnets delivering 117.5 gms of force of attraction on each maxillary central incissor to close a midline diastema Slide 27: 3.Intrusion of posterior teeth in case of anterior open bite Active Vertical Corrector Dellinger AJO 1986 Slide 28: Functional correction Vardimon etal 1989 AJO presented functional appliances with magnets for the treatment of classII and class III problems . Functional orthopaedic magnetic appliances (FOMA II and FOMAIII) for the correction of classII and classIII malocclusion consists of upper and lower magnetic units formed from permanent neodymium –iron – boron magnets, which are placed within acylic appliances directly bonded to teeth. The magnetic force produced are used to move the mandible towards the desired position. Slide 29: Magnetic activator device IV Darendeliler JCO 1995 The MAD IV-a is used in cases where the anterior segment of the maxilla is overdeveloped (gummy smile). Because posterior intrusion and mandibular autorotation are needed, the posterior and anterior magnets are placed in full contact Slide 30: Darendeliler JCO 1995 MAD IV-b is used when an additional extrusive effect is needed in the maxillary anterior region. The anterior magnets are positioned with a vertical opening of 2-3mm, while the posterior magnets are placed in full contact. Slide 31: The MAD IVc Appliance The MAD IV-c is used when only anterior extrusion is needed. The posterior magnets are omitted, and the anterior magnets are placed with an opening of 1-2mm,depending on the severity of the anterior open bite. Slide 32: Magnetic Twin Blocks Clark used magnets in his Twin Block Magnets were embeded in the inclined surface of the twin block in attractive mode When used in the repelling mode it reduces the need for reactivation Used in different ways for treating Class 11 and Class111 malocclusion Slide 33: Whether attracting or repelling magnets are used, reactivation of blocks by acrylic to inclined planes deactivate the magnets. Screw may therefore need to be included in the appliance design for magnetic twin blocks to achieve continuous reactivation of magnetic force. Slide 34: Magnetic Appliance for treatment of Snoring Patients AJO 1998 Bernhold Slide 35: The maximal attractive force between the magnets was 8.5 N (850g). Because of the thickness of the coating acrylic layer in each splint, 0.3 to 0.5 mm, the intermagnet distance produced a gap between the magnets of 0.6 to 1.0 mm, which reduced the force magnitude for mandibular advancement to 5.0 to 6.5 N (500 to 650g). Slide 36: Movement of impacted tooth SandlerAJO 1991 J.P Sandler suggested a technique which involves the use of two magnets 3x3x1mm and a larger 5x5x2mm Great care must be taken to ensure the angle of the pole face of the superior magnet relative to the base magnet . When the angle is changed , the rate of decline of force is very severe. Slide 37: Movement of impacted tooth SandlerAJO 1991 Slide 39: Extrusion of fractured teeth AJO 1997 Bondemark If the fracture line is positioned below the alveolar bone margin and in the coronal third or less of the root and if the apical root fragment is judged to be long enough to support a coronal restoration. Slide 40: Rapid Maxillary Expansion Vardimon et al 1987 A study on monkeys proved that repelling magnets delivered ideal forces for expansion compared to a jack screw appliance. Darendililer et al in 1993 used mid palatal repelling magnets and showed both dental and skeletal expansion in his report. Slide 41: The samarium-cobalt magnets were prepared by attaching an edgewise bracket to the surface of the magnet and plating it with chromium to prevent corrosion of the magnet and with nickel to solder the bracket to the surface. Kawata AJO 1987 Magnetic Edgewise brackets Slide 42: Three kinds of wires are used 0.014, 0.016, and 0.018 inch. 0.014-inch wire is placed for initial leveling and canine retraction. If the 0.014-inch round wire is too light to level the crowded dental arch, then a 0.016-inch wire can be substituted. Slide 43: If the distance between the malpositioned teeth is over 3 mm and the magnetic force is thus not sufficient to retract these teeth, a power chain can be added to assist the magnetic force in the initial stage. When these teeth come closer together, that is, within 3 mm, the power chain is removed and the additional retraction can be done through the available magnetic force. This procedure has reduced treatment time as compared with traditional methods. Slide 44: The orthodontic stimuli provided by the magnetic appliance has reduced the systematic stress reactions seen with conventional orthodontic mechanotherapy. Canine retraction was rapid and consistent as compared with traditional orthodontic appliances. Treatment time was shorter, discomfort was eliminated, and the orthodontic patients were free from periodontal disturbances, root resorption, and caries. Slide 45: Micro Magnetic Retainers Despite the success of fixed retainers to stabilize anterior spacing which are often used in orthodontics , they have no. of undesirable charachteristics: They restrict to gingival tissues, leading to poor oral hygiene. They often fracture because the individual teeth move independently and put excessive strain on the retainer. Slide 46: Micro Magnetic Retainers Directly bonded magnets have a no. of advantages over other types of retainers oral hygiene can be maintained as flossing is not prevented , and there are no wires or ledges close to the gingival margins . The teeth are not splinted together , so sudden differential loading of crowns will not cause the magnets to be dislodged and, therefore, the teeth can move completely physiologically. Recycling of Magnets : Recycling of Magnets Bondemark &Kurol conducted extensive studies on recycling of rare earth magnets concluded that the biocompatability &force stability is not effected Darendililer felt that magnets should not be recycled for ethical reasons Slide 48: Advantages of magnetic appliances Eliminates patient co-operation Produces less pain and discomfort Continuous force exerted Treatment time reduced Reduced periodontal disturbance No friction Less chair side time Better force Better directional control Slide 49: Disadvantages Tarnish &corrosion Cytotoxic effects Bulk of the magnets Taste - Bitter Costly Slide 50:  Materials and Methods: Sixty rare earth magnets composed of neodymiumiron-boron were obtained from the manufacturer.(MMG Mag Dev Limited, Swindon, United Kingdom).The magnets were 4 mm in diameter and 3 mm in height 20 were coated with PTFE (polytetrafluoroethylene) 20 were coated with double layers of parylene 20 were uncoated (control). Slide 51:  Magnetic flux density was measured by using a Gaussmeter To determine which environment had the greatest effect The magnets were divided into soaked and nonsoaked oral environments before testing the effects of ball milling and corrosion testing. Slide 52:  The oral environment was simulated by soaking 30 magnets (10 uncoated,10 PTFE-coated, and 10 parylene coated individually in 35 x 10 mm plastic plates with 6 mL of artificial saliva and stored at room temperature for 4 weeks Slide 53:  At the end of the 4-week period, the artificial saliva was sent for inductive coupled plasma mass spectroscopy (ICPMS) analyses . The flux densities of the soaked magnets were measured and compared with the flux densities of the control nonsoaked magnets. Slide 54:  To simulate an aggressive oral environment, the coated, noncoated, soaked, and nonsoaked magnets were placed in a ball mill (U.S. Stoneware, East Palestine, Ohio) with zirconia grinding pellets, 1 magnet at a time, to represent impact forces such as tooth-to-tooth impact damage Slide 55:  The process was performed at 60 revolutions per minute for 1 hour. After ball milling, the flux densities of the magnets were measured again and recorded. A corrosion test by using the gel chromatography protocol suggested by Matasa was followed Slide 56:  The corrosion solution 4.5 mL of 0.1 mol, lactic acid 85% 3 mL of sodium chloride dissolved in a 3:1 mixture of water and glycerol .Then 5 mL of an aqueous solution of 1% potassium ferrocyanide is added. Then 25 g of Aerosil 200 fine silica known as a jellifying agent Slide 57:  Then 500 mL of the mixture gel was poured into covered plastic plates and sealed with Para film. Stain diameters were measured after 2, 24, 48, and 168 hours. Staining spots were uniform around each magnet and increased in size with time and was measured by using a digital caliper . Slide 58:  Slide 59:  Slide 60:  v Slide 61:  1week Slide 62:  Two major concerns were the release of potentially harmful inorganic ions from the magnets and whether the coatings are protective enough. The gel-chromatography test was carried out to resolve these issues. All sample magnets tested led to different diameters of leached ions after 1 week, with the uncoated magnets leaching more ions than the coated groups. There was a significant difference between uncoated and coated groups (PTFE and parylene) Discussion Slide 63:  The ICPMS analysis (Ion concentration from inductive coupled plasma mass spectroscopy) indicated that significant amounts of iron and neodymium were leached into the corrosive-gel solution but no boron. Slide 64:  In clinical applications, all possible efforts must be made to prevent magnetic devices from corroding and disrupting the magnetic domain, and to prevent possible harmful effects of the corrosion ions per se. Accordingly, with this evidence of uncertain biologic safety, rare earth magnets in dentistry must be used with caution. Slide 65:  PTFE is better coating material than parylene. With aging in artificial saliva , zone of leaching increases from 2-168hrs(1Week), but after 1week-4week there is decrease in leaching of ions. The coating had minimal effect on preventing the leaching of ions. The magnets showed a significant decrease in magnetic flux density after aging in artificial saliva, a corrosive gel, and coating. There was no effect after ball milling. CONCLUSIONS Slide 66:  References Slide 67:  The durability of parylene coatings on Neodymium -iron boron- magnets Abstract: A parylene coating is frequently used to prevent the corrosion of Neodymium -iron boron- magnets when they are used intraorally. This invitro study was designed to test the durability of parylene coating in a simulated oral environment. Single and double parylene coating magnets were subjected to grinding and crushing forces in an industrial ball mill. Results demonstrate that abrasion and wear was visible around the edges after 1 hour of testing noated under scanning electron microscope. EJO 21(1999) Slide 68:  2.Extent and flux density of static magnetic fields generated by orthodontic samarium-cobalt magnets AJO Volume 1995 May (488 - 496) Abstract: The aim of this study was to measure and to analyze the extent and flux density of static magnetic fields generated by commercially available samarium-coblat magnets used in orthodontics. The flux density was measured with a gaussmeter and a Hall probe with the magnets mounted in clinically relevant positions, i.e., in attractive and in repelling positions and also in the single position. Furthermore, the flux density between new and clinically used and recycled magnets was compared. Slide 69: This type of force was achieved by a similar vertical loop which was constantly activated by three parylene-coated neodymium-iron-boron (Nd2Fe14P) block magnets.The rate of tooth movement on the two sides was compared over a period of 3 months. The canines retracted with a constant force moved statistically significantly more than the control canines during the experimental period

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