Introduction　　     More than a decade has passed since magnetic attachments for dentures were manufactured and marketed for the first time in Japan. The magnetic attachments have become increasingly compact, from a rectangular parallelepiped shape of 4.2 mm ´ 3.2 mm ´ 2.5 mm, to a disk shape of 3.6 mm in diameter and 1.2 mm in height. The magnet material has changed from Sm２Co１７ to NdFeB and their clinical use has expanded. Although several reports1-3) discuss the service lives of magnetic attachments, these are experimental predictions and do not deal with the loss of magnetic force in actual clinical cases.     Therefore, the authors studied problems reported by two patients with magnetic attachments and one patient with a magnetic implant. The patients with magnetic attachments visited our annex hospital after more than five years of attachment and complained of a loss of magnetic force. In the case of the magnetic implant, examination revealed considerable damage to the keeper and magnetic assembly. In the present report, we observed the adsorption and keeper surfaces of the magnet structures under a microscope and studied the loss of magnetic force and abrasion in these areas.   　 　 Cases Patient 1　Male aged 63 　 1999  Using a Magfit EX600 with right maxillary first premolar, left maxillary incisors and left maxillary first premolar as supporting teeth, an complete over-denture was inserted. When a metal-base denture was fabricated six months later, a magnetic attachment was implanted for the denture. At annual recall, no abnormality was detected. 2004 The patient visited the hospital, complaining of a low retention of the denture, and upon examination the attachment was revealed to be damaged. 　 　 Patient 2　Female aged 79 　 1994 Using a Magfit 600 with right maxillary second premolar, left maxillary first premolar, and left maxillary second molar as abutment teeth, a complete over-denture was inserted. 2000 Because of periodontal disease, left maxillary second molar was extracted. 2001 Because the magnetic attachment had detached from left maxillary first premolar and was lost, a new Magfit EX600 was inserted. 2002 A new denture was fabricated and a magnetic attachment was moved to this denture. 2003 Coping had come off from right maxillary second premolar because of caries and was reattached. 2004 The patient visited the hospital, complaining of low retention, and examination revealed that coping had detached from right maxillary second premolar and the magnetic attachment at left maxillary second premolar was damaged.  　 　 　 Patient 3    Male aged 53 　 2000 A right and left maxillary canines implant operation was performed and a second operation was performed six months later. 2001 Together insertion of a maxillary denture, a dome-type Magfit IP was attached. 2004 Wrench insertion to check the magnetic keeper could not detect any looseness, however, the magnetic attachment was replaced due to changes noticed on the keeper surface.   　 　 Table 1  Outline of cases            　 Method 　The magnetic forces of the magnetic attachments obtained from the patients were measured on a Dental Magnet Tester　 (DMT-TW, Aichi MI). The surfaces of the magnet keepers and implant keepers were observed under a Digital Microscope (VHX-200, Keyence). 　 　 Results and Discussion   For Patient 1, all of the three Magfit EX attachments no longer had any magnetic force. Under a microscope, all the magnets were found to be damaged at the micro-laser welded sections and the magnet cases were found to be deformed. Metallic corrosion, scratches, and abrasions were also apparent. A magnet case used for an anterior tooth was partially chipped. Damage to the magnet cases might have caused the magnets to corrode and thereby lose their magnetic force. Deformation of the magnet cases and damage to the welded sections seemed attributable to the occlusal force concentrated on the magnet cases. These experimental observations indicate the necessity of stress endurance testing.   For Patient 2, the magnetic force of the Magfit 600 attached 10 years ago was 600 gf, similar to the initial value. Microscopic observation did not show any significant deformation. Although slight corrosion and abrasion were observed, the magnetic force was retained. However, the Magfit EX600, which had replaced a previous attachment five years before, no longer had any magnetic force. AS with Patient 1, the attachment of Patient 2 showed deformation, destruction of the welded sections, and chipping over the entire keeper surface of the magnet case.     For Patient 3, the two Magfit IP attachments showed a magnetic force of 600 g, unchanged from the original. Under a microscope, both the keeper and magnet structure were found to be unevenly abraded. Since Magfit IP is a dome-shaped magnetic attachment, slight movements of the denture seem to have caused the magnet abrasion.   　 　 Conclusion 　Judging from the results of microscope examinations of magnetic attachments used over a long period, it is necessary to recall the users of magnetic attachments periodically and to examine the attachments carefully with regard to the balance of factors including the base teeth, the mucous membrane and the occlusion. If the magnet structure and keeper surfaces are observed and considerable changes are detected, replacing the magnet structure or keeper will greatly affect the long-term prognosis of the denture.   　 　 References 1) Shuiau Y.Y.:Magnetic attachment reserch in asian region.Clinical application of magnetic attachment -abstracts of international symposium-:29-36,2000. 2) Mizutani Y.,Ogura R., Kakumoto Y.et al.: A single case of corroded magnetic attachments to implants. J Jpn Soc Oral Implant 12:584-590,2000. 3) Endo K., Suzuki M. and Ohno H.;Corrpsion Characteristics of ferric austenitic steels for dental magnetic attachment. Dent Mater J 19:34-49,2000.