In vitro effect of microwave irradiation on the retentive force of magnets.

Statement Of Problem: Few studies have addressed the possible effect(s) of microwave irradiation on the magnetic properties of "permanent" magnets during the fabrication of dental and maxillofacial prostheses.

Purpose: The objective of this study was to investigate the influence of microwave irradiation energy on the retentive force of a magnetic attachment system used in maxillofacial prosthetic rehabilitation.

Material And Methods: A heat-polymerized PMMA disk (6 cm in diameter) was fabricated. Seven magnets were placed around the wafer in a circumferential fashion: 1 in the center and 6 surrounding it. The 7 magnets were spaced 2 cm from their respective centers. Seven heat-polymerized PMMA cylinders were also used, and a magnet (counter-magnet) was placed in the center of each even with the cylinder's surface. Once the investment had set (45 minutes after mixing), the 7 counter-magnets in the cylinders were placed against the 7 magnets in the acrylic wafer. A second mixture of investment material was added. The flasks were separated, and the acrylic wafer was removed to accommodate the 7 counter-magnets in the base mold in the same geometric configuration and to serve as a "spacer" for the silicone material. A 1:1 mixture of medical grade elastomer (MDX4-4210) and medical adhesive silicone (type A) was packed and compressed, the molds were reclamped, and the excess silicone was removed. The elastomer/silicone wafer was packed and compressed into the test and base molds, the molds were reclamped, and excess silicone was removed. The first group of magnets, designated Group A, received microwave irradiation for 5 minutes at low power (112 W). This procedure was repeated for each group of magnets at the following polymerizing times (n=14): Group B, 10 minutes; Group C, 15 minutes; Group D, 20 minutes; Group E, 25 minutes; Group F, 30 minutes; and Group G, 35 minutes. Measurements of retentive force (N) at 10 mm/min ramp rate of speed of separation was conducted. The specimen rate read 5.0 points/second. Data were analyzed using a 1-way analysis of variance (alpha=.05); individual mean values were compared using the Tukey test (alpha=.05).

Results: There were no statistical differences in retentive force between groups D, E, and F (20, 25, and 30 minutes, respectively) or between groups A, B, C, D, F, and G (5, 10, 15, 20, 30, and 35 minutes, respectively). When the microwave-irradiated groups A through G were compared with the control group, there was a significant difference (P<.05) in retentive force (N). Group E (25 minutes) showed the largest reduction of retentive force (0.3 N, a reduction of 12%).

Conclusion: If a prosthesis is processed using a microwave and contains samarium cobalt magnets, the retentive force may be reduced up to 12% under specific conditions.