Autores
Matikka H, Keshvari J, Lappalainen R. (2010). Temperature changes associated with radiofrequency exposure near authentic metallic implants in the head phantom-a near field simulation study with 900, 1800 and 2450 MHz dipole. Phys Med Biol. 55(19):5867-81.

Introduction
The increased use of radiofrequency field (RF) emitting devices has increased safety concerns because of the possible coupling of EMF with implants in the human body. Studies have shown that the conductive implant in human tissues could affect the SAR in some regions. The authors have previously studied SAR-enhancements near authentic passive metallic implants and noted that 1g and 10g mass-averaged SAR could be higher near certain authentic implants. But, the authors still don’t know if the changes are physiologically relevant, such as the SAR elevation is significant enough to induce more heating of tissues close to the implant. The current issue was addressed using the FDTD method and Pennes’ bioheat equation.

Objective
The objective of the study was to simulate whether authentic metallic implants in the head region may cause temperature changes in RF near fields where important SAR elevation are seen. Also, the authors investigated whether implant material had any effect on the temperature distribution induced by the RF near field.

Methods
Authentic metallic implants (fixtures, skull plates, earrings, and bone plates) were chosen and embedded in a heterogeneous head phantom because they previously indicated noticeable SAR elevation near the near field of a dipole. Electromagnetic field distributions produced by a dipole emitting 900, 1,800 or 2,450 MHz RF were assessed using a finite difference time domain (FDTD) method. Perfect electric conductivity (PEC) of the implants was assumed in the experiment. The changes in temperature distribution induced by each metallic implant were determined in a thermally steady state.

Results
The relative SAR elevation was indicated by a ratio of the peak SAR value with and without the implant. For the skull plate at 900 MHz, no temperature elevations were seen and the maximum temperatures in all the tissues were similar or lower with and without the implant. Relative SAR1g were slightly lower than those at 1,800 and 2,450 MHz where temperature elevations were noticed. At 1800 and 2,450 MHz, the maximum temperatures induced by the EMF were enhanced by the skull plate although less pronounced at 2,450 MHz frequency. Fixtures had a minor effect on the temperature elevations. At the 900 MHz frequency, the authors noted increases in maximum temperatures of skin, ear, skull and in the location occupied by the implant due to fixtures. The bone plate enhanced a little bit maximal temperature in tissues.

Interpretation
The study measured temperature changes in tissues caused by SAR elevation near metallic implants such as a skull plate, fixtures, and a bone plate in near fields of 900, 1,800 and 2,450 MHz frequencies. When EMF (SAR) was scaled to a power of 1 Watt, the steady-state maximal temperatures of some tissues were noticeably higher in comparison to the absence of the implant. Peak SARs when a metallic implant is inserted in tissues showed that SAR1g is a much better indicator of the thermal effect than SAR10g. As far as the authors are aware, this study is the first to study the effect of implant material (thermal conductivity) on the resulting thermal distribution of tissues in the RF near field. Although differences in thermal conductivities of implant materials were more than 60-fold in some cases, the differences in maximum temperatures were not found to always be large. This indicates that in some cases the changes in steady-state temperatures compared to the case without an implant result essentially from the enhanced SAR or conductivity of the implant, and the specific thermal properties of the implant are irrelevant. However, the authors noted that in other cases, the thermal conductivity of the implant could greatly affect the temperature changes.

Conclusion
The authors concluded that their results provide a broad estimate of the thermal effect of implants. But too many implant-exposure scenarios exist in real-life and it’s impossible to study them all.

 



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