| Current prostheses used by patients with amputations can cause infections and irreversible tissue damage, due to permanent skin penetration or shearing stresses on soft tissue. A prosthetic system utilizing magnetic coupling between the skeleton and prosthesis is in development. By coupling a lower extremity prosthesis indirectly to the bone, force transmission to the skeleton will be improved, reducing soft tissue breakdown of the residual limb. The risk of infection will also be reduced compared to skin-penetrating osseointegrated prostheses, as only one surgery will be needed to attach the implant, and there will be no permanent breach created.;The magnets to be used in the prosthetic device are neodymium-iron-boron (NdFeB) alloys, a type of rare earth magnet and one of the strongest magnets commercially available. The implantation of strong NdFeB magnets in the residual limb introduces the issue of the attraction of ferrous objects to the limb with potentially deleterious effects. The purpose of this study was to evaluate the potential for magnetic shielding to be used to protect the healing residual limb, as well as the magnetic prosthesis, from ferrous materials in the external environment.;The ability of various grades and thicknesses of magnetic shielding (Magnetic Shield Corporation, Bensenville, IL) to attenuate magnetic fields was evaluated at different distances from a magnet of the same grade and dimensions as the one to be implanted. Co-NETICRTM shields with thicknesses of 0.002", 0.004", 0.006", 0.010", and 0.025" were tested, as well as NETIC RTM shields 0.004", 0.010", and 0.030" thick. Of the Co-NETIC RTM shields, the 0.025" thick plate provided the most attenuation when the shield was placed 28.5 mm away from the magnet, with a vertical magnetic field reduction 99.9% +/- 0.3%, and an average horizontal reduction of 99.3% +/- 0.6%. The thickest NETICRTM shield also reduced the magnetic field, with an average vertical reduction of 90.4% +/- 11.4 and an average horizontal reduction of 99.0% +/- 1.0 when the shield was placed 31.5 mm away from the magnet.;These preliminary tests provided the basis for computational modeling performed later, as well as verification of the accuracy of the modeling software. For the unshielded magnet, the mean percentage difference between the experimentally-determined resultant flux density and the computationally-determined flux density was 5.0%. Using the values from the repeatability test for the 1.65 inch diameter 0.010" thick NETICRTM shield, the average percentage difference between the experimentally-determined resultant flux density and the computationally-determined flux density was 20.6%. Using the values for the 42-mm diameter 0.010" thick Co-NETICRTM shield, the average percent difference between the experimentally-determined resultant flux density and the computationally-determined flux density was 27.1%.;After determining the computational model's accuracy, layered shielding configurations were tested to determine if attenuation capabilities of the shields increased when layered. Also, 1.65 inch diameter models were run at multiple heights for both the single layer and double layer shields to quantify how height affects the attenuation capabilities for the different grades of shielding material.;After analyzing both experimental and computational results, a 0.030" thick, 5" tall, 2.48 inch diameter NETICRTM cup proved sufficient to decrease the external flux density to safe levels while conforming to the anatomy of the sheep forelimb, a proposed in vivo model.;Shield evaluation and computational modeling proved the feasibility of using magnetic shields to protect the residual limb post-surgery from injury. This knowledge will be used in the next stages of development and testing of the magnetic prosthetic device with shielding in the sheep model. |
