Passive Implants Research Articles

Effect of shape and coating of a subretinal prosthesis on its integration with the retina

Retinal stimulation with high spatial resolution requires close proximity of electrodes to target cells. This study examines the effects of material coatings and 3-dimensional geometries of subretinal prostheses on their integration with the retina. A trans-scleral implantation technique was developed to place microfabricated structures in the subretinal space of RCS rats. The effect of three coatings (silicon oxide, iridium oxide and parylene) and three geometries (flat, pillars and chambers) on the retinal integration was compared using passive implants. Retinal morphology was evaluated histologically 6 weeks after implantation. For 3-dimensional implants the retinal cell phenotype was also evaluated using Computational Molecular Phenotyping. Flat implants coated with parylene and iridium oxide were generally well tolerated in the subretinal space, inducing only a mild gliotic response. However, silicon-oxide coatings induced the formation of a significant fibrotic seal around the implants. Glial proliferation was observed at the base of the pillar electrode arrays and inside the chambers. The non-traumatic penetration of pillar tips into the retina provided uniform and stable proximity to the inner nuclear layer. Retinal cells migrated into chambers with apertures larger than 10 μm. Both pillars and chambers achieved better proximity to the inner retinal cells than flat implants. However, isolation of retinal cells inside the chamber arrays is likely to affect their long-term viability. Pillars demonstrated minimal alteration of the inner retinal architecture, and thus appear to be the most promising approach for maintaining close proximity between the retinal prosthetic electrodes and target neurons.

Standards for medical devices in MRI: Present and future

The purpose of this review is to define the current standards addressing safety of medical devices in MRI and to describe ongoing standards development efforts. The American Society for Testing and Materials (ASTM International) began developing standard test methods for determining the MR safety of medical devices in MRI in 1997. To date, five ASTM standards addressing testing and marking medical devices and other items for use in the MR environment have been published. International Standards Organization (ISO) 14630, the general requirements standard for nonactive surgical implants, is currently being revised to include information about MR safety of passive implants and to reference the ASTM standards. To address the unique safety issues of active implants, and in particular, active implants with leads, like pacemakers and neurostimulators, a joint working group between ISO TC150/SC6 on active implants and International Electrotechnical Commission (IEC) SC 62B MT40 on magnetic resonance equipment for medical diagnosis is working to develop a technical specification for active implantable medical devices (AIMDs) in MRI. While much progress has been made, work still needs to continue to develop a complete body of test methods to allow the evaluation of the safety of medical devices in the MR environment.

The effect of authentic metallic implants on the SAR distribution of the head exposed to 900, 1800 and 2450 MHz dipole near field

As the use of radiofrequency (RF) electromagnetic (EM) fields has increased along with increased use of wireless communication, the possible related health risks have also been widely discussed. One safety aspect is the interaction of medical implants and RF devices like mobile phones. In the literature, effects on active implants like pacemakers have been discussed but the studies of passive metallic (i.e. conductive) implants are rare. However, some studies have shown that the EM power absorption in tissues may be enhanced due to metallic implants. In this study, the effect of authentic passive metallic implants in the head region was examined. A half-wave dipole antenna was used as an exposure source and the specific absorption rate (SAR, W kg−1) in the near field was studied numerically. The idea was to model the presumably worst cases of most common implants in an accurate MRI-based phantom. As exposure frequencies GSM (900 and 1800 MHz) and UMTS (2450 MHz) regions were considered. The implants studied were skull plates, fixtures, bone plates and ear rings. The results indicate that some of the implants, under very rare exposure conditions, may cause a notable enhancement in peak mass averaged SAR.

Interaction of radio frequency electromagnetic fields and passive metallic implants—A brief review

During the last decade, use of radio frequency (RF) applications like mobile phones and other wireless devices, has increased remarkably. This has triggered numerous studies related to possible health risks due to the exposure of RF electromagnetic (EM) fields. One safety aspect is the coupling of EM fields with active and passive implants in the human body. While interactions with active implants have been quite extensively researched, only a few studies have focused on passive implants. The present article reviews interaction mechanisms and studies of passive metallic, that is, conductive, implants in common external RF EM fields. It is found that implants have been mostly studied numerically, and experimental studies are rare. Furthermore, the studies cover mostly far-field conditions and only a few have studied implants in near fields. A summary of results indicates that a conductive object in tissues may cause notable local enhancement of the EM field and thus enhanced power absorption. The degree of enhancement depends, for example, on the orientation, the dimensions, the shape, and the location of the implant. However, in most of the cases, the field enhancement has not been strong enough to cause remarkable excess heating (more than 1 degrees C) of tissues.

Interaction of mobile phones with superficial passive metallic implants

The dosimetry of exposure to radiofrequency (RF) electromagnetic (EM) fields of mobile phones is generally based on the specific absorption rate (SAR, W kg−1), which is the electromagnetic energy absorbed in the tissues per unit mass and time. In this study, numerical methods and modelling were used to estimate the effect of a passive, metallic (conducting) superficial implant on a mobile phone EM field and especially its absorption in tissues in the near field. Two basic implant models were studied: metallic pins and rings in the surface layers of the human body near the mobile phone. The aim was to find out ‘the worst case scenario’ with respect to energy absorption by varying different parameters such as implant location, orientation, size and adjacent tissues. Modelling and electromagnetic field calculations were carried out using commercial SEMCAD software based on the FDTD (finite difference time domain) method. The mobile phone was a 900 MHz or 1800 MHz generic phone with a quarter wave monopole antenna. A cylindrical tissue phantom models different curved sections of the human body such as limbs or a head. All the parameters studied (implant size, orientation, location, adjacent tissues and signal frequency) had a major effect on the SAR distribution and in certain cases high local EM fields arose near the implant. The SAR values increased most when the implant was on the skin and had a resonance length or diameter, i.e. about a third of the wavelength in tissues. The local peak SAR values increased even by a factor of 400–700 due to a pin or a ring. These highest values were reached in a limited volume close to the implant surface in almost all the studied cases. In contrast, without the implant the highest SAR values were generally reached on the skin surface. Mass averaged SAR1 g and SAR10 g values increased due to the implant even by a factor of 3 and 2, respectively. However, at typical power levels of mobile phones the enhancement is unlikely to be problematic.

A quick guide to safety and compatibility of passive implants and devices in an MR environment

An increasing number of patients with metal implants are being referred for magnetic resonance imaging (MRI) investigations. Implants and devices may be divided into two groups, namely active and passive. This article will focus on passive devices. A device is MR-safe when it is used in the MR environment, but presents no additional risk to the patient or other individuals, although the quality of diagnostic information may be affected. MR procedures may be contraindicated due to various interactions between the MR environment and medical devices, which include torque, translational force, heating, induced electrical currents, magnetic field interactions, artefacts, and misrepresentation. Therefore, before deciding whether any object is MRsafe/ compatible, the intended use and the possible retaining mechanisms must be considered.

First comparisons with laser vibrometry measurements and computer simulation of ear ossicle movements

The dynamic behavior of the ossicular chain is very complex and is frequency-dependent. To date, this has still not been fully investigated or understood. There remains a lack of measurement procedures to pick up the motion of the ossicles and ear drum simultaneously with sufficient resolution. The presented paper reports simultaneous measurements with laser-Doppler vibrometry at two points of the ossicular chain of cadaver specimens. Motions not observed were derived using mechanical simulation models on a computer and then evaluating appropriate mathematical equations. Using a sound stimulus, the displacement velocities of the umbo and stapes footplate were measured, and the corresponding transfer functions were derived by Fourier transform. Results were used for verification of the computer models. In the current investigations these models were refined and allow for the detailed investigation of the dynamic behavior of the ossicular chain, facilitating the optimal design of passive and active middle-ear implants.

Chronic intracochlear electrical stimulation induces selective survival of spiral ganglion neurons in neonatally deafened cats

Ten newborn kittens were deafened by systemic administration of neomycin sulfate. Profound hearing losses were documented by ABR and FFR (500 Hz) testing. At 9–17 weeks of age, the young deafened cats were unilaterally implanted with a multichannel scala tympani electrode. Six of the animals were chronically stimulated at 6 dB above electrically evoked ABR thresholds for 1 h/day for periods of 1 month or 3 months. Stimuli were charge-balanced biphasic pulses (200 μs/phase, 30 pps.) The remaining 4 cats underwent identical deafening and implantion schedules but were not stimulated. Results indicate that administration of neomycin in neonatal cats induced degeneration of hair cells and spiral ganglion cell loss that was bilaterally symmetrical between the two cochleas of each individual animal, although there was variation between animals in the severity of the ototoxic drug effect. In animals receiving passive (unstimulated) implants, morphometric analysis of spiral ganglion cell density showed no significant difference in ganglion cell survival between the implanted cochleas and the contralateral control ears. In contrast, animals that were chronically stimulated for 3 months showed significantly better neuronal survival in implanted and stimulated cochleas as compared to contralateral deafened control ears. The induced conservation of spiral ganglion neurons was observed consistently within the basal cochlear region near the stimulating electrodes. In more apical regions there was no significant difference between the stimulated and control cochleas. The mechanisms underlying this selective conservation of spiral ganglion neurons induced by chronic intracochlear electrical stimulation are uncertain. Since no comparable chronic stimulation studies have been conducted in adults, it is not known whether similar conservation effects could be induced in mature animals.

Two routes of continuous amphetamine administration induce different behavioral and neurochemical effects in the rat

Continuous amphetamine administration has proven to be a useful pharmacological tool. The two systems popularly used for the continuous administration of amphetamine differ in their release characteristics; output of drug from passive diffusion silicone-tubing implants gradually declines over several days, while that of osmotically driven, active pumps remains more stable over time. A matched-dose comparison of the behavioral and neurochemical effects of both systems was performed in which silicone pellets and pumps were filled with appropriate concentrations of drug so as to release equal total quantities over the test period of 5 days. Rat behavior and brain catecholamine levels were affected in quantitatively and qualitatively different ways. The interesting constellation of behavioral and neurochemical changes associated with the use of silicone-tubing systems for the continuous administration of amphetamine may not be confounded by its declining output, but rather caused by it.

The pseudosynovial sheath—Its characteristics in a primate model

The characteristics and long-term fate of the pseudosynovial sheath formed in response to a gliding tendon implant were examined. A primate model was chosen to reproduce the human clinical situation. Hunter passive gliding implants were implanted in 32 digits of eight Macque monkeys. Surgical syndactylism was created to the adjacent active digits to provide passive range of motion of the digits. No implanted digits demonstrated progressive flexion contractures. Radiographs revealed passive excursion of the implants of an average of 2.5 cm. Biopsies were taken at various time intervals and locations for histological examination. The pseudosynovial sheath has three descriptive layers: an intima, media, and adventitia. This sheath becomes mature and stable at 8 weeks. The intima cells contain a glycosaminoglycan substance and have a secretory capacity. The media cells have large amounts of collagen and provide structural and vascular support. The adventitia is a highly vascular structure composed of loose fibrous tissues that demonstrates clefts that may represent gliding planes. The pseudosynovial sheath was found to be a morphologically stable structure that showed no propensity for longitudinal contracture. The sheath appears to have the characteristics necessary to provide a good bed for a tendon graft.