Safety of cornea and iris in ocular surgery with 355-nm lasers.

A recent study showed that 355-nm nanosecond lasers cut cornea with similar precision to infrared femtosecond lasers. However, use of ultraviolet wavelength requires precise assessment of ocular safety to determine the range of possible ophthalmic applications. In this study, the 355-nm nanosecond laser was evaluated for corneal and iris damage in rabbit, porcine, and human donor eyes as determined by minimum visible lesion (MVL) observation, live/dead staining of the endothelium, and apoptosis assay. Single-pulse damage to the iris was evaluated on porcine eyes using live/dead staining. In live rabbits, the cumulative median effective dose (ED50) for corneal damage was 231 J/cm2, as seen by lesion observation. Appearance of endothelial damage in live/dead staining or apoptosis occurred at higher radiant exposure of 287 J/cm2. On enucleated rabbit and porcine corneas, ED50 was 87 and 52 J/cm2, respectively, by MVL, and 241 and 160 J/cm2 for endothelial damage. In human eyes, ED50 for MVL was 110 J/cm2 and endothelial damage at 453 J/cm2. Single-pulse iris damage occurred at ED 50 of 208 mJ/cm2. These values determine the energy permitted for surgical patterns and can guide development of ophthalmic laser systems. Lower damage threshold in corneas of enucleated eyes versus live rabbits is noted for future safety evaluation.

Anterior capsulotomy with a pulsed-electron avalanche knife.

PURPOSE: To evaluate a new pulsed-electron avalanche knife design for creating a continuous curvilinear capsulotomy (CCC) and compare the CCC with a mechanical capsulorhexis. SETTING: Department of Ophthalmology, Stanford University, Stanford, California, USA. METHODS: In this study, CCCs were created in freshly enucleated bovine eyes and in rabbit eyes in vivo. The cutting velocity was adjusted by controlling the burst repetition rate, voltage amplitude, and burst duration. Tissue samples were fixed and processed for histology and scanning electron microscopy (SEM) immediately after surgery. RESULTS: The study included 50 bovine eyes and 10 rabbit eyes. By adjusting the electrosurgical waveforms, gas-bubble formation was minimized to permit good surgical visualization. The optimum voltage level was determined to be +/-410 V with a burst duration of 20 mus. Burst repetition rate, continuously adjustable from 20 to 200 Hz with footpedal control, allowed the surgeon to vary linear cutting velocity up to 2.0 mm/s. Histology and SEM showed that the pulsed-electron avalanche knife produced sharp-edged capsule cutting without radial nicks or tears. CONCLUSIONS: The probe of the pulsed-electron avalanche knife duplicated the surgical feel of a 25-gauge cystotome and created a histologically smooth capsule cut. It may improve precision and reproducibility of creating a CCC, as well as improve its proper sizing and centration, especially in the face of surgical risk factors, such as weak zonules or poor visibility. FINANCIAL DISCLOSURES: Drs. Palanker and Vankov hold patents to the pulsed electron avalanche knife technology, which are licensed to PEAK Surgical by Stanford University. Drs. Palanker and Chang are consultants to PEAK Surgical. Dr. Vankov is an employee of PEAK Surgical. Neither of the other authors has a financial or proprietary interest in any material or method mentioned.

Electrosurgery with cellular precision.

Electrosurgery, one of the most-often used surgical tools, is a robust but somewhat crude technology that has changed surprisingly little since its invention almost a century ago. Continuous radiofrequency is still used for tissue cutting, with thermal damage extending to hundreds of micrometers. In contrast, lasers developed 70 years later, have been constantly perfected, and the laser-tissue interactions explored in great detail, which has allowed tissue ablation with cellular precision in many laser applications. We discuss mechanisms of tissue damage by electric field, and demonstrate that electrosurgery with properly optimized waveforms and microelectrodes can rival many advanced lasers. Pulsed electric waveforms with burst durations ranging from 10 to 100 micros applied via insulated planar electrodes with 12 microm wide exposed edges produced plasma-mediated dissection of tissues with the collateral damage zone ranging from 2 to 10 microm. Length of the electrodes can vary from micrometers to centimeters and all types of soft tissues-from membranes to cartilage and skin could be dissected in liquid medium and in a dry field. This technology may allow for major improvements in outcomes of the current surgical procedures and development of much more refined surgical techniques.

Pulsed electrical stimulation for control of vasculature: temporary vasoconstriction and permanent thrombosis.

A variety of medical procedures is aimed to selectively compromise or destroy vascular function. Such procedures include cancer therapies, treatments of cutaneous vascular disorders, and temporary hemostasis during surgery. Currently, technologies such as lasers, cryosurgery and radio frequency coagulation, produce significant collateral damage due to the thermal nature of these interactions and corresponding heat exchange with surrounding tissues. We describe a non-thermal method of inducing temporary vasoconstriction and permanent thrombosis using short pulse (microseconds) electrical stimulation. The current density required for vasoconstriction increases with decreasing pulse duration approximately as t(-0.25). The threshold of electroporation has a steeper dependence on pulse duration-exceeding t(-0.5). At pulse durations shorter than 5 micros, damage threshold exceeds the vasoconstriction threshold, thus allowing for temporary hemostasis without direct damage to surrounding tissue. With a pulse repetition rate of 0.1 Hz, vasoconstriction is achieved approximately 1 min after the beginning of treatment in both arteries and veins. Thrombosis occurs at higher electric fields, and its threshold increases with vessel diameter. Histology demonstrated a lack of tissue damage during vasoconstriction, but vascular endothelium was damaged during thrombosis. The temperature increase does not exceed 0.1 degrees C during these treatments.

Gene transfer to rabbit retina with electron avalanche transfection.

PURPOSE: Nonviral gene therapy represents a promising treatment for retinal diseases, given clinically acceptable methods for efficient gene transfer. Electroporation is widely used for transfection, but causes significant collateral damage and a high rate of cell death, especially in applications in situ. This study was conducted in the interest of developing efficient and less toxic forms of gene transfer for the eye. METHODS: A novel method for nonviral DNA transfer, called electron avalanche transfection, was used that involves microsecond electric plasma-mediated discharges applied via microelectrode array. This transfection method, which produces synchronized pulses of mechanical stress and high electric field, was first applied to chorioallantoic membrane as a model system and then to rabbit RPE in vivo. Gene transfer was measured by using luciferase bioluminescence and in vivo fluorescent fundus photography. Safety was evaluated by performing electroretinograms and histology. RESULTS: In chorioallantoic membrane, electron avalanche transfection was approximately 10,000-fold more efficient and produced less tissue damage than conventional electroporation. Also demonstrated was efficient plasmid DNA transfer to the rabbit retina after subretinal DNA injection and transscleral electron avalanche transfection. Electroretinograms and histology showed no evidence of damage from the procedure. CONCLUSIONS: Electron avalanche transfection is a powerful new technology for safe DNA delivery that has great promise as a nonviral system of gene transfer.

Design of a high-resolution optoelectronic retinal prosthesis.

It has been demonstrated that electrical stimulation of the retina can produce visual percepts in blind patients suffering from macular degeneration and retinitis pigmentosa. However, current retinal implants provide very low resolution (just a few electrodes), whereas at least several thousand pixels would be required for functional restoration of sight. This paper presents the design of an optoelectronic retinal prosthetic system with a stimulating pixel density of up to 2500 pix mm(-2) (corresponding geometrically to a maximum visual acuity of 20/80). Requirements on proximity of neural cells to the stimulation electrodes are described as a function of the desired resolution. Two basic geometries of sub-retinal implants providing required proximity are presented: perforated membranes and protruding electrode arrays. To provide for natural eye scanning of the scene, rather than scanning with a head-mounted camera, the system operates similar to 'virtual reality' devices. An image from a video camera is projected by a goggle-mounted collimated infrared LED-LCD display onto the retina, activating an array of powered photodiodes in the retinal implant. The goggles are transparent to visible light, thus allowing for the simultaneous use of remaining natural vision along with prosthetic stimulation. Optical delivery of visual information to the implant allows for real-time image processing adjustable to retinal architecture, as well as flexible control of image processing algorithms and stimulation parameters.

Migration of retinal cells through a perforated membrane: implications for a high-resolution prosthesis.

PURPOSE: One of the critical difficulties in design of a high-resolution retinal implant is the proximity of stimulating electrodes to the target cells. This is a report of a phenomenon of retinal cellular migration into a perforated membrane that may help to address this problem. METHODS: Mylar membranes with an array of perforations (3-40 microm in diameter) were used as a substrate for in vitro retinal culture (chicken, rats) and were also transplanted into the subretinal space of adult RCS rats. A membrane was also constructed with a seal on one side to restrict the migration. RESULTS: Retinal tissue in vitro grew within 3 days through perforations of greater than 5 microm in diameter when the membranes were positioned on the photoreceptor side, but no migration occurred if the implant was placed on the inner retinal surface. Histology with light microscopy and transmission electron microscopy (TEM) demonstrated that migrating cells retain neuronal structures for signal transduction. Similar growth of RCS rat retinal cells occurred in vivo within 5 days of implantation. A basal seal kept the migrating tissue within a small membrane compartment. CONCLUSIONS: Retinal neurons migrate within a few days into perforations (> 5 microm in diameter) of a membrane placed into the subretinal space. This may provide a means of gaining close proximity between electrodes in a retinal prosthetic chip and target cells, and thus allow a greater density of stimulating elements to subserve higher resolution. Further studies are needed to explore the long-term stability of the retinal migration.

Precision and safety of the pulsed electron avalanche knife in vitreoretinal surgery.

BACKGROUND: We have developed a new surgical instrument, called the pulsed electron avalanche knife (PEAK; Carl Zeiss Meditec, Jena, Germany), for precise, "cold," and tractionless dissection of tissue in liquid media. OBJECTIVE: To evaluate the 3-dimensional damage zone induced by the PEAK compared with 2 other standard intraocular surgical instruments, diathermy and retinal scissors. METHODS: Damage zone and minimum safe distance were measured in vitro on chick chorioallantoic membrane and in vivo on rabbit retina with the use of propidium iodide staining. RESULTS: The PEAK produced a paracentral zone of cellular structure disruption surrounding a crater and a peripheral zone of structurally intact but abnormally permeable cells. The instrument induced a damage radius that varied from 55 to 300 micro m for the range of voltages and pulses typically used during surgery. For comparison, damage radius for microsurgical scissors was 50 micro m, and for diathermy, 400 to 850 micro m. The PEAK also damaged tissue up to 1.4 mm away by the creation of water flow that formed at the tip of convex probes during collapse of a cavitation bubble. Concave probes, which prevent formation of the water jet, eliminated this effect. CONCLUSIONS: The PEAK operated well within accept-able safety limits and may greatly facilitate both posterior segment surgeries (eg, membrane dissection and sheathotomy) and anterior segment procedures (eg, capsulotomy, nonpenetrating trabeculectomy, and iridectomy).

Effects of the pulsed electron avalanche knife on retinal tissue.

OBJECTIVES: To evaluate the precision of retinal tissue dissection by the pulsed electron avalanche knife (PEAK) and to assess possible toxic effects from this device. METHODS: To demonstrate precision of cutting, bovine retina (in vitro) and rabbit retina (in vivo) were incised with the PEAK. Samples were examined by scanning electron microscopy and histologic examination (light microscopy). To evaluate possible toxic effects in rabbit eyes, 30 000 pulses were delivered into the vitreous 1 cm above the retina. Histologic examinations and electroretinography were performed at intervals up to 1 month after exposure. RESULTS: Cuts in postmortem bovine retina showed extremely sharp edges with no signs of thermal damage. Full-thickness cuts in living attached rabbit retina were similarly sharp and were typically less than 100 microm wide. No signs of retinal toxic effects were detected by histologic examination or electroretinography. CONCLUSIONS: The PEAK is capable of precise cutting through retinal tissue, and there are no demonstrable retinal toxic effects from its use. The precision and tractionless nature of PEAK cutting offers advantages over mechanical tools and laser-based instrumentation. We believe this new device will prove useful in a variety of vitreoretinal surgical applications.