Using 193 nrn Excimer Laser light to reshape the cornea has been shown to be an effective way to correct myopia in man"2 One way to achieve the controlled shape is to create a large diameter Excimer beam, and pass it through a computer controlled ins diaphragm. As the diaphragm closes, progressively less laser pulses reach the outer portions of the treatment area, while all the pulses reach the center region, effectively flattening the cornea. The resultant correction is spherical, and does not correct astigmatism. This paper discusses the use of a motor controlled slit to correct astigmatism in a similar manner to myopia. The mechanism described combines an iris diaphragm and adjustable slit for correcting myopia, astigmatism, or a combination of the two. The slit mechanism also rotates so the slit axis can be aligned with the patient's astigmatic axis. All motions are motor driven under computer control, with encoder feedback to ensure correct positioning. Initial test medium was PMMA, which has ablation characteristics similar to human stromal tissue. Mechanical profile scans and lensometer tests of the PMMA test blocks were performed to verify the correct ablation pattern. Animal eyes were ablated and tested for astigmatic induction.
A new method has been developed for determining intraocular distances. It is
based on interferometry methods in conjunction with the laser Doppler technique,
using partially coherent light. In a first approach two distances are
measured: the axial eye length (distance cornea - retina) and the thickness of
the retina. An accuracy of 10 pm for optical distances is achieved.
Comparisons with the usual ultrasound technique show a good agreement also for
A real-time clinical confocal light microscope provides the
ophthalmologist with a new tool for the observation of the cornea and the
ocular lens. In addition, the ciliary body, the iris, and the sclera can be
observed. The real-time light microscopic images have high contrast and
resolution. The transverse resolution is about one half micron and the range
resolution is one micron. The following observations were made with visible
light: corneal epithelial cells, wing cells, basal cells, Bowman's membrane,
nerve fibers, basal lamina, fibroblast nuclei, Descemet's membrane,
endothelial cells. Observation of the in situ ocular lens showed lens capsule,
lens epithelium, lens fibrils, the interior of lens fibrils. The applications of
the confocal microscope include: eye banking, laser refractive surgery,
observation of wound healing, observation of the iris, the sciera, the ciliary
body, the ocular lens, and the intraocular lens. Digital image processing can
produce three-dimensional reconstructions of the cornea and the ocular lens.
Lens extraction during cataract operation has mainly two consequences: first, a reduction of the refractive power of
the eye and second, the loss of accommodation. Insertion of a monofocal intraocular lens counterbalances the first effect,
accommodation, however, remains lost. Various types of intraocular lenses exhibiting two different focal lengths (bifocal)
are at present under clinical trial. With such lenses one focal length corresponds to the far vision (P.) while the other is
related to the near vision (P )(Fig. 1). In both cases, however, two images are formed simultaneously in the eye: one is
focused on the retina while the other is out of focus. First clinical results indicate that a satisfactory situation can be
obtained both in far and near vision.
Currentmethods ofdetermining the shape and elevation ofthe cornea! surface are inadequate both in terms ofaccuracy and extent
Much of the cornea! surface is missed by these methods -particularly the critical center and most of the periphery. All current
measuring techniques assume this surface to be spherical -which it is not Mathematical assumptions have been employed to
reconstruct the surface. These assumptions fall short of the accuracy required for realistic assessment of the effect of external
influences on cornea! shape. Modern surgical methods designed to modulate the corneal surface curvature or alter the refractive
state of the eye demand precise measurement of this surface -the main refractive element of the human eye. Single side-band
holographic techniques provide the means whereby this demand can be realized. Surface elevations in the sub-micron range can
be readily imaged with this technique.
With the advent of phacoemulsification and foldable intraocular lenses, there is renewed interest
in sutureless cataract wound. We report the use of laser activated tissue glues for the closure of
scleral tunnel cataract incisions. Two glue mixtures were tested in enucleated porcine eyes. Glue
A was composed of hyaluronic acid, human albumin, and indocyanine green dye. Glue B
contained hyaluronic acid, chondroitin sulfate, human albumin, and indocyanine green dye. A
Spectra Physics diode laser (808 nm) with a power density of 7-1 1 watts/cm2 was used for glue
activation. Wound bursting pressures, as determined by the presence of fluid at the wound
margin, was significantly higher with both glue combinations than without the glue (P
Photodynamic therapy (PDT) eiiploying Dihematopor*iyrin ethers (DHE)
(Photofrin II) at pharmacologic lvels, has been denonstrate3 to kill rabbit
lens epithelial cells, in vivo. This in vitro study, reports on the
minimal necessary parameters for rabbit lens epithelial cell death.
Explants of rabbit lenses were incubated in various concentrations of
DHE (1O,, 100, 500, 1000 ug/ml) for 1, 2, or 5 minutes. 30 to 120
Joules/an of collimated 514.5 nm Argon laser light re delivered to the
locier concentrations of 10, 50, and 100 ug,'ml DHE treated cells. One
hundre1 fifteen explants were treated, in all.
Higher concentrations of DHE alone (500 and 1000 ug/ml) were
sufficient to induce cellular swelling. Lower concentrations required light
for cellular effect. Trypan blue staining revealed cell death at these
minimal pa9ieters: DHE 50 ug/ml, incubation 1 minute, 514.5 r Argon light
1.0 Watt/an for 30 sec (30 Joules) . In future studies, these rameters
will be tested in vivo, for their ability to eliminate lens epithelial
proliferation after cataract surgery.
As refractive procedures involving the cornea have proliferated, concerns over the long-term stability and predictability of these procedures have occupied a more prominent role. The authors have applied the procedures and principles of mechanical engineering to mathematically model the cornea and we have used this approach to predict corneal behavior. Strip testing was utilized to measure nonlinear stress/strain relationships in human cornea and scleral tissue. Nonlinear isotropic finite element analysis was utilized to predict corneal behavior under simple insufflation of the globe. Nonlinear anisotropic finite element analysis was used to demonstrate stress patterns and refractive changes in radial keratotomy. The importance of these calculations, as they relate to corneal stability and predictability, is discussed.
Previous attempts to analyze the corneal surface have been limited to geometric interpretation of ring displacement (Placido disc technology). Use of holographic interferometry allows real time optical measurement of the corneal surface. Clinical examples of holographic interferometry are presented.
Endoscopy is a novel method of observation in ocular surgery. It allows a direct viewing of certain internal
structures of the eye which can not be seen through conventional slit lamp/microscope arrangements like the back side of
the iris, the posterior chamber or the fixation area of an IOL. In addition such an instrument is useful in exploratory
orbital surgery because it allows for examination of the scleral wall, the 6 motor muscles and the optic nerve sheet with
minimal trauma to the eye. An endoscopic system can therefore be helpful to check regions which can not be reached
easily during surgery, such as the ciliary body or the pars plana. When a transparent structure of the eye becomes turbid,
e.g., a hazy cornea, observation is impaired and endoscopy could also provide a solution though it is an invasive method.
A number of surgical techniques has been developed to correct ametropia (refractive defaults) of the eye by
changing the anterior corneal radius. Because the air-cornea interface makes up for about two-third of the refractive
power of the eye, a refractive correction is obtained by a suitable photoablation of the cornea. For this purpose, e.g., an
ArF excimer laser which emits a wavelength of 193 nm is being used. After a mechanical removal of the epithelium, the
Bowman's layer and the corneal stroma are photoablated on typically 50% of the central surface of the cornea with
various precomputed shapes. Methods using a variable diaphragm1 or a scanning slit2 are being utilized. After regrowth of
the epithelium, a smooth interface with air develops itself, which can be attributed to a mechanical equilibration. Yet,
SEM studies have shown that with such kind of treatments, irregularities can remain in the new stromal surface (Fig. 1). A
possible explanation for this effect is associated with an inhomogeneous energy distribution of the laser beam profile3. To
some extent, the stromal surface is equalized by the epithelial layer during healing& However, as the corneal epithelium
and stroma have different refractive indices, a scatter of the incident light may result causing a haze in the cornea and a
blur of the image at the retina. In such a case the resolution and the contrast performance of the eye which is expected
from a successful operation, may be reduced. This study is an attempt to quantify the vision blur as a function of the
deformation observed at the epithelium-stroma interface.
More than one hundred fifty clinical excimer laser units designed for corneal surgery have been
developed and sold commercially. Manufacturers include Meditec Lasers in Germany, Summit
Engineering, Taunton Technologies, and Visx in the United States, and Synthelabo in France.
Furthermore a number of prototypes have been built in the USSR and other countries which are being
investigated for their clinical use.
While in the United States and Canada, substantial regulation of these devices has limited their
distribution and use, sales in other parts of the world have been restricted only by market forces. Early
clinical successes have created an enthusiasm for this new technology. In spite of this, substantial
technical issues remain uncertain and have not been carefully studied. Indeed we have accepted certain
parameters for on an almost serendipitous, empirical basis. It is a proper time to pause and consider
the bases for these laser techniques.
This paper presents a methodology for guiding a radiation of ArF excimer laser in a liquid surrounding and confining it to the spots that can be varied in dimension from submicron diameters to tens and hundreds of microns. The approach described here is to confine and guide the excimer laser with variable diameter tapered tubes, thus opening the possibility of applying this laser in vitro-retinal surgery using endo laser techniques. Presently because of a lack of methods to guide the 193 nm ArF radiation in liquid this laser is used exclusively in ophthalmology in topical applications such as in corneal sculpturing. The methodology presented in this paper resolves this problem in a unique way and with impressive results. Specifically, the authors show that with this syringe needle guided excimer laser it is possible to accurately remove retinal tissue without any detectable damage to surrounding cells. Applications of this new technology in retinal surgery are discussed.
The Laser Imaging Workstation (LIWS) is a new tool expected to provide the
Ophthalmologists with improved laser delivery and digital imaging of the retina. It is an
integrated system which allows the physicians to combine diagnosis, treatment planning
and laser delivery into one device with one information media common to all three
treatment phases. In addition of being one common machine for all three treatment
phases, LIWS will be a significantly more precise laser delivery system and can
eliminate or significantly reduce the need for patient cooperation, retro bulbar
anesthesia and delays due to film processing. Due to a new technology, statistical
target tracking, the instrument continuously measures the degree of optical alignment
of the patient's eye with the instrument. If this measurement indicates too much
misalignment, the surgical laser will be shut down at once and, therefore, LIWS
provides also a safe way of doing photocoagulation.
By using optical interference method, the authors have studied the process in which a Q-switched YAG laser induced an eye model to break down. It showed that the mechanical effect of the shock results in local discuption of iris. The authors have investigated the plasma shielding in the lipuid eye model, and put forward both the plasma shielding and the focusing angle of the incident laser as two important factors to protect the retina from injury in high-power laser ophthalmology.
Photorefractive keratomileusis procedures are currently being evaluated for both direct ablation of the human cornea and for the shaping of synthetic lenticules, with the ArF excimer laser offering the promise of reduced photochemical damage of underlying tissues due to the non-penetrative nature of 193 nm radiation. In order to better consider the possible effects of any secondary luminescence produced by the interaction of 193 nm radiation with these materials, an optical system based on a gated diode array spectrometer was constructed and used to record their fluorescence spectra at known laser fluences. A broadband fluorescence peak centered around 290-330 nm was observed upon irradiation of corneal buttons and collagen-based biopolymers. The optical efficiency of the fluorescence cell was calibrated by the use of a scattering material of known reflectivity allowing estimates of the fluorescence quantum yield of this process to be made.
The authors demonstrate the use of a frequency multiplied Q-switched Nd:YAG laser to generate the fifth (213 nm) and the fourth (266 nm) harmonic frequencies to ablate porcine corneas and synthetic collagen materials. This new strategy is discussed for corneal refractive surgery using a solid state laser. The potential for spatially resolved refractive correction based on a 213 nm solid state laser is reviewed in light of new corneal imaging technology that can detect localized refractive errors of the eye. Finally, the authors discuss remaining problems that need to be addressed before this technology can be applied to clinical investigation.
This paper presents an analysis of the effects of axial and transverse displacement on the optical quality and accuracy of lenses created during excimer laser photoablation. Tolerance levels for axial positioning of the cornea prior to and during surgery are presented. The axial tolerance levels are dependent upon a number of parameters which include the intended dioptric correction and laser system cone angle. A collimation lens is introduced as a means of desensitizing the laser system to axial displacement. Transverse displacement tolerances during laser treatment are shown to depend upon the treatment diameter, dioptric correction and acceptable distortion level in the lens ablated into the anterior corneal stroma. A video and computer analysis of transverse motion during seven randomly selected excimer laser refractive surgeries is presented. Although transverse displacement exceeded the tolerance levels presented, it did not appear to affect the quality of correction in the eight patients analyzed.
Photorefractive keratectomy (PRK) using an argon fluoride excimer laser for photoablation of the cornea shows potential for the precise correction of refractive errors in patients. Usually, the epithelium is mechanically removed, and Bowman's layer and stromal tissue are photoablated to precomputed depths and shapes that are based on known ablation rates for these tissues. After four day's time, the epithelium has regrown. Assuming the epithelium to be preoperatively uniform in thickness across the central optical zone, and assuming that it regrows to the same thickness, a theoretical precision of +/- 0.05 diopters is achievable with PRK. Keratometric measurements of the epithelium and of Bowman's layer were made at the 2.0 and 3.6 mm optical zones on 10 fresh cadaver eyes (<21 hours postmortem). In the eyes studied, the epithelium thickness was found to vary across the central optical zone, accounting for the measured refractive differences of 0.5 to 1.8 diopters. Bowman's layer was found to be more prolated than the epithelial surface (ratios: 1.005 compared to 1.033). In addition, the surface of Bowman's layer had a larger degree of astigmatism. Other studies have shown that the epithelium regrowth is a function of the newly exposed corneal topography as the wing cells compensate for irregularities in Bowman's surface. As the preoperative topography of the epithelium cannot be used as a reference surface when computing photoablation depth, intraoperative keratometry of Bowman's surface becomes a necessity in PRK.
Excimer lasers are now being used in surgical applications in the clinical environment. The laser safety measures developed for excimer laser use in the industrial and research setting may not be sufficient for use in the intense, time-urgent environment of the operating room. It is shown that the only major safety concern relates to handing of the compressed halogen gasses.
The potential of an erbium-YAG laser for corneal surgery has been assessed under a range of operating parameters. The ablation threshold has been measured at approximately 0.6 Jcm-2 and the depth of thermal damage evaluated for different pulse durations and energy densities. The minimum damage of
Laser beam diameters within a schematic human eye are computed for nine different fundus lenses and five different laser delivery systems. These beam diameters taken in conjunction with commonly used power, time, and spot size settings for ophthalmic photocoagulation are used to compare the relative radiant exposure at various structures within the eye. Surprisingly large pupillary radiant exposures result from some combinations of fundus lens, spot size, and laser delivery system. A general guideline for spot size selection is to maintain a pupillary radiant exposure of less than 2 J/mm2. This can be accomplished with parfocal type delivery systems by keeping the product of the selected spot size and the lens's laser magnification under 400 micrometers . With defocus type delivery systems pupillary radiant exposure can be kept below 2 J/mm2, with the exception of the Volk Quadra Aspheric lens, if spot sizes above the 500 micrometers setting are avoided. The Quadra Aspheric lens with both parfocal and defocus delivery systems will maintain less than 2 J/mm2 pupillary radiant exposure if the true retinal spot size is kept below 400 micrometers as with the parfocal systems.
Laser surgery on the eye and cornea requires a very high degree of spatial accuracy and reproducibility if
refractive outcomes are to be reliable. We have identified and analysed the components of accuracy that need to be
addressed, and with the results of this analysis designed and built a computer-controlled laser delivery system for
anterior chamber ophthalmic surgery. Design parameters, achievable operating parameters and clinical results are
shown and demonstrated.
Previous research suggests that direct electrical stimulation of the visual cortex with an array of electrodes may be a viable means to provide a limited restoration of a lost visual sense to the profoundly blind. More recent work indicates that this goal may only be achieved by passing currents into the cortex via arrays of electrodes which penetrate into the visual cortex. An electrode array possessing a unique three dimensional architecture is described which could form the cornerstone of an 'artificial vision system'. A system for inserting such complex structures into cortical tissues is also described. Finally, estimates are made regarding the extent of visually guided task performance which may be expected from the limited, pixelized visual sense which could be produced with an artificial vision system.