Sixty-two eyes underwent excimer laser photorefractive keratectomy (PRK) for the correction of myopia at Cedars-Sinai-Medical-Center. The first group of 12 patients are presented with follow up data of ten months postoperatively. The second group of 50 patients are presented with follow up data of three months postoperatively. An in-depth comparison of pre and postoperative refractive data is presented. Comparisons between pre and postoperative corrected and uncorrected Snellen visual acuities are provided in order to asses the functional visual result of the procedure.

Clinical experience with more than ten thousand sighted eyes has demonstrated great promise for correcting myopia with photorefractive keratectomy (PRK). Previously reported techniques have incorporated computer-controlled irises, diaphragms, and apertures to regulate the desired distribution of 193 nm radiation onto the eye. This paper reports on an entirely new approach for performing PRK which utilizes an erodible mask to control the shape transfer process. Compared to the more traditional techniques, the erodible mask offers promise of correcting a broad range of refractive errors. In this paper the erodible mask and associated hardware are described in detail. We describe the shape transfer experiments used to predict the functional relationship between the desired refractive correction and the mask shape. We report on early clinical results from five patients with myopic astigmatism. We conclude that the early shape transfer experiments overestimated the spherical component of the correction by 1.25 diopters and underestimated the cylindrical component by approximately 0.85 diopters. The data suggest there may be biological effects which evoke different healing responses when myopic PRK corrections are performed with and without astigmatism. Clinical trials are proceeding with the mask shapes adjusted for these observations.

Excimer laser photorefractive keratectomy (PRK) is presently under investigation for the correction of myopia. Two companies in the United States, Summit Technology (Waltham, Mass.) and VisX, Inc. (Sunnyvale, Calif.) have developed excimer laser delivery systems and are participating in an FDA study to determine the safety and efficacy of PRK. This is a preliminary report on the refractive and visual results of 51 of 100 eyes treated between October 10, 1990 and March 7, 1991 by the Summit Technology UV200LA excimer laser under the FDA Phase IIB FDA protocol one year after surgery. More detailed information on eight patients treated at Emory University Eye Center (Emory Subgroup) is also reported.

Elliptical photoablations have the advantage of correcting refractive sphere and cylinder without producing transition zones. This document characterizes the geometries of ellipses used for the simultaneous correction of sphere and cylinder. The implementation of elliptical ablations in ophthalmic excimer lasers with imaged apertures is discussed.

Corneal astigmatism, both naturally occurring and iatrogenically induced, is a commonly encountered problem. Examination of corneal topography with instruments that digitize reflected ring images and calculate corneal geometry suggests that corneal astigmatism often deviates from spherocylindrical optics; the observed topography may be highly asymmetrical about the center of the pupil. Currently used incisional procedures are limited in terms of predictability of surgical outcome. The 193 nm excimer laser can be used to alter anterior corneal curvature and flatten the cornea to correct myopia. For correction of astigmatism, a slit-opening in the laser delivery system can be used to selectively flatten the steep meridian. Early results using this procedure for correction of iatrogenically induced high corneal astigmatism are promising. A nationwide multicenter clinical trial is now underway in the United States to evaluate this technique for the correction of naturally occurring astigmatism and compound myopic astigmatism.

In refractive surgery, a number of surgical techniques have been developed to correct ametropia (refractive defaults) of the eye by changing the exterior shape of the cornea. Because the air-cornea interface makes up for about two thirds of the refractive power of the eye, a refractive correction can be obtained by a suitable reshaping of the cornea. Postoperatively, it is usually observed that the corneal region consists of two or more zones which are characterized by different optical parameters exhibiting in particular different focal distances. Under normal circumstances, only the central area of the cornea is involved in the formation of the retinal image. However, if part of the light entering the eye through peripheral portions of the cornea with refractive properties different from the central area can pass the pupil, an out-of-focus `ghost' image may be overlaid on the retina causing a blur. 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 diameter of the central zone, i.e., the optical zone which is of importance for vision.

In this paper, we reported the results of an in vitro pilot study designed to evaluate the possible use of type-I collagen for corneal wound closure via an in situ photo-crosslink process. The wound strength was measured by rising the intraocular pressure (IOP) at a constant rate until wound breakage (blow-up test). In addition, histology was performed to study the wound appearance after photosoldering.

We report on two clinical trials, one involving 10 patients in the United States and the other in Berlin, Germany with 15 patients, all treated for hyperopia with a new laser surgical procedure known as laser thermokeratoplasty (LTK). The procedure involves making a circular array of corneal coagulations using an Ho:YAG laser, fiber-optic handpiece, and contact focusing tip. The extent of steepening of central corneal curvature is controlled by the diameter of the treatment zone. The LTK procedure induces an immediate and significant reduction in hyperopic refractive error. Although the clinical investigations are at an early stage, results of the clinical trials indicate that the LTK procedure may prove to be a reliable means of effecting a permanent change in refraction in patients suffering from hyperopic refractive disorders.

Laser photo thermo keratoplasty (LPTK) is a refractive procedure that changes the curvature of the cornea by laser induced collagen shrinkage. To understand the temperature distribution induced by the laser radiation inside the cornea, a photo-thermal theoretical model is employed to simulate this process. The 4-D space-time temperature distributions within the cornea are calculated and depicted. The effects of the incident fluence and the laser spot size on temperature relaxation rate is discussed. Optimization of incident fluence on the surface of the cornea is made. The comparison between calculated and experimental regions of collagen shrinkage is given. Recommendations for the design of clinical surgery instruments are also presented.

Radial keratotomy microsurgery is a high-precision procedure for modifying the curvature of the cornea. A novel laser technique, using subsurface energy deposition, is shown to be remarkably effective in accomplishing this procedure and related microsurgeries with negligible collateral damage due to thermal or shock waves.

A method of lens extraction after Nd:YAG laser capsulonucleolysis in the eyes with high myopia is described. The scheme and performance specifications of the Nd:YAG laser used for preparing the extracapsular lens extraction are presented. The treatment begins a few months before lens extraction by scleroplasty associated with cryoprofilaxy or Argon laser endocerclage. Twenty-four to forty-eight hours before the operation Nd:YAG laser pulses are applied on the exterior capsule, first in the periphery and then just in the middle of the pupila. A capsulonucleolysis is obtained and the extraction of the transparent lens is performed in extracapsular extraction of the cataract.

A laser system is described that offers manifold applications in ophthalmic surgery. The Nd:YLF solid-state laser emits at a wavelength of 1.053 micrometers . With pulse durations of 40 psec less energy is required to obtain optical breakdown compared to nanosecond lasers. The lower pulse energy allows a substantial confinement of the laser-tissue interaction. With a highly sophisticated delivery system eye movements can be tracked during surgery and a precise placement of the laser focus can be achieved. The high transmission coefficient of ocular structures for wavelengths around 1 micrometers enables the Nd:YLF laser to perform several internal eye surgeries such as cataract fragmentation, intrastromal refractive surgery, internal sclerostomy, or iridotomy. First clinical results of some of these procedures are summarized.

The ISL laser (Intelligent Surgical Lasers, Inc.), a Nd:YLF picosecond pulse laser, is currently being used under investigational device exemption to perform microsurgery of the anterior segment of the eye. At different study sites procedures for cataract fragmentation and iridotomy, as well as for posterior capsulotomy after cataract surgery, are under evaluation. Other potential applications include: sclerostomy ab interno, the cutting of membranes in the anterior and posterior segment of the eye; corneal incisions; and corneal intrastromal effects. We discuss various clinically relevant aspects of the use of this picosecond laser. An overview of different computer controlled laser patterns is given.

The keratoprosthesis is the last solution for corneally blind patients that cannot benefit from corneal transplants. Keratoprostheses that have been designed to be affixed anteriorly usually necessitate multi-step surgical procedures and are continuously subjected to the extrusion forces generated by the positive intraocular pressure; therefore, clinical results in patients prove inconsistent. We proposed a novel keratoprosthesis concept that utilizes posterior corneal fixation which `a priori' minimizes the risk of aqueous leakage and expulsion. This prosthesis is implanted in a single procedure thereby reducing the number of surgical complications normally associated with anterior fixation devices. In addition, its novel design makes this keratoprosthesis implantable in phakic eyes. With an average follow-up of 13 months (range 3 to 25 months), our results on 21 cases are encouraging. Half of the keratoprostheses were implanted in severe burn cases, with the remainder in cases of pseudo- pemphigus. Good visual results and cosmetic appearance were obtained in 14 of 21 eyes.

The development of a matrix with high cell adhesion and ingrowth rates is of importance for artificial cornea. The design of such artificial matrices requires understanding of cellular system colonization processes. After lamellar implementation the more impermeable the material, the larger the diameter and the more anterior placement, the more likely it is that the anterior stroma will starve and become necrotic. One approach to this problem is fenestration or open porosity of the material. Modifications in the thickness, pore size, and their geometric orientation of an expanded polytetrafluorethylene could modify the cellular ingrowth rate. Immunohistochemical study with the monoclonal antibodies AE5 showed normal epithelial differentiation over the polymer. The material in first white opaque became progressively transparent after colonization and demonstrated that an opaque hydrophobic material may become transparent and completely wettable with very low perturbation of the flow through the cornea.

Clinical investigations have suggested the utility of being able to form, in situ, thin lenticules over the exposed corneal surface from the photopolymerization of a semi-liquid collagen gel. In order to be better able to understand and ultimately control the use of this viscous material for investigations of both the biological stability of these collagen films and their potential in synthetic epikeratoplasty we have studied the photochemical properties of the material. In particular, we have investigated the possibility of using light in the visible wavelength range to initiate gelation of bulk collagen gels, the crosslinking process being effected by added photoinitiators.

This paper demonstrates the advantages of three-dimensional reconstruction of the cornea and the ocular crystalline lens by confocal microscopy and volume rendering computer techniques. The advantages of noninvasive observation of ocular structures in living, unstained, unfixed tissue include the following: the tissue is in a natural living state without the artifacts of fixation, mechanical sectioning, and staining; the three-dimensional structure can be observed from any view point and quantitatively analyzed; the dynamics of morphological changes can be studied; and the use of confocal microscopic observation results in a reduction of the number of animals required for ocular morphometric studies. The main advantage is that the dynamic morphology of ocular structures can be investigated in living ocular tissue. A laser scanning confocal microscope was used in the reflected light mode to obtain the two- dimensional images from the cornea and the ocular lens of a freshly enucleated rabbit eye. The light source was an argon ion laser with 488 nm wavelength. The microscope objective was a Leitz 25X, NA 0.6 water immersion lens. The 400 micron thick cornea was optically sectioned into 133, three micron sections. The semi-transparent cornea and the in-situ ocular lens was visualized as high resolution, high contrast two-dimensional images. The under sampling resulted in a three-dimensional visualization rendering in which the corneal thickness (z-axis) is compressed. The structures observed in the cornea include: superficial epithelial cells and their nuclei, basal epithelial cells and their `beaded' cell borders, basal lamina, nerve plexus, nerve fibers, free nerve endings in the basal epithelial cells, nuclei of stromal keratocytes, and endothelial cells. The structures observed in the in-situ ocular lens include: lens capsule, lens epithelial cells, and individual lens fibers.

In this preliminary study the effects of a simple anterior cut of a bovine cornea were investigated by means of double-exposure holographic interferometry. The resulting change of holographic fringe pattern was obvious. Further investigations under controlled conditions are envisaged.

There is a need for precise visual functional analysis that parallels and complements the advance technologies employed in ophthalmic surgery. An instrument is needed which can be coupled to the surgical device which allows precision assessment of vision in some sub portion of the cornea, the entire area of surgical intervention, and the entire cornea. A device is proposed for inclusion which allows (1) monitoring and control of fixation during surgery and visual assessment, (2) precision alignment of the corneal surgical area and the eye lens with the point of fixation (using an achromatic axis), (3) focusing, locating, adjusting, and monitoring an aperture image in the surgical plane, (4) provision of any desired test of vision, and (5) measurement and correction of refractive error and certain aberrations as often as required. The utilization of the hyperacuity paradigms as part of the test battery are also suggested. These can be highly precise reliable benchmarks for characterizing refractive surgery. Hyperacuity testing is well suited for analysis of visual performance in relation to keratorefractive surgery because the procedure is highly resistant to optical image degradation and requires a higher-order analysis by the visual system. The uses of hyperacuity techniques are discussed. These techniques allow assessment of vision through cloudy, distorted, or absorbing optical media, and it is possible to separate quantitatively optical from neural decrements in visual performance. These techniques allow fine detection and measurement of distortion and metamorphopsia. Hyperacuity is also used for precision alignment of the optical elements of the eye and the fovea.

During the past two decades, progress in biophysical technology has made it possible to monitor aging and pre-cataractous as well as cataractous changes in the ocular lens in vivo as well as in vitro. Because they are non-invasive these techniques can be performed in vivo and utilized as screening methods to detect and predict eventual lens opacification and they should also provide increasingly important information to help clarify the cataractogenic process. Three such biophysical approaches can now be employed in vivo; these include lens fluorescence, light scattering measurements, and magnetic resonance imaging (MRI) T₂ analyses. We have utilized two of these methods [fluorescence spectroscopy and MRI measurements of lens water (T₂) phases] to delineate normal age-related and pre- cataractous changes in a series of human subjects and in patients with early lens opacities. The parameters employed for these studies were based on data accumulated from in vitro experiments on human lenses and in vivo and in vitro data from animal experiments.

Objective evaluation of nuclear cataracts would facilitate clinical activities and research studies. This presentation describes a new device using a helium-neon laser beam configured as a sheet of light. Computerized equipment was used for imaging and objective analysis of the lens nucleus. Visual acuity and density measurements of the crystalline lens nucleus were compared in 262 eyes. These included normals and patients with nuclear sclerosis or nuclear cataracts without other visually significant pathology. The coefficient of correlation between acuity and mean nuclear density was 0.78. Thirty-two eyes were examined twice and had no change in visual acuity between the two visits. These paired density measurements had a correlation coefficient of 0.96. Fifteen patients who did have decreased acuity between the two examinations had a correlation between the two measurements of 0.33. The intensity of nuclear sclerosis or cataract, as well as density changes, can be objectively measured and correlated to visual acuity degradation with this instrument.

Future advances in cataract surgery aim to remove the crystalline lens through a small opening such that the capsular bag, once devoid of lens epithelial cells, may be refilled with a clear polymer which may exhibit the elastic properties of a young lens and restore accommodation. Several different lasers are currently being investigated for laser cataract surgery including the excimer, pulsed visible and short infrared, and mid infrared lasers. Taking advantage of the strong water absorption peak at 2.94 micrometers , we have investigated the laser tissue interaction of the Er:YAG (2.94 micrometers ) and Er:YSGG (2.79 micrometers ) which have water absorption coefficients of 13,000 cm^-1 and 7,000 cm^-1, respectively. We have devised a delivery system which measures the ablation time versus radiant energy through a known thickness on a lens nucleus in free air for these two wavelengths. The current presentation compares the ablation rates versus radiant exposure of these two lasers in human lens nuclei. We also show the histopathology from ablated lenses of these two different wavelengths at different radiant exposures. Integration with fiberoptics and clinical applications is discussed.

With most current cataract surgery techniques, an intraocular lens (IOL) is implanted in the capsular bag to emetropize the eye for distance vision. Modern IOLs are made of flexible materials (e.g., silicone and acrylic elastomers) allowing the surgeon to fold and insert the IOL through a smaller limbal incision (4 mm), thus reducing the number of sutures. When using a scleral pocket technique, suturing of such small wounds might not be required. Recently, IOLs having 2 foci (multifocal IOLs) have been introduced. These implants give the patient a second focal plane at normal reading distance, but the double image reduces both visual acuity and contrast sensitivity. However, with all present surgical techniques, the patient loses the natural ability to accommodate. By directing laser energy into a flexible fiber, cataract removal might be performed endoscopically while minimizing trauma to healthy tissue. Bath successfully demonstrated the use of the 308 nm XeCl excimer laser for cataract removal. A significant drawback, however, lies in the fluorescence induced by the 308 nm laser pulses which may cause significant retinal damage. The use of UV radiation also raises serious concerns about carcinogenesis and cataractogenesis risk to both the patient and the surgeon.

Microfluorimetric techniques are nowadays widely used as a diagnostic tool in basic and applied research in biomedicine. They can be employed to study both endogenous and exogenous fluorophores. In the first case, direct indications are obtained on the biomolecules of interest, whereas in the second case indirect information can be deduced on the cellular structures interacting with the fluorophores used as the markers. Moreover, in the case of fluorescent drugs, such as most photosensitizers, microfluorimetric techniques can be exploited to determine the localization sites of the drug itself. Due to the complexity of biological systems, discrimination of the single fluorophores merely based on cw excitation and emission spectral analysis is not always possible. Thus, combination with time-resolved fluorescence analysis can give important additional indications. The potentials of these techniques in ophthalmological research are discussed. The paper reports, as an example of application of time-resolved microfluorimetric techniques, the study of the localization of Photofrin II, a widely used photosensitizing drug, in lens epithelial cells. Indeed, a photodynamic therapy of lens epithelial proliferation frequently following primary cataract surgery has recently been proposed. The potentials of this therapeutic approach strongly depend on a very efficient drug targeting. Microfluorimetric techniques proved very useful to identify the drug localization sites in a series of experiments performed using young adult rabbits as the animal model.

In diabetes nonenzymatic glycation alters collagen throughout the body resulting in the histopathology that underlies diabetic disease in several organs. In the eye such changes in vitreous collagen could contribute to vitreous degeneration and the progression of proliferative diabetic retinopathy. Previous studies have demonstrated early glycation and advanced endproducts in the vitreous of humans with proliferative diabetic retinopathy. Near-infrared Fourier-transform Raman spectroscopy was performed on vitreous obtained at surgery from diabetic patients and from non-diabetic control subjects. The findings were compared to measurements obtained in untreated and glycated (in vitro) rat-tail tendon collagen. The results demonstrated substantial changes in diabetic vitreous collagen resulting from glycation, most likely advanced glycation endproducts. This approach appears to be useful as a means of characterizing the molecular changes induced by diabetes. Furthermore, this technique could be developed as a way of quantifying these changes in vivo in several tissues, so as to gauge the severity of non-enzymatic glycation and monitor the response to therapy.

Laser-induced lesion size is controlled in real time based on a two dimensional reflectance image recorded by a CCD array during lesion formation. A feedback system using components of the reflectance image achieves uniform lesions by compensating for light absorption variability in biological media. Lesions are formed in a phantom using an argon laser to simulate retinal photocoagulation. The tissue model consists of an absorptive high temperature black paint layer and an egg white protein layer. Reflectance images are acquired as the lesion forms at a standard frame rate using a 512 X 512 CCD camera attached to an ITI 151 series image processor and a Sun 3/260 computer. A shutter controlled by the computer is closed when certain preset conditions are met in the images. Results show a low variance in the sizes of the lesions (diameter or depth) produced under different irradiation conditions and the ability to produce lesions of a predefined size under varying illumination conditions. Real time control from reflectance images based on certain parameters is demonstrated as a feasible method of controlling lesion size.

Laser-induced photocoagulation is used routinely to treat a variety of retinal disorders. In this procedure, absorption by the retina and choroid of focused laser light creates thermal lesions that provide the therapeutic effect. The treatment endpoint is determined by the ophthalmoscopical visibility of the coagulation resulting from the increased diffuse reflectivity of the fundus at the lesion site. To date, it is impossible, prior to treatment, to predict the effectiveness of a specific dosage of laser radiation -- the result can be assessed only after the exposure, and only subjectively. To avoid side effects like hemorrhage, macular pucker, and post-coagulative retinal detachments that can result from excessive local heating, a device for automatically controlling the laser exposure parameters is needed. Preliminary research aimed toward development of such a device has now been completed. The technique employs a simple electro-optical detector to monitor the changing reflectivity of the lesion during the laser exposure, and couples that information to a microcontroller which automatically adjusts the laser exposure parameters. The first results of a clinically realistic device are reported.

Previous investigations have demonstrated that when short wavelength light is detected exclusively by short wavelength sensitive mechanisms (SWS or S cone pathways), patients with ocular hypertension (elevated intraocular pressure) or early glaucomatous damage exhibit losses of sensitivity at the fovea and throughout the central 30 degrees of the visual field. We have recently developed a technique for measuring the sensitivity of short wavelength sensitive mechanisms throughout the central visual field by means of a modified automated perimeter. In order to determine the clinical significance and prognostic value of short wavelength sensitivity losses measured with this procedure, we conducted a prospective longitudinal five year investigation of 22 patients with early glaucomatous visual field loss (44 eyes), 38 ocular hypertensive patients (76 eyes) and 62 age-matched normal control subjects (124 eyes). All participants were evaluated annually with standard automated perimetry (AP) and short- wavelength-sensitive perimetry (SWSP). At the beginning of the study, 67 out of 76 eyes in the ocular hypertension patients had normal results for both AP and SWSP tests, while nine out of 76 had normal AP results but abnormal SWSP findings. Five years later, five out of nine ocular hypertensive eyes with initial SWSP abnormalities have developed evidence of glaucomatous visual field loss on standard AP testing. The abnormalities on AP testing occurred in the same general location as those found in earlier years for SWSP evaluations. None of the ocular hypertensive eyes with normal SWSP results in year one developed abnormal AP deficits after five years. Seven out of the 44 eyes of early glaucoma patients demonstrated reproducible evidence of progression of visual field loss on standard AP testing. In all seven instances, the SWSP deficits were larger than the AP abnormalities at the beginning of the study, and the progression of AP sensitivity losses over five years followed the pattern of SWSP deficits obtained in earlier years. These findings provide strong evidence that SWSP abnormalities are an early indicator of glaucomatous damage and are predictive of impending glaucomatous visual field loss for standard AP testing.

Visualization of the optic nerve head cupping is clinically achieved by stereoscopic viewing of a fundus image pair of the suspected eye. A novel algorithm for three-dimensional digital surface representation of the optic nerve head, using fusion of stereo depth map with a linearly stretched intensity image of a stereo fundus image pair, is presented. Prior to depth map acquisition, a number of preprocessing tasks including feature extraction, registration by cepstral analysis, and correction for intensity variations are performed. The depth map is obtained by using a coarse to fine strategy for obtaining disparities between corresponding areas. The required matching techniques to obtain the translational differences in every step, uses cepstral analysis and correlation-like scanning technique in the spatial domain for the finest details. The quantitative and precise representation of the optic nerve head surface topography following this algorithm is not computationally intensive and should provide more useful information than just qualitative stereoscopic viewing of the fundus as one of the diagnostic criteria for diagnosis of glaucoma.

Fresnel prisms are often prescribed for visual field defects in low vision patients. These prisms are made of optical polyvinyl chloride (PVC) and this material increases chromatic dispersion and produces a loss of contrast. In this presentation, the effect of chromatic dispersion on contrast sensitivity is determined. It has been verified that loss of contrast sensitivity is greater at higher spatial frequencies with Fresnel prisms than with glass prisms of the same power. Above 10 prism diopters, Fresnel prisms reduce both contrast sensitivity and visual acuity substantially. However, low vision patients with visual field losses appear not to be affected due to their sometimes very low remaining contrast sensitivity and visual acuity. In the presence of both blurred and diplopic vision, a case report indicating the use of prisms in alleviating diplopia caused by trauma is presented. In spite of a reduction in visual acuity and contrast sensitivity, the patient preferred to have the Fresnel prism on his spectacle lenses for distance viewing. Diplopia appeared to cause more annoyance than the reduction of acuity and contrast sensitivity due to the Fresnel prism.

As people age, so do their photoreceptors. If the visual system has been exposed to sufficient UV radiation combined with other precursors for age-related maculopathies (ARM), then a large number of photoreceptors in central vision stop functioning when the person reaches their late sixties and early seventies. There are channels in the visual system tuned to different bands, approximately one octave, of spatial frequencies. In low vision observers with ARM, the loss of central vision causes a loss in channels sensitive to spatial frequencies above 8 to 10 cyc/deg. Therefore, for ARM observers, words must be magnified to read normal text. I have developed image enhancement filters that compensate for the low vision observer's losses in contrast sensitivity to intermediate and high spatial frequencies. These filters automatically enhance the text displayed on closed-circuit TVs (CCTVs) and render the text in shades of gray more easily perceivable than black and white text. These filters work by boosting the amplitude of the less visible intermediate spatial frequencies more than the lower spatial frequencies. Not only do these image enhancement filters reduce the magnification needed for reading by up to 70%, they also increase the speed that can be used to read text two to four times. A short summary of this research is presented.

The four basic types of laser-tissue interactions are governed by tissue properties (scattering, absorption) and laser parameters (wavelength, energy/power, pulse width, repetition rate). Table 1. summarizes these interactions and the corresponding laser parameters. Lasers may be categorized by their lasing media which may be gas (such as excimer, CO2, argon, metal vapor, He-Ne), solid (such as Nd:YAG, Ho:YAG, Er:YAG, Nd:YLF, semiconductor diode laser, alexandrite, Ti:sapphire), or liquid (such as dye laser). Lasers may also be defined by their emitting spectra (wavelengths) which range from the ultraviolet (such as excimer and highorder harmonic of solid- state lasers) , to the visible (such as argon , metal vapor lasers) , to the infrared (such as c02, YAG lasers). Lasers may be further defined by their operation modes and/or pulse duration which are either continuous wave (CW) or pulsed (such as free running, AO and EO Q- switched). Most of the medical applications of lasers are governed by the tissue reactions to a laser at a specific laser wavelength and pulse duration. For tissues consisting of water the laser penetration depth is inversely proportional to the tissue absorption coefficient which has major peaks at 1.45, 1.93 and 2.94 microns. Examples of medical lasers using the thermal effects caused by a tissue's strong absorption are lasers with wavelengths at 1.3- 1.6 ,.m (Nd:YAG), 1.96-2.1 tm (Ho:YAG) and 2.94 jm (Er:YAG) which are referred to as i.ot" lasers. Alternatively, excimer lasers such as XeCl and ArF, with wavelengths at 308 nm and 193 nm, are referred to as "cold' lasers which may achieve direct bond breaking in tissues with minimal thermal damage to the surrounding tissue. The "cold" laser may also be provided by the high-order harmonic of a solid state laser (such as Nd:YAG and Ti:sapphire) or a dye laser, where frequency conversion techniques using nonlinear crystals have been commonly employed.

This report describes the application of a recently developed spectroscopic technique, near- infrared-excited Fourier transform Raman scattering (abbreviated as near-IR FT-Raman) in the molecular-level characterization of normal and pathological human ocular tissues. The near-IR FT-Raman technique was shown to be particularly well-suited for the noninvasive analysis of intact ophthalmic samples because it exhibits such attractive features as complete fluorescence elimination, great sampling flexibility, high data acquisition speed, and measurement accuracy. For both intact human lenses and corneas, systematic FT-Raman studies were carried out in order to probe detailed molecular changes involved in cataract formation and cornea diseases. FT-Raman measurement was also made for synthetic biomaterials that can be attached to the corneal surface for laser refractive surgery. The technique of near-IR FT- Raman spectroscopy is potentially a nondestructive, non-invasive fingerprinting modality for monitoring lens aging, cataract formation, and corneal disease development.

There are a wide variety of reasons to link spectroscopy with time-series analysis1 and hence with the theory of random processes. While it remains true that the dominant harmonic analysis of spectroscopy is distributional Fourier theory, there are nonetheless good rationales for exploring other decompositions such as the one explored here (the canonical decomposition). One reason which motivates us the the necessity of discriminating tissue types by color spectrum. rfo do this efficiently, one seeks to mininiize the number of characteristic discriininants which describe the spectrum. By treating the spectrum as an instance of a random process, it is well-known that the eigenvalues ) of its canonical decomposition (or Karhunen-Loeve decomposition) , when ordered in decreasing order () )'2 )3 . . .) will typically decay very rapidly, and it follows that usually only the first few (ordered) eigenvalues are needed to characterize the spectrum.

A technique for visualizing the optic nerve, the short and long posterior ciliary arteries and nerves, the ophthalmic vein, and the rectus muscles using a flexible endoscope was developed in human cadavers. Previous reports of orbital endoscopy were limited to the use of a rigid endoscope. Potential applications for this new flexible endoscopic technique include the biopsy of orbital tumors, fenestration of the optic nerve sheath with mid-infrared lasers, photocoagulation of orbital tumors, and retrieval of foreign bodies or slipped muscles.

Laser thermokeratoplasty is a relatively new concept to correct hyperopia and astigmatism by placing coagulation spots into the corneal stroma around the optical center. A comparative study with an Er:Glass- (wavelength (lambda) equals 1.54 micrometers ) and a Cr:Ho:Tm:YAG-laser ((lambda) equals 2.12 micrometers ) has been started using different application systems; a bare fiber tip in contact to the cornea surface in the case of working with the Er:Glass-radiation and a non-contact focusing device in the case of applying the Cr:Tm:Ho:YAG-laser energy. Results in change in the corneal power of refraction up to 7 diopters demonstrate the capability of this method. Histological data of pulse energies up to 30 mJ of Er:Glass-radiation show the depth of the coagulation cones inside the corneal stroma and give proof of no alteration to the endothelium and Descemet's membrane.

A THC:YAG laser (thulium, holmium, chromium-doped YAG crystal) was used to create thermal sclerostomies in 35 glaucomatous eyes of 33 patients. The laser is a long-pulsed (300 microsecond(s) ec), compact, self-contained, solid state laser operating in the near infrared (2.1 (mu) ). A 1 mm conjunctival stab incision was made 12 mm away from the sclerostomy site to allow entry of a specially designed 22-gauge (712 (mu) ) optic probe that delivers energy at a right angle to the long axis of the fiber. Probe insertion produced minimal disturbance of the conjunctiva. Pulse energies of 80 mJ to 120 mJ were used with a repetition rate of 5 pulses/sec. Total energy levels to produce full-thickness sclerostomies ranged from 1.4 to 7.2 J. Subconjunctival 5-fluorouracil injections were administered in 32 eyes. Success rate was 66% at 6 months and 63% at 12 months. Mean intraocular pressure of successful cases was 14 mmHg at 6 months and 13 mmHg at 12 months. Ten cases failed within the initial six months, and one additional case failed by 12 months. Success and failure were analyzed relative to patient risk factors and clinical experience with the procedure.

A fistula from the anterior chamber of the eye into the subconjunctival space can be created by laser application ab externo (laser sclerostomy). The success of the procedure mainly depends on the special application system. The pulsed Erbium-YAG laser (2940 nm) was used as the energy source. The laser energy was guided to the application system via a ZrFl fiber with low attenuation at this wavelength. Because this fiber cannot be used in direct contact to the sclera, an optical coupling unit transmitted the energy to a short quartz fiber. This fiber was inserted in a specially sharpened retractable cannula to guide it into the subconjunctival space. Then the laser energy could be applied directly to the sclera to form the fistula. The procedure was demonstrated in vivo using rabbit eyes. A working fistula with formation of a filtering bleb could be achieved. The trauma to the conjunctiva was as minimal as in a subconjunctival injection. The minor alteration of the conjunctiva in this procedure compared to traditional surgical methods like goniotrepanation or trabeculectomy may cause less scarification and therefore less failure.

Ocular prosthesis requires great skill in craftsmanship and technology to match the appearance of the natural eye for cosmetic reasons. All the effort and cost going into the prosthesis is neutralized by the effect of a fixed pupil size. The possibility of replacing the fixed pupil in the prosthetic eye with a light controlled liquid crystal light valve (LCLV), to match the pupil size of the healthy eye, is discussed.

In this article the appearance, characteristic, and substance of two-way feedback of the human body are given. If we adopt compound physical and chemical stimulation for negative feedback it may improve the therapeutic effect.

Photoablation of ocular tissues can be performed with free running Er:lasers, but their application leads to significantly more thermal tissue damage than photoablation with ArF excimer lasers. This means a serious drawback to clinical investigations on treatments with Er:lasers. However, an essential reduction of the thermal damage zone is achieved by photoablation with a Q-switched Er:YAG laser (TEM_oo, tp equals 100 nsec). Detailed results of our work on bovine cornea are presented.