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.