Paul P. van Saarloos

Laser-induced damage to transparent polymers: chemical effect of short-pulsed (Q-switched) Nd:YAG laser radiation on ophthalmic acrylic biomaterials. I. A review.

The use of short-pulsed lasers in ophthalmic surgery inspired and called for research on the damage inflicted by the laser radiation upon the acrylic polymers from which artificial intraocular lenses are made. The possible release of toxic monomers by laser-induced depolymerization is of great concern but past investigations of this phenomenon have been very limited. The present knowledge of various types of laser-induced damage to transparent polymers is reviewed with particular emphasis on the acrylic materials and intraocular lenses.

Bovine corneal stroma ablation rate with 193-nm excimer laser radiation: quantitative measurement.

Excimer lasers operating at 193 nm may become important surgical instruments in ophthalmology because of their ability to ablate tissue with excellent precision and minimal damage to adjacent tissue. However, the precision is limited by the accuracy of the measurement of the amount of tissue ablated per pulse at the fluence used. A measurement of the ablation rate of bovine corneal stroma over the range of fluences most likely to be useful for corneal surgery (50-400 mJ/cm2) is presented. The technique used produced reproducible results, the data from 47 animal eyes being averaged to further increase the precision. For this range of fluences, these results show a more precise measurement than previously published tissue ablation rate data. These results should be useful in accurately predicting the result of corneal surgery using the excimer laser if species differences are not major.

Histological changes and unscheduled DNA synthesis in the rabbit cornea following 213-nm, 193-nm, and 266-nm irradiation.

PURPOSE To examine the acute outcome of corneal irradiation in adult rabbits for 193-, 213-, and 266-nm laser wavelengths. METHODS Animals were randomly allocated to one of three groups and were treated with 213-nm quintupled Nd:YAG laser, a 193-nm excimer laser, or a 266-nm quadrupled Nd:YAG laser (n = 6 per group, two exposure durations). Thermal damage was assessed histologically and the extent of DNA damage estimated by measuring unscheduled DNA synthesis in corneal epithelial and stromal cells using 3H thymidine autoradiography. RESULTS For the 193- and 213-nm groups, irradiation did not induce thermal damage. Moreover, cells displaying unscheduled DNA synthesis represented < 4% of the total cell numbers with no difference between groups (P > .05). By contrast, the 266-nm laser led to stromal melting and vacuolation; unscheduled DNA synthesis levels were elevated over the other groups (P < .05). CONCLUSIONS Corneal laser ablation with the 213-nm Nd:YAG laser resulted in similarly low levels of thermal and DNA damage to those produced using the clinically accepted 193-nm excimer laser.

Laser-induced damage to transparent polymers: chemical effect of short-pulsed (Q-switched) Nd:YAG laser radiation on ophthalmic acrylic biomaterials: II. Study of monomer release from artificial intraocular lenses

Commercial intraocular lenses and polymer specimens, both poly(methyl methacrylate) (PMMA) and poly(2-hydroxyethyl methacrylate) p(HEMA), were subjected to various levels of irradiation from a Q-switched Nd:YAG laser, and then to extraction and sample-enrichment techniques. The extracts were analysed by capillary gas chromatography. In PMMA samples, residual contents of 0.05-0.89% (wt) MMA were found and it appeared that the laser radiation did not cause a detectable increase of the monomer content. No residual HEMA could be found in p(HEMA) samples before or after laser bombardment. It is concluded that the accidental exposure of lenses to Nd:YAG laser cannot produce a significant release of monomers. Depolymerization induced by laser is a process unlikely to occur at the energy levels used in ophthalmic surgery.

Perspectives on corneal topography: a review of videokeratoscopy

This article reviews the optical principles of computer‐assisted videokeratoscopy (CVK) and provides a guide to the differences between short and long working distance systems. The description of the corneal surface involves a number of complex issues which have yet to be adequately resolved and the importance of alignment and reference axis assumptions to CVK is discussed. With the increasing clinical use of such systems, the debate of such issues has meaning, not just in the research environment but also in routine clinical practice. A number of applications are illustrated.

Corneal topography bow-tie pattern: Artifact of videokeratoscopy?

Purpose: To test the hypothesis that the bow‐tie corneal topography pattern results from corneal asphericity in the presence of astigmatism. Methods: Astigmatic color‐coded power maps using different shape factors were computer generated. Each simulation was based on the calculation of dioptric power at 20 points along each of 180 hemimeridia, for a total of 3600 points. The calculations were made independent of the capture or measurement of videokeratographs. These simulations were compared to power maps taken from clinical records. Results: A shape factor of 1.00 resulted in a spherocylinder color‐coded map with straight‐edged sectors of power. The familiar bow‐tie pattern was generated using an elliptical model with a shape factor of less than 1.00. This pattern was reversed by modeling the cornea as an oblate ellipsoid using a shape factor greater than 1.00. Conclusions: By simple alteration of the amount of corneal asphericity through manipulation of the shape factor, computer simulation showed that this surface characteristic is responsible for the bow‐tie pattern observed in corneal topography power maps.

Comparing methods to view corneal topography data

Background: Graphical display of the many thousands of data points generated by corneal topography systems has progressed from three‐dimensional wire representations, through curvature and power maps, to three‐dimensional subtraction models. Standard colour‐coded corneal ‘topographic’ maps may be misinterpreted as representing corneal profile instead of a form of corneal power. A variety of two‐ and three‐dimensional output methods for displaying corneal topography data are demonstrated.

Development of a versatile stereo scanning laser ophthalmoscope

The scanning laser ophthalmoscope (SLO) is a modern tool which is now widely used to image the fundus of the eye, particularly for assessment of the optic nerve head. We describe a modified SLO capable of producing stereo pairs of the optic disk in real time. A pair of toggling mirrors is used to switch between entry positions of the scanned laser beam into the pupil of the eye thereby creating a stereo base for capturing the two different views required for the pair. Our laboratory prototype is constructed from reflective optics only in the bi-directional part of the beam path, including the focusing and beam shaping unit. Thus, we avoid unwanted back reflections and chromatic aberrations. Light from different laser sources (458 to 1100 nm) can be launched into the SLO, also simultaneously. Collimated beams in beam splitting locations allow for easy modifications. Imaging in fluorescence mode or polarization dependent imaging is also possible. High quality multi-wavelengths stereo pairs of both model and real optic disks were obtained. a lateral resolution of up to 6 micrometer and an axial resolution of up to 65 micrometer was established.

Comparison of different imaging modes for scanning laser ophthalmoscopes

We describe a bench top system for digital scanning laser ophthalmoscopy. This system is used for both regular patient screening and experimental imaging studies. The complete set- up is assembled from a number of modules (e.g. launching, detection, scanning, focusing unit) which may be altered readily to offer a high degree of flexibility in the imaging conditions. Both the launching and the detection unit can be used in a simultaneous, multiple channel configuration. This allows the acquisition of true color and false color images of the back of the eye. In particular, the use of infrared lines permits the investigation of deeper retinal structures. Digital image processing methods can then be used to generate differential images of frames taken with different wavelengths, such as red and infrared. Furthermore, the separate detection channels can also be used to obtain recordings in other imaging modes, such as tightly confocal, loosely confocal, indirect or polarization dependent contrast. We discuss the merits and problems of different imaging modes. Cases are presented where the differential imaging shows clear advantages over the standard (monochromatic) confocal method in the perceptibility of deeper laying structures (choriod).

How detrimental is eye movement during photorefractive Keratectomy to the patient's post-operative vision?

This study aimed to gauge the effect of the patients eye movement during Photo Refractive Keratectomy (PRK) on post- operative vision. A computer simulation of both the PRK procedure and the visual outcome has been performed. The PRK simulation incorporated the pattern of movement of the laser beam to perform a given correction, the beam characteristics, an initial corneal profile, and an eye movement scenario; and generated the corrected corneal profile. The regrowth of the epithelium was simulated by selecting the smoothing filter which, when applied to a corrected cornea with no patient eye movement, produced similar ray tracing results to the original corneal model. Ray tracing several objects, such as letters of various contrast and sizes was performed to assess the quality of the post-operative vision. Eye movement scenarios included no eye movement, constant decentration and normally distributed random eye movement of varying magnitudes. Random eye movement of even small amounts, such as 50 microns reduces the contrast sensitivity of the image. Constant decentration decenters the projected image on the retina, and in extreme cases can lead to astigmatism. Eye movements of the magnitude expected during laser refractive surgery have minimal effect on the final visual outcome.