Noncontact Laser Photothermal Keratoplasty II: Refractive Effects and Treatment Parameters in Cadaver Eyes

BACKGROUND: Noncontact laser photothermal keratoplasty may provide a new alternative for the treatment of myopia, hyperopia, and astigmatism. The purpose of this article is to study the refractive effect that laser photoablation keratoplasty is capable of producing on a normal human cadaver cornea, including the relationship between the keratometric changes and laser treatment parameters. METHODS: The human cadaver eyes were treated with a hohnium laser (pulsed Ho:YAG, 2.10 µp?, 250 µß) coupled to a maskable, polyprismatic delivery system mounted on either an optical bench or a slit-lamp microscope. Using a topographic videokeratography system, we first investigated the refractive effect that noncontact laser photothermal keratoplasty would produce on a normal cadaver cornea. We then studied the keratometric changes produced by different radiant exposure levels at a fixed treatment pattern, as well as by different treatment patterns at a fixed radiant exposure level. Finally, we studied the possible therapeutic application of laser photothermal keratoplasty for correcting high postoperative astigmatism on a cadaver eye model. RESULTS: For the single-pulse 3-millimeter ring of eight-spot treatment, the keratometric power of the cornea initially increased with the radiant exposure and peaked at 26 J/cm2. The refractive effect was increased by projecting an additional set of eight spots equidistant between the first eight spots on the same diameter ring. Eighteen J/cm2 was the minimal radiant exposure required to produce consistent and predictable keratometric changes. The corneas were flattened using treatment patterns smaller than or equal to 3 mm in diameter and steepened using treatment patterns larger than or equal to 5 mm in diameter. A transition zone between 4 and 5 mm was observed in which minimal and unpredictable keratometric changes of the central cornea occurred. The surgically-induced astigmatism (> 10.00 D) was corrected by progressive laser photothermal keratoplasty treatments. CONCLUSIONS: Laser photothermal keratoplasty can acutely steepen and flatten the cornea in human cadaver eyes. [J Refract Corneal Surg. 1994;10:519-528.! RESUME INTRODUCTION: La kératoplastie photothermique sans contact nous donnera peut être une alternative pour le traitement de la myopie, de l'hypermétropie, et de l'astigmatisme. Nous avons étudié l'effet réfraetif de ce laser sur Ia cornée cadavérique humaine et le rapport entre les changements kératométriques et les paramètres du laser. METHODES: Les yeux furent traités par laser holmium (à pulsation Ho:YAG, 2,10 µp?, 250 µß) couplé au système de transmission polyprismatique installé sur un banc optique ou une lampe à fente. En utilisant la vidéokératographie, nous avons évalué l'effet réfractif du laser en variant la lumination radiante et le motif de traitement. On a ensuite étudié la possibilité des corrections dans le cas d'astigmatisme.