Lec 8

AlMughtaribeenUniversity

Refractive errors

Lasers in Ophthalmology : In ophthalmology, various types of lasers are being applied today for either diagnostic or therapeutic purposes. One major diagnostic tool is confocal laser microscopy which allows the detection of early stages of retinal detachment and also glaucoma1 can berecognized in time to increase the probability of successful treatment. In this lecture our interest focuses on therapeutic laser applications.The first indications for laser treatment were given by detachments of theretina. Meanwhile, this kind of surgery has turned into a well-establishedtool and only represents a minor part of today’s ophthalmic laser procedures.Others are, for instance, treatment of glaucoma and cataract. And, recently, refractive corneal surgery has become a major field of research.The targets of all therapeutic laser treatments of the eye can be classifiedinto front and rear segments. The front segments consist of the cornea and lens. The rear segments are given by the vitreous body and retina.

Mr.Gazy Khatmi Email _ [email protected] University - College of Medicien

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Cornea and lens

Cornea and lens together account for the total refraction of the eye.

The power of the cornea is approximately 42 diopters, whereas the total power ofthe eye is roughly 59 diopters. Therefore, about 70 % of the overall refractionarises from the cornea.

The transparency of corneal tissue in the spectral region from 400nm to 1200nm

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4 Common Visions Problems

Myopia Nearsightedness Nearby objects can be seen

perfectly while objects at distance appear blurry

Light rays occur in front of retina

Hyperopia Opposite of myopia Farsightedness Can see distant object

more clearly than nearby objects

Light rays focus behind retina

Astigmatism People who suffer myopia

and hyperopia will suffer astigmastism also

Object both far and near appear blurry

Light entering the eyeball focuses on multiple areas rather than on retina

Presbyopia Common vision

problem for old age Dependent with

reading glasses Human eye loss

flexibility of the lens Weakening in the

muscle

Non-Surgical Correction OptionsGlassesContact Lenses

Surgical Correction Option:LASIK

Laser Eye Interaction

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Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid through the outside surface and the aqueous humour through the inside surface

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List of laser in medicine field :

What is LASIK?LASIK stands for Laser-Assisted In Situ Keratomileusis and is a procedure that permanently changes the shape of the cornea, the clear covering of the front of the eye, using an excimer laser.

LASIK is the most advance form of laser vision correction that is currently available.

How LASIK is Performed Step 1. A suction ring is centered over the cornea of the eye

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Step 2: The microkeratome creates a partial flap in the cornea of uniform thickness

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Step 3:

The corneal flap is folded back on the hinge exposing the middle portion of the cornea.

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Step 4:

The excimer laser is then used to remove tissue and reshape the center of the cornea.

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Step 5:

In the final step, the hinged flap is folded back into its original position.

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LASER TISSUE INTERACTIONLASER TISSUE INTERACTION

LASER

TISSUE

Thermal Effect

Photo-chemical

Ionizing Effect

Photocoagulation Photodisruption Photoablation Photovaporization

Temperature Biological effect

37◦C Normal 45◦C Hyperthermia 100◦C Vaporization, > 100◦C Carbonization > 300◦C Melting

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Photoablation

In Fig. 3.30, the cross-section of a cornea is shown which was exposed to an ArF excimer laser. The removal of tissue was performed in a very clean andexact fashion without any appearance of thermal damage such as coagulation or vaporization. Instead, evidence is given that the tissue was very precisely “etched”. This kind of UV light-induced ablation is called photoablation .

Photoablation was first discovered by Srinivasan and Mayne-Banton(1982). They identified it as ablative photodecomposition, meaning that material is decomposed when exposed to high intense laser irradiation. The ablation depth, i.e. the depth of tissue removal per pulse, is determined by the pulse energy up to a certain saturation limit. The geometry of the ablation pattern itself is defined bythe spatial parameters of the laser beam. The main advantages of this ablation technique lie in the precision of the etching process as demonstrated in Fig. 3.30, its excellent predictability, and the lack of thermal damage to adjacent tissue. biological tissues ablated. Today, photoablation is one of the most successful techniques for refractive cornealsurgery, where the refractive power of the cornea is altered in myopia, hyperopia, or astigmatism

Summary of Photoablation

• Main idea: direct breaking of molecular bonds by highenergy UV photons

Observations: very clean ablation,• Typical lasers: excimer lasers, e.g. ArF, KrF, XeCl, XeF

• Typical pulse durations: 10 ... 100ns• Typical power densities: 107 ... 1010 W/cm2• Special applications: refractive corneal surgery

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Quick Quiz :

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