LASERS IN OPHTHALMOLOGY

Abdelmonem M. Hamed, M.D.Professor of OphthalmologyBenha College of Medicine

Fellow of Baylor College of Medicine, USA

1. Laser Construction2. Laser types3. Clinical application4. Laser Hazards5. Classification of Lasers6. Control Measures

LASERS IN OPHTHALMOLOGY

Laser Construction1

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A pump source A gain medium or laser medium. Mirrors forming an optical resonator.

Pump Source Provides energy to the laser system Examples: electrical discharges, flashlamps,

and chemical reactions. Ex. Excimer lasers use an electrical

discharges.

Gain Medium Major determining factor of the wavelength of

the laser. Excited by the pump source. Where spontaneous and stimulated emission

of photons takes place. Examples of gain medium:

solid, liquid, and gas.

Optical Resonator Two parallel mirrors placed around the gain

medium. Light is reflected by the mirrors back into the

medium and is amplified .

L(light) A(amplification by) S(stimulated) E(emission of) R(radiation)

1.stimulation

3. Amplification of light

2.Emission of radiation

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Laser Output

Continuous Output (CW) Pulsed Output (P)

                       

Ener

gy (W

atts

)

Time

Ener

gy (J

oule

s)

Time

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Laser Fundamentals The light emitted from a laser is

monochromatic (ie. one color/wavelength). Lasers emit light that is highly directional (ie.

narrow beam in a specific direction). The light from a laser is said to be coherent,

(ie. the wavelengths of the laser light are in phase in space and time).

it is more hazardous than ordinary light. Laser light can deposit a lot of energy within a small area.

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Incandescent vs. Laser Light

1. Many wavelengths2. Multidirectional3. Incoherent

1. Monochromatic2. Directional3. Coherent

2.Laser Types

Most common LASERS used in ophthalmology

1. Argon Photocoagulation2. Diode Photocoagulation3. Nd:YAG Photodisruption (yttrium-

aluminium garnet and neodymium)4. Excimer Photoablation

1.Photocoagulation Absorption of the laser light by tissue

pigments>>>>>>>>>The absorbed light is converted into heat >>>>>>>> coagulation of cellular elements.

Argon laser is based on this mechanism.

Therapeutic applications argon laser trabeculoplasty

(ALT) laser shrinkage of iris cyst. Diabetic retinopathy,

Therapeutic applications Intraocular tumours, For sealing of retinal

holes.

Complications of laser photocoagulation accidental foveal burns, macular oedema retinal haemorrhage retinal hole formation opacification of lens accidental corneal burns.

2. Photodisruption

ionization of the electrons of the tissue producing plasma>>>> This plasma expands>>>>>> pressure as high as 10 kilobars>>>>>> exerting a cutting/incising effect upon the tissues.

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Photodisruptioncontin…ed

Therapeutic applications of Nd:YAG laser includecapsulotomy for

opacefied posterior capsule

membranectomy for pupillary membranes.

Iridotomy for ACG

3. Photoablation Lasers based on this mechanism

produce UV light (193-nm) >>> breaks chemical bonds of biologic materials>>> converting them into small molecules that diffuse away.

Photoablation These lasers are called

excimer. These act by tissue modelling (sculpting).

Therapeutic applications of excimer lasers are:PRK, LASIK for correction

of refractive errors PTK for corneal opacity.

cool laser. precise. Refractive eye surgery (myopia, hyperopia, & astigmatism).

Excimer Laser

                                                                                         

                                                                                                                                                       

Excimer laser used for refractive eye surgery.

Gain medium: excited dimer = excimergas ArF (argon + fluorine)

Pump source: via electrical discharge. Produce light in the ultraviolet range.

Exciplex Wavelength (nm)ArF 193

Excimer Laser

The Photoablation Process

Laser light hits tissue>>>>>Collagen molecule excite>>>>>absorbs photons>>>>heated up>>>>chemical bonds break>>>hot gas/plasma of collagen fragments>>>>and water expantion and burst out ( 1-2 microseconds)

3. Clinical application of

laser in othalmology

LASIKCurrently this procedure is being considered the refractive surgery of choice for myopia of up to – 12 D

LASIK Laser in-situ

keratomileusis (LASIK). In this technique first a flap of 90-130 micron thickness of anterior corneal tissue is raised.

After creating a corneal flap midstromal tissue is ablated directly with an excimer laser.

PRK

Remove the epithelium 1st., then laser ablation by excimer laser

Laser trabeculoplasty (ALT)Indications: It should be considered in patients

where IOP is uncontrolled despite maximal tolerated medical therapy

It can also be considered as primary therapy where there is non-compliance to medical therapy

Technique and role of ALT in POAG Its hypotensive effect is caused by

increasing outflow facility, possibly by producing collagen shrinkage on the inner aspect of the trabecular meshwork and opening the intratrabecular spaces

It has been shown to lower IOP by 8-12 mm of Hg

The treatment regime Consists of 50 spots on the anterior half

of the trabecular meshwork over 180°.

Complications These include transient acute rise of

IOP, which can be prevented by pretreatment with pilocarpine and/or acetazolamide;

inflammation which can be lessened by use of topical steroids for 3-4 days.

Less commonly haemorrhage, uveitis, peripheral anterior synechiae and reduced accommodation may occur

YAG Laser iridotomy and capsulomy

Laser photocoagulation of the retina

Macular photocoagulation. Macula is treated by laser only if there is macular oedema

Panretinal photocoagulation (PRP) or scatter laser consists of 1200-1600 spots, each 500 μm in size. Laser burns are applied from the arcades extending peripherally to the equator

Laser photocoagulation of the retina

Endolaser is then applied around the area of retinal tears and holes to create chorioretinal adhesions.

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4. Laser Hazards

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Types of Laser Hazards

1. Eye : corneal or retinal burns, or cataract.

2. Skin : skin burns; while carcinogenesis may occur for ultraviolet wavelengths (290-320 nm).

3. Chemical : Some lasers require hazardous substances to operate (i.e. Excimer lasers).

4. Electrical : Most lasers utilize high voltages.

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5. Classification of Lasers and Laser

Systems

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Laser Safety Standards and Hazard Classification

Lasers are classified by hazard potential. (from class1 to class4), class 4 most dangerous laser

Necessary control measures are determined by these classifications.

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6. Control Measures and Personal Protective

Equipment

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CONTROL MEASURESEngineering Controls

Interlocks Red lamp on the laser room door.

Administrative Controls Standard Operating Procedures (SOPs) Training

Personnel Protective Equipment (PPE) Eye protection

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Common Laser Signs and Labels

WHY IS THE SKY BLUE- due to Ray scattering- much of the shorter wavelength light is absorbed by the gas molecules. - The absorbed blue light is then radiated in different directions. - Since you see the blue light from everywhere overhead, the sky looks blue.

Thank you for your

attention

Email: abdelmonem@hotmail.com