Phacoemulsification

A phaco machine consists of the machine, a hand held probe and a foot switch.. The process of phacoemulsification consists of two main elements -

a. ultrasound energy to emulsify (crush) the cataract and
b. fluidic circuit to remove the crushed matter, i.e. the emulsate through the tiny cut.

Ultrasonic energy is produced by piezoelectric crystal oscillating between 20,000 to 60,000 cycles / sec., situated in the hand piece. This energy is utilised to break the cataract into pieces and to crush them, so that they can be removed through a smaller incision.

Fluid circuit is supplied with a high hung saline bottle which supplies the fluid volume and pressure in the anterior chamber of the eye. The fluid circuit is regulated by a pump. This pump attracts the cataract pieces to the phaco tip (probe). When the tip is occluded, vacuum provides firm grip and also helps to clear the emulsate from the anterior chamber of the eye. The foot switch is used to control the power and vacuum.

The procedure needs a lot of skill and practice to use the correct parameters and control them precisely; to remove the cataract without damaging the other parts of the eye.

Lasers in general

LASER is an acronym for Light Amplification by Stimulated Emission or Radiation. When an electron returns to a lower energy level form a higher energy level it emits radiation. The radiation is released in the form of discrete packets or particles called photons. This radiation may be visible when its wavelength is within 380 to 780 nm range. If the source of light is made to emit light of one definite wavelength, one of the characters of lasers is met. All the electrons need to arrive at the lower energy level at the same time so that the radiation is emitted simultaneously, and this phenomenon is called spatial coherence. Lasers also require a high density of electrons to be effective.

Monochromatic light can be achieved by filters or more popularly by using an optical chamber which is filled by a gas or solid. The length of the chamber is in multiples of the required wavelength. A standing wave is generated in a resonating cavity. Light is reflected with the mirrors at the ends. Laser is fired when a controlled amount of light is allowed through a semi reflective mirror.

Getting the light in the same phase, i.e. spatial coherence is a challenge, as in most of the elements the electrons return to the lower energy state immediately. This is achieved by using a ruby crystal with little chromium, which provides some stability (half life) the excited electrons. Energy is transferred to this crystal by pumping, with a light source.

All the commercial lasers have a property of sustaining an inverted population of electrons.

In Nd:YAG laser (yttrium- aluminium-garnet) laser, the laser releases all the energy from the inverted population each time the laser fires, and the laser therefore becomes pulsed. This is very useful when high energy emissions are required as in photodisruption.

Lasers can be continuous and used at lower energy levels and provides energy for photocoagulation.

Lasers in Ophthalmology

Lasers find many applications in ophthalmology and the major applications are discussed below. For a laser to be effective it must be transmitted through the ocular media so that it can enter the eye and be absorbed by chromophore. Effect of laser will depend on the absorption and the temperature rise at the site, and the responses of different tissues are different.

a. Photocoagulaters - These are continuous wave lasers, and include argon green, blue green, krypton red, diode lasers and adjustable dye. When the temperature rises above certain critical temperature, the tissues coagulate. A continuous beam is produced but a small amount is allowed to escape through. This itself or a separate diode laser beam is used as the aiming beam. The laser is attached to a slit lamp microscope, so that the beam can be directed accurately and the spot size of the beam adjusted. Lasers with indirect ophthalmoscope delivery systems also have been developed which can help treating peripheral retina. The temperatures typically rise by 30^o C, which is sufficient for coagulations.

b. Photodisrupters - This laser produces high energy pulses that are sufficient to disrupt intraocular tissues and membranes. Most popular of them is Nd:YAG laser. This laser delivers high energy in pulses and results in plasma formation which is used to disrupt intra ocular tissues, in simple terms, punch holes. The temperature is raised in the region of 20,000^o C, for a short time (in nanoseconds) and very accurately focused for photodisruption to take effect.

c. Photoablation - the photoablation laser is discussed under excimer laser below.

Excimer Laser

The word excimer comes from excited dimer, which in turn means an excited molecule with two identical components. The laser is composed of a gas mixture with two different molecules. The mixture consists of an inert gas - Ar. Xe or Kr and a halogen (fluoride, chloride or bromide). The application of high voltage current (to the tune of 30,000 eV) leads to formation of unstable inert gas - halide molecules, which dissociates quickly emitting ultraviolet light. The ultraviolet photons break intramolecular bonds without disturbing the neighbouring structures. This process is called ablative photocomposition or photoablation. The laser is first absorbed by the tissues and it results in breakage of intramolecular bonds. The decomposed tissue is ejected by the kinetic energy of the photons out of the surgical plane. The energy density is between 50 and 300 mJ/cm2, the removal of corneal tissues is precise. Each laser pulse ablates a tissue layer of about 0.25 um thickness and converts them into gaseous material.

LASIK

LASIK stands for Laser-in-situ keratomileusis, and is the latest technique of excimer laser refractive surgery. LASIK can correct moderate and high degrees of myopia, with or without astigmatism and hypermetropia. LASIK involves the use of a microkeratome to prepare a corneal flap of a uniform thickness. The excimer laser is then used to ablate the superficial corneal stromal tissue. The flap is then placed back at its original position without sutures.

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