Which of the following reflects wavelength (nanometers) of Laser used for shaping cornea in refractive surgery:
A i.e. 193 - Corneal reshaping refractive procedures such as photorefractive keratectomy (PRK), laser assisted epithelial keratomileusis (LASEK), & laser assisted insitu keratomileusis (LASIK), used to correct refractive errors (myopia and hypermetropia), are ultraviolet-photoablation procedures using Excimer (also k/a argon fluoride/Ar-F- excimer) laser of ultraviolet (193 nm) wave lengthQ. Myopia is treated by ablating the central anterior corneal surface so that it becomes flatter; 10um ablation corrects 1D myopia. Hypermetropia is corrected by ablation of periphery so that the centre becomes steeper. Excimer laser (193 nm) corneal reshaping refractory surgery is of 2 types. In PRK the anterior surface of cornea is ablated by laser, which can correct myopia upto 6D, astigmatism upto 3D and low hypermetropia. The more popular other method is LASIK, in which superficial circular flap of epithelium & stroma is cut with microkeratome (an automated sharp blade), lifting up 120-150 pm thick hinged flap attached on one side. The stromal bed is laser ablated to desired extent ensuring minimal stromal bed thickness of 250 pm (to maintain stability & prevent iatrogenic ectasia). Following this the flap is repositioned without sutures; this can corret hypermetropia upto 4D, astigmatism upto 5D and myopia upto 12D. Modifications to minimize microkeratome related complications include use of 20% alcohol (30-40 seconds) to lift / clear thin epithelial flap (LASEK), use of special microkeratome to cut & separate a thin flap of epithelium & Bowman's layer (Epi - LASIK) and use of femto second laser to pre fashion the anterior flap to a desired size and depth without using a surgical blade Refractive Keratoplasty to modify refractive power of cornea may be done by: (i) radial keratotomy (radial incisions are made in cornea); (ii) arcuate keratotomy (arcuate shaped incisions in steeper or more myopic meridian to reduce astigmatism); (iii) Keratomileusis (tissue resected & modified in shape before replacement) and (iv) epikeratophakia, where donor corneal tissue is attached to the host cornea after removal of epithelium capsulectomy in an eye with a silicone intraocular lens implant is challenging because optical breakdown occurs in silicone at relatively low power, therefore damaging IOL even when the laser is focused posterior to implant. Procedure is even more difficult if capsule is in intimate contact with the implant. Use of corneal contact lens during laser capsulectomy result in steeper convergence & more sharply focused Nd YAG laser beam. - IOL made of PMMA and acrylic are less susceptible to optical breakdown than silicone lenses. Photodisruption (PD) - PD is a mechanical effect produced by highly focused laser (with high electromagnetic field strength), which can actually strip electrons from their nuclei, destroying chemical nature of material producing an entirely different physical state of matter called plasma. - In plasma, the orderly array of molecules is fractured into a random mixture of electron & protons in a process called optical break down, which is basically a miniature lightening bolt or thunderclap. Vapors formed by lightning bolt expands, quickly collapse and produce a miniature thunder clap. Acoustic shock waves produced from thunderclap cause most of the tissue damage. - PD optical breakdown requires electro magnetic fields so powerful they can be produced only by concentrating lasr energy into very brief periods, thereby giving each pulse an extremely high power. Infrared Nd: YAG laser (1064nm) with Q switching or mode locking pulsing is used for PD. Q switching is most commonly used, inexpansive and reliable but cant produce pulses as sho or powerful as mode locking. Whereas, mode locking are expansive and difficult to maintain but more powerful. - Nd: YAG laser is infrared, invisible, so aiming system (a red helium-neon / He-Ne laser) is necessary. However, the patient's eye cause the red aiming beam to bend more than Nd:YAG's infrared laser. Therefore, the focus of both rarely coincide precisely (chromatic aberration). Photodisruption with infrared Nd: YAG laser (1064nm) is used in performing posterior lens capsulectomyQ. Photocoagulation (PC) - In PC, laser light (mostly visible or sometimes infrared) is absorbed by the target tissue or by neighbouring tissue, generating heat that coagulates (denatures) proteins. - It is the most commonly used laser procedure performed during panretinal photocoagulation, argon laser trabeculoplasty, peripheral iridecotomy, and thermal destruction of choroidal neovascular membranes. It is used in treatment of various conditions. Anterior segment PC Posterior segment PC - lridoplasty - Proliferative DR - Iridotomy - Diabetic macular edema - Trabeculoplasty - Choroidal neovascularization - Cyclophoto secondary to age related macular coagulation degeneration - Retinal breaks - Retinal detachments - Types of laser that produce PC include Laser Wave length (nm) Argon Blue- Green 488 Argon-Green 514 Frequency doubled (2x) Nd-YAG (green) 532 Rhodamine 6G Organic tunable Dye laser (Yellow-Red) 570 to 630 Krypton-Red 647 Ruby - Red 694 Diode (near infrared) 810 - Increase in exposure time modestly increases the lesion's diameter and also extends damage deeper into target tissue (eg 10 fold ET doubles diameter). Whereas very brief exposure times (0.01-0.05 seconds), allow little time for heat to dissipate from the burn. Therefore a small area of intense burn is produced, resulting in perforation of delicate ocular structures like neural retina & Bruch's membrane. - An increase in power has strong influence on lesion diameter (doubling power doubles size) but creates more damage and can be painful. In most PC lasers, the energy is concentrated in the centre of beam, with less energy at edges. Therefore, use of excessive power during laser treatment of retina may result in inadveent retinal hole. Also the lower energy at periphery may produce permanent tissue damage even though it does not produce a visible reaction. This means the area of laser damage may extend beyond the area of immediately visible reaction. In sensitive areas such as macula, a small spot size (100pm) is preferred so as to minimize unnecessary adjacent damage. In contrast, larger spot size (200-500pm) is preferred for panretinal photocoagulation. - Laser contact lenses also increase spot size Laser contact lens Multiplication factor of increased spot size Goldman three mirror lens 1.08 Panfundoscopic lens 1.41 Mainster wide angle lens 1.47 Quadr Aspheric lens 1.92 If the spot size is increased, the power needs to be increased. However, because energy is concentrated in centre of beam, it is best to raise power only modestly (no more than two folds at a time) and to test burns to refine the power settings. Photoablation (PA) - PA breaks chemical bonds that hold tissue together, essentially vaporizing the tissue. Chemical bonds are broken by absorption of Photons without any external mechanical pressure. Therefore, the laser is able to remove tissue with more precision and with much less damage to surrounding tissue. - It is most recent light tissue interaction using argon-fluoride (Ar-F) excimer laser producing electro magnetic energy with a wave length of 193 nm (in the extreme ultraviolet). - Photoablation used for shaping cornea in refractive surgery and to treat corneal pathology such as corneal ulcers and scars. - Excimer laser removes --0.1 mm of corneal tissue with each pulse. Beam shaping optics are used to create even beam profile; however, these are ablated slowly & must be replaced periodically. - Irregularities in corneal stroma, presence of keratinocytes, & variable corneal collagen density (depending on altitude, atmospheric pressure, humidity, age and duration of procedure) may cause uneven ablation even when beam profile is uniform.
193
451
532
1064