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Refractive Surgery
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Refractive Surgery
Refractive Surgery
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1
Question
What is the main principle of any refractive surgery procedure?
Answer
To change the refractive state of the eye towards emmetropia, where light rays are perfectly focused on the retina.
2
Question
Which two main components of the eye are targeted in refractive surgery to alter refractive power?
Answer
The cornea and the crystalline lens.
3
Question
What are the four principal types of refractive errors?
Answer
Myopia, hyperopia, astigmatism, and presbyopia.
4
Question
How can refractive surgical procedures be generally categorized?
Answer
They can be categorized as corneal or intraocular procedures.
5
Question
Name the types of keratorefractive procedures used in refractive surgery.
Answer
Incisional keratotomy, surface ablation, lamellar implant, corneal collagen shrinkage, and collagen crosslinking.
6
Question
What are the two main types of intraocular refractive procedures?
Answer
1) Phakic intraocular lens (PICL) implantation and 2) Refractive lens exchange with implantation of monofocal, multifocal, toric, or accommodative intraocular lenses (IOLs).
7
Question
What are the fixation types for Phakic intraocular lenses (PICL)?
Answer
Anterior chamber IOL angle fixated, iris fixated IOL, and posterior chamber IOL sulcus fixated.
8
Question
What part of the eye provides approximately two-thirds of the eye's refractive power?
Answer
The anterior corneal curvature, producing about +48 diopters of optical power.
9
Question
Why is the overall corneal power less than the anterior corneal curvature suggests?
Answer
Because the posterior corneal surface has a negative power of approximately -6 diopters, reducing the total corneal power to about +42 diopters.
10
Question
How are collagen fibrils arranged in the cornea and how does this relate to corneal biomechanics?
Answer
The cornea's collagen fibrils are arranged in approximately 200 parallel networks; this network provides mechanical strength to the cornea, with fibrils more densely packed in the anterior two-thirds.
11
Question
What types of laser-tissue interactions are used in keratorefractive surgery?
Answer
Photoablation, photodisruption, and photothermal effects.
12
Question
Explain photoablation in the context of refractive surgery.
Answer
Photoablation breaks chemical bonds in tissue using excimer laser ultraviolet light (193 nm), allowing precise removal of corneal tissue.
13
Question
What is photodisruption and how is it used in refractive surgery?
Answer
Photodisruption uses a 1053 nm infrared beam to transform tissue into plasma creating microcystic cavities, which allows the formation of flaps or channels in corneal stroma.
14
Question
Describe photothermal effects in refractive surgery.
Answer
Photothermal effects are achieved using a holmium:YAG laser focused on the anterior stroma; absorbed by water, this causes collagen shrinkage.
15
Question
What type of laser is primarily used for corneal ablation in refractive surgery and its wavelength?
Answer
The excimer laser, which emits ultraviolet rays at 193 nm.
16
Question
Contrast excimer lasers with non-excimer solid state lasers used in refractive surgery.
Answer
Excimer lasers use an excited dimer of inert gases and halides releasing UV light at 193 nm, while non-excimer solid state lasers like Q-switched diode pumped lasers operate at 210 nm or 213 nm wavelengths.
17
Question
What are the three photoablating laser delivery types and their characteristics?
Answer
1) Broad-beam lasers: ~7 mm diameter, high energy per pulse, few pulses needed; 2) Scanning-slit lasers; 3) Flying spot lasers: smaller beam (0.5-2.0 mm), higher repetition rate, require tracking mechanisms.
18
Question
Explain the principle of excimer laser photoablation in treating myopia.
Answer
It removes central corneal tissue to reduce the central curvature, thus decreasing the eye's refractive power to correct myopia.
19
Question
How does multizone keratectomy improve treatment of high myopia?
Answer
It uses multiple concentric optical zones to distribute ablation depth, allowing higher myopic corrections with shallower ablation in the central zone and tapering in peripheral zones to reduce symptoms.
20
Question
Describe the pattern of tissue removal in hyperopic refractive treatment.
Answer
Removing a doughnut-shaped portion of midperipheral corneal tissue to steepen the central cornea.
21
Question
How does the ablation zone in hyperopic treatment differ from myopic treatment?
Answer
In hyperopia, the ablation zone is larger than the optical zone, with maximal ablation at the outer edge of the optical zone, unlike central ablation used in myopia.
22
Question
What are the main surface ablation techniques in keratorefractive procedures?
Answer
The main surface ablation techniques include PRK (Photorefractive Keratectomy), LASIK (Laser-Assisted In Situ Keratomileusis), and LASEK.
23
Question
What type of laser is used in Photorefractive Keratectomy (PRK), and why?
Answer
An excimer laser is used in PRK because it can accurately ablate corneal tissue to an exact depth with minimal disruption of the surrounding tissue.
24
Question
What refractive errors can PRK correct, and up to what diopters?
Answer
PRK can correct myopia up to -6D, hypermetropia up to +3D, and astigmatism up to +3D.
25
Question
What are the indications for performing PRK?
Answer
Indications include irregular cornea, thin cornea, epithelial basement membrane disease, previous corneal surgery (e.g., penetrating keratoplasty or radial keratotomy), treatment of some LASIK flap complications, and LASIK enhancement when the stromal bed is insufficient.
26
Question
Describe the basic technique of PRK.
Answer
The visual axis is marked, the corneal epithelium is removed either by mechanical scraping or 20% diluted alcohol, the patient fixates on the laser's aiming beam, and the laser ablates the Bowman's layer and anterior stroma for 30 to 60 seconds.
27
Question
How long does the cornea typically take to heal after PRK, and what aids this process?
Answer
The cornea usually heals within 48 to 72 hours, aided by a bandage contact lens.
28
Question
What is a common postoperative complication of PRK related to the cornea's healing, and how long does it last?
Answer
A subepithelial haze develops within 2 weeks, can persist for 1 to 6 months, and is usually mild, rarely causing diminished visual acuity but may cause nocturnal glare.
29
Question
How can corneal haze, a major risk of PRK, be reduced?
Answer
Corneal haze can be reduced by using adjunctive mitomycin C during or after the procedure.
30
Question
List common complications of PRK.
Answer
Complications include decentration of ablation (causing diplopia, glare, halos, and astigmatism), corneal haze (especially in high myopia), overcorrection, undercorrection, central island formation, optical aberrations, and infectious keratitis.