What is the refractive error range discussed for surgery?
Click to see answer
+4.00 to -10.00 D.
Click to see question
What is the refractive error range discussed for surgery?
+4.00 to -10.00 D.
What is the purpose of clear lens extraction?
To remove noncataractous lenses and insert an intraocular lens to correct spherical refractive errors.
What is the effect of corneal incisions that are closer to the visual axis?
They have the greatest effect on central corneal curvature.
What has superseded the use of cautery in thermokeratoplasty?
The Holmium:YAG laser.
What does the Epikeratophakia procedure involve?
Attaching a lenticule of pre-shaped donor corneal stroma to the surface of the host cornea.
What is a key characteristic of the Epikeratophakia procedure?
The eye is not entered, and the procedure can be reversed by removing the lenticule.
What refractive outcome might be aimed for patients wishing to dispense with glasses for near tasks?
-2.50 D.
What type of refractive error can keratomileusis correct?
High degrees of myopia, but not hypermetropia.
What happens to the refractive index of the lens when silicone oil is present?
It changes the posterior surface of the lens from a converging to a diverging interface.
What is true about the angles formed by parallel lines AB and CD intersecting line EF?
The angles α formed are all equal.
What is the advantage of placing incisions wholly within the graft?
It conserves the wound and has a more predictable effect.
What happens to corneal topography if compression sutures are removed?
The topography will change, potentially leading to overcorrection or degeneration.
What tool allows for adjustments during surgery to leave the cornea roughly spherical?
Keratoscopy.
What advantage does gas filling provide for indirect ophthalmoscopy in a phakic eye?
It may allow indirect ophthalmoscopy without the use of a condensing lens.
How can high degrees of astigmatism after penetrating keratoplasty be corrected?
By removing a deep arcuate wedge measuring 60–90° from the graft–host junction in the flattest meridian.
What are phakic intraocular lenses used for?
To correct refractive errors in phakic eyes.
What is a significant concern regarding phakic intraocular lenses?
The long-term effects on the corneal endothelium, crystalline lens, and iridocorneal angle are unknown.
Why have keratomileusis and keratophakia not been widely adopted?
Because they are technically difficult and the refractive outcome is often unpredictable.
What is a challenge in correcting post-keratoplasty astigmatism?
The meridia of maximum and minimum curvature may not be at right angles to each other.
What refractive change can occur due to the presence of silicone oil in the posterior segment of the eye?
It induces a hypermetropic shift in refraction of +5.00 to +7.00 D.
What is the typical hypermetropia resulting from aphakia?
+4.00 to +6.00 D.
How can relaxing incisions be combined to reduce large degrees of corneal astigmatism?
By placing compression sutures 90° away from them.
What is keratomileusis?
The use of a microkeratome to remove a lamella of anterior corneal stroma, which is then shaped on a cryolathe before being replaced.
How is the success of surgical correction of astigmatism often measured?
By the post-operative tolerance of spectacles or contact lenses to correct the residual refractive error.
What determines the refractive outcome of penetrating keratoplasty?
The dimensions of the donor tissue.
How does silicone oil compare to the crystalline lens in terms of convergence?
Silicone oil is more strongly converging than the crystalline lens.
What material is used in the Intrastromal Corneal Ring procedure?
PMMA (Polymethyl methacrylate) split ring.
What effect does a tight suture placed across the graft–host junction have?
It increases the curvature of the cornea and reduces astigmatism.
Why are linear incisions less favorable?
They pass through cornea of varying thickness and parts are closer to the visual axis.
What condition is epikeratophakia most commonly employed to correct?
Keratoconus.
How long does corneal topography change after discontinuing contact lens wear?
Fifteen weeks for PMMA, ten weeks for gas permeable, and five weeks for soft lenses.
What is the effect of a graft whose diameter exceeds that of the tissue removed in penetrating keratoplasty?
It is more likely to result in myopia.
What is the purpose of corneal incisions in the context of astigmatism?
To reduce astigmatism.
What is the result of gas filling an aphakic eye?
It allows an unaided view of the fundus.
What is the purpose of the radial incisions in the cornea?
To weaken the cornea and allow intraocular pressure to cause the wounds to gape and the cornea to bulge.
What is the outcome for myopes of less than –5.00 D after radial keratotomy?
More than three-quarters end up within 1.00 D of emmetropia.
What is the role of sutures in large corneal incisions?
They appose the wound edges and can induce or reduce corneal astigmatism depending on their tension.
How does the small self-sealing incision in phacoemulsification cataract surgery affect the cornea?
It tends to flatten the cornea in the same meridian post-operatively.
What is the purpose of attaching a silicone buckle during vitreo-retinal surgery?
To indent the wall of the eye and repair a retinal detachment.
What are the three primary trigonometric functions mentioned?
Sine, Cosine, and Tangent.
What is the purpose of the Intrastromal Corneal Ring (ICR) procedure?
To flatten the central corneal curvature and treat myopia up to –4.00 D.
What is a recommended technique for correcting astigmatism after corneal graft sutures are removed?
Incising the graft–host junction over 60–90° where it crosses the steepest meridian.
In what cases might epikeratophakia be useful?
In cases of aphakia or cataract where there is contact lens intolerance or an intraocular lens is contraindicated.
What technique uses an excimer laser to change the anterior curvature of the cornea?
Photorefractive Keratectomy (PRK).
What can cause haloes to be seen around lights after PRK?
If the diameter of the treated area is 3.5–4 mm.
What additional technique can be combined with corneal incisions to address astigmatism?
Radial sutures.
What are nomograms used for in astigmatic correction?
They relate the astigmatic correction from different combinations of clear zone diameter, incision arc, and the number of incisions.
What types of refractive errors can be corrected with Epikeratophakia?
High degrees of hypermetropia and myopia.
What is the consequence of creating anisometropia?
It may make work requiring stereopsis and good visual acuity impossible.
What is the purpose of keratophakia?
To correct aphakia by replacing a lamella of anterior stroma over a shaped lenticule of donor corneal stroma.
What is thermokeratoplasty?
A largely experimental procedure that induces contraction of collagen in the peripheral cornea to increase corneal curvature.
What effect does a transverse or arcuate keratotomy have on the cornea?
It causes the cornea to bulge at the incision site, reducing surface curvature in that meridian.
What effect do longer incisions or incisions closer to the visual axis have?
They have a greater effect on astigmatic correction.
How is the Intrastromal Corneal Ring procedure reversed?
By removal of the ring.
Why is excimer laser correction of hypermetropia more difficult?
More tissue must be ablated peripherally than centrally to make the cornea steeper.
What must be ensured during the Epikeratophakia procedure?
That surgery does not alter the shape of the recipient cornea to avoid unwanted refractive error.
How does epikeratophakia correct keratoconus?
By suturing a lenticule of uniform thickness onto the conical host cornea to compress it and return it to a more normal contour.
Why are temporal incisions considered more astigmatically neutral?
Because the temporal limbus is further from the visual axis.
What configurations are shown for corneal incisions?
Configurations for reducing astigmatism alone or in combination with radial sutures.
What is the purpose of suturing the wound with nonabsorbable mersilene?
To shorten the cornea and steepen the curvature in that meridian.
How do phakic intraocular lenses compare to corneal procedures?
They correct higher refractive errors more predictably while conserving accommodation.
How do longer corneal incisions affect healing?
They are more unstable and slower to heal.
What strategy can be used to correct higher degrees of myopia in PRK?
Multiple concentric treatment zones with smoothing transitions.
How can surgically induced astigmatism be used to neutralize pre-operative corneal astigmatism?
By placing the incision in the steeper axis.
What effect does an encircling scleral buckle have on the globe?
It constricts the globe and increases the axial length.
What is the purpose of an ablatable mask in astigmatism correction?
To shield the cornea and allow deeper ablation of the corresponding area.
How is aphakia commonly corrected today?
By a secondary intraocular lens.
Why is Epikeratophakia no longer used to correct myopia?
Because the refractive outcome was highly variable.
What factors can affect refraction post-surgery?
Ocular incisions, suture placement, scleral buckles, silicone oil, and intraocular gas.
What effect does the degradation of suture material have post-surgery?
It reduces tensile strength and may cause a gradual change in refractive error for up to a year.
What effect does gas in the posterior segment of a phakic eye have?
It greatly increases the refractive power of the posterior surface of the lens and causes a large myopic shift.
How can pre-existing corneal astigmatism be reduced during small incision cataract surgery?
By making the incision in the steepest meridian of the cornea.
What type of incision is considered to have a theoretical advantage due to uniform thickness?
An arcuate incision.
What happens to the refractive power of the cornea when gas fills an aphakic eye?
It makes the posterior corneal surface highly diverging and almost neutralizes the refractive power of the cornea.
For which conditions is clear lens extraction advocated?
High myopia and hypermetropes with presbyopia.
What is the purpose of radial keratotomy (RK)?
To irreversibly flatten the central corneal curvature to reduce its refractive power.
How does age affect corneal healing?
Corneal healing is significantly slower in older individuals than in younger ones.
What happens if a suture is tighter than others?
It induces corneal astigmatism by increasing the corneal curvature in that meridian.