Elk19

+4.00 to -10.00 D.

To remove noncataractous lenses and insert an intraocular lens to correct spherical refractive errors.

They have the greatest effect on central corneal curvature.

The Holmium:YAG laser.

Attaching a lenticule of pre-shaped donor corneal stroma to the surface of the host cornea.

The eye is not entered, and the procedure can be reversed by removing the lenticule.

-2.50 D.

High degrees of myopia, but not hypermetropia.

It changes the posterior surface of the lens from a converging to a diverging interface.

The angles α formed are all equal.

It conserves the wound and has a more predictable effect.

The topography will change, potentially leading to overcorrection or degeneration.

Keratoscopy.

It may allow indirect ophthalmoscopy without the use of a condensing lens.

By removing a deep arcuate wedge measuring 60–90° from the graft–host junction in the flattest meridian.

To correct refractive errors in phakic eyes.

The long-term effects on the corneal endothelium, crystalline lens, and iridocorneal angle are unknown.

Because they are technically difficult and the refractive outcome is often unpredictable.

The meridia of maximum and minimum curvature may not be at right angles to each other.

It induces a hypermetropic shift in refraction of +5.00 to +7.00 D.

+4.00 to +6.00 D.

By placing compression sutures 90° away from them.

The use of a microkeratome to remove a lamella of anterior corneal stroma, which is then shaped on a cryolathe before being replaced.

By the post-operative tolerance of spectacles or contact lenses to correct the residual refractive error.

The dimensions of the donor tissue.

Silicone oil is more strongly converging than the crystalline lens.

PMMA (Polymethyl methacrylate) split ring.

It increases the curvature of the cornea and reduces astigmatism.

They pass through cornea of varying thickness and parts are closer to the visual axis.

Keratoconus.

Fifteen weeks for PMMA, ten weeks for gas permeable, and five weeks for soft lenses.

It is more likely to result in myopia.

To reduce astigmatism.

It allows an unaided view of the fundus.

To weaken the cornea and allow intraocular pressure to cause the wounds to gape and the cornea to bulge.

More than three-quarters end up within 1.00 D of emmetropia.

They appose the wound edges and can induce or reduce corneal astigmatism depending on their tension.

It tends to flatten the cornea in the same meridian post-operatively.

To indent the wall of the eye and repair a retinal detachment.

Sine, Cosine, and Tangent.

To flatten the central corneal curvature and treat myopia up to –4.00 D.

Incising the graft–host junction over 60–90° where it crosses the steepest meridian.

In cases of aphakia or cataract where there is contact lens intolerance or an intraocular lens is contraindicated.

Photorefractive Keratectomy (PRK).

If the diameter of the treated area is 3.5–4 mm.

Radial sutures.

They relate the astigmatic correction from different combinations of clear zone diameter, incision arc, and the number of incisions.

High degrees of hypermetropia and myopia.

It may make work requiring stereopsis and good visual acuity impossible.

To correct aphakia by replacing a lamella of anterior stroma over a shaped lenticule of donor corneal stroma.

A largely experimental procedure that induces contraction of collagen in the peripheral cornea to increase corneal curvature.

It causes the cornea to bulge at the incision site, reducing surface curvature in that meridian.

They have a greater effect on astigmatic correction.

By removal of the ring.

More tissue must be ablated peripherally than centrally to make the cornea steeper.

That surgery does not alter the shape of the recipient cornea to avoid unwanted refractive error.

By suturing a lenticule of uniform thickness onto the conical host cornea to compress it and return it to a more normal contour.

Because the temporal limbus is further from the visual axis.

Configurations for reducing astigmatism alone or in combination with radial sutures.

To shorten the cornea and steepen the curvature in that meridian.

They correct higher refractive errors more predictably while conserving accommodation.

They are more unstable and slower to heal.

Multiple concentric treatment zones with smoothing transitions.

By placing the incision in the steeper axis.

It constricts the globe and increases the axial length.

To shield the cornea and allow deeper ablation of the corresponding area.

By a secondary intraocular lens.

Because the refractive outcome was highly variable.

Ocular incisions, suture placement, scleral buckles, silicone oil, and intraocular gas.

It reduces tensile strength and may cause a gradual change in refractive error for up to a year.

It greatly increases the refractive power of the posterior surface of the lens and causes a large myopic shift.

By making the incision in the steepest meridian of the cornea.

An arcuate incision.

It makes the posterior corneal surface highly diverging and almost neutralizes the refractive power of the cornea.

High myopia and hypermetropes with presbyopia.

To irreversibly flatten the central corneal curvature to reduce its refractive power.

Corneal healing is significantly slower in older individuals than in younger ones.

It induces corneal astigmatism by increasing the corneal curvature in that meridian.