Myopia is a condition of the eye where a distant image is focused in front of the retina
Principle of correction by laser:
By flattening the cornea with the Excimer laser, the images focus directly on the retina.
Hyperopia is a condition of the eye where a distant image is focused behind the retina:
Principle of laser correction:
By increasing the corneal curvature, the image focus is shifted forward and placed on the retina.
The focus along the axes of the astigmatism can be changed.
Three techniques can be used for myopia, hyperopia or astigmatism:
Lasik: laser cutting of a corneal flap and photoablation in the stroma.
Refractive photokeratectomy: after removal of the epithelium, the Bowman’s membrane is phatoablated.
Epilasik: laser procedure after detachment and repositioning of the epithelium.
Presbyopia is a condition where the eye lens’ ability to focus naturally declines with age when reading.
To see up close, the eye lens bulges so that the focus is done on the retina: it is called the accommodation.
The lens loses its flexibility with age and the power of accommodation decreases.
The degree of presbyopia is measured in diopters: from +0.50 diopters at 45 years old to +3 diopters after age 60.
Presbyopia is added to the far-distance vision defects: myopia, hyperopia or astigmatism.
Correction of presbyopia:
Glasses: reading glasses or progressive lenses for distance and near vision.
Presbyopia can be corrected with contact lenses with a vision fluctuation.
Presbyopia can be corrected by an intervention:
– Either a toggle: the dominant eye is corrected for distance vision (emmetropization) and the dominated eye is corrected for near vision (myopization). Only a small proportion of patients can withstand this configuration. A simulation test is done to assess it before surgery.
– Or a multi-focality: of the dominated eye by creating a more convex central area. (Presbylasik)
Phacorefractive surgery : interesting when the lens has lost its transparency or in case of cataract.
The technique is to remove the lens, and insert a multifocal implant.
Excimer laser refractive surgery
Refractive surgery is a surgery used to correct refractive errors and eliminate the dependency on glasses.
This surgical procedure is used to adjust the eye to compensate for myopia, hyperopia and astigmatism.
It is either focused on the cornea with the Excimer laser or done by operating the lens or by placing a refractive implant.
In some cases, a clear lens surgery with placement of an implant that corrects the refractive erroris possible (Prelex).
This technique can be used in case of high myopia, high hyperopia when a laser correction is not possible (corneal parameters incompatible with the degree of correction) nor by additive implant (ocular biometric characteristic incompatible with existing implants) especially in cases of high unilateral ametropia.