Prior to embarking with laser refractive surgery it is important to understand some of the workings of the eye and the function of the components which will be involved. Laser refractive surgery concentrates on the very front of the eye reshaping the cornea to improve vision by reducing refractive error.
The cornea: The clear window at the front of the eye providing most of the eyes focusing power. Light rays from objects pass through the cornea and are partially focused.
The lens: The structure behind the pupil that accounts for some of the remaining focusing power. The lens works with the cornea to focus light rays onto the retina which sends signals to the brain through the optic nerve.
In a normal eye the cornea and lens are able to allow parallel rays of light to pass through and focus upon the retina without effort. There is no refractive error in this state requiring correction and the eye is considered to be emmetropic.
Understanding Refractive Errors
Blurred vision may be caused by different refractive errors such as:
- Myopia (nearsighted)
- Hyperopia (farsighted)
- Presbyopia (inability to focus on near objects)
- Astigmatism (irregular eye shape)
The corneal surface is too steeply curved so images focus in front of the retina causing blurred vision.
- Distance objects appear blurry
- Near objects appear clear
The corneal surface is too flat so images focus behind the retina causing blurred vision.
- Distance objects appear blurry
- Near objects appear blurry
The corneal shape is oval rather than spherical so the light rays focus on multiple points on the retina causing blurred vision.
- Distance vision is blurred
- Commonly associated with myopia and hyperopia
A natural weakening of the elasticity of the internal lens often developing in mid 40s which reduces the focusing ability of the eye.
- Distance vision unchanged
- Near vision becomes increasingly blurry
Optical Aberrations (errors) of the Eye
These are the simplest refractive errors of the eye including nearsightedness (myopia), farsightedness (hyperopia) and regular astigmatism. Overall, these account for approximately 90% of vision problems and may be corrected by glasses, contact lenses or laser refractive surgery. These are the only aberrations which standard or conventional laser platforms can treat. These are more complex refractive errors of the eye. There are many different types of higher order aberrations. An aberramoter is used to measure the amount of each type of higher order aberration in an eye, known as the wavefront of the eye. Every eye’s higher order aberration make up, or wavefront, is unique. These aberrations contribute to vision problems such as halos, glare, doubling, and poor contrast sensitivity. Only laser platforms which have true wavefront-guided laser treatments, such as the Visx Star S4 IR iDesign platform at VICTORIA EYE, can address all of these aberrations. Visual acuity is measured according to the ability to see different sized letters on a Snellen eye chart at a distance of 20 feet. Someone with perfect vision can clearly see a particular line of letters at 20 feet without glasses and is considered to have 20/20 vision. Someone with worse vision, 20/30 for example, would need to stand 20 feet from an object to see it as someone with perfect vision would see it at 30 feet. And yes there are people with better than perfect vision such as 20/15. This reflects their ability to see objects at a distance of 20 feet that the person with perfect vision of 20/20 would have to move to the 15 foot mark to see.
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