pres new page
pres new page
As of April 2013, no method of correcting presbyopia is perfect. The basic problem is that the lens in the eye gets harder with age, puts on layers like an onion throughout life and hence cannot squash up as much for seeing close to. Of all the elements of the eye, it is the lens whose ageing affects the eye function the most.
Bifocal and multifocal (varifocal) glasses work well but multifocal contact lenses and intraocular lenses are less good as they move with the eye. Hence all these systems are somewhat of a "fudge", with parts of the lenses fixed for seeing in the distance and parts for reading. Multifocal intraocular lenses have drawbacks. They are associated with symptoms of halos round lights and a reduced contrast sensitivity (the ability to distinguish an object against a background which is similar to the object itself). Thus multifocal intraocular lenses reduce spectacle dependence but at the expense of clarity. The human eye is very good at adapting to different optics and, after weeks to months, many people can cope well with these imperfect optics. However, every ophthalmic surgeon has patients who cannot get used to these lenses and has to take them out.
Download Article: 6.92mb - Click Here
One system lately is Intracor, which uses a femtosecond laser to make a small central island of 3mm diameter in the cornea. When we read, the pupil gets smaller with accommodation and hence this small steeper area is used for reading through this small pupil. For distance, the pupil gets a bit bigger (depending on the lighting conditions). The cornea beyond the 3 mm area is still focused for distance. Our pupils get smaller with age and also less responsive.
A similar treatment can be done with the excimer laser. With out WaveLight Laser we are able to adjust the "Q value" to make the cornea hyperprolate. This means that the centre part is more for reading and outside this for distance, similar to Intracor and with the same caveats. This is not needed if you start out being longsighted (hyperopic) as the laser treatment steepens the cornea and makes the cornea hyperprolate. Hence these patients can read much better than they would expect. The converse is true for those who are initially short-sighted (myopic) as the tendency is to leave the cornea oblate (like a hamburger bun). With our laser, the wavefront optimised standard treatment keeps the cornea prolate (see pages on wavefront treatments) and hence does not degrade the optics. It is also possible with the "Q adjusted" software to make the cornea hyperprolate as above, and this enables one to read somewhat better. It is best for people who have mobile pupils.
Here are several articles about this taken from the 11th Annual Users meeting for WaveLight in Dublin July 2010: They are a bit technical so good luck!