CL prescriptions and what they mean

[Lynn White]

Contact lenses are a different kettle of fish so I am starting a new thread.

As most people have had RGP's at some point, I will start with those.

RGP itself means rigid gas permeable. This is to distinguish it from old "hard" lenses which were developed in the 60's and made of perspex. These lenses did not allow any oxygen to pass through them, so often caused problems with the eye suffering from oxygen deprivation.

Eventually, new materials were developed that allowed oxygen to pass through them. As soft lenses also allow oxygen to pass through or "permeate", the term rigid gas permeable was invented to distinguish such lenses from soft ones.

So basically a RGP is a hard plastic lens. The overall diameter of such lenses is, on average, about 9.00 mm. The back surface of the lens is described by a term the BCOR - Back Central Optic Radius and is generally of a curvature that more or less matches that of your cornea.

The radius of a circle, or curve, is smaller the more curved the surface is. Think of using a pair of compasses. The closer the pencil and the point is, the smaller and more steeply curved is the circle.

The "average" radius of curvature of the eye is around 7.80 mm. KC eyes have much steeper radii of curvature. Generally one begins to suspect KC when the radius of curvature of the eye drops below 7.00. Its not unusual to have a radius of curvature of 5.00 or even 4.00 with KC, but you would never have this value with a "normal" eye. The curvature of the eye is measured using an instrument called a keratometer.

Now, a simple RGP lens might have the prescription: 6.50/9.00/-6.00. The first number refers to the BCOR, the 2nd to the overall diameter and the last to the power. Therefore this lens has a back curve of radius 6.50mm, an overall diameter of 9.00mm and a power of -6.00D.

Is this all clear so far??

[Arun]

Hi

That makes sense to me. Actually it is very useful. Does that mean the smaller the BCOR the steeper the cone is?

Just out of interest is the BCOR related to the k-value in anyway?

[Lynn White]

Well.. the steeper the cone the smaller the BCOR is.

What exactly do you mean by the K-value?

[jayuk]

Lynn

I have a question for ya......during the latter stages of my KC I tried Piggy Backing..with a Kerasoft and rgp lens (tried varies combinations).....what we found was that all the rules of Contact Lens fitting went out the window......now my understanding was that the first lens inserted was the soft (which was for flattening the surface of the cornea and allowing for a even surface)...how does the rgp then function...ie whats the goal of the fit?.....is it merely the same.....as if the soft lens wasnt there?...and if so...how come the rules of CL fitting went out the window?..

does this make sense?....probably not...lol

[Sweet]

Hhmm i'm kinda going with the idea of piggy backing with mine that it is more for comfort and nothing else as the soft lens is plain. I didn't really think of it evening out the surface, although maybe that is why it is more comfortable?

Doesn't the soft lens not do anything as in you can still get the movement you need to have to see with a RGP and that if it dries out you can always add some lubricating drops and it sorts itself out? And that the biggest problem is with oxygen transmission and lenses drying out?

Maybe! Either way for now it works for me and i wasn't told about it being a different RGP lens as such. Although as i just posted in another topic Moorfields were not happy with using a toric lens but they didn't really say why!

Sweet X x X

[Amarpal]

Just wanted to say thanks for clearing that all up! There are so many numbers on the sheet that comes with my lenses, that I never knew which one was the power. Also, is there a certian 'limit' that is in place regarding the pwer of the lenses (when would you have to stop using RGPs)?
Thanks!

[Arun]

Well.. the steeper the cone the smaller the BCOR is.

What exactly do you mean by the K-value?

I was told that on a corneal topography the k value was some kind of measure of the steapness of the cone or something along those kind of lines, I think.

[Andrew MacLean]

Confused ... you will be!

[Lynn White]

K-value: Ahh right - just wanted to get the context in which it was mentioned. K-value is the actual radies of curvature. As I said b4, the curve of the eye is measured using a keratometer so we would refer to the radii of curvature of the eye as the K readings. On a topography, the contour lines represent different curvatures of the cornea ie if you pick a point on the corneal map, the K value represents the radius of curvature at that point.

Piggy Backs:

B4 I get there.. just let me say that the K readings one gets from the eye determine the back curve of the RGP used. In a normal eye, one tends to fit approximately with a back curve similar to the radius of curvature to the eye. Fitting KC eyes is more of a challenge, as the central area may well be very steep but the periphery may well be still "normal". Added to this, the central point, the apex of the cornea, is often to one side or off centre in KC. This means the lens often ends up sliding to one side, as it gravitates over the eyes apex.

How the RGP fits over the piggy back depends on the soft lens used. There are many kinds of soft lenses, some are very thin - others are much thicker.

This lenses may be used when the main issue is comfort. These mould to the eye very faithfully, so fitting an RGP on top is not that much different to fitting on the naked eye.

A Kerasoft is much thicker and its intention is to actually flatten and reshape the cornea to produce a more regular shape. So two things going on here...in the first place, the kerasoft tends to hold its own shape, so you are now effectively fitting the front surface of the Kerasoft, not your eye. Secondly, as the kerasoft flattens the cornea, the original RGP will now be too steep. One would have to flatten it to match the new situation.

Why PUT the RGP on top of the soft anyway, ie what is the goal of piggybacking?

Well, optically speaking, an RGP is replacing your cornea as the major refracting surface of the eye. It is rigid and holds its own shape and therefore neutralises the effects of the distortions and aberrations produced by KC. Soft lenses by their nature mould to the eye, so many of these aberrations are faithfully reproduced by lens and so still mess up your vision.

Lenses like Kerasoft, Soflex and the Acuity lens do attempt to have some of the moulding effects and rigidity of the RGP's and often work very well. When they don't, combining the two types is an option to try and achieve better vision.

Rules of contact lens fitting apply to the average eye. By their nature, KC eyes are NOT average! Each is very individual and you have to approach each eye with an open mind. Add to this that ANY lens tends to mould a KC eye as it is thinner than normal, and as you mould the eye, its shape changes throwing the careful fit you have just worked on out the window! This is why sometimes you wait ages for a new lens and then it doesn;t work. By the time you get it, your eye has changed shape again! And believe me, its frustrating for fitters as well as yourselves!!

[Sweet]

Ok thanks for that and yes it is really DAM frustrating to us, but i can imagine that after all the work optoms do to fit lenses how you guys feel as well when everything changes and we need to start again!!

Hehe so it is right then that my soft lens being a focus monthly does nothing to the shape of my eye at all which was what i thought, but makes it much more comfortable to put a RGP on.

So as nothing really has changed and i still have the same prescription that i had before piggy backing why am i not allowed to use a toric lens which gave me better vision than the one i just had at Moorfields which isn't a toric lens?

When i drop back in to see them in June for a check up i will be asking what this new lens is that they gave me for piggy backing as they didn't have my notes last time to let me know! That way i can order a spare, because this week i have managed to lose it three times already!! LOL!

Sweet X x X