For those in Contact Lens Practices

[armstrong77]

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Hi

I was just watching some how-to-fit RGP videos recently. They were produced by the CLMA in the early 90s.

They spent a lot of time explaining the clinical pearls of switching those in PMMA to gas-perms.

How often do practitioners encounter PMMA lenses these days? I'm guessing it's very rare...

 

[jefguth]

Hi

I was just watching some how-to-fit RGP videos recently. They were produced by the CLMA in the early 90s.

They spent a lot of time explaining the clinical pearls of switching those in PMMA to gas-perms.

How often do practitioners encounter PMMA lenses these days? I'm guessing it's very rare...

Never seen one. However, if your thinking of creating your own spherical fitting set, nothing lasts like PMMA!

 

[armstrong77]

A few other questions.

1. For RGPs - are 2-step Boston systems still the gold standard for solutions? I know one-steps exist - but do 2-steps still maintain primary market share?

2. The video series I saw (from www.rgpli.org) was created in the early 1990s. They said (probably biased but still) 1/3 of CL wearers were in RGPs. Obviously the figure is not nearly that high today.
- As a percentage, can we definitely say there are more folks in SCLs in 2009 than were in 1991? I'm sure daily disposables are a factor in this.
- What is the current figure for % of CL patients who wear RGPs?

 

[armstrong77]

Hi

I hope someone can answer this RGP fitting question. I'm looking at my notes and wondering if there's an error.

For RGPs, we know:

1. 0.05 mm change in BOZR is equivalent to 0.25 D change in power. So steepening by 0.05 mm, we must increase power by -0.25 D to compensate.

2. We also know that a change in BOZD must be compensated for change in BOZR to retain the same fitting charactersitics (i.e. same sag). So, increasing BOZD by 0.50 mm requires a flattening of BOZR by 0.05 mm to maintain the same fit.

My notes says:

Radius : BOZD : Power
7.55 : 7.50 : -2.75, but we can also fit,
7.50 : 7.50 : -3.00, (compensating for radius change, #1 above)
7.45 : 7.50 : -3.25, (again, compensating for radius change, #1 above)
is equivalent to:
7.50 : 8.00 : -3.25 (blue is change in radius and BOZD, #2 above, but -3.25??)

The problem is my notes says the power is -3.25. Is this correct, or should it be -3.00 to match the change in radius???

My question is - when we change both radius and BOZD (such as the last step), the sag remains the same - does that also require an appropriate change in power? I would think so but my notes say -3.25 (and not -3.00 which you would expect...)

Hope someone can answer.

 

[KHE]

Hi

I hope someone can answer this RGP fitting question. I'm looking at my notes and wondering if there's an error.

For RGPs, we know:

1. 0.05 mm change in BOZR is equivalent to 0.25 D change in power. So steepening by 0.05 mm, we must increase power by -0.25 D to compensate.

2. We also know that a change in BOZD must be compensated for change in BOZR to retain the same fitting charactersitics (i.e. same sag). So, increasing BOZD by 0.50 mm requires a flattening of BOZR by 0.05 mm to maintain the same fit.

My notes says:

Radius : BOZD : Power
7.55 : 7.50 : -2.75, but we can also fit,
7.50 : 7.50 : -3.00, (compensating for radius change, #1 above)
7.45 : 7.50 : -3.25, (again, compensating for radius change, #1 above)
is equivalent to:
7.50 : 8.00 : -3.25 (blue is change in radius and BOZD, #2 above, but -3.25??)

The problem is my notes says the power is -3.25. Is this correct, or should it be -3.00 to match the change in radius???

My question is - when we change both radius and BOZD (such as the last step), the sag remains the same - does that also require an appropriate change in power? I would think so but my notes say -3.25 (and not -3.00 which you would expect...)

Hope someone can answer.

Yowzers.

The correct answer is to find a CL that fits and then over refract and order that. lol

But I'll take a stab at answering your question:

If you increase the optic zone by that much, the lens will fit tighter. As such, you have to flatten the BC to keep the fitting characteristics the same. I would agree with you. The power should be -3.00 not -3.25.

But again.....in the real universe, we fit a CL and then over refract. lol

 

[armstrong77]

Yowzers.

The correct answer is to find a CL that fits and then over refract and order that. lol

But I'll take a stab at answering your question:

If you increase the optic zone by that much, the lens will fit tighter. As such, you have to flatten the BC to keep the fitting characteristics the same. I would agree with you. The power should be -3.00 not -3.25.

But again.....in the real universe, we fit a CL and then over refract. lol

Thanks so much. I thought so. I think my notes had -3.00 initially, then it was scratched out and over-written with -3.25. So the -3.00 should be correct.

 

[eyestrain]

Or you call the CL supplier, tell someone on the phone the refraction, K-readings, etc. and have them ship you one that will be close most of the time.

 

[armstrong77]

Umm... another question.

It seems bitoric RGPs are written in the format:

7.76 -100
---- -----
7.24 -462

Apparently, the above is sufficient to order the lens. Is it??

This example is intended to fit this eye:
Ks 43.75/47.12 (-3.37 x 007) with ocular cyl (Rx): -3.87 x 007

After applying the Mandell corrections (PMMA), there is apparently +0.95 x 007 of residual cylinder correction which is supposed to go on the FRONT surface.

Why is it in the examples I've seen, there is no specification for the front surface power of these bitorics? It seems the order form simply states the two "fractions" above, without specifying the front surface residual cyl correction. i.e. +0.95 x 007 is not written anywhere.

What happened to it? Aren't we supposed to list it? Or am I missing something?

Help!

 

[iris0]

Hi

I hope someone can answer this RGP fitting question. I'm looking at my notes and wondering if there's an error.

For RGPs, we know:


My notes says:

Radius : BOZD : Power
7.55 : 7.50 : -2.75, but we can also fit,
7.50 : 7.50 : -3.00, (compensating for radius change, #1 above)
7.45 : 7.50 : -3.25, (again, compensating for radius change, #1 above)
is equivalent to:
7.50 : 8.00 : -3.25 (blue is change in radius and BOZD, #2 above, but -3.25??)

The problem is my notes says the power is -3.25. Is this correct, or should it be -3.00 to match the change in radius???

My question is - when we change both radius and BOZD (such as the last step), the sag remains the same - does that also require an appropriate change in power? I would think so but my notes say -3.25 (and not -3.00 which you would expect...)

Hope someone can answer.

I looked up my notes. It said that for every modification of 0.2mm in diameter, the radius of base curve is altered by 0.125D or 0.023mm. Increasing BOZD makes the lens steeper. This means that your original note of -3.25 is correct.