Cataract/IOL related

Hi,

We have a few questions.

(1) We'd like to know the definition of IOL power p.  Is the power p defined as either one of the following?  If so, which one is the standard definition used for the labeling of manufactured products?

(a) p = 1/f    or
(b) p = n/f = 1.336/f,

where f is the focal length of IOL in meter (m), and n is the index of refraction of the water in the eye.

(2) My husband is 55, and his affected eye (left) has -8 to -8.5 diopter.  We believe that even after the cataract surgery, the eyesight can still change, maybe becoming more myopic in his case.  We would like to know the likelihood (probability) of this happening, and also how much more myopic.

Thank you very much.

Gwenaelle

[Houman Vosoghi, MD]

You're asking the wrong question I think.  With our current technologies, we are able to aim very close to our desired refraction in the majority of cases and his myopia is highly unlikely to get worse after cataract surgery and will likely resolve if the calculations are accurate (as they are in the vast majority of cases).  The only catch is he should have both eyes done close in time as the brain doesn't like having one eye with -8 while the other is no longer myopic.

HV

[gwenaelle]

Thank you very much for answering our questions.  I understand that for our second question, we most likely don't have to worry about the possibility of his eye getting worse after the cataract surgery.

For the first question, we would still like to know the definition of IOL power, if possible.  Is the power defined as

(a) power = 1/focal length of IOL, or
(b) power = n/focal length of IOL

where n is the index of refraction of the surrounding medium.  Thank you very much in advance.

Gwenaelle

[Houman Vosoghi, MD]

It is a lot more complicated than that but closer to the b than to a as they have to take into account the media in which the lens will sit.  An average lens power in the range of 20-22 diopters means that is the effect of the lens (focusing power of the lens) overall as part of the eye.  If that same lens were measured outside of the eye, you would get a much different power.  If you really want to know exactly what formula is used with the manufacturing of the lenses, you could call up one of the manufacturers and see if they could provide you with their exact formula.


HV

[homecrafter]

May I join in here please?
I am confused as to the IOL numbers.  The 20-22 diopter numbers are on my IOL Master measurement chart .

Where I am confused is when I talk about what my eye is to the Cataract Tech, my cataract eye (that developed 4 months after Vitrectomy in only one eye) she says my cataract eye is a -4.75 and my good eye is a +1 right now.  They would do a "contact lens trial" in my bad eye
with a -3.75 lens to get me to -1.    

Why are my numbers so different from the 20 ish numbers, and what is the correlation?

[gwenaelle]

Thank you very much for giving some of your time and thought on our questions.  I am Gwenaelle's husband.  I still have a few questions related to IOL power.

The first one is the same question we have posed earlier.  I have been thinking about this question on IOL power on and off, and I am still a little bit confused.  According to a few of the textbooks on geometrical optics as well as the following literature,

1. Holladay, http://www.docholladay.com/publications/StandardizingConstants.pdf
2. Holladay, http://www.docholladay.com/pdf/25.pdf
3. Colliac,   http://www.iovs.org/content/31/2/374.full.pdf

it seems clear that the power (p) of any lens including IOL is defined as

     (b)  p = n / f

where n is the index of refraction of the surrounding medium, f is the focal length of the lens in meters (m) in that medium.

Originally, I was inclined to the other definition:

     (a)  p =  1 / f

But now I believe that (b) is the correct definition.  My feeling has changed especially after I realized that the focal length of any lens is different in air and in water, for example.

Now my questions on IOL power are

(1)  Do you agree with me about the definition of IOL power?  In other words, do you also think that (b) is the correct definition?

(2)  If you think that neither (a) nor (b) is correct, and you know the answer, could you please write it down exactly and explicitly?

(3)  Even though I believe that the correct definition of IOL is (b), I think that there are still 3 possibilities for the labeling of IOL power by a manufacturer:

     (i)  p = 1 / fa      in air  (since n = 1 in air)

     (ii)  p = 1 / fw      in water (aqueous humor)

     (iii)  p = n / fw      in water (n = 1.336, aqueous humor)

where fa and fw are the focal length of IOL in air and in water, respectively.  I believe that the most logical choice is (iii), but (i) or even (ii) can be still possible, even though (ii) is actually based on the definition (a).

So, the question is as follows:

Suppose you are putting SN60WF with 10 diopters (D) in one of your patients.

(3-1)  Do you think that 10 D means the power in the air or in the water?
(3-2)  If 10 D means the power in the water, do you think it means (ii) or (iii)?

I would really like to know the answers for all these questions.  If you can give me your answers, I would really appreciate it.  Thank you very very much.

Best regards,