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Thread: power of the crystaline lens

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    power of the crystaline lens

    the lens of the human eye is usually stated to
    have power of 20 DS, or so.
    How is that measured?
    .........
    .. measured when removed from an eye..
    .. the post-cataract-surgery eye needs XX more power ..
    .. another way ..
    ......................
    the power stated, is in air, or when surrounded
    by water?

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    Usually by mathematical equations. All that crazy math ODs learn in school. You know how much power is needed to focus light on the retina. If you subtract the power of the cornea with the axial length of the eye and corneal K readings, what you have left over is the lens.

    The IOLmaster which almost all cataract surgeons use determines the IOL power by using topography (sim K's) and axial length by ultrasound.

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by trifoil View Post
    the lens of the human eye is usually stated to
    have power of 20 DS, or so.
    How is that measured?
    .........
    .. measured when removed from an eye..
    .. the post-cataract-surgery eye needs XX more power ..
    .. another way ..
    ......................
    the power stated, is in air, or when surrounded
    by water?
    If you look at the Nucleus surrounded by the Lens Cortex you have roughly a 6D contribution to power, then if you look at the Lens Cortex surrounded by the Aqueous you have a contribution of 13D to the total power that sums up to about 19D total so I believe there you have it. If you were to look at either the Nucleus or the Cortex even alone in air your powers are in the 100D ranges.

    Figures used were Gullstrand.

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    OptiBoard Professional Ory's Avatar
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    Quote Originally Posted by IndianaOD View Post
    The IOLmaster which almost all cataract surgeons use determines the IOL power by using topography (sim K's) and axial length by ultrasound.
    Just to be a pain in the rear, the IOLmaster actually uses optical interferometry (i.e. lasers) rather than ultrasound. That's why it's measurements are better - it measures to the RPE rather than the internal limiting membrane.

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    IndianaOD writes . Usually by mathematical equations. All that crazy math ODs learn in school. You know how much power is needed to focus light on the retina. If you subtract the power of the cornea with the axial length of the eye and corneal K readings, what you have left over is the lens.
    ...................
    what is confusing to me ...
    since everything behind the cornea has
    a similiar index of refraction, 1.336 to 1.406 ...
    I don't see how any refraction
    is accomplished,
    http://hyperphysics.phy-astr.gsu.edu...n/eyescal.html

  6. #6
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by trifoil View Post
    what is confusing to me ...
    since everything behind the cornea has
    a similiar index of refraction, 1.336 to 1.406 ...
    I don't see how any refraction
    is accomplished,
    http://hyperphysics.phy-astr.gsu.edu...n/eyescal.html
    The parts also have radius measures like 10mm and 6mm, so for instance

    1.406 - 1.336 / 0.006 = 11.66D

    You can see it gets accomplished very quickly with numbers like those.

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    Geeze:

    All that stuff using sonograms and atopsies had nothing to do with this? I find than most odd.
    And the "average" power of the crystaline lens would have to prefaced "at what age? Very high at early ages, lower in mid life and yet quite different after catarct formation.

    Of course once could ask a very good OMD who does a lot of catarcts (not an OD who does none) and if you ask what is his percentage of emetropic results and the average age of his patient's you would at least have what the power of the crystaline lens should be.

    Remember the sonogram will show you what power the lens should be but there is no guarantee that the lens had the correct power before extraction that might have been compensated for by eyeglasses.

    Chip

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    Quote Originally Posted by chip anderson View Post
    Geeze:

    All that stuff using sonograms and atopsies had nothing to do with this? I find than most odd.
    And the "average" power of the crystaline lens would have to prefaced "at what age? Very high at early ages, lower in mid life and yet quite different after catarct formation.

    Of course once could ask a very good OMD who does a lot of catarcts (not an OD who does none) and if you ask what is his percentage of emetropic results and the average age of his patient's you would at least have what the power of the crystaline lens should be.

    Remember the sonogram will show you what power the lens should be but there is no guarantee that the lens had the correct power before extraction that might have been compensated for by eyeglasses.

    Chip

    You could ask an OD who was trained in the physiological optics of the eye. Do you not realize that OMDs really have NO optical education? There seems to be some OMD worship on here by opticians.

    You need to also realize that much of modern knowledge for aberrometry (higher order aberrations) and its application to eye care and surgical technology was developed at optometry schools.

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    Quote Originally Posted by chip anderson View Post
    Of course once could ask a very good OMD who does a lot of catarcts (not an OD who does none)

    Chip
    Chip:

    I have a very good OD friend who does all the IOL calculations for a very good OMD.

    Give it a rest.

    How would you like it if I slammed opticians every chance I got? You'd be howling.

    You, however, never feel its inappropriate to downgrade ODs.

    And you call yourself a Christian?

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    Didn't downgrade O.D.'s just said that a busy cataract surgeon measures lots of these every day. Anything an O.D. knows on this subject will be from books. If one practices ten or 15 years doing implants and doing and calculates the power of same and the crystalline lens one should have a pretty good insight as to what the "average" is.
    One can learn basics in physics and anatomy, but hands on is another thing entirely.
    Don't be so damn sensitive.
    I probably know a lot more about fitting PMMA lenses than almost any O.D. or OMD living, but it doesn't make them stupid or me brighter than they are. I doubt if any of them can fit a true scleral molded lens, or a shell eye/scleral shell. Doesn't mean they are less knowledgeable, just means I've done it a lot and they have probably never done it.
    Chip
    Last edited by chip anderson; 01-13-2009 at 02:49 PM. Reason: self defence

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    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    I hate to say it, but I'm with Chip on this one. There's no need to start yet another pi$$ing contest here. Can one of our resolutions here be to act more like adults and less like adolescents?

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    ATO Member HarryChiling's Avatar
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    act more like adults and less like adolescents?
    For me NO, but I won't make any more negative comments towards OD's. :D Except drk :p
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    What's up? drk's Avatar
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    Oh, yea.

    I'm sensitive and sophomoric?

    Chip is not an OD-basher?

    Black is white and night is day!

    Chip, please limit your discussions, then, to the "non-theoretical". Don't listen to Harry C.'s discussions on free-form lenses, because he never designed or fabricated one. Don't listen to Barry's discussions on refraction, because he's not a refractionist. Don't judge O.D.s, then, because you're not one.

    You are a hypocrite, however.

    You disqualified your own post, of course, since you are not an OMD. And you didn't call out Harry's nice bit of contribution and say "don't ask an optician".

    The original poster did NOT ASK AN OD, HE ASKED THE OPTIBOARD COMMUNITY.

    Finger wag away, Judy, you're always on the side of tsk-tsking and not justice and fairness.

    Outta here. Enjoyed most of it, but the community is a little too "Chippy" for me.

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    All right, let's not **** off good members like drk.

    I, for one, would be sad to see him not post here. I've learned quite alot from him and the other ODs who lend their time and expertise to this board.

    Even I am prone to some strong statements at times. But believe me, I am *not* trying to get anyone goning.

    I'm just trying to (provoke?) get them to try thinking a little differently than usual.

    Peace, everyone.

    Barry
    Last edited by Barry Santini; 01-14-2009 at 07:15 AM.

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    Quote Originally Posted by Barry Santini View Post
    All right, let's not **** off good members like drk.

    I, for one, would be sad to see him not post here. I've learned quite alot from him and the other ODs who lend their time and expertise to this board.


    Barry
    Barry,

    Thanks for saying what I am feeling!

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    Cape Codger OptiBoard Gold Supporter hcjilson's Avatar
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    "Good members" try to get along, and if they wish to argue about semantics, do so by private message without involving or polarizing others. When the flames are fanned, the fire becomes hotter. When reviewing this thread at the specific request of a member, I found a few sentences from one of the protagonists that were inserted only to cause problems and perhaps elicit others in a similar manner. I leave it to you to do the same thing and I think you will find a pattern. Enough said....back to the subject at hand..........
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    Quote Originally Posted by hcjilson View Post
    "Good members" try to get along, and if they wish to argue about semantics, do so by private message without involving or polarizing others. When the flames are fanned, the fire becomes hotter. When reviewing this thread at the specific request of a member, I found a few sentences from one of the protagonists that were inserted only to cause problems and perhaps elicit others in a similar manner. I leave it to you to do the same thing and I think you will find a pattern. Enough said....back to the subject at hand..........
    I agree...everyone should be adult like in conversation and not be so overly sensitive...

    As to the optician who thought to ask the OMD about the power of the crystalline lens, due to his experiences, I did not think he was bashing the OD's....and I am an OD.

    And I don't take it personal when patient questions my qualifications anymore...I had a personal experience one day, that the only doctor in office was me, a lowly OD...as all the MD's were gone for the holiday. One patient was upset there was no OMD available and was told by our receptionists that I was very qualified to diagnose any eye conditions she may have so she came and I checked and found she had retinal tear and small detachment in one eye and I did call our MD on call and we agreed the patient could go back home, her spouse driving and get retinal specialist do retinal repair as they were from out of town. I gave the patient the copy of the exam result and all that and sent them home.

    Once they arrived home they saw their ophthalmologist right away and he proudly declared that nothing was wrong. And that in spite of my note clearly showing where the tear was located.

    The patient was smart enough to get a second opinion and called a retinal specialist, who examined and said she needed retinal repair STAT...

    It was our receptionist who went the extra mile and found out how the patient was doing by calling her...

    And guess what?

    The advent of LASIK screws up the IOL measurement.

    We had patients that had LASIK and then had cataract surgery, and the results was off by a large number...and they can be off in either direction! Can be overminused or overplussed....overminussed is not too bad, as patient could use the myopia to read upclose...

    And no rhyme or reason...got quite a few cataract surgeons scratching their heads!

    And even if we have all the data from BEFORE the LASIK, the results can be off...

    Sometimes they can be right on the dot...

    Our office had been using plano PMMA lens with known base curve and deducted the difference on the eyes to calculate an estimated K reading and compare that to the K reading obtained from keratometry or IOLmaster. Sometimes our surgeons use the average between two readings.

  18. #18
    Master OptiBoarder Darryl Meister's Avatar
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    I'm sorry to be so late to this party; it has been a busy two weeks for me.

    I agree that we have some unnecessary "OD bashing" in this thread, regardless of whether it was intentional or not.

    Further, you certainly do not need to be a surgeon to determine the power of the lens of the eye, particularly since it is measured in situ. In any case, the power is typically determined exactly as IndianaOD indicated, which was the original point of this post.

    I respect and appreciate the contributions that our optometric brethren make to OptiBoard and to the Ophthalmic Optics forum, in particular.

    The Ophthalmic Optics Forum is intended for intelligent dialogue between professionals, including opticians, ophthalmologists, and optometrists. Let's please be mindful of posting comments in this forum that are derogatory in nature or that may be misconstrued by others.
    Darryl J. Meister, ABOM

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    Darrell:

    I think the point I was trying to make was the the power of the crystaline lens of the eye varies. It also varies at diffferent points in life in the same individual. An eye surgeon particularly one familiar with replaceing these lenses with pseudophakic lenses and seeing his results as to the accuracy of these implants would be more likely to know just how applicable statistics and in-situ observations are.

    I once got into a debate with one of our revered speaker ladies who thought the power of the crystaline lens was as little as +12.00 appearently because this was the most common power for post catarct spectacles.

    Chip

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    Master OptiBoarder Darryl Meister's Avatar
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    Chip, Although I'm sure that there was no disrespect intended, I just want to ensure that OptiBoard members aren't offending each other in this forum, deliberately or indeliberately.

    Regarding the accuracy of in-situ measurements, I suspect that the variance in power seen in post-op measurements is probably due just as much to the final position (and tilt) of the implanted IOL relative to the original lens position of the phakic eye.
    Darryl J. Meister, ABOM

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    From Gray's Anantomy




    FIG. 886– Profile views of the lens at different periods of life. 1. In the fetus. 2. In adult life. 3. In old age.

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