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Thread: lenticular astigmatism

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    Question lenticular astigmatism

    I was told that irregular astigmatism (2 or more focal lines are formed that are not 90 degrees apart), is best corrected by RGPs compared to glasses or soft CLs. I understand how RGPs correct irregular corneal astigmatism because of the tear film that is formed. Another cause of irregular astigmatism is a titled crystalline lens. Are RGPs also the best correction for lenticular/internal irregular astigmatism? If so, why?Any help is appreciated in advance.
    Last edited by OptiStudent; 02-10-2007 at 01:26 AM. Reason: clarification

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    ATO Member HarryChiling's Avatar
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    The reasoning behind the RGP's being the best correction, is because you can neutralize any corneal astigmatism with the back surface of the lens (tear film), while the front surface of the lens can be used to neutralize any internal astigmatism.
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    Experience will also tell you that most of the time, if the astigmatism is against the rule (longest corneal meridian nearest 90 degrees) a soft toric lens will be more likely to make the patient and the practioner happy.
    Usually those that have with the rule astigmatism are best served with a rigid lens.

    Chip

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    Thanks!

    Quote Originally Posted by HarryChiling View Post
    The reasoning behind the RGP's being the best correction, is because you can neutralize any corneal astigmatism with the back surface of the lens (tear film), while the front surface of the lens can be used to neutralize any internal astigmatism.
    Thank you for the explanation. I do recall that now.

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    Lenticular astigmatism is not irregular astigmatism. You are correct in your definition of irregular astigmatism, but lenticular, or any astigmatism originating somewhere other than the cornea, is virtually always at meridians 90 apart, while irregular astigmatism usually results from either injury or a disease process such as keratoconus. You also understand why irregular astigmatism can be corrected with an RGP. Higher amounts of corneal astigmatism are also corrected, optically, by a spherical RGP, but mechanically, such a lens will rock on a toric cornea, so a back toric curve is used to acheive an optimum fit. As the others have pointed out, residual astigmatism may then be corrected by a front toric curve. Just to further clarify, a case where there is significant lenticular astigmatism, and little or no corneal astigmatism, a front only toric RGP may be employed, but will need prism ballast since there is no toricity to maintain axis orientation.

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    Just a little side note an aphakic patient (not pseudophake) cannot have residual astigmatism in a rigid lens, with the exception of that rarest of all conditions, retinal astigmatism.

    Chip

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    Thanks Dave!

    "Lenticular astigmatism is not irregular astigmatism. You are correct in your definition of irregular astigmatism, but lenticular, or any astigmatism originating somewhere other than the cornea, is virtually always at meridians 90 apart."

    Yes, I thought it was just me, because I obviously did not fully understand what I read. The book I checked out (of the library), is wrote by an O.D for O.D.s. --Or, experienced CL dispensing opticians, don't get me wrong. Thanks for the clarifications!:)

    On a CL, the back posterior surface is the BC fit to the cornea..The front curve is the power curve (Rx curve)..presently we only make the major meridians 90 degrees apart...So, any residual astigmatism left from an internal astigmatism could only be corrected @ meridians 90 away. I think this is correct and the reason why I did not understand internal astig. correction by the front curve. Internal astig. is NOT an irregular astig. TY! I hope.:o

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    Don't know if this is to your point or not. There is what used to be called "the liquid lens" this being the tear interface between the cornea and the posterior of the lens. This is what corrects almost all corneal astigmatism. Usually the internal or "lenticular" astigmatism is negligable and usually (according to Javal) is 1/2 diopter against the rule.

    In a "normal" eye the patient will have 1/2 diopter with the rule corneal and 1/2 diopter against the rule "lenticular." At one point it was taught to no longer use the term "lenticular astigmatism" as it had nothing to do with the lens. Of course it was actually in the lens but there it was.

    Chip:hammer:

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    There continues to be much misinformation in this thread...

    IMHO, much of it derives from the need for optical schools to *boil-down* the explanations for newbie students.

    Example: The CL & "tear-lens": The *correct* concept is to think of the front surface of the contact as the *new* cornea...as long as that surface is *appropriately chosen, measured, and configured* for that eye, a correct and optimal spherical focus will result for the retina. I think the concept of tear lens helps to lose sight of just what is really going on. Remember, the majority of the refractive power (and subjective impression of clarity) is at this first, air-tear (and then contact-lens) interface.

    Example: Irregular astigmatism: New information, especially from wavefront analysis (I-ZION, etc.) reveals that "axes crossing at other than 90 degrees" is again, an over simplification. The reality is that this type of non-optimal focus is more correctly described (and corrected) through HOA analsis. I think that we, in optical *FLATLAND*, have been trying to describe a phenonmenon of a class of HOAs in first and second order aberration terms (much like the two dimensional creatures of Abbot's book try to describe the appearance of a visitor from the third dimension through their two dimension world). This idea I have gleaned from reading the many papers and abstracts (and lectures @ VEE!) on wavefront anaylsis and correction.

    My two cents...what's yours?

    Barry
    Last edited by Barry Santini; 02-11-2007 at 10:45 AM.

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    Although rare, there is a type of lenticular astigmatism that is irregular. Posterior lenticonus, in which the rear surface of the lens can have an irregular and/or cone shape. It can create irregular spherical and astigmatic errors, and no spectacle or CL can correct it fully. Often, in severe cases, lens extraction is the only solution.

    I've had a few cases of this over the years. It can only be detected through a dilated pupil which allows a full view of the lens to gain a perspective of the curvature of the lens, and biomicroscopic evaluation. Nowadays it might also be detectable with a B-scan.

    Not that this has anything to do with learning to properly fit RGPs, but if you fail to get a good expected acuity, don't forget to consider this possibility. I had been fitting RGPs for 20 years before I had one of these.
    Last edited by fjpod; 02-11-2007 at 09:31 AM.

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    Question: Are all internal astigmatisms irregular astigmatisms?

    My CLSA Manual Vol.2 (2003 lastedt edition), says ""Internal astig. is due to the toricity of the back surface of the cornea and/or the tilting of the crystalline lens w/ respect to the optic axis of the cornea. This is sometimes referred to as "lenticular astigmatism.""

    I previously used the term "tear lens" the CLSA text uses the term "lacrimal lens." During lecture the "concept" of a new corneal surface was explained. In defense of my school, they are preparing me for state boards that do not cover wavefront technology.

    What does IMHO stand for? Best to all.

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    ATO Member HarryChiling's Avatar
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    IMHO = in my humble opinion :)

    Quote Originally Posted by optistudent
    My CLSA Manual Vol.2 (2003 lastedt edition), says ""Internal astig. is due to the toricity of the back surface of the cornea and/or the tilting of the crystalline lens w/ respect to the optic axis of the cornea. This is sometimes referred to as "lenticular astigmatism.""
    I know it as the lacrimal lens as well, but it is easier to explain to the layman as the tear lens and to explain how it helps. If you are studying for anything I would memorize it as the lacrimal lens.

    Quote Originally Posted by optistudent
    My CLSA Manual Vol.2 (2003 lastedt edition), says ""Internal astig. is due to the toricity of the back surface of the cornea and/or the tilting of the crystalline lens w/ respect to the optic axis of the cornea. This is sometimes referred to as "lenticular astigmatism.""
    The CLSA manual was written for opticians so keep in mind it will not cover every detail, like chip explained earlier their is the retina.

    Quote Originally Posted by Barry Santini
    I think the concept of tear lens helps to lose sight of just what is really going on. Remember, the majority of the refractive power (and subjective impression of clarity) is at this first, air-tear (and then contact-lens) interface.
    Seriously, you should not be touching RGP's in any way if you don't understand the concept of the tear lens. The power is effected by the tear lens and the tear lens is created by the fit, unless you fit everyone on K's which isn't realistic, then you will have to take the tear film into consideration. I agree that the concept of RGP's should be that they are replaceing the cornea as the first optical interface, but I also think that leaveing the LL out should only be taught to layman.
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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Quote Originally Posted by HarryChiling View Post
    Seriously, you should not be touching RGP's in any way if you don't understand the concept of the tear lens.
    Harry, I didn't mean to suggest that I don't know anything about RGP fitting (I am a NY State Certified CL fitter, although I do not actively fit lenses at this time). If everyone agrees that the *tear lens* is important conceptually and practically, then we all need to include the refractive effect of the tear film *without* a CL in place when analyzing the complete optics of the eye Agreed?

    Barry

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Barry Santini
    Harry, I didn't mean to suggest that I don't know anything about RGP fitting (I am a NY State Certified CL fitter, although I do not actively fit lenses at this time). If everyone agrees that the *tear lens* is important conceptually and practically, then we all need to include the refractive effect of the tear film *without* a CL in place when analyzing the complete optics of the eye Agreed?
    The tear lens is not an issue when their is no lens on the eye, it does help to fill in imperfections, but it does not create the kind of powers you get when a lens is fit slightly steeper or flatter on the eye. I don't really understand where you are going with this, but I'm game if it gets somewhere

    Quote Originally Posted by Barry Santini
    Harry, I didn't mean to suggest that I don't know anything about RGP fitting (I am a NY State Certified CL fitter, although I do not actively fit lenses at this time).
    I didn't mean to imply that you don't know what you are doing, I was just trying to illustrate the importance IMO of the LL.
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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Quote Originally Posted by HarryChiling View Post
    I don't really understand where you are going with this, but I'm game if it gets somewhere
    Maybe *I* don't know where I'm going with this. Let me figure it out!

    barry

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