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Thread: With your patients wearing short corridor lenses

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    With your patients wearing short corridor lenses

    When you have a patient wearing a short corridor lens, what do you do if their new frame has a much deeper B measurement? Do you keep them in a short corridor or switch to a regular one?

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    Independent Problem Optiholic edKENdance's Avatar
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    This has occurred a lot recently with me. I try to choose best based on age and rx and lifestyle. I've opted for a medium length corridor in all recent cases and the results have been well received.

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    Quote Originally Posted by Happylady View Post
    When you have a patient wearing a short corridor lens, what do you do if their new frame has a much deeper B measurement? Do you keep them in a short corridor or switch to a regular one?

    A lot of patients benefit from a short corridor lens, and extending the B dimension usually only results in more generous add area. Minimum height is just that..minimum.

    The flip-side of that is that intermediate usually suffers, as does width of read area.

    I usually go through a mental check-list of potential advantages, and pitfalls of changing the design, and change only if advantage is apparent. I hope this helps!
    Eyes wide open

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    Master OptiBoarder NCspecs's Avatar
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    It all depends on lifestyle, age of the patient, crankiness level, and whether or not it is someone going from an outdated narrow rectangle to a big ole hipster frame.

    If they aren't making a whole lot of significant POW changes and it ain't broke, I don't fix it. However if we are doing a whole frame makeover I take that as an opportunity to make changes that I think will be in the patient's best interest. I do make sure to talk them through the process and let them know what to expect. Nobody likes irritating surprises when they make a hefty financial investment.
    "Strictly speaking, there are no enlightened beings; only enlightened activity." -Shunryu Suzuki

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    I also put them in a lens like the Shamir Spectrum 16 or a similar offering and it always works.

    Mostly I've learned that variable corridors are a big nono in this particular situation, though they are my friend most of the time.

    This is most troubling when the Add has not increased, if there is a bump in Add value also I find that it isn't a big deal over all even with a variable corridor.

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    What's up? drk's Avatar
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    See if you agree:
    1. I don't finesse to the degree where hyperopes get this lens and myopes get that lens.
    2. Occasionally, like with anisometropia, a short corridor is a good thing, but in general I don't hear people say "I have to look too far down in my glasses (unless they're undercorrected).
    3. Short corridors achieve their greatness by messing something else up. If short corridors were optimal, we wouldn't have "regular" corridors.
    4. Ergo, a normal corridor lens is optimal, and should be encouraged.

    What do you think?

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    Quote Originally Posted by uncut View Post
    A lot of patients benefit from a short corridor lens, and extending the B dimension usually only results in more generous add area. Minimum height is just that..minimum.

    The flip-side of that is that intermediate usually suffers, as does width of read area.

    I usually go through a mental check-list of potential advantages, and pitfalls of changing the design, and change only if advantage is apparent. I hope this helps!
    A+ for this one

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    Quote Originally Posted by drk View Post
    See if you agree:
    1. I don't finesse to the degree where hyperopes get this lens and myopes get that lens.
    2. Occasionally, like with anisometropia, a short corridor is a good thing, but in general I don't hear people say "I have to look too far down in my glasses (unless they're undercorrected).
    3. Short corridors achieve their greatness by messing something else up. If short corridors were optimal, we wouldn't have "regular" corridors.
    4. Ergo, a normal corridor lens is optimal, and should be encouraged.

    What do you think?
    1 agree
    2 somewhat agree but not really, they may not be under corrected, the B may be to Big. In the event you have to much B or a fixed length corridor that's to long (ie the definity with a Min fitting hgt of 18) the pt may find themselves overtaxed to get into the add. Which leads me to
    3 in which I don't really agree with. Shorter corridor lenses (ie unity *gasp!*) have a wonderfull assortment of lengths to pick and choose from with little or no disadvantage depending on the PT needs. These must be used with discretion.
    4 Ergo, a "normal" corridor lens will be optimal in some cases but not all and should also be used judiciously.

    I think with great power comes great responsibility.

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    What's up? drk's Avatar
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    Super post.

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Let's try to define normal, short and long corridors.

    Go!

    B

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    Quote Originally Posted by Barry Santini View Post
    Let's try to define normal, short and long corridors.

    Go!

    B
    Ha! Barry I love your sense of humor. Define normal Bwa hah ha! http://www.optometry.co.uk/uploads/a...13-c-33557.pdf This is a good article on variable length corridors.

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    Quote Originally Posted by Happylady View Post
    When you have a patient wearing a short corridor lens, what do you do if their new frame has a much deeper B measurement? Do you keep them in a short corridor or switch to a regular one?
    I use short corridors always.

  13. #13
    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by drk View Post
    3. Short corridors achieve their greatness by messing something else up. If short corridors were optimal, we wouldn't have "regular" corridors.
    4. Ergo, a normal corridor lens is optimal, and should be encouraged.
    Yes, especially for adds over +1.75.

    Quote Originally Posted by Barry Santini View Post
    Let's try to define normal, short and long corridors.

    Go!

    B
    Newspaper Full page, article body (about 12 point), average light, adds >2.00, distance roughly 40cm for all heights.

    Top third of page (primary to slight upgaze)

    Short corridor- moderate chin lift to clear text (from a blurred state)

    Medium- moderate to severe chin lift

    Long- severe chin lift

    Middle third of page (15˚ of ocular rotation to acquire object)

    Short corridor- slight chin lift to clear text

    Medium- slight to moderate chin lift

    Long- moderate to severe chin lift

    Bottom of third of page (30˚ of ocular rotation to acquire object)

    Short corridor- clear

    Medium- clear

    Long- slight chin lift to clear text

    General rule only.

    Less light, big pupils, high minus, short vertex distance, etc., requires a more aggressive power profile/shorter corridor.

    Alternatively, more light, smaller pupils, high plus, longer VTX, requires a more relaxed power profile and longer corridor.
    Last edited by Robert Martellaro; 08-11-2014 at 11:59 AM.
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  14. #14
    What's up? drk's Avatar
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    See, Robert, you do the #1 finesse thing so well. I still don't understand it.

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    drk,

    Ophthalmic optics is loaded with fine lines.

    What specifically do you need clarified?
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  16. #16
    What's up? drk's Avatar
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    You tried to teach me this before, but I couldn't get it.

    You say myopes need the short corridor and hyperopes the longer.

    I conceptualize it that if "gradient" BD (from the minus lens) "squishes" the image (a big if, I know), you'd need to start with a longer corridor. Alternatively, BD moves things higher, so the minus patient gets a "raised" image on downgaze, anyway, eliminating the worry of too much rotation downwards.

    You are 180 degrees from this, and I never get it.

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    You've got it, but dont know it drk. Myopes = less downward rotation for reading marries best with a shorter corridor.

    B

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    One eye sees, the other feels OptiBoard Silver Supporter
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    It's somewhat counterintuitive, a diagram would help if anyone has one.

    Let's look at a pl sph PAL. Reading depth for the wearer is 15mm. The object that the wearer is looking at is approximately (ignoring minor effects from the add power) 15mm below the primary gaze. PAL design X has a corridor length that allows the wearer to see J1 clearly, without posturing, at a minimum of 15mm. Perfect.

    Change the distance power to -6.00. Due to prismatic effects, the object is now displaced upwards about 2mm. The object appears to the eyeglass wearer to be about 13mm below the pupil instead of 15mm. However, using the the same PAL design as above, there is only about 85% of the power at this point in the corridor, instead of the 100%!

    The result is that the wearer has to lift their chin to make up the difference, aligning the full power of the PAL optics with the object, which is now higher in the corridor where there is less power. To varying degrees, an unhappy wearer, especially if they're an avid reader.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    What an outstanding explanation. A couple more questions:
    1. Would many of you thoughtful dispensers often use a short corridor for a myope, even given a generous height of, say, 20 mm or more? I know there are plenty of lifestyle variables, but to be honest, I wouldn't have even considered a short corridor for most of the deeper frames we seem to be selling these days.
    2. Am I correct in thinking that some lens designs automatically adjust corridor length to allow for this effect? I'm thinking the S fit may do this.

    Appreciate the advice. Like drk, I'm enjoying the education.

  20. #20
    What's up? drk's Avatar
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    It's my understanding that the variable corridors have their own agenda, not Robert's.

    I think their calculus is that there is a "optimal" corridor length based on population studies, such as "the most comfortable angle of downgaze in conjunction with average amount of neck flexure and your average IKEA desktop is...15 degrees depression" PLUS knowing what minimizes "this effect" and maximizes "that feature" in the lens itself.

    I think they would have every PAL set at 20 mm high, if given a choice. Everything else is a compromise, probably.

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    What's up? drk's Avatar
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    For the record, at risk of later ridicule, I officially don't think this myope/short corridor/hyperope/longer corridor effect as described is real, as described.

    What seems to be postulated is a mismatch between prismatic effect and power progression. I think the variables would be interminably interrelated as to offset each other.

    This is (also) where our man Darryl is going to be so missed.

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    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by Fred Dagg View Post
    What an outstanding explanation. A couple more questions:
    1. Would many of you thoughtful dispensers often use a short corridor for a myope, even given a generous height of, say, 20 mm or more? I know there are plenty of lifestyle variables, but to be honest, I wouldn't have even considered a short corridor for most of the deeper frames we seem to be selling these days.
    2. Am I correct in thinking that some lens designs automatically adjust corridor length to allow for this effect? I'm thinking the S fit may do this.

    Appreciate the advice. Like drk, I'm enjoying the education.
    Thanks.

    Q1- The near zone should be positioned in a way that allows the wearer to experience the best possible vision and/or comfort. A shorter corridor in a deep frame, and a longer corridor in a narrow frame may both be desirable in some instances.

    Q2- Some designs account for the prismatic effects, possibly including differences in the stop distance (vertex distance plus the distance from the corneal plane to the center of rotation). Myopes usually have the longest stop distance, hyperopes the shortest.

    To the best of my knowledge, Essilor's Ovation was the first PAL design to modify the corridor length by base curve, using a shorter corridor for flatter BCs, longer for steeper BCs.

    If you mean variable corridors for different frame sizes, see below.

    Quote Originally Posted by drk View Post
    It's my understanding that the variable corridors have their own agenda, not Robert's.
    Variable corridors are like blind love, satisfying the lens/frame relationship, but sometimes ignoring lens/eye/object relationship, and the needs of the wearer.

    I think they would have every PAL set at 20 mm high, if given a choice. Everything else is a compromise, probably.
    For an average amount of panto and vertex distance, I have found that 19mm to 20mm of usable length is usually enough to eliminate the wearer's awareness of the inferior lens/frame boundary.

    Quote Originally Posted by drk View Post
    For the record, at risk of later ridicule, I officially don't think this myope/short corridor/hyperope/longer corridor effect as described is real, as described.

    What seems to be postulated is a mismatch between prismatic effect and power progression. I think the variables would be interminably interrelated as to offset each other.

    This is (also) where our man Darryl is going to be so missed.
    No, no, and sadly yes. Try this- remove your eyeglasses and take one of your trial lenses, the aforementioned -6.00 will do, and look down at the edge of your desk through the bottom of the lens. Note the position of the desk edge compared the the other eye.

    The deviation is the stop distance x prism induced.

    If the reading depth is 15mm, the prism induced will be 9 prism diopters. Because one prism diopter represents a deviation of 1cm at a distance of one meter, and the stop distance averages 27mm (probably more if the wearer is a -6.00 myope due to a longer axial length), the eyes turn up about 2.5mm towards the prism apex (9 x .27). That's more than enough to influence the near optics if it is not compensated for in the lens design, or by choosing an appropriate PAL design.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  23. #23
    What's up? drk's Avatar
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    Let's dumb this down so I can follow.

    Let's use round numbers. Say it's a -10.00 lens, and the patient is looking 20mm down. That's 20 p.d. BD OU. The image shoots up (variably but forget that) a certain linear amount, and let's not get all into the trigonometry.

    But the image has already gone through the lens before it gets to the eye. In fact, it's gone through the lens at the exact point where the power would be correct for that ray. It doesn't go up and THEN through the lens at the wrong point.

    Right?

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    Quote Originally Posted by drk View Post
    But the image has already gone through the lens before it gets to the eye. In fact, it's gone through the lens at the exact point where the power would be correct for that ray. It doesn't go up and THEN through the lens at the wrong point.

    Right?
    The change in location of the object forces the eye to rotate out of position in the corridor, pointing too high for minus powers, and too low for plus powers, compared to the intended design of the PAL, unless the corridor length is optimized for the Rx.

    Same for the inset- distance minus powers requires less inset, plus powers require more inset.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    What's up? drk's Avatar
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    Hmm. You have a point on that inset thing.

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