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Medical Forum / General / Vision / September 2005Toric lens tradeoffs: cylinder, axis, base curve
I'm trying to find lenses close to an ideal prescription of: OD -0.75, -1.00, x115, bc 8.5 OS -0.75, -1.00, x 65, bc 8.5 Tradeoffs: The axis has to be a multiple of 10, so I was told to move closer to 90 degrees (OD = 110, OS = 70). Not sure what the logic is. It seems no one but Ciba has a -1.00 cylinder, but Ciba's base curve is 8.9. Others have bc's of 8.4 to 8.7, but their cylinder specs jump from -0.75 to -1.25. No -1.00. Given that the lens power is lower than the cylinder, I'm thinking that it's important to get close to the correct cylinder/axis, but I'm not sure what mismatching to Ciba's 8.9 bc will cause (dryness?) Or should I try to match the base curve and go with -0.75 cyl? Or -1.25? I realize there are no hard and fast rules, so I'd like to find out what to expect from mismatching base curve vs mismatching cylinder. If the latter, which direction to mismatch. mm when fitting a toric Rx, you start with the closest available contact prescription in a brand that you like best. I would start you in a trials that are -0.75-0.75 X 110 or 120 (right) and 60 or 70 (left). Then you let the patient wear the lenses for awhile (several days to a week) and then recheck them at an appointment where they come in with their contacts on. during the recheck appointment the eyedoc can see how the lenses are rotating on your eyes and make a final cylinder axis determination. likewise the eyedoc can may a final cylinder and sphere power determination at that time. its not important, and even impossible, to have the cylinder corrected exactly. also, the patient will know how comfortable they feel due to their experience wearing them. as you can see this is a multistep process that you have to walk, not run, into. you are getting way too detailed too fast. don't try to pick a brand that is exactly your prescription-- as you know they don't make them at exactly your prescription. also, they likely will rotate on your eyes slightly which makes the starting point irrelevant anyway. even if you find the exact prescription, you might not like the comfort of the lens! go to an eyedoc, or contact lens fitter, and get the fitting done right! they will get you free trial lenses and start you through the process. that way, when you start purchasing your lenses, you will know what you are buying. PS-- i wouldn't start with the CIBA Focus Toric lenses that you are talking about. if I were your eyedoc I would start with SofLens 66 Toric, Biomedics Toric, or Vertex Toric. >go to an eyedoc, or contact lens fitter, and get the fitting done >right! they will get you free trial lenses and start you through the >process. that way, when you start purchasing your lenses, you will >know what you are buying. I appreciate what you are saying, and that would be the ideal scenario. I've already seen an optometrist who took the readings that I posted. Also measured existing lenses, etc. Unfortunately he doesn't carry any lenses; he's more an eye surgeon. My regular optician just said "Can't match your prescription" and was stubborn about offering an explanation on how to get something close. Third stop was another optician who did more measurements, dismissed the optometrist's prescription, and came up with another prescription that doesn't seem correct. Now he's too busy to do a more thorough followup. Given that the initial optometrist's prescription is very close to glasses prescribed a couple years ago, I believe it is correct. I had Dr #3 swap one lens so it's closer to that, and it feels better. Both eyes feel to be about same focus now. The decision on how to 'balance' parameters must be a logical one. Given large the gaps in manufacturers' specs (Ciba *starts* at base curve 8.9, and no one else seems to make -1.00 and 'even' cylinders...?), this decision must be made constantly. I just don't know which are more important. >PS-- i wouldn't start with the CIBA Focus Toric lenses that you are >talking about. if I were your eyedoc I would start with SofLens 66 >Toric, Biomedics Toric, or Vertex Toric. Great, that's what I was looking for. But can you explain why SofLens 66 etc rather than Ciba? I'd assume that means that base curve is more important that I had first thought, and that cylinder mismatch is more forgiving. Is it as simple as that? mm what country do you live in? base curve is relatively unimportant. if a lens is tried, and if doesn't fit properly, an eyedoc may change the base curve to improve the fit but it seldom is the initial reason why a lens is chosen. CIBA Focus lenses are the only disposable lenses that come in -1.00 cylinder powers. its just the way the company choses to make them. using -0.75 makes little difference. I like SofLens 66 because i find it to be a stable fitting, comfortable, affordable lens for most of my patents. it is the number 1 brand of disposable toric lens. CIBA Focus is an older brand of lens-- it frequently rotates and requires refitting. fitting torics is not as simple as comparing some parameters-- you need an experienced contact lens fitter to evaluate the lens fit on your eye. >what country do you live in? <g> That probably did sound a bit third-world. I'm in New York City. I'm sure there is a good optician around here somewhere, but the two that I've seen seem rushed. I'm not sure that the proper tests are being done. Unfortunately I can't just keep paying the fitting and test fee. So here I am. >base curve is relatively unimportant. if a lens is tried, and if >doesn't fit properly, an eyedoc may change the base curve to improve >the fit but it seldom is the initial reason why a lens is chosen. I guess there's a reason for the limited number of base curves available. >CIBA Focus lenses are the only disposable lenses that come in -1.00 >cylinder powers. its just the way the company choses to make them. >using -0.75 makes little difference. OK. I was hesitant to go with other brands due to the jump from -0.75 to -1.25. If it's that small an increment, it sounds like I'm just a step away from planars. >I like SofLens 66 because i find it to be a stable fitting, >comfortable, affordable lens for most of my patents. it is the number 1 >brand of disposable toric lens. CIBA Focus is an older brand of >lens-- it frequently rotates and requires refitting. Understood. I asked them to order a sample pair of B&L lenses today. >fitting torics is not as simple as comparing some parameters-- you need >an experienced contact lens fitter to evaluate the lens fit on your >eye. I also asked the optician to check rotation today. It's like pulling teeth. I get the feeling I shouldn't have to be requesting all of this. >>go to an eyedoc, or contact lens fitter, and get the fitting done >>right! they will get you free trial lenses and start you through the [quoted text clipped - 8 lines] > My regular optician just said "Can't match your prescription" and was > stubborn about offering an explanation on how to get something close. The original prescription should have had a brand of contact lenses as well as a base curve etc. If it didn't, then it was a glasses prescription, not a contact lens prescription. Any contact lens fitter, optometrist or optician can use the glasses prescription as a starting point for trial fitting of contact lenses. The contact lens power may not precisely match the glasses prescription due to the way that contact lenses interact with the eye. > Third stop was another optician who did more measurements, dismissed > the optometrist's prescription, and came up with another prescription [quoted text clipped - 11 lines] > cylinders...?), this decision must be made constantly. I just don't > know which are more important. The decision on how to balance paramenters is based on observing an actual lens on your actual eye. Don't worry about whether a -1.00, -1.25 or -0.75 cyl is used, what axis is used or what base curve. The most important things are: is the vision clear and stable, is the lens comfortable all day, is the fit correct with correct lens movement, centration and corneal coverage, is the health of the cornea maintained after several months of use. Find a fitter who does proper trial fittings and follow ups. You cannot fit toric contact lenses without putting a lens on the eye, checking the overrefraction, the fit and any rotation. Expect to pay a fee for the fitting and followup. Dr Judy >>PS-- i wouldn't start with the CIBA Focus Toric lenses that you are >>talking about. if I were your eyedoc I would start with SofLens 66 [quoted text clipped - 6 lines] > > mm >"_MM" <_MM@nospam.org> wrote in message >> I've already seen an optometrist who took the readings [quoted text clipped - 7 lines] >as a base curve etc. If it didn't, then it was a glasses prescription, not >a contact lens prescription. Base curve was specified (8.5), but he didn't spec the brand. I do trust my optometrist, but he doesn't deal with contacts. I called back and asked him to rewrite the prescription with a spec'd brand. Problem was that the rotation spec was actually too precise. Lens vendors don't know what to do with Axis = 65 and 115. (I understand that the norm is to round toward 90 degrees). Given the problems, I decided to see an optician. That's why I'm having trouble now. >Any contact lens fitter, optometrist or optician can use the glasses >prescription as a starting point for trial fitting of contact lenses. The [quoted text clipped - 4 lines] >lens on your actual eye. Don't worry about whether a -1.00, -1.25 or -0.75 >cyl is used, what axis is used or what base curve. I'm getting that. Given the limited number of base curves available and the fact that it's apparently not critical, I'm not sure why BC has to be specified so closely on a prescription. And if the jumps from -0.75 to -1.25 cylinder is also not that critical, then I think my prescription is dependent only on brand. I assume there's a reason that manufacturers' specs don't overlap but I can't think of what that would be. By the time a Dr specifies BC and cylinder they have practically prescribed a certain brand. > The most important >things are: is the vision clear and stable, is the lens comfortable all day, [quoted text clipped - 6 lines] >overrefraction, the fit and any rotation. Expect to pay a fee for the >fitting and followup. I have paid their fees and I've been trying the (Ciba) sample lenses with limited success. Dryness after about 6 hours and feeling like they are rotating occasionally. I haven't worn torics before, so I don't know whether this is unusual or correctible. Unfortunately, getting the optician to follow up is like pulling teeth. They've already run my credit card. <g> At this point I know what the prescription should be for any given brand. I'd love to just get the prescription written for B&L's and order them online, but I doubt that they'll do that. Thanks to both of you for the info. Very helpful. I don't suppose anyone knows a good optician in NYC? That would simplify things. >>The original prescription should have had a brand of contact lenses as >>well [quoted text clipped - 8 lines] > vendors don't know what to do with Axis = 65 and 115. (I understand > that the norm is to round toward 90 degrees). If your optometrist doesn't fit contacts, didn't do a trial fitting and then wrote a Rx without a brand name and for parameters that don't exist then he didn't write a contact lens prescription. Did you pay an extra fee for the contact lens Rx? If so, you should ask for it back. A contact lens Rx should be based on observing an actual lens on a actual eye and be for a lens that actually exists. > Given the problems, I decided to see an optician. That's why I'm > having trouble now. [quoted text clipped - 19 lines] > I can't think of what that would be. By the time a Dr specifies BC > and cylinder they have practically prescribed a certain brand. It is not possible to correctly specify BC, cyl and axis without specifying brand. >> The most important >>things are: is the vision clear and stable, is the lens comfortable all [quoted text clipped - 13 lines] > they are rotating occasionally. I haven't worn torics before, so I > don't know whether this is unusual or correctible. If you can't wear for more than 6 hours, then return and demand follow up. The fee paid covers fitting and followup, they are obliged to provide that or refund the fee if they can't followup. > Unfortunately, getting the optician to follow up is like pulling > teeth. They've already run my credit card. <g> At this point I know > what the prescription should be for any given brand. No you don't. The prescription will vary with the brand and can only be determined through fitting. I'd love to just > get the prescription written for B&L's and order them online, but I > doubt that they'll do that. They cannot do that without fitting the B&L and doing followup. > Thanks to both of you for the info. Very helpful. I don't suppose > anyone knows a good optician in NYC? That would simplify things. Or find an optometrist who fits contacts. Dr Judy >> Base curve was specified (8.5), but he didn't spec the brand. I do >> trust my optometrist, but he doesn't deal with contacts. I called [quoted text clipped - 7 lines] >didn't write a contact lens prescription. Did you pay an extra fee for the >contact lens Rx? If so, you should ask for it back. No extra fee. I had seen him about eye strain (10 hours at a computer monitor with progressive lens glasses). I concluded that the best approach was weak lenses. Hence the -0.75. So far the lenses are working even better than I expected--when they aren't rotating or drying out. >>>The decision on how to balance paramenters is based on observing an actual >>>lens on your actual eye. Don't worry about whether a -1.00, -1.25 >>>or -0.75 cyl is used, what axis is used or what base curve. Not sure I follow. I would have thought that astigmatic correction would be the same for glasses and lenses. As for base curve, I concluded that it must not be important or manufacturers would make a range of BCs. I think Ciba's lowest base curve is 8.9. Some others seem to make only one base curve. If it's 'one size fits all,' I don't know why BC even needs to be spec'd exactly on prescriptions. I did notice that there was a line on the thicker part of the lenses, so I asked the optician to check whether that was rotated to the bottom as I wore the lenses. I was surprised that he didn't mention the line or test for positioning before. >If you can't wear for more than 6 hours, then return and demand follow up. >The fee paid covers fitting and followup, they are obliged to provide that >or refund the fee if they can't followup. I requested samples of B&L 66 lenses. They were reluctant but I think they ordered them. >> Unfortunately, getting the optician to follow up is like pulling >> teeth. They've already run my credit card. <g> At this point I know >> what the prescription should be for any given brand. > >No you don't. The prescription will vary with the brand and can only be >determined through fitting. Starting from here: OD -0.75, -1.00, x 110 bc 8.5 OS -0.75, -1.00, x 70 bc 8.5 Ciba makes -0.75 -1.00 x110/x70 and their lowest BC is 8.9 That seems like the obvious choice for Ciba. B&L makes BC = 8.5, but the cylinder needs to be -0.75 or -1.25. So two logical choices for B&L (and most other manufacturers). Am I missing something? The increments seem so wide that there is usually only one lens per manufacturer that will be anywhere close (two if cylinder is rounded upward). Since I'm fending for myself, I'd like to make sure I understand this. >They cannot do that without fitting the B&L and doing followup. What should the followup entail? I'll make sure it gets done. >> Thanks to both of you for the info. Very helpful. I don't suppose >> anyone knows a good optician in NYC? That would simplify things. > >Or find an optometrist who fits contacts. That's probably what I should have done. I'm hoping that the B&L lenses will be the last of it. once again, you are getting into the details too deeply. in broad and general strokes, here are the steps in choosing a contact lens (from the eyedocs point of view) 1. find the closest contact lens prescription to the patients in a brand of lens that, in your experience, fits best is will be most accepted by the customer. 2. get a trial pair of that brand 3. have the patient wear that pair of lenses for a short time and return for a follow-up visit 4. at the follow-up visit, assess the visual acuity, assess the contact lens fit using a slit lamp, and have the patient describe the comfort of the lens over the period of time they wore the lenses. if the lenses fit well and are comfortable but rotate on the eyes, then adjust the prescription accordingly. if the lenses are not comfortable, try another brand. Do not focus on the base curve. It is relatively unimportant for soft disposable contact lenses unless the eyedoc observes a problem at the follow-up exam! Do not worry about getting the exact cylinder correction and axis. They are not available in all cylinder axes and powers anyway! the contacts will likely rotate slightly on your eye anyway making that level of precision irrelevant. slightly under or overcorrecting your astigmatism by 0.25D wound probably not cause any problems anyway. this should answer your questions. BTW, optometrists do not do surgeries (at least in the US). and opticians do not fit contact lenses (in the US but not in other countries). i thought you mentioned your location was in NYC in an earlier post. if this is true, then you have me really confused about the types of professionals you are seeing. choosing an eyedoc to take care of fitting your toric contact lenses what you need to do. don't try to simply do it yourself and buy your lenses over the internet (which is illegal in the US but not in other countries) because you may get something that fits poorly and gives poor vision. and your original prescription that specifies a base curve without any other information is incomplete. whoever provided that prescription did it wrong. |
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