Rx from K-reading.

[ituryu]

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Guys please can anyone give me the guideline for accurately generating a spectacle Rx from your keratometric reading, I seem to have mine mixed up or I am not too sure. I am using an average of 42.50D for normal population, but my main problem is generating the sphere.
Thanks, and please do not refer me to a textbook except it's online, cause I don't have one if not I would not be putting up this post.

 

[UABopt]

Guys please can anyone give me the guideline for accurately generating a spectacle Rx from your keratometric reading, I seem to have mine mixed up or I am not too sure. I am using an average of 42.50D for normal population, but my main problem is generating the sphere.
Thanks, and please do not refer me to a textbook except it's online, cause I don't have one if not I would not be putting up this post.

I could be completely wrong here, but I am not exactly sure why you would want to determine an RX completely just from K’s. As far as I understand, the keratometer measures only the surface curvature of the central 3mm of the cornea (and that at only the two principle meridians)… and even the Dioptric value that is obtained is an estimate based on the measured curvature and population averages.

It is possible however to estimate the total astigmatism of the eye based on Javal’s rule, which basically just says that the average person has about 0.50D of against the rule lenticular astigmatism that must be added to the corneal astigmatism determined by the Delta K’s.

I suppose you could theoretically calculate a sphere power from the K’s if you knew the axial length and lenticular power of the eye in question. I don’t know why you would want to do that though, it does not seem very practical. Perhaps I am missing something here.

I see you are basing your answer on an average of 42.50D…. In this case I would say if for example your K’s were 44.00/45.5@90 in this case the cornea of the eye in question has 1.50D too much power in the 180 meridian and 3.00D too much power in the 90 meridian. Which is compound myopic astigmatism.
You would calculate:
-1.50X180 as your Delta K’s we must now account for lenticular astigmatism
adding 0.50D of against the rule astigmatism gives you:

-1.00X180 as your cyl power (assuming you don’t use the formula 1.25(Corneal astigmatism)+(+0.50X180) <-for the against the rule lenticular astigmatism). Also technically if the cyl power is not either exactly in the 180 or the 90 meridians you will have to calculate what the power of the against the rule astigmatism from javal’s rule is in that meridian, but I am not going to go into how to do that.

Your sphere power would be 42.50-44.00 giving you -1.5D… to correct this eye you would need -1.50-1.00X180. This could all be completely wrong it has been a while since I did any of these calculations. This would all be much easier to explain if I could draw lens crosses. OH well…. I hope that helps a little, perhaps you should invest in a text book.

 

[ituryu]

Now this wasn't exactly the guideline I was hoping to get, and that's not saying you are wrong cause I seem not to understand it, one of my lecturer's gave this in school, but if we look at it, what I understood seemed logical:
Using 44.00/45.50@90, and taking the average K-reading from whatever population which clinical research I have no idea of been 42.50; you find the difference from but K- readings, and you get:
-1.50@180/-3.00@90, and not even considering Javal's rule, you transpose to a cross cyl. thus:
-1.50x90/-3.00x180, and then transpose to your spherocyl:
-1.50-1.50x180, and this become your starting point in your subjective refraction!and believe me this came out in one of my examsand I asked questions about certain things and even referred to neglecting Javal's ATR compensation, I got a C for all that, funny I did not fail it!
Now I believe for PCIOL Pxts, this is not all that accurate, but the K-reading if autorefraction is not convenient as I have seen in my attachment, and the possibility of making a good sphcyl Rx from K reading seems fair, btu the ground rule is not all that clear to me. Yes I did think of going to my lecturers, but most of them are not approachable, while the ones that are approachable are very busy. And pls don't suggest that I should go to my mate, they don't even know what it implies or plan to use or understand most of the things, all they do is memorise the whole thingand reproduce it again during the examsin fact the name Javal has no meaning to them . It's the honest truth!!!

 

[UABopt]

Now this wasn't exactly the guideline I was hoping to get, and that's not saying you are wrong cause I seem not to understand it, one of my lecturer's gave this in school, but if we look at it, what I understood seemed logical:
Using 44.00/45.50@90, and taking the average K-reading from whatever population which clinical research I have no idea of been 42.50; you find the difference from but K- readings, and you get:
-1.50@180/-3.00@90, and not even considering Javal's rule, you transpose to a cross cyl. thus:
-1.50x90/-3.00x180, and then transpose to your spherocyl:
-1.50-1.50x180, and this become your starting point in your subjective refraction!and believe me this came out in one of my examsand I asked questions about certain things and even referred to neglecting Javal's ATR compensation, I got a C for all that, funny I did not fail it!
Now I believe for PCIOL Pxts, this is not all that accurate, but the K-reading if autorefraction is not convenient as I have seen in my attachment, and the possibility of making a good sphcyl Rx from K reading seems fair, btu the ground rule is not all that clear to me. Yes I did think of going to my lecturers, but most of them are not approachable, while the ones that are approachable are very busy. And pls don't suggest that I should go to my mate, they don't even know what it implies or plan to use or understand most of the things, all they do is memorise the whole thingand reproduce it again during the examsin fact the name Javal has no meaning to them . It's the honest truth!!!

I am no expert on this subject mater, but it seems to me that your method for performing the calculation works just fine…. Other than the fact that Javal’s rule has not been accounted for. Again I believe this calculation may be okay purely as an academic exercise, but I imagine that it would have very little clinical significance if any at all. After a certain level say about 2.00D or so most refractive error is axial in nature anyhow, that is to say that there are some people, though perhaps not the norm, with steep corneas that are hyperopic and some people with flat corneas that are myopic. A keratometer is an instrument that is good to use for its intended purpose, namely to measure the surface curvature of the central 3mm of the cornea, and from that also determine corneal astigmatism. If you have no autorefractor available then you should just use retinoscopy to determine a starting point for your subjective refraction, a skilled retinoscopist is thought by many to be more accurate than an autorefractor anyhow. If a retinoscope is also not available for whatever reason, then I would use the clock dial chart to estimate a good starting point. Though again I would never profess to be an expert on this subject.
Am I just completely wrong?

 

[ituryu]

This is the point I am trying to make you see the rule is not constant or well defined, and you cannot determine if the eye is hyperopic or astigmatic, note you are not using ocular biometry. Try and see through this.

 

[Ben Chudner]

Just another example of how little our schools prepare us for the real world. I understand the need to teach optics so that OD's have a strong foundation, but problems like this have no application in a clinical setting. I haven't used Javal's rule since boards, nor have I needed to get out a piece of paper to try to figure out a patient's optical cross. This is information that you need to build a foundation for future learning, but I would recommend not killing yourself over remembering this stuff. Good luck in your future studies.

 

[ituryu]

Just another example of how little our schools prepare us for the real world. I understand the need to teach optics so that OD's have a strong foundation, but problems like this have no application in a clinical setting. I haven't used Javal's rule since boards, nor have I needed to get out a piece of paper to try to figure out a patient's optical cross. This is information that you need to build a foundation for future learning, but I would recommend not killing yourself over remembering this stuff. Good luck in your future studies.

Ben I feel you on this, but from my industrial attachment, I have had Pxts especially PCIOL Pxts whom I don't get anty good Ret. reflex, and autorefraction not possible except autokeratometry, and the rule some have is making half the cyl component value their sphere now this can't make a good decision on if the Pxt is myopic or hyperopic, not consideriing if the Pxt has a good depth of focus.
Rules that don't work definitely!!!
But thanks for your reply. PS can you define Best Visual Acuity?