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Front surface toric VS Back surface toric

This is a discussion on Front surface toric VS Back surface toric within the Toric Contact Lenses forums; My current lens type is "front surface toric" with the following data: BC: 8.6 Diametet: ...


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  #1 (permalink)  
Old 06-19-2009, 10:35 AM
Contact Lenses Forum - Freshman
 
Join Date: Jun 2009
Posts: 1
Default Front surface toric VS Back surface toric

My current lens type is "front surface toric" with the following data:

BC: 8.6
Diametet: 14.5
Power: -12.50
Cyl Power: -0.75
Axis: 180

My friend suggest me to change the lens type to "back surface toric" due to better stablisation.

Is that true? And, will the lens prescription data change if I choose "back surface toric"?

Please advise. Thanks
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  #2 (permalink)  
Old 09-15-2010, 09:27 AM
Contact Lenses Forum - Senior
 
Join Date: Dec 2007
Posts: 205
Default Front surface toric VS Back surface toric

Quote:
Originally Posted by blackhorse View Post
My current lens type is "front surface toric" with the following data:

BC: 8.6
Diameter: 14.5
Power: -12.50
Cyl Power: -0.75
Axis: 180

My friend suggest me to change the lens type to "back surface toric" due to better stablisation.

Is that true? And, will the lens prescription data change if I choose "back surface toric"?

Please advise. Thanks
Hi blackhorse.

According to conforma.com--ahem--"Back Toric designs are normally utilized on those patients with both significant corneal toricity (>2.50 Diopters) and refractive cylinder approximately 1.5 times the corneal astigmatism . . . "

I'm not sure what "corneal toricity" is, but it's measured in diopters (">2.50 Diopters"), and so are the power and cylinder measurements in your prescription, so it's one of those.
Then I noticed that conforma.com said " . . . and refractive cylinder approximately 1.5 times the corneal astigmatism" so it's something different than the cylinder. (Deductive reasoning at it's best, folks.)

So, in order to benefit from back toric lenses, the "toricity" or power should be more than 2.5 diopters. Check.

Then the cylinder measurement must be approximately 1.5 times the astigmatism. By that I'm guessing they mean the "toricity." Your cylinder measurement is -0.75. Is that 1.5 times greater than your astigmatism of -12.50? No, so you're not a textbook case.
However, that's assuming my calculations are correct, and I'm certainly not confident of that.

So after all that, my recommendation is to ask your eye doctor.

Unless someone else comes along with a better answer.
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  #3 (permalink)  
Old 09-16-2010, 02:43 PM
Contact Lenses Forum - Senior
 
Join Date: Dec 2007
Posts: 275
Default Pardon My Corneal Toricity

Quote:
Originally Posted by BigBanker1 View Post
Hi blackhorse.

According to conforma.com--ahem--"Back Toric designs are normally utilized on those patients with both significant corneal toricity (>2.50 Diopters) and refractive cylinder approximately 1.5 times the corneal astigmatism . . . "

I'm not sure what "corneal toricity" is, but it's measured in diopters (">2.50 Diopters"), and so are the power and cylinder measurements in your prescription, so it's one of those.
Then I noticed that conforma.com said " . . . and refractive cylinder approximately 1.5 times the corneal astigmatism" so it's something different than the cylinder. (Deductive reasoning at it's best, folks.)

So, in order to benefit from back toric lenses, the "toricity" or power should be more than 2.5 diopters. Check.

Then the cylinder measurement must be approximately 1.5 times the astigmatism. By that I'm guessing they mean the "toricity." Your cylinder measurement is -0.75. Is that 1.5 times greater than your astigmatism of -12.50? No, so you're not a textbook case.
However, that's assuming my calculations are correct, and I'm certainly not confident of that.

So after all that, my recommendation is to ask your eye doctor.

Unless someone else comes along with a better answer.
I was curious about this corneal toricity, so I looked it up. However, every time the phrase "corneal toricity" came up, it was part of a highly technical journal, it seemed. So I thought I'd just try looking up the word "toricity" and to my surprise, it was almost always preceded by the word "corneal," so I guess that's about the only kind there is.

Finally, I found this--

The cylinder or cylindrical component of a spectacle prescription is an indication of the amount of astigmatism in an eye. Astigmatism is the amount that the refractive structures of the eye (primarily, the front and back surfaces of the cornea and the front and back surfaces of the crystalline lens) vary or deviate from sphericity or a spherical shape. That is, astigmatism indicates the degree of toricity or cylindrical shape of all of the eye’s refractive components, combined.

So "toricity" is another word for "cylindrical shape."

Was that so hard?
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  #4 (permalink)  
Old 09-20-2010, 11:46 AM
Contact Lenses Forum - Senior
 
Join Date: Dec 2007
Posts: 205
Default

Quote:
Originally Posted by Unclelar View Post
I was curious about this corneal toricity, so I looked it up. However, every time the phrase "corneal toricity" came up, it was part of a highly technical journal, it seemed. So I thought I'd just try looking up the word "toricity" and to my surprise, it was almost always preceded by the word "corneal," so I guess that's about the only kind there is.

Finally, I found this--

The cylinder or cylindrical component of a spectacle prescription is an indication of the amount of astigmatism in an eye. Astigmatism is the amount that the refractive structures of the eye (primarily, the front and back surfaces of the cornea and the front and back surfaces of the crystalline lens) vary or deviate from sphericity or a spherical shape. That is, astigmatism indicates the degree of toricity or cylindrical shape of all of the eye’s refractive components, combined.

So "toricity" is another word for "cylindrical shape."

Was that so hard?
So "corneal toricity" is a phrase that describes how much like a cylinder one's cornea is shaped.

Like you said, Unclelar. "Was that so hard?"
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  #5 (permalink)  
Old 09-20-2010, 12:07 PM
Contact Lenses Forum - Ph.D.
 
Join Date: Nov 2009
Location: near Hamburg, Germany
Posts: 2,144
Default

Quote:
Originally Posted by blackhorse View Post
My current lens type is "front surface toric" with the following data:

BC: 8.6
Diametet: 14.5
Power: -12.50
Cyl Power: -0.75
Axis: 180

My friend suggest me to change the lens type to "back surface toric" due to better stablisation.

Is that true? And, will the lens prescription data change if I choose "back surface toric"?

Please advise. Thanks
Blackhorse, your astigmatism is quite minor (-0.75D) and many eye care specialists would not correct that with contact lenses. I don't think a back toric design would improve lens axis stability as the astigmatism/toricity you have is so small.

knotlob
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  #6 (permalink)  
Old 09-20-2010, 12:15 PM
Contact Lenses Forum - Senior
 
Join Date: Dec 2007
Posts: 205
Default

Quote:
Originally Posted by Knotlob View Post
Blackhorse, your astigmatism is quite minor (-0.75D) and many eye care specialists would not correct that with contact lenses. I don't think a back toric design would improve lens axis stability as the astigmatism/toricity you have is so small.

knotlob
You're right, Knotlob. Blackhorse's astigmatism is quite minor. I thought I'd read someplace (Maybe here.) that if your astigmatism prescription is less than 1.00, you don't really have to correct it. Would you say that was an accurate statement?
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Old 09-20-2010, 03:04 PM
Contact Lenses Forum - Ph.D.
 
Join Date: Nov 2009
Location: near Hamburg, Germany
Posts: 2,144
Default

Quote:
Originally Posted by BigBanker1 View Post
You're right, Knotlob. Blackhorse's astigmatism is quite minor. I thought I'd read someplace (Maybe here.) that if your astigmatism prescription is less than 1.00, you don't really have to correct it. Would you say that was an accurate statement?
Yes, I think many contact lens practitioners would probably not correct for the astigmatism using a toric lens due to the extra cost and potential difficulties with comfort and alignment for a relatively small improvement in vision. They would probably increase the spherical correction by half of the cylinder value if torics were not prescribed.

Some toric contact lens manufacturers don't supply cylinder correction values less than -1.25D anyway.

Easy enough to correct in spectacles I guess.

knotlob
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  #8 (permalink)  
Old 09-20-2010, 04:10 PM
Contact Lenses Forum - Bachelors Degree
 
Join Date: Dec 2007
Posts: 570
Default

Quote:
Originally Posted by Knotlob View Post
Yes, I think many contact lens practitioners would probably not correct for the astigmatism using a toric lens due to the extra cost and potential difficulties with comfort and alignment for a relatively small improvement in vision. They would probably increase the spherical correction by half of the cylinder value if torics were not prescribed.

knotlob
Wait, run that by me again? Increase the spherical correction by half of the cylinder value? What does that mean?
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  #9 (permalink)  
Old 09-20-2010, 05:15 PM
Contact Lenses Forum - Ph.D.
 
Join Date: Nov 2009
Location: near Hamburg, Germany
Posts: 2,144
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Quote:
Originally Posted by Zoey View Post
Wait, run that by me again? Increase the spherical correction by half of the cylinder value? What does that mean?
Blackhorse's prescription is:
BC: 8.6
Diameter: 14.5
Power: -12.50
Cyl Power: -0.75
Axis: 180

If the optician prescribes a non toric/spherical prescription only, it is normal practice to add on half of the cylinder value (which is not being corrected) to the spherical prescription.

So with the above figures, spherical power is -12.5D, cylinder is -0.75 (but not applied). So the spherical only prescription will be (-12.5 + (0.5*-0.75)) = -12.88D.

knotlob
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  #10 (permalink)  
Old 09-21-2010, 10:28 AM
Contact Lenses Forum - Bachelors Degree
 
Join Date: Dec 2007
Posts: 566
Default

Quote:
Originally Posted by Knotlob View Post
Blackhorse's prescription is:
BC: 8.6
Diameter: 14.5
Power: -12.50
Cyl Power: -0.75
Axis: 180

If the optician prescribes a non toric/spherical prescription only, it is normal practice to add on half of the cylinder value (which is not being corrected) to the spherical prescription.

So with the above figures, spherical power is -12.5D, cylinder is -0.75 (but not applied). So the spherical only prescription will be (-12.5 + (0.5*-0.75)) = -12.88D.

knotlob
Thanks for explaining, Knotlob. I can see why Zoey got confused.
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  #11 (permalink)  
Old 11-01-2010, 02:28 PM
Contact Lenses Forum - Senior
 
Join Date: Dec 2007
Posts: 205
Default

Quote:
Originally Posted by Knotlob View Post
Blackhorse's prescription is:
BC: 8.6
Diameter: 14.5
Power: -12.50
Cyl Power: -0.75
Axis: 180

If the optician prescribes a non toric/spherical prescription only, it is normal practice to add on half of the cylinder value (which is not being corrected) to the spherical prescription.

So with the above figures, spherical power is -12.5D, cylinder is -0.75 (but not applied). So the spherical only prescription will be (-12.5 + (0.5*-0.75)) = -12.88D.

knotlob
It's good that we've got someone on board who can Do The Math.
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  #12 (permalink)  
Old 11-02-2010, 04:01 PM
Contact Lenses Forum - Bachelors Degree
 
Join Date: Dec 2007
Posts: 570
Default

Quote:
Originally Posted by Orionebula View Post
Thanks for explaining, Knotlob. I can see why Zoey got confused.
That helps a little. I'm still a bit confused, but less so.
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  #13 (permalink)  
Old 11-02-2010, 05:17 PM
Contact Lenses Forum - Senior
 
Join Date: Aug 2010
Posts: 289
Default

What we need to know is, the spectacle Rx. Without that to start with we can only guess.
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