Fine tune of Astigmatism with Toric IOL

To my understanding,the AcrySof IQ toric have 3 models SN6AT3-5 to take care the astigmatism with 1.5, 2.25 and 3 diopter cylinder power.
If I have a WTR corneal astimgatism to be 0.9 D and a surgeon choose a SN6AT3(1.5D power), does it means that I will over-corrected by 0.6 D.
In order to fine tune it, will the surgeon place the cataract incision to increase my WTR astigmatism by some amount (e.g. 0.6D) to match the 1.50D toric lens.

Another question is how much difference with the calculation using Haigis or Hoffer formula. I tried to input the same parameters of the 2 formulas. I found that resulting myopia from the formulas has almost 1.0 diopter difference.

Thanks

[Ray T Oyakawa, MD]

You really need to discuss this with you doctor.

Your residual astigmatism with a T3 would be 0.37 D at 90 if your steep axis is at 90 and your surgeon makes his incision at 180 or Zero with an SIA of 0.5 D.

Regarding the formulas--you would have to really study this to understand them--most ophthalmologist do not understand them.  However,  Hoffer is best for short eyes (hyperopic) and Haigis is a more advanced formula.   The use of the formulas depends on the surgeon' personalized A-constant.

Holladay 2 is also an advance formula which is very good.  I use Holladay 2, Haigis, and SRKT (only for long eyes).  It is difficult to personalize the constants for Haigis (I use some constants optimized by other doctors I know.).  This can be done by Dr. Hill in the US and another doctor in Germany.  It takes about 100 cases.

Dr. O.

[taurus1203]

I rephrase my question to make more sense

To my understanding,the AcrySof IQ toric have 3 models SN6AT3-5 to take care the astigmatism with 1.5, 2.25 and 3 diopter cylinder power.
If I have a WTR corneal astimgatism to be 0.9 D and a surgeon choose a SN6AT3(1.5D power), will the actual power will smaller than 1.5D, let say 1.2D.
If it is actual effective power of the IOL is 1.2D , does it means that I will over-corrected by 0.3 D.
In order to fine tune it, will the surgeon place the cataract incision to increase my WTR astigmatism to 1.2D to match IOL.

Another question is how much difference with the calculation using Haigis or Hoffer formula. I tried to input the same parameters of the 2 formulas. I found that resulting myopia from the formulas has almost 1.0 diopter difference.

Thanks

[Ray T Oyakawa, MD]

You really need to discuss this with you doctor.

Your residual astigmatism with a T3 would be 0.37 D at 90 if your steep axis is at 90 and your surgeon makes his incision at 180 or Zero with an SIA of 0.5 D.

Regarding the formulas--you would have to really study this to understand them--most ophthalmologist do not understand them.  However,  Hoffer is best for short eyes (hyperopic) and Haigis is a more advanced formula.   The use of the formulas depends on the surgeon' personalized A-constant.

Holladay 2 is also an advance formula which is very good.  I use Holladay 2, Haigis, and SRKT (only for long eyes).  It is difficult to personalize the constants for Haigis (I use some constants optimized by other doctors I know.).  This can be done by Dr. Hill in the US and another doctor in Germany.  It takes about 100 cases.

Dr. O.

[taurus1203]

Dr Oyakawa,
Thanks a lot for detail explanation and help me to predict the residual astigmatism.
I think you use a more accurate 1.03 D for T3  as published by Acrysof.
I guess your computation of the residual astigmatism is : 0.9 + 0.5 - 1.03 = 0.37.

If I DON'T use the toric lens and have the cataract incision at the 90 degree and assuming the SIA is 0.5 then my residual astigmatism will be
0.9 - 0.5 = 0.4

That means the residual astigmatism will be almost the same (0.37 against 0.4) for both scenario.
Am I correct ?

I just want prepare myself have a little knowledge to the coming discussion with the surgeon.

Thanks again

[Ray T Oyakawa, MD]

Yes, but use the acrosoft toric calculator.  With the rule astigmatism is a range and unless the steep axis is at 90, your calculation is incorrect since it is a vector calculation.

Dr. O.

[taurus1203]

My prescription of the glass is 180 deg, -1.75 cylinder
I think my steep axis is at 90 deg and I guess my calculation is ONLY correct in this scenario.
Is it ?
Thanks

[Ray T Oyakawa, MD]

Your glasses prescription is not used in the calculation.  You need your corneal ks and your calculation is correct only when the incision is on the steep axis or 90 degrees from it.

Dr. O.