- Ridley's post op refraction in 1949 was -24.0 +6.0 x30 - your patients will expect better
- Four things you need to know to calculate correct IOL power:
- 1) Desired postoperative SE
- Usually -0.5 to -1.00 is the plan. Why?
- if you are left myopic post op. someplace in front of your head is in focus
- -1.00 gets you about 20/40 at far and you can see well at mid distance
- A spectacle overcorrection of -1.00 will nearly eliminate induced IOL mag.
- Sometimes however you may not want a SE of -1.00
- if the other eye is stable (post op or no cataract) and you want to match it
- patient prefers to have 20/20 at far without correction (good luck)
- 2) The length of the eye
- AEL device uses ultrasound to estimate the length of the eye
- Measure both eyes
-
| Remeasure if |
greater than 0.3 mm difference between eyes
AEL < 22 or > 25
|
- Full of potential error, pushing too hard, not axial, etc...
- Error on device at VA can be, as high as +/- 0.5 mm. according to Dr. Ossoinig
- With AEL < 22 or > 25 consider sending to echo for immersion beware of staphyloma
- 3) The power of the cornea
- Keratometric measurement of both eyes -- should be about the same
-
| Remeasure if |
greater than 1 diopter difference between eyes
K ave < 40 or > 47
mires are irregular |
- 4) The post operative position of the IOL
- The more anterior the IOL is placed the less power the IOL needs for emmetropia
- Goal is to place a posterior chamber (PC) lens
- These can end up in the bag (best) or sulcus (anterior to ant. capsule)
- Inadvertent placement in the sulcus creates a 0.75 d myopic shift
- Always plan to have available anterior chamber (AC) lenses
- These are placed anterior to the iris w/haptics that settle into the angle
- These are used when the posterior capsule is not agreeable to holding an IOL
- When too small they can tilt and when too large they can hurt
Estimating the IOL power for emmetropia:
- Formulas started with a theoretical model by Fydorov, Collenbrander et all, 1970s
- Based on geometric optics
- Power = N/(AEL-ACD) - N/(N/K-ACD)
-
| where: |
Power is the expected power of IOL for emmetropia post op
N is the aqueous and vitreous refractive index
ACD is the post operative depth of the IOL
AEL is the axial eye length as measured via an ultrasound device
Ave-K is average of the two keratometric axes |
- But you don't know the ACD or post operative depth of the lens pre op!
- Useful Formulas use regression analysis or other tricks to estimate ACD
-
| 1) SRK |
| Power = A-constant - 2.5(AEL) - 0.9(Ave-K) |
|
- Classic regression formula develped in 1980 by Sanders, Retzlaff, and Kraff
- where: A-constant is a parameter of the type of lens and ACD
-
| eg. |
PC lens -- Alcon acrysoft A constant of 118.9
AC lens -- Alcon MTA series A constant of 115.3 |
- note that an error of x in axial eye length results in an error of 2.5x in IOL power
- falls apart in predictive value with eyes w/AEL < 22 and > 24.5
- 2) Other algorithms
- AEL device has many algorithms to choose from to give power for both PC and AC lenses
- Some also use ant chamber depth to estimate IOL power
- Error is greatest with with high myopes and high hyperopes
- Estimate the power for both the AC and the PC lens compare several formulas
- High myopes end up more myopic and high hyperopes end up more hyperopic post op than predicted
-
Selecting the IOL power for your patient
- The SRK computes the lens power for emmetropia; but, you probably want -1.00
- you can use the AEL device printout to find the lens for any desired post op. SE
- roughly a change of the IOL from the emmetropic value of 1.4 results in SE change of 1.0
- eg. SRK gives 19 diopters for emmetropia, about 20.5 will give -1.00 SE post op.
- If your estimated IOL power is unusual you are probably wrong
- Double check your calculations and when applicable look at experience with other eye's IOL
- ask yourself was the patient very myopic or hyperopic as a young person (eg in the big war)
- then if convinced that the calculations are right, make sure the lens is stocked
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