Calculating the power of Ant. Chamber IOL
This is an interesting dilemma faced by many surgeons.
When one encounters a complication and once decides to implant an ACIOL:
I am putting this dilemma in the form of question with multiple answers coming from various surgeons:
I was recently operating a Morgagnian cataract and had planned to implant a PMMA IOL. I usually do an SICS and envelope capsulotomy for these cases. The A-constant of the PMMA IOL was 118.4 and the required power was +22.0D. However I noted inferior 180 degrees zonular dialysis and the inferior half of the bag was loose. So I went to plan B after doing an adequate ant vitrectomy, intracameral pilocarpine and a peripheral iridectomy, I planned to implant an anterior chamber IOL. Now the A constant of the ACIOL was 115.3. So what power of ACIOL should I implant, also keeping in mind that the effective lens position now changes?
The best way is to enter the A constant into your biometer, and calculate the power of the IOL for the given axial length & K readings. Indeed, the difference between AC & PC IOL varies depending upon the power of the IOL. For myopes it is smaller and for hyperopes it is larger. Hence, it is best to go through the Biometer readings.
IOL power for ACIOL is +19.0D.
The A constant of AC IOL is less only because of its altered position as compared to the PC IOL. In this case the power should be +19.0
You can calculate the ac IOL power by using the primitive SRK formula IOL power =A-2.5L-0.9K where u can use the A constant for A and axial length and keratometry for L and K reading respectively.
In ACIOL power calculation A-constant takes care of relative position of IOL. By default it’s made for anterior position (so A constant usually ~3D less than PCIOL). You can either do re IOL power calculation using A-constant 115.3 or simply deduct the difference of two A-constant i.e. 3.1D from PCIOL power as K & AL remain unchanged. In this case ACIOL power should ~18.9 or 19D.
You have to reduce the power of the lens by 3.1 to compensate for the A Constant in your case then the power will be +19.0 and then a further 1 to 1.5 for the anterior position of the lens. So in your case the power will be 17.5 or 18.0. The only problem is that usually all these lenses are not available when you actually require them.
IOL power calculation by SRK formula is done as follows:
IOL power= A constant of the particular IOL minus 2.5x AL minus 0.9x Av. K.
(A= Axial Length, K= Keratometry value). As the A- Constant of the ACIOL is 3.1 D (118.4 -115.3= 3.1) less than the A- Constant of the PCIOL, the ACIOL power would be 19.0 D (22D-3D).
By simply deducting the difference between the two A-Constants,you get 3.1. This is to be deducted from the PC IOL power. So, you should choose an AC IOL of 19.0 D for this patient. Hope this helps.
There are two ways to manage the calculations of IOL power if you shift from a PC IOL of A constant 118.4 to an AC IOL of A constant 115.3 on the table
1. the most scientific method is to have different calculation for the two A-constants in the primary workup. Most of the present day A scan have the capability of showing three sets of reading with three different A constants.
2. In case you have calculations with only one A-constant which is 118.4, it is to be remembered that the A constant changes depending on the ELP (Effective Lens Position). An AC lens requires less diopteric power than a PC lens to bring light to focus on the retina. Therefore, an anteriorly placed lens will have a lower power and a lower A constant than a posteriorly placed lens. A simple calculation would be to subtract the A constant of AC IOL from that of a PC IOL, and this value is to be subtracted from the power of the PC IOL. i.e. 118.4 -115.3 = 3.1, so IOL power of 22 will become 22 3.1 = 18.9 or 19 D
April 19, 2008. Presenting my research data as poster in Wilmer Eye Institute.