With newly developed methods of IOL power calculation, we are approaching 90% or higher accuracy of achieving +/- 0.5 D of the intended refractive target.
The best formulae include:
- The Barrett Universal formula uses improved algorithms
- The Hill-RBF method relies on artificial intelligence with large data sets
- The Ladas Super Formula incorporates both of these novel methods as well as crowd-sourced big data to continuously hone its results.
But even with these techniques of calculation, there is still a need to adjust the IOL power in unusual cataract cases such as choosing a sulcus IOL or AC IOL in cases of compromised capsular support. (scroll down for full video)
Changing from one IOL design to another
The A-constant of the IOL reflects many variables such as effective lens position, lens geometry, refractive index and more. If we are planning to implant a particular IOL in the capsular bag but then decide to switch to a different IOL, we may need to adjust the power in order to keep the same refractive outcome. This may be the case if a particular power of a specific IOL is not available or if we decide to switch from one brand to another to better match the patient’s tissue.
The difference in A-constant between the two IOL types is the same as the adjustment needed to the IOL power.
For example, if we are planning to insert an IOL of power +20 D and A-constant of 119.2 in the capsular bag, but then we decide to switch to a different IOL design with an A-constant of 118.7 while still implanting it in the capsular bag, we need to drop the IOL power by 0.5 D to +19.5 D to have the same refractive outcome as originally planned. The difference of A-constants (119.2 – 118.7 = 0.5) is the same as the difference in the IOL powers (20.0 – 19.5 = 0.5).
Choosing the AC IOL power
This also applies for choosing an appropriate IOL power for an anterior chamber IOL. If the original IOL for in-the-bag placement calls for a power of +20 D with an A-constant of 119.2, but then we decide to implant an AC IOL with an A-constant of 115.7, we need to drop the anterior chamber IOL power to +16.5 to have the same refractive result. This drop from +20 to +16.5 is 3.5 D, which is the same as the difference in A-constants (119.2 minus 115.7).
Sulcus IOL placement
In the vast majority of cases, we can implant the desired IOL in the capsular bag as planned. But in some cases, notably with weakness of the capsule, we may desire to place the IOL in the sulcus. The sulcus IOL must be suitable for this position and is typically of the three-piece variety. The entire IOL including the haptics and the optic can be placed in the ciliary sulcus. Alternatively, we can do some type of optic capture through the capsulorrhexis such as keeping the haptics in the sulcus while pushing the optic posteriorly through the capsulorrhexis, or the reverse. When we change the effective lens position of the optic, we need to adjust the IOL power to achieve the intended refractive target. If the optic is in the plane of the sulcus, then its power should be adjusted. However, in cases in which the haptics are in the sulcus but the optic is behind the capsulorrhexis, less adjustment or no adjustment is needed because the effective lens position of the optic is still “in-the-bag.”
- Haptics in Sulcus / Optic Capture through Capsulorhexis = use in-the-bag power
- because the optic is still technically behind the anterior capsular rim
- Haptics sutured to back of iris = use in-the-bag power
- because the sutures are always a little looser than you think and the optic is more posterior than a true sulcus IOL
- Haptics and Optic in the Sulcus = calculate sulcus IOL power
- because the optic is clearly in front of the anterior capsular rim in the sulcus
The first step is to make sure that the A-constant of the sulcus IOL model is about the same as the A-constant for the original IOL. If not, the adjustment based on A-constant needs to be made. This will give the appropriate IOL power for in-the-bag placement of the three-piece IOL, but because we will be placing it in the ciliary sulcus, we will need to further adjust this.
As the IOL moves more anterior in the eye, a lower power is needed for the same refractive outcome. This means that the sulcus IOL will need to have a power lower than the same IOL placed in the capsular bag. This varies with the power of the original IOL and can be calculated specifically or an approximation can be used. This “Rule of 9s” says that IOL powers can be grouped into groups, split at IOL powers 9, 18 and 27. The IOL power is reduced, respectively, by 0.5 D, 1 D, and 1.5 D, as shown below:
Zonular laxity and post-vitrectomy eyes
In eyes with suspected zonular laxity such as those with pseudoexfoliation syndrome, the IOL optic may sit further posterior in the eye than expected. With loose zonules, the entire lens and iris diaphragm can be pushed forward, and a relatively shallow anterior chamber can be seen during the preoperative consultation. During cataract surgery when the relatively heavy and thick cataractous lens is replaced by the thin and light IOL, the optic can sit further posterior in the eye than would have been predicted by the preoperative anterior chamber depth. In this case, addition of 0.5 D to the IOL power can help buffer against a post-op hyperopic surprise.
In eyes that have previously had a posterior vitrectomy for retinal disease, the lack of an anterior hyaloid face can also cause the IOL optic to sit more posterior after cataract surgery. Again, adding 0.5 D to the IOL power can prove to be of benefit.
Long and short eyes
With the original SRK regression formula, a reasonable number of patients could achieve close to the desired refractive target after cataract surgery. This formula tended to do better with average eyes but was less accurate with eyes that were longer or shorter than typical. An adjustment was needed to increase accuracy, and so the SRK II was developed, which called for an adjustment to be made for short and long eyes.
Skip forward to modern times, and we have more accurate ways of doing these calculations. The Wang-Koch axial length modification can help hone our results in long, myopic eyes, and formulae such as the Holladay 2 and Haigis, which incorporate a measured anterior chamber depth, can give us more accurate results in short, hyperopic eyes. Newer methods such as the Ladas Super Formula, available at www.IOLcalc.com, will make these adjustments automatically without requiring user prompting.
Finally, and perhaps most importantly, we need to incorporate the patient’s preferences into our IOL power selection. Patients with a history of myopia are likely to be most comfortable with a result that errs on the side of slight myopia when aiming for plano. Some hyperopic patients, on the other hand, have a strong desire to maintain good distance vision and are not as bothered by being left slightly hyperopic.
Our IOL calculations are becoming more and more accurate, but they do not yet account for all possible scenarios. Surgeons will still need to adjust the IOL power in certain cases in order to achieve the best postoperative refractive result.
- originally planned IOL:
- Alcon SN60WF +18.0 for plano (A=119.2) single-piece design
- change to three-piece design:
- Alcon MA60AC +17.5 for plano (A=118.7)
- to get from 119.2 to 118.7 we subtract 0.5, so apply this to the IOL power +18.0 minus 0.5 = +17.5
- Alcon MA60AC +17.5 for plano (A=118.7)
- change to sulcus power for Alcon MA60AC
- use Rule of 9s: subtract 0.5 D from IOL power = +17.0
Final result is: implant Alcon MA60AC +17.0 in the sulcus for post-op goal of plano.
click below to learn about the technique of Sulcus IOL placement: