The geometry of the IOL is important for IOL calculations and it must be taken into consideration. Yet, we learn very little about this important topic and how it can make a large difference in certain eyes. Most of our IOLs tend to be biconvex in design where the power of the lens is split between the anterior (front) and posterior (back) surfaces. There are times, however, when it becomes important to use other designs such as meniscus or plano-convex and we must account for these changes.
Biconvex IOLs are the most common and for 90% or more of our patients, this is the IOL geometry that will be used. Therefore, most of our IOL calculation methods make the assumption that we will be using this type of IOL. With the typical biconvex design, the dioptric power is split between the anterior and posterior optic surfaces. There is an effort to keep the anterior power relatively constant for most of the power range while the posterior surface changes to adjust the overall dioptric power. There are also some anterior asymmetric biconvex designs where the anterior lens surface changes throughout the power range, but this can make the IOL calculations more challenging.
Looking at the examples from the Alcon AcrySof MA60 series IOLs, we see that between the powers of +6 to +30, we can order either the MA60BM or the MA60AC. What is the difference? The key is to look at the fifth letter in the name, either B (MA60BM) or A (MA60AC). The B = Biconvex and the A = Anterior asymmetric biconvex. These IOLs have different suggested A-constants with the MA60BM being higher since the effective lens position is more posterior in the eye. I find that there is also less variation in the A-constant across the power range which makes for more accurate IOL calculations. Thus I prefer the MA60BM. Professor Wolfgang Haigis explains it well:
The MA60BM has (in the range of 15 to 25 D) a constant anterior refractive power with different powers produced by curvature changes of only the posterior surface, whereas the MA60AC is roughly comparable with an equiconvex lens. The mean position of the anterior lens surface of the MA60AC is therefore closer to the cornea than that of the MA60BM. Accordingly, the mean effective lens position (ELP) of the MA60BM is farther from the cornea. This fact is reflected in the different A‐constants given to these lenses by their manufacturer (MA60BM: A=118.9; MA60AC: A=118.4)Source: Professor Wolfgang Haigis from this site.
Meniscus IOLs are required when the IOL power is very low or even negative, like we would use in highly myopic patients with long axial lengths. These IOLs behave very differently and they have wildly different A-constants. You may be surprised to see that while the calculated A-constant (for SRK/T) for a typical MA60BM (biconvex) is 118.9, it is 126.6 for the plus power MA60MA and 103.6 for the minus power MA60MA, according to the ULIB database. Newer calculators like Barrett Universal II and the Ladas 2.0 Artificial Intelligence will take this into account and specify a meniscus IOL.
Plano-convex IOLs have an advantage in eyes with silicone oil because the flat posterior surface of the IOL will limit the refractive surprise when it is against the silicone oil in the vitreous cavity. These IOLs are typically PMMA (poly methyl methacrylate) and are non-foldable and non-injectable, thus requiring larger scleral incisions for insertion. These types of IOLs are not commonly used outside of this scenario of silicone oil in the vitreous cavity.
We all know how to insert the IOL correctly and of course, we always avoid the dreaded “S-sign” seen when the IOL is upside down in the eye. But how does the refractive power of the eye change when it is upside down? For the MA60AC there is very little change because the power is split between front and back surfaces, though the haptic angulation will change the ELP. For the MA60BM there will be about a -1 diopter change and for a plano-convex IOL there will be a +2 diopter surprise. Bottom line: Don’t insert the IOL upside down!
Click below to learn more about this topic and see the injection of a meniscus IOL: