A fan of CataractCoach has asked for input regarding the IOL calculations for his patient with steep keratometry. When he did the calculations with his biometer and then compared it to the Barrett Universal and the Ladas Artificial Intelligence 2.0 calculators, each came up with a different prediction.
The patient has cataracts and a long-standing refraction of about -8 OD and -6 OS. There is no recent myopic shift from nuclear sclerosis as the cataract changes are primarily cortical. The K values are quite high at about 50 diopters in each eye. Here is the print-out from the surgeon’s biometer:
This surgeon has his biometer configured to use the SRK/T for all calculations and then has chosen to do a print-out with four different IOLs. This will work reasonably well for most average eyes, but I would prefer the print-out to show four different formulae for one IOL. It is easy to switch from one IOL to another by virtue of the A-constant.
The IOL power varies 1 to 1 with the A-constant so that if the IOL power is +20.0 at an A-constant of 119.0 (single-piece acrylic), then:
- we change to an IOL with A-constant 118.5 (3-piece acrylic), the IOL power must drop by 0.5 D since the A-constant dropped by 0.5 D
- we change to a different single-piece acrylic with A-constant 119.5, the IOL power must increase by 0.5 D since the A-constant increased by 0.5 D
- we change to an ACIOL with A-constant 115.5, the IOL power must drop by 3.5 D since the A-constant dropped by 3.5 D
- for changing to a sulcus IOL, first change to the 3-piece acrylic IOL power as shown above and then use the Rule of Nines to determine sulcus IOL power
The SRK/T formula is the theoretical formula (hence the T notation in the name) from three surgeons: Sanders, Retzlaff, and Kraff. It is in the same family of third generation theoretical formulae such as the Holladay 1 and Hoffer Q. All three of these fomulae use the K and axial length value to determine the effective lens position (ELP), which is where in the eye (in the anterior-posterior direction) the IOL optic will end up after surgery.
The SRK/T formula in particular relies heavily on the K value to determine the ELP — and that does make sense in general: if the K is flat (low K value like 38), the ELP will likely be more anterior and a lower IOL power will be determined. If the K is steep (high K value like 50), the ELP will likely be more posterior and a higher IOL power will be determined.
However, when we look at the actual measured anterior chamber depth (ACD) from the biometer, it is 2.64 mm in the right eye. The third generation formulae do not use the ACD as an input variable so this value of 2.64 mm is not taken into account by the SRK/T (or the Holladay 1 or the Hoffer Q for that matter).
If we use a fourth generation formula which allows more input variables, then this ACD value helps to determine the ELP for more accurate calculations. These formulae include the Holladay 2, the Haigis, and Barrett Universal II.
Finally, there are newer methods of IOL power calculation which evolve with time since they are based on artificial intelligence neural networks and machine learning. These include the Ladas Super Formula AI 2.0 and the Hill-RBF which continue to improve with time.
How powerful is this technology? A recent graduate of our UCLA ophthalmology residency program compared the different calculations using a neural network based on the framework of the Ladas Super Formula AI method and the results were clearly the best, even with just 100 eyes. With 1000 eyes, it was even better. And in the near future, we will be working with 100,000 eyes and even millions of eyes.
Getting back to this challenging case, our last calculation method is with the Ladas AI 2.0 which is available to all ophthalmologists worldwide at IOLcalc.com for free. This Super Formula has the benefit of many thousands of eyes worth of data and results.
The patient underwent successful cataract surgery and the +10.5 IOL was implanted and the patient achieved the desired plano outcome. The Barrett Universal II was also quite close and would have left the patient about -0.4 diopters of myopia. The SRK/T was far off and would have resulted in a post-op refraction of about -2.5 diopters of myopia.
Change can be difficult to accept, but you must accept that artificial intelligence methods of IOL calcs will be the future.
try it for yourself at IOLcalc.com
If you have any doubts about this, just remember that there is not a human on the planet who can beat artificial intelligence computers at Chess, Chinese Go, or Japanese Shogi. And doing accurate IOLcalcs is next.
Disclosure: Uday Devgan MD is a principal in Advanced Euclidean Solutions which owns the Ladas Super Formula AI and runs IOLcalc.com (access to do your own IOL calcs is free)
