Even with the best methods of IOL calculations (new generation formulae, artificial intelligence methods, and more), there will still be some patients where it is very difficult to predict the ideal power. A good example of this are patients with small eyes and high hyperopia. The IOL Calc Quiz from last week was a highly hyperopic patient who fell into this category.
If we end up with a refractive surprise from the surgery of the first eye, how can we learn from this and help be more accurate for the second eye?
We are assuming that the surgery of the first eye went well, with the IOL in the capsular bag with appropriate overlap of the capsulorhexis over the optic. If the IOL optic is not being held securely by the capsulorhexis, it can move anterior and induce a myopic shift or it can tilt and induce astigmatism.
There are many factors which influenced the results of the first eye:
- the lens geometry is often overlooked, but it plays an important role: the geometry (and A-constant) can be different for a +30 D IOL compared to a +20 D IOL even when comparing the IOLs of the same make and model
- the patient’s healing response including the way that the capsular bag contracted in the post-op period is important and difficult to predict
- the eventual effective lens position (ELP) of the IOL optic is one of the toughest factors to predict in IOL calculations. This is particularly true in small eyes where a small move can make a significant difference.
- the specific biometer used including the calibration is important. The same way that your bathroom scale may measure differently from your neighbor’s, two biometers of the same brand and model may measure differently unless they are calibrated to the same reference
All of these factors can be wrapped up in a specifically honed A-constant for this particular patient. This is NOT an A-constant that you will use for other patients, but rather it is just for the second eye of this patient. Since this patient has similar biometry for both eyes (similar axial lengths, keratometry, and AC depth), we can certainly use the results of the first eye to help improve the accuracy for the second eye.
We enter the biometry for both eyes and then adjust the A-constant for the right eye which already had cataract surgery. We know that OD received a +31.0 D IOL and after a month ended up being -1.25 for the refraction. In this case, we need to adjust the A-constant of the right eye to 117.9 to produce this result of a +31.0 D IOL giving -1.31 (very close to -1.25). This new A-constant is applied to the left eye and we enter our desired goal of plano OS.
Then applying this adjusted A-constant to the other eye gives a prediction of either +29.5 (for a +0.17 goal) or +30.0 (for a -0.20 goal). Since the surgeon supplied IOL calcs for an extended depth of field IOL (EDOF IOL), we recommend a +30.0 for the left eye since EDOF IOLs work well with a pinch of myopia. For a monofocal IOL I would likely choose +29.5 D OS since these higher hyperopes usually don’t mind ending up +0.25 or even +0.50 post-op.
The patient would then have a very nice monovision arrangement of -1.25 OD and plano OS. If this was not well tolerated (as sometimes myopic results are not great with EDOF IOLs), then LASIK or PRK could be done OD to deliver plano in that eye as well.
Click below to hear this explanation in video format: