Updated: Jul 12, 2020
Doctor X was much excited to implant his first presbyopic correction lens, and so was the patient. Together they chose one of the best lenses, that promised to give the opportunity to junk the glasses that the patient hates to wear. With the best optical biometry done with an experienced biometrician, and all necessary tests done to rule out any ocular pathology that may hinder the patient having a sub optimal outcome, nothing but the best outcome was predicted for the patient.
The post operative outcome surprisingly, turned out to be a dampner. The patient landed up with a hyperopic surprise. The doctor is now in a dilemna, should he go for the second eye targeing a refractive number that would balance the surprise in the first eye ? Or should he first try to find out what went wrong in the first eye, so that he is sure that a mistake, if there at all had been in biometry of first eye, would not be repeated again. Doctor X, decided to go for the second option, deciding to take the pain of investigating what went wrong.
To start with, and within the scope of this write up, there would be three aspects worth looking into. First, where is the IOL sitting in the eye or the Effective Lens Position. Second, was the axial length rightly measured ?, and Lastly, was the right IOL power implanted.
1. ELP - Biometry is not about measuring the right IOL power. It is at best, only half of the story. Even, a right IOL power implanted may give a wrong result if it is not sitting at the right place in the eye. How far an IOL sits from the cornea, is typically called the ELP of the IOL, which has a direct impact on the post operative outcome. So is the IOL implanted sitting at the right place. A pseudophakic anterior chamber depth can provide an idea. Without this information, it may be impossible to understand the right cause of the unexpected outcome. In one such study, and on an average eye, a .1mm of deviation in the ELP was found to impact the final outcome by .19 dioptres. Pseudophakic anterior chamber depth, thought in some cases is difficult to obtain, may have an answer to the unexpected outcome in this case.
2. Axial Length - Was the Axial Length of the operated eye in this case rightly meseaured ? Can we re measure the AL and see if the pre and post surgery values matching ? Absolutely. We should match pseudophakic AL with the phakic AL measured pre surgery. To do that we have to measure the AL in the pseudophakic mode. All biometers employ a correction factor for the material of the IOL implanted - PMMA/Silicone/Acrylic. However, if this measurement is being done in ultrasound biometry, chances are that it will account for a PMMA IOL only. To make this pseudophakic AL measurement work for acrylic IOLs, you have to know the correction factor for the IOL implanted. On an average the correction factor for acrylic IOL is .2, meaning you have to add .2mm to the AL that you have measured on the pseudophakic mode. However, to be precise on the correction factor of the specific IOL implanted, we need to know - a) the velocity of sound in the IOL, b) the central optic thickness of the IOL dioptre implanted. Armed with these two information, we can then calculate the correction factor that needs to be added on the pseudophakic AL measured.
3. Right IOL Power - This is the most common question that customers have,once they face a refractive surprise. Time and again I have come across customers raising their eyebrows and asking if the labelled IOL power was right enough. Possible, but rare. With all companies investing millions of dollars in the manufacturing process, proper checkpoints are created to reduce this possibility to extremely rare incidence. But it is possible to find out if the IOL implanted in the eye, was the power that was originally intended to be implanted. To that extent, the only information that is required is the IOL power and corresponding central optic thickness from the manufacturer to start with. A pseudophakic measurement of IOL optic thickness (Lens Thickness, LT) should then help relate to the IOL dioptric power. One challenge is that, measuring psedophakic Lens thickness may itself be a challenge. Add to this, and due to the sensitive nature of measurement, any eye movement will lead to wrong measuremnt of central optic thickness and thereby refer to the wrong IOL power.
Nevertheless, a thorough investigation of refractive surprise is important to help us not repeat the same mistake again. Biometry is said to be the art of applying mathematics to biology. With this art being mastered, the world of presbyopia and astigmatism correcting lenses would immensely benefit patients.