Imprecise IOL selection is what makes cataract surgery outcomes so much less predictable than LASIK outcomes. Now, a device promises to change that.
by Arthur Cummings, MD, FRCS
One of the biggest barriers to the widespread acceptance of multifocal IOLs is the inherent variability in cataract surgery’s refractive outcomes. Patients in multifocal IOLs are notoriously unaccepting of more than small amounts of residual refractive error; and surgeons who would prefer not to deal with dissatisfied patients—especially those who have paid out of pocket for their procedure—generally avoid implanting multifocal lenses.
But it is becoming more difficult to ignore multifocal IOLs. In the past, surgeons simply removed the cataract and sent the patient back to his optician for glasses to correct any residual distance refractive error. But now, many cataract patients want to see well at both distance and near—without glasses.
Challenges
Currently, cataract surgeons use a variety of formulas to estimate the required IOL power for a given eye. This power estimation requires knowing where in the eye the IOL will be located. Lens formulas have been created to determine such “effective lens position” (ELP). Defined as the distance between the IOL and the cornea, ELP is estimated from factors including corneal curvature and axial length, which must be measured before surgery, along with preoperative refractive power and anterior chamber depth.
IOL power calculation formulas work reasonably well in many eyes, but surprises are common. Indeed, whereas 92% of LASIK procedures achieve an outcome within one-half diopter of the target correction, that is true in less than 60% of cataract surgeries.1,2
As both a refractive and cataract surgeon, I have long been interested in the refractive outcomes of cataract surgery in patients with prior LASIK, PRK, or even RK. While these patients typically want to remain free of spectacles, they also present a significant challenge in IOL power calculation.
For these patients, most surgeons utilize a vision correction calculator such as the one available at ASCRS.org. In addition to the data required for IOL calculation in virgin eyes, the calculations for post-refractive eyes incorporate the pre-refractive surgery keratometry, the refraction, and the treatment that was performed (if available). Then, what the calculator provides is not a single number but a wide range of possible answers—something that concerns many surgeons. Even when the patient’s pre-refractive surgery parameters are available, there is considerable uncertainty with respect to the correct IOL power. And without a history, the process of choosing an IOL is much more complicated.
Can the Problem Be Fixed?
Choosing an IOL for these patients is the problem that motivated the formation of a new company, ClearSight Innovations, Ltd. While working on ray tracing for laser refractive procedures, Eugene Ng, MD, and I speculated that we could better reconstruct the anterior and posterior surfaces of the cornea and crystalline lens if we could measure all the optical interfaces more accurately and with different wavelengths. Dr. Ng and I thought that amalgamating data from all these interfaces would enable surgeons to better predict ELP and thereby improve cataract surgery outcomes. To achieve this, our team developed ClearSight’s Mirricon device for mathematically modelling ocular tissues in three dimensions.
The goal of the Mirricon device is to give surgeons the ability to select IOLs without having to enter data into a formula. By estimating ELP with far greater accuracy than is currently possible, it is designed to enable the average surgeon to get extraordinary results, much the way a femtosecond cataract laser enables a beginning surgeon to achieve a beautifully centered, perfectly circular capsulorhexis. A clinical proof-of-concept study of the Mirricon device is currently underway on more than 100 eyes. Data is not back yet on all 100, but we do know the Mirricon device was able to collect accurate data in all of the previously unoperated eyes, whereas a number of those same eyes could not be measured by either of two leading optical biometry devices.
The number of people who have had laser refractive surgery is in the vicinity of 16 million worldwide.3 At some point, almost all of them are going to require cataract surgery and will be looking for good refractive outcomes. Clearly, they present an unmet clinical need, and we are excited that ClearSight’s Mirricon device may help them regain visual function. And beyond these neediest of patients, there are millions of multifocal and even monofocal patients who will see better with more accurate IOL calculation.
Arthur Cummings, MD, FRCS, is medical director of Wellington Eye Clinic in Dublin, Ireland. He has a financial interest in ClearSight Innovations.