He first visited us for consultation about cataract surgery with multifocal lenses because things had been hard to see for a year. Both eyes had extreme myopia exceeding −10 D plus astigmatism of over 2.0 D, and an exceptionally long axial length of about 30 mm. Cataract surgery in high myopia (long eyes) carries its own technical risks, of course, but also a very high likelihood of postoperative refractive error, so surgery aiming for emmetropia (exact distance focus), as with multifocal lenses, demands considerable precision. So, although it makes this a little long, this time we also describe our clinic's preparation for and handling of "refractive error."
In the preoperative interview he said he plays golf, drives, and does important computer work, so he wanted to see clearly both far and near; he had researched lenses himself in advance and was interested in options such as the trifocal PanOptix.
As for the cataracts, he had nuclear cataracts, worse in the right eye, with fairly strong cortical opacity; they did not look advanced enough for us to recommend surgery ourselves, and with full correction vision could just about be brought up to 1.2. Even so, especially in the right eye, as the OPD-Scan III results below show, internal aberrations had increased to 0.500 µm and he complained strongly of difficulty seeing due to cataract, so we planned surgery starting with the right eye.
As for the lens, I had thought we could center on PanOptix, as he wished, and choose according to the interview, but with 30 mm axial lengths in both eyes he was unfortunately outside PanOptix's manufactured power range (normally made down to +6.0 D), so we explained this and asked him to consider monofocal lenses again. Still, he wished for surgery with multifocal lenses, aiming for life without glasses, and after discussion the surgery was done with the WELL FUSION System®: MiniWELL Ready in the dominant right eye and MiniWELL PROXA in the non-dominant left eye.
In general, intraocular lens power is found by calculating the lens power expected to achieve the target refraction. For ordinary eyes our clinic mainly uses the Barrett Universal II (BU II), SRK/T and Haigis formulas, and the AI-based Hill-RBF, Kane and EVO formulas, choosing between them by case or evaluating the results of several formulas together to decide the power, but in fact more than 30 formulas exist. Recently the Japan Ophthalmologists Association published a notice to reduce the risk of postoperative refractive error, "On the use of intraocular lens power and toric power calculation formulas" (Japanese); interested readers can see it via the link.
Ordinary formulas calculate from a lens-specific A-constant that reflects the lens's fixed position after surgery (its depth from the cornea), and are prediction formulas combining not only mathematical and theoretical elements but also regression elements from past real results, for example data showing that using lens X gave refraction Y. Recently, formulas incorporating artificial intelligence, so-called AI, have also been developed, but they are still predictions, so a certain amount of error is unavoidable, and especially for people like him, whose axial length exceeds 30 mm and for whom data are scarce, that is, with non-average eye size and proportions, the risk of refractive error is higher.
For example, calculating his lens with the online tool for the Barrett Universal II formula, said to be accurate for long eyes, gives the following results.
It is a little hard to read, but the left side is the right eye and the right side the left eye. The closer the target refraction (Refraction) is to 0, the more clearly distance is seen; but if it overshoots 0 into plus values (lower down) and becomes hyperopic, the eye theoretically has no point of focus anywhere from infinity to near, so we surgeons often choose the "first minus" lens power, slightly on the minus side of 0 (with TECNIS Synergy, incidentally, choosing the lens power whose target refraction has an absolute value closest to zero is recommended). For his right eye, for example, the first candidate is the +5.5 D lens, expected to give the first-minus refraction of −0.39 D, and 1/0.39 = 2.56 m is the distance focal length.
Whereas the normal power of the human crystalline lens is about +20 D, typical intraocular lenses are made from +6.0 D to +30.0 D, so modern cataract surgery not only removes the cataract opacity itself but, by replacing it with a lens of appropriate power, corrects the eye to a refraction that is easier to live with: it also has an aspect of refractive surgery. Depending on the target refraction, people with long eyes, that is, high myopia, receive low-power lenses such as +6 or +7 D to reduce that myopia, while those with short eyes, that is, hyperopia, conversely receive high-power lenses such as +29 or +30 D, bringing postoperative refraction close to zero. Incidentally, we normally focus at near by having the ciliary muscle thicken the roughly +20 D crystalline lens to increase its power. For example, making it 25 D gives focus at 1/(25 − 20) m = 25 cm by calculation, and the state in which that muscle weakens or the lens hardens and cannot change its thickness is so-called presbyopia.
To get back on track: for his right eye, for example, a +5.5 D lens with a target refraction of −0.39 D is recommended to correct the high myopia from the 29.80 mm axial length to near zero, but as the BU II results above show, this formula calculates lenses of 5.0 D or less as plano-convex (meniscus) lenses. Ordinary lenses are biconvex, but ordinary low-power lenses of +5.0 D or less are plano-convex meniscus lenses, and building in that correction is one reason the BU II formula rarely produces refractive error even in long eyes. You can see that, whereas the difference in target refraction between the +6.0 D and +5.5 D lenses above is 0.30 D, the difference between the +5.5 D and +5.0 D lenses suddenly becomes more than double, at 0.63 D.
However, many recent premium lenses, not only MiniWELL, are biconvex even at +5.0 D or less, so when biconvex low-power lenses are used, the correction above may actually become a source of error; our clinic therefore also uses its own regression formula derived from pre- and postoperative data for long eyes to decide the lens power. Deciding the power without considering such differences in lens shape could be a risk for refractive error, but perhaps because the error is small and the people affected are limited, I have not yet heard any presentation or discussion addressing this point at meetings in Japan or abroad.
With technical advances in measurement systems, lens power is now also decided by intraoperative measurement, achieving accuracy equivalent to, and under some conditions better than, prediction formulas, and our clinic too has introduced an intraoperative aberrometer (wavefront aberrometer) to reduce refractive error even slightly.
Specifically, during surgery, after the cataract is removed and just before the intraocular lens is inserted, a wavefront aberrometer attached to the surgical microscope measures total ocular aberration, and the result is checked against global data in real time to decide the lens power; this is ALCON's ORA™ system. The figure below is an example of an intraoperative measurement: it predicts that with a +17.0 D MiniWELL, postoperative refraction will be −0.17 D (*a different patient from this case).
I will describe the ORA™ system in detail another time and explain only his results here. For the right eye's lens power, we went into surgery with our regression formula predicting a refraction of −0.27 D with a +5.0 D lens (the BU II formula above predicted hyperopia of +0.24 D with +5.0 D). In intraoperative measurement, as in the middle row of the table below, from total spherical aberration of +2.40 D (Sphere) and astigmatism of −0.43 D (Cylinder), the ORA™ system recommended a lens power of +5.00 D (Suggested Power) with a predicted postoperative refraction of −0.24 D (Predicted Refr., far right); our prediction and the ORA system™'s prediction almost matched, so we could use the +5.00 D lens with considerable confidence, and as a result he was very satisfied with good vision of 1.5 / 1.0 for distance and near, with no refractive error.
For the left eye's lens type, we had prepared not only MiniWELL PROXA but also standard MiniWELL so he could decide based on how the right eye saw, but he still needed near vision highly and wished for surgery with PROXA. For lens power, our regression formula calculated that a +4.0 D lens would give a target refraction of −0.25 D (BU II above predicted +0.29 D), so we prepared powers around it for surgery. As a result, measurement with the ORA™ system below also recommended a Suggested Power of +4.00 D with a predicted refraction of −0.06 D, almost emmetropic, so here too we used +4.0 D with confidence, and in postoperative testing he was satisfied with good uncorrected vision of 1.0 / 1.2 for distance and near, especially near vision at 30 cm. As for the difference between his eyes, just as the vision tests show, distance was easier with the right eye's MiniWELL Ready and near with the left eye's MiniWELL PROXA, and he said he did not particularly notice photic phenomena such as halos and glare with the left eye's PROXA.
Our clinic places "accuracy", including in diagnosis and treatment, at the core of its philosophy, and we strive to bring refractive error to zero as far as possible; even so, in people whose eye shape deviates from the norm, or who have had corneal refractive surgery such as LASIK, unexpected refractive error can occur.
Besides our own regression formula, we sometimes take what might be called an analog, AI-like approach, searching our database by hand for people with similar axial length, corneal power, anterior chamber depth and lens thickness and deciding the lens power from their results, but unfortunately even then the result is sometimes not the same. In other words, factors not captured by existing parameters, such as posterior corneal power, or zonular strength and vitreous pressure, which may affect lens position, may also be related to postoperative refraction.
Of course, for unacceptable refractive error, we may ultimately have no choice but to opt for a lens exchange, but an exchange inevitably increases the physical, financial and time burden on the patient, so we will continue to refine optimization to improve the accuracy of lens power prediction and strive to bring postoperative refractive error as close to zero as possible.
2022.04.29