10 IOL Refractive Surprises
10.1 Introduction
The goal of cataract surgery is to improve visual function and thus postsurgical quality of life. Patient expectations for precise refractive outcomes after cataract surgery are higher than ever. Advances in ocular imaging (e.g., measurement of anterior chamber depth and axial length, assessment of corneal topography), intraocular lens (IOL) design (monofocal, multifocal, toric, and accommodative IOLs), and microsurgical techniques (small-incision phacoemulsification, femtosecond-assisted cataract extraction) have moved cataract surgery from being not only a therapeutic procedure but also a refractive one.
Nowadays, the goal of cataract surgery extends beyond the traditional restoration of the clarity of the lens to correction of the refractive aberrations of the eye. The refractive cataract surgeon seeks to achieve the best possible uncorrected visual acuity in order to reduce spectacle dependence and improve patients’ quality of life and productivity. Thus one metric for success of cataract extraction and IOL implantation lies in achieving predictable refractive outcomes. Refractive surprises after cataract surgery are disappointing for both the patient and the surgeon. This chapter discusses potential errors that may lead to post-IOL implantation refractive surprises, along with preventive measures to avoid such outcomes.
10.2 Determining IOL Power
Several values are required for determining IOL power:
Axial length (AL).
Corneal refractive power—steep and flat keratometric values (Ks).
Postoperative IOL position in the eye (effective lens position [ELP]).
IOL A-constant.
The accurate predictability of postcataract refractive outcomes is based on the aforementioned metrics. Currently, on average 70% of eyes are ± 0.5 diopters (D) of the attempted correction, whereas 85% of eyes are ± 1 D of the attempted correction as planned preoperatively. 1 , 2 In post–corneal refractive surgery eyes the percentage drops to 50% for achieving ± 0.5 D of attempted correction. 3 A major advance in achieving predictable outcomes has been in our ability to obtain more accurate AL measurements with noncontact, partial coherence interferometry. 4 , 5 , 6 Another advance is that, although two-variable IOL power calculation formulas were used for more than 3 decades, today’s more advanced formulas use additional variables in order to better predict IOL power (Haigis, Holladay, etc.). The one important variable that cannot be measured preoperatively is ELP, which is thought to be affected by the diameter and shape of the capsulorhexis. 7 , 8
10.3 Sources of Error Resulting in Refractive Surprises
Based on the regression formula for IOL power calculation, the primary potential sources of error include AL (a 1 mm error will lead to ± 2.5 D of refractive error), corneal power (1 D of error in the average keratometric reading will lead to a ± 0.9 D refractive error), and the A-constant of the IOL. Other variables include the surgeon factor, wound healing, and an effective lens position, which may not be easily quantified. The relative contribution of each of these variables is shown in Table 10-1.
Measurement | Error of | Will result in this change in IOL power |
Axial length | 1 mm | 2.5 D |
Corneal power | 1 D | 0.9 D |
A-constant | 1 | 1 D |
10.4 Axial Length
The AL is the distance between the anterior surface of the cornea and the fovea; it is usually measured by A-scan ultrasonography or optical coherence biometry. The AL is the most important factor in IOL calculation. A 1 mm error in AL measurement results in a refractive error of approximately 2.5 D in an average eye. 9 In cases where there is a significant difference in AL between the two eyes of a patient, when the refraction is similar, the surgeon should consider repeating the evaluation. AL measurements should be performed routinely in both eyes prior to cataract surgery as an extra validation of accurate AL assessment.
10.4.1 Ultrasonography
In A-scan ultrasound biometry, a high-frequency sound wave penetrates through the eye and is reflected back toward the probe. Encountering any media interfaces results in the production of “echoes.” Such echoes make it possible to calculate the distance between the probe and various structures within the eye. 10
Two types of A-scan ultrasound biometry are currently in use, contact applanation biometry and immersion A-scan biometry. Contact applanation biometry requires placing an ultrasound probe on the central cornea. Although this is a convenient way to determine the axial length for most normal eyes, errors in measurement almost invariably result from the probe indenting the cornea and shallowing the anterior chamber. Because the compression error is variable, it cannot be compensated for by a constant. IOL power calculations using such measurements will lead to an overestimation of the IOL power, especially in shorter eyes. In contrast to contact applanation biometry, in immersion A-scan biometry no direct pressure is applied on the globe. Instead, a saline-filled scleral shell is placed between the probe and the eye. Depending on the operator, a mean AL shortening of 0.25 to 0.33 mm has been reported in applanation biometry measurements compared to immersion axial length measurements. 3 Such a difference would account for a difference in the calculated IOL power of up to 1 D. In general, immersion biometry has been shown to be more accurate than contact applanation biometry. 11
10.4.2 Partial Coherence Interferometry
Partial coherence interferometry relies on the principle of light interference to measure the time required for infrared light to travel to the retina. This transit time is then translated to an AL measurement. No contact with the globe is required; thus corneal compression artifacts are eliminated. Compared with ultrasonography, partial coherence interferometry provides more accurate and reproducible AL measurements. 4 , 12 However, AL measurements with this technique are limited in the presence of a dense cataract or other media opacities, and, in such cases, ultrasonography is more reliable.
The most obvious advantage of partial coherence interferometry over ultrasound biometry is that the AL measurement is performed through the visual axis because the patient is asked to fixate on a laser spot during the measurement. In highly myopic or staphylomatous eyes, this can be particularly useful given that it is often difficult to measure the true AL through the visual axis with an ultrasound probe alone.
It should be noted that the AL obtained with partial coherence interferometry is slightly longer than that obtained with ultrasound biometry. This is because partial coherence interferometry measures the distance from the corneal surface to the retinal pigment epithelium, whereas ultrasound biometry measures the distance to the anterior retinal surface. This explains why modern IOL biometry devices use refined IOL constants unique to their mechanism of operation in their IOL calculations.
10.5 Corneal Power
The central corneal power is the second important factor in determining IOL power. Central corneal power can be measured by manual keratometry or corneal topography. Errors in measuring corneal power occur because of warpage from contact lens wear, surface drying, large amounts of astigmatism, or previous refractive surgery.
10.6 IOL Power Calculation Formulas
Regression formulas are used to predict the appropriate IOL power for emmetropia based on the axial length of the eye, the refractive power of the cornea, and the estimated lens position. The most commonly used regression formula is the SRK formula:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
