The number of patients who have had refractive surgery continues to grow, and ophthalmologists are increasingly confronted with the management of post–refractive surgery patients with other ocular conditions, such as cataract, glaucoma, retinal detachment, corneal opacities, and irregular astigmatism. Calculation of the intraocular lens (IOL) power presents a particular challenge in this population.
Intraocular Lens Calculations After Refractive Surgery
Although numerous formulas have been developed to calculate IOL power prior to cataract surgery for eyes that have undergone refractive surgery, these cases are still prone to refractive surprises. Currently, there is no infallible way to calculate IOL power for a patient who has undergone refractive surgery. Although the measurement of axial length should remain accurate after refractive surgery, determining the keratometric power of the post–refractive surgery cornea is problematic. The difficulty arises from several factors. Small, effective central optical zones after refractive surgery (especially after radial keratotomy [RK]) can lead to inaccurate measurements because keratometers and Placido disk–based corneal topography units measure the corneal curvature several millimeters away from the center of the cornea and possibly outside the modified treated zone. In addition, the relationship between the anterior and posterior corneal curvatures may be considerably altered after refractive surgery (especially after laser ablative procedures), leading to inaccurate results. Generally, if standard keratometry readings are used to calculate IOL power for a previously myopic, post–refractive surgery eye, the postoperative refractive error will be hyperopic, because the keratometry readings are erroneously steeper than the true central corneal power.
A variety of methods have been developed to better estimate the central corneal power after refractive surgery. None is perfectly accurate, and different methods can lead to disparate values. As many methods as possible should be used to calculate corneal power, and these estimates should be compared with each other, with standard keratometric readings, and with corneal topographic central power and simulated K readings.
Newer corneal topography and tomography systems not based on the Placido disk imaging claim to directly measure the central corneal curvature; such technology may make direct calculation of IOL power after refractive surgery more accurate. In addition, intraoperative wavefront aberrometer systems use Talbot-Moiré–based interferometry to obtain real-time aphakic IOL calculations—an approach that has been shown to increase accuracy and improve refractive outcomes in cataract surgery.
Prior to cataract surgery, patients need to be informed that IOL power calculations are less accurate when performed after refractive surgery and that, despite maximum preoperative effort by the surgeon, additional surgery, such as surface ablation, laser in situ keratomileusis (LASIK), IOL exchange, or implantation of a piggyback IOL, may be required to attain a better refractive result if the patient is unwilling to consider corrective glasses or contact lenses to correct the refractive outcome. Cataract surgery done after RK frequently induces short-term corneal swelling with flattening and hyperopic shift. For this reason, in the event of a refractive “surprise,” an IOL exchange should not be performed in post-RK eyes until the cornea and refraction stabilize, which may take several weeks to months. Corneal curvature does not tend to change as much following cataract surgery performed after photorefractive keratectomy (PRK) or LASIK; thus, it may be possible to plan and perform an IOL exchange or refractive surgical procedures earlier in these patients.
Eyes With Known Pre– and Post–Refractive Surgery Data
One method for calculating IOL power following refractive surgery is the clinical history method, in which pre–refractive surgery refraction and keratometry values, if available, combined with the current refraction and keratometry readings, are used to approximate the true post–refractive keratometry values for the central cornea. Unfortunately, even with these measurements, this approach has not been proven to be accurate. Despite this, pre–refractive surgery information should be kept by both the patient and the surgeon. To assist in retaining these data, the American Academy of Ophthalmology has developed the K-Card with its partner, the International Society of Refractive Surgery. The card is available in PDF form on the Academy website (www.aao.org/patient-safety-statement/kcard).
The key concept is to understand what changes occur on the corneal surface with refractive surgery. To use the historical method, the ophthalmologist should have the pre–refractive surgery refraction and keratometry readings, and the change in spherical equivalent can be calculated at the spectacle plane or, better yet, at the corneal plane. The post–refractive surgery refraction must be stable and obtained several months after the refractive surgery but before the onset of induced myopia from the developing nuclear sclerotic cataract. For example:
Preoperative average keratometry: 44.00 D
Preoperative spherical equivalent refraction (vertex distance 12 mm): –8.00 D
Preoperative refraction at the corneal plane: –8.00 D/(1 – [0.012 × –8.00 D]) = –7.30 D
Postoperative spherical equivalent refraction (vertex distance 12 mm): –1.00 D
Postoperative refraction at the corneal plane: –1.00 D/(1 – [0.012 × –1.00 D]) = –0.98 D
Change in manifest refraction at the corneal plane: –7.30 D – (–0.98 D) = –6.32 D
Postoperative estimated keratometry: 44.00 – 6.32 D = 37.68 D
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.