CHAPTER 11
Considerations After Refractive Surgery
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.
IOL 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. 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 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 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. 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 when cataract surgery is performed after photorefractive keratectomy (PRK) or LASIK; thus, it may be possible to perform an IOL exchange earlier in these patients.
Eyes With Known Preand Post-Refractive Surgery Data
It is important for ophthalmologists to understand 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. 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 (AAO) has developed the K Card with its partner, the International Society of Refractive Surgery (ISRS); the card is accessible to ISRS members at the following URL: http://isrs.aao.org/resources.
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
Eyes With No Preoperative Information
When no preoperative information is available, the hard contact lens method can be used to calculate corneal power. This method is quite accurate in theory but, unfortunately, not very useful in clinical practice. The corrected distance visual acuity (CDVA, also called best-corrected visual acuity, BCVA) needs to be at least 20/80 for this approach to work. First, a baseline manifest refraction is performed and then a plano hard contact lens of known base curve (power) is placed on the eye, and another manifest refraction is performed. If the manifest refraction does not change, then the cornea has the same power as the contact lens. If the refraction is more myopic, the contact lens is steeper (more powerful) than the cornea by the amount of change in the refraction; the reverse holds true if the refraction is more hyperopic. For example:
Current spherical equivalent manifest refraction: –1.00 D
A hard contact lens of known base curve (8.7 mm) and power (37.00 D) is placed Overrefraction: +2.00 D
Change in refraction: +2.00 D – (–1.00 D) = +3.00 D
Calculation of corneal power: 37.00 D + 3.00 D = 40.00 D
The ASCRS Online Post-Refractive IOL Power Calculator
A particularly useful resource for calculating IOL power in a Post-Refractive surgery patient has been developed by Warren Hill, MD; Li Wang, MD, PhD; and Douglas D. Koch, MD. It is available on the website of the American Society of Cataract and Refractive Surgery (ASCRS) (http://iol.ascrs.org/) and directly at http://iolcalc.org.
To use this IOL calculator, the surgeon selects the appropriate prior refractive surgical procedure and enters the patient data, if known (Fig 11-1). The IOL powers, calculated by a variety of formulas, are displayed at the bottom of the form, and the surgeon can compare the results to select the best IOL power for the individual situation. This spreadsheet is updated with new formulas and information as they become available and, at this time, probably represents the best option for calculation of IOL powers in Post-Refractive surgery patients. For more detailed IOL power calculation information, see BCSC Section 3, Clinical Optics.
Figure 11-1 The data screen of the post-keratorefractive IOL power calculator of the ASCRS. The surgeon enters the patient’s Pre-refractive surgery data (if known) and the current data into the data form. After the “calculate” button at the bottom of the form is clicked, the IOL power calculated by a variety of formulas is displayed. (Note: In this illustration, accessed August 23, 2013, the “calculate” button was activated with no patient data entered so as to show the final appearance of the screen; the form itself is updated periodically and available at http://iolcalc.org/.) (Used with permission from the