Joel M. Solano, MD and John P. Berdahl, MD
Intraoperative wavefront aberrometry is a method of measuring the refractive error of an eye at the time of cataract surgery to aid in the determination of the best intraocular lens (IOL) for the desired refractive target. Currently, there are 3 devices that couple with the operating microscope to allow for such wavefront measurements: ORA (Alcon; Food and Drug Administration [FDA] approved), HOLOS (Clarity Medical Systems, Inc; FDA approval pending), and Aston (under development at Solihull Hospital and Aston University, Birmingham, United Kingdom). This chapter will discuss how wavefront aberrometry grows the ophthalmic surgeon’s confidence in selecting an IOL compared with previously available methods and provide guidelines for optimization of data acquisition during surgery.
INTRAOCULAR LENS POWER CALCULATIONS
The standard of care in ophthalmology is to use IOL power formulas when selecting an IOL at the time of cataract surgery. These formulas rely on measuring axial length, corneal power, and anterior chamber depth. They are known to have a high predictive value in eyes with normal, previously unoperated anatomy. However, even the newer generation formulas have suboptimal performance when predicting postoperative refractive outcomes for patients with extremes of refractive error or those with a history of refractive surgery. The challenges in IOL prediction for these patients include variability in effective lens position and difficulty with acquiring true corneal powers. Keratometers and topographers measure only the anterior corneal curvature and assume a standard relationship between the anterior and posterior curvatures. With refractive surgery, the relationship between anterior and posterior corneal curvatures is altered, and thus the predicted corneal powers are inaccurate. When making a cornea more oblate, as is done with myopic refractive surgery, the estimated IOL power can lead to postoperative hyperopia and the unpleasant “refractive surprise.” Several options are available to help combat the difficulty with post–refractive surgery IOL selection, including the contact lens method, historical method, nomogram method, targeting myopia method, and the widely accepted American Society of Cataract and Refractive surgery online post–keratorefractive IOL power calculator (http://iolcalc.org/). These are all viable options for the preoperative selection of the IOL power. Intraoperative aberrometry is the only method for IOL selection that can take place during surgery and will account for both the anterior and posterior curvatures as well as any surgically induced astigmatism.1–6
ASTIGMATISM
Astigmatism can be managed at the time of cataract surgery with either incisional corneal surgery or toric IOL placement. The complete treatment of cylinder requires conceptualizing the astigmatism as a vector with power and direction. In the visual system, astigmatism can be induced at the anterior cornea, posterior cornea, or the lens. When removing the lens during cataract surgery, we are left with correcting the total corneal astigmatism, but it is important to remember that topography and keratometry are based on the anterior surface only. Koch et al have shown that the posterior cornea contributes to the total corneal power, and this must be accounted for when treating astigmatism at the time of cataract surgery.7 They showed that the mean magnitude of astigmatism of the posterior cornea was -0.3 ± 0.15 diopter (D; range -0.01 to -1.10 D). Their data allows for the generalization that the posterior cornea adds against-the-rule astigmatism, but the range of data is such that some patients can have with-the-rule astigmatism on the posterior cornea. Thus, when trying to account for this when treating corneal astigmatism, many patients will be left with unexpected cylinder. One approach to better managing astigmatism is to actually measure both the anterior and posterior power contributions from the cornea. At this time, measuring the posterior curvature preoperatively has been difficult.
Surgically induced corneal astigmatism (SIA) also needs to be factored in when managing cylinder in the cataract patient. SIA can be measured and tracked over time for an individual surgeon, but the range of data can be such that using an average will lead to postoperative surprises.
INTRAOPERATIVE WAVEFRONT ABERROMETRY
Intraoperative wavefront aberrometry is a method of determining the refractive error of an aphakic or pseudophakic eye during cataract surgery. The aim of intraoperative aberrometry is to measure the optical system after the corneal incisions have been made so that the optics are as close to the postoperative state as possible while the measurements are acquired. Indeed, corneal wound healing and remodeling can change the power of the cornea; however, this approach remains as close to the final postoperative state as one can obtain before selecting and placing the IOL. The advantage of delaying the measurements for IOL power calculation until this point is that the total power of the cornea is included, as opposed to the preoperative method, in which just the anterior surface curvature is measured and the power calculated based on an assumption between the anterior and posterior surfaces. In addition, the measurements are captured after the corneal incisions have been created, which allows for a patient-specific account of SIA, thus obviating the need for the surgeon to calculate SIA. A search of the ophthalmology literature shows 3 aberrometers that are either available or in development: ORA (FDA approved), HOLOS (FDA approval is pending), and Aston (under development at Solihull Hospital and Aston University, Birmingham, United Kingdom).
ORA
ORA (formerly Wavetec’s Optiwave Refractive Analysis system) is currently the only intraoperative wavefront aberrometer commercially available in the United States. The device attaches to the cataract surgeon’s operating microscope and is capable of capturing phakic, aphakic, and pseudophakic refractions.
After phacoemulsification and removal of the lens cortex, the capsular bag and anterior chamber are filled with a cohesive viscoelastic and the images are captured. There is an art to quick and accurate image capture, and it is up to the surgeon to decide which values are most reproducible. On occasion, the data at the time of image acquisition are substantially different from the expected values calculated preoperatively. Several repetitions of image capture are then performed, and the surgeon will decide which measurements have the most similar spherical equivalents to guide the surgeon’s selection. Additionally, the surgeon may compare preoperative and intraoperative measures of magnitude and direction of the cylinder for guidance.
HOLOS
HOLOS is an intraoperative wavefront aberrometer that is currently being developed by Clarity Medical Systems, Inc. HOLOS is not yet commercially available and is pending FDA approval.
The instrument uses a sequentially shifting wavefront device to rapidly sample the wavefront and acquires data in real time. All data is acquired real-time, and therefore there is no need to pause and have the surgical assistant push a data capture button.8
ORA: IMAGE CAPTURE
Many surgeons use ORA almost exclusively in the aphakic or pseudophakic state, but the device is able to capture data during the phakic state as well. In the phakic state of the eye, the measurements can be taken before incision or at the time of limbal relaxing incision or enhancement. When capturing data during the aphakic state, the user must ensure a clear path for the wavefront for IOL selection. Once all of the cataract is removed, the surgeon should make sure the posterior capsule is clean and use capsular polish as needed. Another important aspect to good data acquisition is ensuring that all dispersive viscoelastic is removed, including the viscoelastic that sometimes forms a shell deep to the corneal endothelium. Once the lens capsule has been cleaned and the dispersive carefully removed, the bag is then filled with a cohesive viscoelastic. By filling the bag and anterior chamber with viscoelastic to the point where it just starts to egress from the main incision, the intraocular pressure is usually left close to 20 mm Hg. An intraoperative tonometer is used to ensure that the pressure is between 15 and 21 mm Hg and can be adjusted accordingly with more or less cohesive viscoelastic.
Figure 10-1. Intraoperative aberrometry measures an aphakic wavefront.
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