Pursuit of Perfection

How the OPD-Scan III assists me in providing nearly perfect refractive cataract surgery results.

By Jonathan D. Solomon, MD

Refractive cataract surgery is a growing field. For surgeons looking to tap into this market, management of astigmatism is the first step. An estimated one quarter of all patients over the age of 40 manifest significant astigmatism,^1,2 while nearly 20% of cataract patients have more than 1.5 diopters of preoperative corneal astigmatism.³

Accurate Meridian Measuring

Several approaches can be implemented to reduce cylinder at the time of cataract removal. Regardless of your surgical preference, identification of the steep corneal meridian is imperative. By most accounts, available toric IOLs demonstrate remarkable rotational stability, with nearly 100% of lenses falling within 10° of the intended orientation (Figure 1).^4,5 Therefore, when there is postoperative residual astigmatism, I‘ve always suspected there was an inherent problem with the precise identification of the steep meridian.

Figure 1. Analysis of postoperative orientation with the OPD-Scan III Toric Summary.

Proposed methods for accurate meridian recognition include placement of preoperative reference marks, usually at the surgical limbus, at the 3- and 9- o’clock positions (some surgeons include the 6- o’clock position), while the patient is upright to reduce the effect of cyclotorsional rotation. Subsequent marks are then placed at the steep keratometric meridian, either at the surgical limbus or the peripheral cornea. Alternatively, distinct emissary vessels or other easily identifiable anatomic landmarks can be used as axis markers. Recently, a modification of this method has been applied using a photograph and subsequent triangulation of preplaced reference marks.⁶ However, this adds a considerable level of complexity to the preoperative analysis.

OPD-Scan III Benefits

I recently instituted a rapid and reproducible method of identifying the correct placement of limbal relaxing incisions or toric intraocular lenses using the OPD-Scan III. OPD stands for optical path difference. An earlier version, the OPD II (3D Wave), was an indispensable diagnostic tool in my clinic for more than 5 years. Following software and hardware upgrades, the combined skiascopic-wavescan topographer, autorefractor and pupillometer, is now capable of both flash and infrared photography.

The placido flash image (Figure 2), captures the Purkinje-Sanson reflex and provides a high contrast photograph of the surgical limbus. This enables identification of emissary or limbal vessels for traditional anatomic documentation. A retro-illuminated image of the crystalline lens can also be captured through a dilated pupil. I consider this photograph to be most valuable. By exploiting easily recognizable lenticular characteristics, preoperative placement of horizontal reference marks becomes obsolete.

Figure 2. High-resolution eye image used to identify emissary vessels at the time of traditional axis location.

The preregistered Toric Summary, another software upgrade, allows identification of the steep meridian. The distance from the lenticular landmarks are then measured in degrees. As part of the Toric Summary, there is a photopic eye image complete with topographic overlay (Figure 3). The combination of the two maps virtually eliminates the chance of cyclotortional marking error. When seated at the operating scope, I merely align the steep meridian by locating the lenticular reference marks and use my favorite toric marker for orientation.

Figure 3. Toric Summary demonstrating lenticular landmarks used as reference marks, and the location of the steep meridian is then measured in degrees. The photopic eye image complete with axial topographic overlay verifies the steep axis.

I am always trying to improve my accuracy, and when emmetropia is the goal I am within one half a diopter 89% of the time. The same precision is mandatory when correcting astigmatism. Prior to the introduction of the OPD-Scan III, 95% of my toric lenses were within 5° of the intended axis. Since we have made the change, I have reduced the axis-error and, in effect, my enhancement rate by 50%.

Nevertheless, some cases still require enhancements. Because I routinely operate on-axis, the need for vector analysis is rarely a problem as it relates to surgically induced astigmatism.In my hands, when intraoperative aberrometry is performed, on-axis incisions have a tendency to cause the steep meridian to slide a few degrees during aphakic or pseudophakic measurements, presumably due to stromal edema. I therefore again rely on the OPD-Scan III to analyze the position of the IOL postoperatively, and prior to enhancement. Returning to the Toric Summary, the retro-illuminated image of a dilated pupil unequivocally identifies the axis of the IOL (Figure 4).

Figure 4. Toric Summary-Orientation of an Alcon AcrySof toric IOL in relation to steep meridian.

Suppose the pupil dilates poorly. The OPD-Scan III is particularly effective. When the corneal wavefront profile is subtracted from the total ocular wavefront analysis, the end result is another very important derivation—the internal OPD. Complete with axis and magnitude, I‘m able to easily identify the orientation of the IOL (Figure 5). The separation of the corneal and lenticular astigmatism is particularly useful for those who choose to operate off-axis. There‘s no more guessing. Vector analysis is a precise endeavor.

Figure 5. Comparison of the axial topography to the internal OPD. Notice the neutralization of the astigmatism.

The backbone of the lifestyle IOL market is the toric lens, and the advanced imaging and data sets obtained using the OPD-Scan III provide a wealth of information that enable me to optimize my refractive outcomes, and in turn, grow my premium IOL practice. ■