Gray-to-gray is the new white-to-white for ICL sizing
Precision in refractive surgery is a cornerstone of achieving optimal visual outcomes, whether the selected procedure uses a cornea- or lens-based approach.
For phakic IOL implantation specifically, accurate sizing and placement of the lens are crucial to ensure a proper fit and rotational stability within the eye. Traditional sizing methods rely on white-to-white (WTW) measurements to assess the horizontal distance between the visible scleral edges (limbus) of the eye. There is growing recognition, however, that gray-to-gray (GTG) measurements may offer a more accurate and reliable method for phakic IOL sizing, potentially improving surgical outcomes. One main reason is that the limbus is a zone — referred to as the gray zone — and understanding how to navigate it is key to an accurate measurement for implant sizing.
Source: Vance Thompson, MD
Many modern formulas that apply AI use WTW measurements. Understanding the gray zone can greatly help the accuracy of this important measurement in sizing for the EVO ICL (STAAR Surgical). My preference is a mid-gray zone to mid-gray zone measurement performed with the patient lying down under a microscope. I use my excimer laser microscope.
ICL sizing: Basics and approaches
Sizing for the EVO ICL determines the lens vault, or the distance between the ICL’s posterior surface and the anterior surface of the crystalline lens. Ideally, the vault should be between 250 µm and 750 µm. A lower vault increases the risk for anterior subcapsular cataract formation, and a higher vault increases the risk for pupillary block, angle crowding and pigment dispersion.
For decades, anterior chamber depth and external WTW measurements have been the keystones for ICL sizing. With WTW, the horizontal diameter of the cornea is measured from the 3 o’clock to 9 o’clock limbus typically using either a caliper or an automated device. In my experience, biometers measure the widest WTW, topographers the second widest and epithelial mapping devices the third widest. The latter are the closest to the manual WTW that respects the gray zone. Manual mid-gray zone to mid-gray zone measurements as a WTW measurement are lower than both automated biometer and topographer WTW measurements, reducing the chance of an excessive vault that some experience when relying on automated devices alone. Calipers should be used under an operating microscope, not at the slit lamp (Figure 1).
One significant issue, however, is the variability in WTW measurements between devices (Figure 2). Generally, automated measurements overestimate the WTW diameter (Micheletti). In my experience, the Lensar (Lensar) measures 0.5 mm higher than the Pentacam (Oculus), whereas the Pentacam can measure up to 0.2 mm to 0.3 mm higher than manual measurements. The STAAR Online Calculation and Ordering System is based on a nomogram with the Orbscan (Bausch + Lomb), and a discrepancy in the actual vs. measured WTW diameter may lead to incorrect sizing and inadequate vault.
GTG measurements focus on the limbus, which is often referred to as the gray zone or sometimes the blue zone because of its blue-gray appearance from light scatter. The posterior border of the limbus corresponds to the internal junction of the cornea and sclera overlying the trabecular meshwork in all meridians. It may be used as an external landmark for incisions in both cataract and glaucoma surgery.
The GTG approach to ICL sizing acknowledges that the limbus is a zone that can vary in width from 1 mm to 2 mm. This differs from a WTW approach, which considers the limbus as a fixed point. I like to measure from the mid-gray zone to the mid-gray zone under high magnification. In my experience, focusing on this specific area achieves a more consistent and precise measurement that accounts for the natural variation in the limbus.
Unlike WTW measurements that can vary significantly depending on the device, GTG measurements performed under an operating microscope offer a level of precision that is difficult to achieve with traditional methods. GTG measurements do not require advanced technology like high-frequency ultrasound biomicroscopy and OCT and therefore may lower the barrier to entry for surgeons new to ICL implantation. All that is needed is a pair of manual calipers and an operating microscope. I like to take patients into the laser suite, where I have beautiful microscopes with a fixation light. I also find it is a nice opportunity for patients to get comfortable with topical anesthesia and a lid speculum, which also will be used during ICL surgery.
For most eyes, high magnification is required when performing GTG measurements. For eyes with large corneal diameters, however, I may end up needing to use mid-magnification. To obtain the most accurate measurement, the calipers should touch the limbus rather than hover above it. Measurements should be repeated until you are satisfied that you have a precise mid-gray zone to mid-gray zone measurement. WTW measurements should be performed as a backup.
Bridging the gap between technology and technique
In my practice, we are performing more ICL surgery than ever before. Not only does the procedure benefit a wide range of patients, but it also has advantages over laser vision correction, including preservation of the corneal epithelium, endothelium and stroma, when proper patient selection and preoperative planning are respected. Further, there is no risk for postoperative ectasia, a reduced risk for induced higher-order aberrations and a decreased chance of surgically induced dry eye.
By focusing on the gray zone of the limbus and integrating GTG measurements into our surgical planning, we have achieved greater consistency and accuracy of our measurements, enhancing the precision of ICL sizing and increasing safety and improved outcomes. More research is needed to compare GTG measurements with other advanced imaging techniques, but our personal experience suggests that whether you rely on a formula or manual measurements, the GTG could become a standard in ICL sizing.
- Reference:
- Micheletti M. Assessment of measurement variability across automated biometry devices. Presented at: American Society of Cataract and Refractive Surgery meeting; April 5-8, 2024; Boston.
- For more information:
- Vance Thompson, MD, founder of Vance Thompson Vision, Sioux Falls, South Dakota, can be reached at vance.thompson@vancethompsonvision.com.
Collapse