PRESCRIBING FOR ASTIGMATISM
IS IT TIME TO SINK THE BOAT ANCHOR?
GRACE LIAO, OD; BRITNEY KITAMATA-WONG, OD; & TIMOTHY B. EDRINGTON, OD, MS
A few decades ago, a colleague of Dr. Edrington’s referred to keratometers as boat anchors, implying that they were useless in prescribing GP contact lenses. Today, many of our colleagues view keratometers as archaic instruments that should be replaced by computerized topographers. Is it really time to phase keratometers out completely? Let’s discuss the pros and cons of the two.
Measured Ks (keratometry) or simulated Ks (topography) are generally the starting point for corneal GP fitting. Both are excellent in quantifying the corneal curvature and corneal toricity with normal corneas (Mehravaran et al, 2014; Dehnavi et al, 2015). They can also be used to measure noninvasive tear breakup time and to assess for on-eye lens flexure.
Measuring Base Curve A manual keratometer can be used to measure the base curve of a GP lens without the complicated conversions necessary when using a topographer (Kumbar et al, 2012). This offers an inexpensive alternative to radiuscopes, which are now limited in availability. Keratometers can also measure off-eye lens warpage of corneal and scleral GP lenses.
Irregular Corneas Some argue that manual keratometry provides more accurate measurements of the steep and flat meridians of an irregular cornea (Hashemi et al, 2015) because it focuses on the central 3mm of the cornea. Topography takes into account a much larger area of the cornea, which can lead to inaccurate selection of the flat and steep meridians.
Office Footprint The office footprint for a keratometer is smaller. Additionally, the cost to own and operate a keratometer is less compared to a topographer as it does not require special software or computer systems to process data.
The Big Picture Topography measurements generate corneal curvature values beyond the central 3mm measured by manual keratometers. Therefore, topography can be useful in detecting subclinical cases of keratoconus and can provide a better understanding of the corneal contour. While manual keratometry provides an idea of the degree of corneal irregularity, topography provides a better description of the corneal contour across the corneal surface. Topographers are also useful in tracking changes or progression in corneal conditions such as corneal ectasias.
Specialty Fits Topography software offers information about elevation and sagittal height, which can be very useful in specialty lens fitting. For certain types of specialty lens fitting, such as orthokeratology, topography is arguably the most critical piece of information as it tracks changes in curvature and treatment centration over time.
Extended Ranges An added benefit of a topographer is the extended range of dioptric values by which corneal curvatures can be measured. Even with extending the range, a keratometer is limited to values of 30D to 68D. Topographers have the capability of measuring curvatures upwards of 100D.
EHR Compatibility A benefit of computerized topography is the ability to integrate captured images directly into an electronic medical records system.
We highly recommend purchasing a corneal topographer for your practice if you have not already. Overall, topography may be more useful in the modern age, but we believe that it is important to keep both tools in our arsenal. CLS
We’d like to thank Randy Kojima for his assistance with this article.
For references, please visit www.clspectrum.com/references and click on document #253.
Dr. Liao is a Cornea and Contact Lens Resident at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). She is a member of the American Optometric Association (AOA), the California Optometric Association (COA), and the American Academy of Optometry.
Dr. Kitamata-Wong is a Cornea and Contact Lens Resident at the SCCO at MBKU. She is a member of the AOA, the COA, and the San Mateo County Optometric Society.
Dr. Edrington is a professor at the Southern California College of Optometry at Marshall B. Ketchum University. He has received honoraria for the STAPLE program. You can reach him at firstname.lastname@example.org.