Prescribing for Astigmatism
Cracking the Bitoric Code
BY COLTON HEINRICH, OD; ELAINE CHEN, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Patients who have significant corneal toricity and a high cylinder refraction can be challenging to fit. Such patients often benefit from bitoric GP lenses. Thankfully, toric calculators developed by Drs. Robert Mandell and Thomas Quinn make designing bitoric lenses relatively straightforward.
Base Curve Toricity Versus Corneal Toricity
Consider a bitoric lens when the corneal toricity is greater than 1.50D. For less than 2.00D of corneal toricity, Dr. Mandell recommends designing the initial lens base curve (BC) toricity to match the corneal toricity to optimize the fit and minimize lens rotation. As corneal toricity increases, undercorrect the BC toricity by approximately 0.50D to enhance tear exchange. Dr. Quinn recommends having at least two-thirds of the corneal toricity in the BC for rotational stability.
Considerations for Overall Lens Diameter
Dr. Quinn determines the initial lens overall diameter (OAD) based on upper eyelid positioning with respect to the superior limbus. If the upper eyelid covers the superior limbus, he recommends a larger OAD (9.4mm to 10.2mm); if the upper lid does not cover the limbus, prescribe a smaller OAD. Dr. Mandell suggests utilizing lid aperture size to guide OAD selection.
Both doctors recommend maintaining the lens-to-cornea fitting relationship by preserving the sagittal depth: when increasing the OAD, flatten the BCs; when decreasing the OAD, steepen the BCs.
TIPS FROM THE EXPERTS
From Dr. Mandell:
• The success of the method lies in allowing fitters to understand the lens-to-cornea relationship throughout the fitting procedure. It reduces the fitting to the same process that is used in fitting a spherical contact lens, which is easily understood.
• Eliminating the conversion back to a sphero-cylindrical state reduces the potential for errors.
From Dr. Quinn:
The GPLI online calculator helps explain bitoric design step-by-step. The critical lens parameters required to design an initial lens using my toric calculator include keratometry (or sim K) values, spectacle refraction, and recommended OAD.
Considerations for Lens Power
Once the OAD and BCs are chosen, determine the lens power by calculating the tear lens and combining it with the vertexed spectacle lens power in each meridian using optical crosses. With changes in BCs, the power of the lens would require a change in each meridian (steepen add minus, flatten add plus power).
If the dioptric difference in BC is equal to the dioptric difference in power, the lens is considered a spherical power effect (SPE) bitoric, and rotation on the cornea will not induce unwanted residual astigmatism. When the difference in BC and power are not equal, the lens is a cylinder power effect (CPE) bitoric. Note that when prescribing a CPE, lens rotation and rotational instability may degrade patients’ vision.
Dr. Mandell’s and Dr. Quinn’s bitoric fitting calculators are available online through the GP Lens Institute’s website (www.gpli.info). Dr. Clarke Newman’s bitoric nomogram has also been recently featured on the GPLI website. CLS
Special thanks to Drs. Bob Mandell and Tom Quinn for taking the time to share their immense bitoric GP wisdom.
Drs. Heinrich and Chen are the cornea and contact lens residents at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). Dr. Edrington is the cornea and contact lens residency coordinator at SCCO. He is also a Fellow of the American Academy of Optometry and a Diplomate in its Cornea, Contact Lenses, and Refractive Technologies Section.