The Scleral Lens Vault
Adjusting Sagittal Depth When Fitting Scleral Lenses
BY GREGORY W. DENAEYER, OD, FAAO
Sagittal depth (or height) is what determines the vaulting capability of a scleral lens. The radii and width of the scleral lens back-surface curves define its sagittal depth. The back-surface curves can be spherical or aspheric. Sagittal depth can be increased by decreasing the radius and increasing the width. Conversely, sagittal depth can be decreased by increasing the radius and decreasing the width. Sagittal depth can also be increased or decreased by adjusting the asphericity of the back-surface curves.
Most eyecare practitioners fit scleral lenses using diagnostic lenses with standard design parameters that have specified sagittal depths. Fitting involves determining which diagnostic lens has a sagittal depth that adequately vaults over the corneal surface for the chord diameter of the lens being used.
Getting Measurements Through Imagery
Measuring the sagittal height using placido corneal topography or optical coherence tomography can help estimate the sagittal depth needed for fitting purposes.
Options in emerging technologies include the EyePrintPro (EyePrint Prosthetics, LLC), which allows practitioners to take an impression of the eye that is later scanned to determine lens vault and fit. The soon-to-be-released sMap3D (Precision Ocular Metrology, LLC) and the Eye Surface Profiler (Eaglet Eye b.v.) are non-placido-based topographers that measure scleral topography and sagittal depth, which can be used for initial lens selection and customized fitting.
During the fitting process, you can adjust sagittal depth by using steeper or flatter diagnostic lenses. There are occasions when the best-fitting diagnostic lens needs adjustment beyond the standard parameters to improve the scleral lens-to-eye fitting relationship.
Changes to any of the back-surface curves will change the ordered lens’ sagittal depth compared to the diagnostic lens that is evaluated in situ. Flattening the back-surface curves will reduce the sagittal depth, while steepening the back-surface curves will increase the sagittal depth of the scleral lens.
Some scleral lens systems that use steps for limited peripheral curve adjustment automatically compensate with design adjustments to keep the sagittal depth constant. For other scleral designs that allow more complete customization, you can make slight changes to the base curve or first peripheral curve to compensate for peripheral curve adjustments.
Design-specific calculators can help with these adjustments. Sagittal depth calculators can be used to provide a gross estimation of how to adjust the parameters to obtain the desired scleral lens sagittal depth. The calculators’ accuracy may be limited because they won’t contain proprietary aspheric curve parameters that significantly affect the lens fitting properties.
Future innovation and improvements with measuring scleral topography will increase practitioners’ accuracy of scleral lens fit and adjustment. CLS
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio and a consultant to Visionary Optics, Alcon, B+L, and Aciont. He is also a shareholder in Precision Ocular Metrology LLC. You can contact him at email@example.com.