The Scleral Lens Vault
Selecting Scleral Lens Diameter
BY GREGORY W. DENAEYER, OD, FAAO
The diameter of a GP lens determines its function. GP lenses that land solely on the cornea typically have some movement that allows tear exchange with blinking. A scleral lens, on the other hand, has a diameter that is large enough relative to the cornea that it lands on the sclera, and there is little to no tear exchange. This allows a scleral lens to hold a fluid reservoir.
The Scleral Lens Education Society has defined corneo-scleral lenses as GP lenses that rest partly on the cornea and partly on the sclera; it has defined scleral lenses as GP lenses that rest entirely on the sclera. Scleral lenses are further subdivided as mini-scleral lenses, which have a diameter up to 6mm larger than the horizontal visible iris diameter (HVID), and large scleral lens, which have diameters greater than 6mm larger than the HVID. For an average cornea (11.8mm [Caroline and André, 2002]), mini-scleral lenses range in diameter from 15mm to 17.8mm, and large scleral lenses range from 17.9mm to 24mm.
Selecting the Right Diameter
When selecting scleral lens diameter, consider the following ocular characteristics.
HVID is one of the most important factors when choosing scleral lens diameter. Depending on the lens design and the amount of settling into the bulbar conjunctiva, a scleral lens that is approximately 16mm in diameter will completely vault most corneas that have an HVID of 12.3mm or less. For patients who have relatively larger HVIDs (Figure 1), you may need to modify the design or switch to a larger-diameter lens to avoid limbal bearing. Fluorescein evaluation or optical coherence tomography imaging will help you determine whether the scleral lens is landing on the limbus.
Figure 1. A keratoconus patient who has a large HVID of 13.32mm.
Scleral Asymmetry The sclera is nonrotationally symmetrical, and its asymmetry increases with increasing distance from the limbus (van der Worp, 2010). Mini-scleral lenses have an advantage in that spherical designs are less likely to exhibit mismatch between the scleral lens haptic and the sclera because they fit relatively closer to the limbus, where the sclera is more symmetrical. Large scleral lenses may require back-surface toric haptics to achieve scleral alignment.
Irregularity The sagittal depth of a scleral lens increases with increasing diameter. Patients who have severe ectasia, such as keratoglobus, may require large scleral lenses to achieve enough sagittal depth to properly vault the cornea. Successfully fitting these patients usually requires scleral lens diameters between 18mm to 20mm.
Aperture Size Patients who have small apertures and sunken orbits may not physically be able to apply a large scleral lens. For those patients, start by fitting a mini-scleral design. If patients continue to have difficulties with lens application, decrease the diameter by 1mm to 2mm.
A mini-scleral design of approximately 16mm will successfully fit most patients who are being managed for corneal surface irregularity or ocular surface disease. A 16mm lens will adequately vault the cornea and will avoid the asymmetry further out on the sclera that can create a mismatch between a spherical lens haptic and the scleral surface.
Fit large-diameter scleral lenses for managing severe ectasia or if limbal bearing occurs with the best-fit mini-scleral lens. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #220.
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio, and a consultant to Visionary Optics, B+L, and Aciont. You can contact him at firstname.lastname@example.org.