The Scleral Lens Vault
Vaulting the Cornea
By Gregory W. Denaeyer, OD, FAAO
Scleral contact lenses (diameters of 15mm to 24mm) by definition are fit to vault over the cornea and rest on the sclera/bulbar conjunctiva, enabling them to hold a fluid reservoir. Typically, scleral lenses are fit to vault the central cornea by 100 microns to 400 microns.
The sagittal depth of a scleral contact lens determines the amount of vault that it will have when fit to the anterior surface. Diagnostic lenses help evaluate the amount of vault that a scleral lens has relative to a patient’s eye.
There are two ways to measure the vault of a scleral lens. One involves using a slit beam from a biomicroscope to compare the fluorescein-stained reservoir between the lens and the cornea with the known thickness of the lens. The other is to measure the vault using optical coherence tomography (OCT) imaging.
Factors That Influence Vault
The following factors affect the depth of scleral lens vault:
Back-Surface Curves The back-surface curves, including the base curve and peripheral curves, all contribute to lens sagittal depth. The base curve has the most influence on sagittal depth versus any one peripheral curve because it has the widest area. Sagittal depth can be increased by steepening the back-surface curves or by widening their area.
Lens Diameter Sagittal depth increases with increasing contact lens diameter.
Lens Design The design can influence the sagittal depth of a scleral lens. For example, a reverse curve will increase sagittal depth compared to a standard geometry design.
Settling Scleral lenses rest on the bulbar conjunctiva, which is spongy in nature. Over time, a scleral lens will depress this tissue, causing it to sink and lose vault.
Consider the following in determining the optimal amount of scleral lens vault:
Ocular Surface Disease In cases of ocular surface disease, a scleral lens not only acts to protect a patient from exposure, but the fluid reservoir continuously bathes the anterior ocular surface. For this application, is more vault and fluid reservoir better? Sonsino and Mathe (2013) reported that the amount of vault was not a factor for successfully using scleral lenses to manage dry eye.
Vision Some practitioners have assumed that patients may lose some visual acuity with increasing vault of a scleral contact lens. However, at least one study has shown no correlation between scleral lens vault and visual acuity (Sonsino and Mathe, 2013).
Reservoir Debris Debris can build up in the fluid reservoir, which can cause some patients’ vision to become foggy. Some patients have to remove their lenses if their vision is significantly affected. Lowering the vault to reduce the amount of potential debris that such patients have to look through may lessen their symptoms. Evidence that patients fit with less vault tend to have less interrupted lens wear (McKinney et al, 2013) supports this strategy.
Blinking Scleral lenses that have a high vault can impede a patient’s ability to blink smoothly or completely.
Oxygen Using theoretical calculations, Langis et al (2012) concluded that to avoid corneal swelling, the central corneal vault of a scleral lens should not exceed 200 microns when using hyper-Dk materials (>150) and limiting central thickness to 250μm. Their calculations consider the fluid reservoir as a barrier to oxygen transmission. However, another possibility is that the reservoir acts as an oxygen depot that is continuously renewed by atmospheric oxygen that permeates the lens (Sonsino and Mathe, 2013; Caroline, 2013). This could explain why clinically significant corneal edema isn’t observed when the above hypothetical criteria are not met in eyes that have healthy corneal endothelium. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #218.
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.