Appropriate Scleral Lens Sagittal Depth
Category: Contact Lens
Contributor and Photographer: Tressa Larson OD FAAO
Scleral contact lenses rest on the sclera, vaulting over the entire cornea and limbus to avoid irritating or scarring those tissues. The amount of space between the lens and the cornea is typically referred to as sag. Sagittal depth actually refers to the entire contact lens from the center of the lens to the plane that the edges circumscribe. Various brands of scleral contact lenses allow you to manipulate the sag in different ways in order to achieve the desired vault over the cornea. Some brands allow you to actually specify the sagittal depth where others require you to manipulate the base curve. A steeper base curve typically results in increased sagittal depth where a flatter base curve reduces it. Regardless of how you achieve clearance over the corneal surface, there is an ideal amount of space between cornea and lens. Larger diameter sclerals typically have deeper sag. We must clear the cornea by enough space that if there are small variations in corneal shape or if the lens settles deeper into the squishy conjunctival tissue that we remain vaulted over the cornea and the limbus. Too much sag is not desirable because it reduces oxygen transmission (think Dk/t where the thickness of the saline layer between the cornea and the lens is t) and reduces BCVA. Good sagittal depth for a scleral lens is typically between 100-500 um centrally. Typical scleral lens thickness is between 250-500 um so if you know the center thickness of your lens you can use this to eyeball it in the slit lamp, but it is much more satisfying to measure it with OCT. In the slit lamp image you see here, the saline layer has debris in it making it appear speckled which made it easier to appreciate. Having fluorescein in this saline makes it much, much easier to see.
Ophthalmic Atlas Images by EyeRounds.org, The University of Iowa are licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.