Why the correct answer is right
Scleral lenses are designed to vault over the cornea and rest entirely on the conjunctiva, which is a soft, spongy tissue layer covering the sclera. Because this tissue is compressible, the weight of the lens and the constant pressure from blinking cause the lens to gradually sink deeper into the tissue over the first few hours of wear. This process is called "settling," and an OCT (Optical Coherence Tomography) typically measures a decrease in corneal clearance of about 100 to 200 microns during this time.
Why the other options are incorrect
The lens will not lift or gain clearance because gravity and eyelid tension are constantly pushing the lens toward the eye, not away from it. Expecting no change in clearance is a common fitting mistake that can lead to the lens eventually touching the sensitive cornea once it settles. While some lenses may experience minor flattening or flexure, the primary and most significant change measured by an OCT is the vertical sinking into the conjunctival tissue.
Memory aid
Think of a scleral lens like a heavy mattress being placed on a thick, plush carpet. At first, the mattress sits high on top of the fibers, but over time, its weight causes it to sink down into the soft rug. Just like the mattress settles into the carpet, the scleral lens settles 100-200 microns into the "carpet" of the conjunctiva.
Real-world application
When you are fitting a patient, you must over-estimate the initial clearance to account for this inevitable settling. If you want 150 microns of clearance at the end of the day, you should aim for approximately 300-350 microns of clearance at the initial application. Using an OCT allows you to precisely track this change and ensure the lens never makes contact with the corneal surface.
