Why the correct answer is right
Sagittal height, often called "sag depth," is the vertical distance between a flat reference plane (the chord) and the highest point of a curved surface (the apex). In OCT profilometry, this measurement tells us exactly how "deep" the anterior chamber is or how high the cornea vaults. It is a three-dimensional measurement of volume and space rather than just a two-dimensional width.
Why the other options are incorrect
The width of the cornea is a horizontal measurement known as the chord diameter or HVID (Horizontal Visible Iris Diameter). The refractive index refers to the eye's ability to bend light based on tissue density, which is a material property, not a geometric distance. Pupil diameter measures the opening of the iris and does not describe the physical curvature or depth of the corneal dome.
Memory aid
Think of an archery bow. The bowstring represents the flat "chord" or reference plane, while the distance from that string to the center of the wooden bow represents the "sagittal depth." The more the bow is curved, the higher the "sag" or distance from the string to the wood.
Real-world application
This measurement is the most important factor when fitting scleral contact lenses. Because scleral lenses are designed to "vault" over the sensitive cornea without touching it, an optician must know the eye's sagittal height to choose a lens with a slightly deeper sag. This ensures a consistent layer of soothing saline solution remains between the lens and the eye surface.
