4.1 Scleral Anatomy, Terminology & Sagittal Depth Principles

4.1 Scleral Anatomy, Terminology & Sagittal Depth Principles

A scleral lens creates a fluid reservoir over the cornea and limbus while transferring its load to conjunctiva over sclera. That bearing relationship, not a brand name or a single diameter, defines the modality.

Use Bearing-Based Terminology

The Scleral Lens Education Society recommends these terms:

• A corneal lens rests entirely on the cornea.
• A corneo-scleral lens shares bearing between cornea and sclera.
• A scleral lens rests entirely on sclera and vaults the cornea and limbus.
• A mini-scleral lens is up to 6 mm larger than the horizontal visible iris diameter (HVID).
• A large-scleral lens is more than 6 mm larger than HVID.

Therefore, a 16.5 mm lens cannot be classified as mini- or large-scleral from diameter alone. On an eye with 12.0 mm HVID it is 4.5 mm larger and fits the mini-scleral size definition, provided it lands entirely on sclera. The same diameter on an unusually small cornea must be interpreted relative to that eye and its actual bearing.

Understand Ocular Sagittal Height

Sagittal height is the perpendicular depth from a reference chord to the ocular surface. It is affected by corneal curvature and eccentricity, HVID, limbal shape, scleral elevation, and the chord at which it is measured. A sagittal value without its chord is incomplete.

For a simple sphere of radius r measured at semi-chord y, sagittal depth s can be represented as:

s = r - square root of (r squared minus y squared)

A real anterior eye is not a sphere, so this formula teaches the geometry but does not replace profilometry, tomography, or diagnostic fitting. Small changes in diameter can create large changes in sag because the landing chord moves farther out on the eye.

Base Curve and Sag Are Related but Not Synonyms

In a simple corneal GP, base curve is a primary fitting variable because the lens bears on the cornea. Modern scleral designs contain separate optical, transition, limbal, and landing zones. A laboratory may hold total sag constant while changing the back optic zone radius, or a base-curve change may alter sag if other parameters are not compensated. The correct exam principle is to use the manufacturer’s design logic and specify the intended clinical change rather than assume that every base-curve adjustment produces the same clearance response.

Recognize Scleral Asymmetry

The anterior sclera generally becomes less rotationally symmetric at larger chords. Nasal, temporal, superior, and inferior elevations can differ, and focal obstacles such as pingueculae, pterygia, filtering blebs, or extraocular-muscle anatomy can create local elevation changes. Do not infer a universal nasal-versus-temporal pattern or a fixed micron difference in every eye.

Clinical clues that a spherical landing zone is mismatched include:

• sectoral blanching or vessel compression;
• edge lift or bubble entry in another meridian;
• lens decentration or rotation;
• localized rebound redness after removal;
• discomfort that increases during wear;
• a focal obstacle beneath the haptic.

A toric landing zone addresses approximately orthogonal meridional differences. Quadrant-specific, free-form, impression-based, or scan-driven designs address non-orthogonal or focal elevation differences.

Think in Functional Zones

Design targets are manufacturer- and patient-specific. A clearance range printed for one lens family is not a universal definition of success. Evaluate the lens after settling, compare with the laboratory fitting guide, inspect the cornea and limbus, and document the measured or estimated reservoir.

Select a Diagnostic Lens

Use available HVID, keratometry or tomography, ocular-surface profile, diagnosis, and the diagnostic set’s instructions. Insert the lens with sterile preservative-free filling solution, verify that no bubble is trapped, and evaluate central clearance, limbal relationship, landing-zone alignment, movement, rotation, comfort, and vision.

If central touch is present, select a design change that increases sagittal relationship according to that system. If clearance is excessive, reduce it while preserving limbal vault and safe landing. If one meridian compresses while the perpendicular meridian lifts, consider toricity. For a focal pinguecula or bleb, consider a notch, micro-vault, smaller diameter, or free-form landing zone according to anatomy and laboratory capability.

Order in the Laboratory’s Language

“SAG +100 microns” does not mean the same thing across all designs. Some laboratories change total sagittal depth; others change a named vault zone, limbal curve, or landing-zone angle. Record the diagnostic lens identifier, diameter, material, base curve or vault label, power, landing geometry, over-refraction, rotation marks, and the exact observed problem. State the desired result: for example, increase clearance over the cone while preserving limbal clearance, or relieve nasal compression without increasing temporal edge lift.

Exam Traps

• Diameter alone does not establish bearing or the mini-versus-large category.
• Base curve is not a universal proxy for total scleral-lens clearance.
• A spherical diagnostic lens can reveal toricity but should not be expected to align on every eye.
• Numerical targets are design guides, not substitutes for settled biomicroscopy and ocular-health assessment.