21.3 Scleral Contact Lens Design, Sagittal Depth Fitting & Clearance Evaluation

Scleral Contact Lens Design, Sagittal Depth Fitting & Clearance Evaluation

Core Clinical Mandate: Scleral contact lenses have revolutionized the visual rehabilitation of severe irregular astigmatism and the therapeutic management of ocular surface disease. By completely vaulting the fragile corneal epithelium and the sensitive limbal stem cells, sclerals land exclusively on the conjunctival tissue overlying the sclera. The COMT must master scleral lens classification, sagittal depth fitting biophysics, the three anatomical clearance zones, settling dynamics, and complication management including mid-day fogging and conjunctival blanching.

Scleral Classification, Anatomic Rationale & The Fluid Reservoir

The Fundamental Scleral Paradigm

Unlike corneal RGP lenses, which sit directly on the cornea and transfer mechanical friction during the blink, a scleral lens completely bridges the cornea and limbus without physical contact. It creates an enclosed, liquid-filled chamber—the post-lens fluid reservoir—filled with sterile, preservative-free 0.9% sodium chloride saline:

• Optical Neutralization: The fluid reservoir neutralizes virtually all irregular corneal astigmatism, high-order aberrations, keratoconic distortion, and post-keratoplasty graft-host steps by creating a perfectly spherical front refracting surface.
• Therapeutic Hydration: In severe ocular surface disease (e.g., neurotrophic keratitis, persistent epithelial defects, Stevens-Johnson syndrome, ocular graft-versus-host disease [GVHD], severe Sjögren syndrome), the fluid reservoir provides 12 to 16 hours of continuous, non-evaporating corneal hydration, shielding the epithelium from eyelid friction and desiccation.

International Scleral Lens Classification (SLS Standard)

The Scleral Lens Education Society (SLS) categorizes rigid lenses based on overall diameter (OAD) and anatomical landing profile:

1. Corneo-Scleral Lenses ($12.0 \text{ to } 15.0 \text{ mm}$):
   • Sized slightly larger than the visible iris diameter (VID).
   • Land partially on the peripheral cornea and partly on the anterior sclera/conjunctiva.
   • Provide minimal vault depth; limited to regular astigmatism or mild corneal ectasias.
2. Mini-Scleral Lenses ($15.0 \text{ to } 18.0 \text{ mm}$):
   • Sized up to $6.0 \text{ mm}$ larger than the cornea (most common clinical sizes are $15.5 \text{ to } 16.5 \text{ mm}$).
   • Completely vault the cornea and limbus, landing entirely on the anterior bulbar conjunctiva/sclera.
   • The clinical workhorse for moderate-to-severe keratoconus, pellucid marginal degeneration, and post-LASIK ectasia.
3. Large / Full Scleral Lenses ($>18.0 \text{ mm}$, up to $24.0 \text{ mm}$):
   • Sized substantially larger than the limbus, extending into the conjunctival fornices.
   • Feature immense sagittal vaulting capacity ($>5000 \ \mu\text{m}$).
   • Mandatory for advanced globus keratoconus, severe ocular cicatricial pemphigoid, symblepharon management, and profound terrien marginal degeneration.

The Sagittal Depth (Sag) Fitting Philosophy vs. Corneal Curvature

In standard corneal RGP fitting, the practitioner selects a Base Curve based on Keratometry ($K$). In scleral lens fitting, corneal curvature is largely irrelevant to lens clearance.

The Sagittal Depth Concept

Scleral lens fitting is governed by Sagittal Depth (Sagittal Height, Sag)—the vertical perpendicular distance (in micrometers, $\mu\text{m}$) from the flat landing plane of the lens haptic to the highest posterior apex of the lens optic zone:

• An eye with high corneal elevation or deep anterior segment geometry requires a high-Sag lens ($4600 \text{ to } 5200+ \ \mu\text{m}$).
• A flatter, shallower eye requires a low-Sag lens ($3800 \text{ to } 4200 \ \mu\text{m}$).
• In modern spline-engineered scleral designs, changing the base curve modifies only the internal optical power of the lacrimal lens; it does not change corneal clearance. Clearance is controlled directly by selecting higher or lower Sagittal Depth steps (available in increments of 25, 50, or 100 $\mu\text{m}$).

Scleral Fitting Paradigm:
CORNEAL RGP  ──> Fitted to CURVATURE (Base Curve / K-readings in mm or Diopters)
SCLERAL LENS ──> Fitted to SAGITTAL DEPTH (Total Vault / Sag in Micrometers [µm])

The Three Functional Anatomical Zones & Clearance Dynamics

A modern scleral lens is divided into three distinct, independently adjustable zones:

1. Zone 1: Optical Vault (Central Corneal Clearance)

• Initial Insertion Clearance: Upon insertion, the lens must show 250 to 350 µm of central fluid vault.
• The Settling Phenomenon: The rigid lens rests upon the spongy, compressible bulbar conjunctiva and underlying Tenon's capsule. Over the first 2 to 4 hours of continuous wear, the lens sinks into this tissue by 100 to 150 µm.
• Final Settled Clearance Target:
  • Optimal Range: 150 to 200 µm of fluid clearance across the entire cornea after complete settling.
  • Under-Vaulting ($<100 \ \mu\text{m}$): Sinking risks late central or paracentral corneal touch, leading to epithelial abrasions, pain, and lens rejection.
  • Over-Vaulting ($>350 \ \mu\text{m}$): Excessive fluid depth increases metabolic barrier resistance, inducing corneal hypoxia, endothelial acidosis, microcystic edema, and mid-day tear fogging.

2. Zone 2: Transition Zone (Limbal Clearance)

• Anatomical Target: The corneoscleral junction (limbus) houses the delicate Palisades of Vogt and limbal epithelial stem cells.
• Target Clearance: The lens must maintain 50 to 100 µm of fluid clearance over the limbus.
• Complications of Limbal Bearing: If the transition zone drops and bears down on the limbus, the mechanical pressure causes localized ischemia, limbal stem cell deficiency (LSCD), conjunctivalization of the cornea, and deep corneal vascularization.

3. Zone 3: Landing Zone (Haptic / Scleral Alignment)

• Anatomical Substrate: Rests entirely on the bulbar conjunctiva, episclera, and scleral shell.
• The Toric Sclera: Extensive profilometry research demonstrates that human sclera beyond a 15 mm diameter is highly asymmetric and non-rotationally symmetric in over 95% of the population. The sclera is typically flatter in the horizontal meridian and steeper in the vertical (or inferotemporal) meridian.
• Haptic Geometries:
  • Spherical Haptics: Acceptable only in small diameters ($<14.5 \text{ mm}$). In larger lenses, a spherical landing causes the lens to rock, bearing heavily in the flat meridian while gaping open in the steep meridian.
  • Toric Haptics: The standard of care, incorporating independent flat and steep landing meridians $90^\circ$ apart.
  • Quadrant-Specific & Free-Form Haptics: Individually lathed across 4 to 8 quadrants or derived directly from 3D scleral topography (e.g., sMap3D, Pentacam CSP, EyePrintPRO) to accommodate pingueculae, pterygia, or post-glaucoma trabeculectomy blebs.

Biomicroscopic Clearance Evaluation & Anterior Segment OCT

Slit-Lamp Optic Section Technique

To evaluate scleral lens clearance quantitatively at the slit lamp:

1. Fill the scleral bowl with sterile, preservative-free sodium fluorescein mixed with 0.9% unpreserved saline prior to insertion.
2. Set the slit lamp to a 45° angle with a narrow optic section (slit width ~0.2 mm) at high illumination.
3. Observe the three-layer "sandwich":
   • Layer 1 (Outer): The non-fluorescent, clear contact lens.
   • Layer 2 (Middle): The neon-green fluorescein tear reservoir.
   • Layer 3 (Inner): The grey-white optical section of the patient's cornea.
4. The Center Thickness Ratio Method: Use the known Center Thickness (CT) of the contact lens (typically engraved on the lens vial, e.g., $300 \ \mu\text{m}$) as a physical caliper:
   • If the green tear reservoir is equal in thickness to the $300 \ \mu\text{m}$ lens section, clearance is approximately $300 \ \mu\text{m}$.
   • If the tear reservoir is half the thickness of the lens, clearance is approximately $150 \ \mu\text{m}$.
   • Alternatively, compare against the patient's central corneal thickness (typically ~500 to 550 µm).

Anterior Segment OCT (AS-OCT) Verification

AS-OCT provides objective, micron-accurate measurements of clearance across the 360° profile. Calipers are placed from the posterior lens boundary to the anterior corneal epithelium centrally, paracentrally, and limbal-wise, allowing exact tracking of settling rates and haptic landing angles.

Clinical Complication Management: Blanching, Impingement, Edge Lift & Mid-Day Fogging

1. Conjunctival Vascular Blanching vs. Edge Impingement

• Vascular Blanching (Heel Compression):
  • Appearance: Blood flow within the conjunctival and episcleral vessels is completely pinched off under the landing zone, creating white, bloodless, ischemic tracks.
  • Mechanism: The landing zone is overall too steep or tight. Upon lens removal, blood rushes back into the compressed vessels, causing massive rebound conjunctival hyperemia and pain.
  • Management: Flatten the landing zone / haptic curves.
• Edge Impingement (Toe-Down Bearing):
  • Appearance: The inner heel of the landing zone lifts slightly, but the sharp outer edge ("toe") digs directly into the conjunctiva, creating a localized circular furrow or indentation ring with focal vessel engorgement at the perimeter.
  • Management: Flare the outer edge of the haptic upward (flatten the outer landing zone).

2. Edge Lift (Edge Standoff / Toe-Up)

• Appearance: The edge of the lens stands away from the conjunctiva, creating a shadow or gap.
• Consequences: Causes foreign body sensation on blinking, permits external air bubbles to be pumped into the fluid reservoir during ocular saccades, and invites mucous and lipid debris into the chamber.
• Management: Steepen the landing zone / haptic curves to bring the edge flush against the globe.

3. Mid-Day Fogging (MDF)

• Clinical Presentation: The patient reports crystal-clear 20/20 vision for the first 2 to 4 hours of wear, followed by progressive smoky, milky, or hazy vision that worsens through the afternoon. Removing the lens, rinsing out the fluid bowl, refilling with fresh saline, and reinserting restores instantaneous 20/20 clarity.
• Pathophysiology: Accumulation of lipids, non-polar proteins, mucin strands, and desquamated epithelial cells trapped inside the post-lens fluid reservoir. The microscopic particulates scatter light entering the eye.
• Risk Factors:
  • Excessive central vault ($>350-400 \ \mu\text{m}$), which reduces tear reservoir circulation.
  • Loose or asymmetric haptics that actively pump outer tear lipids and meibomian secretions into the vault during blinks.
  • Coexisting untreated blepharitis, meibomian gland dysfunction (MGD), or ocular surface inflammation.
• Management Algorithm:
  1. Reduce Central Vault: Decrease sagittal depth to achieve an ideal settled clearance of $150 \text{ to } 200 \ \mu\text{m}$.
  2. Optimize Haptic Alignment: Switch from spherical to toric or quadrant-specific haptics to seal out tear pumping.
  3. Viscous Filling Strategy: Fill the scleral bowl with a combination of 70% unpreserved saline mixed with 2 to 3 drops of high-viscosity preservative-free artificial tears (e.g., carboxymethylcellulose 1%, Celluvisc).
  4. Aggressive Eyelid Hygiene: Initiate warm compresses, hypochlorous acid lid scrubs, and oral doxycycline or omega-3 fatty acids for MGD.

Patient Handling Protocols, Insertion Bubbles & Safety Rules

Scleral Insertion Protocol

1. Filling Solution: Must use strictly non-preserved sterile 0.9% sodium chloride saline (e.g., ScleralFil, LacriPure, Purilens). Preserved multi-purpose solutions, re-wetting drops containing benzalkonium chloride (BAK) or polyquad, and tap water are strictly contraindicated. Trapping preservatives against the cornea for 14 hours induces severe toxic keratopathy.
2. The Tripod / DMV Plunger Technique: The patient places the lens on a large hollow DMV suction plunger, fills the bowl to an overflowing positive meniscus with unpreserved saline, and bends the neck so the face is strictly parallel to the floor (chin to chest).
3. Insertion Air Bubbles: If the patient tilts their head even slightly, saline spills out, and an air bubble is trapped inside the fluid reservoir. An air bubble resting over the cornea causes localized corneal desiccation, optical blur, and epithelial dimple veiling. If an air bubble is seen on slit-lamp inspection, the lens must be removed immediately and re-inserted.

Removal Protocol

A small, solid DMV removal plunger is moistened with saline and applied strictly to the inferior or peripheral one-third of the lens at an angle. Applying the plunger to the exact center of a scleral lens attempts to pull against an airtight hydraulic vacuum, causing severe ocular pain and potential corneal abrasions. Tilting the plunger at the peripheral edge breaks the hydraulic seal effortlessly.