Soft Contact Lenses

SCL Materials & Properties - Material Marvels

• Materials:
  • Hydrogels (e.g., HEMA): O₂ via water.
  • SiHy (Silicone Hydrogels; e.g., Lotrafilcon): O₂ via silicone. Higher Dk.
• Key Properties:
  • $Dk$ (Oxygen Permeability): Material's O₂ capacity.
  • $Dk/t$ (Oxygen Transmissibility): $Dk$/thickness. Corneal health.
    • Daily Wear (DW): $Dk/t > \textbf{24} \times 10^{-9}$ (📌 Daily 24)
    • Extended Wear (EW): $Dk/t > \textbf{87} \times 10^{-9}$ (📌 Extended 87)
  • WC% (Water Content): Hydrogels: ↑WC → ↑Dk. SiHy: Dk less WC-dependent.
  • Modulus: Stiffness. ↑Modulus → handling ease, ↓comfort.
  • Wettability & Deposit Resistance.

⭐ Holden-Mertz: $Dk/t > \textbf{24} \times 10^{-9}$ (DW), $Dk/t > \textbf{87} \times 10^{-9}$ (EW) to prevent corneal edema.

SCL Types & Modalities - Lens Lineup

• Key SCL Categories:

  Modality / Schedule	Daily Disposable (DD)	Reusable (Bi-weekly/Monthly)	Extended Wear (EW)
  Wear Time	Single day use	Daily wear, regular cleaning	EW: Up to 30 nights (SiHy approved)
  Replacement	Daily	2 weeks / 1 month	Per lens type (e.g., monthly for some EW)
  Pros	↑Hygiene, convenience	Cost-effective (lens)	Max convenience
  Cons	↑Cost (annual)	Cleaning regimen, deposit risk	↑Hypoxia, ↑Infection risk
  Common Specialty Lens Availability	Toric, Multifocal	Toric, Multifocal, Custom	Toric, Multifocal

⭐ Extended wear SCLs, especially older generation high water content ionic lenses, carry a significantly higher risk of microbial keratitis (MK), particularly Pseudomonas. Newer SiHy lenses offer better oxygen permeability, reducing hypoxic stress.

SCL Fitting Principles - Finding Fit

• Goal: Optimal vision, comfort, ocular health.
• Key Parameters: Base Curve (BC) for sagittal depth, Diameter (Dia) for overall size.
• Assessment (3-Point Check): Evaluate on eye.
  • Corneal Coverage: Complete, extends beyond limbus.
  • Centration: Well-centered post-blink, minimal lag.
  • Movement: Ideal 0.2-0.4mm with blink.
• Tight Fit: <0.1mm movement; conjunctival drag, vessel blanching, discomfort.
• Loose Fit: >1.0mm movement; decentration, edge lift-off, unstable vision.
• Over-Refraction (OR): Refine spherical/cylindrical power post-fit.

⭐ A tight SCL may initially feel comfortable but can lead to serious complications like corneal hypoxia and neovascularization.

SCL Complications - Red Eye Risks

Differentiating key SCL-related red eye conditions:

⭐ Pseudomonas aeruginosa is the most common organism in SCL-related microbial keratitis, causing rapid, severe ulceration.

Other risks:

• Corneal Neovascularization: Vessels >1.5mm into cornea (chronic hypoxia).
• Superior Epithelial Arcuate Lesion (SEAL): Mechanical trauma from lens edge.
• Tight Lens Syndrome (TLS): Acute pain, halos, corneal edema, often after sleep.

SCL Care Regimens - Clean Routine

• Goal: Prevent microbial keratitis.
• Key Systems:
  • MPS: Rub & rinse crucial.
  • H2O2: Preservative-free; neutralize (e.g., catalytic disc) for ~6 hours.
• Cleaning Steps:

⭐ Never use tap water for rinsing lenses or cases due to Acanthamoeba keratitis risk.

High‑Yield Points - ⚡ Biggest Takeaways

• Materials: Hydrogels (HEMA) and Silicone Hydrogels (SiHy); SiHy for high oxygen permeability (Dk/t).
• Water Content: High water in hydrogels: ↑ Dk, but more deposits & dehydration.
• Pros: Great initial comfort, easy adaptation, good for intermittent/disposable wear.
• Cons: Less durable, prone to deposits, may blur vision with astigmatism.
• Key Risks: Microbial Keratitis (Pseudomonas), GPC, Corneal Neovascularization, Sterile Infiltrates.
• Fitting: Based on K-readings & HVID; SCLs are larger than cornea.