Contact Lenses

Soft Contact Lenses

The most commonly prescribed type. Made from flexible hydrogel or silicone hydrogel materials that conform to the shape of the cornea.

- Daily Disposable — Worn once and discarded. Lowest risk of infection, no cleaning needed.

- Bi-Weekly (2-week) — Replaced every two weeks. Requires daily cleaning and storage.

- Monthly — Replaced every 30 days. Requires daily cleaning and proper storage.

- Extended Wear — FDA-approved for overnight wear (up to 6 nights for some lenses, 30 nights for others). Higher risk of complications.

Rigid Gas-Permeable (RGP/GP) Lenses

Made from firm, oxygen-permeable materials. They maintain their shape on the eye and create a tear lens that masks corneal irregularities.

- Provide sharper vision than soft lenses, especially for astigmatism

- More durable and longer-lasting than soft lenses

- Require an adaptation period

- Excellent for keratoconus and irregular corneas

Scleral Lenses

Large-diameter GP lenses (14.5-24mm) that vault entirely over the cornea and rest on the sclera. A fluid reservoir between the lens and cornea provides comfort and optical correction.

- Indicated for keratoconus, severe dry eye, post-surgical irregular corneas

- Very stable on the eye with minimal movement

Hybrid Lenses

Combine a GP center with a soft skirt (e.g., SynergEyes). Provide the optical quality of a GP with the comfort of a soft lens.

DK and DK/t

- DK — Oxygen permeability of the lens material itself

- DK/t — Oxygen transmissibility (DK divided by thickness). This is what actually matters for corneal health.

- Higher DK/t = more oxygen reaches the cornea

Hydrogel Materials (HEMA-based)

- Water content ranges from 38% to 79%

- Oxygen transmission depends primarily on water content — higher water = more oxygen

- BUT higher water content also means more protein deposits and faster dehydration

Silicone Hydrogel Materials

- Oxygen transmission through the silicone component, NOT water

- DK/t values 5-7 times higher than conventional hydrogel

- Allow extended/overnight wear with reduced hypoxia risk

- Lower water content (24-48%) but higher oxygen permeability

GP Materials

- Fluorosilicone acrylate — most common modern GP material

- Very high DK values (typically 30-180)

- No water content — oxygen transmits through the material itself

Key Relationship:

For hydrogels: Higher water content → Higher oxygen → But also more deposits and dehydration

For silicone hydrogels: Oxygen depends on silicone content, NOT water content

Base Curve (BC)

The curvature of the back surface of the lens, measured in millimeters.

- Determines how the lens sits on the cornea

- A flatter BC (larger number) creates a looser fit with more movement

- A steeper BC (smaller number) creates a tighter fit with less movement

- Proper fit allows 0.5-1.0mm of movement with each blink for soft lenses

Diameter (DIA)

The overall size of the lens from edge to edge.

- Soft lenses: typically 13.8-14.5mm

- GP lenses: typically 8.5-10.0mm

- Scleral lenses: 14.5-24mm

Signs of a STEEP (tight) fit:

- Minimal or no movement

- Lens may adhere to the cornea

- Conjunctival indentation at lens edge

- Patient may report comfort initially but develop redness over time

Signs of a FLAT (loose) fit:

- Excessive movement (>1mm)

- Lens decenters or falls off the cornea

- Lens edge lifts off the sclera

- Inconsistent vision with blinking

Toric Contact Lenses (for Astigmatism)

Toric lenses have different powers in different meridians. They must maintain proper orientation on the eye.

Stabilization Methods:

- Prism Ballast — Thicker at the bottom, held in place by gravity and lid forces

- Thin Zone (Dual Thin Zone) — Thin areas at top and bottom; lids hold the lens in position

- Truncation — Bottom edge is cut flat to rest on the lower lid

If a toric lens rotates, the axis of correction shifts and vision blurs.

Multifocal Contact Lenses (for Presbyopia)

- Simultaneous Vision Designs — Both distance and near images are on the retina at the same time; the brain selects the appropriate image

- Center-near (aspheric) — Near power in the center, distance in periphery

- Center-distance — Distance power in the center, near in periphery

- Concentric rings — Alternating zones of distance and near

- Monovision — One eye corrected for distance, other for near. Simple but reduces binocular vision and depth perception.

Care Systems:

- Multipurpose Solution (MPS) — Cleans, rinses, disinfects, and stores in one solution

- Hydrogen Peroxide — 3% H₂O₂ that must be neutralized (catalytic disc or tablet) before insertion. More effective disinfection than MPS.

- Enzymatic Cleaners — Protein removal tablets used weekly with some lens types

Complications Opticians Should Recognize:

- Giant Papillary Conjunctivitis (GPC) — Inflammatory bumps under the upper lid from deposit/mechanical irritation. Symptoms: itching, mucus, lens intolerance.

- Corneal Neovascularization — Blood vessel growth into the cornea from chronic hypoxia. Requires switching to higher DK/t lenses or reducing wear time.

- Corneal Ulcer (Microbial Keratitis) — Serious infection. Symptoms: pain, redness, white spot on cornea, discharge. Refer immediately.

- Contact Lens-Related Dry Eye — Symptoms: discomfort, dryness, reduced wear time. Manage with rewetting drops, reduced wear time, or switch to daily disposables.

When to Refer to the Doctor:

- Eye pain, redness, or discharge

- Sudden vision changes

- White spot on the cornea

- Persistent discomfort despite proper lens care

- Any signs of infection

Patient Education:

- Never sleep in lenses unless approved for extended wear

- Never use tap water with contact lenses

- Replace lenses on schedule

- Replace lens case every 1-3 months

- Always wash and dry hands before handling lenses

Vertex Distance Compensation

Contact lenses sit directly on the cornea (vertex distance = 0mm), while spectacles sit ~12-14mm away. For prescriptions over ±4.00 D, the power must be compensated.

Formula: Fc = Fs / (1 - d × Fs)

- Fc = contact lens power

- Fs = spectacle power

- d = vertex distance in meters

Key Rules:

- High minus spectacle Rx → less minus in contacts (e.g., -10.00 spectacles ≈ -9.00 contacts)

- High plus spectacle Rx → more plus in contacts (e.g., +10.00 spectacles ≈ +11.00 contacts)

Additional Differences:

- Contact lens Rx includes base curve (BC) and diameter (DIA) — not needed in spectacle Rx

- Cylinder power/axis may differ due to vertex distance effects

- Contact lens Rx has an expiration date (typically 1-2 years)

- The FTC Contact Lens Rule requires prescribers to release the contact lens Rx automatically