Introduction to Contact Lenses
Contact lenses are thin optical devices worn directly on the corneal surface to correct refractive errors. As a COA, you will assist with contact lens fittings, educate patients on proper wear and care, and troubleshoot common issues. A solid understanding of lens types, parameters, and fitting principles is foundational for your certification exam.
Soft Contact Lenses
Soft contact lenses are made from flexible, water-containing hydrogel or silicone hydrogel materials. They drape over the cornea and extend slightly beyond the limbus, conforming closely to the eye's shape. Imagine placing a thin, damp tissue over a rounded surface; it settles and molds to the contour beneath it.
Key characteristics of soft lenses:
- Comfortable from the first wear due to flexibility
- Larger diameter (typically 13.8-14.5 mm), covering the entire cornea
- Available in spherical, toric (astigmatism), and multifocal designs
- Oxygen transmission depends on material: silicone hydrogel allows significantly more oxygen than traditional hydrogel
Gas Permeable (GP) Lenses
Gas permeable lenses (also called RGP or rigid gas permeable lenses) are made from firm, oxygen-permeable plastic. They are smaller than the cornea (typically 9.0-10.0 mm diameter) and float on the tear film rather than conforming to the corneal shape. Think of a GP lens as a small, rigid disc resting on a cushion of tears.
GP lens advantages:
- Superior optical quality, especially for irregular corneas and higher astigmatism
- Excellent oxygen transmission through the material itself
- More durable and longer-lasting than soft lenses
- Tear exchange occurs with each blink, flushing debris from beneath the lens
The trade-off is a longer adaptation period. Patients may need 1-2 weeks to become comfortable with the lid sensation of a rigid lens.
Essential Contact Lens Parameters
Base Curve (BC)
The base curve is the radius of curvature of the lens's back (concave) surface, measured in millimeters. A smaller base curve means a steeper lens; a larger base curve means a flatter lens. The base curve is selected to match the patient's corneal curvature for optimal alignment.
- A lens that is too steep (tight) restricts tear circulation, causes discomfort, and may trap debris
- A lens that is too flat (loose) moves excessively, decenters, and may fall out
Diameter
The diameter is the overall width of the lens from edge to edge. Soft lenses have larger diameters that extend past the limbus for full corneal coverage. GP lenses have smaller diameters that fit within the corneal borders. Proper diameter selection ensures adequate centration and comfortable edge interaction with the eyelids.
Power
The power (measured in diopters) is the refractive correction built into the lens. For patients with astigmatism, toric lenses include both sphere and cylinder power along with an axis. Multifocal lenses incorporate multiple power zones for patients who also need reading correction.
Wearing Schedules
Contact lens wearing schedules define how long lenses can be worn and how frequently they must be replaced:
| Schedule | Replacement | Wear Pattern |
|---|---|---|
| Daily disposable | New lens each day | Removed nightly, discarded |
| Two-week | Every 14 days | Removed nightly, cleaned daily |
| Monthly | Every 30 days | Removed nightly, cleaned daily |
| Extended wear | Per manufacturer | Can be worn overnight (higher infection risk) |
Daily disposables offer the lowest risk of infection and deposit buildup since there is no cleaning or storage involved. Extended wear lenses carry higher risk because overnight wear reduces corneal oxygen supply.
Why This Matters for COAs
You will record lens parameters during fittings, assist the physician with trial lens selection, and teach patients insertion and removal techniques. Understanding how base curve, diameter, and material affect fit and comfort allows you to troubleshoot patient complaints and recognize when a lens modification is needed.
Key Takeaways
- Soft lenses conform to the cornea; GP lenses float on the tear film
- GP lenses provide superior optics for irregular corneas and high astigmatism
- Base curve determines lens steepness: too steep traps tears, too flat causes excess movement
- Diameter differs between soft (larger, past limbus) and GP (smaller, within cornea)
- Daily disposable lenses carry the lowest infection risk
- Vertex distance conversion is needed for prescriptions over +/- 4.00 D