Contact Lens Fundamentals
Contact lenses are medical devices worn on the cornea or within the conjunctival fornix to correct vision or for therapeutic purposes. As a CPOA, you may assist with contact lens fittings, perform preliminary measurements, educate patients on care, and help troubleshoot lens-related complaints. A solid understanding of lens types, parameters, and nomenclature is foundational to this role.
Types of Contact Lenses
Soft Lenses
Soft contact lenses are made of hydrogel or silicone hydrogel materials. They drape over the cornea, are comfortable on insertion, and are the most commonly prescribed lenses worldwide.
| Feature | Hydrogel | Silicone Hydrogel |
|---|---|---|
| Oxygen transmission (Dk/t) | Lower | Much higher |
| Corneal health | Risk of hypoxia with extended wear | Approved for extended/overnight wear |
| Deposits | More lipid/protein deposits | Less but different deposit types |
| Comfort | Very comfortable, high water content | Sometimes stiffer; high-modulus issues |
Replacement Schedules
- Daily disposable (DD): fresh lens each day; ideal hygiene; no solutions needed
- Bi-weekly (2-week) replacement: replaced every 2 weeks; nightly removal and cleaning
- Monthly replacement: replaced monthly; requires daily cleaning and disinfection
- Extended wear (EW): approved for continuous wear up to 7 or 30 days; higher infection risk
💡 Clinical Tip: Daily disposable lenses eliminate the need for lens care solutions and significantly reduce the risk of contact lens-related infection (keratitis). They are preferred for patients with allergies, poor compliance, or dry eye.
Rigid Gas-Permeable (RGP) Lenses
RGP lenses are made of oxygen-permeable rigid materials. They provide superior optics for high astigmatism and irregular corneas (keratoconus) but require an adaptation period. Key advantages:
- Excellent oxygen transmission
- Sharper vision in irregular astigmatism
- Longer lifespan (1-2+ years)
- Resists protein deposits
Specialty Contact Lenses
- Toric lenses: correct astigmatism in soft or rigid form; have stabilization features to prevent rotation
- Multifocal lenses: simultaneous vision or alternating designs for presbyopia
- Orthokeratology (Ortho-K): reverse-geometry RGP lenses worn overnight to reshape the cornea temporarily; patient sees without lenses during the day; used for myopia control
- Scleral lenses: large-diameter RGP lenses that vault over the cornea and land on the sclera; used for keratoconus, irregular corneas, and severe dry eye
- Therapeutic (bandage) lenses: soft lenses used to protect the cornea (abrasions, bullous keratopathy) or promote healing after surgery
Key Lens Parameters
Understanding lens parameters is essential for ordering, verifying, and fitting contact lenses:
| Parameter | Abbreviation | Description |
|---|---|---|
| Base curve radius | BCR / BC | Curvature of the posterior lens surface; must approximate the corneal curvature |
| Diameter | D / OAD | Overall lens size; soft ~13.8-14.5 mm; RGP ~9-10 mm |
| Power | D or Rx | Sphere, cylinder, and axis for toric; add power for multifocal |
| Oxygen transmissibility | Dk/t | Measure of how much O2 passes through the lens; higher is better for corneal health |
| Water content | % | For soft lenses; higher water = more comfortable but may dehydrate on the eye |
| Center thickness | CT | Affects lens oxygen transmission and durability |
Fitting Principles
Contact lens fitting begins with a comprehensive eye exam and keratometry (K readings) to measure corneal curvature. A trial lens is applied, and the fit is evaluated by:
- Centration: lens should center over the cornea
- Movement: with the blink, soft lenses should move 0.25-0.5 mm; RGP lenses slightly more
- Coverage: lens should fully cover the cornea
- Comfort: the patient's subjective experience
If the base curve is too flat, the lens moves excessively and may fall out. If too steep, the lens is immobile and restricts oxygen, risking corneal staining and hypoxia.
🔑 Key Point: Always verify vision with trial lenses in place using a distance acuity chart. Over-refraction (refraction with the contact lens on the eye) may be needed to fine-tune the prescription, especially for RGP and toric lenses.
Contraindications to Contact Lens Wear
- Active ocular infection or inflammation
- Severe dry eye (borderline cases may tolerate specialty lenses)
- Inability to handle lenses (manual dexterity issues)
- Significant corneal pathology that makes fitting impossible or risky
- Certain medications (isotretinoin reduces tear production)
Key Takeaways
- Soft lenses are most common; silicone hydrogel offers higher oxygen transmission than hydrogel
- Daily disposables have the lowest infection risk; extended wear has the highest
- RGP lenses provide better optics for irregular corneas but require adaptation
- Scleral lenses vault the cornea and are used for keratoconus and severe dry eye
- Key parameters: base curve, diameter, power, Dk/t, water content
- Soft lens fit assessment: centration, movement (0.25-0.5 mm with blink), coverage