Contact lenses are a primary vision correction modality for millions of patients, and CPOs frequently assist with contact lens evaluations, fittings, and patient education. A solid understanding of lens types, parameters, and fitting principles is essential for the CPO exam.
Types of Contact Lenses
Soft Contact Lenses
Soft contact lenses are made from hydrogel or silicone hydrogel materials. They are flexible, drape over the cornea, and are comfortable on initial wear. Key advantages include:
- Comfortable immediately on insertion (minimal adaptation period)
- Stable on the eye during sports and activity
- Available in disposable modalities (daily, biweekly, monthly)
- Silicone hydrogel materials provide high oxygen transmissibility (important for corneal health)
Soft lenses are available in spherical, toric (for astigmatism), and multifocal designs. Their main limitation is that they conform to the cornea, so they cannot fully correct irregular corneal surfaces.
Rigid Gas Permeable (GP) Lenses
Gas permeable (GP) lenses are rigid and smaller than the cornea. They maintain their shape on the eye and create a tear fluid layer between the lens and the cornea, neutralizing corneal surface irregularities. Key advantages include:
- Superior optical clarity, particularly for irregular corneas (keratoconus, post-surgical)
- More durable and longer lasting
- Better corneal oxygenation per unit area
- Custom designed for each patient
GP lenses require an adaptation period of 1 to 2 weeks as the patient gets used to the awareness of lens edges. They are also more likely to dislodge with activity.
Key Contact Lens Parameters
Contact lens prescriptions include parameters that define the physical properties of the lens:
| Parameter | Description | Typical Range |
|---|---|---|
| Base Curve (BC) | Radius of curvature of the posterior lens surface; must match the corneal curvature | 8.0 to 9.2 mm (soft) |
| Diameter (D) | Overall size of the lens | 13.8 to 14.5 mm (soft); 8.5 to 10 mm (GP) |
| Power | Optical correction (sphere, cylinder, axis for toric) | Varies widely |
| Dk/t | Oxygen transmissibility through the lens; higher is better for corneal health | Silicone hydrogel: 100+ units |
| Water content | Percentage of water in hydrogel materials; affects oxygen delivery and deposits | 25% to 75% for hydrogels |
Wearing Schedules and Modalities
Daily wear (DW): Lenses are worn during waking hours and removed at night. Most recommended for corneal health.
Extended wear (EW): FDA-approved lenses worn for up to 7 days continuously, including sleep. Higher risk of infection and corneal hypoxia compared to daily wear.
Continuous wear (CW): High-Dk silicone hydrogel lenses approved for up to 30 days continuous wear. Still carries infection risk; not recommended for all patients.
Replacement schedules: Daily disposables are replaced every day (no cleaning required); biweekly and monthly lenses require proper cleaning and storage solutions.
Patient Selection and Contraindications
Not all patients are suitable contact lens candidates. Key considerations include:
- Suitable candidates: Motivated patients with adequate tear production, healthy corneas, and good manual dexterity for lens handling.
- Relative contraindications: Severe dry eye, recurrent corneal erosions, active ocular infections, inability to handle lenses safely.
- Absolute contraindications: Active ocular infection, severe corneal scarring in the lens-bearing zone, inability to comply with care instructions.
Key Takeaways
- Soft lenses (hydrogel and silicone hydrogel) are comfortable from day one; GP lenses require adaptation but provide superior optical clarity for irregular corneas.
- Key parameters include base curve, diameter, power, and Dk/t (oxygen transmissibility).
- Lens movement of 0.5 to 1 mm per blink indicates an appropriate fit for soft lenses.
- Daily wear (removal each night) is recommended over extended wear to minimize infection risk.
- Silicone hydrogel materials have high oxygen transmissibility, improving corneal health compared to traditional hydrogels.