What is Dk/t and why does it matter for contact lenses?

Dk/t is a measure of oxygen transmissibility through a contact lens, where Dk is the oxygen permeability of the lens material and t is the lens thickness. A higher Dk/t means more oxygen passes through the lens to reach the cornea. The cornea depends on atmospheric oxygen because it is avascular. Insufficient oxygen leads to corneal hypoxia, causing neovascularization, edema, and increased infection risk. Silicone hydrogel lenses have Dk/t values over 100 units, significantly higher than traditional hydrogel lenses, making them safer for extended or daily wear.

What is the difference between daily disposable and daily wear lenses?

Daily disposable lenses are replaced with a new pair every day; they require no cleaning or storage solutions. Daily wear refers to the wearing schedule, meaning lenses are removed every night for sleep, regardless of whether they are daily disposable, biweekly, or monthly replacement lenses. A monthly replacement lens worn on a daily wear schedule is removed each night, cleaned, and stored, then reused for up to 30 days. Daily disposable lenses on a daily wear schedule offer the convenience of no cleaning and the hygiene benefit of fresh lenses each morning.

How do you assess contact lens fit on a slit lamp?

Soft lens fit is assessed by observing lens centration, coverage of the limbus, and movement with blinking and lateral gaze. A well-fitting soft lens should: center over the cornea with complete limbal coverage (edge slightly beyond the limbus on all sides), move 0.5 to 1 mm with a straight-ahead blink, and not show excessive tightness (minimal or no movement) or excessive looseness (more than 1.5 mm of movement). The push-up test helps assess tightness. GP lens fit is assessed using fluorescein patterns with a cobalt blue light to visualize the tear layer under the lens.

Why might a patient with keratoconus benefit from GP lenses over soft lenses?

Keratoconus causes progressive thinning and irregular steepening of the cornea into a cone shape. Soft lenses drape over this irregular surface, conforming to it rather than correcting it. The irregular corneal surface still transmits optical aberrations through the soft lens, limiting visual acuity. GP lenses, being rigid, maintain a spherical shape over the irregular cornea and create a smooth, regular anterior optical surface using the tear film between the lens and the cornea. This often provides much sharper vision than spectacles or soft lenses for keratoconus patients. Specialty lenses such as scleral lenses are often preferred for advanced keratoconus.

What does it mean when a contact lens base curve is 'too steep'?

If the lens base curve is steeper (smaller radius of curvature) than the cornea, the lens fits too tightly, like a cap that is too small. There is insufficient tear exchange beneath the lens, creating a stagnant reservoir that accumulates debris and reduces oxygen delivery. On slit lamp assessment, a steep lens shows little or no movement with blinking. Prolonged wear of a steep lens causes corneal hypoxia, vascularization, and discomfort. The base curve should be flattened (larger radius number) to allow adequate lens movement and tear exchange.

Contact Lens Basics for the CPO Exam

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

Base curve selection is critical. If the lens base curve is too flat relative to the cornea, the lens will be loose, move excessively, and cause discomfort. If too steep, the lens will be tight, reduce tear exchange, and risk hypoxia. For soft lenses, a flat-to-align fit is preferred; the lens should move 0.5 to 1 mm with each blink.

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.

Sleeping in contact lenses, even extended-wear lenses, significantly increases the risk of microbial keratitis (corneal infection). The risk is approximately 6 to 8 times higher with overnight wear versus daily removal. Patients should be counseled to remove lenses if they feel they might fall asleep.

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.