Understanding Keratoconus
Keratoconus is a progressive corneal ectasia (thinning and protrusion) that causes the cornea to assume a cone-shaped contour rather than its normal smooth dome shape. This irregular surface creates irregular astigmatism that cannot be fully corrected with glasses or standard soft contact lenses. GP contact lenses are typically the primary visual correction because the rigid lens surface replaces the irregular cornea as the primary refracting surface.
Cone Classifications
Keratoconus cones are classified by size, shape, and location, which directly influences fitting strategy:
Nipple Cone
- Small diameter (approximately 5 mm)
- Centrally located on the cornea
- Steep apex with relatively normal peripheral cornea
- Often easier to fit because standard GP designs can vault the small cone while landing on the more normal surrounding cornea
Oval (Sagging) Cone
- Larger diameter (approximately 5-6 mm or more)
- Displaced inferiorly and temporally
- More challenging to fit because the larger area of distortion makes achieving stable lens centration difficult
- The decentered cone often causes the GP lens to ride low or tilt
Globus Cone
- Involves more than 75% of the corneal surface
- Most advanced form with the greatest fitting challenge
- May require scleral lenses when standard GP designs cannot achieve adequate fit
The Three-Point Touch Pattern
The ideal GP lens fit for keratoconus is the three-point touch pattern, evaluated using fluorescein under cobalt blue light:
- Apical touch (or light bearing): Gentle contact at the cone apex, seen as a dark area of thin fluorescein (or light bearing). Some practitioners prefer slight apical clearance to avoid apical scarring
- Mid-peripheral clearance: A ring of fluorescein pooling around the cone apex, indicating the lens vaults over the paracentral cornea
- Peripheral touch: Two areas of bearing at the flatter peripheral cornea, typically superior and inferior, providing stability
This pattern creates a stable lens position that distributes bearing across three points rather than concentrating pressure on the cone apex alone.
Avoiding Apical Scarring
Excessive bearing on the cone apex leads to apical scarring, a permanent opacity at the thinnest point of the cornea:
- The cone apex is the thinnest, most vulnerable area of the keratoconic cornea
- Chronic mechanical pressure from a lens resting heavily on the apex causes epithelial and stromal damage
- Once scarring develops, it permanently reduces vision through the optical center
- Modern fitting philosophy trends toward apical clearance rather than bearing to prevent this complication
Specialty Lens Designs
Rose-K
Rose-K is the most widely used specialty GP lens design for keratoconus worldwide:
- Features multiple aspheric curves that match the complex corneal contour
- Available in standard, nipple, and post-graft versions
- Peripheral curves can be adjusted independently to optimize edge lift and comfort
Piggyback Systems
A piggyback system places a GP lens on top of a soft lens:
- The soft lens acts as a cushion, improving comfort
- The GP lens provides the optical correction
- Used when GP lens bearing on the cone causes discomfort or when the apex is too sensitive for direct GP contact
Hybrid Lenses
Hybrid lenses (like SynergEyes) combine a GP center with a soft skirt, offering GP optics with soft lens comfort.
Scleral Lenses
For advanced keratoconus where corneal GP lenses cannot achieve adequate fit, scleral lenses vault entirely over the cone and land on the sclera.
Fitting Process
- Obtain keratometry or topography: Measure the steepest and flattest meridians to characterize the cone
- Select initial trial lens: Start steeper than the flat K reading (many practitioners use the steep K as a starting point)
- Evaluate fluorescein pattern: Look for three-point touch with minimal apical bearing
- Assess movement and centration: The lens should center well and move 1-2 mm with blinks
- Perform over-refraction: Determine the final power over the trial lens
- Adjust as needed: Modify base curve, diameter, or peripheral curves based on the pattern
Key Takeaways
- Keratoconus causes irregular astigmatism best corrected with GP contact lenses
- Nipple cones are small and central; oval cones are larger and decentered inferiorly-temporally
- The ideal fit is a three-point touch pattern with light apical bearing and peripheral stability
- Excessive apical bearing causes permanent apical scarring
- Rose-K is the most widely used specialty GP design for keratoconus
- Advanced cases may require piggyback, hybrid, or scleral lens systems