What is three-point touch in keratoconus fitting?

Three-point touch is the ideal GP lens fluorescein pattern for keratoconus: gentle bearing at the cone apex, fluorescein pooling (clearance) in the mid-periphery around the cone, and bearing at two peripheral points on the flatter surrounding cornea. This distributes lens weight across three points for stability.

What is the difference between a nipple cone and an oval cone?

A nipple cone is small (approximately 5 mm), centrally located, and surrounded by relatively normal cornea, making it easier to fit with standard GP designs. An oval cone is larger, displaced inferiorly and temporally, and affects a broader area of the cornea, making stable lens centration more challenging and often requiring specialty designs.

Why should you avoid fitting keratoconus lenses too flat?

A flat fit concentrates all bearing on the cone apex, the thinnest part of the cornea. This chronic mechanical pressure causes epithelial damage and eventual apical scarring, a permanent opacity that reduces central vision. Fitting should begin steeper than the flat K reading to achieve proper clearance over the apex.

Keratoconus Contact Lens Fitting: GP Design, Three-Point Touch, and Cone Types

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 cone type determines the fitting approach. Nipple cones are generally more responsive to standard GP designs because the small cone is surrounded by relatively normal cornea. Oval cones require specialty designs (like Rose-K) or scleral lenses because the larger, decentered distortion challenges conventional fitting.

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 the lens too flat (flat K or flatter) for keratoconus. A flat fit concentrates all bearing on the cone apex, causing discomfort, lens instability, and eventual apical scarring. The fitting should begin steeper than the flat K reading to achieve the three-point touch pattern.

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

When the fluorescein pattern shows heavy apical bearing with a steep peripheral seal-off (a "bull's eye" pattern), the base curve needs to be steepened, not flattened. Flattening in this situation worsens apical bearing. Steepening lifts the center of the lens off the apex.

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