GP Lens Management Guide – 8

Keratoconus

It is often perceived that you have to be a contact lens expert to fit keratoconus patients. Nothing could be further from the truth. Keratoconus fitting requires trial-and-error fitting, common sense and patience. The result can often be a very appreciative patient. The different fitting relationships are described below:

Fitting Philosophy – Description

Three-Point Touch

  • Very mild apical touch centrally with paracentral clearance
  • Mid-peripheral bearing
  • Peripheral clearance
  • Bearing is present centrally and at two points mid peripherally
  • This method allows more even distribution of the lens across the corneal surface
Apical Clearance

  • Apical clearance centrally minimizes any further compromise to the fragile apex of the cone
  • Peripheral seal off and adherence are possible with this method
Apical Bearing

  • A large zone of apical bearing assists in centration
  • Excessive pressure on the fragile region of the cone may enhance corneal scarring

Keratoconus Fitting Pearls

  • Use a diagnostic fitting set. Contact your CLMA member laboratory about fitting set information. There are numerous designs, including Soper, McGuire, Rose K, ComfortKone and others.
  • The most popular philosophy is to obtain three-point touch. Select a base curve radius equal to the steep “K” reading. Evaluate the fluorescein pattern via a cobalt blue in combination with a Wratten filter with the biomicroscope. (A Burton Lamp is also very beneficial in viewing keratoconic fluorescein patterns.) If apical clearance is present, flatten the base curve progressively until minimal apical touch is present. This resulting bulls-eye pattern should show apical bearing, paracentral clearance, mid-peripheral bearing and peripheral clearance.
  • Good centration is imperative with these designs.
  • The optical zone diameter should decrease as the condition progresses. One commonly used philosophy is to select an OZD equal to the base curve radius in millimeters. For example: BCR = 7.00mm; OZD = 7.00mm
  • Multiple peripheral curves are usually necessary to correspond with the rapidly flattening mid-peripheral and peripheral cornea. The peripheral curve should be flatter and wider than conventional designs to minimize peripheral seal-off.
  • Bitoric lenses are rarely indicated due to the irregular astigmatism of the cornea.
  • One suggested fitting philosophy is presented below:
Stage 1 Average K < 45.00D Conventional lens design
Stage 2 Average K = 45D – 50D Overall diameter = 9.0mm
OZ diameter = base curve radius
Tetracurve design
Stage 3 Average K = 50D – 55D Overall diameter = 8.6 – 8.8mm
OZ diameter = base curve radius
Tetracurve or pentacurve design
Peripheral curve radius = 12.00mm
Peripheral curve width = .3 – .4mm
Stage 4 Average K = 55D – 60D Overall diameter = 8.0mm
OZ diameter = base curve radius
Pentacurve design
Peripheral curve radius = 12.00mm
Peripheral curve width = .3 – .4mm

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