Lens Fitting Principles and Techniques

Introduction

• Overview of contact lens fitting using various diopter levels.
• Importance of proper lens fitting for comfort and effectiveness.

Diopter and Corneal Contact

• First Lens: Diopter flatter than K (41 diopter lens)
  • Centers on the cornea but displaces laterally.
  • Not an ideal fitting situation.
• Second Lens: 42 diopter base curve, exactly on flat K.
  • Shows 20 microns clearance.
  • Lens centers well with touch points at 3 and 9 o'clock.
  • Ideal fit with proper centering after blinking.

Mid-Peripheral Contact Points

• Importance of mid-peripheral contact along the horizontal meridian.
• Ideal clearance between 10 and 20 microns, with contact at 3 and 9 o'clock.
• Varies with corneal eccentricity (value examples: 0.20, 0.40, 0.60).

Corneal Eccentricity and Fitting Challenges

• Using central keratometric readings can be challenging.
• Different eccentricity values affect lens fit:
  • Example: 42 diopter lens may fit with 0.40 eccentricity but not with 0.20 or 0.60.
• Suggests measuring cornea periphery for better fit.
• Use of topographer and axial map to determine base curve.

Vertical Movement and Lens Design

• Lens should move unobstructedly along the vertical meridian.
• Importance of avoiding obstruction to prevent discomfort.
• Ideal clearance of 40 microns at 12 and 6 o'clock.

Astigmatism and Lens Fitting

• Patients with with-the-rule astigmatism are ideal for rigid gas permeable lenses.
• Example: K readings 41.75 x 43.75, 2 diopters of with-the-rule astigmatism.
• Expect touch points at 3 and 9 o'clock and unobstructed vertical movement.

Application of Fits Using Different Diopters

• Example 1: 41.75 lens - too flat, centers poorly.
• Example 2: 42.75 lens - better fit with adequate clearance.
• Example 3: 43.75 lens - too steep, risks of staining and keratitis.

Determining Lens Power

• Use spherical over-refraction or SAM-FAP rule.
• SAM-FAP Rule:
  • Steeper than flat K: Add minus power.
  • Flatter than flat K: Add plus power.