GP lens eye Rounds

LOW ASTIGMATISM: Lens Design

Jessica Conroy OD, FAAO

Necessary for all GP fits²

• Spectacle refraction
• Keratometry measurements
• Lid anatomy
• Lower lid position with respect to the inferior limbus
• Horizontal Visible Iris Diameter (HVID)
• Pupil size in dim and bright light

Spherical GP²

• Single Base Curve Radius (BCR) with one overall spherical power
• Many different philosophies for determining BCR to begin with; power of the contact lens is determined after selecting BCR and incorporating SAM-FAP
• Other parameter selection (i.e., Overall Diameter, Optic Zone Diameter, etc.) are dependent on patient’s measurements and lid-anatomy
• General Rules:
  • Lid Attached Fit: GP lens fit that promotes attachment of the GP to the upper eyelid
    • During the blink, the upper eyelid moves over the upper edge of the GP
    • For this type of fit, the upper eyelid must overlap the superior limbus
    • Generally fit “on-K – 0.50D flatter-than-K” with larger OAD
  • Interpalpebral Fit: GP lens fit that promotes attachment of the GP to the upper eyelid
    • For this type of fit, the upper eyelid will sit above the superior limbus
    • Generally fit “0.50D steeper-than-K” with smaller OAD
  • Overall Diameter = HVID – 2mm
  • Optical Zone Diameter = pupil size in dim + 2mm

Front Toric GP¹

• Single BCR with overall spherical power; Application of patient’s residual astigmatism on front surface
  • Will verify with one BCR
  • Will verify with two powers
    • Document powers in terms of “Sphere -Cylinder x Axis”
• Overall, spherical power is determined after BCR selection (SAM-FAP); power of front-toricity = amount of residual astigmatism remaining once spherical power has been applied to lens
• Typically stabilized by prism ballast or prism-ballast truncation
  • Prism Ballast: Prism incorporated into inferior lens design
    • Empirical: Order based on patient’s refractive values, keratometry values, and anatomy measurements
      • Once lens arrives, note rotation of toric markers
    • Diagnostic:
      • With spherical diagnostic set:
        • Fit “steeper-than-K” due to prism increasing weight of lens (and likely resulting in slightly inferior decentration)
        • Select lens with ideal lens-to-cornea fit relationship
        • Perform toric over-refraction
      • With toric diagnostic set:
        • Fit “steeper-than-K” due to prism increasing weight of lens (and likely resulting in slightly inferior decentration)
        • During fit process, note rotation of base-apex line
        • If rotation it observed, incorporate power change using LARS principle
    • Note: Prism Ballasted lenses tend to rotate 10-15 degrees nasal.
  • Prism Ballast and Truncation
    • Typically, most desirable lens design for front toric GPs due to good rotational stability
      • Incorporate prism and truncate lens to ensure inferior edge is not too thick/ heavy
    • Fit process very similar to that of prism ballast

References

1. Bennett ES, Sorbara L, Kojima R. Gas Permeable Lens Design, Fitting, and Evaluation. In Bennett ES, Henry VA. Clinical Manual of Contact Lenses, 5^th ed. Wolters Kluwer, Philadelphia, PA, 2020:134–185.