Here is a basic cheat sheet to troubleshooting that problematic RGP patient. We’ve all had the RGP lens wearing patient, for whom we order lenses for based on their refraction and keratometry readings. Sometimes, the lenses fit perfectly, and other times, to our disappointment, they do not. With many warranties only covering one re-make, we need to scrupulously examine the causes of a poorly fitting lens, and ensure that the next lens that arrives in the mail is a perfect fit.
The most common elements that must be examined before beginning a troubleshooting process include:
- Is the base curve too flat?
- Is the base curve too steep?
- Is there too much edge lift?
- Are the edges too tight?
- Is this a lid fit? Or an intrapalpebral fit?
The first critical step is to look at the fluorescein pattern!
Does the fluorescein pattern look like A or B?
A. Flat Lens
- Touch or bearing in the middle surrounded by clearance.
- Darkness indicates pressure on the cornea.
- Smaller darker areas mean even greater degrees of flatness (the smaller the area, the flatter the fit).
- Large amounts of movement and de-centration.
- Blinking causes a lot of movement, and the lens is dropped inferiorly if too flat.
- For lenses that do not lid attach, the lens is pulled up on a blink, and then dropped back into position.
B. Steep Lens
- Central area of clearance.
- Much more comfortable because they fit tighter to the cornea.
- Lack of lens movement.
- Good initial comfort, but complaints of blurry vision or redness and irritation by the end of the day.
- Centers well.
Steps to Correcting for Lens Types:
A. Flat Lens:
- Increase the diameter first by 0.4mm.
- Steepen the base curve by 0.1mm.
- Don’t forget to change the power! Flatten – add plus.
B. Steep Lens:
- Decrease the diameter first by 0.4mm.
- Flatten the base curve by 0.1mm.
- Don’t forget to change the power! Steepen – add minus.
(For every half step change in BC, change power by a quarter)
For a Toric Lens:
A. With-the-rule astigmatism
B. Against-the-rule astigmatism
Lens Diagrams from the “Manual of Contact Lens Prescribing and Fitting“
- Keep in mind when changing diameter, the average cornea is 11.8 mm.
- The first lens should be chosen based on flattest K.
- When a lens is pushed up with the lower lid, a flat lens will drop towards the limbus.
- If the over-refraction shows more minus than the spec RX suggests, the lens is too steep (-0.50D = .1mm steep).
- Larger lenses tend to be more comfortable and more stable than small, so increasing diameter is preferred.
- If changing base curve, you must make changes of at least 0.05mm to cause a change in the fit.
- Avoid altering the optic zone.