Alignment Fit for Mid Peripheral Zone #
While the center of the lens needs to provide vision, the mid-periphery of the lens needs to provide the positioning and movement. Remember the horizontal meridian is the flattest (in with-the-rule astigmatism) and you should expect that the lens will touch here most closely. This will allow the lens to move up and down, and keeps if from moving from side to side.1
Steep Mid Peripheral Fit #
When the middle of the lens is too tight, it will seal off the tear film in the center of the lens and inhibit the exchange of oxygen. A tight middle will also limit the movement of the lens and may cause rubbing of the cornea. This will lead to trouble.1
Flattening the center of the lens will loosen this area of the lens and allow greater movement.1
Objective findings like 3 & 9 O’Clock Staining or “H”-pattern or subjective feedback like Fogging, Photophobia or Hazing, Halo’s and Fogging or also supports the steep Fluorescein findings.
Flat Mid Peripheral Fit #
When there is not enough interaction between the back side of the lens and the cornea, there will not be the opportunity to hold the lens in place. When this happens, the lens can slip from side to side and movement will not be consistent.1 It might also be grabbed by the top lid, keeping it up under the lid as there aren’t enough grip for the lens to hold it in place on the cornea.
Steepening the center of the lens will tighten this area, allowing more grip and less lens movement.1
Objective findings like 3 & 9 O’Clock Staining or “H”-pattern also supports the flat Fluorescein findings.
1. Laäs C, Eksteen C, Booysen D. The art of contact lens fitting. 2005:23-26
