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Fitting & Verification
Rigid Lenses

LARGER LENSES WITH WIDE PERIPHERAL CURVES

General Characteristics:
The lens diameter is approximately 9-10 mm and contains a large optic zone of 7-8 mm. It often assumes the shape of a tricurve lens with the radius of the intermediate curve 1 mm flatter than the base curve and the radius of the peripheral curve 1 mm flatter than that of the intermediate curve. With blinking the lens attaches to the upper lid and is lifted high and may even override the superior limbus. As we�ll see later on, a high riding lens may also be achieved through the use of a minus carrier. A larger lens of this type may be indicated by a flat, large cornea-over 11 mm or flatter that 44.00 diopter, or by a large palpebral aperture of 10 mm or greater. It can also be useful when smaller lenses result in poor centration. When made of PMMA, these lenses can only be tolerated by about 20-30% of patients. However considerably greater success is achieved when the newer gas permeable materials are used.

Advantages:
  1. The larger optic zone can minimize or eliminate the annoying flare often seen with smaller lenses.
  2. The wider peripheral curves hold a large reservoir of tear fluid that aids in cushioning the lens and assists in tear exchange.
  3. The larger size enables the lens edge to remain under the upper lid during blinking.
  4. Spherical lenses can correct for up to 3.00 to 4.00 diopters of corneal astigmatism.
  5. The lenses center well, are stable, and are easy to handle.

Disadvantages:

  1. There is an increased risk of corneal molding due to the bulk of the lens which can induce up to 5.00 to 6.00 diopters of corneal astigmatism.
  2. Not practical for patients with small palpebral fissures, or small, steep corneas.
  3. Lenses can only be tolerated by about 20-30 percent of patients. Although it is less comfortable at first, a surprising degree of adaption can be achieved after a few weeks.
  4. Peripheral curves must be precisely designed or lenses can become very loose.

    (The newer gas permeable materials have solved many of the problems associated with large PMMA lenses. The advantages still stand.)


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