Fitting Keratoconus
Keratoconus is a bilateral non-inflammatory corneal ectasia with an incidence of approximately 1 per 2,000 in the general population (1). The treatment of Keratoconus can be implemented by the use of spectacle lenses, contact lenses of various kinds and surgery.
In more advance Keratoconus cases where severe irregular astigmatism with resultant higher order aberrations are present, the use of Rigid Gas Permeable (RGP) lenses are indicated to restore the patients vision. Unfortunately in some Keratoconus cases, contact lens intolerance develops and many patients consider Penetrating Keratoplasty (cornel graft). Lim, et al found that 83% of patients opting for Penetrating Keratoplasty was due to intolerance to contact lens wear (2).
However, Smiddy, et al found that 87% of the Keratoconus cases referred for Penetrating Keratoplasty to the Wilmer Institute at the Johns Hopkins Medical Institution could be successfully refitted with contact lenses. Of the successfully fitted cases ultimately 31% needed keratoplasty after an average of 38.4 months of lens wear and 69% did not require Keratoplasty over an average follow-up interval of 63 months of wearing contact lenses.
Of the postoperative penetrating Keratoplasty eyes 60% had to wear contact lenses for best vision. Their conclusion was that Penetrating Keratoplasty can be delayed or avoided in many Keratoconus patients by using proper fitting contact lenses and further that Keratoconus eyes often need contact lenses after Keratoplasty (3).
Piggyback system
One option to improving the comfort of RGP lenses on Keratoconic and irregular corneas is to use a piggyback system. A piggyback system comprises of a soft contact lens first placed on the cornea and then followed by a RGP lens fitted on top of the soft lens. With the birth of Silicone Hydrogel soft lenses and hyper DK RGP materials, piggyback systems have become a safe and viable option for vision correction of the compromised cornea. Tsubota, et al found that the oxygen pressure under piggybacked oxygen-permeable hard contact lenses was 95 +/- 14 mmHg after 5 minutes wear which was almost three times higher than the 34 +/- 14 mmHg when PMMA and low water-content lenses were used (4). Clare O'Donnell first reported on the fitting of a hyper-Dk piggyback contact lens system in Keratoconus and other irregular corneas in 2004 (5).
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