OM t of c.iii - Vision Research Coordinating Center - Washington ...

2/1/99 Chapter 1 Background page 1-14
optically corrected. Spectacles cannot adequately correct the irregular astigmatism
present in keratoconus, so before contact lenses, surgery was the only recourse. After
diagnosing keratoconus and establishing a need for improved vision beyond that which
can be achieved with spectacles, rigid contact lenses are the method of choice for
optimizing vision and managing the irregular astigmatism (corneal distortion)
associated with keratoconus (Mandell, 1988). The ultimate objective is to provide the
best vision, comfort and wearing time possible while minimizing any adverse effect on
corneal physiology from the contact lens.
Scleral contact lenses were used a century ago to treat corneal distortion and
keratoconus and were made of glass and later of a plastic, polymethylmethacrylate
(PMMA) (Gould, 1970). From approximately 1950 through the mid-1970s, PMMA
contact lenses provided the most common management of keratoconus and the fitting
methods used today were developed (Hall, 1963; Voss and Liberatore, 1962; Soper and
Jarrett, 1972). More recently, soft contact lenses have been used in the earliest stage of
the disease (Koliopoulos and Tragakis, 1981; Lundh, 1978).
Because of the inability of soft contact lenses to correct significant amounts of
irregular astigmatism, gas permeable rigid corneal lenses are required for mild,
moderate, and advanced keratoconus (Raber, 1986; Rosenthal, 1986; Belin et al., 1988;
Gasset and Lobo, 1975; Mackie, 1977; Buxton, 1978; Mobilia and Foster, 1979; Raber,
1983; Maguen et al., 1983; Cohen and Parlato, 1986). These lenses may be fitted flat
(touching the center or apex of the cornea ) or steep (not touching, or clearing the
corneal apex).
The piggyback approach, a rigid lens fitted over an underlying soft lens
(Baldone, 1984) is currently only used in very complicated, advanced stages of the
disease due to excessive corneal edema and inconvenience of lens care systems.
While contact lenses provide improved vision, they also have the potential to
damage the cornea. Scarring of the central cornea may occur in keratoconus without
contact lens wear (Krachmer et al., 1984), but it is possible that contact lenses can hasten
axial scarring (Korb et al., 1982). Whether by causing hypoxia, entrapment of biodebris
and tears, or physical trauma, the contact lens may induce deep central corneal
abrasions. Due to the loss of epithelial layers, disruption of the anterior limiting
membrane and Bowman’s membrane, and disruption of the regular anterior stroma
collagen structure (Pataa et al., 1970), the central cornea may become scarred—and thus
the vision reduced—secondary to contact lens wear in keratoconus.
1.6.1 Fitting Methods: Flat Lenses vs. Steep Lenses in Keratoconus
The major techniques for fitting rigid lenses in keratoconus (as described by Korb
et al., 1982) are: