RGP RGLs for Postsurgical Corneas
BY LORETTA B. SZCZOTKA, OD, MS
Rigid gas permeable reverse geometry lenses (RGL), originally advocated for use in orthokeratology, have become increasingly popular postsurgical contact lens designs. RGLs incorporate secondary curvatures that are usually 3.00D to 6.00D steeper than their central curvatures, although peripheral curves have been fitted postsurgically up to 15.00D steeper than the central posterior curve. These designs can help maintain alignment along a plateau or oblate-shaped cornea, which occurs in 31 percent of eyes after penetrating keratoplasty and in most eyes after myopic RK, ALK, PRK or LASIK (Fig. 1). Several RGL designs are available, including: Plateau (Menicon USA, Ltd.); OK Series (Contex Inc.); RK-Bridge (Conforma); NRK lens (Lancaster Contact Lens, Inc.); and RK Design (Rooney Optical).
Shifting to Reverse Geometry
The decision to fit an RGL design instead of a standard or aspheric RGP design (which progressively flattens in the periphery) depends on the surgical procedure performed, the corneal contour, patient comfort and previous RGP lens performance. There's no one correct design selection; many are often physiologically and visually acceptable. The following are my guidelines for choosing standard or reverse geometry RGP designs.
Standard RGPs work well on:
K Symmetrical corneas with no isolated areas of abrupt corneal contour changes surrounding the graft, flap or surgical optical zone.
K Eyes having a higher-than-average preoperative corneal eccentricity (e>0.55). On an eye with rapid peripheral flattening (e=0.75), the secondary and peripheral curve systems of a standard RGP design may be sufficient to allow satisfactory midperipheral corneal alignment.
K Corneas with a smooth, unaltered periphery (PRK, ALK, LASIK).
Reverse geometry RGPs work well on:
K Corneas with abrupt contour changes surrounding the graft or surgical optical zone, which encourages rigid lens decentration.
K Eyes with asymmetrical, surgically altered corneal peripheries (PK and RK).
K Eyes with a low preoperative corneal eccentricity (eg. e=0.3) because the periphery approaches that of a sphere. After grafting or ablating the central cornea, a large disparity may exist between the peripheral cornea and the RGP lens if it was fit appropriately to the central cornea.
K Eyes having a large disparity between the central and peripheral corneal curvatures, necessitating a better alignment fluorescein pattern than a standard RGP can provide.
Oblate Cornea Options
I occasionally use aspheric designs on oblate corneal shapes. The Boston Envision lens (Polymer Technology Corp.), a low eccentricity peripheral aspheric design, has been successful in many cases. However, high eccentricity aspheric RGP lenses flatten excessively in the lens periphery and are counter-productive to achieving good midperipheral alignment on oblate shapes. Watch for an aspheric lens that decenters because visual acuity can be jeopardized from unanticipated power changes positioned over the entrance pupil. CLS
FIG. 1: Oblate corneal shape post-PK.Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.