An ideal keratoconus corneal RGP fit should center well to provide optimal vision and move approximately 0.5–1.0 mm with blink to allow for oxygen-rich tear exchange beneath the lens. The ideal fluorescein pattern is called “3 point touch ” and should show light touch at the apex with surrounding pooling and midperipheral touch with pooling of the 0.5–1.0 mm edge of the lens. The resultant fluorescein pattern is a bull’s eye pattern of concentric rings (see Fig. 16.2). The midperipheral touch helps to distribute the weight of the lens across a larger area rather than placing the full burden on the apex of the cone. Patients are able to see well in flat fitted lenses that push on the apex of the cone since the reshaping causes a reduction in HOA [3]. In 1982, Korb et al. published a paper describing apical scarring as a result of apical fit, flat lenses [4]. The much larger CLEK study also reviewed the effect of RGP wear and corneal scarring and found that the risk of scarring in keratoconic patient was two times more likely in contact lens wearers compared to noncontact wearers. However, patients with more severe ectasia were more likely to wear contact lenses than patients with milder ectasia so a causal relationship could not be established [5]. Due to this ongoing debate, many practitioners do not fit apically flat lenses. In instances where three-point touch is not achievable, the corneal epithelium cannot tolerate the lens, or lens retention is a problem, another lens modality should be chosen.

There are many different RGP manufacturers that design and produce corneal RGP lenses specifically for keratoconus. For example, Acculens makes the Accukone^® , Blanchard makes the CentraCone^® , and Lens Dynamics makes the Dyna Cone^® . In addition to the different designs that are specific to each manufacturer, many will carry the Rose K^® , Rose K2^®, and Rose K2 IC^® lenses that were designed by an optometrist from New Zealand named Paul Rose. The line of Rose K lenses is widely available and is currently distributed in over 88 countries [6]. The Rose K and Rose K2 designs can be used for mild to severe keratoconus with base curves available between 4.30 and 8.60 and diameters between 7.9 and 10.4 mm. The Rose K2 differs from the previous Rose K design in that the Rose K2 has aspheric curves to reduce HOA and a larger optic zone to improve acuity in low light. Additionally, Rose K and Rose K2 are now available with Asymmetric Cornea Technology (ACT) . This allows the inferior quadrant of the lens to be steeper in order to correspond to the steepest portion of the cornea.

The Rose K2 IC design is a larger diameter corneal RGP that works well for mild to moderate keratoconus. It is available in base curves between 5.70 and 9.3 and diameters between 9.4 and 12.0 mm [7]. New wearers may adapt more easily to this design for two reasons. The first is that a large diameter lens often centers well on the cornea and a well-centered lens causes less lens awareness than a decentered lens. Also, large diameter lenses tend to move less than smaller diameter lenses and less movement allows for less lens awareness. The challenge with this design in more advanced keratoconus is that as the central base curve needs to be adjusted steeper, the sagittal depth increases significantly due to the large diameter and the lens may suction, or adhere, to the cornea. An adhered corneal RGP is always considered an unacceptable fit as the epithelium cannot tolerate the pressure of the lens or the reduced oxygen levels that occur when there is no longer oxygen-rich tear exchange beneath the lens. Additionally, while this design works well for mild to moderate keratoconus, it does not work well for nipple cones. Due to the large diameter, this lens creates more pressure on the cornea than smaller diameter lenses. A broad cone can disperse this pressure over a larger area and the epithelium will be able to tolerate it. A nipple cone disperses the pressure over the much smaller area of ectasia and epithelial erosions can occur at the apex. Nipple cones can be fit successfully with small diameter cornea RGP s or one of the other modalities discussed later.

Keratoconus designs can be carved from a variety of gas permeable material buttons. Most labs make these designs from common RGP materials such as Bausch and Lomb’s Boston ES, EO, XO, or XO2^® or Menicon’s Menicon Z^® . These materials vary in their composition, wetting angle, and oxygen permeability (Dk) (Table 16.1). The wetting angle quantifies the wettability of a solid surface by a liquid. The closer the wetting angle is to zero, the more hydrophilic the surface of the lens. Dk refers to oxygen permeability of the material and the higher the number, the more oxygen permeable. Both Dk and wetting angle improved as Boston ES^® evolved to EO^®, then XO^®. RGP lenses for keratoconus are now commonly made with XO2^® and Menicon Z^®.

The advantages to the corneal RGP lenses are that they are fairly simple to fit and are well tolerated by the cornea due to the high Dk materials available. They are also easy to insert, remove, and clean and are relatively inexpensive compared to other options. The natural deposit resistance of the RGP materials is also preferred over the soft lens materials for atopic patients. Some insurance companies will cover corneal RGP lenses for patients with keratoconus.

This lens modality has limited success when the apex of the cone is decentered unusually inferior. The lens will tend to center over the apex of the cone, which can cause the visual axis to be outside the optic zone of the lens. This results in poor acuity and glare complaints. An inferiorly decentered lens is also difficult for a patient to adapt to since the upper lid will hit the edge of the lens with each blink and push the lens into the cornea.

These lenses may also fail in cases of extreme ectasia where lenses tend to have poor stability and are poorly retained. They are also not ideal for patients that work in dusty environments (i.e., construction, landscaping, farming), since debris can get under the lens causing cornea abrasion and discomfort.