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 advanced keratoconus cases where severe irregular astigmatism with resultant higher order aberrations are present, the use of Rigid Gas Permeable (RGP) lenses is indicated to restore the patient’s vision.  Unfortunately, in some kKeratoconus cases, contact lens intolerance develops, and many patients consider Penetrating Keratoplasty (Corneal 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 keratoconus 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 an 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).

Biofinity Silicone Hydrogel lens

Comfilcon A (Biofinity) is a third-generation polymer with no TRIS-based derivatives. It uses a unique long-chain siloxane macromer combined with other components to result in a lens that features high oxygen permeability (DK 128) and a relatively low modulus (0.75 MPa). The Comfilcon A material is inherently wettable with no internal wetting agent or surface treatment required.  It has a fundamentally different chemistry that breaks the relationship between oxygen permeability and water content that other silicone hydrogel materials follow. (6) (7) (8)

The combination of high oxygen permeability and low modulus make the Biofinity lens an ideal piggyback lens.  The low modulus allows the lens to follow the irregular keratoconus topography more closely without fundamentally changing the pre-fit Sim K-values. This low modulus of the lens allows the practitioner to trial fit the keratoconus RGP lenses without the Biofinity lens on the eye and only introduce the Biofinity lens at the final stages of fitting with minimal impact on the RGP fit.

Image 1 shows the piggyback system with the Biofinty lens placed directly on the cornea and the Keraton RGP lens fitted on top.

Image 2 shows the Oculus tangential topography map of an irregular cornea due to Pellucid Marginal Degeneration with no contact lens on it.

Image 3 shows the Oculus tangential topography map of the same cornea with a Biofinity contact lens fitted on it.

Conclusion

This hyper-Dk piggyback contact lens combination of Biofinity soft lenses and HDS 100 RGP lenses satisfy the ocular and visual requirements for patients with compromised corneas for cases where regular RGP lens wear has been unsuccessful. This system can be implemented in most cases requiring visual rehabilitation like Keratoconus, Pellucid Marginal Degeneration, Penetrating Keratoplasty (corneal grafts), and most cases of unsuccessful refractive surgery.

Biofinity Silicone hydrogel lenses’ hyper DK and low modulus nature make this lens the ideal soft lens choice when a dual-lens fitting system is needed to fit the compromised cornea.

Bibliography

1. Keratoconus. Rabinowitz, Yaron S. 4, s.l. : Survey of Ophthalmology, January-February 1998, Vol. 42, pp. 29-319.
2. Characteristics and functional outcomes of 130 patients with keratoconus attending a specialist contact lens clinic. Lim, N and Vogt, U. s.l. : Eye, 2002, Vols. 16, 54-59.
3. Keratoconus. Contact lens or keratoplasty? Smiddy WE, Hamburg TR, Kracher GP, Stark WJ. 4, Apr 1988, Ophthalmology, Vol. 95, pp. 487-492.
4. A piggyback contact lens for the correction of irregular astigmatism in keratoconus. Tsubota K, Mashima Y, Murata H, Yamada M. Jan 1994, Ophthalmology, pp. 101(1):134-9.
5. A Hyper-Dk Piggyback Contact Lens System for Keratoconus. O’Donnell, Clare. 1, Jan 2004, Eye & Contact Lens: Science & Clinical Practice:, Vol. 30, pp. 44-48.
6. B, Tighe. Trends and developments in silicone hydrogel materials. Editorial. www.siliconehydrogels.com. [Online] Sept 2006.
7. Carnt, Nicole. 3rd Generation Silicone Hydrogel Lenses. Editorial. http://www.siliconehydrogels.org. [Online] May 2008.
8. Chou, Brian. The Evolution of Silicone Hydrogel Lenses. www.clspectrum.com. [Online] Jun 2008.