Silicone hydrogels were first introduced in 1999. This represented a breakthrough in technology in terms of oxygen transmissibility. In a traditional hydrogel material oxygen diffuses to the cornea by means of dissolving into the water component of the lens. Water itself only has a Dk of 80, and as such there is a limit to the oxygen transmissibility of this material. A typical hydrogel lens with 58% water has a Dk/t of 25.5 x 10-9.
Silicone hydrogels transmit oxygen to the eye through the silicone, rather than the water component of the lens. This results in much higher levels of transmissibility. They meet the minimum Dk/t criteria for oedema free daily wear with the suggested range being from 24 x 10-9 to 35 x 10-9.1,2 As such the many hypoxic changes that were seen through overwear of hydrogels such as neovascularisation, corneal microcysts, corneal oedema and limbal hyperaemia are a thing of the past when silicone hydrogels are worn.3,4
However, in spite of the advancement in oxygen delivery, challenges remained with the early silicone hydrogels with regard to their suitability for comfortable daily wear. Their intrinsically higher modulus having the potential to lead to mechanical complications such as superior epithelial arcuate lesions (SEAL) and contact lens induced papillary conjunctivitis (CLIPC).5
Mechanically induced complications from first generation SHs: superior epithelial arcuate lesion (SEAL) and CL induced papillary conjunctivitis (CLIPC)
Focus for material development and contact lens selection for patients has now moved towards ensuring the correct balance of material properties are achieved in order to deliver comfort as well as health benefits for the wearer. This section outlines current thinking and new research findings in this important area.
References
1. Holden, BA and Mertz, GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci. October 1984; 25:10 1161-1167
2. Harvitt DM and Bonnano J. Re-Evaluation of the Oxygen Diffusion Model for Predicting Minimum Contact Lens Dk/t Values Needed to Avoid Corneal Anoxia. Optom Vis Sci. 1999; 76:10 712-719
3. Fonn D et al. The ocular response to extended wear of a high Dk silicone hydrogel contact lens. Clin Exp Optom. 2002; 85;3 176-182
4. Dumbleton KA et al. Vascular Response to Extended Wear of Hydrogel Lenses with High and Low Oxygen Permeability. Optom Vis Sci. 2001; 78:3 147-151
5. Dumbleton KA. Non-inflammatory Silicone Hydrogel Contact Lens Complications. Eye & CL. 2003; 29:1 S186-189
