By Roxana Hemmati, OD, Cornea and Contact Lens Fellow, University of Houston, Texas Eye Research and Technology Center
The tear film is a key component to the health of the ocular surface; it provides lubrication, nutrition, and protection. When any contact lens is placed on the eye, tear circulation under the lens is needed to create a proper contact lens-cornea relationship.
It’s a well-known clinical fact among contact lens practitioners that tight fitting contact lenses lead to complications. These complications stem from the interference of the lens with normal ocular surface physiology, primarily caused by the entrapment of the post-lens tear film. This may precipitate conditions such as microbial keratitis, CLARE, peripheral corneal ulcers, and lens-induced edema.
SynergEyes, the sole provider of hybrid contact lenses in the United States, has a 2nd generation design lens, UltraHealth, which is intended for keratoconus and irregular cornea applications. This newest iteration boasts several improvements over earlier hybrid lens designs; most notably an enhanced back surface profile which widens the landing zone over 2 regions of the lens to improve patient comfort and lens movement while preventing corneal adherence.
Although well established in the specialty contact lens market, little is known about the physiologic environment, specifically, tear circulation under hybrid lenses. In order to assess these improvements, a study was conducted to determine the post-lens tear exchange of the UltraHealth hybrid lens.
This study included 17 previously diagnosed keratoconus patients. Through the use of corneal topography, anterior segment OCT, and slit lamp examination, a best-fit UltraHealth lens was selected for each subject. Utilizing the Fluorotron Master FS2 Fluorophotometer, we were able to obtain and quantify the decay of post-lens high molecular weight fluorescein (HNaFl) over time, allowing us to calculate the rate of tear exchange.
An initial baseline scan of the UltraHealth lens filled with unpreserved saline was taken to measure the natural fluorescence of the cornea/hybrid lens system. The lens was then removed and re-inserted with HNaFl and unpreserved saline mixture. Fluorotron measurements were taken at multiple time periods to assess the decrease of HNaFl from the post-lens tear film reservoir.
The median tear exchange rate was found to be 2.03 %/min, with a range between 0.021 %/min to 10.95 %/min. The strong positive correlation seen with the fluorophotometer data confirms that the enhanced back surface design of this 2nd generation hybrid lens allows for improved tear exchange.
Skidmore et al. conducted a similar study on scleral contact lens tear exchange and reported a much lower tear exchange rate between 0.02 to 0.28 %/min.
With this new data on hybrid lens tear exchange, we can see that the hybrid contact lens is superior to the scleral contact lens with regards to post-lens tear exchange, although it is still unclear what the required tear exchange rate is to maintain corneal health.
Both hybrid lenses and scleral lenses, among other lens modalities, are valuable options in managing irregular corneas, but it is important that practitioners consider these physiological advantages of the UltraHealth hybrid lens.
Roxana T. Hemmati, OD is a Cornea and Contact Lens Post-Doctorate Fellow with The Texas Eye Research and Technology Center at The University of Houston College of Optometry.