WET-SHIPPING GP LENSES
The Initial GP
Non-Wetting Lens Syndrome
This FDA approval may help eliminate
a common wetting problem with GP contact lenses.
By Patrick J. Caroline, FCLSA, FAAO, and Mark P. André,
For optimum performance of today's oxygen permeable (GP) lenses, the surfaces must remain wet and deposit free. Clinical experience has taught us that poorly wetting lens surfaces may exist initially (upon dispensing of a new lens) or after weeks or months of lens wear.
Of particular interest to practitioners is the initial non-wetting lens syndrome that occurs immediately upon insertion of newly manufactured GP lenses (Figures 1a and 1b). This phenomenon may result from a number of factors that can occur during GP lens fabrication and/or shipping.
|Figure 1a and 1b. Initial GP
During lens fabrication a waxy compound known as blocking pitch fixes the lens button to a brass arbor prior to the cutting of the anterior (power) curve and posterior (base) curve. Additionally, a heated and formed wax is often used as a polishing medium for removing residual lathe marks, a technique known as pitch polishing. Following surface polishing and deblocking (removing the lens from the brass arbor) any residual wax is removed with a solvent, which dissolves the compound from the surfaces of the semi-finished lens. If removal is incomplete, residual wax will remain and may result in localized or diffuse areas of non-wetting (Figure 2).
Initial non-wetting may also occur secondary to excessive or improper solvent exposure. Laboratory solvents contain aromatic as well as oily components. When the solvent is exposed to lens surfaces, the plastic swells from the aromatic component, allowing the oily component to penetrate the plastic (Figure 3). If the solvent is completely released, the lens will hydrate and establish proper tear to lens interaction when placed on the eye. Residual solvents remaining within the lens matrix will temporarily compromise lens hydration.
|Figure 2. Scanning electron micrograph of residual waxy pitch that
appears as snowflake pattern on the surface of a GP lens.
||Figure 3. Scanning electron micrograph of numerous bleach-like blotches created by excessive solvent
The Contact Lens Case
The contact lens storage case itself can contribute to initial GP non-wetting secondary to the case manufacturing process. The plastics from which modern cases are produced often contain small amounts of hydrophobic oils that can contaminate the anterior lens surface during dry storage. Additionally, cases are manufactured using an injection molding technique that often requires a hydrophobic silicone spray to facilitate removal of the case from the mold.
If some of the hydrophobic oil remains on the surface of the case, it may transfer to the anterior lens surface during dry storage. This is especially true during summer months when cases may be exposed to excessive heat during shipping, further leeching oils from within the case.
Traditionally, GP contact lenses have been shipped to practitioners in a dry (non-hydrated) state due to FDA restrictions that discouraged wet-shipping of GP lenses. The FDA discouraged the practice of wet shipping because long-term sterility of the solution within the case had not been established. Studies today have shown that safe, long-term storage of RGP lenses is possible through the introduction of new preservatives and more stable solution formulations. Recently Alcon Laboratories became the first solution manufacturer to receive FDA approval for laboratory wet-shipping with new Unique pH RGP Solution (Figure 4). Wet-shipping provides numerous advantages in that it:
- Loosens residual manufacturing waxes not removed in the final phase of lens production
- Helps purge residual manufacturing solvent from the lens surfaces
- Eliminates direct contact between the anterior lens surface and residual oils that may be present in the wells of the case
- Allows in-office verification of lens parameters in a more natural hydrated state
Figure 4. Alcon's new Unique pH GP
A variety of environmental and/or handling factors may also contribute to surface non-wetting and should be routinely investigated, including:
- Tear components
- Environmental contaminants
- Lanolin-based skin moisturizers and soaps
- Cosmetics and cosmetic removers
Clinical experience has shown that laboratory wet-shipping can dramatically lessen the initial non-wetting of GP lenses. With the recent introduction of Alcon Unique pH, we now have a solution specifically approved for wet-shipping GP lenses to help practitioners resolve this occasional problem.
Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.
Mark André is director of contact lens services at the Oregon Health Sciences University.
Contact Lens Spectrum, Issue: October 2002