Menicon has demonstrated that the intrinsic properties of its tisilfocon A (Menicon Z) gas permeable (GP) material consistently exhibits Dk levels of over 180 Fatt units, and as such, is the GP contact lens material with the highest Dk in the world.
In a whitepaper prepared in February 2023,1 Menicon Senior Technical Director, Steve Newman stated that although the company initially took “a conservative approach to making any permeability claims” about its contact lens, it had now demonstrated, via “over a quarter of a century of successful patient outcomes around the world” that the Menicon Z material achieves “the highest Dk levels, without adversely affecting clinical performance”.
Menicon Z is used to manufacture all types of lens designs and configurations, from corneal lenses to scleral shells. The whitepaper described the material as “a thermoset copolymer derived from fluoro-methacrylate and siloxanylstyrene, bound by crosslinkers. Available in blue and red tints, as well as in clear form, they also have an ultraviolet (UV) absorber, benzotriazole, incorporated in the material”.
By combining this material with plasma surface treatment, Menicon said it had achieved a high wetting material that demonstrated optimal in-vivo performance. “This combination proved to be advantageous in overnight orthokeratology where the maintenance of a pristine surface is paramount to clinical performance and safety.”
Menicon Z lenses were approved by the United States Food and Drug Administration (FDA) (P990018) for continuous wear of up to seven days in 2000. They are now the only FDA approved GP material in the USA for up to 30 days continuous wear.
Copy Claims
Mr Newman said that since patents for Menicon Z began to expire, similar material formulations have been synthesised and brought to market with claims of “world’s highest Dk”.
To assess these claims, Menicon had its own Menicon Z material tested against Optimum Infinite samples, a tisilfocon, a material manufactured by Contamac (Essex, United Kingdom). Menicon claims that Optimum Infinite had previously been described as having a Dk of 200 Fatt units.
The testing, which included evaluation of all relevant material properties, was conducted at Menicon’s research facilities in Japan. Both Menicon Z and Optimum Infinite were found to share a common formulation basis and exhibit similar characteristics, including a permeability of around 180+ Fatt units.
An external comparative Dk study was conducted in 2019, by Professor William J. Benjamin of Material Performance Assessments, LLC. Similar to Menicon’s in-house assessment, the study found oxygen permeability of around 180+ Fatt units for both Menicon Z and Optimum Infinite.
Standards Addressed for Marketing
Professor Benjamin addressed the need to establish GP Dk standards in a paper published in 2022,2 noting an inherent tendency for manufacturers to overstate Dk values of various GP materials in their marketing.
In collaboration with the Contact Lens Manufacturers Association (CLMA) in the United States, Benjamin et al. entered into an extensive assessment of available GP lens materials and then established standards that all laboratories could use for comparison against their own materials and when referencing oxygen transmissibility (Dk/t) outcomes in their respective lens designs.
Menicon has welcomed the standards, which, it says have enabled the company to continue to market Menicon Z material as having the highest Dk in the world.
In the whitepaper Mr Newman explained the challenges Menicon faced when developing the material – a project that commenced back in 1987.
“Increasing Dk levels substantially via the addition of various silicone monomers is usually associated with a softer, more malleable material that may not provide the most accurate or consistent fitting capabilities,” he wrote.
“Additionally, softer, high Dk GP materials are harder to produce by lathing means and tend to inhibit the ability of the manufacturers to obtain the highest quality parameter tolerances and surface finishes.
Menicon was able to increase the Dk substantially without compromising the optimal physical hardness and stiffness requirements.”
He said Menicon’s choice of material design had “been validated by the introduction to the market of a facsimile to Menicon Z (Optimum Infinite) which shares both the USAN name and similar Dk performance.
“Whilst initially taking a conservative approach to making any permeability claims, in this report Menicon has been able to demonstrate that the intrinsic properties of the tisilfocon A (Menicon Z) GP material has consistently exhibited Dk levels of over 180 Fatt units, thus being the GP contact lens material with the highest Dk in the world.”
References
1. Newman, S., Tisilfocon A., Menicon’s breakthrough gas permeable material was originally registered and referenced with a permeability range of 163 ~ 172 but it’s true permeability is confirmed at 180+. 19 February 2023.
2. Benjamin W.J., and the Dk Reference Study Group. Revised Oxygen Permeability (Dk) of Reference Materials. Investigative Ophthalmology & Visual Science 47:ARVO E- Abstract 97/B385, 2006.
