Why High Dk/t Matters in the Daily Disposable Modality

July 07, 2017

The high oxygen transmissibility (Dk/t) of silicone hydrogel (SiHy) allows more oxygen to pass through the lens to the cornea than hydrogel (Hy) lenses, which helps maintain optimal ocular health.¹

As Desmond Fonn, MOptom, and Deborah Sweeney, PhD, explain in “The Benefits of Silicone Hydrogel Daily Disposable Lenses: A look at why silicone hydrogel daily disposable lenses are our preferred option for daily wear”…

The SiHy material’s high oxygen transmission has “essentially eliminated hypoxia-related consequences seen with hydrogels when used for either extended wear or daily wear.”²

In fact, several highly respected thought leaders in the industry have made the case for high Dk/t SiHy lenses, regardless of modality…

Brien Holden, PhD, Serina Stretton, PhD, Percy Lazon de la Jara, PhD:

“Dk really matters…the clinical imperative remains to maximize oxygen transmissibility for all lens wearers.”³

Noel A. Brennan, PhD, and Philip B. Morgan, PhD:

“The models and some scientific data suggest that the cornea is more hypoxic during daily wear of hydrogels than during continuous wear of silicone hydrogels. To avoid hypoxia in daily wear, we recommend using silicone hydrogels.”⁴

“Evidence supports the routine prescribing of silicone hydrogel contact lens materials in preference to conventional hydrogels for daily wear use to avoid deleterious clinical sequelae due to corneal hypoxia.”⁵

Deborah Sweeney, PhD, regarding silicone hydrogel lenses:

“The superior health benefits of these materials for all modalities in terms of Hypoxic consequences have been thoroughly documented.”⁶

Low risk perception

Still, some practitioners point to the low risk of hypoxia-related consequences in the daily disposable (DD) modality, regardless of whether the patient wears SiHy or Hy lenses. The rationale: both SiHy and Hy DD lenses meet the Dk/t level for daily wear necessary to avoid corneal edema. Holden and Mertz determined that Dk/t level in their seminal 1984 study.⁷

In addition, a recent retrospective analysis of clinical studies conducted at Brien Holden Vision Institute (Diec et al, 2017) appears to support the low-risk perception of the DD modality. The analysis found “no clinically significant differences in physiological variables… and adverse event rates were not significantly different between the two materials” (SiHy and Hy).⁸

HEALTH MATTERS TO PATIENTS

Patients rank ocular health second among the most important deciding factor when choosing between two lens brands. (Vision quality #1.)*

A Hanover Research survey* conducted on behalf of CooperVision provides insights into what lens wearers consider the most important factors when choosing a brand or type of contact lens.

The survey resulted in 1,193 complete responses from contact lens wearers between 16 and 64 years old.

One of the survey’s key findings: patients care about health.

When choosing between two lens brands**, only vision quality ranked higher than health in respondents decision-making process.

Price ranked last (fifth) as a determining factor. Not surprisingly then, 95% of those health-conscious lens wearers are also willing to pay a higher price for contacts that ensure eye health.***

*December 2015 contact lens wearer survey. Data on file.

**n=1175

***n=564

Responsible prevention

Compared to extended wear lenses, the relatively low risk of hypoxia-related consequences in the DD modality has led some practitioners to conclude that there’s little need to prescribe silicone hydrogel over hydrogel lenses.

However, observations about the DD modality and patients explain why silicone hydrogel should now be the material of choice among prescribers of one day lenses.

There is no “average” patient. Why compromise?

While the Holden and Mertz study only addressed “the average eye,” eye care practitioners would likely agree that there is no such thing as an average patient.

Some corneas are simply thirstier for oxygen than others. And there is a range of physiologic responses to hypoxic stimulus.²

The bottom line: Practitioners and patients don’t have to compromise. Health matters to patients, too (see the related feature section). Choosing SiHy, the lens material that is best able to prevent hypoxia altogether, simply makes sense.

SiHy is advised for at-risk patients

Fonn and Sweeney advise that “it is especially sensible to prescribe SiHy contact lenses for patients who have greater oxygen demands and who need to wear thicker lenses.”² Thicker lenses can significantly impact oxygen transmissibility.⁹

The Diec et al analysis supports that guidance. It explains that regardless of lens material preference, SiHy contact lenses should be considered “to prevent hypoxia-related complications with at-risk patients.”⁸

Longer wear time makes oxygen transmissibility even more important

Another factor influencing the choice of SiHy over Hy DD contact lenses is daily wear time.

In “A multi-country assessment of compliance with daily disposable contact lens wear” (Dumbleton et al, 2013)¹⁰, survey participants included eligible DD contact lens patients in Australia, Norway, the U.K. and the U.S.

The findings¹⁰ include:

14 hours is the median daily wear time for 1 day contact lens patients across all four countries.
U.S. 1 day lens wearers habitually wear their lenses for at least 15 hours per day and seven days per week.

These findings further support high Dk/t SiHy lenses as preferable to hydrogels in minimizing, if not eliminating the risks of corneal swelling, and other hypoxia-related consequences.

What about SiHy lens comfort? Isn’t that still a concern?

Early generation SiHy lenses failed to match hydrogel for comfort. That has changed. Today’s advanced SiHy lenses are now just as comfortable to wear as Hy⁸—without compromising the high oxygen transmissibility that made SiHy lenses a success.

“Irrespective of prescription, as SiHy DD lenses can meet all of the other requirements of a ‘best’ lens including comfort, vision, and ease of handling, we believe that it should be the material/lens of choice in preference to a hydrogel.”

Desmond Fonn, MOptom, and Deborah Sweeney, PhD²

Conclusion

In explaining their preference for SiHy lenses in the DD modality, Dr. Fonn and Professor Sweeney conclude:

“Practitioners will always select the ‘best’ lens for each patient. Based on current fitting trends, it will most likely be a DD lens dependent on prescription. Irrespective of prescription, as SiHy DD lenses can meet all of the other requirements of a ‘best’ lens including comfort, vision, and ease of handling, we believe that it should be the material/lens of choice in preference to a hydrogel.”²

See also:

Infographic: Why is silicone hydrogel (SiHy) the right choice for 1 day contact lenses? Summarizes key evidence for choosing SiHy daily disposable contact lenses over hydrogel for your patients.
Patient Comfort: The New Reason to Choose Silicone Hydrogel 1 Day Contact Lenses. In an analysis of previously conducted clinical trials, silicone hydrogel daily disposable contact lenses were found to be just as comfortable as hydrogel daily disposable lenses.
What are contact lens wearers willing to pay? 95% of wearers who say ocular health is important are willing to pay more for contact lenses that ensure eye health.

Sources:

1. Brennan NA. Beyond Flux: Total Corneal Oxygen Consumption as an Index of Corneal Oxygenation During Contact Lens Wear. Optom Vis Sci. 2005 Jun;82(6):467-72. Applies to SiHy lenses with Dk/t of 15 or higher.

2. Fonn D, Sweeney D. The benefits of silicone hydrogel daily disposable lenses: A look at why silicone hydrogel daily disposable lenses are our preferred option for daily wear. Cont Lens Spectrum. 2015;30:42-45.

3. Holden BA, Stretton S, Lazon de la Jara P, et al. The future of contact lenses: Dk really matters. Cont Lens Spectrum. Published February 1, 2006. Accessed March 6, 2017.

4. Brennan NA, Morgan PB. Reader's Forum: Another view of oxygen transmission. Cont Lens Spectrum. Published December 1, 2005. Accessed March 6, 2017.

5. Morgan PB, Brennan NA, Maldonado-Codina C, et al. Central and Peripheral Oxygen Transmissibility Thresholds to Avoid Corneal Swelling During Open Eye Soft Contact Lens Wear. J Biomed Mater Res Part B: Appl Biomater; 2010 Feb;92(2):361-65.

6. Sweeney D. Have Silicone Hydrogel Lenses Eliminated Hypoxia? Eye Contact Lens. 2013;39(1):53-60.

7. Holden BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci. 1984;25:1161-67.

8. Diec J, Tilia D, Thomas V. Comparison of silicone hydrogel and hydrogel daily disposable contact lenses. Eye Contact Lens. 2017. DOI: 10.1097/ICL.0000000000000363 [Epub ahead of print].

9. Lira M, Pereira C, Real Oliverira ME, Castanheira EM. Importance of contact lens power and thickness in oxygen transmissibility. Cont Lens Anterior Eye. 2015;38(2):120-26.

10. Dumbleton KA, Richter D, Woods CA, et al. A multi-country assessment of compliance with daily disposable contact lens wear. Cont Lens Anterior Eye. 2013;36(6):304-12.