In the early 2000s, the contact lens community was buzzing with the emergence of silicone hydrogel materials. The first FDA-approved silicone hydrogel lenses hit the market in the late 1990s, and soon thereafter, all major manufacturers were launching products that utilized this material with unprecedented high oxygen permeability (Dk).
At the time, overnight wear was a more common occurrence in soft contact lens wearers, and the eyecare community was interested in finding safe ways for patients to sleep in their contact lenses.1 It was hypothesized that by allowing more oxygen to the eye and reducing hypoxic corneal changes, rates of corneal inflammatory events and microbial keratitis associated with overnight contact lens wear could possibly decrease.
What is silicone hydrogel?
Need a refresher on how silicone hydrogel materials are different than their traditional hydrogel counterparts and why they allow significantly more oxygen to flow through the lens?
Here is a quick review of contact lens materials
Hydrogel materials are composed of water-loving (hydrophilic) HEMA (2-hydroxyl-ethyl methacrylate) polymers.2 For a hydrogel material, water is responsible for transporting oxygen through the lens since it has a higher Dk than the hydrogel material itself. To increase Dk in a hydrogel material, therefore, water content needs to be commensurately increased, or lens thickness would need to be decreased.
This means that hydrogel materials have a finite maximum Dk they can reach—they can’t be composed entirely of water, and they can’t be zero millimeters thick. Subsequently, silicone became a research interest in the 1970s as a means to augment hydrogel materials with the potential to enhance certain properties.2
Silicone has a much higher Dk than water—a silicone hydrogel material can be up to five times more oxygen permeable than a hydrogel material. Initially, the main barrier to utilizing silicone in soft contact lenses was its hydrophobic nature—it didn’t blend well with hydrophilic hydrogel monomers. This problem was overcome by pairing polar groups with silicone, resulting in a more homogeneous mixture when combined with hydrogel.
Because of these inherent material property differences, hydrogel and silicone hydrogel materials have some differing general properties:
- Hydrogel materials: Due to their high water content, tend to be less stiff (i.e., lower modulus) than silicone hydrogels.
- Silicone hydrogel materials: Based on their hydrophobic nature, tend to have poorer wettability and more deposits.
When designing a hydrogel or silicone hydrogel material for contact lens use, a manufacturer understands the disadvantages of each material and can compensate for and utilize design features that help offset anticipated disadvantages.
Mounting evidence against overnight wear
In the decades that have followed the debut of silicone hydrogel materials, evidence has suggested that silicone hydrogel overnight wear is as or riskier than with hydrogel materials.
While silicone hydrogel materials do reduce the risk of hypoxic complications associated with overnight wear, like corneal neovascularization and stromal swelling,3-5 they have not had a significant impact on the rate of microbial keratitis in contact lens wearers.6-10 In fact, some analyses have suggested that people wearing silicone hydrogel materials overnight actually have a higher risk of corneal inflammatory events compared to those wearing hydrogel materials overnight.11-13
“It is difficult to attribute causation of infectious or inflammatory events to just oxygen alone.”
Most analyses have concluded that the risk of microbial keratitis and corneal infiltrative events is multifactorial, and hypoxia is not the only contributing factor.6,11 As an eyecare community, we have learned that material type may not be the primary risk factor for contact lens-associated inflammation and infection. Rather, evidence suggests overnight wear itself is the primary risk factor for both.7,10,12-17
Interest in overnight contact lens wear has waned, likely due to acknowledgment of the evidence described above, along with the rise in availability and parameter expansion of daily disposable modalities. Further, it’s been demonstrated that daily disposable contact lens wear can be associated with a significantly reduced risk of associated inflammatory events,12,15,16 and less severe disease and microbial keratitis-associated vision loss.10,18
Are silicone hydrogel materials safe in daily disposables?
When daily disposables first became available, most featured hydrogel materials. As new daily disposable brands have been released, a trend towards silicone hydrogel materials has been seen during these respective product launches.
It does beg the question if the patient is not intending to sleep in the lenses—is there still valid concern about what kind of material the daily disposable is? Do the ultra-high Dk values of silicone hydrogel materials offer any substantial benefits to the patient, or are hydrogel daily disposables still good options?
There is no considerable evidence to suggest that a patient who is compliant with daily disposable or daily reusable contact lens wear might have worse ocular health outcomes with a hydrogel option.
As long as a patient is wearing a hydrogel material on a daily wear basis with their eyes open, multiple studies have shown that there are no concerns for unacceptable hypoxic stress.19-23 As well, it has been reported that there are no substantial comfort differences when comparing hydrogel to silicone hydrogel materials.6,24
What this all means for patients: compliance above all
Therefore, when choosing a contact lens brand, it may be less imperative to consider the material type and more important to ensure you are fitting a daily disposable option for both patient comfort and compliance. Most soft contact lens wearers have several daily disposable options that can fulfill their prescription, lifestyle, and budget needs. In instances where a daily disposable isn’t a realistic option, effective patient education on why overnight wear should be avoided is critical.
“When it comes to fitting contact lenses, the material does matter.”
Innovation in material technology has led to materials that have essentially eliminated hypoxic complications that were regularly encountered in years past. Now that hypoxia has been taken mostly out of the equation, eyecare professionals (ECPs) can be encouraged to recommend daily disposable wear whenever possible and provide meaningful compliance education to ensure that new complications don’t overshadow the success of these new technologies.
