Soft Lens Material Choices and Selection
Silicone hydrogel lenses are a good option for many patients, but should they be our only choice?
By Gregory W. DeNaeyer, OD, FAAO
|Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio, and a consultant to Visionary Optics, B+L, and Aciont. You can contact him at firstname.lastname@example.org.|
There has been no better time in history to be a contact lens practitioner. The lens designs and modalities that are currently available provide unprecedented options that we can offer to our contact lens patients.
In the last two decades, the contact lens industry has moved toward frequently replaced soft contact lenses in place of conventional (quarterly or annual replacement) hydrogels for the correction of ametropia and presbyopia. The current trend within the soft lens arena is the accelerating global shift from the use of hydrogel to silicone hydrogel (SiHy) as the lens material of choice (Nichols, 2010).
SiHy lenses have a proven physiological benefit over typical hydrogels particularly for daily wear, and the SiHy market has continued its rapid growth; as of 2010, SiHy lenses represent 73 percent of new lens fits in the United States (Morgan et al, 2010). If this trend continues, SiHy lenses could approach 100 percent of soft lens new fits in the not-too-distant future. With that in mind the question we must ask is: Are we better off if our only option is a SiHy material?
A Short SiHy History
SiHy lenses were first introduced in 1999 for extended wear/continuous wear with the promise of having fewer complication rates, particularly microbial keratitis. This was based on the fact that SiHy lenses have oxygen transmissibility that is in some cases four to five times higher than that of typically available hydrogel lenses.
In reality, most of us chose to use SiHy lenses for daily wear rather than for extended/continuous wear despite these safety and health promises, and now we see SiHy materials slowly penetrating even the daily disposable lens segment.
What has 12 years of clinical experience with SiHy lenses taught us about their ability to meet the promise of a safer lens wearing experience? Unfortunately, many clinicians soon learned through direct experience that continuous wear was not safer with SiHy lenses with respect to microbial keratitis, and these observations were validated in large scale epidemiological studies by Stapleton et al (2008), Dart et al (2008), and Szczotka-Flynn et al (2009).
Some patients wearing hydrogel lenses experience lens-induced neo-vascularization and limbal hyperemia, especially in cases involving high refractive error and/or astigmatism. The availability of SiHy designs—including toric, multifocal, and daily disposable options—have given practitioners the ability to resolve these clinical signs by switching these patients from hydrogel to SiHy materials.
Oxygen and the Cornea
There is no question that SiHy lenses have virtually eliminated hypoxia-related complications that some patients experienced when wearing low-Dk/t hydrogel lenses (Radford et al, 2009). The two most notable clinical signs of hypoxia included limbal redness and vascularization. However, it is important to understand what the threshold transmissibility is that prevents hypoxia-related complications for daily wear patients.
Morgan and Brennan (2007) have suggested that this transmissibility (Dk/t) threshold would be 20 units across the entire lens for daily wear. In fact, Morgan and Brennan postulate that transmissibility above this level follows the law of diminishing returns. In other words, increased transmissibility above this threshold does not significantly improve the physiological status of the cornea. Although many current hydrogel designs do not meet this threshold across the entire lens, the transmissibility wars among different SiHy lenses is unwarranted as the lowest oxygen transmissibility of any SiHy lens is sufficient for daily wear patients.
The addition of silicone to hydrogel polymers to increase oxygen permeability has been fraught with complications from the start. Most notably, the diminished wettability caused by the hydrophobic nature of silicone has largely been overcome by surface treatments, the addition of wetting agents, or with the use of newer lens care products. However, other lens-related complications have resurfaced after the wider adoption of SiHy lenses.
SiHy lenses have a relatively higher modulus as compared to hydrogel materials, and it generally increases with increasing Dk (Morgan and Brennan, 2009). The higher modulus of some SiHy lenses has, at least in part, been associated with mechanical complications such as contact lens papillary conjunctivitis (CLPC), conjunctival splits, superior epithelial arcuate lesions (SEALs), and unintentional orthokeratologic effects (Morgan and Brennan, 2009). Recent epidemiological evidence shows that SiHy lens materials, unlike hydrogel lens materials, are an independent risk factor for the development of corneal inflammatory events (CIEs) (Chalmers et al, 2011). CLPC and symptomatic CIEs are particularly concerning complications because they can lead to discontinuation of contact lens wear, requiring pharmacologic intervention or eventually dropout from contact lens wear.
With all of this in mind, at some point we have to start to reconsider the cost versus benefit of SiHy lenses based on individual patient needs.
Comfort and Dropout
Multiple lens properties contribute to contact lens comfort including material modulus, contact lens design (lens edge shape and thickness profile), and surface properties. Wetting agents, contact lens care solutions, the patient's physiology, and replacement schedule all factor into the contact lens comfort equation. There is debate about whether SiHy lenses are more comfortable as compared to hydrogel lenses, and the data is highly confounded based on differing study designs that make it difficult to assess the cumulative research outcomes (Brennan et al, 2002; Chalmers et al, 2009; Cheung et al, 2007; and others. For complete list visit www.clspectrum.com).
There are many variables that contribute to the comfort of lens wear. Ideally, researchers would be able to study each of these by manipulating one variable at a time on a common prototype (Young et al, 2007). Additionally, it's a stretch to attribute new SiHy lens comfort directly to increased transmissibility. If we concede and conclude that SiHy lenses are more comfortable, then why as recently as 2007 are contact lens dissatisfaction (26.3 percent) and permanent discontinuation (24 percent) as high as historical rates with hydrogel lenses, considering that the primary self-reported reason for lens dissatisfaction and discontinuation were ocular symptoms (dryness and discomfort) (Richdale et al, 2007)?
Into the Future
We now have 12 years of experience with silicone hydrogel materials. The increased transmissibility of SiHy materials has pleasantly reduced hypoxic signs in some of our patients wearing low-Dk hydrogel lenses. Unfortunately, in other ways, SiHy lenses have been somewhat disappointing. They have failed to reduce the incidence of MK with extended wear/continuous wear. Despite suggestions that SiHy lenses are relatively more comfortable, rates of dissatisfaction and discontinuation remain high. SiHy lenses also may put some patients at greater risk for mechanical-related complications or corneal inflammation that may lead to dropout.
The perception that SiHy lenses are the be-all and end-all material has put tremendous pressure on contact lens manufacturers to use SiHy for all of their new designs. When you consider this, it's not surprising that 73 percent of new fits are SiHy lenses (Morgan et al, 2010); it's the only choice that practitioners have if they want to use new lens designs. This has created a positive feedback loop with almost unstoppable momentum.
Wouldn't we all be better off if we had an improved hydrogel or a novel lens material to offer to some of our patients? Wouldn't it be great if we had a lens with the benefits of both hydrogel and silicone hydrogel materials? If we did, SiHy lenses would be more of a niche product for select patients that require increased transmissibility to maintain a normal physiological response.
It's hard to say what's going to happen in the next five years, but I hope that when I walk into my diagnostic soft lens room that I'm not limited to only a SiHy selection. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #198.