Why haven’t SiH contact lenses conquered the world (yet)?

Kathy Dumbleton. MSc MCOptom FAAO (Dip CL), FBCLA- Senior Clinical Scientist, CCLR Senior Researcher Centre for Contact Lens Research (CCLR) University of Waterloo, Ontario, Canada

Morgan et al. reported that in 2007 more than half of new soft lens fits in the United States and several eastern European countries were with silicone hydrogel lenses [1]. However the same article reported that these materials only accounted for 27% of all new fits worldwide, a figure which many find disappointing given the many documented advantages associated with silicone hydrogel lenses. The number of currents wearers refitted with silicone hydrogel lenses are probably even lower.

So why haven’t silicone hydrogel contact lenses conquered the world (yet)?
This is a question that manufacturers, researchers and eye care practitioners are often asked. Unfortunately it is not an easy question to answer, even though many opinions are frequently proposed. This editorial considers several factors, whether genuine or not, which are perceived to have limited the growth of silicone hydrogel lenses in the market place and discusses how these may have changed over time.

We have all been guilty of the old adage “if it ain’t broke, don’t fix it!” Unfortunately many busy eye care practitioners find themselves choosing to simply keep their patients in the same lens materials and designs rather than taking the additional time to explain the benefits associated with newer technologies which may be available for them. It is easy to assume that our patients are happy with their lenses if they don’t actually complain to us about reduced wearing times, red eyes or dry eyes for example. And why should the patient know that it could be different, that they could increase their comfortable wearing time or reduce the redness of their eyes, when what they are currently wearing is all they have known? An approach to elicit this information from the patient is to have a discussion with the patient, asking “open” rather than “closed” questions. For example questions like “how would you describe your contact lens wear” or “what seems to be the main problem with your contact lenses/ eyes/ vision?” are more likely to solicit responses indicating that a refit of lens material or design could be beneficial. There are also many occasions where even the patient does not realize that their current lenses may not be performing as well as they could. If the practitioner is more proactive in refitting with silicone hydrogel lenses at routine follow-up examinations, this will likely result in more satisfied patients who will hopefully endorse these lenses to fellow contact lens wearers and stay in lens wear longer.

Any new technology comes at a higher price, especially when it is first introduced to the market place. Silicone hydrogel lenses were no exception. When practitioners first started to fit these lenses to their patients, they were reserved for patients with specific demands, for example continuous wear, complications from long term hypoxia etc. As more lens materials and designs have become available, the costs have generally decreased but in most cases silicone hydrogel lenses are unit for unit more expensive than their conventional hydrogel counterparts. Unfortunately it is all too often the eye care practitioners who seem to decide what their patients can afford to pay for their lenses. The practitioners who have been most successful with switching their patients to these newer technologies tend to be those who are non-judgemental with their patients and take the time to inform them about “what’s new” and to explain why the advantages of silicone hydrogel lenses will frequently prevail over the additional expense. The perceived greater costs are however decreasing with time and with advancement in manufacturing techniques, so hopefully price will not be an obstacle for much longer.

Another issue with silicone hydrogels when they were first introduced was the limited parameter availability. Fortunately this has been addressed with the addition of extended power ranges for inventory lenses with spherical silicone hydrogels now being available in powers ranging from +8.00DS to -12.00DS and a custom silicone hydrogel being available from +20.00 DS to -20.00DS. Additional base curves and a range of diameters also facilitate optimal lens fitting for a wide range of corneal curvatures and sizes. Practitioners now have a choice of toric lens designs in silicone hydrogel lenses for their astigmatic patients and the parameter availability for these lenses is rapidly increasing. Even our presbyopic patients can be fitted with multifocal silicone hydrogel lenses with additional designs likely to be released in the coming months. So, although cited as a reason for not fitting silicone hydrogel lenses, with the current lenses and designs that are widely available, this must be considered a weak rationalization.

Although silicone hydrogels can be worn for daily, extended and continuous wear, the one modality that has been missing to date is a daily disposable format. Daily disposable lenses offer numerous advantages over reusable lenses, regardless of lens material. They have been shown to be associated with higher rates of compliance and lower rates of complications than conventionally replaced lenses. Many practitioners are choosing a daily disposable modality for this reason and these lens types are gaining popularity rapidly worldwide. Although still less commonly prescribed in North America, daily disposables have been growing in popularity in recent years. One reason may be the concerns associated with solution use, particularly with respect to recent recalls of products and the fusarium and acanthamoeba keratitis scares [2,3]. Using a new lens for every wearing period all but eliminates the risk of these complications with contact lens wear.

Soon after silicone hydrogel lenses started to be prescribed for daily wear, reports of corneal staining started to appear in the literature when certain combinations of care products and silicone hydrogel materials were combined4-6. While not all patients exhibit these solution incompatibilities, their frequent occurrence and notoriety may have deterred many practitioners from refitting more of their patients with silicone hydrogel lenses. Most practitioners switching their patients lens type do not necessarily consider a change in care products at the same time. If the patient then goes on to experience solution sensitivity staining or other complications, it is usually the lens material rather than its combination with a specific care product which is blamed. Manufacturers are developing care products specifically for use with these newer materials and there are now products available which have been reported to be compatible with the majority of silicone hydrogel lenses, including the hydrogen peroxide care systems as well as multipurpose disinfecting systems.

With some silicone hydrogel lens materials, deposition may be an issue with some patients and this has been cited as a reason for not fitting more patients with silicone hydrogel lenses. A number of modifications and advancements have been made to improve the surface wettability of silicone hydrogel lenses. While the tear film still comes into contact with the hydrophobic silicone of the lenses and in some patients this can result in the formation of lipid deposits, the increased material choice should allow better compatibility and a reduction in lens deposits. Fortunately not all patients' experience these problems, but those that do can use in eye lubricant drops, and / or ensure that they are rubbing and rinsing their lenses, with separate surfactants if necessary, in order to prevent the build up of these deposits. Since several silicone hydrogel materials are now available, a change to an alternative lens type may also solve the problem.

When silicone hydrogel lenses were initially introduced there were several anecdotal reports of discomfort especially when they were prescribed as replacements for conventional hydrogel lenses. The literature also describes patient comfort responses to vary with different silicone hydrogel contact lens designs [7,8]. Many of these initial complaints may have been associated with sub-optimal lens fitting characteristics and initial lens designs. In recent studies conducted at the CCLR, we have shown initial comfort and adaptation on refitting with silicone hydrogel lenses to be good [8,9] and no differences in the overall ratings between the silicone hydrogel lenses investigated [9]. Despite the constant upgrading of silicone hydrogel lens materials and designs, some practitioners do however remain reluctant to try these lenses on their patients because of possible concerns of greater lens awareness and therefore decreased comfort. With greater experience and time, hopefully these concerns will be dispelled and adaptation and comfort will be cited less frequently as reasons for not prescribing theses lenses.

The principal advantage of silicone hydrogel lenses is their excellent ability to transmit oxygen, which has resulted in the elimination of hypoxic complications. However, despite the higher levels of oxygen available to the cornea, mechanical, inflammatory and infectious complications still occur. Mechanical complications, such as superior epithelial arcuate lesions (SEAL) did appear to occur more frequently with silicone hydrogel lenses in their early days when only lenses with high modulus of elasticity and limited parameters were available [10,11]. These mechanical complications have been greatly reduced with the newer materials and designs available, but practitioners may still be reluctant to fit the lenses for fear of mechanical complications occurring. Probably of greater concern however is the risk of inflammation and infection. It was hoped that by providing the cornea with more oxygen, these complications would occur less frequently. This has not proved to be the case with the currently available silicone hydrogel lenses [12,13], and it seems that it is the patient and their wearing habits that have more of an impact on the risk of experiencing inflammation and infection rather than the lens material worn. This should not discourage practitioners from refitting their patients however since it does appear that when these complications do occur in silicone hydrogel lens wearers they are generally less severe or respond to management more rapidly [14].  

Unfortunately the expression “once bitten twice shy” may also able why many eye care practitioners have been reluctant to give silicone hydrogels a second chance if they were not always successful with their early experiences with these lenses. Remember as with all technologies that advance there are now a wider range of lens materials and designs available using silicone hydrogel materials and what once was an obstacle may now just be a mere perception.

References

1. Morgan PB, Woods CA, Knajian R, Jones D, Efron N, Tan K, Pesinova A, Grein H, Marx.S, Santodomingo J, Runberg S, Tranoudis IG, Chandrinos A, Itoi M, Bendoriene J, van der Worp E, Helland M, Phillips G, González-Méijome J, Belousov B, Mack C. International contact lens prescribing in 2007. Contact Lens Spectrum 2008.
2. Joslin CE, Tu EY, Shoff ME, Booton GC, Fuerst PA, McMahon TT, Anderson RJ, Dworkin MS, Sugar J, Davis FG, Stayner LT. The association of contact lens solution use and Acanthamoeba keratitis.
   
3. Saw SM, Ooi PL, Tan DT, Khor WB, Fong CW, Lim J, Cajucom-Uy HY, Heng D, Chew SK, Aung T, Tan AL, Chan CL, Ting S, Tambyah PA, Wong TY, Schein OD, Katz J. Risk factors for contact lens-related fusarium keratitis: a case-control study in Singapore. Comparing contact lens and refractive surgery risks.
   
4. Epstein AB. SPK with daily wear of silicone hydrogel lenses and MPS. Contact Lens Spectrum 2002;17:30.
   
5. Jones L, MacDougall N, Sorbara LG. Asymptomatic corneal staining associated with the use of balafilcon silicone-hydrogel contact lenses disinfected with a polyaminopropyl biguanide-preserved care regimen. Optom Vis Sci 2002;79:753-761.
   
6. Carnt N, Willcox MDP, Evans V, Naduvilath TJ, Tilia D, Papas EB, Sweeney DF, Holden BA. Corneal staining: the IER matrix study.
   
7. Brennan NA, Coles ML, Ang JH. An evaluation of silicone-hydrogel lenses worn on a daily wear basis. Clin Exp Optom 2006;89:18-25.
   
8. Dumbleton K, Keir N, Moezzi A, Feng Y, Jones L, Fonn D. Objective and subjective responses in patients refitted to daily-wear silicone hydrogel contact lenses. Optom Vis Sci 2006;83:758-768.
   
9. Dumbleton, K., Woods, C. A., Jones, L., and Fonn, D. Comfort and adaptation to silicone hydrogel lenses for daily wear.  7-12-2006. AAO, Denver, program number 060066.
   Ref Type: Conference Proceeding
   
10. Dumbleton K. Noninflammatory silicone hydrogel contact lens complications. Eye Contact Lens 2003;29:S186-S189.
    
11. O'Hare N, Naduvilath T, Sweeney, D., and Holden, B. A clinical comparison of limbal and paralimbal superior epithelial arcuate lesions (SEALs) in high Dk EW. Invest Ophthalmol Vis Sci 42[4], S595. 2001.
    Ref Type: Abstract
    
12. Szczotka-Flynn L, Debanne SM, Cheruvu VK, Long B, Dillehay S, Barr J, Bergenske P, Donshik P, Secor G, Yoakum J. Predictive factors for corneal infiltrates with continuous wear of silicone hydrogel contact lenses.
    
13. Szczotka-Flynn L, Diaz M. Risk of corneal inflammatory events with silicone hydrogel and low dk hydrogel extended contact lens wear: a meta-analysis.
    
14. Schein OD, McNally JJ, Katz J, Chalmers RL, Tielsch JM, Alfonso E, Bullimore M, O'Day D, Shovlin J. The incidence of microbial keratitis among wearers of a 30-day silicone hydrogel extended-wear contact lens.