Silicone Hydrogels at Academy 2004

Alisa Sivak, MA, DipEd

Alisa assists the Centre for Contact Lens Research by writing and editing publications, reports, grant applications, and educational communications.

Held in Tampa, Florida, this year’s annual meeting hosted a record number of presentations (38 total) relating to silicone hydrogels, including twenty posters, twelve paper presentations and six continuing education sessions.

Clinical Signs and Symptoms

Tampa, Florida, USA

Several presentations by researchers at the CCLR, the Ohio State University and CIBA Vision highlighted the novel properties of silicone hydrogel materials responsible for elimination of lens-induced hypoxia and its associated effects (e.g. corneal swelling, microcysts, hyperemia and vascularization), and an increased ability to overcome common contact lens-related problems such as bacterial binding, dehydration and mechanical complications.

Reports comparing conventional hydrogels to high-Dk silicone hydrogels demonstrated less limbal and conjunctival redness but more conjunctival staining during daily wear with silicone hydrogels (Maldonado-Codina et al., Eurolens Research) and less perilimbal redness with overnight wear of silicone hydrogels, but more than is found without lens wear (Situ et al. CCLR). Santodomingo et al.’s ( Aston University) study found very few differences in ocular physiology and tear film characteristics between the silicone hydrogels tested.

Results from a large continuous wear study of more than 5000 silicone hydrogel lens wearers (Chalmers, private practitioner and McNally, CIBA Vision) have revealed the potential for silicone hydrogels to overcome patients’ dryness and discomfort symptoms associated with their previous lens wear history. Examination of the reasons for discontinuation of wear with silicone hydrogels in this study group were high refractive errors, patients’ high satisfaction with previous lenses, and age <30 years. Less than 0.2% of wearers discontinued because of redness or pain, and the majority of wearers achieved a full CW schedule.

The use of silicone hydrogels in reducing dry eye symptoms was emphasised in a CE session on Dry Eye Management (Cupryn and Karpecki, private practitioners) and was further supported by three studies reporting higher comfort and less redness and dryness with silicone hydrogel lenses compared to HEMA-based lenses (Dumbleton et al., CCLR; Wagner et al., Nova Southeastern University College of Optometry; Liu et al., TERTC). Interestingly Liu et al. found these positive characteristics of silicone hydrogels occur despite an overall decrease in non-invasive tear break-up time and wettability, and suggested that other material characteristics may be more important than absolute wettability and apparent surface wetting when determining initial lens comfort.

Comfort, Wettability, Protein Deposition and Staining

Although the ability of a contact lens to retain water is thought to be a direct result of the water content of the lens material, Guillon et al. (Optometric Technology Group) reported it is also associated with specific lens formulations. Silicone hydrogels have superior water retention as a result of their low water content, but some silicone hydrogels retain water more efficiently than others. In addition Chamberlain et al. (OTG) reported that some silicone hydrogels achieve better in vivo wettability than those made from low-Dk hydrogels and these effects are enhanced when lenses are pre-soaked in Opti-Free Express (Ketelson et al., Alcon Research Ltd.).

Results of a study involving symptomatic soft lens wearers, showed that some specific combinations of silicone hydrogel and lens care systems can cause increased staining, though all combinations tested produced similar patterns of subjective comfort and dryness (Jones et al., CCLR). In addition, both hydrogel and silicone hydrogel lenses pre-soaked in PHMB-based systems on a DW basis produced statistically significant staining, compared to those pre-soaked in a polyquad solution (Garofalo et al., Alcon Research Ltd.).

Overall silicone hydrogel lenses deposit low levels of protein compared to non-silicone lenses and characterization of these protein deposits indicates that the bulk and surface compositions of different silicone hydrogel materials attract different patterns of deposition (Forrest et al., University of Waterloo/CCLR). Jones et al. (CCLR) reported results showing that rewetting drops containing surfactants are able to reduce protein and denatured lysozyme deposited on silicone hydrogels during CW.

Corneal Swelling and other physiological changes

Corneal swelling, one of the most acute complications of lens-induced hypoxia, occurs to a far less extent during overnight wear with silicone hydrogels compared to conventional hydrogels. Brennan (Brennan Consultants) reported that the overnight corneal swelling response to a silicone hydrogel toric lens is comparable to that with a silicone hydrogel spherical lens and is substantially less than the swelling found with a hydrogel toric lens. Fonn et al.’s (CCLR) investigation of the differences in corneal swelling between silicone hydrogel and conventional hydrogel lenses has revealed a possible sympathetic corneal swelling response whereby a non lens-wearing eye swells to significant levels in apparent response to the swelling in a contralateral, hydrogel lens-wearing eye, and is higher than when the same control eye is paired with a silicone hydrogel lens.

Bergenske et al. (Pacific University) reported that topographical and refractive changes associated with silicone hydrogel lens wear may be the result of inadvertent wearing of inverted lenses. Their study found that subjects are unable to distinguish between inverted and non-inverted silicone hydrogel lenses when it comes to comfort, though topographical changes are noticeably different.

Two studies examined the impact of silicone hydrogels on ocular surfaces at the cellular level. Campbell and Connor, Jr. (Southern College of Optometry) assessed the health of the ocular surface in subjects wearing silicone hydrogels by measuring the density of conjunctival goblet cells and found no significant disruption of cells at the ocular surface after 3 months of CW. Edmonds (private practitioner) et al. (OSU) reported data from a pilot study finding that silicone hydrogels can have a positive effect on endothelial cell density in subjects who had previously worn low-Dk lenses and have the potential to relieve some polymegethism and pleomorphism in eyes with a long history of hydrogel lens wear.

Merchea et al. (OSU) used a Hartmann-Schack type aberrometer to quantify changes in ocular aberrations associated with CW of silicone hydrogels and found a reduction in total RMS after 3 months of lens wear, mostly due to changes in the 2nd order zernike terms.

Complications

Chalmers (private practitioner) and McNally (CIBA Vision) presented the results of a study aimed at identifying predictive factors and risk behaviours for the development of complications from CW. Their findings indicated that young age (under 20 years), incomplete hand washing behaviour and swimming while wearing lenses place wearers at greater risk of developing corneal infiltrates.

Dumbleton and Jones (CCLR) offered a course reviewing the etiology, risk factors, clinical presentation and management of complications observed with silicone hydrogels. Management strategies may include temporarily stopping lens wear, applying a topical antibiotic, referral to medical care, application of ocular lubricants, follow-up, and waiting until the epithelium is intact and patient is asymptomatic before resuming wear. Strategies for preventing recurrence include reducing wearing time, changing lens design, improving lid hygiene or using comfort drops.

Morgan et al.s’ (Eurolens Research/Manchester Royal Infirmary) 12-month hospital-based epidemiological study of the incidence of non-severe and severe keratitis with contact lenses indicates that there is a significantly higher incidence of severe keratitis in wearers who sleep in contact lenses compared to those who wear them on a DW basis – though silicone hydrogel lenses carry a lower risk of severe keratitis compared to hydrogel lenses worn for EW. This and several other studies highlighted the need for practitioners to remain vigilant when managing their continuous wear patients. The importance of rapid and correct diagnosis and treatment of microbial keratitis was emphasized strongly at the meeting.

Specialty Uses for Silicone Hydrogels

Several case reports highlighted the clear benefits of using silicone hydrogels in piggyback lens systems and as bandage lenses. Comparison of the performance of various silicone hydrogel and non-silicone hydrogel piggyback options for a variety of GP lenses to determine the effect of blinking on levels of corneal hypoxia indicated that blinking adds stress with low-Dk combinations and has less of an effect (or negative effect) with high-Dk combinations (Florkey et al., OSU).

Feibaum (private practitioner) et al. presented a case report examining the use of silicone hydrogels and intra-corneal ring technology as mechanical devices to correct or stabilize and improve visual acuity in patients with clear corneal keratoconus. Silicone hydrogels proved to be a good alternative for a GP-intolerant patient implanted with INTACS.