SILICONE HYDROGEL UPDATE

Unique Applications for Silicone Hydrogel Lenses

Custom lenses will give practitioners a wide range of lens designs for diverse clinical applications.

By Beth T. Kinoshita, OD, FAAO, Matthew Lampa, OD, FAAO, & Patrick Caroline, FAAO

Dr. Kinoshita is an assistant professor of optometry at Pacific University in Forest Grove, Oregon and serves as chief of contact lens services. Dr. Lampa is an assistant professor of optometry at Pacific University in Forest Grove, Oregon. Mr. Caroline is an associate professor of optometry at Pacific University in Forest Grove, Oregon. He is also a consultant to Paragon Vision Sciences.

Today, it is clear that the ultimate destiny of silicone hydrogel lenses in clinical practice will follow two very different paths. One will be the traditional path in which silicone hydrogel lenses will be used to correct normally occurring myopia, hyperopia, astigmatism and presbyopia. To that end, the use of the modality is off to a noteworthy start.

The second, significantly less traveled path involves custom silicone hydrogel lens designs and approvals for a wide range of unique indications. These custom designs will offer flexibility in prescribing options that will meet the unique needs of our patients. Some of these indications include therapeutic/bandage lenses, piggyback lenses, everted high-minus lenses for orthokeratology and custom, lathable, silicone hydrogel lenses for pathologic and irregular corneas.

Currently, only a small number of laboratories have the equipment and expertise required to manufacture the complex geometries of custom soft contact lenses. However, in the near future, the demand for custom soft contact lens designs in silicone hydrogel materials will entice more laboratories, including GP laboratories, to enter the marketplace. CIBA Vision, Lagado Corporation and Contamac US, Inc. currently have lathable silicone hydrogel materials.

We predict that the number of new lathable silicone hydrogel materials will significantly increase over the next two years. These non-surface-treated materials can have a theoretical Dk of up to 300 with the biggest obstacle being the wetting properties of the lenses. Manufacturers will need to develop unique strategies to overcome the inherent hydrophobicity of silicone hydrogel materials. The ability to independently manipulate base curve, power, diameter, thickness, optical zone and all other lens parameters will open the door for a wide range of lens designs for unique clinical applications (Table 1).

Custom Silicone Hydrogel Lenses for Atypical Ametropia

CIBA Vision was the first soft lens manufacturer to offer custom silicone hydrogel lenses when it introduced the O₂Optix Custom (sifilcon A, 32-percent water, Dk = 82) in 2007. Today, the lens is available in an expanded range of parameters (base curve radii from 7.4mm to 9.2mm, ±20.00DS powers and overall diameters ranging from 13.2mm to 14.8mm). This array of parameters, along with the increase in oxygen transmissibility, has afforded other options to correct diverse refractive errors, including aphakia. The wide range in base curves, powers and diameters of the O₂Optix Custom allows the ability to fit atypical corneal shapes beyond the range of what currently available silicone hydrogel products can fit (Figure 1).

Figure 1. A photokeratoscopy view of a large 12.70mm cornea (left) and a small 10.60mm cornea (right). The 12.7mm cornea would be best fit with a custom soft contact lens with an overall diameter of at least 14.7mm whereas the 10.6mm cornea would require a custom soft lens with an overall diameter of approximately 12.6mm.

Custom HEMA-based toric soft contact lenses are currently available through a number of laboratories. These lenses can be manufactured in almost any sphere, cylinder and axis combination. However, HEMA-based lenses have had limited long-term success for irregular corneas due to decreased oxygen transmissibility with the increase in center thickness required to mask many corneal irregularities. As experience with lathable silicone hydrogel materials grows, the indications for custom silicone hydrogel toric designs will include high regular and irregular astigmatism, keratoconus, pellucid marginal degeneration, post-keratoplasty, post-trauma, post-ocular infection and post-refractive surgery.

Irregular Astigmatism

Perhaps the most exciting application of custom silicone hydrogel lens designs is managing irregular corneas. While GP lenses will always be the material of choice for irregular corneas, GP lenses can exhibit significant limitations in terms of patient comfort and the centration of optics.

Current silicone hydrogel lens designs have enjoyed only limited success in managing irregular corneas due to the limitations in overall parameter specifications such as varying center thickness, independent central and peripheral curvature designs, and reverse geometry designs.

One unique application for custom silicone hydrogel soft lenses may be keratoconus. Unlike traditional soft contact lenses, custom keratoconus designs incorporate a tri-curve posterior lens surface: a steep central zone to align the apex of the ectasia, a paracentral zone to align the more "normal" midperipheral/peripheral area and a scleral curve. The optical success of the modality is based on the center thickness of the lens, which is often approximately 0.30mm to 0.40mm — about four times the thickness of traditional soft contact lenses. These lenses can be manufactured in a wide range of parameters with base curve radii up to 5.0mm, powers up to –40.00D and diameters up to 16.0mm (Figure 2).

Figure 2. Slit lamp photograph of a custom soft contact lens for keratoconus with a center thickness of 0.40mm to aid in masking a portion of the corneal irregularities.

Post-Refractive Surgery

A significant number of patients who undergo refractive surgery procedures fail to achieve emmetropia or are left with irregular corneal astigmatism. Contact lenses are often the best option for visual rehabilitation for these patients. Custom post-refractive surgery soft lenses can be manufactured in a reverse geometry configuration to better emulate the oblate (flat central curvature relative to normal "steep" periphery) shape of myopic post-refractive surgery corneas. The design features of a custom soft reverse geometry lens include a thick central optical zone to provide more stable optics and masking of regular and irregular astigmatism over the flatter central cornea (Figure 3). Silicone hydrogel materials will partly address the oxygen concerns related to the thicker reverse geometry lens design. Like all other custom silicone hydrogel lenses, these lenses will be manufactured in a wide range of base curves, diameters and powers.

Figure 3. The photokeratoscopy view of a patient post-myopic radial keratotomy with irregular corneal astigmatism. The patient was fit with a custom reverse geometry soft contact lens with a center thickness of 0.35mm. The photokeratoscopy image on the right was taken over the soft contact lens. Note the more regular mires.

The three primary concerns in fitting soft contact lenses following refractive surgery (such as radial keratotomy or LASIK) are hypoxia, peripheral neovascularization and excessive apical vaulting over the flattened central cornea (Figure 4). The recent advances in lathable high-Dk silicone hydrogel lenses will dramatically lessen the physiologic concerns related to hypoxia and neovascularization. The higher-Dk materials will also permit the use of thicker lens designs to aid in better optical correction and surface neutralization.

Figure 4. HEMA-induced peripheral neovascularization following radial keratotomy.

Piggyback/Hybrid Designs

The practice of placing a rigid lens on top of a soft lens was first reported by Baldone in the mid-1970s. Traditional piggyback systems consisted of standard HEMA soft contact lenses and either PMMA or low-Dk silicone/acrylate lenses. It's not surprising that this combination of lenses frequently resulted in hypoxia and neovascularization, thereby limiting the modality's usefulness. However, with the introduction of high-Dk silicone hydrogel lenses and stable high-Dk GP materials, the dual lens system is now enjoying a resurgence, particularly for patients unsuccessful with soft lenses or GP lenses alone.

Practitioners primarily use piggyback lenses to improve patient comfort or to aid in GP lens positioning when the anterior surface of the cornea is compromised. In a number of post-myopic corneal surgery patients, especially those with excessively flat central corneas, it may be helpful to create a new steeper surface to the eye by utilizing the increased center thickness of a high-plus-powered silicone hydrogel lens.

Historically, we've been concerned about the physiologic performance of thicker high-plus lenses, especially on a compromised cornea. However, a study by Kollias and Mutti (2002) found that +6.00D powered silicone hydrogel lenses induced corneal swelling comparable to that induced by no lens wear. They also noted no significant change in corneal shape or refractive error. Therefore, using plus-powered high-Dk silicone hydrogel lenses to create a new anterior fitting surface is just one more technique for managing patients who have excessively flat corneal topographies.

Some of the difficulties encountered with traditional piggyback lenses include decentration of the rigid lens and dislodgement of the rigid lens by the upper lid. Custom piggyback designs incorporate a circular depression into the center of the anterior surface of the soft lens. A high-Dk GP lens is then fit within the recessed area of the soft lens. This design provides excellent visual correction by virtue of the centered GP optics as well as enhanced comfort through the bandage effect of the soft lens. Because these lenses must be manufactured with greater central and peripheral lens thickness, a silicone hydrogel platform of lenses would help eliminate the physiologic concerns associated with HEMA.

Hybrid lenses are a natural evolution of the piggyback modality. In 1985, Saturn II hybrid lenses by Precision-Cosmet were introduced. Since then the modality has continued its evolution in manufacturing and rigid/soft lens bonding technology. However, the low Dk of the HEMA skirt continues to be an obstacle to long-term successful lens wear. The next logical step for this platform of lenses will be the incorporation of a high-Dk silicone hydrogel skirt.

Custom Orthokeratology

In 2004, Caroline et al reported "orthokeratology-like" topographical and refractive changes in a patient who had inadvertently worn a high-minus silicone hydrogel lens inside out. Further research performed at Pacific University showed up to 1.50D of myopia reduction in patients wearing everted –9.00D to –10.00D silicone hydrogel lenses on an overnight basis (Figure 5). This is currently an off-label use of commercially available silicone hydrogel lenses. This work raises the question: can custom silicone hydrogel lenses be designed for increased myopia reduction?

Figure 5. Soft lens orthokeratology induced by overnight wear of an everted –10.00D silicone hydrogel.

Other Applications

Therapeutic contact lenses have been an integral part of the contact lens field for more than 35 years. Uses for these lenses include pain management, mechanical protection to promote wound healing, drug delivery vehicle, and occlusion therapy for inoperable diplopia or amblyopia therapy. Traditionally, HEMA lenses managed a wide range of corneal anomalies. However, due to the continuous wear requirements, these lenses failed to provide the necessary oxygen to avoid further complications such as hypoxia, neovascularization and severe microbial keratitis.

In March 2003, CIBA Vision received FDA approval for the use of its Night & Day (Dk = 140) as a therapeutic lens for two primary indications. The first indication was for corneal protection and pain management following ocular surgery; the second was for the treatment of acute or chronic ocular pathologies. Since then, two other silicone hydrogel lenses, Bausch & Lomb's PureVision lens (Dk = 101) and Vistakon's Acuvue Oasys (Dk = 103) have been approved for therapeutic indications. While these lenses perform well for most therapeutic indications, there are situations in which practitioners may prefer an FDA-approved custom silicone hydrogel lens for therapeutic application. These include large-diameter lenses for superior limbic keratoconjunctivitis, bleb leakage, to overlay a tissue adhesive or for post-scleral patch graft following pterygia surgery or Mooren's marginal ulcers.

Silicone hydrogels for prosthetic application is currently unsuccessful because the pigment to simulate iris detail is not well retained by this material. In the future, we're hopeful that a new family of pigments will be created to allow the use of custom silicone hydrogel lenses for prosthetic use.

Looking to the Future

The contact lens industry is forever moving forward. This allows us to offer patients something new and innovative each year. Looking ahead to the next few years shows no exception.

In June 2008 at the British Contact Lens Association annual exhibition and conference, Johnson & Johnson Vision Care United Kingdom introduced a daily disposable silicone hydrogel. The 1-Day Acuvue TruEye (narafilcon A, Dk/t = 118), available only in the United Kingdom and Ireland this autumn, will offer one base curve (8.5mm) and one diameter (14.2mm). As with the other silicone hydrogel Acuvue brand lenses, the new daily disposable will contain Hydraclear and will come with full Class 1 UV blocking.

Finding the balance between a high-oxygen-permeable soft lens material while maintaining on-eye comfort, wettability and optical clarity are all factors when considering the performance of a contact lens. As the challenges in producing lathable silicone hydrogel materials are conquered, a myriad of parameters and lens design options will be available to practitioners and patients. CLS

To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #154.