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Specialty Applications of Silicone Hydrogel Contact Lenses

The continuous evolution of silicone hydrogel lenses helps more and more patients every year.

CARMEN CASTELLANO, O.D., F.A.A.O., ST. LOUIS, MO.

Unless a patient initiates an interest in contact lenses (CL), most practitioners consider patients who have a refractive error for spectacle lens wear first. As a result, these practitioners may not even discuss CLs as an option. We now have silicone-hydrogel CLs available to address oxygen deprivation and dry eye and correct astigmatism and presbyopia, allowing us to fit many more patients in contact lenses.

Here, I discuss how silicone-hydrogel CLs can meet patients ocular' needs, the role of custom-designed silicone-hydrogel CLs and how to select and fit this important eyecare device.

MEETING PATIENTS' NEEDS

Although custom-designed PMMA lenses have been available since 1948, and later custom GP's, many patients and practitioners have resisted these CLs due to comfort and fitting difficulty or concerns about oxygen transmission. The key to making a silicone hydrogel available in a custom design is the ability to lathe the material. Fortunately CL manufacturers have begun to offer more custom-designed silicone-hydrogel options to quell comfort and fitting difficulties and alleviate corneal hypoxic issues:

• hypoxic stress and hyperemia. Researchers have long documented the effects of oxygen deprivation on the health of the anterior surface. Bulbar and limbal hyperemia may be a function of a poorly fitted or damaged CL or even surface deposits. Or, hyperemia may occur as a result of hypoxia. In fact, some researchers have suggested that hypoxia is the factor most likely responsible for hyperemic response to CL wear.¹

In addition to bulbar and limbal hyperemia, hypoxic stress may also cause corneal edema, neovascularization, endothelial cell polymorphism and cell loss, as well as visually significant corneal distortion.

Since the approval of the first silicone-hydrogel CL in the United States in 1999, the issue of just how much oxygen transmission is necessary to eliminate hypoxic stress has been somewhat controversial. A 1984 study showed the minimum Dk/t required to be 24 for daily wear and 87 for overnight wear. But a 1998 study showed the necessary Dk/t to be 35 and 125, respectively. So, we know that increasing the oxygen transmission of a CL has a positive effect on corneal physiology as long as wettability and lens quality are maintained.^{2, 3}

Certainly, since the advent of silicone-hydrogel CLs several years ago, practitioners have seen evidence of the benefits of increased oxygen. Patients are now often able to wear their CLs for longer periods than they used to with traditional hydrogel CLs (even as continuous wear) and as a result, report less dryness and redness.

Due to the addition of silicone, wettability and solution compatibility are an issue for some patients. As a result, eyecare companies have developed several multipurpose solutions specifically for use with silicone-hydrogel CLs in an effort to improve wettability and, ultimately, comfort.

• modulus. Silicone-hydrogel CLs often display a higher modulus of elasticity, or "stiffness," when compared with traditional hydrogel CLs. This may affect fitting characteristics and sometimes the initial comfort of a silicone-hydrogel CL.

To get a patient through a potential increase in awareness during the first few days of wear, simply educate him that this may occur. Also, inform the patient that the higher modulus of the silicone-hydrogel CL may aid in handling.

On the other hand, the higher modulus of a silicone-hydrogel CL has been associated with certain complications, such as CL papillary conjunctivitis (CLPC), superior epithelial arcuate lesions (SEALS), limbal epithelial hypertrophy (LEH) and mucin-ball accumulation beneath the CL surface. This also increases the potential for edge fluting — the tendency for a lens edge to stand off the eye, creating lens awareness.

Two commonly seen effects of conventional hydrogel CLs may actually reverse when patients are refit with silicone-hydrogel CLs. The literature references several studies that have shown small increases in myopia occur (0.50 diopters or less) with thick (0.10 mm or thicker) conventional hydrogel CLs. One study showed this phenomenon to reverse upon refitting with a silicone-hydrogel CL. In addition, microcysts observed in corneas wearing conventional hydrogels have been shown to dissipate upon refitting to silicone hydrogels.⁴

THE ROLE OF CUSTOM DESIGNS

The parameters of readily available silicone-hydrogel CLs have recently expanded to include higher powers, more CL options for astigmats and even multifocal designs. While these parameters are adequate for many patients, some patients still need custom-designed CLs.

Recently, CL manufacturers have recognized this need. As a result, many of them currently offer you the ability to design a soft CL that has a wide array of parameters seen in conventional hydrogel materials.

• special needs. The potential of the custom-designed silicone hydrogel CL allows you to offer "marginally successful" patients a means to achieve more stable visual acuity and greater comfort.

• keratoconus. Due to the unusual steepness of advanced keratoconus, standard CLs simply don't fit well enough on many of these patients. Piggyback systems that use high-Dk silicone-hydrogel CLs as the base, however, have shown to be highly successful in keratoconus patients. These lens systems often allow patients who have significant corneal disease to tolerate GP CLs and gain the benefits of improved vision. The steeper options now available may allow for more patients to achieve success with piggyback designs in cases of keratoconus. Plans are even in the works for a keratoconus-specific custom silicone-hydrogel CL.

SELECTION AND FITTING

A made-to-order silicone-hydrogel CL is a tremendous help to special-needs patients, beyond the offering of power requirements that are outside the ranges of currently available CLs. In addition, a custom design may aid those who have abnormally large or small corneas or unusually flat or steep eyes. By being able to more precisely adjust these parameters to fit unusual situations, you will not only be able to provide patients with CL designs that improve physiology, but also provide better visual stability and visual quality.

The two methods for a first time, custom silicone-hydrogel CL:

• horizontal visible iris diameter (HVID) method. A well-fitted CL should center well, covering the limbus 360°, without causing vessel impingement. It should also display minimal but adequate movement. You can best achieve this by carefully selecting the initial CL diameter based on the HVID of the cornea. Some topographers allow you to measure the HVID with excellent accuracy but, if you do not have such a device, a pupil gauge or CL reticule (jeweler's loupe) may be adequate. Choose a lens that has a diameter approximately 3mm larger than the HVID.

In our office, we've found that a base curve based on the flattest corneal curvature and diameter is a good choice. For a 13.2mm diameter CL, initially select an 8.3mm base curve for corneas 8.0mm or flatter, 8.0mm for corneas between 8.00- and 7.50mm and a 7.7mm base curve for corneas steeper than 7.50mm. (See "Initial CL Selection" below and "Selection of Initial Base Curve for Custom Silicone Hydrogels," page 58.) Once you evaluate the initial CL on the eye, make adjustments if necessary, based on your observation of the fit.

• arc length method. Drs. Becherer and Davis described this method for prescribing. Determine the arc length using the radius of curvature (flat k) and HVID. This method includes a nomogram to help you select the appropriate diameter and base curve.^{5, 6}

The nomogram is somewhat different for each material, so results may vary if stiffer materials are used to evaluate this method. Researchers could, however, develop new nomograms to apply to various materials.

ON THE HORIZON

CL manufacturers are constantly expanding the options available to today's difficult-to-fit patients. Custom designs on the horizon include torics, multifocals and CLs designed for pediatric aphakics and post-refractive surgery patients. As a result, stay abreast of these latest developments, and be sure to offer virtually all your patients the options of CLs.

As the old adage goes: You'll never know unless you ask. OM

1. Guillon M, Bilton S, Bleshoy H, et al. Limbal changes associated with hydrogel contact lens wear. J Br Contact Lens Assoc. 1985;8:15-19.

2. Holden BA, Sweeney DF, Swarbrick HA, et. al. The vascular response to long-term extended contact lens wear. Clin Exp Optom. 1986;69: 112-196.

3. Holden BA, Sweeney DF, Swarbrick HA, et al. The vascular response to long-term extended contact lens wear. Clin Exp Optom. 1986;69:112-196.

4. Ross G, Nasso M, Franklin V, Lydon F, et. al. Silicone Hydrogels: Trends in Products and Properties. Presented at BCLA 29th Clinical Conference & Exhibition, Brighton, UK; June 3-5, 2005.

5. Andre M, Davis J, Caroline P. A New Approach to Fitting Soft Contact Lenses. Eyewitness. Fourth Quarter, 2001.

6. Douthwaite WA. Initial Selection of Soft Contact Lenses Based on Corneal Characteristics. CLAO J. 2002 Oct;28 (4):202-5.

7. McMonnies CW, Chapman-Davies A. Assessment of conjunctival hyperemia in contact lens wearers. Part II. Am J Optom Physiol Opt. 1987;64:251-255.

8. Euromcontact. Seeing things clearly: An economic model of the optical retail industry in Europe. 2001, London Business School.

9. Guillon M, Bilton S, Bleshoy H, et al. Limbal changes associated with hydrogel contact lens wear. J Br Contact Lens Assoc. 1985;8:15-19.

10. Ross G, Nasso M, Franklin V, et al. Silicone Hydrogels: Trends in Products and Properties. Presented at BCLA 29th Clinical Conference & Exhibition, Brighton, UK; June 3-5, 2005.

11. Dumbleton K, Keir N, Moezzi A, Feng Y, et al. Objective and subjective responses in patients refitted to daily-wear silicone hydrogel contact lenses. Optom Vis Sci. 2006 Oct.;83(10)758-68.

Dr.Castellano practices at The Koetting Associates, specializing in contact lenses. He also currently serves as adjunct assistant professor at the University of Missouri-St. Louis School of Optometry, Pacific University College of Optometry and Department of Ophthalmology and Visual Science at the Washington University School of Medicine.

1. Bulbar and limbal hyperemia may be a function of:
A. a poorly fitted contact lens.
B. a contact lens that has been damaged.
C. hypoxia.
D. all of the above.

2. Which of the following is likely NOT a result of hypoxic stress?
A. bulbar and limbal hyperemia.
B. corneal neovascularization.
C. giant papillary conjunctivitis.
D. corneal edema.

3. Studies have shown that microcysts seen with conventional hydrogel wearers:
A. are of no consequence.
B. will usually dissipate upon cessation of lens wear.
C. will often dissipate upon refitting to silicone hydrogels.
D. both B and C.

4. The first silicone-hydrogel lens was approved in the US in:
A. 1971.
B. 1987.
C. 1999.
D. 2003.

5. In a 1984 study, researchers found that the minimum Dk required for safe daily wear (DW) and extended wear (EW) to be:
A. DW: 24 and EW: 87, respectively.
B. DW: 35 and EW: 125, respectively.
C. DW: 80 and EW: 175, respectively.
D. No minimum thresholds were found in this study.

6. "Modulus" is a term used to describe what feature of a contact lens material?
A. wettability.
B. oxygen transmission.
C. stiffness.
D. thickness.

7. The higher modulus of elasticity seen in silicone-hydrogel contact lenses has been associated with which of the following?
A. contact lens papillary conjunctivitis (CLPC).
B. superior epithelial arcuate lesions (SEALS).
C. limbal epithelial hypertrophy (LEH).
D. all of the above.

8. There is NO evidence to suggest that "myopic creep" seen in some conventional hydrogel lens wearers is reversible with silicone hydrogels.
A. true.
B. false.

9. PMMA contact lenses first became available in:
A. 1948.
B. 1884.
C. 1934.
D. 1996.

10. What development has been key to the introduction of a custom-made silicone-hydrogel lens?
A. the invention of a unique surface treatment.
B. the ability to lathe the material.
C. the ability to cast mold the material.
D. the ability to decrease the modulus.

11. In fitting a custom soft contact lens, the two key parameters are:
A. base curve and diameter.
B. lens power and water content.
C. diameter and modulus.
D. lens power and surface treatment.

12. Which rule-of-thumb most closely matches the recommendation of this article for lens diameter when custom designing a silicone hydrogel?
A. The diameter should be two times the flattest base curve in millimeters.
B. The diameter should be approximately 3mm larger than the horizontal visible iris diameter
C. The diameter should be three times the scotopic pupil size in millimeters.
D. The diameter should always be 14.0mm

13. Piggyback contact-lens systems may be most beneficial in the treatment of which of the following ocular conditions?
A. conjunctivitis
B. glaucoma
C. keratoconus
D. pinguecula

14. Based on recommendations in this article, select the custom lens of first choice for the following example: Spec Rx: -6.50D – 0.50 x 180
Flattest K-Reading: 7.60mm
HVID: 10.5mm
A. diameter 13.2mm/BC 8.0mm/Power -6.25D.
B. diameter 14.0mm/BC 8.9mm/Power -6.50D.
C. diameter 14.0mm/BC 7.8mm/Power -5.75D.
D. diameter 14.8mm/BC 9.2mm/Power -6.25D.

15. Regarding horizontal visible iris diameter (HVID):
A. accurate measurement is necessary in order to select the appropriate base curve of a custom silicone hydrogel.
B. It plays little role in designing a custom silicone hydrogel.
C. It may be measured using some corneal topographers.
D. A and C.

16. "Arc length designing" refers to a method of selecting contact-lens parameters based on:
A. flattest K and HVID.
B. power and lid position.
C. pupil size and age.
D. none of the above.

17. For a patient with a large corneal diameter (HVID) of 12mm, the most appropriate) custom diameter would likely be:
A. 13.2mm.
B. 14.0mm.
C. 14.8mm.
D. any should be fine.

18. Fluting may occur as a result of the higher modulus of a silicone-hydrogel contact lens. This refers to:
A. the tendency for the lens to tear easily.
B. the tendency for the lens edge to stand off the eye, often creating lens awareness.
C. a lens that is too tight.
D. none of the above.

19. Silicone-hydrogel lenses are currently available in the following designs:
A. single vision spheres.
B. torics.
C. multifocals.
D. A, B and C.

20. Lens movement is of no importance when fitting a silicone-hydrogel lens.
A. True.
B. False.