HYBRID LENS DESIGN
A Lens that Succeeds Where Others May Fail
By William Winegar,
Both a gas permeable lens and a soft lens, this design may be the right lens for difficult-to-fit corneas.
A practicing ophthalmologist developed herpes zoster in the right eye, resulting in severe corneal scarring and irregular astigmatism. He could not tolerate rigid gas permeable lenses, which decentered due to the irregular corneal contour.
A professional golfer with moderate keratoconus was referred to our office after a 10-year history of rigid gas permeable lens wear. His lenses at that time caused frequent episodes of severe epithelial abrasions and variable and unstable visual acuity.
Both of these individuals found a solution in the SoftPerm contact lens. This lens features a central rigid gas permeable portion composed of alkalyne styrene and a low water content thin soft lens flange. It is a viable alternative when other modalities fail in treating problem contact lens cases.
I fit the ophthalmologist in the first case with SoftPerm contact lenses in 1991, and he currently wears the lens all waking hours. Acuity corrects to 20/25, and the lens fits well with good centration and approximately 0.25mm of lens movement in primary gaze. I have seen no negative slit lamp findings to date.
I chose parameters strictly on fluorescein pattern evaluation and manifest overrefraction. The ideal fluorescein pattern demonstrates apical clearance in the gas permeable center of the lens. I recommend using large molecular weight fluorescein rather than standard sodium fluorescein, which stains the soft skirt of the SoftPerm lens.
The range of base curves available with the SoftPerm lens, as well as its design, helped us provide the golfer in the second case with stable, clear visual acuity. He wears the contact lens 14+ hours per day with excellent comfort. We fitted the lenses with maximum apical vault, and no further episodes of abrasions have occurred in the last seven years. Fluorescein patterns and overrefraction determined the fit.
The SoftPerm contact lens has a hybrid design, but it adapts like a conventional daily wear soft contact lens. Precision Cosmet originally manufactured the lens, and Wesley Jessen redesigned it to its current parameters which are shown in Table 1.
TABLE 1: SoftPerm Lens Parameters
Indications for fitting the SoftPerm lens include irregular corneal astigmatism and corneal dystrophies, such as healed interstitial keratitis, map dot dystrophy, keratoconus and pellucid marginal degeneration. The SoftPerm lens offers excellent centration on irregular corneal topographies and may be the only alternative for patients with pathological irregular astigmatism because it is comfortable, aligns centrally on the cornea and provides stable, consistent visual correction.
Fitting the Hybrid Lens
Proper evaluation and fitting of the SoftPerm contact lens requires interpretation of the fluorescein pattern to maximize the relationship between the base curve and corneal bearing. I recommend a simple technique that employs the use of high molecular weight fluorescein and a viscous, unpreserved wetting agent such as Celluvisc.
Place the SoftPerm lens into a sterile flat pack with Celluvisc for a few moments. Remove the lens and fill its posterior surface with two drops of the high molecular weight fluorescein, then insert the lens directly onto the patient's cornea (Figure 1). Anesthetize the eye prior to insertion to minimize reflex tearing. Evaluate the fluorescein with the blue cobalt filter in the biomicroscope. Wait approximately 30 to 60 seconds after insertion to view the most accurate pattern.
Figure 1: The Softperm lens with two drops of high molecular weight fluorescein on the posterior surface.
The most desirable pattern reveals apical vault. Any significant bearing dramatically reduces the prognosis for a successful response. Apical touch will lead to a frank epithelial defect. Apical vault is essential for the hybrid contact lens.
Determine power by an overrefraction. You can easily verify the base curve with a radiuscope. Apply the lacrimal lens principles to adjust the base curve radius after initial fit. For example, a 7.30mm -3.00D SoftPerm contact lens refit with a 7.10mm base curve would require a power of 4.25D.
As a daily wear contact lens, initially monitor the wearing time of hybrid lenses. Recommended wearing schedule is four hours on day one and three additional hours added each day. Schedule patients to return for follow-up five days after initial dispensing, and instruct them to wear the hybrid lenses for a minimum of four hours on the day of their follow-up visit. Use high weight molecular fluorescein to evaluate the corneal response at the follow up.
Careful evaluation of neovascularization is critical. I recommend follow-up care for SoftPerm contact lens wearers every six months.
Keratoconus SoftPerm Contact Lenses
Wesley Jessen introduced steeper base curve SoftPerm lenses specifically for the keratoconus patient. Base curves of 6.90mm, 6.70mm and 6.50mm allow a wider range of choices for the mild to moderate conic corneas. The steeper base curve design of the keratoconus SoftPerm lens has an optical zone diameter (OZD) of 6.5mm, as compared to the 7.0mm OZD in the standard lens. It yields excellent centration and stability for the keratoconus patient. SoftPerm keratoconus lenses may be the only alternative for hyper-sensitive contact lens intolerant keratoconus patients.
For mild keratoconus patients (steep K less than 48.00 diopters), begin by trial fitting a hybrid lens whose base curve radius is approximately 1.25 diopters steeper than the flat K. Use my previously outlined technique to insert the SoftPerm lens, and evaluate the high molecular weight fluorescein pattern over one to two minutes. You have the proper base curve when you see complete apical vault and no air bubbles are sequestered behind the lens (Figure 2). Apical touch equal to or greater than 2.0mm indicates a flat contact lens fit (Figure 3).
Figure 2: A Softperm lens fit with the proper base curve.
Figure 3: Apical touch indicating a flat lens fit.
On more moderate keratoconus patients, (steep K greater than 50.00 diopters) begin with the 6.90mm SoftPerm contact lens. Evaluate the fluorescein pattern as previously discussed to determine the proper base curve.
I recommend a hydrogen peroxide system to maximize results with a SoftPerm lens. Allow the lens to "wet" in unpreserved lubricant for improved comfort and wettability prior to insertion.
The SoftPerm lens is not a panacea for all contact lens problems, but it does offer a lens design every practitioner must consider for difficult-to-fit contact lens patients.
William Winegar is the Chief of the Contact Lens Service at the Bascom Palmer Eye Institute at the University of Medical Schools Department of Ophthalmology in Miami. He has published articles in numerous ophthalmic journals and lectured extensively on contact lens technology.