Modulus and its Effect on Contact Lens
Higher lens modulus and lens design characteristics may impact fitting success for some patients.
The degree of a contact lens material's flexibility, or its resistance to being deformed, is its modulus of elasticity, commonly referred to simply as modulus. You can think of modulus as the propensity for a hydrogel or silicone hydrogel lens to act like a rubber band its tendency to "snap back" after being stretched or deformed. Comparing the material modulus values of contact lenses can be helpful in clinical practice, with higher modulus values representing greater lens stiffness and greater resistance to shape change (Table 1).
Impact of Lens Modulus
Modulus is evident clinically in how well a contact lens holds its shape during lens handling and by the degree to which it conforms to (or lays down on) the cornea and peri-corneal sclera when worn. Higher modulus is associated with better handling, ease in establishing whether the lens is inside out and ease of application and removal. On the other hand, higher modulus may have a clinical downside related to mechanical effects associated with lens wear by some patients, including contact lens papillary conjunctivitis (CLPC), superior epithelial arcuate lesions (SEALs), limbal epithelial hypertrophy (LEH), generation of mucin balls and resistance to full draping on the anterior ocular segment, resulting in edge fluting (or edge standoff). Fluting is usually associated with immediate and consistent awareness of the lens on the eye.
The following clinical case highlights the phenomenon of edge fluting with higher modulus lens materials.
A Clinical Case
A 27-year-old myopic female patient successfully wearing Acuvue 2 (Vistakon) hydrogel lenses agreed to be refit into silicone hydrogel lenses to take advantage of the physiological benefits provided by a higher-oxygen-transmissible lens. Her eyes were virtually identical in refraction and corneal topography and so I present only the left eye here. Of particular note is that her corneas were steep, consistent with the refractive myopia of –7.50D (Figure 1).
We attempted to fit the patient with first-generation silicone hydrogels (both lotrafilcon and balafilcon materials), but both attempts resulted in edge fluting. We also diagnostically fit various silicone hydrogel and hydrogel lens brands, using different base curves within brands when available, which demonstrated the influence of lens modulus on lens fitting performance (Figures 2 and 3). Fluting occurred with each base curve of the Night & Day (CIBA Vision) lens, the PureVision (Bausch & Lomb) lens and the flatter base curve of the Acuvue Advance (Vistakon) lens. The steeper base curve of the Acuvue Advance lens allowed for complete draping of the lens on the ocular surface. Complete draping also occurred with hydrogel lenses (Acuvue 2 in each base curve and Proclear [CooperVision]). Ultimately, the patient opted to not make a change from her habitual contact lenses and she continues to enjoy successful, regular daily wear of hydrogel lenses.
The evaluation of various lenses on this patient's eye demonstrates the potential for both material modulus and lens design features to affect lens fit. The edge fluting that occurred with some of the lenses fit in this case likely resulted from the greater peripheral lens thickness of the higher minus lenses coupled with the higher modulus. Both modulus and design worked together to inhibit the lenses' ability to drape the anterior segment.
Lens design, including base curve and diameter choices, seems to have a greater impact as the lens material modulus increases. In addition, this case demonstrated the significance of having a choice of base curves with a lens of lower modulus to avoid edge fluting for patients who have relatively steep corneas.
This case demonstrated that some lenses, based upon their higher material modulus in the available designs, did not fulfill an acceptable on-eye fitting criterion (owing to edge fluting) on a patient who had steep corneas and moderately high myopia. This case is also a reminder that hydrogel lenses still play an important role as a vision correction option for some patients.
Dr. Snyder is an adjunct professor of optometry at the University of Alabama at Birmingham. Since submitting this article he has joined Bausch & Lomb as Director of Professional Relations.