HISTORY AND BACKGROUND

Soft contact lenses were regulated as drugs when initially approved by the FDA in 1971. But for more than four decades, the FDA has regulated contact lenses intended solely for the correction of ametropia as medical devices, with requirements related to rigorous preclinical and clinical testing and careful oversight of design control and manufacturing processes.

Most contact lenses (daily wear) are registered with Class II (moderate risk) guidelines, while extended-wear contact lenses are registered as Class III (high risk) devices. Class II follows the 510(k) process, evaluating new lenses for substantial equivalence, to ensure new investigational contact lens products are as safe and effective as a predicate (existing approval) device under scientifically rigorous protocols for extensive in-vitro and clinical testing. Class III mandates a Premarket Approval Application (PMA), requiring even greater safety and efficacy testing than Class II devices, and typically involves a clinical trial for up to one year and even post-market surveillance in some cases.¹

Today, more than 88% of the world’s contact lens wearers use soft contact lenses, while the remaining approximately 12% use gas permeable (hard or rigid) contact lenses.² Soft contact lenses are typically prescribed under specific state laws in the United States by specifying the brand, base curve, diameter, and power.

DIFFERENCES IN LENS MATERIALS, MANUFACTURING, AND PACKAGING

Soft contact lenses (SCLs), made from numerous hydrogel and silicone-hydrogel materials of widely varying water contents, are cut on a lathe or molded and then hydrated in solution. The nearly isotonic solution may contain special buffering and wetting agents. Each manufacturer uses a somewhat different manufacturing process for each lens brand.

There are also differences in the packaging container and storage solution from one manufacturing process to another. This not only differentiates the packaging, but occurs due to differences in how the lens adapts and may change shape within the package during the manufacturing process—different materials expand in various directions and shapes as they absorb the specially formulated packaging solutions that are utilized. The lens is then autoclaved for sterility and to assure that it is stable on the shelf or in storage.

LENS PARAMETERS

Typically, contact lenses are measured in saline for quality control, which is challenging due to the need to control osmolarity, pH, and temperature of the solution used to maintain the lens shape and because measurement methods may vary from company to company. Although two manufacturing companies may sell SCLs with the same labeled base curve, diameter, and powers, the lenses are likely to perform differently on the same eye. One contact lens company may measure the base curve with one method, while the other company employs another method, and both methods would conform to the ISO standards. This is true for lens diameter and power as well. Two materials may swell in different directions as they hydrate.

Sagittal or sag depth (the total depth of the back of the lens determined by the posterior curvature and diameter) is the most important lens dimension controlling the fit of the lens to the eye.³ If it is not replicated when switching from one lens to another, the likelihood of a misfit or improper fit increases. Two lenses may appear similar from only base curve and diameter numbers, but may differ in sag depth due to the factors listed above. The CooperVision Biofinity lens and the Bausch + Lomb PureVision lenses, for example, have the same labeled parameters—8.6 mm base curve and 14.0 mm diameter lens—but have a 125 microns difference in sag depth on the back surface. This amount is nearly a 33% difference in depth, which can be a clinically significant difference.⁴ And, although both lenses have the same labeled parameters, beyond the sag depth difference, they also have different elastic modulus (stiffness) and may have different thickness profiles, both of which can affect lens flexure on the eye.

All soft contact lenses will settle on the cornea and conjunctiva differently depending on the factors listed above, which can change the fit, movement, and the optical power of the lens. These materials, like many others, also vary in their surface wettability and lubricity (slipperiness), which can affect comfort and vision.⁵

The diameters of soft contact lenses are measured by the manufacturer at room temperature, and these hydrogels may change shape on the eye at a higher temperature. Young et al.⁶ measured soft lenses at room and eye temperature. Twenty-four reusable and daily disposable lens types (10 hydrogel and 14 silicone hydrogel) were measured for diameter at 20°C (room temperature) and 34°C (eye temperature). All lenses decreased in diameter when raised to eye temperature. The largest mean diameter changes with silicone hydrogel and hydrogel lenses were with CooperVision Avaira (0.33 mm) and Bausch + Lomb SofLens Daily Disposable (0.69 mm). The smallest mean changes for silicone hydrogel and hydrogel lenses were with Johnson & Johnson Vision Care 1-DAY ACUVUE TruEye (0.04 mm) and Bausch + Lomb SofLens 38 (0.11 mm), respectively. There was a wide range in diameter change between the different lenses tested. This study demonstrates the levels of shrinkage of the lens on the eye in current soft contact lenses may vary substantially. In many cases, these shrinkage levels may be expected to have significant effects on clinical performance.

Based on the shrinkage measurements, it was possible to estimate the true on-eye parameters of the various lenses. It is notable that the range of actual on-eye designs is greater than the range implied by the labeled parameters. Based on labeled parameters, only two of the lenses were outside of the base curve range (8.40 mm to 8.80 mm), and only two lenses were outside of the diameter range (14.0 mm to 14.2 mm). In contrast, the calculated ranges for on-eye base curve and diameter are 0.5 mm and 0.9 mm, respectively.

This variation results from the wide range in thermal shrinkage, and is likely to result in varying lens fitting characteristics and, potentially, offers the practitioner some useful flexibility in being able to fit the wide range of corneal curvatures within the population. The study by Young et al.⁶ evaluated the range of thermal shrinkage in current soft contact lenses and showed this to be remarkably large. Many of these base curve and diameter changes can be expected to have significant effects on clinical performance.

Wolffsohn and colleagues⁷ fit 39 subjects with three different silicone hydrogel daily disposable contact lenses. Their conclusion was that changing to a different lens brand with similar parameters will not necessarily result in a comparable lens fit and performance.

COMFORT

Comfort can change dramatically from one lens brand to another due to a number of factors, including material, peripheral lens design, thickness, and the shape of the edge.⁸ This is important, as lens comfort is the major reason for patients’ discontinuation of contact lens wear.⁹

Surface wettability and lubricity varies from material to material, and these differences can significantly affect lens comfort depending on the patient’s tear film chemistry and ocular surface condition.^5,10

Each company’s manufacturing process also results in a different edge shape (Figure 1), which can interact differently with the eye and affect the interaction of the contact lens with the conjunctiva, thereby affecting comfort. Research has shown that edge designs may influence the fitting characteristics of a lens, including movement.⁸ In addition, some edge designs may affect the conjunctiva and cause Giant Papillary Conjunctivitis (GPC), which may exacerbate excessive lens movement, creating symptoms of severe itching, mucus discharge, and blurred vision, and can affect the ability of an individual to continue contact lens wear successfully.

OXYGEN TRANSMISSION

The passage of oxygen through a contact lens is critical in maintaining normal corneal physiology and health. Soft contact lenses vary in terms of their oxygen transmission or Dk/t, the amount of oxygen that passes through the lens. Different lens materials, designs, center thickness, and overall lens thickness profiles (and subsequently different lens brands) are the rate-limiting factors. It is well-established in the medical literature that lack of the proper amount of oxygen to the cornea can lead to corneal edema or other serious ocular consequences, such as neovascularization.^11,12

HEALTH AND SAFETY

The cornea is one of the most sensitive tissues in the body and is responsible for the majority of the eye’s refractive, or focusing, power. If it is injured or damaged by contact lens wear, vision may be temporarily impaired or permanently lost.¹³ Lenses that do not properly center, do not entirely cover the cornea and limbus, do not move adequately, and do not have proper sagittal depth may irritate the eye or cause undesirable bubbles under the lens that may cause discomfort and eye irritation. A lens that decenters, with the lens edge contacting the limbus, may cause neovascularization. These new blood vessels may be associated with scar tissue and, if they extend into the visual axis, may lead to a permanent loss of vision.

Lens movement on the cornea is needed to exchange tears behind the lens and flush out debris to optimize the ocular surface and corneal health. Health of the cornea, limbus, and conjunctiva are dependent upon the fit of the lens for proper tear exchange and oxygen transmission of the lens. Insufficient tear flow behind the lens can trap irritating debris and microorganisms and too little oxygen behind the lens may disrupt vision or cause adverse events, such as corneal edema, neovascularization, and corneal endothelial polymegethism and/or polymorphism. Ensuring proper lens movement and fit, as well as monitoring for changes in the health of the cornea, require careful examination with a biomicroscope.¹⁴

VISION

Contact lens power may be different from spectacle lens power due to vertex distance. In addition, the lens may also flex on the eye and possibly change its power as a result.

Lenses also vary in the amount of spherical aberration (SA), the change in power across the lens surface that may create distortions or other visual effects. The power profile across contact lenses of the same labeled central power from different companies may vary as well.¹⁵ These power profiles may vary even more widely for multifocal/bifocal SCLs.¹⁶ Subsequently, the implication is that all -3.00 D or +3.00 D (or any other power) contact lenses are not identical. As a result, it is not uncommon for a patient to need one power for a given contact lens brand and a different power for another brand. This can even occur with two different brands from the same manufacturer.

In addition, a poorly centered lens may also disrupt vision. The power of a contact lens on the eye is a function of the manner in which the lens flexes on the eye, hydration changes, and the corneal topography.¹⁷ These changes and differences may alter the patient’s vision through aberration of the contact lens/eye combination.¹⁸ Some manufacturers do not design SA correction into their lenses, and others include SA adjustments in their designs in varying amounts to correct SA in the contact lens and/or for the average eye. Thus, a -4.00 D power lens from one company may have a different optical effect than another company’s -4.00 D lens, and so forth, across the power range.

When switching from one manufacturer’s lens to another, this spherical aberration may subsequently require a clinically significant power change in order to properly and optimally correct a patient’s vision. In certain instances, the variance in optical properties creates such discrepancies in the patients’ visual outcomes that it makes them unusable.

OTHER FACTORS LEADING TO DIFFERENCES IN LENS EFFECTS

Light transmission across the light spectrum may vary from material to material for several reasons, including whether or not they have UV-blocking capabilities. This could affect vision in some high- or low-light situations and when the spectral transmission of the lens has been changed. Some brands of lenses include UV Class 1 and Class 2 UV blockers, while many do not.

A so-called generic contact lens could be a different material compared with a previously worn lens. Some soft contact lens materials absorb and release preservatives found in contact lens care (CLC) solutions more than others. These CLC solutions are used to clean, rinse, and disinfect contact lenses. The release of these chemical ingredients from contact lenses onto the surface of the eye may cause sensitivity for some patients and for others can cause an ocular reaction with the sensitive cells of the cornea. Multipurpose (preserved) solution interactions have been shown to cause corneal infiltrates or corneal staining.¹⁹

Given the aforementioned probable differences between the physical contact lens fit, vision, and ocular outcomes, changing from one contact lens brand to a so-called generic contact lens with a high degree of efficacy is improbable, even if there was a regulatory process that would allow such a product to be sold.²⁰ ■

REFERENCES

1. U.S. Food & Drug Administration. CFR-Code of Federal Regulations Title 21. Available at https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=801.109 ; last accessed Oct. 20, 2020.
2. Morgan P, Woods C, Tranoudis IG, et al. International contact lens prescribing in 2017. Contact Lens Spectrum. 2018;33:28-33.
3. Young G. Mathematical model for evaluating soft contact lens fit. Optom Vis Sci. 2014;91:e167-176.
4. van der Worp E, Mertz C. Sagittal height differences of frequent replacement silicone hydrogel contact lenses. Cont Lens Anterior Eye. 2015;38:157-162.
5. Tighe, B. Silicone hydrogels: Structure, properties and behaviour. In: Sweeney DF, ed. Silicone Hydrogels: Continuous-wear Contact Lenses. Oxford: Butterworth-Heinemann; 2004:1-27.
6. Young G, Potts M, Sulley A. The effect of temperature on soft contact lens diameter. Eye Contact Lens. 2016;42:298-302.
7. Wolffsohn J, Hall L, Mroczkowska S, et al. The influence of end of day silicone hydrogel daily disposable contact lens fit on ocular comfort, physiology and lens wettability. Cont Lens Anterior Eye. 2015;38:339-344.
8. Turhan SA, Toker E. Optical coherence tomography to evaluate the interaction of different edge designs of four different silicone hydrogel lenses with the ocular surface. Clin Ophthalmol. 2015;9:935-942.
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10. Nichols JJ, Willcox MD, Bron AJ, et al. The TFOS International Workshop on contact lens discomfort: executive summary. Invest Ophthalmol Vis Sci. 2013;54:TFOS7-TFOS13.
11. Langham M. Utilization of oxygen by the component layers of the living cornea. J Physiol. 1952;117:461-470.
12. Holden BA, Mertz GW, McNally JJ. Corneal swelling response to contact lenses worn under extended wear conditions. Invest Ophthalmol Vis Sci. 1983;24:218-226.
13. Stapleton F, Carnt N. Contact lens-related microbial keratitis: how have epidemiology and genetics helped us with pathogenesis and prophylaxis. Eye (Lond). 2012;26:185-193.
14. Jones LW, Jones DA. Non-inflammatory corneal complications of contact lens wear. Cont Lens Anterior Eye. 2001;24:73-79.
15. Wagner S, Conrad F, Bakaraju RC, et al. Power profiles of single vision and multifocal soft contact lenses. Cont Lens Anterior Eye. 2015;38:2-14.
16. Montes-Mico R, Madrid-Costa D, Dominguez-Vicent A, et al. In vitro power profiles of multifocal simultaneous vision contact lenses. Cont Lens Anterior Eye. 2014;37:162-167.
17. Plainis S, Charman WN. On-eye power characteristics of soft contact lenses. Optom Vis Sci. 1998;75:44-54.
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19. Bron AJ, Argueso P, Irkec M, Bright FV. Clinical staining of the ocular surface: mechanisms and interpretations. Prog Retin Eye Res. 2015;44:36-61.
20. U.S. FDA. What is the approval process for generic drugs? Available at: https://www.fda.gov/drugs/generic-drugs/what-approval-process-generic-drugs ; last accessed Oct. 20, 2020.