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Navigating the Soft CL Galaxy at Hypergel Speed

2 CPD in Australia | 0.25G in New Zealand | 3 May 2017

 

By David Stephensen

There’s a complex galaxy of stars out there in the soft contact lens sky. It pays to broaden your perspective beyond the silicone hydrogel portion of that sky if you’re looking to satisfy the vision needs of every patient.

When we look at the night sky we see millions of stars and comprehending the view can be challenging. We can try to make sense of these stars by grouping them by size and brightness. Or we can take a more systematic approach, and make patterns out of the groups of stars before us.

Over the last 50 years, the number of soft contact lens polymers and designs have bloomed, creating their own galaxy of options. Today there are so many unique stars in the contact lens sky that it can be confusing to the everyday contact lens practitioner when selecting a product for each patient’s needs.

Classifying the Soft Contact Lens Sky

The introduction of the Soflens by Bausch and Lomb in 19711 lit the first star in the soft contact lens sky. Over many years, this was followed by variants of hydrogel contact lenses then more recently, by silicone hydrogel contact lenses. As time has moved on, many new soft contact lens stars have been added and we can now start to organise these soft contact lens stars in our minds by constellation, such as silicone hydrogel stars vs. hydrogel stars. We can also look at the effects realised during contact lens wear to help classify these soft contact lens stars. Importantly, as we move into the future, soft contact lenses will increasingly need to be classified by their compatibility with the modern screen based lifestyle.2

A recent new entrant into the hydrogel space is the Bausch and Lomb Biotrue contact lens (nesofilcon A). This has been described as a hypergel material representing the ability of nesofilcon A to carry a high water content while maintaining its hydration characteristics throughout the wearing day.3,4

Oxygenation

Through the 1970s the hydrogel contact lens stars were investigated to examine the effects on the eye in terms of variables of corneal swelling, corneal curvature and visual efficacy.5,6,7,8 The investigation of corneal physiology lead to a growing fascination with oxygenation of the cornea under a soft contact lens, potentially to the detriment of other factors that determine the wearer’s individual satisfaction with contact lenses.

All contact lenses create a relative level of hypoxia in the cornea compared to open eye wear. In order to describe the oxygen performance of contact lenses there have been various measures proposed from Dk/t through to oxygen flux and oxygen consumption.9 There has been a tendency for these measures to be used independently to ascribe clinical effects to individual contact lens products, but in reality, if we are going to describe the oxygen characteristics of the stars in our soft contact lens sky we need to be using terminology that accurately describes the effect on the eye.

In the early period of hydrogel contact lens wear the use of Dk/t created a reasonable model of oxygen performance. As very few contact lens polymers were available, the main modulation of oxygen response was through the modification of the thickness of the contact lens. A thinner contact lens gave a significant reduction in corneal oedema.7 The problem with Dk/t arose as more stars appeared in the soft contact lens sky. As Dk/t increases, the cornea does not increase its appetite for oxygen. This means that the effect on the eye of a Dk/t increase is not linear. For example, the Dk/t of 130 at the centre of a -3.00 PureVision 2 silicone hydrogel contact lens does not provide the eye with four times the oxygen of a Biotrue OneDay hypergel style hydrogel contact lens with a Dk/t of 42 at the centre of a -3.00 lens.9,10,11

So, if Dk/t is a poor descriptor of our soft contact lens stars, then what is a more useful way of clinically categorising oxygen performance in our minds when selecting a soft contact lens for our patients?

The answer to that question is that we can use either oxygen flux or oxygen consumption as our clinical measure. Oxygen flux describes the amount of oxygen that enters the cornea through the contact lens, and oxygen consumption describes the amount of oxygen consumed by the cornea through physiological metabolism.9 Oxygen flux and oxygen consumption are both non-linear functions, and demonstrate the diminishing return of increasing Dk/t as the available oxygen and oxygen consumed do not increase with increasing Dk/t.9

A real-world analogy for this can be seen in the apparent atmospheric oxygen available for respiration at different altitudes. In this example the open eye environment is represented by the apparent atmospheric oxygen available at sea level. The closed eye environment is represented by the peak of Mount Everest, an altitude at which most people require bottled oxygen to survive due to the low apparent available atmospheric oxygen. In contact lens terms, the sea level would represent no contact lens on the eye, and the peak of Mount Everest would represent a sealed PMMA contact lens.

As we climb through altitudes between these two extremes, our perception of the available atmospheric oxygen changes and breathing becomes more difficult. If we take this apparent oxygen as a function of Dk/t one would think that a Dk/t of 42 as represented by the hypergel Biotrue One Day contact lens11 would be the equivalent of standing at an elevation of around 5,000m, or on top of a mountain in the Ecuadorian Andes.12,13 In fact, the hypergel Biotrue One Day contact lens provides the cornea with the same oxygen level that is achieved by standing on top of a 300m high hill, much like standing on top of Mount Coot-tha in Brisbane… the latter would be a much more enjoyable experience during which one barely notices the change in apparent atmospheric oxygen while enjoying dinner at the Summit Restaurant. As the hypergel Biotrue One Day contact lens provides 93 per cent of the cornea’s oxygen requirement during open eye wear, then oxygen is not a critical clinical consideration for contact lens selection.14 This means that our eyes do not have to immediately turn to the silicone hydrogel section of the soft contact lens starry sky. In fact, we can open our eyes to the whole gamut of soft contact lenses available – including hydrogel contact lenses.

Figure 1: Laboratory Dehydration Model3

 

Hydrogel Contact Lenses

Once we again include hydrogel contact lenses in our frontline clinical options it becomes apparent that they have other significant advantages over silicone hydrogel contact lenses for our patients.

The quest for oxygen drove the development of silicone hydrogel contact lenses, with the aspiration to improve ocular surface physiology, decrease the infection rate associated with contact lens wear, and improve comfort.15 Over time it has been demonstrated that these intended benefits do not fulfil this original aspiration.

Certainly, there are benefits to ocular surface physiology as the supply of oxygen approaches that of the open eye. Chronic use of low oxygen flux hydrogel contact lenses, such as the Soflens, did create changes in terms of corneal stromal striae, endothelial blebs and other markers of corneal oedema.8 These physiological changes were reduced to the point of near elimination with the introduction of silicone hydrogel contact lenses.14 However, prior to this the importance of contact lens design itself was noted as a significant factor in reducing the levels of corneal oedema.7 So, contact lens design is a factor that also needs to be considered.

The reduction in clinical evidence of corneal oedema was made at the cost of other new forms of ocular surface compromise associated with high Dk/t contact lenses. These can be divided into a group that represents clinical challenge and a group that represents cosmetic challenge.

Figure 2: A comparison of hydrogel, silicone hydrogel and hypergel materials4,22

 

Clinical and Cosmetic Challenges

As the oxygen permeability and oxygen flux of a silicone hydrogel contact lens increases, the water content of that contact lens decreases. In turn the contact lens modulus alters and tends to become more rigid. This alteration in contact lens modulus created some new clinical entities associated with silicone hydrogel contact lenses and an increase in levels of other contact lens related complications.15

The mechanical effect of silicone hydrogel contact lenses on the tarsal surface of the conjunctival and other associated factors creates an increased incidence of contact lens papillary conjunctivitis.15,16,17 This has been noted to occur at a greater frequency for neophytes compared to adapted wearers.16 Contact lens papillary conjunctivitis is an irritative condition that may be associated with increased rates of cessation of contact lens wear. Contact lens papillary conjunctivitis has been noted to occur at lower frequencies with hydrogel contact lenses than with silicone hydrogel contact lenses.17 Selection of clinically appropriate hydrogel contact lenses for neophyte wearers may improve the rate of successful continuation of contact lens wear. In other words, we could say that appropriate clinical judgement may improve the contact lens ‘drop in’ rate.

Mechanical adverse effects on the bulbar conjunctival surface have also been noted with the use of silicone hydrogel contact lenses. Increased levels of conjunctival staining with the instillation of sodium fluorescein has been observed in patients wearing silicone hydrogel contact lenses.16,17,18 Comparisons of silicone hydrogel and hydrogel daily disposable contact lenses show a statistically significant increase in the degree of conjunctival staining with the silicone hydrogel daily disposable contact lenses.18 This increased level of staining does not extend onto the corneal surface as both hydrogel and silicone hydrogel contact lenses exhibit the same degree of staining of the corneal epithelium post wear.18 Silicone hydrogel contact lenses also exhibit an increased rate of conjunctival compression compared to hydrogel contact lenses, even in daily disposable contact lenses for which the modulus of the materials between the two classes are similar. In part this may be related to contact lens edge design. However it does imply a difference in on-eye performance, favouring the hydrogel materials.18 The presence of conjunctival compression may also form a cosmetic inhibition to continuation of contact lens wear for some patients.

There are more significant conjunctival complications associated with the silicone hydrogel soft contact lens stars, such as superior epithelial arcuate lesions (SEALs) and conjunctival splits. Both these formations create a compromise to the corneal and conjunctival epithelial integrity respectively.15 Conjunctival splits are thought to arise from mechanical effects from the silicone hydrogel edge and, while asymptomatic, do create cosmetic concerns. SEALs are of greater concern as the compromise to epithelial integrity may present a mechanism for subsequent infection. Additionally, SEALs represent a significant source of discomfort to the patient. In a situation in which equivalent hydrogel contact lens products are available, commencement of contact lens wear using a hydrogel product should be contemplated.

Of potentially greater significance to the ocular health is the significantly increased incidence of corneal infiltrative events that are associated with the use of silicone hydrogel contact lenses.15,16,17,18 Even in daily disposable contact lens wear the incidence of corneal infiltrative events with relatively low modulus silicone hydrogel contact lens materials has been reported to be 2.68 times that of comparable modulus hydrogel and hypergel contact lenses.18 Corneal infiltrative events include both symptomatic and asymptomatic corneal infiltrative events, these lesions may require therapeutic intervention and a disruption to the normal contact lens wearing schedule as part of the management. Corneal infiltrative events with silicone hydrogel contact lenses have been noted to occur more frequently along the superior and inferior periphery of the cornea. This may be due to the combination of the lid margin pressure and the silicone hydrogel material in these areas. Additional factors that may enhance the incidence of corneal infiltrative events include increased levels of bacterial adhesion with some silicone hydrogel contact lens materials as well as increases in ocular surface temperature under silicone hydrogel contact lenses.16 In the absence of any likely hypoxic scenario, hydrogel and hypergel contact lens materials should be considered when fitting neophyte patients to reduce the incidence of corneal infiltrative events.16

Figure 3: HyperGel is a next generation contact lens material inspired by the natural physiology of the eye21

 

Visual and Comfort Performance

There is a practical clinical interest in classifying the stars in the soft contact lens sky in terms of their visual and comfort performance on the eyes of our patients. The visual performance of a contact lens is a function of the stability of the polymer and a gentle interaction with the tear film. Polymer stability is important to ensure that the contact lens form and refractive index remain stable throughout the wearing day. The tear film provides the front refracting surface of the eye and a uniform and stable tear film across the anterior surface of the contact lens when worn on the eye.19,20 Newer polymer technology has enabled some of the newest stars in the contact lens sky to make significant gains in on-eye visual function.

Polymer dimensional stability is, in large part, a function of the ability to maintain a constant water content throughout a wearing day. Different contact lens polymers lose different amounts of their mass as water throughout the day. Investigations comparing silicone hydrogel and hydrogel contact lenses to novel hypergel contact lenses show some interesting results. The novel form hypergel contact lens starts with a water content of 78 per cent, mirroring the water content of the cornea. Through a wearing day the hypergel daily disposable contact lens loses only 1.52 per cent of this original water content by weight, compared to a 7 per cent water content loss by weight with other silicone hydrogel daily disposable contact lenses.19

When compared to other novel silicone hydrogel polymers such as the new water gradient silicone hydrogel contact lenses, the hypergel polymers also retain a greater percentage of surface water content with wear. This metric can be assessed through measurements of surface refractive index that show that with wear, the water gradient silicone hydrogel contact lenses’ surface refractive index increases to a value similar to that of the silicone hydrogel core. Conversely, the surface refractive index of the nesofilcon A hypergel contact lens remained relatively constant throughout the wearing period.3 This is important for maintaining contact lens stability for periods of lower blink rate, such as during the use of screen based equipment.

Stability of Superficial Tear Film

Grading the visible soft contact lens stars by the stability of the superficial tear film provides another way in which we can predict the stability of visual performance throughout the day. Tear film stability can be assessed using a placido disc based reflection system. Small disruptions in the placido ring margins can be detected using a software-based video algorithm to determine the tear break up time. In a study comparing silicone hydrogel daily disposable contact lenses to hydrogel and hypergel contact lenses, the tear break up time was significantly better for both the hydrogel and hypergel contact lenses than the silicone hydrogel contact lenses.20 The implication of this finding is that during wear, a hydrogel or hypergel contact lens will provide a significantly better quality of vision through the reduced degradation of low contrast visual acuity.19 This effect is maintained through the wearing day, and should lead to increased patient satisfaction with contact lens performance on the eye.

The soft contact lens stars in the silicone hydrogel region of the soft contact lens sky have been blooming in number over the past 18 years. However, as time goes on there is an increasing level of evidence that should prompt the everyday contact lens practitioner to review whether a silicone hydrogel contact lens will in fact provide their patients with the best performance during daily wear.

Silicone hydrogel contact lenses encounter a greater frequency of corneal infiltrative events compared to hydrogel contact lenses.15,16,17,18 They also encounter greater levels of contact lens papillary conjunctivitis than hydrogel contact lenses, particularly in neophyte patients.16 Events such as SEALS and conjunctival compression are characteristics of silicone hydrogel contact lenses, but they are rarely seen in hydrogel contact lenses.15,18 Patient comfort between silicone hydrogel and hydrogel contact lenses in a daily disposable modality does not show any degree of patient preference for silicone hydrogel contact lenses.18 Hydrogel contact lenses are able to maintain their water content, physical form and surface refractive index more consistently than silicone hydrogel or water gradient contact lenses over a wearing day.3,19 The external tear film is significantly more stable with hydrogel contact lenses than silicone hydrogel contact lenses, providing an improved and more consistent visual performance for our contact lens wearers.20

Conclusion

When you are looking at the various stars in the soft contact lens sky, it pays not to only become fascinated with its silicone hydrogel portion. The hydrogel soft contact lens stars still have characteristics that make them of great practical and clinical interest in providing our patients with comfortable, visually functional and safe contact lens wear. If there are no oxygen flux issues for an individual patient then hydrogel contact lenses, particularly in the newer forms such as the hypergel Biotrue Oneday family of contact lenses, provide us with a bright new and shiny, soft contact lens star upon which to fix our clinical gaze.

 
 
   David Stephensen B.App.Sc(Hons)(Optom) B.Bus Graduate, Certificate in Ocular Therapeutics, CASA CO conducts a specialty contact lens practice with his primary practice located in the southside Brisbane suburb of Moorooka. Dr. Stephensen is a Fellow of the Cornea and Contact Lens Society of Australia and is now a Honorary Vice President of the Cornea and Contact Lens Society and the Chair of the Case Report Section for the Fellowship programme. He has interests in anterior segment eye conditions, toric intraocular lenses, contact lens correction for presbyopia, orthokeratology and in contact lens correction of abnormal corneas – particularly pellucid marginal degeneration, post graft, and keratoconus.


References
1. The Bausch and Lomb Story, Bausch and Lomb, 2017; Accessed 13/2/2017 http://www.bausch.com/our-company/about-bausch-lomb/the-bausch-lomb-story
2. Delo C. U.S. Adults Now Spending More Time on Digital Devices Than Watching TV, Advertising Age, 1/8/2013, Accessed 15/2/2017 (http://adage.com/article/digital/americans-spend-time-digital-devices-tv/243414/)
3. Schafer J, Steffen R, Reindel W, Chinn J. Evaluation of surface water characteristics of novel daily disposable contact lens materials, using refractive index shifts after wear, Clinical Ophthalmology, 2015, 9: 1973–1979
4. Data on File, Bausch and Lomb, April 2013
5. Harris MG, Sarver MD, Polse KA. Corneal Curvature and Refractive Error Changes Associated with Hydrogel Contact Lenses, Am J Optom & Physiol Optics, 1975, 52(May): 313-319
6. Harris MG, Sarver MD, Brown LR. Corneal Edema with Hydrogel Lenses and Eye Closure: Time Course, Am J Optom & Physiol Optics, 1981, 58(1): 18-20
7. Mertz G. Corneal Thickness Response to Ultra Thin Bausch and Lomb Soflens (Polymacon) Contact Lenses, Am J Optom & Physiol Optics, 1978, 55(6): 380-383
8. Polse KA, Sarver MD, Harris MG. Corneal Edema and Vertical Striae Accompanying the Wearing of Hydrogel Contact Lenses, Am J Optom & Physiol Optics, 1975, 52(March): 185-191
9. Brennan N, Morgan P. Clinical Highs and Lows of Dk – Part 1 – Has Oxygen run out of Puff?, Optician 2009, Vol 238(6209): 16-20
10. http://www.bausch.com/ecp/our-products/contact-lenses/myopia-hyperopia/purevision2-contact-lenses (accessed 14/2/2017)
11. http://www.bausch.com/ecp/our-products/contact-lenses/myopia-hyperopia/biotrue-oneday-lenses (accessed 14/2/2017)
12. Baillie K, Altitude Air Pressure Calculator, Accessed 14/2/2017: http://www.altitude.org/air_pressure.php
13. List of Mountains by Elevation, Wikipedia, Accessed 14/2/2017: https://en.wikipedia.org/wiki/List_of_mountains_by_elevation
14. Data on File, Bausch and Lomb, November 2011
15. Brennan N, Morgan P. Clinical Highs and Lows of Dk – Part 2 – Modulus, design, surface – more than just fresh air, Optician 2009, Vol 238(6218): 26-30
16. Santodomingo-Rubido J, Wolffsohn JS, Gilmartin B. Adverse Events and Discontinuations During 18 Months of Siliconee Hydrogel Contact Lens Wear, Eye and Contact Lens, 2007, 33(6): 288–292
17. Long B, McNally J. The Clinical Performance of a Siliconee Hydrogel Lens for Daily Wear in an Asian Population, Eye & Contact Lens, 2006, 32(2): 65–71
18. Diec J, Tilia D. Comparison of Siliconee Hydrogel and Hydrogel Daily Disposable Contact Lenses, Eye & Contact Lens, 2017, 0: 1–6
19. Timberlake GT, Doane MG, Bertera JH. Short term, Low-Contrast Visual Acuity Reduction with in vivo Contact Lens Drying, Opt Vis Sci, 1992, 69(10): 755-760
20. Walther H, Subbaraman LN, Jones L. Novel In Vitro Method to Determine Pre-Lens Tear Break Up Time of Hydrogel and Siliconee Hydrogel Contact Lenses, 2015, Poster Presentation LOTJ number: 2017-02-049
21. Cox_Poster D925_BTOD – ARVO Lens Dehydration 05 03 12
22. Bergmanson J. Clinical Ocular Anatomy and physiology, 14th ed. Houston TX: Texas Eye and Research Technology Center 2007.
LOTJn: 2017–03–171

Fast Facts about Contact Lenses

Melanie Kell

Italian architect, mathematician and inventor Leonardo da Vinci (1452-1519) has been credited as the first person to have come up with the concept of contact lenses. In 1508 he produced sketches suggesting the optics of the human eye could be altered by placing the cornea directly in contact with water. However it wasn’t until the late 1880s that the first glass contact lenses were produced. In 1936 lenses evolved to be made from a combination of glass and plastic and then, in 1948, the first all plastic lenses were introduced, resembling the modern gas permeable lenses in use today. In 1959 Czech chemists Otto Wichterle and Drahoslav invented the first hydrophilic hydrogel soft contact lens material, a development that led to the 1971 launch of Bausch + Lomb's ‘SofLens’ brand contacts, the first FDA-approved soft contact lenses in the United States… and so the starry sky began to shine.1

1. Today, the top four features that patients want from their contact lenses include:2

  • All day comfort
  • UV protection
  • Resistance to protein deposits
  • A lens that mimics the natural eye

2. Some lenses can make the eye feel dry, itchy and uncomfortable and can even cause vision problems.2

3. Twenty two per cent of Australian patients don’t wear their contact lenses every day as they believe their eyes need to rest and breathe.2

4. Approximately 20 per cent of lens wearers are likely to discontinue use, despite advances in silicone hydrogel lenses.3

5. Eye care professionals are the most influential partner in the decision to wear contact lenses.2

References
1. www.allaboutvision.com/contacts/faq/when-invented.htm
2. Bausch + Lomb data on file. Project Marple. A study of the Australian contact lens market consisting of 415 20-minute online interviews conducted with contact lens wearers.
3. Gallup. 2013 Gallup study of the US consumer contact lens market. September 2013.

' soft contact lenses will increasingly need to be classified by their compatibility with the modern screen based lifestyle '