Article

IN FOCUS WITH CURRENT CONTACT LENS MATERIALS AND DESIGNS

An overview of new contact lens technologies, with a focus on what lens characteristics impact wearing comfort.

More than 140 million people wear contact lenses (CLs) all over the world. The CL market is estimated at approximately $7.2 billion globally, and the U.S. market is valued at slightly greater than $2.5 billion (Nichols, 2017). Furthermore, as of the third quarter of 2016, U.S. sales grew nearly 5% during the first nine months of 2016, and the worldwide sales grew at a similar rate of 4% to 5% (Nichols, 2017). Despite this buoyant trend, one of the major issues that is plaguing the CL industry is discontinuation of lens wear, the major cause of which has been attributed to CL discomfort (CLD). CLD can be induced by many factors (Nichols et al, 2013) that can be patient-related, environmental, or CL-related.

Over the past decade, digital device use has increased significantly; approximately 30% of American adults spend more than nine hours daily using a digital device (The Vision Council, 2015). Overuse of these devices results in eyestrain, neck/shoulder/back pain, headache, blurred vision, and dry eyes. Additionally, reduced humidity and increased air flow are associated with CLD. Although it is commonly believed that conditions such as high altitude, air-conditioned environments, pollution, climate, and temperature impact CLD, there is no direct evidence to support this (Dumbleton et al, 2013).

This article provides a brief summary of the current understanding of the CL material properties that are relevant to CLD and an overview of various materials and designs available in the market.

MATERIAL-RELATED PROPERTIES RELEVANT TO LENS DISCOMFORT

The CL material-related factors that can play a role in wearing comfort include a combination of bulk properties (water content and ionicity, dehydration, oxygen transmissibility, modulus, and mechanical factors) and surface properties (wettability, surface modification, and coefficient of friction) of the material (Jones et al, 2013). According to Coles and Brennan (2012), the only material-related factor that has been demonstrated to correlate with CLD is coefficient of friction. Other factors such as oxygen transmissibility, wettability, modulus, and lens dehydration have only weak links with CLD. Other physical CL properties that seem convincingly related to CLD include replacement frequency, design, thickness/bulk, and edge configuration (Jones et al, 2013).

Several studies have shown that there is an improvement in comfort with increasing replacement frequency (Jones et al, 2013). When it comes to lens design, thin, tapered-edge designs interact less with the lids, thereby resulting in improved comfort (Maissa et al, 2012). Lenses that have complex and thicker designs are less comfortable and result in higher reports of CLD (Young et al, 2011). Reduced lens movement results in increased perceived comfort, while higher lens mobility results in reduced comfort (Young, 1996). Materials that have higher water content are associated with an increase in CLD; however, this has only been shown with traditional hydrogel lens materials (Nichols and Sinnott, 2006; Ramamoorthy et al, 2008).

Although deposition of tear film components such as proteins and lipids did not correlate with discomfort, one study showed that the amount of denatured protein had a good correlation with subjective comfort (Subbaraman et al, 2012). Traditionally, deposition on CLs was believed to reduce comfort during CL wear; however, based on the current evidence, it appears that not all deposits have this effect (Omali et al, 2015). The deposition of tear proteins such as lysozyme and lactoferrin on CLs could not only be not harmful, but beneficial to CL wearers, particularly when these proteins remain in their native state.

In light of these findings, over the past few years, companies have employed innovative technologies to develop CLs with improved surface and bulk properties. Certain lenses have also been launched with the focus on reducing eyestrain and improving comfort when using digital devices. Further research is needed to determine whether these properties result in improved clinical performance with these lenses.

DO SILICONE HYDROGELS OUTPERFORM TRADITIONAL HYDROGELS?

The advent of silicone hydrogel (SiHy) CLs essentially eliminated hypoxic signs, whether worn on a daily wear or overnight basis (Sweeney et al, 2004). SiHy materials now account for approximately 55% of all soft lens fits worldwide and 62% in the United States alone (Morgan et al, 2017). Practitioners still choose to fit traditional hydrogel materials for several reasons.

SiHys are inherently more hydrophobic compared to traditional hydrogels due to the incorporation of siloxane moieties (Sweeney, 2000), resulting in issues related to wettability (Figure 1). Due to their hydrophobic nature, certain SiHys deposit high levels of few lipids (Lorentz and Jones, 2007) and also result in increased denaturation of the deposited protein (Omali et al, 2015). Furthermore, the incorporation of siloxane groups increases the modulus and stiffness (Jones et al, 2006), resulting in mechanical complications, especially with lower-water-content SiHys (Dumbleton, 2002). These mechanical complications include superior epithelial arcuate lesions (Holden et al, 2001), mucin balls (Figure 2) (Dumbleton et al, 2000), conjunctival epithelial flaps (Bergmanson et al, 2012), CL-associated papillary conjunctivitis (Skotnitsky et al, 2002; Skotnitsky et al, 2006), and corneal erosions (Dumbleton, 2003).

Figure 1. Contact lens surface with poor wettability.

Figure 2. Mucin balls associated with SiHy wear.

Moreover, many practitioners have been in situations in which they refit patients from successfully wearing a hydrogel material into a SiHy, and the patients were unable to obtain similar levels of comfort, resulting in them returning to the traditional hydrogel material. As a result, a significant proportion of the patients continue to use hydrogel lenses, particularly daily disposables.

As SiHy materials transmit higher levels of oxygen, it was believed that there would be a reduction in the rate of corneal infections, even when worn on an extended wear basis (Holden et al, 2003). Contrastingly, studies have shown that the risk of microbial keratitis is similar regardless of lens material (Schein et al, 2005; Stapleton et al, 2013; Stapleton et al, 2008). In fact, several studies show that the use of SiHy materials results in an almost 2X increase in relative risk for infiltrative keratitis (Chalmers et al, 2011; Chalmers et al, 2012; Chalmers et al, 2010; Szczotka-Flynn and Diaz, 2007).

When it comes to comfort, there is evidence of increased comfort when patients are refit with SiHy materials (Riley et al, 2006; Chalmers et al, 2005; Dumbleton et al, 2008); however, these studies lack appropriate controls and were not conducted as randomized clinical trials (Jones et al, 2013). Furthermore, reports suggest that the introduction of SiHy materials provided only a minimal increase in overall lens comfort (Jones et al, 2013; Guillon, 2013).

In summary, although SiHy materials have provided a multitude of benefits, certain issues still remain. This has led some practitioners to rethink the option of fitting only SiHy materials (Brennan, 2009; Young, 2011; Bergenske et al, 2005). This holds true for lenses used on a daily wear basis, in which the oxygen requirement for the cornea is adequate for most wearers, especially in low-to-moderate prescriptions (Brennan, 2005; Morgan et al, 2010). The market has seen the introduction of daily disposable lenses in both material types, and manufacturers have developed newer SiHy lenses with improved properties.

SCLERAL CONTACT LENSES

Historically, scleral CLs were prescribed only when other “traditional” CLs did not provide adequate vision correction or for certain corneal conditions such as ectasia. However, over the last decade, CL manufacturers have offered a variety of scleral designs that have a wide range of applications.

Scleral lenses have a bearing on the sclera and therefore have a diameter of 15mm and above. Mini-sclerals are typically between 15mm and 18mm in diameter, while full scleral lenses are >18mm in diameter (more than 6mm bearing on the sclera). Generally, smaller-diameter sclerals bind to the cornea due to suction caused by the vacuum and may pose a challenge to fitters (Rathi et al, 2012). This suction is reduced or eliminated by air ventilation (fenestrated) or fluid ventilation (non-fenestrated) by creating channels on the posterior surface (Pullum et al, 2005). The fenestrations may cause air bubbles to develop within the visual axis, resulting in compromised vision; however, fluid-ventilated lenses overcome this. Current scleral lenses are manufactured using high-Dk materials that increase the oxygen supply to the cornea when compared to old polymethyl methacrylate (PMMA) materials.

Scleral lenses can be extremely useful when traditional CLs do not improve vision. They can be effectively used in patients who have undergone penetrating keratoplasty secondary to corneal ectasia (Pullum et al, 2005; Pecego et al, 2012); in these cases, the vault provided by the traditional corneo-scleral lenses is not adequate. Furthermore, scleral lenses reduce higher-order aberrations in irregular corneas such as in keratoconus and post-refractive surgery. Moreover, the large diameters help improve lens centration and stability, making them attractive options for incorporating complex designs for certain astigmatic and presbyopic patients.

Another major indication of scleral lenses is for patients who have severe ocular surface disease (Pullum and Buckley, 2007), particularly in the pediatric population (Gungor et al, 2008). These lenses provide protection by reducing ocular surface dehydration and also help to keep the ocular surface moist (Kok and Visser, 1992); therefore, they can be helpful in conditions such as Sjögren’s syndrome, Stevens-Johnson syndrome, graft-versus-host disease, and neurotrophic corneal disease. They can also be used for patients who have incomplete lid closure due to eyelid coloboma, exophthalmus, ectropion, nerve palsies, and after lid retraction surgery (Pullum et al, 2005).

Scleral lenses can also be helpful for patients who are actively involved in sports and for cosmetic purposes such as aniridia and ptosis (Shah-Desai et al, 2011).

CONTACT LENSES FOR MYOPIA CONTROL

Approximately 1.5 billion people around the world (~22% of the global population) are myopic. In the United States, myopia has doubled in the last 30 years, and it is estimated that approximately 33% of the population is myopic. Moreover, approximately 2% are near-sighted at school entry, and 15% are myopic upon entering high school; an earlier onset is linked to faster progression, which in turn contributes to increased myopia severity and a higher risk of associated ocular pathologies such as cataract, glaucoma, and retinal degenerations (Pan et al, 2013; Lee et al, 2008; Saw et al, 2005).

While there are various strategies and treatment options to prevent myopia progression, recently there has been a dramatic increase in the trend of prescribing CLs for myopia control (Nichols, 2015). The following section provides an overview of various CL design options for myopia control, which is currently an off-label use in the United States.

Orthokeratology (OK) Studies published during the mid-to-late 2000s reported approximately 50% less axial elongation for two years of OK lens wear compared to controls (Cho et al, 2005; Walline et al, 2009). Recent studies conducted for a period of two to five years show 32% to 42% myopia control using OK lenses (Chan et al, 2014; Santodomingo-Rubido et al, 2012; Hiraoka et al, 2012). Furthermore, partial correction using OK lenses can also be effective, with high myopes of –6.00D or more partially treated by –4.00D showing a 76% reduction in myopia progression over one year when compared to spectacle wear (Charm and Cho, 2013). In a recent study, children were fitted with a novel dual-focus multifocal OK lens design in one eye and a conventional OK lens in the fellow eye. After 26 days, short-term changes occurred in the multifocal OK lens wearing eyes that were considered anti-myopic (Loertscher, 2013).

Multifocal Contact Lenses Although primarily designed for presbyopia correction, concentric and aspheric bifocal and multifocal designs can slow myopia progression. Concentric bifocal lenses reduced myopia progression and axial length in children who had esophoric fixation disparities (Aller et al, 2006). These lenses have a center-distance design surrounded by concentric zones of near and distance powers. Aspheric designs change the spherical aberrations caused by the lenses, and this impacts the shape of the peripheral image focus, which in turn reduces myopia progression (Atchison, 2006). Another study that investigated a center-distance aspheric multifocal design reported a 50% reduction in myopia progression and a 29% axial length reduction when compared to a single-vision-wearing control group (Walline et al, 2013).

Dual-Focus Design Anstice and Phillips (2011) reported on a dual-focus center-distance concentric design soft CL that has a central optic zone diameter of 3.36mm surrounded by concentric rings of alternating near and distance focus, with the near add power fixed at +2.00D. This study found a 30% or more reduction in myopia in the eye wearing this design compared to the other eye fitted with a single-vision distance-corrected lens. Nevertheless, there has been some debate about the reduced visual quality that is caused by the induction of positive spherical aberration when using this design (Kollbaum et al, 2013; Cheng et al, 2013).

Peripheral Plus Powered Aspheric Design Another lens design that has been used for myopia control is an aspheric soft CL with a central optic to provide refractive correction surrounded by progressively increasing relative positive power to reach +1.00D difference from the optic and +2.00D at the edge of the peripheral treatment zone, with a 9mm total treatment zone diameter (Sankaridurg et al, 2011). When using this design for one year, Chinese children in this study showed 34% and 33% less increase in myopia and axial elongation, respectively, when compared to spectacle-wearing controls. These results show that CLs designed to induce peripheral hyperopia can impact the central refractive development and potentially reduce the myopia progression rate.

In summary, it appears that OK and certain soft CL designs are effective in slowing myopia progression and seem to be an attractive, efficient, and safe means of myopia control.

COLORED CONTACT LENSES

Cosmetic CLs account for 4% of total soft CL fits in the United States, while in some Asian countries, such as Taiwan and Korea, this is as high as 58% and 41%, respectively (Morgan et al, 2017). Colored CLs are used mainly for cosmetic purposes, but they also have prosthetic, therapeutic, and special effects applications. There are primarily three types of colored soft CLs that are used to alter eye color: transparent tinted lenses, dot matrix computer-generated lenses, and custom hand-painted soft lenses. The traditional “opaque” cosmetic CLs alter the appearance of the iris color.

Of late, the market has seen an increase in the “limbal ring” CL. It is believed that a prominent limbal ring increases the attractiveness of a person’s face (Peshek et al, 2011; Shyu and Wyatt, 2009). Limbal ring CLs magnify the eye appearance by adding pigments either on the surface or within the bulk of the limbal CL material. The pigments located on the front surface of the CL will interact with the palpebral conjunctiva, while those on the back surface will interact with the cornea. A study that investigated the pigment location on seven different daily disposable limbal ring CLs found that except for one lens, all other tested lens materials revealed pigments on the surface (Osborn-Lorentz et al, 2014).

Most complications associated with cosmetic CL wear are due to poor user compliance (Young et al, 2014; Steinemann et al, 2003; Singh et al, 2012). A recent survey showed that the ocular complications were most common (61%) in the age group between 18 and 25 years, with one-third of complications seen in first-time lens wearers; half did not even receive proper care instructions. One-quarter of the lenses were purchased illegally from unlicensed stores (Gaiser et al, 2017). Back in the late 1990s, “annular tinted contact lens syndrome” was reported, which consisted of reversible corneal irregularities due to the use of tinted CLs (Bucci et al, 1997). A study that compared a lens material that had colorants with a lens that did not have pigment reported significant difference in perceived comfort. The authors attributed this difference to the increased surface roughness and altered surface wettability of the material that had pigments (Steffen and Barr, 1993). Moreover, bacteria tend to bind to surfaces that have increased roughness (Kodjikian et al, 2008). Another study that evaluated the impact on ocular physiology of pigments added during lens fabrication found that the addition of pigments on one particular lens material had no impact on corneal swelling or hyperemia during open-eye wear (Moezzi et al, 2016).

One of the major issues related to cosmetic CLs is when they are sold illegally. A 2014 American Optometric Association survey showed that 53% of cosmetic CL wearers could purchase lenses without a prescription. The sale of CLs in the United States requires a valid prescription that includes the brand name, lens parameters, and expiration date. A federal law based on the Federal Trade Commission’s 2004 Fairness to Contact Lens Consumers Act states that any establishment that sells cosmetic CLs should verify the prescription by contacting the prescribing practitioner. However, this is not always possible when the sales occur through the internet or through overseas suppliers. Therefore, practitioners should be aware of the illegal sale of cosmetic CLs, which is turning out to be a public health issue. Better oversight of unlicensed CL vendors is needed.

LENS DESIGNS FOR PRESBYOPIA

Presbyopic patients have a variety of CL options for vision correction, including supplemental spectacle correction over CLs, monovision, and multifocal CLs. In the United States, approximately 16% of soft CLs are prescribed as multifocal/monovision designs, while globally it is around 11% (Morgan et al, 2017). The prescribing of multifocal/monovision soft CLs has been continually increasing; they account for more than 30% of soft lens fits in Canada, Switzerland, and Germany. Moreover, multifocals are the preferred option when compared to monovision (Morgan et al, 2017).

Concentric or Annular Designs These are designed with a small central annular zone that contributes to distance or near power. This is surrounded by a peripheral annulus, which provides either near or distance vision.

Aspheric Designs These designs have a gradual change in curvature on either the anterior or posterior surface based on the geometry of conic sections. In the center-near design, the highest plus power is in the geometric center and gradually decreases toward the periphery, incorporating controlled levels of negative spherical aberration. In the center-distance design, the lowest plus power is in the geometrical center and increases in power toward the periphery, incorporating controlled amounts of positive spherical aberration.

Diffractive Designs A central zone focuses images at distance by refraction of light and focuses images at near through diffraction principles created by zone echelettes. Equal amounts of light pass through both the distance and near elements of the lens, so diffractive designs are considered truly pupil-independent (Bennett, 2008). Approximately, 40% of light is distributed to each foci—distance and near—and the rest is lost to dispersion and scattering (Legras et al, 2010). This design involves a loss in image contrast caused by the fraction of light that goes into higher diffraction orders, and this could result in a significant visual compromise in low levels of illumination (Montés-Micó et al, 2011) and glare (Toshida et al, 2008).

Over the past few years, there have been reports of improved success with the use of multifocals compared to monovision (Woods et al, 2009; Richdale et al, 2006; Ferrer-Blasco and Madrid-Costa, 2011). With growing evidence of the success of newer multifocal designs, they appear to be an attractive choice; if they don’t work as well as anticipated, monovision or modified monovision can be excellent alternatives.

CONTACT LENSES FOR ASTIGMATISM

Around 50% of patients have refractive astigmatism (≥ 0.75DC) (Young et al, 2002), and approximately one-third of CL wearers require astigmatic correction. These patients are sometimes fitted with spherical lenses, resulting in less-than-optimal vision correction. Over the past decade, toric soft CLs have improved immensely and have become an attractive option for astigmatic correction. In the United States, the percentage of soft CLs that are prescribed as toric designs is approximately 25%, while globally it is around 22% (Morgan et al, 2017).

Patient success with toric soft CL wear is dependent upon several factors. Comfort, clear and stable vision, and good corneal physiology are all important; however, the primary reason for patient success or failure with toric designs is dependent on the rotational stability of the lenses (Goldsmith and Steel, 1991). Therefore, it is critical to stabilize the rotation of a soft toric lens, as this determines the fit and wear success.

To achieve this, two parameters are important: 1) the correcting cylinder needs to be positioned at the appropriate axis; and 2) lens rotation during, or after, a blink needs to be minimized. CL manufacturers employ several methods to stabilize on-eye toric lens rotation, and they broadly fall into two categories: prism ballast and dynamic stabilization (also referred to as thin-zone or double slab-off). Other types of stabilization methods include peri-ballast and truncation.

CONCLUSION

CL materials and designs have evolved immensely over the past few decades, and prescribing newer lenses can potentially help practitioners address common problems. New CL materials intended to improve ocular surface biocompatibility, and novel CL designs intended to improve fit, comfort, and visual performance, are emerging. It is beneficial to stay abreast of the novel innovations and technologies in CL materials and designs to offer the best to patients. CLS

For references, please visit www.clspectrum.com/references and click on document #258.