Extended Wear
Kathy Dumbleton
Lyndon Jones
▪ INTRODUCTION
While the majority of patients choose to wear their lenses on a daily basis, removing them at the end of each day before going to sleep, this modality does not offer the convenience of permanent vision correction sought by many contact lens wearers. The opportunity for day and night lens wear has therefore been attractive to contact lens wearers since its very inception. This modality of contact lens wear is referred to as extended (up to 6 consecutive nights) or continuous (up to 30 consecutive nights) wear.1 Overnight lens wear first became a reality some 30 years ago,2,3,4 but its success has been extremely turbulent over this time.
Overnight contact lens wear has historically been associated with a high rate of complications5,6,7 and, as a result, practitioners and patients alike have become concerned about the potential safety issues associated with extended wear. Hypoxic complications, resulting from poor oxygen supply to the cornea, were common.8,9 Fortunately, with the advance of silicone hydrogel materials and high-oxygen-permeability rigid gas permeable (GP) materials, these complications are now relatively rare.1,10,11,12 Unfortunately, the major concern with overnight lens wear, corneal infection, still remains.13,14,15,16,17
For overnight lens wear to be successful, contact lenses must not only be convenient, but they must also be safe and comfortable. While the risk of microbial keratitis remains the major source of anxiety associated with extended and continuous wear, comfort and dryness are also major limiting factors for patients desiring the convenience of this modality of lens wear.
▪ HISTORY OF EXTENDED WEAR
Throughout the 1970s and early 1980s, manufacturers released a variety of materials that were intended for overnight wear. These early materials were often worn for up to a month at a time without being removed and achieved great commercial success, with John de Carle reporting success with over 2,000 patients in the early 1970s and other authors reporting similarly high levels of clinical success up to the mid-1980s.18,19,20,21,22 As a result of such positive data, extended wear (EW) for cosmetic use for up to 30 days was approved by the Food and Drug Administration (FDA) in 1981, sparking an explosion in the number of patients being fitted with lenses for overnight wear. However, very soon afterward, reports of corneal ulceration with significant vision loss began appearing in journals,23,24 and the safety of overnight wear was questioned in both peer-reviewed journals and the lay media. The Contact Lens Institute, in the United States, sponsored studies to investigate the relative risk and incidence of infectious keratitis. The results from these studies were published in 19895,6 and clearly demonstrated that overnight wear of lenses carried with it a significantly increased risk of corneal infection. As a result, the FDA immediately reduced the approved length of time for overnight wear without removal from 30 to 7 days.
In the mid-1980s, it was believed that the corneal infections observed with overnight wear were probably because of poor hygiene and compliance and that the principal factor driving
such infection rates was because of patients reinserting poorly disinfected lenses. It was hypothesized that using lenses on a disposable or frequent replacement basis, in which the lenses were inserted once only and then discarded upon removal, would likely have an impact on the infection rates reported. Such a concept became a clinical reality with the introduction of disposable EW lenses to the United States in 1987. The first published large-scale study appeared to support such a concept,25 but soon thereafter reports of infectious keratitis started to appear.26 The final proof that disposability had no impact on the rate of ulceration with conventional hydrogel materials worn overnight came with the publication of a paper in 1999,7 which showed that the rate of ulcerative keratitis was exactly that found 10 years previously in the United States,6 before disposability was commonplace. This publication clearly showed that overnight wear with conventional soft lens materials should be discouraged because of the increased risk that such a modality had on the development of sight-threatening keratitis.
such infection rates was because of patients reinserting poorly disinfected lenses. It was hypothesized that using lenses on a disposable or frequent replacement basis, in which the lenses were inserted once only and then discarded upon removal, would likely have an impact on the infection rates reported. Such a concept became a clinical reality with the introduction of disposable EW lenses to the United States in 1987. The first published large-scale study appeared to support such a concept,25 but soon thereafter reports of infectious keratitis started to appear.26 The final proof that disposability had no impact on the rate of ulceration with conventional hydrogel materials worn overnight came with the publication of a paper in 1999,7 which showed that the rate of ulcerative keratitis was exactly that found 10 years previously in the United States,6 before disposability was commonplace. This publication clearly showed that overnight wear with conventional soft lens materials should be discouraged because of the increased risk that such a modality had on the development of sight-threatening keratitis.
Despite this, patients still seek methods to liberate them of spectacles, with refractive surgery being extremely popular. Patients still sleep in lenses overnight even when told not to do so, with an estimated 40% of patients occasionally or frequently sleeping in their lenses.27 Clearly, patients desire a lens that can be worn overnight and will do so whether their practitioner sanctions it or not. To determine the potential safety of materials to be worn overnight, at least from the perspective of hypoxia, requires a detailed knowledge of the oxygen requirements of the cornea.
▪ CORNEAL OXYGEN REQUIREMENTS AND OXYGEN TRANSMISSIBILITY
The cornea is avascular and derives most of its oxygen supply from the atmosphere. Any contact lens acts as a potential barrier to oxygen transport, and the ability of a material to transport oxygen through the lens is a major factor in determining the clinical success of that material. The most widely cited figures for the minimum acceptable oxygen transmission (Dk/t) are 24 × 10−9 units for daily wear and 87 × 10−9 units for overnight or EW.28 More recently, a level of 125 × 10−9 units has been reported as a requirement to prevent stromal anoxia during closed-eye conditions.29 While these values are often quoted, it must be reiterated that these values are “averages,” and patients exhibit widely different corneal oxygen requirements.30,31
In addition to understanding the corneal requirements for oxygen, an understanding of the factors relating to the lens that control oxygen transmission is also crucial. Oxygen delivery to the cornea through the lens depends on both the oxygen permeability (Dk) of the material and the thickness (t) of the lens in question. The Dk of conventional hydrogels is directly related to the amount of water that a polymer can hold, as the oxygen dissolves into the water phase of the material and diffuses through the lens from the anterior to the posterior lens surface. The Dk increases logarithmically with increasing water content of the material,32 and can be determined from the water content using either the non-edge-corrected Fatt formula (Dk = 2.0 × 10−11 e0.0411WC)33 or the boundary- and edge-corrected Morgan and Efron formula (Dk = 1.67 × 10−11 e0.0397WC),34 in which WC is the quoted water content of the material concerned. The units of Dk are 10−11 (cm2/sec)(mL O2/mL × mm Hg) or “barrer.” The term Dk/t describes the oxygen transmissibility of a lens and gives a quantitative indication of the amount of oxygen that a lens-wearing eye will receive through the lens and is a more clinically useful number than Dk, which gives no indication of the effect of lens thickness or lens design.35 The units of Dk/t are 10−9 (cm/sec)(mL O2/mL × mm Hg).
Once the Dk/t of the lens in question is known and it is appreciated that such a lens material and design would provide the cornea with suitable levels of oxygenation, then the issue becomes more related to the suitability of the patient to safely adapt to overnight wear.
▪ PATIENT SELECTION FOR EXTENDED AND CONTINUOUS WEAR
Patient selection is crucial for success when prescribing contact lenses for any wearing modality, but is particularly important for EW and continuous wear (CW). Practitioners are fortunate to have a wide array of contact lens designs and materials available, such that almost every patient can be successfully fitted. However, care must still be taken to select only those patients suitable for overnight lens wear and then to prescribe the most appropriate lens type for their individual optical, physiologic, vocational, and environmental needs.
A thorough history is essential, not only to assess the patient’s motivation and reasons for an overnight wearing modality, but also to evaluate his or her general and ocular health. Systemic disease, medications, allergies, dry eyes, and previous inflammation or infection may contraindicate how contact lenses are worn, and information about the patient’s occupation, work environment, and leisure pursuits may also be crucial.
Suitable Candidates
While this appraisal is not intended to serve as an exhaustive list, there are several good reasons for considering an overnight wear modality. A group of obvious candidates for CW are patients with high refractive errors who are vulnerable as a result of their unaided visual performance. These patients benefit enormously from being able to see clearly at all times, particularly when waking during the night. Other prospective patients include those who have an active lifestyle or occupation in which spectacle wear is hazardous or impractical. These groups may include members of the emergency workforce, who often undertake shift work with unpredictable hours and schedules. CW may also be beneficial for parents of young children who demand functional vision within seconds of waking, day and night. There may also be situations where hygiene is a concern and patients are unable to disinfect or handle their contact lenses each day in a sanitary manner because of location. Examples include outdoor enthusiasts and military personnel.
An overnight wear modality may also be used for several therapeutic and bandage applications,36,37,38,39,40 and in certain binocular conditions, where the likelihood of improving corrected visual acuity in the amblyopic eye is much greater with continuous visual correction. A group of potential candidates for EW or CW also worthy of mention is those individuals who are considering refractive surgery. These modalities of lens wear can be offered either in the short term, such that patients can experience 24-hour visual correction, or as a permanent alternative to irreversible surgical procedures. In addition, many current contact lens wearers admit to occasionally or regularly sleeping while wearing their lenses, and those individuals who report doing this should be proactively counseled on the options of EW and CW, when appropriate.
Unsuitable Candidates
Unfortunately, not all prospective EW and CW candidates are suitable, because of their lifestyle, general health, or ocular appearance. Patients who have a history of noncompliance with instructions for wearing time, replacement frequency, and lens care should probably be avoided, as the consequences of being noncompliant when wearing lenses overnight are potentially higher than in a daily wear mode, and these individuals have also been reported to be at greater risk of infection and inflammation.41,42,43 Several studies have also reported a higher prevalence of infiltrative complications in smokers,6,44,45 and while smoking is not strictly a contraindication to overnight wear, these patients should be counseled with respect to this factor. Another activity that has been reported to be associated with a higher risk of complications among lens wearers is swimming,46,47 and for this reason EW and CW modalities should be avoided for regular swimmers. General health is also a consideration, and individuals with systemic conditions associated with increased inflammation or a slower healing response may be better suited to a daily wear modality.
 ▪ FIGURE 16.1 Scar remaining from a resolved contact lens peripheral ulcer (CLPU). |
There are also several ocular conditions that can preclude overnight wear with contact lenses. Patients with chronic blepharitis or meibomian gland dysfunction typically have a higher bacterial load (especially gram positive organisms) on the ocular adnexa,48,49 increasing their risk for developing corneal infection or inflammation. Severely symptomatic dry eye patients should also be avoided, as their likelihood of successful wear is unlikely, and patients with chronic desiccation staining may also be better with a daily wear modality, as breaks in the epithelial barrier may lead to corneal infection.50 The decision to fit patients with a history of inflammation will be dependent on the most likely cause of the infiltrates. Corneal scars should be regarded with great suspicion, particularly if they have the typical circular appearance indicative of a resolved contact lens peripheral ulcer (Fig. 16.1). Once there has been one corneal inflammatory response event, there is a much higher risk of the patient developing a further inflammatory event,45,51,52,53 and overnight wear should either be avoided or the patient monitored extremely closely.
Once a decision has been made to fit lenses on an overnight basis, then the choice of contact lens material becomes the next important issue.
▪ MATERIAL SELECTION FOR EXTENDED AND CONTINUOUS WEAR
Currently, practitioners have four major options for fitting patients who desire overnight wear:
Conventional Hydrogel Materials
As described above, oxygen diffuses through conventional hydrogel materials through the water phase. Unfortunately, this reliance on water to maximize Dk has been a severely limiting factor for the development of hydrogels for overnight wear, as water has a Dk value of only 80 barrer,54 and thus the oxygen diffusion through the lens is limited. Using the Morgan and Efron formula,34 it can be seen that the most basic of soft lens polymers, poly-HEMA, has a Dk of only 9 to 10 barrers. To increase the Dk of a conventional hydrogel contact lens material beyond that of poly-HEMA, it is necessary to incorporate monomers that will bind more water into the polymer.35,55,56 These higher water content materials typically use HEMA or methyl methacrylate (MMA) as the “backbone” monomers, with more hydrophilic monomers such as N-vinyl pyrrolidone (NVP) or methacrylic acid (MA) increasing the water content to 60% to 70%, providing Dk values of close to 30 barrers.35 55,56 Table 16.1 reports the Dk/t values for several commonly prescribed conventional hydrogel lenses, using the Morgan and Efron formula34 to derive the Dk values from the published water contents and center thicknesses of
−3.00 D lenses. It must be remembered that these Dk/t values will be lower for positively powered lenses and high minus lenses because of the increased lens thickness inherent with such lens designs.
−3.00 D lenses. It must be remembered that these Dk/t values will be lower for positively powered lenses and high minus lenses because of the increased lens thickness inherent with such lens designs.
TABLE 16.1 COMMON CONVENTIONAL HYDROGEL CONTACT LENS MATERIALS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Inspection of Table 16.1 clearly shows that conventional hydrogel lens materials provide woefully inadequate oxygen transmissibilities for safe, edema-free overnight wear, given the required Dk/t values reported above for overnight wear. This awareness of the shortcomings of conventional hydrogel materials resulted in the development of novel materials that would provide increased amounts of oxygen to the corneal surface.
Silicone Elastomers
The first group of materials to provide significantly enhanced oxygen transmission was based on silicone rubber, and these “silicone elastomers” became clinically available in the early 1970s.57,58 These lenses provided sufficient oxygen transport to the ocular surface for overnight wear, with Dk values >300 barrers,59 and they were used for both therapeutic and pediatric applications for over 20 years.60 However, despite their exceptional oxygen transmission and durability, several major limitations were associated with their use in clinical practice. Fluid is unable to flow through these materials, resulting in frequent lens binding to the ocular surface,61 and the lens surfaces are extremely hydrophobic, resulting in marked lipid and mucous deposition.62,63 A silicone elastomer lens is still available (Silsoft, Bausch & Lomb), but its clinical usage is very low because of its high cost, limited parameter availability. and poor surface wettability.
Rigid Gas-Permeable Materials
In the 1960s and early ’70s, the only rigid lens material available was polymethyl methacrylate (PMMA). Despite their low cost and excellent biocompatibility, PMMA lenses gradually lost their popularity because of their lack of oxygen permeability, and in 1978 the first truly gaspermeable lens material (Boston 1) was introduced, which incorporated a silicon-containing monomer commonly called TRIS, which resulted in a marked increase in oxygen permeability.64,65 Over the next decade polymer chemists started to increase the silicone content of rigid GP lenses in an attempt to increase oxygen permeability.66 This strategy worked well, until a Dk value in the mid-1950s was reached, at which point the silicone content was so high that the surface acquired a small but significant electrostatic charge.66,67 This negative charge attracted positively charged lysozymes from the patient’s tear film and, after a few months of lens wear, tenacious protein deposits bound to the lens surface, preventing the surface from wetting properly and inducing inflammatory changes in some patients. In addition, such materials often displayed poor dimensional stability,68 were relatively brittle,69 easily scratched,70 and occasionally exhibited lens “crazing” because of poor or variable polymerization procedures.71,72,73,74
In an attempt to reduce surface deposition but maintain gas permeability, manufacturers started to produce fluorosilicone acrylates in the late 1980s, in which fluorine was added to enhance wettability and oxygen permeability to levels above that previously available in silicone acrylates.59,75 Studies have demonstrated that fluorosilicone acrylates deposit less protein than silicone acrylates76 while maintaining high levels of oxygen transmission.
Silicone acrylate GP materials for overnight wear were initially fitted in the early 1980s and proved relatively successful.77,78,79 However, some patients still showed hypoxic complications when lenses were worn for extended periods of time.80 Improved manufacturing methods have now resulted in the development of several sophisticated GP lens materials that have Dk values of over 100 barrer (Table 16.2), which provides adequate oxygenation for overnight wear in the
majority of patients.81,82,83 Of these, the Menicon Z material is the only GP material that is FDA approved for up to 30 nights CW and has proven to be successful when worn in this way.84,85,86,87
majority of patients.81,82,83 Of these, the Menicon Z material is the only GP material that is FDA approved for up to 30 nights CW and has proven to be successful when worn in this way.84,85,86,87
TABLE 16.2 COMMON RIGID LENS MATERIALS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Silicone Hydrogel Materials
Since the development of silicone acrylate GP and silicone elastomeric soft lens materials, the advantages of incorporating siloxane groups into contact lens materials, from an oxygen transmission perspective, have been well known. Since the late 1970s, manufacturers have tried to incorporate silicone into conventional HEMA-based hydrogel materials to develop high-Dk hydrogels. However, the chemistry required to successfully achieve this is very complex, and it was not until the late 1990s that this became commercially possible.
Eight silicone hydrogel lens materials are currently available, with their major features being summarized in Table 16.3. As described above, the incorporation of siloxane groups into hydrogel materials is complex, as silicone is inherently hydrophobic. A huge impediment to the development of silicone hydrogel lenses is related to the decreased surface wettability, increased lipid interaction, and accentuated lens binding previously seen in silicone elastomers. To make the surfaces of silicone hydrogel lens materials hydrophilic and more wettable, techniques incorporating plasma into the surface processing of the lens have been developed.65,92,93,94 More recent techniques have involved incorporating hydrophilic monomers into the lens material that “migrate” to the surface of the lens and aid wettability.95,96 The purpose of these surface modifications is to mask the hydrophobic silicone from the tear film, increasing the surface wettability of the materials and reducing lipid deposition. In addition to complications induced by poor surface wettability, the incorporation of siloxane moieties results in an increase in the modulus or “stiffness” of the lens materials, resulting in silicone hydrogel materials being significantly “stiffer” than their conventional hydrogel counterparts.
Space limitations prevent an extensive review of the technology behind these materials, and fuller reviews can be found elsewhere.1,56,65,97 However, the differences that do exist are fairly closely related to the company that manufactures them, and thus a brief overview of the lenses will be provided by dividing them into the companies who currently have commercially available lenses.
Bausch & Lomb
Bausch and Lomb’s PureVision material, balafilcon A, is a homogeneous combination of the silicone-containing monomer polydimethylsiloxane (a vinyl carbamate derivative of TRIS) copolymerized with the hydrophilic hydrogel monomer NVP.92,98,99,100 PureVision lenses are surface treated in a reactive gas plasma chamber, which transforms the silicone components on the surface of the lenses into hydrophilic silicate compounds.65,92,99,101 Glassy, discontinuous silicate “islands” result,99,102 and the hydrophilicity of the transformed surface areas “bridges” over the underlying balafilcon A material. PureVision is one of only two lenses approved for up to 30 days of CW, and clinical trials have shown the lens to be effective when used in this way.103,104,105 It is also one of only three silicone hydrogels approved for use as a therapeutic bandage lens, and several studies have demonstrated its value when used in this manner.36,106,107
CIBA Vision
CIBA Vision has four silicone hydrogel lenses. Focus Night & Day material, lotrafilcon A, employs a co-continuous biphasic or two channel molecular structure, in which two phases persist from the front to the back surface of the lens,93 and their O2OPTIX and AirOptix Aqua
lenses (lotrafilcon B) are based on very similar technology. These lenses are manufactured via a cast molding process, as are all the other silicone hydrogel lenses, with the exception of CIBA Vision’s final offering, the O2OPTIX Custom lens (sifilcon A), which is lathe-cut.108 This lens is available in a wide variety of parameters108 and is an ideal option for patients who require silicone hydrogels but are outside the conventional parameter range offered by the majority of frequent replacement lenses, thus needing a custom-made option.109,110 The surfaces of all four lenses are permanently modified in a gas plasma chamber using a mixture of trimethylsilane oxygen and methane to create a permanent, ultrathin (25 nm), high refractive index, continuous hydrophilic surface.93,99,102,111,112,113 Focus Night & Day is approved for up to 30 nights of CW and successful results have been reported with the lens used in this manner52,105,114,115,116,117,118,119 and it is also approved for use as a therapeutic lens.37,38,39,40,106,120,121 On a daily wear basis both lotrafilcon materials have proven to be clinically successful.122,123,124,125
lenses (lotrafilcon B) are based on very similar technology. These lenses are manufactured via a cast molding process, as are all the other silicone hydrogel lenses, with the exception of CIBA Vision’s final offering, the O2OPTIX Custom lens (sifilcon A), which is lathe-cut.108 This lens is available in a wide variety of parameters108 and is an ideal option for patients who require silicone hydrogels but are outside the conventional parameter range offered by the majority of frequent replacement lenses, thus needing a custom-made option.109,110 The surfaces of all four lenses are permanently modified in a gas plasma chamber using a mixture of trimethylsilane oxygen and methane to create a permanent, ultrathin (25 nm), high refractive index, continuous hydrophilic surface.93,99,102,111,112,113 Focus Night & Day is approved for up to 30 nights of CW and successful results have been reported with the lens used in this manner52,105,114,115,116,117,118,119 and it is also approved for use as a therapeutic lens.37,38,39,40,106,120,121 On a daily wear basis both lotrafilcon materials have proven to be clinically successful.122,123,124,125
TABLE 16.3 SILICONE HYDROGEL LENS MATERIALS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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CooperVision
CooperVision offers the Biofinity lens, which is manufactured from comfilcon A and reportedly has a higher oxygen permeability than would be predicted from its water content,97,126 implying that the chemistry on which it is based is different from that employed in other silicone hydrogels. To date, little data have been published on the lens, but its performance appears comparable on overnight wear to other silicone hydrogels.127,128
Johnson & Johnson (Vistakon)
Johnson & Johnson’s Acuvue Advance material, galyfilcon A, has the highest water content of all the silicone hydrogel materials (47%) and thus the lowest Dk and is only approved for daily wear. It has an ultraviolet (UV) blocker, with a reported class 1 UV protection, blocking >90% of UVA and >99% of UVB rays.129,130,131 Acuvue OASYS lens (senofilcon A) also has class 1 UVblocking capabilities.131 The Acuvue Advance lens material was the first nonsurface-treated silicone hydrogel to become a commercial reality, closely followed by Acuvue OASYS. The senofilcon A material used to manufacture the Acuvue OASYS lens is based on similar chemistry to that of the galyfilcon A material in Acuvue Advance. Both materials incorporate a long chain high molecular weight internal wetting agent based on polyvinylpyrrolidone (PVP), which is designed to provide a hydrophilic layer at the surface of the material that “shields” the silicone at the material interface, thereby reducing the degree of hydrophobicity typically seen at the surface of siloxane hydrogels.129,130 The Advance lens internal wetting agent is termed Hydraclear, and that used for the OASYS lens is HydraClear Plus, implying that more PVP is probably incorporated. The OASYS lens has been well received by the profession, clinically appears to deposit less than Acuvue Advance, and has been particularly successful in subjects with symptoms of contact lens-induced dryness.132,133,134
Menicon
The newest “kid on the block” is the PremiO lens from Menicon (asmofilcon A).135 This lens is currently only available in a limited number of markets, and no data are yet available on its clinical performance. The lens uses a patented polymerization system to combine the siloxane and hydrophilic monomers (Menisilk) and uses a novel plasma surface treatment that, according to Menicon, combines the benefits of both plasma coating (as exemplified in the lotrafilcon A and lotrafilcon B materials from CIBA Vision)94,102,136,137 and plasma oxidation (as seen in the surface treatment process used with the Bausch & Lomb balafilcon A material).94,100,102 It is packaged in a unique heart-shaped blister and has an oxygen permeability that is higher than would be predicted from its water content.
▪ CLINICAL PERFORMANCE OF SOFT CONTACT LENS MATERIALS WORN OVERNIGHT
Hypoxia and acidosis are perhaps the greatest challenges for any soft lens worn on an overnight basis. Acidosis results from hypercapnia (increase of carbon dioxide) and is often associated with hypoxia.138,139 In addition, oxygen flow and the release of carbon dioxide waste products are impeded by the contact lens, and all these factors result in significant stresses on the cornea.
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