Disposable and Planned Replacement Soft Contact Lenses

Paul White, O.D., Ronald K. Watanabe, O.D.
AUGUST 1998

How do these lenses stack up against their conventional counterparts?  Here's a look at their similarities, their differences and their irrefutable impact on the contact lens industry.

In 1988, when Vistakon introduced Acuvue soft contact lenses to the United States, the vast majority of analysts predicted that disposable and planned replacement soft contact lenses would eventually command up to 15 percent of the corporate soft contact lens market. This prediction was attained by 1990, and today, disposable and planned replacement lenses comprise over 70 percent of manufacturers' revenue.

Even though disposable and planned replacement contact lenses are made from essentially the same materials as conventional lenses, they are the products of much more sophisticated manufacturing procedures which produce relatively accurate lenses quickly and inexpensively. The greatly reduced per lens cost to practitioners and patients permits more frequent and economically feasible lens replacement, while the good lens parameter accuracy and repeatability allows patients themselves to replace lenses with a high probability of reproducible results.

In the late 1980s and early 1990s, there were limited lens parameters and designs available. This has changed dramatically over the past five years, and so has the reproducibility of disposable planned replacement contact lenses. Today, all of the larger and some of the smaller soft contact lens manufacturers offer disposable and planned replacement lenses with expanded ranges of base curve radii, powers, lens diameters and thicknesses. Spherical, toric and multifocal designs are now available, and other options include visibility and handling tints, cosmetic enhancers, opaque cosmetics and ultraviolet filtration. As lens parameters and designs have increased, so too has the practitioner's ability to properly fit and satisfy patients' needs with disposable and planned replacement lenses. Very extensive marketing campaigns targeting both practitioners and patients, including free trial pairs, have contributed to the demand for disposable and planned replacement contact lenses.

Defining Replacement Frequency

Conventional soft lenses are used until they fail to provide satisfactory vision, comfort, fit or tissue response, which generally occurs after about 12 months of wear. According to Food and Drug Administration (FDA) regulations, any product labeled "disposable" is a one-time use product. Therefore, a disposable contact lens must be discarded upon removal, whether this is after one day or one week of wear. The contact lens profession has often maintained a different interpretation -- disposable lenses are considered to be those that are replaced biweekly or more frequently, and planned replacement lenses are those that are replaced at one- to three-month intervals. The actual replacement interval is prescribed to patients by their practitioners in accord with the patients' best interest (Table 1). Of course, some patients do not comply with manufacturers' or practitioners' instructions and establish their own replacement plan.

Present Market

At the beginning of 1988, all soft lenses were conventional. By 1993, about 55 percent of the then 25 million contact lens wearers in the United States still used conventional soft lenses and about 15 percent used lenses in each of the disposable, planned replacement and rigid gas permeable categories. The total number of contact lens wearers had remained fairly constant for many years, including the first five years after the availability of disposable and planned replacement lenses. Today, there are about 32 million contact lens wearers in the United States, of whom about 26 million wear soft and six million wear RGP lenses (Table 2). This almost 30 percent increase of contact lens wearers in five years is due to an expanding teen-age and twenties population and to disposable and planned replacement soft lenses, which appeal to all age groups and which have reduced the contact lens dropout rate.

In 1993, manufacturers' sales of soft contact lenses totalled about $600 million. From 1995 through 1997, manufacturers' U.S. soft contact lens sales increased about $200 million to total approximately $1.1 billion. Although an estimated 39 percent of patients still wear conventional soft contact lenses, manufacturers' sales dollars for them is only 25 percent of the total. Disposable lenses account for about 64 percent of manufacturers' soft lens sales, and planned replacement lenses account for about 11 percent. Manufacturers and practitioners derive more income overall and per patient from disposable and planned replacement lenses than from conventional soft lenses. Manufacturers' "per wearer revenue" in 1997 is estimated to be twice as much ($52) for disposable and planned replacement contact lenses than for conventional soft contact lenses ($26). Studies indicate that practitioner revenues from lens sales increase in relation to replacement frequency.

Bifocal soft contact lenses account for only about $17 million of the estimated $340 million manufacturers received in 1997 from specialty soft contact lens sales. Toric lenses account for about $155 million and cosmetic colored lenses for about $135 million. While the bifocal contact lens market has not grown significantly for a number of years, toric and cosmetic lenses have each grown about 35 percent in the past two years. In the past six years, the number of annual U.S. soft toric new fits has almost doubled. About one-third of the total U.S. contact lens population, or 11 million people, have significant astigmatism. It is estimated that about four million of these astigmats wear soft spherical or RGP lenses, while three million wear soft conventional, and 40 percent wear planned replacement lenses. Only about one percent of disposable and planned replacement toric soft lenses are prescribed for two-week replacement or less, 33 percent are prescribed for one- to three-month replacement, and 66 percent are prescribed for semiannual replacement. This less frequent replacement schedule compared to spherical lenses is probably related to the increased cost of toric lenses.

General Benefits

In addition to economics, there must be other benefits of disposable and planned replacement contact lenses over conventional soft lenses to cause such a dramatic marketplace shift over such a short period of time. What are some of these general benefits?

Replacing lenses at shorter intervals denies protein and other deposits the opportunity to build up on the lens surfaces. This reduces the chance of eye health problems resulting from the deposits and from the solutions which bind to the deposits (Fig. 1). Optical clarity depends upon clean lens surfaces and a stable pre-lens tear film. Lens deposits may cause lens surfaces to become more hydrophobic and to wet unevenly with a poor or quickly evaporating pre-lens tear film.

Replacing lenses at shorter intervals also reduces the chance of wearing damaged lenses. If patients find that a lens has become damaged, they can discard it and immediately start wearing a new lens. Because these lenses are sold in multi-packs, patients almost always have backup lenses in case one is lost or damaged. With conventional lenses, however, patients may wear a damaged lens until a new one is ordered, which may cause eye irritation or mechanical complications.

Replacing lenses more frequently simplifies lens care. Enzymatic cleaning is not needed if replacement is frequent enough, and multipurpose, all-in-one solutions can be used to clean and disinfect the lenses between replacements. One-time use disposable lenses require no care at all. Simplicity of lens care probably increases patient compliance, which has historically been very poor with conventional soft contact lenses.

Long-term storage of contact lenses facilitates contamination by microorganisms, so frequent replacement is ideal for patients who wear lenses only for specific activities or special events.

Patients who travel can take extra lenses with them, which reduces the need for multiple bottles of solutions. Also, the worry of losing a lens while away from home is eliminated. This is especially true when traveling to areas where access to eye care is difficult.

Patients who are unsure of whether or not they want contact lenses can often try them free, courtesy of manufacturers' offers. This also helps the practitioner to determine a successful fit before ordering lenses. A free trial pair is usually dispensed with a follow-up scheduled one or two weeks later.

Many practitioners have large inventories, which enable patients to take lenses home the day of the fitting, eliminating the need for a separate dispensing visit. You'll also know immediately if the lenses are comfortable and performing well. This is more difficult with conventional diagnostic lenses, which may not have the exact power or other parameters the patient needs. If patients notice that their vision or comfort is not optimal during the trial period, another trial lens may be used prior to ordering. This reduces the need for returns and exchanges.

Possible Disadvantages

In most cases, the yearly cost of disposable and planned replacement contact lenses is higher than that of conventional lenses. However, patients must remember that they are not only buying more lenses, but they are also buying the benefits of the modality. Although the lens expense may be greater, so too is the value. For many patients, a reduced cost of contact lens care solutions offsets some of the added expense.

To save on costs, some patients will extend the replacement plan that their doctor has prescribed for them. These lenses are not intended for long-term use and may become damaged or develop deposits more easily. Remind patients that extending the life of their lenses inappropriately may negate some of the benefits of disposable and planned replacement lenses.

A common misperception of disposable lenses is that lens care is not needed. This is true if the lens is used only once, but most patients reuse their lenses. Lenses that are reused must be cleaned and disinfected after each use to prevent complications.

Most of these lenses are thin, so they are somewhat difficult to apply and remove, especially for new contact lens wearers and for those with weak prescriptions. Patients may have more difficulty in determining whether or not a lens is inverted, so some companies place markings on their contact lenses to help patients determine lens inversion (Fig. 2).

Additional Indications

• Because of their benefits, disposable and planned replacement soft contact lenses may be preferable for any patient. However, there are certain situations where the benefits are enhanced.
• Patients with comfort or vision problems due to heavy deposits are good candidates, as are patients with a history of deposit-related complications, such as giant papillary conjunctivitis (GPC).
• Patients who tear or lose lenses frequently enjoy immediate backup lens availability.
• Part-time wearers can eliminate the issue of long-term storage of their lenses.
• Due to the ease of in-office trials and the ability to fine-tune lens power, evaluating monovision patients is easier.
• Patients who work in dirty, dusty environments or who work with harmful fumes or chemicals can replace lenses before they become contaminated. In some instances, exposure to fumes or chemicals contraindicates the use of contact lenses.
• While the cornea is undergoing rehabilitation from corneal warpage due to poorly fitting lenses, disposable and planned replacement lenses may provide acceptable temporary vision, often equal to or superior to spectacles, until the patient's cornea stabilizes and the patient can be refit into a more permanent lens design.
• Disposable lenses can be used for short-term extended wear use while the cornea is healing from a mechanical abrasion or a surgical procedure.
• Conditions such as keratoconus may require a piggyback system of contact lens wear, where a rigid lens is placed on top of a soft lens on the eye. Disposable lenses are a good choice for the soft lens because they can be replaced easily if damaged by the rigid lens.
• Placing trial lenses that have a prescription on the eyes of ametropic patients can facilitate in-office testing of visual fields or frame selection.

Ocular Health

The first principle of health care is to do no harm. How do disposable and planned replacement contact lenses compare to conventional soft lenses with regard to this principle? A contact lens may be used as an optical patch and as a bandage. As a patch, a contact lens reduces the availability of oxygen to the cornea and the dissipation of carbon dioxide from the cornea. The patch effect creates different amounts of hypoxia and interferes with the cornea's normal aerobic metabolic cycle. This leads to edema, a decrease of glycogen reserves and an increase of lactic acid. Lactic acid decreases the cornea's pH, which may result in stromal and endothelial reactions (Fig. 3). As a bandage, the contact lens creates pressure on the underlying tissues and reduces wetting of the ocular surface and dissipation of material between the contact lens and the cornea. The contact lens may also become contaminated with organic and inorganic deposits, and may become scratched, chipped or ripped. The bandage effect may lead to problems of desiccation, mechanical abrasion and chemical reaction with solutions and toxins from the breakdown of trapped debris. While disposable and planned replacement contact lenses compared to conventional soft contact lenses do not change the patch hypoxia situation, they do significantly reduce many of the bandage inflammatory reactions.

Hypoxia, Corneal Infections and Extended Wear

Except for silicone elastomer, present soft contact lens materials reduce the oxygen supply to the cornea, reduce tear pumping and produce Dk/L values lower than that of the uncovered tear film. Hypoxia can lead to corneal changes such as epithelial and stromal edema, epithelial and stromal thinning, microcysts, neovascularization and polymegethism (Fig. 4). Although these changes are usually reversible, the sequelae may predispose the patient to more serious complications, such as microbial keratitis. Practitioners should be able to differentiate sterile infiltrates and infectious ulcers (Table 3).

FIG. 4:  Corneal microcysts from hypoxia

TABLE 3  Differentiation of sterile infiltrates and microbial corneal ulcers. Characteristic sterile infiltrate infectious ulcer Location peripheral central Size small large Redness localized diffuse Pain / Photophobia mild severe Discharge serous mucopurulent AC Reaction absent to mild moderate to severe

The critical oxygen requirement (COR) determined by Holden and Mertz (1984) established the minimum Dk/L values needed to avoid significant corneal swelling with daily wear and extended wear contact lenses. The ideal minimum daily wear value is 24 x 10^-9 , while the ideal extended wear value is 87 x 10^-9. This roughly three-to-one ratio indicates that only about one-third of the available oxygen under open eye conditions exists under closed eye conditions. They also established a somewhat acceptable but marginal extended wear value of 34 x 10^-9.

Many conventional, disposable and planned replacement soft lenses satisfy or approximate the ideal minimum daily wear Dk/L value. No soft contact lens presently available in the United States comes close to the ideal extended wear value, but some of them approximate the marginally acceptable extended wear Dk/L value. The ideal values for extended wear represent the minimum Dk/L required for no corneal swelling greater than three to four percent, which occurs during sleep even without a contact lens. The acceptable extended wear value indicates the minimum Dk/L required for eight percent or less overnight edema, which would allow for full recovery from the overnight edema while wearing the lens during the day. Many clinical studies have shown that overnight wear of conventional soft contact lenses causes more complications than does daily wear. Contributing factors are increased oxygen debt and corneal edema greater than eight percent, reduced flushing of metabolic debris, increased lens deposits, poorer surface rewetting of lenses and increased contact time with soiled lenses. The following are some of the landmark studies that substantiate this.

The FDA conducted a retrospective clinical study for all hydrogel lenses of more than 22,000 patients which indicated an incidence of microbial keratitis about four times higher for hydrogel extended wear than for daily wear. A survey of members of the Contact Lens Section of the AOA reported similar results. Such information cannot be ascertained from the smaller studies (400 to 600 eyes), conducted for FDA approval of a specific lens. The Contact Lens Institute (CLI), which is composed of many of the major contact lens and solution manufacturers, sponsored a major epidemiological study on contact lens induced ulcerative keratitis which revealed that microbial keratitis had a 0.20 percent incidence with extended wear soft lenses and a 0.04 percent incidence with daily wear soft lenses -- a 5 to 1 ratio. This was based on lens type (extended wear or daily wear), regardless of whether or not the patient slept with the lenses on. When this latter factor was considered, the ratio was between 10 and 15 to 1. These low incidences in terms of the then four million extended wear wearers yield an estimated 13,000 cases of extended wear contact lens induced microbial keratitis per year, which is estimated to be a third of all microbial keratitis from combined causes. There is also some dose response between overnight wear of present soft contact lenses and the development of ulcerative keratitis (i.e., the longer the overnight lens use, the greater the risk). This is regardless of patients' contact lens care and hygiene, which has a smaller relationship to the development of ulcerative keratitis. The problem is caused primarily by sleeping with lenses in the eyes.

Over the years, a number of studies have investigated how these findings with conventional soft contact lenses compare to results of disposable and planned replacement soft contact lens modalities used for extended wear. The results have varied from equal, somewhat greater or somewhat lesser ulcerative keratitis with disposable and planned replacement versus conventional soft lenses used for extended wear.

It is almost unanimously accepted by contact lens researchers, however, that the risk of corneal ulcers is significantly greater with any type of extended wear soft lens than it is for daily wear and that the risk is probably at least the same with disposable versus conventional soft lenses. Therefore, most practitioners discourage extended wear of soft lenses or severely limit their duration.

Inflammatory Reactions

While corneal ulcers represent the most severe contact lens induced tissue complications, a wide range of other tissue complications associated with conventional contact lens wearers have been found in daily wear or extended wear disposable and planned replacement soft lens users as well. But there is a much lower incidence and severity of inflammatory reactions, such as GPC and superficial punctate keratitis (SPK), with disposable and planned replacement lenses.

GPC is caused primarily by mechanical irritation of the superior tarsal conjunctiva and secondarily by an autoimmune reaction to the patient's mucoproteins on the lens. Normal micropapillae have a diameter of less than 0.3mm, macropapillae have a diameter of 0.4mm to 0.9mm, and giant papillae have a diameter of 1.0mm or greater. The enlarged papillae are collections of lymphocytes and plasma cells. In addition to the enlarged papillae, contact lens induced papillary conjunctivitis is characterized by hyperemia, reduced transparency and increased production of mucus by the tarsal conjunctiva. Patient symptomatology includes decreased comfort, increased lens movement, hazy vision and itchiness. Signs and symptoms increase directly with the severity of the papillary conjunctivitis. Topical antihistamines, non-steroidal anti-inflammatory drugs (NSAIDs), and mast cell stabilizers may reduce symptoms, but it's necessary to eliminate the cause. Improved patient care of conventional soft lenses and reduced wearing time may help, but disposable and planned replacement lenses are often the best answer to prevent or to remedy GPC because more frequent lens replacement reduces both the primary mechanical irritation and the secondary autoimmune reaction causes.

SPK manifests as corneal staining which occurs when sodium fluorescein is retained in gaps on the epithelial surface which are caused by the absence, damage, displacement or breakdown of cells (Fig. 5). Possible causes are mechanical trauma, desiccation, metabolic interference or chemical toxicity and hypersensitivity. Mild staining may be asymptomatic, but as the epithelial disruption increases, patients report discomfort, pain, increased lacrimation and photophobia. With extensive and deeper epithelial disruption, fluorescein may enter the corneal stroma.

Mechanical trauma may be caused by a foreign body such as a torn, scratched or coated contact lens that wedges itself against the cornea or by fingernails during lens insertion or removal. Metabolic interference caused by hypoxia may break down or change the selective permeability of epithelial cells over a wide corneal area. Chemical toxicity and hypersensitivity staining is most often a result of reactions to contact lens solutions or to lens deposits and substances adherent to the deposits. Severe epithelial disorder associated with solution toxicity and hypersensitivity can result in pseudodendrites, which appear as raised, gray epithelial plaques with serpentine shapes and light staining, or superior limbic keratoconjunctivitis (SLK), which manifests as an inflammatory reaction of the superior cornea and adjacent bulbar conjunctiva. A deep or broad area of epithelial disruption reduces the ability of the epithelium to function as a barrier to infection, which may lead to ulcerative keratitis.

Compared to conventional soft lenses, disposable and planned replacement lenses reduce the incidence and severity of SPK. From a mechanical standpoint, torn or scratched lenses are replaced more quickly, and more frequent replacement reduces the buildup of deposits. From a chemical toxicity and hypersensitivity aspect, more frequent replacement reduces the adsorption and absorption accumulation of chemicals from solutions and other sources.

Conclusion

Disposable and planned replacement soft lenses have been a watershed event for manufacturers, practitioners and patients. Although such lenses have many advantages compared to conventional soft lenses, they also have areas of similarity and of disadvantage. We remain hopeful that some of the disadvantages will be reduced or eliminated with future materials, designs and manufacturing procedures.

Ronald K. Watanabe, O.D., is an assistant professor at The New England College of Optometry and chief of contact lens services at the New England Eye Institute.  Paul White, O.D., had been for 30 years a professor of optometry at the New England College of Optometry and chief of contact lens services.