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Overview of contact lenses

Overview of contact lenses
Literature review current through: Jan 2024.
This topic last updated: Oct 19, 2022.

INTRODUCTION — An estimated 40.9 million people aged 18 and older in the United States (one in six adults) wear contact lenses, with 93 percent wearing soft lenses and the remainder rigid gas-permeable (RGP) lenses [1].

The types of available contact lenses, indications for their use, and appropriate care to decrease the risk of infection or trauma will be reviewed here. The complications with contact lens use are discussed separately. (See "Complications of contact lenses".)

LENS TYPES — Contact lenses may be categorized by their compositional material, wearing schedule, disposal schedule, permeability, water content, and type of correction (figure 1 and figure 2). With many new lens types available, there are alternatives to help most patients achieve comfortable lens wear with clear vision. New types of contact lenses are continually being introduced with the intent to decrease risks of infection, inflammation, and conjunctival trauma while maximizing vision correction and convenience of use [2].

Hydrophilic/soft lenses — Soft lenses account for more than 90 percent of prescribed contact lenses in the United States and worldwide (figure 2 and figure 1) [3,4].

Soft lenses are used to correct a variety of refractive errors, including myopia, hyperopia, astigmatism (toric lenses), and presbyopia (multifocal lenses). Not every prescription is available in every material or brand. Certain refractive errors, caused by keratoconus or other corneal distortions, may not be correctable with soft lenses.

Soft lenses are generally quite comfortable and easier to adapt to than rigid lenses. Patients wearing soft lenses need regular follow-up care and must be compliant with lens care regimens to avoid serious eye problems (see 'Guidelines for prevention of infectious keratitis' below). The most common reason for patients to discontinue contact lens wear relates to symptoms of lens awareness or dryness while wearing lenses. Other reasons include inadequate visual acuity, allergic reactions, and difficulty handling lenses [5].

Soft lenses are made of various plastic polymers that absorb water (hydrophilic). These materials differ in terms of oxygen permeability (expressed in Dk units, where D stands for diffusion and k for solubility), water content (varying between 20 and 70 percent water by weight), surface quality (wettability), ultraviolet absorption, and structural consistency (stiffness or modulus). The US Food and Drug Administration (FDA) has developed a system for classifying soft lenses (table 1).

All soft lenses absorb water, as well as a variety of other substances: chemicals in contact lens solutions, tear secretions, makeup, and airborne chemicals or vapors. Oily substances from the eyelids or facial creams that come in contact with the lens can coat the lens surface.

Prior to 1996, the polymer in all soft lenses was primarily 2-hydroxyethyl methacrylate (HEMA)-based, which is still used in several current lenses. Polymers with silicone hydrogel (SiHy), which are more highly oxygen permeable, were introduced in 1999 and are now the most common material in newer types of lenses [6]. Lenses with higher oxygen permeability are generally considered a healthier option [7], although case-control studies have not demonstrated that SiHy lenses decreases the risk of microbial keratitis [8] or of nonulcerative contact lens-related disorders seen in an emergency setting [9].

Physical characteristics of different SiHy lenses are detailed in a table (table 2). Dk/t is a measure specific to the individual lens, rather than its material, and refers to the oxygen permeability (Dk) of the material normalized for the thickness of the lens (t). The higher the Dk/t value, the more breathable the lens. Patients using higher Dk/t lenses develop less corneal edema during all-day or overnight wear compared with HEMA-based lenses [7]. Other material characteristics include water content, surface quality (wettability), ultraviolet absorption, and modulus (stiffness). The higher modulus lenses will feel stiffer, and lower modulus lenses floppier, when handled.

Soft lens materials provide a good medium for microbial growth, so lens cleaning and disinfection are required with approved systems. Additionally, if patients are not compliant with care techniques, red-eye reactions, discomfort, and allergic reactions can occur. Proper disinfection procedures are essential to minimize the chance of contamination and the potential of complications. (See 'Lens care and use' below.)

Replacement (disposal schedule) of soft lenses — Because soft lenses can absorb chemicals, and deposits can bind to the surface, all soft lenses require a replacement schedule based on the individual patient and the type of lens prescribed. The distribution of soft lenses fit by their recommended disposal schedule (also called modality) is shown in the figure (figure 3).

Quarterly – Three-month replacement lenses are generally more customized designs that are expensive to replace more frequently. Patients are educated to adhere to recommended cleaning and disinfection routines to maintain clean lenses and optimal corneal health. These lenses are dispensed in a four-pack for a one-year supply.

Monthly – Numerous brands of soft lenses are available in this modality. Again, daily cleaning and disinfection are required. Many patients find the monthly replacement routine easy to remember and lens cost is reasonable.

One to two weeks – Similar to the one-month disposable lenses, these require daily cleaning and disinfection. However, studies have shown that a majority of patients using these lenses are not compliant with their prescribed replacement schedule [10].

Daily – Daily disposable lenses are commonly and increasingly prescribed. In 2019, this modality became the most commonly prescribed modality of soft lens wear. Because they are disposed after each day’s use, no disinfection is required. Advantages of daily disposal include: no possibility of contact of disinfection solution with the eye, a new lens worn each day, and the need for cases and solutions and a nightly disinfection regimen. Daily disposable lenses are associated with reduced risk of corneal infiltrative events compared with monthly or one- to two-week soft lens disposal [11,12].

Special use soft lenses

Tinted soft lenses – Soft lenses can be tinted for cosmetic, therapeutic, or prosthetic purposes.

Cosmetic tints – Many soft lenses can be made in a variety of colors. The tints may be transparent to enhance natural eye color or can be opaque to dramatically change the color of the iris.

Therapeutic tints – These special tints are used for highly light-sensitive patients or to enhance color perception in patients with color deficiencies. Though these lenses do not fully compensate for color blindness, they are tinted red and worn to help color-deficient patients identify reds and greens more readily.

Prosthetic tints – Soft lenses can be tinted or hand-painted to improve cosmesis in patients with scarred corneas or to create an artificial pupil in patients with aniridia, albinism, or damaged/distorted pupils.

Bandage lenses – Soft lenses are used as bandage lenses in cases of corneal laceration, corneal exposure injury, and during the healing phase after some ocular surgery such as photorefractive keratectomy (PRK).

Piggyback fitting – In cases of highly irregular corneal curvature, as in keratoconus, a soft lens is placed on the cornea and a rigid contact lens is placed over it. The soft lens provides a more regular surface for the gas-permeable (GP) lens to ride upon and also acts to protect the cornea from irritation due to excessive movement of the RGP lens.

Soft lenses for keratoconus or irregular astigmatism – These specially designed lenses are custom-made with greater center thickness than standard soft lenses. The increased thickness makes the lens more rigid in an effort to correct more optical irregularity similar to rigid lenses [13].

Lenses to slow myopic progression – One soft multifocal lens, MiSight, has gained FDA indication to slow myopia progression in myopic children [14]. In 2021, the FDA approved ACUVUE Abiliti, an orthokeratology (ortho-K) design, for the “management of myopia.”

Rigid gas-permeable lenses — RGP contact lenses hold a specific shape, although they do have a small amount of flexure. Compared with soft contact lenses, RGP lenses generally provide better visual acuity and are more durable but require a longer period of time for adaptation. Although most patients will adapt to RGP lenses after four to seven days of wear, studies show there is significant individual variation (7 to 30 days) in the time for full adaptation [15-17]. RGP lenses are generally replaced after two to three years of use.

Prior to the development of RGP lenses, "hard" lenses were made of a plastic material, polymethylmethacrylate (PMMA). This material provided no oxygen permeability and depended on tear circulation to provide oxygen to the cornea. With the development of RGP lenses, PMMA materials are now rarely used.

Because of their rigidity, RGP lenses are often used to achieve optimal visual acuity in patients who have not had satisfactory acuity with soft lenses. RGP lenses are also generally better for those who have some degree of "dry eyes." RGP lenses are also used for ortho-K to provide good daytime acuity without the need to wear any correction during waking hours (figure 4) [18-21].

RGP lenses comprise approximately 5 to 10 percent of all contact lens fits in the United States, including 1 percent for Ortho-K [6]. There are more than 40 different RGP materials used today. As with soft lens materials, each type of RGP lens has unique characteristics in regard to wettability, oxygen transmission, and flexure [5]. Many of these lenses are more gas permeable than soft lenses.

Most RGP wearers use their lenses during the day. Some lenses are approved by the FDA for one week of continuous wear, and at least one brand has approval for 30 days of continuous wear.

RGP lenses are ordered by the eye care practitioner and made on a custom basis by an RGP lab. Providers can specify various parameters to optimize fit and comfort including diameter, base curve, power, peripheral curves, thickness, edge design, optical zone, as well as material and color.

Within the last 15 years, computer-guided lathes have been developed that allow manufacture of highly customized lenses. RGP lenses can be ordered in various diameters to fit the interpalpebral opening (7 to 9 mm), the corneal diameter (10.0 to 11.5 mm), the corneoscleral area (12 to 14 mm), or the sclera (15 to 24 mm).

Custom RGP designs may be the only vision correction option for patients with irregular corneal topography who are not adequately correctable with soft lenses or spectacles. Specialty GP designs include:

Reverse geometry – Steeper curvature peripherally than centrally

Quadrant-specific curves – Different curves in each quadrant of the lens

Toric/bitoric – Different curves horizontally versus vertically (on front and/or back surface)

Aspheric curves – Placed on front or back surface

Multifocal lenses – eg, aspheric, segmented, concentric

Ortho-K/corneal reshaping – Reverse geometry lenses of custom or proprietary design to temporarily change the corneal curvature to improve unaided acuity

Scleral lenses — Scleral RGP lenses are larger diameter (up to 24 mm) lenses prescribed to completely vault over the cornea. These lenses are designed to extend beyond the cornea to rest on the conjunctiva overlying the sclera and are comfortable to wear. Because they completely vault the corneal surface, they retain a reservoir of tears between the lens and the cornea. Scleral lenses are helpful for correcting vision in cases of irregular or distorted corneas in patients with conditions such as keratoconus, post-penetrating Keratoplasty (corneal transplant), post-refractive surgery, corneal scarring, and ocular surface disease (severe dry eyes, graft-versus-host disease, Stevens-Johnson syndrome, and ocular pemphigoid) [22,23].

Management and fitting of scleral lenses requires fitters to be specially trained to evaluate the unique characteristics and potential complications that can occur with these lenses. In addition, patients wearing scleral lenses require special techniques and solutions for application and removal, wear, and care.

Hybrid contact lenses — Hybrid contact lenses have a RGP central portion fused to a peripheral soft skirt. The first lens of this type (SoftPerm) was made of very low permeability materials and had limited parameters. The latest version of this innovative design (SynergEyes/Duette) is comprised of more permeable materials (both central RGP and peripheral soft) and with various parameters to allow fitting over a large range of unique corneal shapes [24,25]. These lenses are worn during the day and disposed of after six months of use.

The various designs of hybrid lenses can be made to correct for myopia, hyperopia, astigmatism, presbyopia (multifocal design), keratoconus, post-surgical eyes, and other irregular astigmatism cases.

Advantages of hybrid lenses are excellent acuity, greater comfort compared with RGP lenses, and a wide range of parameters and designs. Disadvantages of hybrid lenses are more difficult application and removal, and higher costs than other lenses.

Orthokeratology, overnight corneal reshaping

Mechanism and use — Ortho-K is the process of wearing a specially designed RGP contact lens while asleep to temporarily effect a change in the corneal curvature resulting in clear vision during waking hours without correction. In addition to various specific brand names, this technique is also referred to as:

Corneal refractive therapy (CRT)

Overnight corneal reshaping (OCR)

Vision-shaping treatment (VST)

Ortho-K

After wearing Ortho-K lenses, the cornea is reshaped via fluid forces created under the lens (figure 5). Studies demonstrate the changes effected by Ortho-K are epithelial, not stromal [19,26,27]. It is not a structural "bending" of the cornea, but rather a redistribution and relative thinning/thickening of the epithelial layer. The attached pre-/post-topography for Ortho-K demonstrates similarity to post-laser-assisted in situ keratomileusis (LASIK) topography with central flattening surrounded by a ring of steepening.

Ortho-K fitting techniques

Empirical – Pretreatment findings (refraction, central keratometry readings, horizontal visible iris diameter, and corneal eccentricity) are given to the lab who manufactures the calculated lens parameters.

Diagnostic – Lenses of known parameters are evaluated on the patient to assess movement, centration, and fluorescein pattern to determine the optimal lens parameters.

Topography-based – Baseline corneal topography is imported into a lens-design software program to simulate actual lens fit and custom-design the optimal lens parameters.

Patients go through a very short and easy adaptation to the treatment. Most patients report awareness of the lenses while their eyes are open but minimal to no awareness of the lenses while the eyes are closed. Application is similar to traditional RGP lenses, but removal is usually assisted with a suction device because of the larger diameter (from 10.0 to 11.8 mm).

Indications and efficacy — Candidates for Ortho-K include:

Myopes with less than 6.00 D of spherical correction and less than 1.50 D of astigmatism

Current contact lens wearers reporting discomfort (dryness and itching)

LASIK noncandidates

Children who are progressive myopes [28]

Post-LASIK patients who are under-corrected

Unaided vision improves significantly after the first night of wear. In a crossover study comparing vision-related quality of life (VR-QOL) with Ortho-K versus soft contact lenses worn during the day, approximately 70 percent of the subjects preferred Ortho-K and chose to continue this modality following the study [20]. Another study compared VR-QOL between Ortho-K wearers and subjects who received LASIK and found comparable ratings between the two groups [29].

Ortho-K has been shown to slow myopic progression [30-40]. A study that monitored axial length in children 8 to 16 years of age over a two-year period found that Ortho-K suppressed axial elongation in myopic children, suggesting that this treatment can slow the progression of myopia to a certain extent [36]. A consensus of meta-analysis studies indicate that children wearing Ortho-K show 40 to 60 percent less myopic progression and axial length increases than children wearing spectacles to correct their myopia [38,40-43]. In the United States, Ortho-K has FDA clearance to correct refractive error as indicated above. Ortho-K does not have FDA indication to slow myopia progression, therefore prescribing for such an indication is considered “off-label.”

Reversibility and safety — An important aspect of Ortho-K is reversibility. If patients elect to discontinue use of Ortho-K lenses for any reason, refraction will return to pretreatment levels within 5 to 10 days [44-46]. Changes to corneal topography are also reversible, but studies show it may take longer to return to baseline (3 to 12 weeks). Isolated studies have demonstrated that in some cases, topographic changes may not completely return to baseline [45,47].

Serious complications are rare but include microbial keratitis and corneal ulcers in patients using Ortho-K lenses. The overall incidence of microbial keratitis has been reported as 7.7 per 10,000 patient-years of wear [48]. Case reports, many outside the United States, indicate that most adverse events were due to patient and/or prescriber noncompliance, deviation from use of FDA-approved products, and the absence of certification training of the respective practitioners. In the United States, lens materials must be FDA-approved for overnight wear. The FDA requires certification training for practitioners before the manufacturers can ship the devices to them.

In a retrospective study of 300 children and adults over a 4.5-year period, there were three minor adverse events that required medical treatment [49,50]. They involved central epithelial defects from debris being trapped under the lens, and there was no loss of best-corrected visual acuity on follow-up after treatment. The potential for complications is small and can be minimized if patients are compliant with lens care, cleaning, and disinfecting procedures; providers and patients are diligent in performing proper follow-up visits; adequate lubricants are used; and lenses are replaced on a regular basis.

Increasing axial length associated with myopic progression is associated with increased risk of vision-threatening complications such as myopic maculopathy, peripheral retinal degeneration, retinal detachment, glaucoma, and cataract [51-53]. Risk of these complications has been shown to be higher than the risk of serious complications associated with ortho-K [54,55]. Ongoing study is essential to establish safety, risks, long-term effects, and the efficacy in the retardation of myopia progression.

Multifocal lenses — Each of the lens types described above (soft, RGP, and hybrid) are available in multifocal designs in various materials to correct for both distance and near vision needs simultaneously [56,57]. In late 2019, a soft multifocal lens, MiSight, was approved by the FDA to slow myopia progression in children.

Lens coatings — A relatively new development is a surface treatment to create a very wettable, more lubricious lens surface. This coating is composed of Hydra-Polyethylene Glycol (HPEG) and is approved for use on gas-permeable and soft lenses. The coating makes the surface very slippery and more resistant to bacterial adhesion and minimizes mucous or protein deposits [58-60].

Future developments — Investigation is ongoing to develop new designs, materials, and applications for contact lenses.

Drug delivery systems – Lenses impregnated with drugs for slow sustained release are being evaluated for antiinfective, antiinflammatory, and pressure-lowering indications [27,61,62].

Biometric monitoring – Contact lenses embedded with electronic sensors will be able to monitor various individual attributes such as blood sugar, intraocular pressure, driver alertness, and others.

Wavefront-corrected optics – Wavefront error measured for an individual patient is incorporated onto the front surface optics of a contact lens. Made in a lens design that is rotationally stable on the eye (soft or GP, but more likely in a scleral GP lens).

Enhanced optics – Specially designed lenses in combination with external wearable technology are being developed for heads-up display, telescopic/microscopic optics, or night-vision optics.

CONTACT LENS FITTING AND FOLLOW-UP — Contact lens fitting should be done by licensed clinicians who may be optometrists, ophthalmologists, or opticians. A thorough pre-fitting evaluation can identify risk factors and target the best lens and lens care combination to provide long-term comfort, good vision, and easy maintenance.

A thorough pre-fitting evaluation should include:

Refraction and visual acuity

Keratometry

Corneal topography

Biomicroscopic evaluation of the cornea, conjunctiva, and eyelids

Empirical and/or diagnostic fitting

Detailed instruction about application/removal, lens care, and solutions

Face-to-face, technician and patient

Pre-recorded video instructions

Written instructions

Links to YouTube videos

After initial dispensing, follow-up evaluations should include:

Biomicroscopic evaluation of the cornea, conjunctiva, and eyelids with and without contact lenses in place

Visual acuity

Review of care, handling, and disinfection procedures

New contact lens wearers should generally be seen within the first week, at one month, at six months, and annually thereafter. This frequency may vary with the type of lens, wearing schedule, and condition being treated.

LENS CARE AND USE

Length of lens use and wear — The absorption characteristics of soft lenses and their tendency to accumulate surface deposits can affect oxygen permeability and surface quality. Therefore, soft lenses allow only a limited time of safe and healthy usage and should be worn and disposed of on an individually prescribed schedule.

One-day lenses – These lenses are designed for one-day wear and are dispensed in a large supply (30 or 90 pairs). They are ideal for people who want to wear contact lenses intermittently, for those who place convenience as a high priority, and for those who have sensitivity to disinfecting solutions, since they do not require use of disinfecting solutions.

Two-week disposable – These lenses are worn for a maximum of two weeks. Most should be removed each night and cleaned and disinfected, but some have US Food and Drug Administration (FDA) approval for six days and nights of continuous wear.

Monthly disposable – Most of these lenses are also worn during the day and removed each night, although some have approval for 30 days of continuous wear.

Quarterly disposable – These lenses, designed to be replaced every three months, are generally custom-made lenses for prescriptions for high dioptric or astigmatic powers that are outside the parameters available in standard lenses.

The majority of soft lenses are prescribed for use only during waking hours and not intended to be worn while sleeping. However, there are soft lenses with FDA clearance for overnight/extended wear for up to 30 days of continuous use. Patients wearing lenses overnight must be informed of the risks, educated properly, and monitored carefully to prevent complications associated with overnight wear. Patients should be advised that there is evidence of increased risk of microbial keratitis with overnight contact lens use [63-66]. They should be informed of the warning signs of microbial keratitis (pain, redness, blurred vision, photophobia) and told to notify their clinician immediately if any of these symptoms occur. (See "Complications of contact lenses".)

Individual patient response to overnight/extended wear of soft lenses varies. While some patients can wear lenses continuously for 30 days without complications, others may show lens deposits, debris accumulation under the lens, corneal edema, corneal infiltrates, or other potentially serious complications. These apparently minor complications can potentially result in sight-threatening conditions. Length of lens wear should be individually prescribed for each patient based on corneal response, with emphasis on long-term eye health. Making patients aware of the symptoms and procedures to follow is essential to ensure prompt attention and treatment of any symptoms.

Potential complications — Serious conditions associated with contact lenses include corneal ulcers [8], corneal abrasions, and infectious keratitis (see "Complications of contact lenses"). Evaluation by an eye care provider should be arranged promptly if patients experience blurred vision, pain, or photophobia. Contact lens wear and/or lens care solutions can provoke many eye reactions. Other sources of eye irritation are lens deposits (chemical or mechanical), foreign bodies trapped under the contact lens, and allergic reactions to preservatives in the care solutions (see "Complications of contact lenses"). Infection risk can be minimized by following proper procedures for contact lens care.

Common problems include [8,67]:

Redness

Itching

Contact lens-induced dry eye (CLIDE)

Allergic reactions

Conjunctivitis (microbial, mechanical, allergic, or inflammatory) [67]

Contact lens-induced acute red eye (CLARE)

CLARE refers to a sudden red eye reaction in a contact lens wearer of unknown etiology. Possible causes are lens deposits, low-grade infection, chemical sensitivity to solution, or external chemical contamination of the contact lens.

Patients experiencing minor redness or itching without pain or blurred vision should be treated initially with removal of contact lenses and lubricating eye drops (artificial tears or over-the-counter antihistamine drops). They should be advised to report any increase in severity of symptoms or development of blurred vision or pain. After resolution of minor irritations, future episodes may be prevented by changing the replacement schedule, reducing wearing time, or changing the lens type or disinfection regimen.

Solutions are used to clean and disinfect contact lenses. Reports of serious infections have been related to contaminated lens cases and inadequate patient disinfectant technique [68]. (See "Clinical manifestations and diagnosis of Fusarium infection", section on 'Keratitis' and "Free-living amebas and Prototheca", section on 'Keratitis'.)

Guidelines for prevention of infectious keratitis — Following national and international reports of Acanthamoeba keratitis and Fusarium keratitis, the FDA reviewed proper care of contact lenses. Citing the risk of eye infections and corneal ulcers, with the potential to cause blindness, the FDA has issued recommendations and guidelines for the safe use of contact lenses and associated care products [69]. These recommendations follow:

Replace your contact lens storage case every three to six months.

Always wash your hands before handling contact lenses to reduce the chance of getting an infection.

Remove the lenses immediately and consult your eye care professional if your eyes become red, irritated, or your vision changes.

Always follow the directions of your eye care professional and all labeling instruction for proper use of contact lenses and lens care products.

Use contact lens products and solutions recommended by your eye care professional.

Do not use contact lens solutions that have gone beyond the expiration or discard date.

Rub and rinse your contact lenses as directed by your eye care professional.

Clean and disinfect your lenses properly following all labeling instructions provided with your lens care products.

Do not "top-off" the solutions in your case. Always discard all of the leftover contact lens solution after each use. Never reuse any lens solution.

Never use nonsterile water (distilled water, tap water, or any homemade saline solution). Exposure of contact lenses to water has been associated with Acanthamoeba keratitis, a corneal infection that is resistant to treatment and cure.

Do not put your lenses in your mouth to wet them. Saliva is not a sterile solution.

Clean, rinse, and air-dry your lens case each time lenses are removed. You may want to flip over your lens case while air drying so excess solution may drain out of the case. Contact lens cases can be a source of bacterial growth.

Do not transfer contact lens solutions into smaller travel size containers. This can affect the sterility of the solution, which can lead to an eye infection. Transferring solutions into smaller size containers may also leave consumers open to accidentally using a solution not intended for the eyes.

A synopsis of these recommendations is presented in a table (table 3). Additional contact lens information from the FDA is available.

Soft lens solutions

Multipurpose solutions — Multipurpose solutions (MPS) are the most commonly used, accounting for 89 percent of solutions used in the United States versus 11 percent for peroxide systems in 2008 [6]. MPS are intended to be "all-in-one" solutions that are used to rinse, clean, disinfect, and store lenses during their overnight soaking. Success with MPS depends on patient compliance with directions, particularly in regard to sufficient rinsing or, preferably, using a rub technique to achieve adequate disinfection [70,71]. Individual patient tolerance is variable, and some find that MPS provoke an allergic or toxic reaction when exposed to the eye. Available MPS in the United States are shown in a table (table 4). Generic versions are also available at various retailers.

While MPS offer several conveniences compared with other contact lens solutions (ready availability, one bottle, ease for travel), MPS are also more likely to cause allergic or sensitivity reactions, and corneal staining is more prevalent than with peroxide systems.

Peroxide systems — Peroxide systems have gained in popularity over the last few years. They use hydrogen peroxide to disinfect and passively oxidize surface deposits. These systems require that the peroxide solution be "neutralized" prior to lens use, and different techniques (one- or two-step) are used for accomplishing this. Peroxide systems are very effective in disinfecting contact lenses and are associated with lower risk of microbial keratitis and corneal infiltrative events compared with multipurpose solutions [72]. Commercially available peroxide systems in the United States are shown in a table (table 4).

Peroxide disinfection, compared with MPS, provides no direct exposure of preservatives to the eye, and there is a lower likelihood of sensitivity reactions. Lens cleaning is passive, by oxidation, and there is less risk for noncompliance resulting in infection. However, peroxide systems lack the convenience factors of MPS, are more difficult to travel with, and pose the potential for irritation from accidental exposure to non-neutralized solution.

Rigid gas-permeable solutions — Solutions to be used with rigid gas-permeable (RGP) lenses vary in viscosity, wetting agent, and preservatives. One to three bottles contain solutions designed for wetting/conditioning, disinfecting, and cleaning. Available solutions in the United States are shown in a table (table 4).

Lubricating drops — There are numerous brands of drops that are designed to be used with contact lenses as lubricants at frequencies varying from once per day to hourly. These may be labeled as "contact lens drops," artificial tears, or lubricating drops. Available lubricants differ in their viscosity and whether or not they are preserved, and individual patient preference and tolerance will determine which is most acceptable. Compatibility with patient's eyes, individual contact lens brand, and other solutions patients use are also factors.

Preservative-free saline — Due to specific lens requirements or patient sensitivity, preservative-free saline may be recommended for lens application or for lens rinsing. These are available in single-use vials from 3 to 15 mL from various manufacturers. Examples include Nutrifil, Purilens, LacriPure, and Scleralfil.

Application and removal technique — Application of all types of lenses involves, with clean hands, placing the prepared lens directly on the center of the cornea while holding the lids and lashes to create an opening large enough to clear the diameter of the lens.

Removal techniques vary with the lens type:

Soft lenses are removed by pinching the edges of the lens at the four and eight o'clock positions with the thumb and index finger while holding the upper lid out of the way. Instilling a drop of wetting solution or artificial tears makes removal easier.

If the lens is not readily removed, it is possible the lens is in the eye but not on center. The most likely place to find the dislocated lens is under the upper lid (usually folded). To check for this, have the patient look straight down while holding the upper lid up as high as possible. If found, the lens can be pinched out from there. It may be necessary to evert the lid to find a lens that has adhered to the inner surface of the upper lid.

RGP lenses can be removed with a rubbery contact lens removal tool (suction cup) or by manipulating the upper and lower lid simultaneously together against the edges of the lens. If an RGP lens is off-center, it is best to try to manipulate the lens position through the closed lids to a position of easy access, usually the temporal sclera, and then use the suction cup device.

Hybrid lenses are removed similar to soft lenses, except that the area of pinching must be smaller, at the edge of the soft portion of the lens. Pinch the soft portion with both fingers at the six o'clock position. With this lens, it is critical that the upper lid be held away from the surface of the lens to allow for removal. Again, lubricating drops inserted in the eye can help remove a lens that is not coming out easily.

SUMMARY AND RECOMMENDATIONS

Contact lenses may be categorized by their compositional material, wearing schedule, disposal schedule, permeability, water content, and type of correction. (See 'Lens types' above.)

Soft lenses can be classified by a number of properties, including oxygen permeability, water content, and structural consistency (table 1). Silicone hydrogel lenses, with increased oxygen permeability, are often considered preferable. (See 'Hydrophilic/soft lenses' above.)

Rigid gas-permeable (RGP) contact lenses hold a specific shape, although they do have a small amount of flexure. Compared with soft contact lenses, RGP lenses generally provide better visual acuity and are more durable, but they typically require a gradual increase in wearing time, which is called adaptation. (See 'Rigid gas-permeable lenses' above.)

Orthokeratology (Ortho-K) involves wearing special lenses overnight to correct spherical and astigmatic refractive error within specified ranges. Ortho-K should be prescribed and monitored by certified practitioners. Ongoing study is essential to establish safety, risks, long-term effects, and the efficacy of Ortho-K to slow or stop myopia progression. (See 'Orthokeratology, overnight corneal reshaping' above.)

Contact lens fitting is performed by optometrists, ophthalmologists, or certified opticians, who also provide follow-up evaluation. (See 'Contact lens fitting and follow-up' above.)

Multipurpose solutions (MPS) are the most commonly used solutions in the United States to rinse, clean, disinfect, and store lenses during their overnight soaking. Compared with peroxide systems, MPS are more convenient but are more likely to cause allergic or sensitivity reactions and are associated with a higher rate of microbial keratitis. Peroxide disinfection systems provide no direct exposure to preservatives but are somewhat less convenient to use. (See 'Multipurpose solutions' above and 'Peroxide systems' above.)

Complications of contact lens wear are common. Conditions associated with contact lenses include corneal abrasions, various noninfectious complications, and microbial/infectious keratitis which can cause permanent vision loss. (See 'Potential complications' above and "Complications of contact lenses".)

Infection risk can be minimized by following proper procedures for contact lens care. Proper use of contact lenses and hygienic recommendations were issued in a guideline from the US Food and Drug Administration (FDA) and are summarized in the table (table 3). (See 'Guidelines for prevention of infectious keratitis' above.)

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