For photophobia, grey tints are often prescribed because they distort all colors equally. Polaroid lenses minimize glare while driving, boating, or skiing by blocking horizontal light waves. The sun's harmful, ultraviolet UVA and UVB rays may cause skin cancer, photokeratitis, pinuecula (Fig. 6.50) and pterygium (Fig. 6.49a), while hastening the onset of cataracts and macular degeneration. Tinted lenses, including polaroid lenses, should have a UV filter added to remove 98–100% of these rays. Branded photochromic glass lenses and Transitions plastic lenses darken in sunlight and have a UV filter.

Sports injuries, especially basketball, baseball, ice hockey, and racket games, are the leading causes of blindness in children. Protective eye wear could prevent 90% of these sportsrelated injuries.

Contact lenses

Plastic contact lenses, invented in 1947, are now worn by 34 million Americans, usually as an alternative to spectacles, to correct myopia, hyperopia, astigmatism, and presbyopia (Fig. 2.21).

Fig. 2.21 Plastic contact lens.

Candidates for contact lenses

This text will discuss soft lenses since they account for 95% of fittings. Hard and gaspermeable contacts may be preferred less often for cases of dry eye, astigmatism, and irregularly shaped corneas in keratoconus (see Figs 6.42 and 6.43)

Relative contraindications to contact lens wear:

•significant allergies

•lid margin infection

•conjunctivitis

•dry eyes

•very young children or elderly

Fitting contact lenses

Keratometry (Fig. 2.23)

After the refraction for spectacles, the corneal curvature is measured with a keratometer. This determines whether to fit a flatter or steeper lens (see Fig. 2.27). The keratometer also reveals distortion of the cornea from unhealthy contact lens wear (Fig. 2.24) or other corneal diseases.

Power (P), base curvature (BC), and diameter (DIA) are the three basic variables that are usually required to order all types of soft lenses (Fig. 2.25).

Determination of lens power

The power of a contact lens is not always the same as the patient’s spectacle correction. Place the contact lens with the spectacle power on the eye. Then, refine it with an over-refraction. The lens should completely cover the cornea and extend just beyond

Fig. 2.22 Contacts are great for almost every sport.

Fig. 2.23 Keratometer.

Fig. 2.24 Circular images projected on a damaged cornea have distorted keratometric readings.

Fig. 2.25 (a) 13.5 mm diameter.

(b) 14.5 mm diameter.

the entire limbus (corneal-conjunctival junction) (Fig. 2.26) and move 0.5 to 1.0 mm on each blink. If adequate concentration is not achieved, a different base curve or diameter may be tried.

Types of contact lenses

Most people wear contacts during the day only ("daily wear"). Less often, sleep in lenses ("extended wear") are used, since they have a rate of infection that is 5 times as great as daily wear. Lenses may be replaced yearly, but are more commonly disposed of every 2 weeks to three months ("frequent replacement") or on a daily basis ("disposable"). The frequency of replacement depends on comfort and the rate of mucus accumulation (Fig. 2.28).

Astigmatism lenses are preferred when the astigmatism correction is –0.75 or more. They are elliptical in shape with markings at 90° or 180° axis and weighted at 6 o'clock so they don't rotate (Fig. 2.29). When placed on the eye, these lines should line up close to those axes or an adjustment is made in the axis (Fig. 2.30).

Presbyopic bifocal contact lenses are not highly successful but may be tried for motivated patients—often over age 40—who have problems focusing up close (Fig. 2.31). An alternative to a bifocal contact lens in correcting a presbyopic patient is to use a standard spherical contact lens, making one eye focused for near and the other focused for distance. This is called monovision. Usually, the eye with the clearest vision is chosen for distance.

Fig. 2.29 Lens properly aligned on eye with center marking at 180°.

Fig. 2.26 Contact lens properly overlapping limbus.

Fig. 2.27 (a) Steep base curve 8.2 mm.

(b) Flat base curve 9.1 mm.

Fig. 2.28 Mucus deposits on contact lenses.

Fig. 2.30 Lens settled on to eye rotated 10° counterclockwise.

Fig. 2.31 Acuvue bifocal contact lens with concentric zones of alternating near and far vision.

The iris color could be enhanced with transparent tinted soft lenses or changed to a different color with opaque tinted soft lenses (Fig. 2.32).

No patient should leave the office without feeling adept at lens insertion and removal, realizing the importance of good handwashing techniques, and having knowledge about the use and differences between disinfecting, cleaning, and rinsing (saline) solutions (Figs 2.33–2.35).

Fig. 2.34 Remove lens by sliding it off cornea onto sclera and then gently pulling it off using thumb and index finger.

Fig. 2.32 Colored contact lenses.

Fig. 2.33 Place contact lens directly on the cornea using the tip of the index finger for the contact, the middle finger to hold lower lid down, and the finger of the other hand to lift upper lid.

Fig. 2.35 Contact lens solutions.