Contact Lenses

A contact lens sits on the tear film, moves with the eye and almost touches the cornea. Those three facts explain nearly everything that makes it optically different from a spectacle lens: it changes the power needed, the size of the image and the effort of near work, a rigid lens forms a lens out of the tears behind it, and a lens resting on living tissue has to let oxygen through.

#Contact lenses and spectacles

Moving the correction from about 12 mm in front of the eye to the cornea changes the power it needs (Chapter 6): a myope needs less minus in a contact lens, and a hyperope more plus. A −8.00 D spectacle correction becomes about −7.25 D at the cornea, and +8.00 D becomes about +8.75 D; below about ±4 D the difference is negligible. The image also changes size (Chapter 14): a myope sees the world larger through contact lenses than through spectacles, and a hyperope smaller.

Because a contact lens moves with the eye, the eye always looks through its optical centre. There is no prism when the eyes turn, so no vertical imbalance in anisometropia, no ring scotoma, and no frame in the way. Near work changes too, for two reasons. A myope looking down and inward through minus spectacles gains base-in prism, which spares some convergence; a contact lens moves with the eye and gives none. And because the spectacle lens sits 12 mm in front of the eye, bringing a target from distance to 40 cm changes the vergence arriving at the cornea of a −8.00 D myope by only about 2.1 D, against the 2.5 D an emmetrope needs. A contact lens, at the cornea, gives no such saving. Myopes therefore accommodate and converge more with contact lenses than with spectacles, and hyperopes less, which is why a myope in early presbyopia often notices reading difficulty first in contact lenses:

#Materials and oxygen

The cornea takes most of its oxygen from the air through the tear film, so a lens must let oxygen through. A material’s oxygen permeability is its Dk; what reaches the cornea depends on the transmissibility Dk/t, which falls as the lens gets thicker. Holden and Mertz estimated that avoiding corneal swelling in open-eye wear needs a Dk/t of about 24 × 10⁻⁹ units, and that keeping overnight swelling to the 4% the closed eye shows without any lens needs about 87 × 10⁻⁹; later work put the value needed to avoid stromal hypoxia overnight higher still.

• Hydrogel soft lenses carry oxygen dissolved in their water, so their Dk rises with water content but remains modest.
• Silicone hydrogel lenses transmit oxygen through silicone and reach several times the Dk of hydrogels, enough for overnight wear, although they have not reduced the risk of infection.
• Corneal rigid gas-permeable lenses (mostly fluorosilicone acrylates) are smaller than the cornea and pump tears beneath them with every blink. Their predecessor, PMMA, let no oxygen through at all, and caused central corneal oedema.

Chronic hypoxia shows as epithelial microcysts, stromal oedema with striae and folds, limbal redness and, eventually, corneal neovascularisation.

#Rigid lenses and the tear lens

A rigid lens keeps its shape, so the space between its back surface and the cornea fills with tears. The tears form a lens of their own, the tear lens, whose front surface is the back of the contact lens and whose back surface is the cornea. Its power is the difference between the lens’s base curve and the cornea’s curvature, both expressed in keratometric dioptres. A lens fitted steeper than K traps a plus tear lens, so the contact lens needs extra minus; a lens fitted flatter than K forms a minus tear lens and needs extra plus. The mnemonic is SAM-FAP: steeper add minus, flatter add plus. Near normal corneal curvatures each 0.1 mm of radius is worth about 0.50 D.

Fluorescein shows the tear lens directly: in cobalt blue light, thick layers of tears glow green and areas of touch look dark. A steep lens pools tears centrally, sometimes with bubbles, and bears on the mid-periphery; a flat lens touches centrally, pools at the edge and moves too much; an aligned lens shows a thin, even layer, with a bright ring under its flatter edge:

To find the power of a rigid lens, move the spectacle prescription to the corneal plane, then correct for the tear lens:

For a −8.00 D myope with K = 43.00 D fitted with a base curve of 43.50 D: −8.00/(1 + 0.012 × 8) = −7.30 D at the cornea, and the lens is 0.50 D steeper than K, so it needs −0.50 D more: about −7.75 D.

#Masking corneal astigmatism

The tear lens fills whatever shape lies behind the rigid lens. On a toric cornea it takes up the toricity, and because tears and cornea have almost the same refractive index (1.336 and 1.376), the corneal astigmatism nearly vanishes optically: the eye now has a spherical front surface, the front of the contact lens. Only the astigmatism of the rest of the eye, the back surface of the cornea and the crystalline lens, remains:

So a patient whose spectacle cylinder matches their corneal astigmatism does well in a spherical rigid lens, while one with significant internal astigmatism is left with it (the comparison should strictly be made at the same plane, which matters only for large cylinders). Above about 2.5 to 3 D of corneal astigmatism, a spherical lens rocks on the toric cornea and a toric back surface fits better; because a toric back surface itself induces cylinder (about half its back toricity), such lenses are usually bitoric. Soft lenses drape over the cornea and form almost no tear lens, so they do not mask corneal astigmatism: low astigmatism, up to about 0.75 D, is often left uncorrected with a spherical lens of the spherical equivalent, and more needs a soft toric lens.

#Soft lenses: the fit

A soft lens is larger than the cornea, usually 13.5 to 14.5 mm across, with a base curve of about 8.3 to 9.0 mm. It drapes over the eye, and what decides how tightly it fits is its sagittal depth, the height of the lens over its diameter, compared with the depth of the eye over the same width. A steeper base curve or a larger diameter makes a deeper lens and a tighter fit:

A well-fitting soft lens centres, covers the cornea in every direction of gaze, and moves a fraction of a millimetre with each blink. A tight lens hardly moves and resists the push-up test; vision clears briefly just after a blink, and the limbus may be red or indented by the end of the day. A loose lens moves too much, decentres, and its edge may stand off or flute; vision blurs just after a blink. An inside-out lens behaves like a loose one. To loosen a tight lens, flatten the base curve or reduce the diameter.

#Soft toric lenses

A soft toric lens works only if it keeps its orientation on the eye. It is stabilised by prism ballast (a thicker base below), by thin zones above and below that the lids hold in place, or by truncation, and it carries reference marks, often at 6 o’clock, so the examiner can see how it has settled. If the marks have rotated, the axis ordered must allow for it. The rule, as the examiner sees the lens, is LARS: left add, right subtract. A lens that settles 10° to the left (clockwise) carries its axis 10° lower, so the next lens is ordered with its axis 10° higher:

A rotated toric lens leaves $2C\sin \theta$ of astigmatism (Chapter 13), so stable rotation matters more the higher the cylinder. Rotation that varies from blink to blink cannot be compensated, and needs a different design.

#Presbyopia

Presbyopic contact lens wearers have three main options:

• Reading glasses over distance contact lenses: simple, with full binocular vision.
• Monovision: one eye, usually the dominant one, corrected for distance and the other for near. Most people adapt, but stereopsis and binocular summation suffer, which matters for some occupations and for night driving.
• Multifocal lenses. Soft multifocals work by simultaneous vision: concentric or aspheric zones put a focused and an out-of-focus image on the retina together, and the brain attends to the sharper one, at a cost in contrast and with haloes, all depending on pupil size. Rigid translating bifocals rest on the lower lid, so the near segment rises in front of the pupil as the eye looks down.

#Special lenses

• Keratoconus: a rigid lens replaces the irregular cornea with a smooth optical surface and corrects irregular astigmatism that spectacles cannot. When corneal lenses are uncomfortable or unstable, options include a rigid lens over a soft one (piggyback), hybrid lenses with a rigid centre and soft skirt, and scleral lenses.
• Scleral lenses rest on the sclera and vault the whole cornea over a reservoir of saline. Besides irregular corneas, they protect and hydrate the ocular surface in severe dry eye, graft-versus-host disease, Stevens–Johnson syndrome and exposure.
• Orthokeratology uses a reverse-geometry rigid lens, worn overnight, to flatten the central cornea by thinning its central epithelium. Most of the correction, of low to moderate myopia, is lost within a few days if wear stops, although full recovery can take weeks. In children it slows axial elongation, roughly halving it in trials, but overnight wear carries the risk of microbial keratitis, including Acanthamoeba.
• Bandage lenses protect a healing epithelium after abrasions, recurrent erosion or surgery, and relieve pain (but not an abrasion in a contact lens wearer, which needs antibiotic cover against Pseudomonas and is neither patched nor bandaged). Infants left aphakic after cataract surgery are often corrected with high-plus contact lenses, such as silicone elastomer lenses, which transmit oxygen well enough for extended wear.
• Cosmetic and prosthetic lenses change or mask the colour of the iris or cover a disfigured eye. Decorative lenses bought without a fitting are a recognised cause of serious infection.

#Complications

Microbial keratitis is the most serious. Its annual incidence is about 2 per 10,000 daily wearers of hydrogel lenses but 20 to 25 per 10,000 when hydrogel or silicone hydrogel lenses are worn overnight, and overnight wear is the largest single risk factor; silicone hydrogels have not removed it. Poor lens-case hygiene, smoking and water exposure add to the risk. Pseudomonas aeruginosa is the commonest organism. Acanthamoeba keratitis follows exposure of lenses to tap water, swimming pools, hot tubs or home-made saline; it causes pain out of proportion to the signs, radial perineuritis and a ring infiltrate. Outbreaks of Fusarium and Acanthamoeba keratitis have been traced to particular multipurpose solutions. Contact lens wearers with a painful red eye need urgent assessment, and a lens wearer’s corneal ulcer is treated as bacterial until proved otherwise.

Other complications include:

• Giant papillary conjunctivitis: large papillae on the upper tarsal conjunctiva, with itching, mucus and excessive lens movement, a response to mechanical irritation by the lens and to deposits on its surface (it is also seen with exposed sutures and ocular prostheses). It improves with frequent replacement or daily disposable lenses, better hygiene or a break from wear.
• Sterile infiltrates and the contact lens peripheral ulcer: small, peripheral and less painful than infection.
• Hypoxic changes: microcysts, oedema and neovascularisation, managed with higher-Dk materials and less overnight wear.
• Corneal warpage, especially from rigid lenses: an irregular topography that can mislead refraction, keratometry and biometry. Lenses are left out before refractive surgery or cataract biometry, soft lenses for one to two weeks and rigid lenses for at least three, longer if the topography is still changing.
• Dry eye, superior limbic keratoconjunctivitis and solution reactions, and with rigid lenses, staining of the cornea at 3 and 9 o’clock and, after years of wear, ptosis. Long-term hypoxia changes the endothelium (polymegethism).
• Lens care prevents much of this: rub and rinse lenses with fresh solution, never water; never top up old solution; replace the case regularly; and neutralise hydrogen peroxide systems before the lens goes back in the eye.

#Recap

• At the cornea, myopes need less minus and hyperopes more plus. Eye movements induce no prism with contact lenses; myopes accommodate and converge more with them than with spectacles.
• Oxygen reaching the cornea depends on Dk/t: about 24 × 10⁻⁹ for daily wear and 87 × 10⁻⁹ for overnight wear (Holden–Mertz).
• Tear lens = base curve − K (keratometric dioptres): steeper add minus, flatter add plus; about 0.50 D per 0.1 mm. Lens power = spectacle power at the cornea minus the tear lens.
• A spherical rigid lens masks corneal astigmatism; the internal astigmatism remains. Soft lenses do not mask it.
• A deeper soft lens (steeper base curve or larger diameter) fits more tightly. Tight: little movement, vision clears after a blink.
• Soft toric rotation, as the examiner sees it: left add, right subtract.
• Presbyopia: monovision costs stereopsis; simultaneous-vision multifocals cost contrast.
• Overnight wear is the main risk factor for microbial keratitis. Pseudomonas is commonest; think Acanthamoeba after water exposure.

#Test yourself

#Further reading

• American Academy of Ophthalmology, Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation, chapter on contact lenses.
• E. S. Bennett and V. A. Henry (eds.), Clinical Manual of Contact Lenses. Wolters Kluwer.
• B. A. Holden and G. W. Mertz, Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Investigative Ophthalmology & Visual Science 25 (1984) 1161–1167.
• F. Stapleton et al., The incidence of contact lens–related microbial keratitis in Australia. Ophthalmology 115 (2008) 1655–1662.