Refractive Surgery

Refractive surgery corrects the eye’s error inside the eye or on its surface, so that no lens has to be worn. There are two approaches: reshape the cornea, which supplies two-thirds of the eye’s power, or add or replace a lens. Each is applied optics with a biological price, and choosing between them rests on a few measurements and some arithmetic.

#Reshaping the cornea with a laser

The excimer laser (argon fluoride, 193 nm, ultraviolet) breaks molecular bonds in the corneal stroma and removes a fraction of a micrometre of tissue with each pulse, with only minimal thermal damage to the tissue around: photoablation. To correct myopia it removes more tissue centrally, flattening the cornea; for hyperopia it removes a ring of tissue in the mid-periphery, deepest at the edge of the optical zone, steepening the centre (hyperopic treatments need a larger total diameter and regress more); for astigmatism an elongated, cylindrical ablation flattens the steepest meridian or steepens the flattest. The depth needed to correct D dioptres of myopia over an optical zone S millimetres wide is given by Munnerlyn’s formula:

So −6.00 D over a 6.0 mm zone takes about 72 µm, and widening the zone to 6.5 mm takes 85 µm. A larger zone reduces haloes at night, when the pupil may be wider than the treated area, but costs tissue. The femtosecond laser (near-infrared, about 1030 to 1060 nm) works differently: by photodisruption each focused pulse creates a tiny plasma that expands into a bubble of water vapour and carbon dioxide; rows of bubbles cleave the stroma beneath an intact surface, cutting a flap or a lenticule.

#PRK, LASIK and SMILE

• Photorefractive keratectomy (PRK, and its variants LASEK and trans-epithelial PRK) removes the epithelium and ablates the surface. There is no stromal flap, so no flap complications and less weakening of the strong anterior stroma, but recovery is slower and painful, and corneal haze can form, peaking one to three months after surgery, mainly after larger corrections; mitomycin C (usually 0.02%) applied briefly to the stroma reduces it.
• LASIK ablates the stroma beneath a hinged flap, cut by a microkeratome or a femtosecond laser. Vision recovers within a day, with little pain. The flap brings its own complications and severs more corneal nerves, so dry eye is common for months.
• SMILE (small-incision lenticule extraction, approved in the United States for myopia with up to 3 D of astigmatism) uses the femtosecond laser to cut a lens-shaped lenticule within the stroma, which is removed through a small incision, without a flap or an excimer laser.

PRK is preferred for thin corneas, epithelial basement membrane dystrophy, and patients at risk of eye trauma, such as those in contact sports or the military. LASIK typically treats myopia up to about −10 D, astigmatism up to about 5 or 6 D and hyperopia up to about +4 to +5 D; hyperopic corrections regress more.

#How much cornea must stay

A cornea that is thinned too much, or that was weak to begin with, can bulge forward over months or years: ectasia, a progressive steepening and thinning that resembles keratoconus. The laser removes tissue, and a LASIK flap adds little strength, so planning counts what is left:

Most surgeons leave at least 250 to 300 µm under a LASIK flap. A newer measure, the percent tissue altered, (flap + ablation) ÷ corneal thickness, flags risk at 40% or more. Most important of all is the shape of the cornea before surgery: subtle inferior steepening, asymmetry or posterior elevation on topography and tomography (forme fruste keratoconus) is the strongest risk factor, together with young age, thin corneas and high corrections. Established ectasia is managed like keratoconus: cross-linking to halt progression; rigid, hybrid or scleral lenses for vision; ring segments or topography-guided PRK with cross-linking in selected eyes; and keratoplasty if these fail.

#Aberrations

Spectacles correct only defocus and astigmatism, the lower-order aberrations. An aberrometer measures the whole wavefront leaving the eye. In a Hartmann–Shack aberrometer, light from a spot on the retina emerges through an array of tiny lenslets, each focusing its piece of the wavefront to a spot; where the wavefront is tilted, the spot moves, so the pattern of spots maps the local slope of the wavefront. The result is described as a sum of Zernike polynomials, each with a characteristic shape and blur. Beyond defocus and astigmatism come the higher-order aberrations: coma, trefoil and spherical aberration.

The normal cornea is prolate, flatter toward the periphery (asphericity Q about −0.26), which keeps its spherical aberration small. Myopic ablation flattens the centre more than the periphery and makes the cornea oblate, so positive spherical aberration rises, and with it haloes and glare when the pupil dilates at night. Hyperopic ablation does the opposite. A decentred ablation adds coma.

Modern ablation profiles limit these effects. Wavefront-optimised profiles add pulses in the periphery to preserve more of the cornea’s asphericity and so induce less spherical aberration; wavefront-guided profiles treat the whole wavefront measured in that eye, sphere, cylinder and higher-order aberrations; topography-guided profiles regularise an irregular cornea. Centration matters: in eyes with a large angle kappa, typically hyperopes, the pupil centre lies well away from the visual axis, and the ablation is centred nearer the corneal light reflex.

#Complications of LASIK

• Flap problems: an irregular, thin or buttonholed flap, a free cap, or folds (striae) that blur vision and may need the flap lifting and refloating.
• Diffuse lamellar keratitis: sterile inflammation in the interface in the first few days, with a fine granular haze (“sands of the Sahara”). It is treated with intensive topical steroids, and irrigation of the interface if severe.
• Pressure-induced stromal keratopathy: a steroid response raises the intraocular pressure and drives fluid into the interface. It can look like diffuse lamellar keratitis, but more steroid makes it worse. Applanation over the central cornea reads falsely low because of the fluid, so pressure is measured peripherally; treatment is to stop the steroid and lower the pressure.
• Central toxic keratopathy: central stromal opacity and thinning, with a hyperopic shift, a few days after surgery; unlike diffuse lamellar keratitis it does not respond to steroids, and it usually clears slowly on its own.
• Infection: within the first days usually Gram-positive bacteria, including MRSA; weeks later, atypical mycobacteria or fungi. It is managed by lifting the flap, culturing and irrigating the interface.
• Epithelial ingrowth under the flap edge (more likely after enhancements and hyperopic treatment), dry eye, and night-vision symptoms: haloes, glare and starbursts. With microkeratomes, a steep cornea (over about 48 D) risks a buttonholed flap and a flat one (under about 41 D) a free cap.
• Under- or over-correction and regression, sometimes needing an enhancement.

#Lenses inside the eye

For high myopia beyond the safe range of laser surgery, or a cornea too thin for it, a phakic IOL adds a lens while keeping the crystalline lens. The implantable collamer lens sits behind the iris, in front of the crystalline lens; the space between them, the vault, should be about 250 to 750 µm. Too little vault risks anterior subcapsular cataract, too much risks angle closure and pigment dispersion; older designs need a peripheral iridotomy against pupillary block, while newer ones have a central hole. Iris-fixated lenses clip to the mid-peripheral iris in the anterior chamber and need a deep enough chamber and regular checks of endothelial cell counts. Phakic IOLs need an adequate anterior chamber depth: for the collamer lens, at least 3.0 mm (endothelium to lens) in the United States and 2.8 mm in some other markets, and about 3.2 mm for iris-fixated lenses, with an endothelial cell count above the age-adjusted minimum.

Refractive lens exchange removes a clear crystalline lens and implants an IOL. It suits presbyopic patients, especially hyperopes, but gives up accommodation and, in high myopes, raises the risk of retinal detachment.

#Incisions and cross-linking

Radial keratotomy (RK), now abandoned, flattened the central cornea with deep radial cuts. Its legacy is a cornea whose refraction fluctuates through the day, drifts toward hyperopia over the years and can rupture with trauma. Astigmatic keratotomy and limbal relaxing incisions are still used: an incision across the steep meridian flattens it, and the meridian 90° away steepens by roughly the same amount, so the spherical equivalent is barely changed (a coupling ratio of about 1).

Corneal cross-linking is not a refractive procedure, though it is sometimes combined with one (for example, topography-guided PRK with cross-linking). Riboflavin is soaked into the stroma and activated with ultraviolet A light (about 370 nm), creating new bonds between collagen fibres that stiffen the cornea and halt the progression of keratoconus and ectasia. Riboflavin absorbs most of the ultraviolet within the stroma; the standard protocol requires at least 400 µm of stroma, so that the endothelium beneath is protected.

#Who is a candidate

• An adult with a stable refraction (little change over at least a year) and realistic expectations.
• Normal corneal topography and tomography, and enough corneal thickness for the planned correction.
• A healthy ocular surface: dry eye, blepharitis and exposure are treated first. Uncontrolled autoimmune or collagen vascular disease, pregnancy and a history of herpes simplex keratitis are relative contraindications.
• An understanding that presbyopia will still come; monovision is an option, and should be tried with contact lenses first.

#Recap

• Excimer (193 nm): photoablation. Femtosecond (about 1030 to 1060 nm): photodisruption, for flaps and lenticules.
• Munnerlyn: depth ≈ $S^{2}D/3$ µm. Residual stromal bed = thickness − flap − ablation (keep at least 250–300 µm); percent tissue altered ≥ 40% is risky.
• Abnormal preoperative topography is the most important risk factor for ectasia.
• PRK for thin corneas, basement membrane dystrophy and trauma risk; LASIK recovers fastest; SMILE avoids a flap.
• Myopic ablation makes the cornea oblate: more positive spherical aberration, haloes at night. Decentration adds coma.
• Diffuse lamellar keratitis: more steroid. Pressure-induced stromal keratopathy: stop the steroid, measure IOP peripherally.
• Phakic IOLs for high myopia or thin corneas; vault 250–750 µm. Cross-linking needs at least 400 µm of stroma.

#Test yourself

#Further reading

• American Academy of Ophthalmology, Basic and Clinical Science Course, Section 13: Refractive Surgery.
• C. R. Munnerlyn, S. J. Koons and J. Marshall, Photorefractive keratectomy: a technique for laser refractive surgery. Journal of Cataract and Refractive Surgery 14 (1988) 46–52.
• M. R. Santhiago et al., Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography. American Journal of Ophthalmology 158 (2014) 87–95.
• L. N. Thibos et al., Standards for reporting the optical aberrations of eyes. Journal of Refractive Surgery 18 (2002) S652–S660.