Post-refractive Surgery

Introduction

For nearly half a century, vision scientists around the world have struggled with the challenge of surgically correcting human refractive error. Typically by employing mechanical manipulation of the corneal tissue, the delicate art of refractive surgery has been greatly refined over the past 40 years, and although great strides have been made in recent years, the nature of ocular surgery and the inherent complications have left a wake of patients with suboptimal visual results. For a number of these patients, contact lenses provide the best visual correction and restoration of binocular vision ( , ).

Modern refractive surgery techniques boast high success rates and few complications ( , , ), but the journey here has not been without adverse outcomes. Many experimental and poorly understood procedures – including keratophakia, keratomileusis, epikeratophakia, thermal keratoplasty, automated lamellar keratoplasty, and radial keratotomy – have been performed on millions of eyes. These outdated procedures, specifically radial keratotomy (RK), have been shown to create structural changes in the cornea that can render the tissue ineffective at correctly refracting light for adequate retinal focus ( ).

Whilst some patients had successful outcomes from these older procedures, many were left with permanently scarred and/or irregular corneal shapes. More recent surgical procedures, such as photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK) and laser epithelial keratomileusis (LASEK) have provided improved outcomes, and advancements in these procedures have further lessened the associated complications ( ). Regardless, there are still complications seen even with advanced refractive surgery procedures such as small incision lenticule extraction (SMILE), requiring susceptible individuals to need fitting with custom-made contact lenses as refractive treatment.

Since the late 1990s, millions of people have undergone refractive surgery in the United States. In 2010 alone, 800,000 refractive surgeries were performed ( ), and although satisfaction amongst patients is greater than 95% (Solomon 2009), there are still complications such as undercorrection/overcorrection, higher-order aberrations and dry eye syndrome that require patients to be fitted with specialty contact lenses ( ). The compounding number of patients who ultimately need specialty contact lenses after refractive surgery is driving the need for clinicians who can fit them.

The various refractive surgery procedures can be classified into one of six categories ( Table 23.1 ).

Because the cornea is the most powerful refracting surface of the eye and, in terms of practicality, is the easiest tissue to manipulate in order to change eye power, most modern refractive procedures modify the shape of the cornea to achieve the desired refractive outcome, although some newer technologies alter the internal optics of the eye ( ).

Postoperative corneal shape is influenced by:

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  type of refractive error being corrected (myopia, hyperopia, astigmatism or presbyopia)

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  surgical technique employed

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  individual wound-healing response

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  other potential intraoperative influences.

RK is rarely carried out these days, but there are still many patients who underwent RK who require contact lenses. The fitting philosophies of the specialty lenses used for these cases can be carried over to other post-refractive surgery corneas such as PRK and LASIK/LASEK.

General Principles of Post-surgical Corneal Topography (see Chapter 8 )

It is not within the scope of this book to describe each refractive surgical procedure in detail. Instead, the focus is on postsurgical corneal topography and the contact lens designs required for each. Data on corneal topography are the most important data required when fitting a contact lens on a post-surgical eye; it is the most accurate technique used to measure the front corneal surface curvature and shape ( , ).

The following should be considered when taking and evaluating a corneal topography image:

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  Corneal mapping should be done with a clear and robust tear film to avoid any artifact in the data collection.

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  If needed, artificial tears can be instilled to smooth out the tear film prior to imaging.

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  A topographical map should be viewed:

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    in axial mode to show the corneal power ( Fig. 23.1a ) when choosing the BOZR of a soft, RGP or piggyback lens system

    

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    in elevation mode to show the overall shape of the cornea (see Fig. 23.1b ) when fitting scleral and hybrid lenses; this is specifically important in determining whether to choose an RGP, scleral or hybrid lens.

Radial keratotomy

RK involves cutting deep (90% corneal thickness), equally spaced, radial incisions into the cornea. The incisions extend from the paracentral area of the cornea, 1.5–2.5 mm from the centre, to just short of the limbus. The number of incisions is directly related to the severity of the myopia – more incisions indicate a higher initial refractive error. Radial incisions result in a central corneal flattening ( Figs. 23.2a , b and c), while transverse or arcuate incisions correct astigmatism and hexagonal incisions to correct hyperopia by steepening the central cornea ( Fig. 23.2d ) ( ). The general shape of the post-RK cornea consists of an area of central flattening, a midperipheral ‘knee’ and adjacent peripheral steepening (which is sometimes quite similar to the preoperative peripheral topography).

Throughout the years, there have been a number of explanations presented for the central flattening effect noted in RK ( , ) but the currently accepted model is the ‘wound gape model’ ( ); the radial incisions create wounds that gape open under the force of the intraocular pressure and stresses within the corneal tissue. These gaping incisions are first filled with an epithelial plug and finally with scar tissue.

After the procedure, there is often instability of the refractive error due to weakening of the corneal structure, as well as a hyperopic shift ( ) and diurnal fluctuations in vision and corneal shape ( , , ). reported on 10 years of data from the Prospective Evaluation of Radial Keratotomy (PERK) study. They found the hypermetropic shift to be unique to RK and called it the ‘hyperopic shift’. This occurred in approximately 43% of individuals in whom hyperopia increased by 1.00 D or more over a period between 6 months and 10 years. It was unrelated to any ageing phenomenon (i.e. latent hyperopia) but was an unexplained ongoing effect of the flattening procedure.

The amount of wound gape and the subsequent corneal flattening were influenced by a number of biological and surgical factors that include:

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  patient age at the time of surgery

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  number, length and depth of the incisions

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  preoperative shape factors

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  intraocular pressure, stresses and biochemical properties within the corneal tissue

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  individual wound-healing response.

Corneal topography after PRK and LASIK

The hallmark of corneal topography after PRK and LASIK/LASEK is a flattened central cornea over a chord of 5–7 mm ( Fig. 23.3 ). This ablated area is surrounded by a 0.5- to 1.5 mm zone that extends across the treated portion of the cornea into the normal untreated midperipheral cornea. As with all surgical procedures, complications can compromise the depth, position and contour of the ablation zone.

Choosing a Contact Lens After RK

There are five classes of lens that can be used to fit post–refractive surgery corneas, with special considerations made to each in the context of postsurgical fitting:

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  custom soft lenses

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  custom RGP lenses

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  piggyback lenses

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  scleral lenses

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  hybrid lenses.

Computerised corneal topography provides the shape and power of the cornea such that the lens type can be decided.