The Alpins method: A breakthrough in astigmatism analysis

by Keith J. Croes

Abstract

Refractive surgery is an emerging subspecialty of ophthalmology. About half of the world's population are candidates for corneal refractive surgical correction of nearsightedness (myopia) or farsightedness (hyperopia), and about half of these people have astigmatism of sufficient amount to warrant concurrent correction. A new method, developed by Noel Alpins, MD, assists in the planning and evaluation of corneal refractive surgery on people with astigmatism. The Alpins method determines a goal for astigmatism correction and a vector (steepening force) required to achieve that goal. From this, the method allows the calculation of the principal components by which an operation fails to achieve its goal, and other components that assist in the comparative analysis of the results of astigmatism surgery for individuals and groups of individuals. The Alpins method, available in a computer program called ASSORT, would be a valuable addition to current refractive lasers and corneal topography instruments.

Introduction

The ideal cornea of the eye is a perfect dome with a base that is a perfect circle. Alas, we live in an imperfect world. For people with astigmatism, the dome is not spherical and the base is elliptical to one degree or another.

The cornea -- the clear front window of the eye -- has a major role in focusing light on the retina, the light-sensitive tissue in the back of the eye, which can be likened to the film in a camera. Any distortion of the cornea results in the defocusing of light and a blurred image on the retina.

Since the late 1970s, eye surgeons have been taking advantage of the refractive power of the cornea to correct nearsightedness (myopia) and farsightedness (hyperopia). The ophthalmic subspecialty that has emerged is called refractive surgery. Refractive surgery represents a growing market worldwide with a huge potential impact both in economic and human terms.

The complicating factor

The various corneal refractive procedures that have been developed over the past two decades have one thing in common: to correct myopia, they flatten the dome of the cornea; to correct hyperopia, they steepen the dome of the cornea.

The most common corneal refractive procedures are photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). In PRK, an excimer laser is used to ablate the surface of the cornea, steepening or flattening it as appropriate for the individual patient. In LASIK, the surgeon first uses an instrument called a microkeratome to cut a flap on the front of the cornea. The flap is folded back, the excimer laser is used to make an ablation on the corneal bed (again steepening or flattening as appropriate) and the corneal flap is returned to its original position. In essence, LASIK is PRK performed under a corneal flap.

Myopia and hyperopia in the absence of astigmatism are said to be spherical. The correction of spherical myopia and hyperopia by changing the shape of the cornea is a relatively straightforward process. In myopia, the focal point of converging light rays lies in front of the retina. In hyperopia, the focal point is behind the retina. A flattening of the cornea reduces its refractive power, which in the myope pushes the focused image back to the retina where it belongs. Similarly, steepening the cornea increases its refractive power, moving the focal point forward to the retina, much to the delight of the hyperope.

When astigmatism is thrown into the equation, however, refractive surgery becomes immensely more complicated. In planning, implementing and analyzing refractive surgery, astigmatism has challenged the best minds in ophthalmology.

Until now. Or more accurately, until the July 1993 issue of the Journal of Cataract and Refractive Surgery, which contained the article "A new method of analyzing vectors for changes in astigmatism," by Australian ophthalmologist Noel Alpins(1). Alpins would subsequently write a number of papers(2,3,4) elucidating what collectively can be called the Alpins method of astigmatism analysis. Alpins' vector-analysis methods are protected by U.S. patents 5,514,124, 5,749,867 and 5,740,815. You can find and read these patents using the IBM Patent Server. Alpins' methods also have a number of international patents and patents pending.

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