Incisional surgery: Astigmatic keratotomy and limbal relaxing incisions

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CHAPTER 29 Incisional surgery

Astigmatic keratotomy and limbal relaxing incisions

Thomas Kohnen, Oliver K. Klaproth

Chapter outline

INTRODUCTION 179
EPIDEMIOLOGIC CONSIDERATION AND TERMINOLOGY 179
ANATOMICAL CONSIDERATIONS 179
FUNDAMENTAL PRINCIPLES 179
GOALS OF SURGERY 180
INDICATIONS FOR SURGERY 180
PREOPERATIVE ASSESSMENT 180
ANESTHESIA 181
OPERATION TECHNIQUES 181
INTRAOPERATIVE COMPLICATIONS 181
POSTOPERATIVE CARE 181
POSTOPERATIVE COMPLICATIONS 181
ASSESSMENT OF SURGERY: SELF-EVALUATION; RESULT OF SURGERY 181
REFERENCES 181

Introduction

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Astigmatism is one major reason for reduced visual acuity and optical quality. Its correction may be performed with most of the known procedures of refractive surgery, such as laser in situ keratomileusis, toric phakic intraocular lenses, or the refractive lens exchange. Incisional surgical procedures based on corneal relaxation principles are limbal relaxing incisions (LRI) and astigmatic keratotomy (AK). Both aim for the correction of astigmatism. LRI is usually performed as an initial or re-treatment for correction of lower astigmatism after refractive laser surgery, phakic intraocular lens implantation, cataract surgery, or refractive lens exchange. As opposed to LRI, AK offers the option to correct higher astigmatism, i.e. after penetrating keratoplasty. Principles, indications, techniques, and results of both procedures will be explained below.

Epidemiologic consideration and terminology

As opposed to a spherical cornea, where refractive power of all meridians is identical, regular toric corneas are defined by two perpendicular meridians with different (highest and lowest) refractive powers. This results in two longitudinal displaced orthogonal focus lines and a circle of least confusion between them instead of one focal point. Thus retinal images are blurred and drawn-out (Fig. 29.1).

image

Fig. 29.1 Sturm’s conoid, showing the principle of toric optics and resulting regular astigmatism in an positive refractive power lens, represented by the principal planes HH’1,red and HH’2,blue of its meridians 1 and 2, which are located 90° to each other. y – image. F – focal points. f – focal lenghts. Δf – difference in focal lengths of the two meridians: astigmatism.

Astigmatism with more than two meridians, non-perpendicular meridians, or no regular pattern at all is defined as irregular astigmatism. Influence of astigmatism on visual acuity is approximately half of the influence of a defocus aberration of the same magnitude1.

Anatomical considerations

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