Incisional surgery: Astigmatic keratotomy and limbal relaxing incisions

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


See Video image


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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Jun 4, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Incisional surgery: Astigmatic keratotomy and limbal relaxing incisions

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