ASTIGMATIC CONSIDERATIONS IN CATARACT SURGERY

  • 12/08/2021
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I. INTRODUCTION

Contemporary cataract surgery has evolved from a procedure with the simple focus of removing an obstruction of the visual axis to a refractive procedure. Minimal spectacle dependence is expected by more and more patients. Control of astigmatism and proper intraocular lens (IOL) selection are imperative to a good refractive result. There are 2 astigmatic considerations in cataract surgery: control of astigmatism induced by the surgery and correction of preexisting astigmatism. Patient parameters and preference determine the refractive goal of the surgery. Emmetropia, monovision, mild residual astigmatism, or pseudoaccommodation should be discussed in detail prior to surgery.

II. MINIMIZING ASTIGMATISM INDUCED BY CATARACT INCISIONS

A. General Principles

An incision bisects fibers within a tissue and results in relaxation of tension of those fibers due to separation of the wound edges by causing tissue gape. Tissue structure and internal distribution of tension determine resultant tissue deformation. The size, location, and architecture of a cataract incision determine its astigmatic effect. Larger and deeper incisions cause larger changes in corneal and scleral tissue. Experiments with various size incisions by Koch, Kuglen, and others showed that induced astigmatism is proportional to the cube of the incision length.2 Incisions less than 3 mm produced clinically in- significant cylinder of <0.5 D.

Sutures or tissue adhesive reverses tissue gape. Sutures with appropriate tension and location restore tissue architecture and correct induced astigmatism. Tight or improperly placed sutures compress, displace, and flatten tissue and cause central optical zone steepening in that meridian. Loose sutures leave the meridian flattened. Even properly sutured wounds relax over time, causing against-the-wound drift. The time during which the wound “drifts” depends on its size; the bigger the wound, the longer the duration. These changes overlap with the natural aging process in which the cornea drifts toward against-the-rule (ATR) astigmatism.

Figure 7-1. Scleral groove incisions for ECCE. (1) “Traditional” or “smile” incision. (2) Straight incision. (3) “Frown” incision.

B. Classical Extracapsular Cataract Extraction Incisions

Historically, cataract incisions encompassed up to 180 degrees of the cornea. A classic extracapsular cataract extraction (ECCE) incision consists of a superior partial thickness scleral groove posterior to the limbus, a scleral tunnel, and an internal corneal “lip.” The initial groove can curve parallel to the limbus (“traditional” or “smile”), be straight, or curve away from the limbus (“frown”) (Figure 7-1). The latter is less prone to against-the-wound drift but makes surgery more technically challenging.Longer tunnels provide more stability but may also lead to decreased mobility of instruments, or “oarlocking.”

Curved “smile” incisions are the most common type of ECCE incisions. There are many ways to close these incisions. Suture material should be nonabsorbable. Sutures should induce 3 to 5 diopters (D) of with-the-rule (WTR) astigmatism because the incision typically drifts 1 to 3 D (up to 5 D) over several years. Astigmatism is managed by selective suture removal starting 1 month after surgery. Running “baseball,” horizontal and vertical mattress, and Masket variation sutures have been proposed to reduce surgically induced astigmatism. These techniques do not limit late ATR astigmatic drift. The vertical meridian can be relaxed in the presence of preexisting WTR astigmatism by leaving intentional wound gape. Each quarter millimeter of gape corrects 1 D of astigmatism up to 4 to 5 D.

C. Clear Corneal Incisions

The advent of phacoemulsification and foldable IOLs and the introduction of smaller incisions decreased surgically induced astigmatism. Incisions of 3 mm or less induce less than 0.5 D, usually 0.2 to 0.4 D. Temporal scleral and corneal incisions caused no detectable astigmatism from postoperative day 1, while 3-mm superior incisions took less than 1 month to stabilize.Nasal 3.5-mm corneal incisions induced slightly more astigmatism than temporal incisions. Temporal 1- and 2-plane clear cornea incisions induced the least astigmatism and were easy to perform, quickly becoming popular. Triplanar incisions with deep posterior grooves induce astigmatism and can be used to correct ATR astigmatism. Many surgeons place their incisions on the steep meridian to reduce preexisting astigmatism.Another approach is to use astigmatically neutral temporal incisions and place additional incisions as described next.

Phacoemulsification burns can cause severe induced astigmatism. Heating collagen to above 60°C leads to its coagulation and local shrinkage, leading to wound gape. The defect is difficult to correct because wound edges are disturbed and a watertight closure may cause high astigmatism. A wound burn should be watertight but tissue edges should not be approximated. Contemporary phaco machines minimize heat production but burns do occur, especially in sleeveless microincisional phaco.

*Dikutip dari Buku Essentials of Cataract Surgery 2nd Ed, halaman 69-71

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