Opposite clear corneal incisions

The mark of a novel but potentially enduring innovation is “why didn't I think of that?” In their article in this issue, Lever and Dahan make such a mark in the advanced management of pre-existing astigmatism, applicable to cataract refractive surgery, refractive lensectomy, and phakic intraocular lens (IOL) surgery. During the modern evolution of the refractive aspects of lenticular surgery, through the periods of intracapsular cataract extraction (ICCE) and larger incision extracapsular cataract extraction (ECCE), we learned that the corneal meridian on which the extended incisions were based flattened during the healing process.1 Pathologically, large incisions heal with added tissue, with consequent extension of the radius of curvature of that central meridian.2 The invariable consequence of that era of surgery was against-the-rule (ATR) astigmatism of a significant degree. Corneal topography then added another dimension in the comprehension of changes in corneal shape.3 This was especially applicable as algorithms soon provided information about the more peripheral cornea and surgically induced changes therein, which could affect the optical zone of the cornea.3 Incisions for lenticular surgery reverted to the cornea because small incisions became practicable without enlargement to accommodate foldable IOLs.4 As small enough incisions did not induce astigmatic change, the issue of managing pre-existing astigmatism became especially relevant.5 Clear corneal incisions, whether temporal or oblique, do not affect the shape of the cornea's optical zone if correctly engineered and less than 3.0 mm ± 10%.1,6 How, then, should pre-existing astigmatism be eliminated? There were 2 favored options; either center the clear corneal incision on the steep corneal meridian and extend its form by an arcuate component7 or, using a “standard clear corneal incision,” perform limbal or clear corneal arcuate incisions across the steep corneal meridian.8 Both systems are effective, especially the latter, if paired incisions are used. An alternative method to isolate the optical zone of the cornea9 has not achieved universal acceptance. Corneal topographic maps after clear corneal incisions reveal that localized flattening of the cornea central to the incisions invariably and understandably occurs. The wider the incision, the greater the effect.1 Thus, if 3.0, 4.0, 5.0, and 6.0 mm clear corneal incisions are compared topographically, as the incisions extend so does the flattening effect to produce V-shaped steep hemimeridia extending on both sides of the incision.1 The shorter the incision, the more localized the effect. Thus, an incision of less than 3.0 mm will not influence the shape of the optical zone of the cornea. Taking this process to its logical conclusion, a 12.0 to 14.0 mm incision as used for ICCE/ECCE will produce ATR astigmatism with a flat “whole” meridian centered on the incision and a steep meridian at 90 degrees to the flat meridian. A clear corneal incision of 4.0 to 5.0 mm will create hemimeridional change. If, however, the clear corneal incision is repeated diametrically opposite the initial incision and if both incisions are placed on the steep meridian, symmetry will occur in the flattening of the steep meridian. This is the concept of opposite clear corneal incisions (OCCI) proposed by Lever and Dahan, and it really states the obvious. They are to be applauded for bringing the concept to our attention. The OCCI technique is simple to perform and can be varied according to the astigmatic requirement. As with other incisional techniques, there is an element of art as well as science for we all stamp our individuality on the same type of incision. It is the principle that is important. Technically, the technique requires a stable anterior chamber, best achieved with the new generation of ophthalmic viscosurgical devices10 whose dispersive characteristics add to their cohesive nature to ensure that the chamber is stable during the incision process. Lever and Dahan, on the other hand, favor the anterior chamber maintainer to achieve the same end. We also have to remember that incisions in themselves are subject to subsequent modification by thermal and mechanical processes, which should be avoided at all costs.11 Opposite clear corneal incisions with appropriate preoperative corneal mapping represent a real advance in the applications of refractive lenticular surgery.