In laser corneal refractive surgery, as in life, there is a constant decision-making process, a “yin and yang“ to provide not only the safest possible treatment, but also the one that will provide the highest long-term visual quality. When considering the laser corneal surgical treatment options for compound myopic astigmatism, many surgeons feel that unimeridional astigmatic ablations are best. At least, it is the only option that has been presented to the surgeon to use. In general, astigmatic patients tend to increase the basal heart rate of surgeons because they know the refractive accuracy and overall results of these patients generally trail those of spherical eyes. However, a different approach from unimeridional ablations has now given me greater control of my own anxiety of treating what previously seemed to be monstrous refractive cases.
THE STATUS QUO
So, what is a unimeridional ablation, and what is another option? To start, compound myopic astigmatism (regular) occurs when the two principle corneal meridians, which are 90&Mac251; apart, have brought both focal points into focus in front of the retina (Figure 1). The steeper corneal meridian brings the focal point more anterior to the retina than that which is focused by the flat corneal meridian. A unimeridional ablation will collapse the first or more anterior focal point onto the second by flattening the steepest meridian, thus creating a spherical corneal surface (Figure 2). The resulting surface, at this point, still creates a defocused image on the retina due to residual myopia. The remaining spherical ablation will flatten the corneal surface to bring the refractive state of the eye to emmetropia; subsequently, the remaining singular focus point will be brought precisely onto the retina. This basic premise summarizes what all laser manufacturers have achieved to date: they have treated the astigmatic portion of the compound astigmatic patient in the steep meridian. An advantage afforded by this method is central tissue conservation, although a drawback could be a relatively small short axis of the ablation with a relatively abrupt contour change between the treated and untreated portions of the cornea over the entrance pupil. If the short axis is smaller than the scotopic pupil, and the change is significantly abrupt, it could lead to unwanted scotopic symptoms.
AN ALTERNATIVE
One alternative to treating in a unimeridional fashion is to treat in either a cross-cylinder or bitoric approach as demonstrated by Arturo Chayet, MD, of Mexico, and Paolo Vinciguerra, MD, of Italy.1-3 Although one could treat compound myopic astigmatism with two crossed cylinders, as is done with mixed astigmatism, it has been my experience that a bitoric approach best achieves the ablation for the highest quality vision in both photopic and scotopic conditions. In simplest form, the bitoric (50/50 split) ablation dissects the refraction by treating one-half of the astigmatism in minus cylinder format (Figure 2), one-half in plus cylinder format (Figure 3), and finally, treating the spherical equivalent of the refraction. Splitting the cylinder value can be done at any ratio; it is most commonly done at either 50/50 or 70% minus ablation and 30% plus ablation. An example of a 50/50 bitoric ablation is as follows:
The surgeon can combine the minus cylinder ablation with the minus spherical ablation, and achieve this result: Plano + 2 X 090 and -4 -2 X 180. One also has the latitude to adjust the spherical myopia treated based upon nomogram adjustment for surgeon factor for sphere and astigmatic ablation coupling.
The main goal of the splitting allows for the maximal possible width of the short axis of the minus cylinder ablation. For example, using the S3 laser (VISX, Santa Clara, CA), if we employ the 6.5-mm zone with blend capabilities to 8 mm, we can verify the difference in shape of the following treatments. For a unimeridional approach, the short axis of the minus cylinder will maximally be 5 mm, but with the bitoric ablation, the short axis of the minus cylinder ablation will reach 6 to 6.2 mm. It is interesting to note that the unimeridional ablation will essentially be 8 mm in long axis and 5 mm in short axis with blend to 8 mm, whereas the bitoric ablation will be 9 mm in long axis and 6 mm in short axis with blend to 8 mm. The differences become even more striking if the blend is not used for the unimeridional ablation, and the surgeon obtains a 6.5- by 5-mm ablation surface.
PROS AND CONS
The purpose of the bitoric ablation is to achieve corneal surface diopter changes over a larger corneal surface area in a more gradual fashion. In essence, the surgeon establishes a more gradual rate of curvature change over this larger surface area in order to minimize abrupt refractive surface changes, and therefore minimize unwanted scotopic visual symptoms, decrease regressive effects at sharp contours, and establish a more suitable optic over the long term. The higher the amount of astigmatism, the better the bitoric ablation actually works. Is there a price to pay? Certainly—the surgeon does remove approximately 10% more central tissue with a bitoric ablation than a unimeridional ablation.
The future of corneal laser surgery appears to be computer-guided ablations in the form of topography and wavefront. I firmly believe these will be beneficial, but I also believe that less sophisticated, more cognitive approaches such as a bitoric ablation for the treatment of astigmatism will improve the visual function of patients undergoing refractive surgery. However, this technology will require the lead and interest of manufacturers to become reality.
John F. Doane, MD, FACS, is in private practice with Discover Vision Centers in Kansas City, Missouri, and is Clinical Assistant Professor of the Department of Ophthalmology, Kansas University Medical Center. He does not hold a financial interest in any of the materials mentioned herein. Dr. Doane may be reached at (816) 350-4539; jdoane@discovervision.com1. Chayet AS, Montes M, Gomez L, et al: Bitoric laser in situ keratomileusis for the correction of simple myopic and mixed astigmatism. Ophthalmology 108:303-308, 2001
2. Chayet AS, Magallanes R, Montes M, et al: Laser in situ keratomileusis for simple myopic, mixed and simple hyperopic astigmatism. J Refract Surg 14:S 175-176, 1998 (2 suppl)
3. Vinciguerra P: Correction of astigmatism with a cross cylinder ablation, in Buratto L, Brint S (eds): Lasik Surgical Techniques and Complications. Thorofare, NJ, Slack, Inc., 2000
