Refractive Surgery Complications and Complexity

MAY 2011 CATARACT & REFRACTIVE SURGERY TODAY 45 A cornerstone of successful laser vision correction is the accurate centration of the ablation zone over the entrance pupil. Symptoms of poor visual acuity, poor quality of vision, loss of contrast sensitivity, halos/glare/starbursts, and multiple images (monocular polyopia) have all been reported after both PRK and LASIK in which a significant decentration has been observed.1 In general, the widespread adoption of eye-tracking systems for routine use during laser procedures as well as greatly expanded ablation zone diameters effectively eliminated cases of decentration. Small, subclinical decentrations still occur, however, typically with minor and easily managed symptoms and side effects (Figure 1). For the purpose of this review, decentrations will be broadly divided into three categories: large (> 1 mm), moderate (0.5-1 mm), and small subclinical (< 0.5 mm). The decentration of an ablation zone is defined as the difference between preand postoperative corneal topography measurements using tangential maps, as described by Mrochen, Kaemmer, and others.2 Using this method, the area of the intended laser ablation surrounded by a region of approximately zero power is calculated, and the eccentricity is then determined by the distance between the center of the flattened zone and the center of the entrance pupil. Other methods to characterize the extent of decentration include the center of flattest power,3,4 a vector center-ofmass formula,5 and Fourier analysis of corneal power components.6 LARGE DECENTRATIONS Although uncommon today, large decentrations can be the most challenging to manage. Seiler and McDonnell presented the first case of diametral ablation to compensate for a grossly decentered ablation in 1995. Noting even then that decentrations of less than 1 mm are usually well tolerated by patients, the authors described a laser profile delivered at an opposite but equal distance 180o from the topographic location of the primary correction.7 Pallikaris later proposed a theoretically similar approach not using a laser. He performed an arcuate keratotomy across the axis of decentration, also 180o from the center of the primary ablation.8 Both techniques had the same goal: the normalization of corneal topography in an effort to reduce visual symptoms and improve the patient’s BCVA. Currently, there are two schools of thought regarding how to treat large decentrations with a laser. Each has advantages and disadvantages. The wavefront-guided approach can simultaneously normalize the corneal topography and address the patient’s residual refractive error. The primary limitation of this method is that large decentrations tend to be associated with high amounts of induced higher-order aberrations. As a result, it is difficult or even impossible to capture the necessary data to formulate a treatment plan. The second option is a topography-guided approach, which builds on the Visx Decentered Ablations