OCULAR SURGERY NEWS EUROPE/ASIA-PACIFIC EDITION 4/1/2008
Surgeon describes best techniques for correcting a decentered ablation
Ming Wang, MD, describes decentered ablations and the course of action needed to remedy them.
Ming Wang, MD

Introduction

Significantly decentered excimer ablations result in loss of best corrected visual acuity due to irregular astigmatism and cause symptoms such as glare, night vision difficulty, ghosting and diplopia. Possible causes of decentration include poor fixation due to poor patient instruction, anxiety, over-sedation, blurry vision due to high refractive error or the exposed stromal bed causing difficulty seeing the laser’s target. It can also be due to improper stabilization of the patient’s eye with a Thornton ring during ablation. In order to prevent decentration, careful preoperative and intraoperative instructions are key, especially with regard to the fixation target. My special guest in this column is Ming Wang, MD, to explain how to handle this complication.

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Custom-Corneal Ablation Pattern (Custom-CAP, Visx) received U.S. Humanitarian Use Device approval for the treatment of decentrations in 2002. Elevation data is obtained using the Humphrey Atlas (Carl Zeiss Meditec), and a software program allows simulation of surgeon-directed ablations of chosen location, shape, size and depth to improve corneal topographic appearance. Although effective, Custom-CAP does not address the refractive error. While most surgeons consider an improvement in BCVA and reduction of symptoms a surgical success, many patients are frustrated by the lack of improvement or, in some cases, worsening of uncorrected vision.

Wavefront-driven custom treatment may be used to correct decentrations, assuming the technology currently available is able to detect the irregularities reliably. Shack-Hartmann aberrometers may fail when attempting to measure eyes with considerable irregularity, due to limitations of the lenslet array. While decentrations may increase higher-order aberrations, attempting to correct the aberrations may not fully correct the topographical errors. These systems assume a normal prolate cornea in treatment planning, and the refractive error corrections may be less accurate. Thus, these treatments may be less effective than topographically directed treatments.

Re-treatment using conventional enhancement techniques rarely fully corrects the problem and typically increases the effective decentration. This occurs because the neural axes (visual axis and line of sight) and the optical axis (geometrical) are not aligned in cases of decentration. Image placement on the fovea requires the eye to rotate, making full correction of the optical problem unlikely when all measurement and planning occurs on the visual axis. Conventional technology is not able to decouple these axes and treats solely on the visual axis information.

The advancement of Scheimpflug imaging to create three-dimensional models of corneal shape may be the missing link to accurate topographically driven treatments. These systems measure the corneal shape directly and with greater accuracy than placido or slit scanning methods. Combining precise topographical measurements with sophisticated software programs, such as the Corneal Integrated Planning and Treatment Algorithm (CIPTA, Ligi) software, may enable treatment of irregular astigmatism.
Link to Article at pconsupersite.com

Moose's Comment: Not a lot of progress has been made in treating people with decentered ablations. While you can find lots of surgeons who claim to treat decentered ablations and other forms of irregular astigmatism the scientific data does not support the idea that the chance of a successful outcome are high.