The Impact of Centroid Shift and Registration on Clinical Outcomes

Currently, the most important concept in corneal refractive surgery is quality of vision. Customized ablation, improving contrast sensitivity, decreasing higher-order aberrations and reducing glare—all top priorities to corneal surgeons and patients undergoing surgery—are made possible by improvements in iris registration (IR).

Custom treatments with IR yield more accurate ablations and, therefore, better postoperative visual acuities, because the wavefront-guided ablation can be placed on the exact location where the wavefront measurement was taken. Patients with unusual prescriptions (eg, high degrees of astigmatism or a significant amount of higher order-aberrations) tend to benefit most from custom treatment with IR.

Anxiety, which plays a key role in the amount of rotational cyclotorsion and fixation difficulty, occurs in some individuals. Because matching the iris under the laser is extremely important, anxious patients may benefit from IR because it uses fixed landmarks on the eye (ie, iris crypts, outer iris boundary) to determine treatment registration. While IR does not replace the need for patients to fixate, it does provide the surgeon with the assurance that the laser recognizes the placement of the patient's eye and will provide accurate treatment. Without this technology, there is no real way to ensure that the laser ablation is being placed in the correct location.

IR also provides surgeons with a higher comfort level, ensuring that the surgeon is putting the ablation pattern as close as possible to the measured wavefront. In my experience, the Visx system (Santa Clara, California, and Advanced Medical Optics, Santa Ana, California) with IR is the most accurate mechanism currently available (Figure 1). While other technologies recognize ink marks on the sclera—which can diffuse as the tear film moves over the eye—the Visx system captures patient's unique natural ocular features to register treatment.

Wavefront measurements vary with pupil location and size and are determined through identification of iris crypts. IR measures wavefronts registered to a fixed landmark for accuracy and transfers the information to the laser. During treatment, the camera in the laser locates the iris crypts and rotates the treatment placement to make sure that the ablation is correctly located. For custom ablation, this fixed feature must be identifiable both at the time of measurement and at the time of treatment. Surgical results are dependent upon this.

Although pupil tracking may accurately track the eye, it will not yield a stable reference point from which to apply a wavefront, as the pupil is a moving target. Pupil position is not accounted for in most systems. Significant misplacement of the ablation profile for measurement of surgery—even with iris tracking—may occur. Misplaced treatments can result in induced refractive error (ie, cylinder, induced higher-order aberrations, decreased visual acuity), therefore physicians should use IR whenever possible.

Current generations of wavefront-controlled ablations (ie, Visx Advanced Customvue) have markedly improved patient outcomes. Precise registration has improved visual acuity and contrast sensitivity and now allows us to all treat irregular corneas. Pathological eyes that underwent previous refractive surgery can finally be treated. We can now treat decentered ablations, small optical zones, higher-order aberrations and irregular topographies.

One complicated case is custom ablation on form fruste keratoconus patients. LASIK, conventional PRK, or optimized ablation are not suitable procedures for these patients because an optimized ablation is not going to treat the higher-order aberrations. If performed, the surgeon may induce coma and create an unhappy patient. A better way to treat the form fruste keratoconus patient is with a registration system. Cyclorotation and pupil shift should be considered and ablations should be placed in the correct position; these are all possible with an IR system.

The current generation of wavefront-controlled ablations have markedly improved patient outcomes (eg, more precise ablations, accurate treatment placement, predictable outcomes, reduced retreatment rates, reduction in higher order aberrations, improved quality of vision.) Precise registration allows us to treat irregular corneas (eg, form fruste keratoconus) decentered ablations, and small ablation zones. Finally, the submicron ablation accuracy of the excimer laser will not reach its potential without customized ablations, accurate centration, cyclotorsion management and registration. Pupil tracking and optimized ablations alone cannot realize the potential of the excimer laser. IR with customized ablation offers improved accuracy. The technology will afford better outcomes in the future.

Julian Stevens, FRCS, FRCOphth, is a consultant ophthalmic surgeon, and director of refractive surgery at Moorfields Eye Hospital, in London. He is a member of the Cataract and Refractive Surgery Today Europe Editorial Board. Dr. Stevens states that he is a paid consultant for AMO/Visx. He may be reached at JulianStevens@compuserve.com.