Topographically customized ablation treats irregular astigmatism

Well-defined, three-dimensional ablation maps are transferred to a flying-spot excimer laser, which performs customized treatment. treatment.

---Post-myopic PRK treatment with a small optical zone and a central island (left) and post-CIPTA elevation anterior map (right), showing enlargement of the optical zone and resolution of the central island.

---The same case is presented in tangential (true) keratometric maps (pre- and post-CIPTA, on left and right respectively).

ROME – Interactive software for topographically driven photorefractive keratectomy (Corneal Interactive Programmed Topographic Ablation; CIPTA) is giving excellent results in the treatment of irregular astigmatism, according to a study carried out by Carlo Sborgia, MD, Giovanni Alessio, MD, and Gabriella La Tegola, MD, of Bari University Eye Clinic. Results of the research were presented by Dr. Alessio here at the Rome ’99 meeting.

“Photorefractive keratectomy [PRK] assisted by corneal topography is to date the most successful way to treat corneal irregularities or astigmatism by smoothing the corneal surface,” Dr. Alessio said. “By transferring the programmed ablation from a corneal altimetric tomograph [Orbscan; Bausch & Lomb] to a flying spot excimer laser [Laserscan 2000; Laser Sight], the CIPTA software provides a customized laser ablation that also can deal with difficult cases of irregular astigmatism.”

The CIPTA software was created by Giuseppe D’Ippolito, an engineer at Ligi s.r.l., in Taranto, Italy, in collaboration with Chuck Broadus, an engineer at Orbtek (Salt Lake City, Utah), and with clinical evaluation by the medical staff at the University of Bari.

Three-dimensional representation

---Ablation map of the previous patient: untreated areas are represented in green, treated zones in yellow; darker areas correspond to deeper ablation.

---Patient with hyperopia and astigmatism, with elevated K-angle and visual axis far from corneal apex was treated by means of CIPTA for correcting ametropia and recentering corneal apex on visual axis: preoperative (left) and postoperative (right) elevation anterior maps.

“The aims of CIPTA are to correct ametropia by changing the corneal profile (flattening or steepening) and to obtain the ‘ideal’ shape of each cornea so as to achieve the best quality of vision,” Dr. Alessio said.

A corneal elevation map acquired with the Orbscan topographer provides the actual three-dimensional shape of the cornea and not just a mathematical model of it, he said. By measuring corneal height above a reference surface at every point, both in the center and in the periphery, Orbscan can model any irregularity of the corneal surface, he said.

A new, ideal three-dimensional shape of the cornea, calculated by the CIPTA software, is geometrically represented by an “aconic surface” that can have the same asphericity as the original corneal shape, Dr. Alessio said. “The volume of the ablation derives from the intersection of the three-dimensional shape of the cornea and the best aconic surface for the patient,” he said. “The transition zone of intersection between the two surfaces has a constant slope in all directions, and this minimizes the risk of regression.”

Entering the data

---Axial power keratometric maps of the same case: preoperative (left) and postoperative (right).

---Ablation map shows the treated area.

CIPTA involves several interactive steps. All the treatment parameters (center and axis of refractive treatment, minimal useful refractive diameters, transition zone, refractive correction) are suggested by the software but defined by the surgeon. A good choice of such parameters is necessary for planning the ablation profile.

Centering of the ablation is important to obtain a symmetrical cornea. Different centering points can be chosen: the pupil, the corneal apex, the fixation point (the corneal reflex of the fixation target of the instrument), the thinnest corneal point, or any other point identified by use of the computer cursor or by Cartesian coordinates.

Refractive diameters are selected based on both the pupillary diameter in scotopic conditions and the refractive error.

The choice of treatment axis is very important. The surgeon can select the axis of the patient’s best aconic surface or respect the actual visual axis.

The desired refractive correction and the asphericity coefficient are fed into the computer, and the level of ablation (PRK or laser in situ keratomileusis) and the transition zone (both slope and width) are defined.

“At the end of this interactive pro- cess,” Dr. Alessio said, “we can see the ablation map, in which the untreated zones are represented green and the ablated areas in yellow-red. Darker areas correspond to deeper ablations. The customized altimetric ablation profile is then transferred to a flying spot excimer laser, which performs the treatment. A flying-spot laser with a small spot size (800 µm) can provide ablations of any shape and depth, and can even reach corneal zones as peripheral as the limbus.”

Case report

---Decentered myopic PRK (elevation anterior map).

---Post-CIPTA elevation map, showing a recentered treatment.

---Ablation map of this case.

“At the Bari University Eye Clinic, we have been using CIPTA since October 1997,” Dr. Alessio said. “We have treated more than 100 eyes, with the first 28 eyes having a follow-up of 18 months. All patients improved uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA) and quality of vision; only one eye lost one Snellen line of BCVA at 18 months. At that time, the mean percentage of astigmatic correction in the hyperopic group was 89% and in the myopic group 85%.”

“Patients are very satisfied with their visual performance,” he continued. “Stability of refraction is an important and constant parameter in eyes that underwent [topographically controlled] ablation; the reason could be related to the large ablation and to the constant slope of the blend zone. Whereas at first we treated irregular (primitive or post surgical) astigmatism only, now we treat all kinds of ametropia, regular or irregular, and so far our results have been improving.”

To illustrate the effectiveness of CIPTA, three cases were presented:

A male patient, age 45, underwent a myopic PRK for –8.00 –1.50 x 20°, 2 years earlier. He presented with glare, halos and a BCVA of 20/25 with a correction of –1.00 –2.00 x 30°. “We carried out CIPTA centered on the pupil with an enlargement of the optical zone. Postoperative UCVA was 20/32 and BCVA 20/20 with a refractive error of plano –1.00 x 180°. The patient had no more glare and halos,” Dr. Alessio said.

A female patient, age 54, had hyperopic with-the-rule astigmatism and a large k-angle. UCVA was 20/200 and BCVA was 20/32 with +3.50 +2.00 x 100°. CIPTA centered on the line of sight was performed. Postoperatively, UCVA improved to 20/32 and BCVA was 20/20 with a refraction of +1.25 sphere.

A male patient, age 43, had a decentered PRK for –8.00 sphere 3 years earlier. In January 1998, he had a refraction of plano –1.75 x 95°. “We carried out CIPTA, centered on the pupil, to correct the refractive error and to reposition in the center of the cornea the new refractive area. Postoperative UCVA was 20/20, with a plano refraction,” Dr. Alessio said.

For Your Information:

• Carlo Sborgia, MD; Giovanni Alessio, MD; and Gabriella La Tegola, MD, can be reached at Università di Bari, Sezione di Ottica Fisiopatologica, Dipartimento di Oftalmologia e otorinolaringnoiatria, Bari, Italy; (39) 080-5592435; fax: (39) 080-5592839; e-mail: alessio@otticafisiopat.uniba.it. Dr. Alessio has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.