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Up Front | Apr 2008

A Potential Alternative to Penetrating Keratoplasty

The topography neutralization technique has been well received by patients who hope to avoid or delay corneal transplantation.

Two-stage, topography-guided laser vision correction may be a potentially effective treatment alternative for several corneal indications. Based on topography instead of refraction, this laser vision correction technique consists of a topographic smoothing (stage one) and an algorithm-based refractive adjustment (stage two). It can be used after LASIK, PRK, or radial keratotomy (RK) to enlarge optical zones. Topography-guided laser vision correction can also be used to retreat decentered ablations and irregular corneas or as the primary treatment of asymmetrical astigmatism in normal corneas. In keratoconic eyes, topography-guided laser vision correction may provide an alternative to penetrating keratoplasty (PK).

PATIENT CASES
Topography-guided correction was used to retreat a patient who had undergone four-cut RK and LASIK 9 years prior. Preoperatively, this patient had a UCVA of 20/50 with 20/30 BCVA. One month postop, UCVA and BCVA measured 20/30 and 20/50, respectively.

Topography-guided ablation was also used in a post-LASIK patient with a previous decentered ablation. Preoperatively, the patient's BCVA was 20/20 and the decentration was 0.92 mm. Three months after topography-guided treatment, UCVA was 20/20 with a decentration of 0.15 mm.

In a total of 34 eyes retreated for decentered ablations, we found that the average preoperative decentration was 0.92 mm, which improved to 0.3 mm postoperatively (Figure 1). More than one-quarter of these patients also experienced a gain of one or more lines of BCVA. Seventy-one percent had no change (Figure 2).

For patients who meet the specific criteria, topography-guided PRK using the Allegretto Eye-Q 400 Hz excimer laser (WaveLight Technologie AG, Erlangen, Germany) has also shown positive results.

Patients with keratoconus can be considered for this procedure if they are contact lens–intolerant, on the corneal transplant list, and have a BCVA of 20/40 or better. Additionally, they must have reproducible topographies (Allegretto Topolyzer; WaveLight Technologie AG), and a calculated residual corneal thickness of more than 300 µm of stroma after the planned ablation.

We have performed transepithelial topography-guided PRK on 15 keratoconic eyes. We used the topographic neutralizing technique (TNT) to improve stability in two cases, and mitomycin C is applied in all cases for 15 seconds. A bandage contact lens was then placed over the eye postoperatively, and topical steroids were administered for 3 months.

We have found that TNT-based ablation steepens the flatter areas of the periphery and flattens the steeper, central areas. The calculations for this procedure also takes into account changes induced in the sphere and cylinder.

OUR RESULTS
In our 15 cases, TNT ablation resulted in decreased steepening but increased myopia in eyes with large cones. Those outcomes, however, led to an improvement in refractive predictability for topographic ablations.

At 6 months follow up, seven eyes were within 0.50 D of the intended correction, and 100% were within 2.00 D. BCVA was unchanged in 10 eyes and improved in two. Three eyes, however, lost one or more lines.

FOLLOW-UP
At 12-month follow-up, we found that 13% of patients experienced a gain of 2 lines of BCVA; 25% gained one line, and 50% had no change.

Twelve percent lost one line, although no patients lost two lines or more. In terms of haze, 88% of patients had clear vision, and 12% experienced either 1+ or 2+ grade haze.

MULTISTEP PROCESS
TNT is performed in four steps: (1) we analyze the plano treatment for smoothing the cornea needs analyzed, (2) we identify the cylinder induced by the plano treatment and the amount of astigmatism treatment needed to address it, (3) we add a myopic treatment to the center to compensate for the first hyperopic ablation, and then (4) we add the manifest refraction for the final treatment calculation.

This treatment needs long-term follow-up to determine if outcomes are only temporary. Still, it has been well received by patients who hope to avoid or delay corneal transplantation. Expectations, however, should be carefully managed, especially in patients with steep eyes or advanced cones.

Additionally, surgeons should understand that this is a complex procedure that will require more study before it can be considered a viable treatment for widespread use.

David T.C. Lin, MD, FRCSC, is a Clinical Associate Professor of Ophthalmology at the University of British Columbia, a surgeon at Pacific Laser Eye Center, and a fellow of the Royal College of Physicians and Surgeons of Canada. Dr. Lin states that he was a paid consultant to Wavelight. He may be reached at info@pacific-laser.com.

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