Effect of Depth Upon the Smoothness of Excimer Laser Corneal Ablation

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The Argon Fluoride (ArF) Excimer laser is currently used for experimental reshaping of the front surface of the cornea for the correction of myopic refractive error (photorefractive keratectomy, PRK) and for smoothing corneal irregularities (phototherapeutic keratectomy, PTK). Both PRK and PTK are in FDA clinical trials, but are more readily available outside the U.S.A. These techniques have achieved reasonable success in spite of early reports that deeper ablation procedures can cause reduced corneal clarity (haze) or a result which is less accurate or tends to regress. We studied how different depths of excimer tissue removal affect the smoothness of the ablation zone of the rabbit cornea. Dutch belted rabbits were immediately sacrificed by pentobarbital overdose. Their corneas were de-epithelialized with a knife or by the excimer laser and then were photoablated. A 4.5-mm circular ablation beam was delivered to each denuded area, but the beam was masked by positioning a steel blade to partially block the laser beam, thus creating variable ablation depths corresponding to 0.0, 12.5, 37.5, and 62.5 µm. The eyes were fixed in situ by topical and anterior chamber application of glutaraldehyde and the corneas were excised after 5 min and placed in glutaraldehyde for tissue processing. The corneas were whole-mounted and examined by scanning electron microscopy (SEM). The resultant micrographs show increasing irregularity of the ablated surface as a function of depth. The irregularity appeared to be due largely to the inhomogeneities of the anterior stroma, which is known to be layered by alternately directed collagen fibrils. The presence of a less regular surface after deeper ablations may contribute to the increased corneal haze noted by other investigators for higher refractive corrections when more tissue is removed.

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