Elsevier

Ophthalmology

Volume 108, Issue 4, April 2001, Pages 666-672
Ophthalmology

Posterior corneal curvature changes after myopic laser in situ keratomileusis

Presented in part at the Annual Meeting of the American Academy of Ophthalmology, in Orlando, Florida, October 1999.
https://doi.org/10.1016/S0161-6420(00)00581-9Get rights and content

Abstract

Objective

To assess the posterior corneal power and asphericity changes after myopic laser in situ keratomileusis (LASIK) and to correlate these changes with the amount of correction and the residual stromal bed thickness.

Design

Prospective nonrandomized (self-controlled) comparative study.

Participants

Fifty-seven eyes of 14 women and 15 men, mean age at the time of surgery 33 ± 9 (range, 19–53) years with a spherical equivalent (SEQ) of −1.00 to −15.50 (mean, −5.07 ± 2.81) diopters (D).

Intervention

All procedures were accomplished with the Keratom II Coherent-Schwind excimer laser and the Moria Model One microkeratome (150-μm head). Subjective refractometry, Orbscan slit scanning corneal topography analysis and pachymetry were performed before and 3 months after LASIK for myopia (n = 35, −1.00 to −15.50 D, mean −4.75 ± 3.07 D) or myopic astigmatism (n = 22, sphere 0.00 to −9.75 D, mean −4.75 ± 2.36 D; cylinder −0.75 to −3.50 D, mean −1.68 ± 0.86 D). Intended ablation depth ranged from 12 to 108 (mean, 48 ± 22) μm. Topographic raw data were decomposed into a set of Zernike polynomials as published in detail previously, and parameters potentially indicative for detection of a “mild keratectasia” were derived.

Main outcome measures

Posterior central corneal power and asphericity before and after LASIK were compared, and changes of these variables were correlated with the SEQ change (ΔSEQ) and the residual corneal bed thickness (RBT).

Results

The mean RBT after LASIK was 280 ± 42 μm. Overall, change of posterior power (−6.28 ± 0.22 D/−6.39 ± 0.23 D, P = 0.02) was statistically significant, and change of asphericity (0.98 ± 0.07/1.14 ± 0.20, P < 0.0001) was highly significant. In eyes with RBT ≤250 μm, the average change of posterior central power (−0.20 ± 0.10 D vs. −0.08 ± 0.18 D) was significantly greater than in eyes with RBT >250 μm (P = 0.003). The change of posterior corneal power correlated significantly with ΔSEQ (P = 0.004) and the RBT (P = 0.002).

Conclusions

Increased negative keratometric diopters and oblate asphericity of the posterior corneal curvature suggest that mild “keratectasia” of the cornea may be common early after LASIK. Further studies with longer follow-up are required to clarify whether this biomechanical deformation is progressive and whether a residual bed thickness of >250 μm can completely prevent it.

Section snippets

Subjects

In this prospective nonrandomized (self-controlled) comparative trial, 57 otherwise healthy eyes (28 right, 29 left, 28 bilateral) of 14 women and 15 men were included that had been nonconsecutively treated by one surgeon (ES) from March 1998 to February 1999 with LASIK in the Centro Medico Docente La Trinidad, Caracas, Venezuela, for myopia (n = 35, −1.00 to −15.50 D, mean, −4.75 ± 3.07 D) or myopic astigmatism (n = 22, sphere 0.00 to −9.75 D, mean, −4.75 ± 2.36 D; cylinder −0.75 to −3.50 D,

Refractive outcome

All the LASIK procedures were uneventful. At 3 months postoperatively, the SEQ ranged from −2.00 to 1.00 (mean, −0.24 ± 0.58) D. For pure myopia, it ranged from −2.00 to 1.00 (mean, −0.19 ± 0.52) D; for myopic astigmatism, it ranged from −2.00 to 1.00 (mean, −0.32 ± 0.69) D (sphere −2.00 to +1.00 D, mean, −0.24 ± 0.72 D; cylinder −0.25 to −0.75 D, mean, −0.50 ± 0.13 D). In eyes with RBT ≤250 μm, it ranged from −2.00 to 1.00 (mean −0.04 ± 0.87) D; in eyes with RBT >250 μm, it ranged from −2.00

Potential and limitations of the orbscan topography/pachymetry system

With conventional Placido-disc technology, the cornea is illuminated by a set of concentric mires, which are reflected off the cornea and imaged by a video camera positioned at the videokeratoscope axis, and the deformation of the mires is used for calculation of anterior corneal curvature. In contrast, the Orbscan is a pancorneal slit-scanning-topography system in which two scanning slit lamps project 40 calibrated beams onto the eye, angled at 45° to the left and to the right of the video

Conclusions

The ability to individually assess the posterior corneal curvature may have important practical value for the detection of mild iatrogenic keratectasia after lamellar keratorefractive surgery. Increased negative keratometric diopters that did correlate well with the amount of intended correction and the residual bed thickness and oblate asphericity of the posterior corneal curvature suggest that mild “keratectasia” of the cornea may be common early after LASIK. Further studies are required to

Acknowledgements

The authors thank Anja Feuerlein for entering data into the database.

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