Patterns of Corneal Astigmatism in Eyes Post Myopic Laser Keratorefractive Surgery.

Purpose: In the absence of a measured posterior keratometry input, toric intraocular lens power formulae in patients with a history of myopic laser keratorefractive surgery (M-LVC) predict the posterior corneal astigmatism. These formulae assume that the posterior corneal curvature is essentially unchanged post M-LVC and hence its characteristics in this demographic of patients would be the same as virgin eyes. We aim to describe astigmatism measured by a swept-source optical coherence tomography biometer in such patients.

Patients and methods: Retrospective consecutive case review of eyes with prior M-LVC, followed by subsequent cataract surgery. Pre-operative biometric data was collected using the IOLMaster 700 (Carl Zeiss Meditec). Magnitude and orientation of the steep and flat meridian of standard keratometry (SK), posterior keratometry (PK) and total keratometry (TK) were recorded. Relationships between corneal astigmatism, age, and gender were assessed. Statistical analysis was performed using linear regression, Mann Whitney U-test and Chi squared test.

Results: A total of 341 eyes (218 patients) were studied. Mean SK was 39.65D ± 2.16D. Mean PK was -5.82D ± 0.24D. Mean TK was 39.15D ± 2.34D. SK astigmatism (SKA) was predominantly vertical (57.8%, n = 197 eyes) with a mean magnitude of 0.85D ± 0.47D. Posterior corneal astigmatism (PCA) was predominantly vertical (91.5%, n = 312 eyes) with a mean magnitude of 0.29D ± 0.14D. TK astigmatism (TKA) was predominantly vertical (42.2%, n = 144 eyes) with a mean magnitude of 0.83D ± 0.47D. There was a positive correlation between magnitude of TKA with SKA and PCA (P < 0.001). A negative correlation between age and magnitude of SKA, PCA and TKA was noted (P < 0.001). No statistically significant relationship with gender was noted.

Conclusion: PCA orientation of post M-LVC eyes is similar to published data of virgin eyes. This reinforces the assumption made in toric intraocular lens power formulae that posterior corneal astigmatism in these eyes share the same characteristics as virgin eyes.