Quantification and Analysis of Risk Factors associated with Opaque Bubble Layer Formation using Visumax Femtosecond Laser
1. Paul Zhao¹
2. Federico Luengo Gimeno²
3. Hla M Htoon²
4. Chee Hian Tan³
5. Donald T Tan¹
6. Jodhbir Mehta¹
¹Singapore National Eye Centre, Singapore
²Singapore Eye Research Institute, Singapore
³National University of Singapore
Purpose: To describe a repeatable and objective method of measuring the extent of opaque bubble layer (OBL) formation and analyse the risk factors associated with an OBL created by the VisuMax femtosecond laser.
Method: This was a prospective, cohort study on 215 consecutive eyes which had laser in situ keratomileusis for myopic astigmatism. Quantitative analysis of the size of the OBL was performed using Adobe Photoshop 7.0 software. A pilot study was done on 100 eyes by 2 different observers to check for consistency of photoshop processing and analysis. Patient demographics, preoperative visual acuity, refraction, keratometry, intraocular pressure, pachymetry, flap size and thickness, ablation zone and the surgeon performing the procedure were documented.
Results: Analysis of 50 digital images processed by 2 different investigators showed good consistency with a mean difference of only 0.49% (95%CI 0.04 to 0.93) in measured OBL size. Enrolled subjects had a mean age of 32.6 +/- 7.1 years and 63.3% were females. The opaque bubble layer covered a mean of 55.62% +/- 29.4% of the corneal flap. Multivariate linear regression analysis controlling for factors (age, gender and surgeon) showed that increasing central corneal thickness was associated with a larger area of OBL. ( B = 0.158, p= 0.025) and the size of the OBL in Indians was also found to be significantly smaller as compared to Chinese. ( B = -20.578, p= 0.021).
Conclusion: Thicker corneas were associated with a larger opaque bubble layer and Indians tended to have a smaller area of OBL formation as compared to Chinese who underwent the procedure.
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