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Multi-optional algorithm helpful for LASIK enhancement
The algorithm can help a surgeon decide whether to do surface ablation on the flap or flap re-lift.
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 Amar Agarwal |
In recent years, LASIK outcomes have improved with the use of better algorithms, ablation profiles, and preoperative and intraoperative iris recognition. Despite these advancements, some post-LASIK patients still have significant visual symptoms and poor uncorrected visual acuity and require an enhancement procedure. We developed and evaluated an algorithm to decide between flap re-lift and surface ablation on the flap for LASIK enhancement.
Method
We retrospectively analyzed 20 eyes that underwent enhancement by surface ablation on the flap and 20 eyes that underwent enhancement by flap re-lift. Significant parameters were used to create a prospective algorithm to decide between surface ablation on the flap and flap re-lift.
The study consisted of three parts.
In the first part of the study, a retrospective medical record assessment of the enhancements was done. Cases were noted if they had (a) spherical equivalent of ±0.5 D or less and astigmatism of 0.5 D or less at 3 months and 6 months after enhancement, (b) stable refractions with follow-up with a variation of 0.25 D or less in sphere, 0.25 D or less in cylinder and 15° or less between the 3-month and 6-month follow-ups, and (c) complete follow-up until 6 months after enhancement. Twenty cases of flap re-lift and 20 cases of surface ablation on the flap were randomly selected from these patients. The minimum residual bed thickness noted in the measurements was used for further calculations and was henceforth called post-LASIK residual bed thickness. The estimated ablation for enhancement (based on the residual refractive error and the optical zone treated) and estimated post-enhancement residual bed thickness (based on post-LASIK residual bed thickness minus ablation to be performed) were noted from medical records. Postoperative spherical equivalent and complications after 3 months and 6 months were noted.
In the second part of the study, an algorithm was created. Based on the significant differences between flap re-lift and surface ablation on the flap, preferred guidelines were created. These results were used as response points to create a multi-optional decision-making flowchart to choose between the two procedures.
The third part of the study was a prospective evaluation of the algorithm. The multi-optional algorithm was created based on clearly defined response protocol derived from pachymetric and ablation parameters found to be significantly different between the two groups (Figure 1). The same flowchart was used for choosing between flap re-lift and surface ablation on the flap in the prospective part of the study.
 Figure 1. Multi-optional sequential algorithm to decide between surface ablation on the flap and re-lift for enhancement. Images: Agarwal A
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Discussion
Surface ablation on the flap emerged as the procedure preferred in clinically critical situations such as previous ablation of more than 100 µm and estimated post-re-ablation residual bed thickness between 250 µm and 275 µm. As is the standard safety convention, ablation was not recommended in a case with post-LASIK residual bed thickness of less than 250 µm. In situations in which the post-LASIK residual bed thickness is more than 250 µm but would fall to less than 250 µm after ablation on the bed, no excimer ablation was recommended. However, in such a situation, there can also be a contradicting view suggesting that a surface ablation on the flap can still be performed as long as it is limited to the flap and does not involve the residual stroma. This is based on the finding that the post-LASIK flap does not seem to contribute to the biomechanical strength in an extent similar to the unablated normal cornea. However, we feel that whatever strength is being provided by the flap should not be further compromised. Furthermore, the limit of 250 µm is not always safe. If the first ablation had decreased the residual stroma to a zone near 250 µm, it is better to avoid further weakening related to excimer ablation.
The ideal regression equation in a scattergram comparing the attempted and achieved correction should be y = x + 0 (y = achieved correction, x = attempted correction), or the line of equivalence. After enhancement in both the groups, the regression equation and the R–squared (which is a measure of the fit of data to the plotted equation) improved, suggesting desired outcome. Spherical equivalent changes achieved in our cases were also comparable in both the groups, with achievement of correction within the target range.
The efficacy of an enhancement procedure is often difficult to measure if only spherical equivalents are analyzed. This is because many patients will have mixed astigmatism due to decentered ablations. Furthermore, the averages will even out, as there will be both hyperopic (overcorrected) and myopic (undercorrected) cases. It is therefore useful to look at the absolute values of spherical errors and their change toward zero as an efficacy of the enhancement procedure. We analyzed the change in absolute spheres in both the groups.
The presence of mixed astigmatism would reduce the spherical equivalent to values close to zero, challenging the indication for enhancement. For example, a spherical equivalent of +0.25 D can represent both +0.25 diopters cylinder/0 diopters cylinder and +2 diopters sphere/–3.5 diopters cylinder. The former case will have a better quality of vision and may not need an enhancement. Therefore, we also used defocus equivalent to analyze the outcomes. The results of both absolute spherical errors and defocus equivalent outcomes were comparable in intergroup analysis and good in intragroup assessment.
For wider applicability of the algorithm, a final adjustment can be done without altering the pachymetric guidelines. This suggested algorithm can include the cases excluded in our study due to grossly decentered flap, flap complications, previous surface ablation or a visually significant scar on the flap. These cases are best dealt with surface ablation. This algorithm is given in Figure 2.
 Figure 2. Modified algorithm including non-pachymetric situations in which surface ablation may be more useful than flap re-lift. |
Conclusion
In conclusion, a judicial selection of cases by this algorithm or a similar one can prevent a surface ablation in cases in which a re-lift may suffice and avoid biomechanical complications related to re-lift in cases in which surface ablation may be more useful.
- Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Eye Hospital and Eye Research Centre. Prof. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; email: dragarwal@vsnl.com; website: www.dragarwal.com.
- Disclosure: No products or companies are mentioned that would require financial disclosure.