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  5. FP509 : Validity of Percentage Tissue Altered (PTA) as a Screening Formula for Post- LASIK...

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Dr. Karan Bhatia, B15311, Dr. Renuka Rati, Dr. Aniket Shastri, Dr. Satyamurthy K V

ABSTRACT

Purpose – To calculate the percentage tissue altered (PTA) in post-laser assisted in situ keratomileusis (LASIK) eyes and to validate its role as an independent factor to evaluate ectasia in Indian population.

Design – Retrospective Study

Methods – 333 eyes with normal pre-operative corneal topography by combined Placido and Scheimpflug Imaging based topography system (SIRIUS) underwent LASIK between 2011 and 2014 at a tertiary level teaching hospital in south India. Pre-operatively patient’s refraction, flap thickness (FT), ablation depth (AD), residual stromal bed and central corneal thickness (CCT) were recorded. The formula used was – PTA = (FT+AD)/CCT. PTA was grouped into <0.4 (low risk), 0.4-0.45 (moderate risk) and >0.45 (high risk). All patients were called for follow up and underwent a topography to look for ectasia.

Results – 60.1%, 29.1% and 10.8% patients had PTA of <0.4, 0.4-0.45 and >0.45 respectively. However, after a minimum follow up of 2 years, none of the patients had any sign of ectasia.

Conclusion – Careful selection of patients is mandatory before proceeding for LASIK. The role of PTA >0.4 as an independent risk factor for post-LASIK ectasia is to be evaluated further. Other factors or a modified formula suitable for Indian eyes needs to be investigated.

Key words – LASIK, Percentage Tissue Altered, Ectasia

Introduction

LASIK has emerged as the gold standard surgery for correction of refractive errors, especially myopia. The number of these surgeries has increased in recent years. Post-LASIK ectasia is a known, but rare complication which can scare any refractive surgeon. It occurs when surgery is done is a weak/thin cornea or when normal cornea is weakened beyond the safety limits. Various risk factors have been described in literature like abnormal corneal topography and tomography, residual stromal bed, central corneal thickness, ectasia risk score system scores, high myopia, age etc. Recently, the role of percentage tissue altered (PTA) has come up. Santhiago et al first described it as PTA = (FT + AD)/ CCT, where FT – flap thickness, AD – ablation depth, CCT – preoperative central corneal thickness. According to him, PTA > 0.4 was the most important risk factor. [1,2]

This aim of this study was to calculate the PTA in post-LASIK eyes and to investigate whether PTA could be an independent factor for post-LASIK ectasia in the Indian population.

Methods

This retrospective study was done analysing patients between 18-40 years of age, operated for myopia and myopic astigmatism with normal bilateral pre-operative corneal topography by combined Placido and Scheimpflug Imaging based topography system (SIRIUS) by laser in-situ keratomileusis from 2011-14 at a tertiary level teaching eye hospital in south India. The study was approved by the institutional review board as per the tenets of the Declaration of Helsinki. Eyes having history of any previous intra-ocular surgery were excluded from the study.

The Pre-operative data recorded included patient’s age, sex, refraction, spherical equivalent (SE),date of surgery, flap thickness (FT), ablation depth (AD), residual stromal bed (RSB) and central corneal thickness (CCT). Degree of myopia was graded by amount of SE as low (<-4 D), moderate (-4 to -6 D) and high (>-6 D) Percentage tissue altered (PTA) was calculated as described by Santhiago et al as PTA = (FT+AD)/CCT.[1] PTA < 0.4, 0.4-0.45 and >0.45 was considered as low, moderate and high risk for ectasia respectively. All data was formulated in Microsoft excel sheet and analysis was done. All patients were called for a follow up visit aftera minimum period of 2 years and underwent corneal topography scan with SIRIUS to look for signs of ectasia.

A normal preoperative scan was defined as having regular pattern without any asymmetry or mild asymmetry (steeping <0.5D and without any skewed radial axis). Post-operative ectasia was defined as progressive inferior steeping, increasing myopia and astigmatism, worsening of uncorrected and best corrected visual acuity.[1]

Results

Three hundred and thirty three eyes with normal pre-operative corneal topography by SIRIUS who underwent LASIK between 2011 and 2014 were retrospectively analysed. The average age was 23.35 years, with females 1.6 times more involved in the study. The spherical equivalent ranged from -0.625 to -12 D with an average of -4.95 D. 34.53 %, 41.74%, 23.72% eyes had low (<-4 D), moderate (-4 to -6 D) and high (> -6 D) degrees of myopia retrospectively.

The mean attempted laser ablation was 123.09 µ+ 9.52(SD) and the mean planned RSB was 334.45 µ + 36.05(SD). 60.06%, 29.13% and 10.81% patients had PTA of <0.4, 0.4-0.45 and >0.45 respectively[Table 1]. However, after a minimum follow up of 2 years, none of the patients had any sign of ectasia.

Discussion

Most patients who develop post-LASIK ectasia have identifiable risk factors or irregular topographic patterns. The presence of ectasia in patients with normal pre-operative corneal topography raises an alarm. PTA as an independent risk factor for developing post-LASIK ectasia is a new emerging concept, which was first described by Santhiago et al. The explanation was that the anterior 40% stroma contributes maximum strength to the cornea, which gets modified after laser refractive surgery. The flap itself does not give any significant biomechanical contribution. Its removal causes corneal weakening as the 40% threshold is crossed.According to them, PTA ismore sensitive than RSB and that ectasia had occurred in corneas with normal thickness, normal RSB and even normal topography. However, a high PTA did not mean that ectasia will occur. It merely meant that these eyes carried an increased risk of ectasia. Most ectasia cases manifest in the first 18 months.[1]

We retrospectively analysed 333 eyes of patients aged between 18-40 years who were operated for myopia and myopic astigmatism with normal bilateral pre-operative corneal topography by combined Placido and Scheimpflug Imaging based topography system (SIRIUS) by laser in-situ keratomileusis from 2011-14 at a tertiary level teaching eye hospital in south India and took corneal tomography scans to look for ectasia after a minimum follow up period of 2 months. In our series, we found that 40% cases had a PTA>0.4 and that none of them had any sign of ectasia after the minimum defined period. Our current results proved PTA cannot be relied on as an independent factor to predict post-operative ectasia. Hence, around 133 eyes benefitted from the procedure, without any complication; who otherwise would have been rejected for LASIK.

Santhiago et al screened normals by placido based corneal topography. However, nowadays newer methods like Corneal tomography are available, which take into account both the anterior and posterior elevation indices. This makes a better pre-operative screening possible. Hence, the risk of post-operative ectasia decreases further.[1]

Saad et al also calculated PTA in post-LASIK American eyes and did not find anyectatic eye over a mean follow-up of 30 months.[3]

Our strength in the study is that we have analysed Indian eyes with a good sample size and follow up period. Our study is limited as it is a retrospective one.

With recent advances in technology, it is possible to make flaps more predictable with the use of femtosecond laser. The flap architecture using a microkeratome is that of meniscus and with femtosecond laser a planar one.[4,5] Moreover, with the use of femtosecond laser the flap construction error reduces from 24µ (microkeratome) to 6µ.[5] This makes the flaps more predictable and, hence, the PTA more accurate.[4,5,6]

Conclusion

Careful selection of patients is mandatory before proceeding for LASIK. The role of PTA >0.4 as an independent risk factor for post-LASIK ectasia is to be evaluated further. Other factors or a modified formula suitable for Indian eyes needs to be investigated. 

References

  1. Santhiago MR et al. Association Between the Percent Tissue Altered and Post-Laser In Situ Keratomileusis Ectasia in Eyes with Normal Preoperative Topography. AJO.2014;87-95.
  2. Devi SK, Singh R, Azimeera S, Vanathi M. Post Lasik Ectasia: Recent Concepts. DOS Times. 2017;22(4); 17-19.
  3. Saad A, Binder PS, Gatinel D. Evaluation of the percentage tissue altered as a risk factor for developing post-laser in situ keratomileusis ectasia. J Cataract Refract Surg. 2017;43(7):946-951
  4. Santhigo MR, Kara-Junior N, Waring GO. Microkeratome versus femtosecond flaps: accuracy and complications. CurrOpinOphthalmol. 2014; 25(4):270-4
  5. Von Jagow B, Kohnen T. Corneal architecture of femtosecond laser and microkeratome flaps imaged by anterior segment optical coherence tomography. J Cataract Refract Surg. 2009;35(1):35-41
  6. Jhang Y, Chen YG, Xia YJ. Comparison of corneal flap morphology using AS-OCT in LASIK with the WaveLight FS200 femtosecond laser versus a mechanical keratome. J Refract Surg. 2013;29(5):320-4

Tables –

Table 1: showing percentage of PTA

Risk of Ectasia PTA Number (%)
Low <0.4 200 (60.06%)
Moderate 0.4-0.45 97 (29.13%)
High >0.45 36 (10.81%)
Total 333

PTA – Percentage Tissue Altered

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