Transepithelial surface ablation

March 24, 2015

PRK has been an established method for laser vision correction for nearly 30 years, however, its popularity has reduced somewhat due to the advent of LASIK. With a recent renewed interest in surface ablation techniques some modifications have been made to alleviate disadvantages of the procedure. In this article, the authors highlight their clinical experience of TESA in myopic eyes with or without astigmatism.

Transepithelial surface ablation

Clinical experience in myopic eyes with or without astigmatism

By Dr Amir Hamid, FRCOphth, CertLRS, Dr Sajjad Mughal, FRCS, CertLRS and Mr Arif Sokwala, BSc

Photorefractive keratectomy has been an established method for laser vision correction for nearly 30 years.1,2 Its popularity amongst both patients and surgeons has reduced since the advent of laser‑assisted in situ keratomileusis (LASIK).

However, there has been a renewed interest in surface ablation as a method for correcting refractive errors due to the advantages of maintaining corneal biomechanical strength and elimination of flap-related complications and optimization of the methods for epithelial removal. The disadvantages of surface ablations have traditionally been postoperative pain, delayed epithelial healing and stromal haze. Modifications to PRK have been introduced to alleviate these issues. The key area that has been addressed in this respect is that of epithelial removal prior to excimer laser ablation. Ethanol has commonly been used as an alternative to mechanical debridement with preservation of the epithelial flap. An even more recent development is Transepithelial PRK (t-PRK) where epithelial removal is performed via laser phototherapeutic keratectomy (PTK) either through a two step or single step process.

Epithelial removal with surface ablation (two step vs single step)

t-PRK (2 step method)

Several studies have looked at the results of 2-step surgery using older generation broad beam laser systems in comparison to ethanol assisted PRK.3–5 The results of two studies4,5 demonstrated better outcomes but in one there were overcorrections.3 In these studies the epithelium was ablated using a broad beam (even) PTK profile. A PTK treatment was initially performed to ablate the epithelium followed by a PRK procedure for correction of refractive error.


Transepithelial surface ablation (TESA)
(1 step method)

The Schwind Amaris excimer laser (Schwind eye-tech-solutions, Kleinostheim, Germany) achieves epithelial and stromal ablation in a single uninterrupted step that consists of uniform precise epithelial removal followed by stromal ablation.

TESA differs from the earlier two-step methods by using a customized epithelial profile derived from population based high frequency digital ultrasonography. These studies demonstrated that the corneal epithelium is thicker in the periphery.6 This customized profile ablates 55 um centrally and 65 um peripherally for a typical 8 mm total ablation zone. There is a further compensation for differential ablation between epithelium and stroma. Furthermore, TESA will also compensate for the phenomena of ‘epithelial masking’ in areas of stromal irregularity to achieve smoother ablations.

Visual outcomes of single step t-PRK/TESA

Fadlallah et al.7 found the visual outcomes comparable between single step t-PRK and a conventional alcohol-assisted PRK group. The postoperative mean sphere and mean astigmatism in the single step t-PRK group was -0.21 ± 0.61 and +0.43 ± 0.62 respectively. There was significantly less postoperative pain and rapid complete epithelial healing in the single step t-PRK group. Uncorrected distance visual acuity (UDVA) was not significantly different between the two groups at 3 months. Postoperative corneal haze can also occur after PRK and their study found at postoperative 3 months, 10% of eyes in the single step t-PRK group had grade 1 haze compared to 26% in the alcohol-assisted PRK group.


Aslanides et al.8 found both single step t-PRK and the alcohol-assisted PRK groups to have safe outcomes. Their primary finding was that in the single step t-PRK group, patients had less early postoperative pain and photophobia on the third postoperative day with rapid epithelialization. Patients in this group also had better vision by 3 Snellen lines on this day. Corneal haze was significantly less at 1, 3 and 6 months (0.2 versus 0.43) but by year 1 there was no haze present in both groups. At postoperative 1-month, there was no significant difference in the unaided Snellen visual acuity (0.94 versus 0.97). Similarly, Fadlallah et al.7 also found no significant difference in visual acuity between the 2 groups at the 1 month and 3 month postoperative periods.
Luger et al.9 found that between the single step t-PRK group and an alcohol-assisted PRK, the postoperative mean spherical equivalent (SE) 1 year after surgery was +0.07 D ± 0.23 and +0.01 D ± 0.27 respectively and 97% of eyes in both groups achieved an UDVA of 0.1 logMAR or better.

TESA: Our results

We recently presented our unpublished clinical results at the 15th International Schwind Users Meeting 2014 in Vancouver, Canada,9 and at the XXXII Congress of the ESCRS 2014 in London, UK.10 In our retrospective analysis of patients treated in our Optimax Laser Eye Clinics in UK by five surgeons, 399 eyes underwent single-step laser epithelial removal and stromal ablation using the transepithelial PRK nomogram of the Amaris laser’s ORK-CAM software (Schwind eye-tech-solutions).

All eyes underwent ablation with an Aberration-Free algorithm with the Schwind Amaris at a repetition rate of 750 Hz pulse with 1050 Hz eye tracking. The laser ablation was centred on the pupillary axis. The intended refractive aim for all eyes was emmetropia and there were no retreatments included. Adjunct mitomycin C was not used in any patient.

The preoperative manifest SE was –3.88 ± 1.47 D (range: –1.25 to –8.00 D). At 1 month the postoperative manifest SE was reduced to –0.20 ± 0.53 D (range: –4.88 to 1.88) and at 3 months it was –0.17 ± 0.18 D (range: 0.88 to –1.25 D). The manifest SE was within 0.50 D and 1.00 D of emmetropia in 89% and 99% of eyes, respectively (Figure 1). At 3 months, the preoperative manifest sphere was reduced from –3.58 ± 1.44 D (range: –0.50 to –7.75 D) to –0.05 ± 0.33 D (range: +1.25 to –1.00 D) and the preoperative manifest astigmatism was reduced from –0.60 ± 0.53 (range: 0 to –3.50 D) to –0.25 ± 0.25 D (range: 0 to –1.75 D). UDVA of 20/25, 20/20 and 20/16 or better was achieved in 20%, 45% and 24% of 399 eyes, respectively (Figure 2). A gain of 1 or more lines was observed in 25% of eyes. Postoperative corneal haze of ≥1.5 was observed in 2% of eyes only. Figures 4 and 5 display the visual outcomes of attempted versus achieved SE and stability of SE over time.

TESA in summary

TESA is effective at producing very safe and predictable visual outcomes for mild to moderate simple myopia or compound myopic astigmatism. We have also been able to demonstrate refractive stability and safety (clinically significant corneal haze is uncommon).
The role of the corneal epithelium as a smoothing agent in relation to any underlying stromal topographical irregularity allows transmission of the smoothness of the aspheric ablation profile to the underlying stroma. This yields more predictable refractive results and less induction of clinically significant corneal aberrations.

When t-PRK/TESA is used instead of ethanol assisted PRK there is the theoretical benefits of reduced postoperative dry eye, chronic ocular surface disease and recurrent corneal erosions. This is due to the reduced levels of keratocyte apoptosis and hence reduced generation of proinflammatory mediators.11,12 this may also explain the lower levels of postoperative pain reported and more rapid epithelial healing.8

TESA represents an effective new method of surface ablation which is safe, predictable, stable and effective. Intraoperatively it is a very easy experience for a patient to undergo as it is painless, no touch technique (bladeless).


References

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6.    D.Z. Reinstein et al., J. Refract. Surg., 2008;24:571–581.
7.    A. Fadlallah et al., J. Cataract Refract. Surg., 2011;37:1852–1857
8.    I.M. Aslanides et al., Clin. Ophhalmol., 2012;67:973–980
9.    http://www.eye-tech-solutions.com/en/home/information-centre/events/ (S. Mughal et al.)
10.    http://www.escrs.org/london2014/programme/free-papers-details.asp?id=21925 (S. Mughal et al.)
11.    W.-J. Kim, S. Shah and S.E. Wilson, J. Refract. Surg., 1998;14:526–533.
12.    J.Y. Oh, J.M. Yu and J.H. Ko, Invest. Ophthalmol. Vis. Sci., 2013;54(6):3852–3856.