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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 109  |  Issue : 4  |  Page : 167-171

Assessment of posterior corneal surface changes after photorefractive keratectomy in moderate myopia


Lecturer of Ophthalmology, October 6 University, Giza, Egypt

Date of Submission23-Dec-2016
Date of Acceptance30-Nov-2016
Date of Web Publication19-Apr-2017

Correspondence Address:
Tamer H El-Sersy
11 Montaza St., Cairo, 11311
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2090-0686.204728

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  Abstract 


Aim
The aim of this work was to study the changes that occur in the posterior corneal surface after photorefractive keratectomy (PRK) in moderate myopia using pentacam.
Patients and methods
This prospective study included 30 myopic eyes of 17 patients with spherical equivalent −2.00–6.00 D who underwent PRK using Visx S4 Excimer Laser. Pentacam examination was carried out for all our eyes at preoperatively, and 1 week and 3 months postoperatively. Pentacam was used to compare preoperative and postoperative measurements of the anterior and posterior corneal curvatures, and asphericity of the posterior corneal surface.
Results
This study included 30 myopic eyes of 17 patients, seven women and 10 men. Patients’ ages ranged from 28 to 40 years; the mean age was 30.3±3.03 years. No statistically significant changes were found in the posterior horizontal corneal curvature. A statistically significant change in the posterior vertical corneal curvature from 6.1±0.3 to 6.06±0.2 mm 1 week after PRK was found, but it increased after 3 months to 6.2±0.3 mm. No statistically significant changes in the posterior mean corneal curvature were observed either at 1 week or 3 months after PRK. A significant change in the Q-value (asphericity) was found both at 1 week and 3 months postoperatively.
Conclusion
After PRK, changes in the posterior corneal surface affect mainly the vertical curvature asphericity with forward elevation of the posterior corneal surface.

Keywords: corneal surface, keratectomy, myopia, photorefractive, posterior


How to cite this article:
El-Sersy TH. Assessment of posterior corneal surface changes after photorefractive keratectomy in moderate myopia. J Egypt Ophthalmol Soc 2016;109:167-71

How to cite this URL:
El-Sersy TH. Assessment of posterior corneal surface changes after photorefractive keratectomy in moderate myopia. J Egypt Ophthalmol Soc [serial online] 2016 [cited 2020 Jul 13];109:167-71. Available from: http://www.jeos.eg.net/text.asp?2016/109/4/167/204728




  Introduction Top


Photorefractive keratectomy (PRK) was the first type of corrective eye surgery to use a laser rather than a blade to remove corneal tissue. Excimer laser was developed in the early 1970s and modified for ophthalmic use in the early 1980s. PRK became less popular following the development of LASIK, a procedure that allowed patients to have their vision corrected without the need for extended recovery from surgery [1]. Trans-PRK is a relatively new modification of the conventional PRK, in which the epithelial removal is achieved with an excimer laser instead of alcohol and manual scraping. This is supposed to create a smoother crater allowing relatively rapid healing of the epithelium with a resulting faster visual recovery and less discomfort [2].

Pentacam can analyze the anterior and posterior corneal surfaces on the basis of the true elevation measurement from limbus to limbus. The data are collected in around 2 s, and −25 000 data points are taken. The data can be represented as elevation data or radius of curvature data [3].

Aim

The aim of this work was to study the changes that occur in the posterior corneal surface after PRK for the correction of myopia using Pentacam.


  Patients and methods Top


This was a prospective intervention case-series study that involved 30 myopic eyes of 17 patients subjected to PRK. All patients provided written informed consent for their medical information to be included in study analyses. Pentacam examinations were performed preoperatively, and at 1 week and 3 months postoperatively.

Patients with systemic or ocular disease that may interfere with the healing process of the cornea, such as rheumatoid arthritis, dry-eye syndrome, anterior or posterior uveitis, keratoconus, corneal dystrophy or degeneration, glaucoma, and retinal diseases, lens opacity, history of severe ocular trauma, and previous ocular surgery were excluded from the study.

All patients were subjected to the following:


  1. Assessment of history.
  2. Complete ophthalmological examination:
    1. Slit-lamp examination of the anterior segment.
    2. Manifest and cycloplegic refraction assessment.
    3. Measurement of the uncorrected visual acuity (UCVA) and best-corrected visual acuity.
    4. Measurement of the intraocular pressure.
    5. Fundus examination with examination of the peripheral retina.
    6. Contact lens wearers were instructed to remove them for at least 2 weeks before the Pentacam examination.


The patients underwent PRK using Visx S4 Excimer Laser (Abbott Medical Optics Inc., Santa Ana, California, USA) with PRK platform. After standard disinfection and ocular draping with instillation of topical benoxinate hydrochloride 0.4% as a topical anesthetic, the cornea was exposed to 20% ethyl alcohol for 15 s with the aid of a well. Thereafter, the epithelium was removed manually in a centripetal manner using a blunt hockey blade. The diameter of epithelial removal was 8 mm. Thereafter, 1050-Hz excimer laser, with variable beam sizes from 0.65 to 6.5 mm, was used to achieve the desired ametropia.

A topical application of mitomycin-C 0.02% (0.2 mg/ml) diluted in balanced salt solution was instilled in each eye, with a microsponge placed over the ablated stroma for 15–30 s immediately after laser ablation. The corneal surface and the entire conjunctiva were then vigorously irrigated with 20 ml cold, balanced salt solution to remove the residual mitomycin-C. At the end of the procedure, a bandage contact lens was applied followed by one drop of tobramycin–dexamethasone suspension 0.1%.

Patients were instructed to use topical prednisolone acetate suspension 1% four times daily, tapered over 4 weeks according to corneal haze and intraocular pressure, ketorolac tromethamine ophthalmic solution 0.4% three times daily for 2 weeks, moxifloxacin hydrochloride ophthalmic solution 0.5% four times daily for 1 week, and ibuprofen 400 mg tablet orally twice daily for 4 days [4].

Bandage contact lens was removed on the fourth to sixth postoperative day, guided by epithelial healing and patient’s comfort.

Pentacam examination using Oculus Pentacam (The Pentacam software version 6.02r10, Oculus Pentacam, Münchholzhäuser Straße 29, 35582 Wetzlar, Germany) was performed preoperatively and at 1 week and 3 months after PRK for the assessment of the following:
  1. Preoperative and postoperative corneal thickness (pachymetry).
  2. Preoperative and postoperative anterior and posterior corneal elevation by fitting the corneal shape to a best-fit sphere.
  3. Preoperative and postoperative asphericity of the posterior corneal surface, which corresponds to the Q-value.


Correlations of significant changes on the posterior surface were assessed with refraction, residual bed thickness, the depth of laser ablation, and with each other.

Statistical analysis was carried out using the SPSS statistical package (version 11; SPSS Inc., Chicago, Illinois, USA).


  Results Top


This study included 30 myopic eyes of 17 patients diagnosed with moderate myopia subjected to PRK. Seven cases were female and the remaining 10 cases were male in this study. Patients’ ages ranged from 28 to 40 years, with a mean age of 30.3±0.03 years.

Preoperative refraction ranged from −2.00 to −6.00 D spherical error and from −0.25 to −3 D cylindrical error, with a mean spherical equivalent of −4.5±1.11 D. Preoperative pachymetry ranged from 480 to 580 μm, with a mean of 519.5±33.5 μm. The preoperative radius of the posterior horizontal meridian ranged from 5.8 to 7.1 mm, with a mean of 6.5±0.3 mm. The preoperative radius of posterior vertical meridian ranged from 5 to 6.3 mm, with a mean of 5.9±0.3 mm. The preoperative mean radius of the corneal back ranged from 5.5 to 6.3 mm, with a mean of 6.1±0.2 mm. The preoperative flat posterior keratometric power ranged from 5.1 to 6.2 D with a mean of 5.9±0.2 D and the steep posterior keratometric power ranged from −5.5 to 6.8 D, with a mean of 6±0.3 D. The mean posterior keratometric power of the corneal back ranged from −5.3 to 6.5 D, with a mean of −6.1±0.2 D. The preoperative asphericity ranged from −0.82 to 0.06, with a mean of −0.19±0.17 ([Figure 1] and [Figure 2]).
Figure 1 Pentacam examination (four-map display).

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Figure 2 Elevation maps of the posterior corneal surface of the right eye of the same patient (a) preoperatively, (b) 1-week postoperatively, and (c) 3-month postoperatively.

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Contact lens was removed in average by the sixth postoperative day, and there was a strong positive correlation (+0.929) between the depth of ablation and the period needed to achieve healing of the epithelium.

There was a strong positive correlation (+0.904) between wider optical zone and postoperative pain on the first postoperative day. Pain was significantly worse by the end of the first week.

Minimal and nonsignificant haze was noticed in patients and it tended to fade away by the end of the sixth month of the study.

No complications were recorded in any of the patients in our cases. The UCVA was significantly better on the first day, with a mean of 0.7±0.23.

This study suggests that PRK affects the vertical asphericity of the posterior corneal surface ([Table 1] and [Table 2]).
Table 1 Operative outcomes

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Table 2 Comparison of anterior and posterior corneal elevations preoperatively and at 1 week and 3 months postoperatively by fitting the corneal shape to a best-fit sphere

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  Discussion Top


This study included 30 myopic eyes of 17 patients subjected to PRK. Pentacam examination was performed preoperatively and at 1 week and 3 months postoperatively. Pentacam was used to compare preoperative and postoperative measurements of anterior and posterior corneal elevations, and asphericity of the posterior corneal surface. This prospective study was carried out to evaluate the effect of PRK on the posterior corneal surface and to assess its changes at 1 week and 3 months after PRK using the oculus pentacam.

The study included myopic patients eligible for PRK, with the exclusion of any patient unfit for PRK and pediatric patients as the size and shape of the cornea change throughout infancy and childhood. Moreover, eyes with previous corneal or refractive surgery were excluded to avoid other factors that may affect the results. Assessment of the posterior corneal surface in terms of curvatures, asphericity, and elevation maps was performed in all patients preoperatively and at 1 week and 3 months postoperatively. Correlation of these changes with refraction, residual bed thickness, and the amount of laser ablation was determined. Millimeter was used for the evaluation of corneal curvatures rather than diopter as the former is more representative for anatomical changes and to avoid the confusion caused by the use of negative and positive signs with diopters.

PRK showed superior visual results during the early postoperative period. There was no statistically significant difference in the outcomes of UCVA by months. Similar results were obtained by Hatch et al. [4], as they compared PRK with thin-flap LASIK as regards visual outcome. Our visual outcome is better than that obtained by Wang et al. [2] as 80% of trans-PRK eyes in our study achieved UCVA of 1.00 by the end of the first postoperative month compared with only 39% of the patients in their study; this shows the superior outcome to the single-step trans-PRK provided by Schwind Amaris 500E compared with the older Schwind ESIRIS that utilized phototherapeutic keratectomy mode followed by PRK in the study by Wang. Another older study carried out by Clinch et al. [5] to compare trans-PRK with mechanical removal of epithelium showed more humble results, with only 17.9% of patients in the trans-PRK group reaching a UCVA of 1.0 by the end of the first postoperative month.

Emmetropia (±0.5 D) was achieved in all patients included in the study by the end of the sixth postoperative month. These results are comparable to the results by Wang et al. [2].

Pain was significantly worse in our study with an average score of 3.2±1.1 until the end of the first postoperative week, and then subsided markedly such that no significant difference in pain was observed. Similar average pain (3±1.2) score was found by Buzzonetti et al. [6] as they utilized Nidek CXIII Excimer Laser (Nidek, Fremont, California, USA) to perform trans-PRK.

Haze was not a significant issue in our study, and in all patients it regressed by the end of the study. These results could be compared with those obtained by Hashemi et al. [1], with a slightly higher score of haze.

Our study showed a significant forward elevation of the posterior corneal surface; this finding is in accordance with that obtained by Lee et al. [7].


  Conclusion Top


After PRK, changes in the posterior corneal surface affect mainly the vertical curvature asphericity with forward elevation of the posterior corneal surface.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Hashemi H, Taheri SMR, Fotouhi A, Kheiltash A. Evaluation of the prophylactic use of mitomycin-C to inhibit haze formation after photorefractive keratectomy in high myopia: a prospective clinical study. BMC Ophthalmol 2004; 4:12.  Back to cited text no. 1
    
2.
Wang DM, Du Y, Chen GS, Tang LS, He JF. Transepithelial photorefractive keratectomy mode using SCHWIND-ESIRIS excimer laser: initial clinical results. Int J Ophthalmol 2012; 5:334–337.  Back to cited text no. 2
    
3.
Naroo SA, Cervino A. Corneal topography and its role in refractive surgery. In: Naroo SA, editor. Refractive surgery: a guide to assessment and management. Oxford, United Kingdom: Butterworth–Heinemann; 2004.  Back to cited text no. 3
    
4.
Hatch BB, Moshirfar M, Ollenon AJ, Sikder S, Mifflin MD. A prospective, contralateral comparison of photo refractive keratectomy (PRK) versus thin-flap LASIK: assessment of visual function. Clin Ophthalmol 2011; 5:451–457.  Back to cited text no. 4
    
5.
Clinch TE, Moshirfar M, Weis JR, Ahn CS, Hutchinson CB, Jeffrey JH. Comparison of mechanical and transepithelial debridement during photorefractive keratectomy. Ophthalmology 1999; 106:483–489.  Back to cited text no. 5
    
6.
Buzzonetti L, Petrocelli G, Laborante A, Mazzilli E, Gaspari M, Valente P, Francia E. A new transepithelial phototherapeutic keratectomy mode using the NIDEK CXIII excimer laser. J Refract Surg 2009; 25:S122–S124.  Back to cited text no. 6
    
7.
Lee H, Kang DSY, Ha BJ, Choi JY, Kim EK, Seo KY, Kim T-I. Changes in posterior corneal elevations after combined transepithelial photorefractive keratectomy and accelerated corneal collagen cross-linking: retrospective, comparative observational case series. BMC Ophthalmol 2016; 16:139.  Back to cited text no. 7
    


    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]



 

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