• Users Online: 39
  • Home
  • Print this page
  • Email this page
Home Current issue Ahead of print Search About us Abstracting and Indexing Editorial board Archives Submit article Instructions Subscribe Contacts Login 


 
 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 12  |  Issue : 3  |  Page : 254-259

Accuracy of different topographic instruments in calculating corneal power after myopic photorefractive keratectomy


1 OcularTissue Engineering Research Center; Labbafinejad Medical Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran
2 Labbafinejad Medical Center; Ophthalmic Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran
3 Ophthalmic Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran
4 Department of Medical Education, School of Medical Education, Shahid Beheshti University of Medical Sciences, Tehran, Iran

Date of Submission19-Apr-2016
Date of Acceptance31-Aug-2016
Date of Web Publication13-Jul-2017

Correspondence Address:
Masumeh Kalantarion
Department of Medical Education, School of Medical Education, Shahid Beheshti University of Medical Sciences, Valiasr St., Tehran 19857-17443
Iran
Sahba Fekri
Ophthalmic Research Center, Shahid Beheshti University of Medical Sciences, Boostan 9 St. Pasdaran Avenue, Tehran 16666
Iran
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jovr.jovr_74_16

Rights and Permissions
  Abstract 

Purpose: To compare the corneal power measurements obtained using different topographic instruments after myopic photorefractive keratectomy (PRK).
Methods: Patients with myopia who were candidates for corneal refractive surgery were sequentially included. Pre-PRK and six months post-PRK corneal powers were measured using Javal manual keratometer, Orbscan II, Galilei, Tomey TMS4, and EyeSys 2000 topographers. Measured values were compared with those obtained using the clinical history method (cHM).
Results: This study included 66 eyes of 33 patients. The lowest keratometric measurements were obtained using the Galilei topographer (42.98 ± 1.69 diopters, D) and the highest measurements were obtained using the Javal manual keratometer (43.96 ± 1.54 D) preoperatively. The same order was observed postoperatively. Effective refractive power (EffRP) measured using EyeSys was most similar to the values obtained using cHM (Icc, intraclass correlation coefficient = 0.951), followed by the total corneal power measured using the Galilei system (Icc = 0.943). The values obtained using the adjusted EffRP formula (EffRP – 0.015*Δ Refraction – 0.05) were more consistent with the values obtained using cHM (Icc = 0.954) compared to those obtained with the adjusted average central corneal power formula measured using the Tomey system (Icc = 0.919).
Conclusion: Post-PRK corneal powers measured using the adjusted EffRP formula were the most similar to values obtained using cHM.

Keywords: Corneal Topography; EyeSys Corneal Analysis System; Galilei Dual Scheimpflug Analyzer; Photorefractive Keratectomy


How to cite this article:
Baradaran-Rafii A, Fekri S, Rezaie M, Salehi-Rad S, Moradi A, Motevasseli T, Kalantarion M. Accuracy of different topographic instruments in calculating corneal power after myopic photorefractive keratectomy. J Ophthalmic Vis Res 2017;12:254-9

How to cite this URL:
Baradaran-Rafii A, Fekri S, Rezaie M, Salehi-Rad S, Moradi A, Motevasseli T, Kalantarion M. Accuracy of different topographic instruments in calculating corneal power after myopic photorefractive keratectomy. J Ophthalmic Vis Res [serial online] 2017 [cited 2017 Sep 24];12:254-9. Available from: http://www.jovr.org/text.asp?2017/12/3/254/210631


  Introduction Top


Photorefractive keratectomy (PRK) is an approved method for application of excimer laser in corneal refractive surgery. It has been shown to be effective and safe in the correction of refractive errors.[1],[2] However, there are still significant challenges including inaccurate measurement of corneal power and intraocular pressure after corneal laser refractive surgery.[3],[4],[5],[6],[7] Intraocular lens (IOL) power calculation is based mainly on the measurement of corneal power, axial length, and estimated lens position after surgery,[8],[9],[10],[11] and is known to be less predictable after refractive surgery.[12],[13],[14] A significant source of error is the fact that most keratometers measure the central corneal radius of curvature and assume a sphero-cylindrical cornea, which is incorrect after myopic refractive surgery,[14] leading to overestimation of the corneal refractive power and consequently, hyperopic outcomes.[12],[15] Furthermore, keratometric measurement errors occur due to an altered corneal refractive index.[14] Since the cornea becomes flattened after myopic laser surgery, the anterior chamber is incorrectly assumed to be shallow in the widely used third generation IOL formulas, such as the Holladay, Hoffer Q, and SRK/T, while it actually has remained negligibly altered.[16]

Various methods have been developed to improve the accuracy of corneal power estimation. For many years, the clinical history method (cHM) has been considered to be the gold standard; however, preoperative measurements are not always available and postoperative refraction may not be accurate and stable because a myopic shift may occur during the progression of cataract. Although newer and more promising approaches including topographic techniques have been introduced, there are still significant conflicts among the outcomes.[17] The aim of this study was to compare the accuracy of cHM with different techniques (manual keratometry, Orbscan II, Tomey, EyeSys, and Galilei) for corneal power calculation after myopic photorefractive keratectomy.


  Methods Top


Patients who were included in the study were candidates for photorefractive surgery aged between 18 to 40 years; had stable myopic refraction for at least 12 months; had no history of systemic disease, autoimmune disease, or immunosuppressive therapy; were not pregnant or lactating; were not using topical ophthalmic medications; and had no contraindication for PRK. Patients who had unstable refraction after surgery or were lost to follow up were excluded. Soft contact lens usage was ceased at least two weeks prior to surgery. The study protocol was approved by our University Ethics committee, and written informed consent was obtained from all participants.

Complete eye exam, manifest refraction, cyclorefraction, intraocular pressure (IOP) measurement, dilated fundus examination, measurement of corneal curvature by Javal manual keratometer (Haag-Streit, Switzerland), Orbscan II (Bausch & Lomb, Rochester, NY), Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems, Switzerland), Tomey TMS-4 (Tomey, Japan), EyeSys 2000 corneal Analysis System (EyeSys Vision, Houston, TX), and pachymetry (Nidek, Japan) were performed for all patients preoperatively and six months post-PRK after stability of refraction.

Scientific Definitions

  • clinical history method (cHM): This is the method of determining the real corneal power following PRK or laser in-situ keratomileusis (LASIK) using preoperative keratometric data as well as preoperative and postoperative refraction:
    Keff = Kpre – ΔRef [18]
    Where Keff is the corneal power to be included in calculation formulas, Kpre is the preoperative mean corneal power, and ΔRef is the change in refraction
    These are measured as spherical equivalent.
  • Average central corneal power (AccP): This is the average of the mean powers of the central Placido rings over the central 3.0 mm of the cornea, as displayed by the Tomey Topography Modeling System.
  • Adjusted AccP (adj AccP): This is one of the formulas for determining the real corneal power following PRK/LASIK using the AccP from the Tomey device:
    Adj AccP = AccP - 0.16*ΔRef [19]
  • Effective refractive power (EffRP): This is the effective refractive power of the cornea within 3 mm that is measured by the EyeSys device:
  • Adjusted EffRP (adj EffRP): This is one of the formulas to determine the real corneal power following PRK/LASIK using the EffRP from the EyeSys device [20]
    Adj EffRP = EffRP – 0.015*Δ Ref – 0.05[20]
  • Total corneal power (TcP): This is the total corneal power within 4 mm measured using the Galilei device.
  • Refraction correction (Rc): This is the corrected refraction at the corneal surface (if vertex distance = 0.012 m).
  • Rc = Spectacle refraction/(1- 0.012*spectacle refraction).


Surgical Technique

After prep, drape, and instillation of three drops of topical tetracaine 0.5%, the corneal epithelium was gently debrided after exposure to ethanol 15% for 20 seconds. Laser ablation was performed using the Nidek Ec-5000 excimer laser machine (Nidek co. Ltd., Japan). The optical zone was 6.0 mm up to 6.5 mm with a transition zone up to 1.5 mm. Treatments were conventional. Immediately after ablation, the ocular surface was rinsed with chilled saline for 10 seconds. This was followed by one drop of topical chloramphenicol 0.5%, a topical nonsteroidal anti-inflammatory drug, and an extended wear soft bandage contact lens. Patients who needed ablation for more than 75 micrometers had a pledget soaked in mitomycin c 0.02% that was applied for 10–30 seconds then rinsed with 50 mL balanced salt solution. After PRK, patients were treated with topical chloramphenicol 0.5% and betamethasone 0.1% four times a day for two weeks. Then, betamethasone was replaced by topical fluorometholone four times a day, which was tapered in two months. Follow-up examinations were scheduled at one day, three days, seven days, one month, three months, and six months postoperatively. No additional surgery was performed during follow-up.

Statistical Analysis

Data was analyzed using the SPSS version 21 statistical software (SPSS Inc., chicago, IL). Normality was checked using the Kolmogorov-Smirnov test. Normally distributed data were expressed as mean ± standard deviation, median, and range, with 95% confidence interval (95% cI). To evaluate the difference between the two sets of data, we used repeated measure analysis of variance. Multiple comparisons were made using the Bonferroni method. Intraclass correlation (Icc) analysis was used to evaluate the difference in measurements between the two sets. In the last step to have a measure of agreement, we used the 95% limit of agreement (95% LOA) and Bland-Altman graph. P values less than 5% (P < 0.05) were considered statistically significant.


  Results Top


A total of 66 eyes of 33 patients (16 men and 17 women) were included in the study. Patients' mean age was 26.9 ± 5 years, ranging from 20 to 40 years. Mean spherical equivalent refraction and mean central corneal thickness were -3.69 ± 1.7 D (range, -7.7 to -2.2 D) and 539 ± 22.1 μm (range, 500 to 589 μm) respectively, at baseline.

Keratometric values at baseline and post-PRK are shown in [Table 1] and [Table 2], respectively. In general, the lowest keratometric measurements were obtained using the Galilei topographer and the highest measurements were obtained using the Javal manual keratometer. Using Icc analysis, EyeSys values, followed by Galilei values, were most consistent with the cHM values compared with other systems [Table 3]. Bland–Altman plots with 95% LOA are shown in [Figure 1]. Total corneal power measured using Galilei had a higher correlation with cHM measurements compared to Orbscan II keratometric values. Moreover, similar results were seen between the manual keratometer and Orbscan II. Adj EffRP formula (using the EyeSys system) was the best predictor of post-PRK corneal power (95% LOA: -0.14 to 0.32) [Table 3].
Table 1. Baseline keratometric readings using different methods

Click here to view
Table 2. Post-PRK keratometric measurements using different methods and formulas

Click here to view
Table 3. correlation of post-PRK keratometric values measured using different methods and formulas with clinical history method as gold standard

Click here to view
Figure 1. Bland–Altman plots (95% limits of agreement) illustrate the differences in corneal power measurements between values obtained using Javal manual keratometer, Orbscan II, Tomey, and Galilei topographers versus values obtained by clinical history method. Manual KR, manual keratometer; cHM, clinical history method; TcP, total corneal power measured by Galilei topographer.

Click here to view



  Discussion Top


Despite improvements in topographic techniques and formulas, there are still significant differences in corneal power measurements after refractive surgery. In this study, we showed that corneal power obtained by using EyeSys, which was used in the adjusted EffRP formula, was the most consistent with cHM values. This was followed by TcP values obtained by using Galilei (Icc, 0.954 vs 0.943) in comparison with other keratometric techniques. In contrast, manual keratometry and Orbscan II were the least consistent. (Icc = 0.912).

The manual keratometers measure only four discrete paracentral points on the anterior cornea. These systems assume that the central cornea is spherical and that the posterior cornea has a radius of curvature 1.2 mm smaller than its anterior counterpart.[21] Earlier topographic devices, including Tomey, calculate the keratometric values within the 3-mm zone; as they are based on the reflection, they fail to consider the posterior plane of the cornea. However, Orbscan II uses a slit scanning method in addition to the Placido disc, which can calculate posterior corneal power using the triangulation method.[21]

Hussein et al [22] developed a method for measuring corneal power by topography within the pupil area and showed that mean central keratometry, using topography in patients with small ablation zones and those with greater corrections, was different from manual keratometry. In contrast, Seitz et al [23] reported that manual keratometry following PRK in myopic patients was the best choice. In the current study, there was not significant difference between post PRK mean keratometric values using Javal, Orbscan II, and Tomey.

Seitz et al [23] and Sonego-Krone et al [24] suggested Orbscan as a criterion for determining the real corneal power following myopic LASIK. In addition, Qazi et al reported satisfactory results after using Orbscan II for determining the real corneal power after refractive surgery.[21] However, the accuracy of this method is not confirmed yet. In the present study, within the 3-mm zone, values obtained using the Orbscan II and manual keratometry were not significantly different from each other, but they were significantly different from those obtained using EffRP, TcP, and cHM (P < 0.01).

The EyeSys system is one of the topographical methods that calculate the mean corneal power within a 3-mm zone. In our study, the mean EffRP calculated using EyeSys was 40.92 ± 2.02 D after PRK, which was significantly lower than Tomey (41.14 ± 2.00 D) and manual keratometry values (41.21 ± 1.99 D) (P < 0.05). The difference may be because the central flat spots are considered in the EyeSys system, unlike in manual keratometry and Tomey system. However, these values were greater than those obtained using cHM (40.62 ± 2.01 D) and TcP 1-4 mm (40.45 ± 2.13 D).

The Galilei Dual Scheimpflug Analyzer is an optical system for corneal topography and three-dimensional analysis of the anterior eye segment based on a revolving dual-channel Scheimpflug camera and a Placido disc. Hua et al [25] reported a high correlation among TcP calculated using Galilei, ray tracing method, IOL master keratometric values, and cHM in post-LASIK eyes. In a review article, Koch [26] reported that IOL calculation formulas based on TcP measurements using with either Scheimpflug or OcT technology may be more promising because they use the patient's actual data, not an average value obtained by regression, and do not require historical data. Schuster et al [27] also showed that using Scheimpflug technology (Pentacam HR) for corneal power measurement and IOL calculation yielded improved postoperative results for patients with previous corneal refractive surgery. In the present study, we also found that there was a considerable correlation between TcP and cHM (Icc = 0.943).

In summary, we showed that the adjusted EffRP formula (using EyeSys system) was the most consistent with cHM (Icc = 0.954, -0.14 to 0.32 95% LoA) followed by TcP (using Galilei system) post-PRK. Although improvements have been achieved using more developed topographic instruments and newer formulas for corneal power measurement after refractive surgery, there are still significant differences among various methods.

Financial Support and Sponsorship

Nil.

Conflicts of Interest

There are no conflicts of interest

 
  References Top

1.
Waring GO, Durrie DS. Emerging trends for procedure selection in contemporary refractive surgery: consecutive review of 200 cases from a single center. J Refract Surg 2008;24:S419-23.  Back to cited text no. 1
    
2.
Rajan MS, Jaycock P, O'Brart D, Nystrom HH, Marshall J. A long-term study of photorefractive keratectomy; 12-year follow-up. Ophthalmology 2004;111:1813-24.  Back to cited text no. 2
    
3.
Rosa N. Standards for reporting results of refractive surgery. J Refract Surg 2001;17:473-4.  Back to cited text no. 3
[PUBMED]    
4.
Rosa N, capasso L, Lanza M, Romano A. Axial eye length evaluation before and after myopic photorefractive keratectomy. J Refract Surg 2005;21:281-7.  Back to cited text no. 4
    
5.
De Bernardo M, capasso L, Rosa N. Algorithm for the estimation of the corneal power in eyes with previous myopic laser refractive surgery. cornea 2014;33:e2.   Back to cited text no. 5
    
6.
cennamo G, Rosa N, La Rana A, Bianco S, Sebastiani A. Non-contact tonometry in patients that underwent photorefractive keratectomy. Ophthalmologica 1997;211:341-3.  Back to cited text no. 6
    
7.
Rosa N, cennamo G. Goldmann applanation tonometry after PRK and LASIK. cornea 2001;20:905-6.  Back to cited text no. 7
    
8.
Rosa N, Furgiuele D, Lanza M, capasso L, Romano A. correlation of changes in refraction and corneal topography after photorefractive keratectomy. J Refract Surg 2004;20:478-83.  Back to cited text no. 8
    
9.
Rosa N, Lanza M, capasso L, Lucci M, Polito B, Romano A. Anterior chamber depth measurement before and after photorefractive keratectomy: comparison between IOL master and Orbscan II. Ophthalmology 2006;113:962-9.   Back to cited text no. 9
    
10.
Rosa N, capasso L, Lanza M. Power calculation after laser refractive surgery. J cataract Refract Surg 2009;35:1653.   Back to cited text no. 10
    
11.
Rosa N, capasso L, De Bernardo M, Lanza M. IOL power calculation after refractive surgery. Ophthalmology 2011;118:2309.  Back to cited text no. 11
    
12.
Seitz B, Langenbucher A, Nguyen NX, Kus MM, Küchle M. Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmology 1999;106:693-702.  Back to cited text no. 12
    
13.
Koch DD. New options for IOL calculations after refractive surgery. J cataract Refract Surg 2006;32:371-2.  Back to cited text no. 13
    
14.
Hamilton DR, Hardten DR. cataract surgery in patients with prior refractive surgery. curr Opin Ophthalmol 2003;14:44-53.  Back to cited text no. 14
    
15.
Chan cc, Hodge c, Lawless M. calculation of intraocular lens power after corneal refractive surgery. clin Exp Ophthalmol 2006;34:640-4.  Back to cited text no. 15
    
16.
Haigis W. Intraocular lens calculation after refractive surgery for myopia: Haigis-L formula. J cataract Refract Surg 2008;34:1658-63.   Back to cited text no. 16
    
17.
De Bernardo M, capasso L, caliendo L, Paolercio F, Rosa N. IOL power calculation after corneal refractive surgery. Biomed Res Int 2014;2014:658350.   Back to cited text no. 17
    
18.
Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy. J Refract Surg 1995;11:490-3.  Back to cited text no. 18
    
19.
Awwad ST, Manasseh c, Bowman RW, cavanagh HD, Verity S, Mootha V, Mcculley JP. Intraocular lens power calculation after myopic laser in situ keratomileusis: Estimating the corneal refractive power. J cataract Refract Surg 2008;34:1070-6.   Back to cited text no. 19
    
20.
Hamed AM, Wang L, Misra M, Koch DD. A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002;109:651-8.  Back to cited text no. 20
    
21.
Qazi MA, cua IY, Roberts cJ, Pepose JS. Determining corneal power using Orbscan II videokeratography for intraocular lens calculationafter excimer laser surgery for myopia. J cataract Refract Surg 2007;33:21-30.  Back to cited text no. 21
    
22.
Husain SE, Kohnen T, Maturi R, Er H, Koch DD. computerized videokeratography and keratometry in determining intraocular lens calculations. J cataract Refract Surg 1996;22:362-6.  Back to cited text no. 22
    
23.
Seitz B, Langenbucher A. Intraocular lens calculations status after corneal refractive surgery. curr Opin Ophthalmol 2000;11:35-46.  Back to cited text no. 23
    
24.
Sónego-Krone S, López-Moreno G, Beaujon-Balbi OV, Arce cG, Schor P, campos M. A direct method to measure the power of the central cornea after myopic laser in situ keratomileusis. Arch Ophthalmol 2004;122:159-66.  Back to cited text no. 24
    
25.
Hua YJ, Huang JH, Pan c, Wang QM. [Assessments of total corneal power and intraocular lens power in post-LASIK eyes]. Zhonghua Yi Xue Za Zhi 2012;92:2339-44. [Article in chinese]  Back to cited text no. 25
    
26.
Koch DD. The Enigmatic cornea and Intraocular Lens calculations: The LXXIII Edward Jackson Memorial Lecture. Am J Ophthalmol 2016;171:xv-xxx.   Back to cited text no. 26
    
27.
Schuster AK, Schanzlin DJ, Thomas KE, Heichel cW, Purcell TL, Barker PD. Intraocular lens calculation adjustment after laser refractive surgery using Scheimpflug imaging. J cataract Refract Surg 2016;42:226-31.  Back to cited text no. 27
    


    Figures

  [Figure 1]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

Top
 
 
  Search
 
Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
Access Statistics
Email Alert *
Add to My List *
* Registration required (free)

 
  In this article
Abstract
Introduction
Methods
Results
Discussion
References
Article Figures
Article Tables

 Article Access Statistics
    Viewed201    
    Printed8    
    Emailed0    
    PDF Downloaded69    
    Comments [Add]    

Recommend this journal


[TAG2]
[TAG3]
[TAG4]