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 Table of Contents  
ORIGINAL ARTICLE
Year : 2014  |  Volume : 107  |  Issue : 3  |  Page : 153-159

Comparison between the Scheimpflug camera and the topographic modeling system in the corneal assessment before refractive surgery


Department of Ophthalmology, Faculty of Medicine, Alexandria University, Alexandria, Egypt

Date of Submission09-Mar-2014
Date of Acceptance14-Jun-2014
Date of Web Publication30-Dec-2014

Correspondence Address:
Mohamed M. A. Lolah
el Eskandrany Street. Moharam Beck, Alexandria, 21515
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2090-0686.148123

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  Abstract 

Purpose
To compare corneal measurements by a Scheimpflug camera and a topographic modeling system (TMS 5) in patients seeking refractive surgery.
Design
Prospective comparative study between two instruments.
Setting
RoayaLasik Center, Alexandria, Egypt.
Patients and methods
This study included 30 eyes of patients seeking refractive surgery, selected randomly, between 18 and 35 years old without any history of corneal surgery. An informed consent was obtained from all patients before participation in the study. All the eyes were assessed by the Scheimpflug camera (WaveLight ALLERGO Oculyzer) and the TMS 5. The corneal measurements of the two assessments were compared.
Results
The mean central corneal thickness (CCT) measurements by the Scheimpflug camera and the TMS 5 were 549.3 ± 28.8 and 537.3 ± 26.6 μm, respectively, with a statistically significant difference between the two devices (P < 0.001). There was a statistically significant difference between their mean keratometric readings (P < 0.001). Astigmatism and the anterior maximum elevation mean readings showed no statistically significant difference between the two devices, although the difference between posterior maximum elevation mean readings in the two devices was statistically significant (P < 0.001).
Conclusion
CCT readings of the TMS 5 were thinner than the CCT readings of the Scheimpflug system. The keratometric readings were higher in the TMS 5. No statistically significant differences were found in astigmatism or maximum anterior elevations readings between the two devices, whereas there was a statistically significant difference in the maximum posterior elevations readings between the two devices.

Keywords: refractive, Scheimpflug, TMS 5


How to cite this article:
Lolah MM, El Massry AA. Comparison between the Scheimpflug camera and the topographic modeling system in the corneal assessment before refractive surgery. J Egypt Ophthalmol Soc 2014;107:153-9

How to cite this URL:
Lolah MM, El Massry AA. Comparison between the Scheimpflug camera and the topographic modeling system in the corneal assessment before refractive surgery. J Egypt Ophthalmol Soc [serial online] 2014 [cited 2019 Aug 25];107:153-9. Available from: http://www.jeos.eg.net/text.asp?2014/107/3/153/148123


  Introduction Top


Corneal assessment is very important before refractive surgery; it can aid the decision of whether the patient is eligible for surgery or not and it can also determine the type of surgical technique and avoid postsurgical late complications [1,2].

Earlier versions of topographic modeling system (TMS) topographer used a Placido disk, which could only assess the corneal surface curvature [3].

The inclusion of a scanning slit in the Placido disk (OrbscanII) enables analysis of the anterior and posterior corneal surface by measurement of the corneal thickness map [4,5].

The Oculyzer (WaveLight ALLERGO) uses a Scheimpflug-based system that can provide three-dimensional images by rotation of the Scheimpflug camera [6]. It can provide a complete image of the anterior segment of the eye in 2 s [7].

In the Scheimpflug system, 25 000 (hazard ratio: 138 000) true elevation points can be calculated by providing a three-dimensional image of the anterior eye segment [8,9].

A new technology of including the rotating Scheimpflug camera in the Placido disk in the TMS 5 topographer (Topographic Modeling System, version 5) can enable the calculation of up to 7300 points obtained by the Placido disk and up to 40 960 points obtained by the Scheimpflug camera [10].

The aim of this study was to compare the corneal measurements [pachymetry, keratometry, best-fit sphere (BFS), and the maximum elevation point] in a 3.0 mm central zone provided by a system using solely the Scheimpflug camera and a system using the Placido ring topographer and the Scheimpflug camera in patients seeking refractive surgery.


  Patients and methods Top


The study was approved by the ethics committee of the Alexandria University Faculty of Medicine. Patients attending the Roayeacenter of Refractive Surgery, Alexandria, Egypt, seeking refractive surgery were included in this study. Exclusion criteria were previous ocular surgery or ocular pathology other than refractive error. Patients who wore contact lens were required to stop wearing the lenses for at least 72 h before examination. Placido-Scheimpflug system (TMS5- Tomey corp. GmbH, Germany) and the Scheimpflug camera (WaveLight ALLERGO Oculyzer, Oculus. GmbH, Germany) system in each patient. Repeatability was ensured for both systems as each eye was examined three times on each device. Diurnal variation was avoided by performing all the measurements at the same time of the day.

Examination

Three examinations of each eye were started with the Placido-Scheimpflug system (TMS 5 - Tomey corp. GmbH, Germany) and then three examinations with the Scheimpflug camera (WaveLight ALLERGO Oculyzer, Oculus. GmbH, Germany), both of which are noncontact methods. With both systems, the patient's chin was placed on the chin rest and the forehead was pressed against the forehead strap. In TMS 5, the Ring-Topo mode performs a measurement by aligning the reflection of the laser light at the center of the first mire ring automatically. The unit avoids the offset of the alignment in addition to problems caused by the patient blinking; it also has a very short image capture time of 0.5 s. In the Scheimpflug mode, it can automatically capture multiple slices by focusing the alignment light on the center of the cornea. This measurement takes from 0.5 to 1 s. As the slit light used is emitted inside the cone, similar to conventional TMS models, the TMS 5 can capture an image without a dark room. The combination of ring topography with Scheimpflug topography leads to more accurate and reliable results and eliminates the mistrace that may sometimes be encountered with a conventional TMS device. In case of the Scheimpflug camera, the room lights are switched off for all examinations to obtain a reflex-free image. The patient's eye is then aligned along the visual axis by a central fixation light (blue circular ring in the Scheimpflug camera). The automatic release mode reduces operator-dependent variables, it automatically determines when the correct focus and alignment with the corneal apex has been reached, and then captures a scan. The software used a quality factor (QS) in choosing values for analysis depending on 95% of the population values. The data were extracted using WaveLight ALLERGO software and the Data Table Tool of the TomeyExamViewer software. Corneal thickness was assessed in 10 reference positions: central, superior, nasal superior, nasal, nasal inferior, inferior, temporal inferior, temporal, temporal superior, and the thinnest point.

Statistical analysis

The results were tabulated and the differences between the devices were analyzed using the paired Student t-test. Pearson correlation coefficients were determined to show the correlations between data. The Bland-Altman method was used to assess the agreement in variables between the two devices, and 95% limits of agreement (LOA) were calculated. Evaluation of the repeatability of both the systems was performed using intraclass correlation coefficients (ICCs). All data were analyzed using SPSS software (version 18; SPSS Inc., Chicago, Illinois, USA) and (Microsoft corp. Leeds, United Kingdom).


  Results Top


This study was carried out on 30 eyes of 16 healthy individuals, seven men and nine women, mean age 24.8 ± 12.5 years (range 18-35 years).

[Table 1] shows the mean central corneal thickness (CCT) readings and [Table 2] shows the interdevice differences. The mean CCT measurements by the two devices were 549.3 ± 28.8 and 537.3 ± 26.6 μm, respectively; this difference, 11.9 ± 12.7 μm, was highly significant, P value less than 0.001, meaning that the CCT measurement by the second device was significantly thinner than the first one. However, according to the Pearson correlation, the CCT measurements by the two devices were strongly correlated (P < 0.001 and r = 0.9); 95% LOA showed poor agreement (lower −12.9 μm, upper 36.8) [Figure 1]. The mean thinnest point readings in the Scheimpflug camera were higher than the mean thinnest point readings of TMS 5, with a mean difference of 17.2 ± 9.7 μm, with P less than 0.001 indicating statistical significance. Pearson correlation reports (P < 0.001 and r = 0.94), 95% LOA lower value -1.8 μm, upper value 71.2 μm. Fortunately, the other reference point readings of the corneal thickness showed higher values in the Scheimpflug system with a highly significant difference (P < 0.001) in comparison with the TMS 5 readings. Poor agreement was also found in these readings between the two devices. According to the keratometric readings, the mean flattest-k of the Scheimpflug and the TMS 5 was 42.5 ± 1.3 and 42.8 ± 1.3 D, respectively; the mean steepest-k was 43.9 ± 1.5 and 44.8 ± 1.3 D, respectively. The differences in the means of the flattest-k and the steepest-k of the two devices were 0.27 ± 0.35 and 0.5 ± 0.6 D, respectively (P < 0.001), and the correlation was excellent (Kf, r = 0.96, Ks, r = 0.92); the 95% LOA showed almost poor agreement [Figure 2]. However, the mean readings of astigmatism of the Scheimpflug system were 1.4 ± 0.8 and 1.6 ± 1.0 D for the TMS 5, with a mean difference of 0.2 ± 0.8 D (P > 0.001), there was no statistically significant difference in the astigmatic readings between the two devices; 95% LOA showed good agreement (upper −1.89 D, lower 1.48 D). In terms of the elevation data, the mean anterior BFS reading was 7.9 ± 0.2 mm for the two devices, with an excellent correlation, r = 0.99, with good agreement (upper 0.05 mm, lower −0.06 mm) [Figure 3]. The mean readings of maximum anterior elevation of the Scheimpflug (6.1 ± 3.2 μm) were higher than those of the TMS 5 mean readings (5.8 ± 3.2 μm); the mean difference was 0.4 ± 2.8 μm, which was not significant (P > 0.001), and showed good agreement (upper 5.8 μm, lower -5.1 μm) [Figure 4]a. The posterior BFS mean reading of the Scheimpflug and the TMS 5 was 6.6 ± 0.3 and 6.7 ± 0.2 mm, respectively; the mean difference was 0.2 ± 0.1 mm (P < 0.001), with poor agreement (upper −0.019 mm, lower −0.43 mm). However, the mean readings of Scheimpflug maximum posterior elevation (17.3 ± 6.5 μm) were higher than those of TMS 5 (9.6 ± 4.3 μm) and the mean difference was 7.6 ± 4.6 μm, with a statistically significant difference (P < 0.001), and poor agreement (upper −16.7 μm, lower −1.4 μm [Figure 4]b.
Figure 1:

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Figure 2:

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Figure 3:

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Figure 4:

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Table 1 Mean Scheimpflug camera and TMS 5 readings for CCT, thinnest point, peripheral corneal thickness, K, and elevation


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Table 2 Difference between Scheimpflug camera and TMS 5 in CCT, thinnest point, peripheral corneal thickness, K, and elevation measurements


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Repeatability

Evaluation of the repeatability was performed using ICCs. All readings had excellent repeatability (ICCs 94-99%) and, for the MEP readings, ICCs were 92 and 85% for the Scheimpflug system and TMS 5, respectively. Three successive measurements during the same visits showed good agreement for the two devices.


  Discussion Top


Corneal assessment is very important for refractive surgery to determine the amount and the type of correction. Accurate corneal measurements can help determine the safety of refractive surgery and avoid iatrogenic ectasia [10].

The Scheimpflug system has a rotating camera that offers a noninvasive means of assessing the anterior and posterior corneal surface with the anterior chamber of the eye. It can acquire 12, 25, and 50 images in a single scan [11].

The recent TMS 5 system differs from the previous versions by the inclusion of a rotating Scheimpflug system in the Placido disk, enabling the analysis of the posterior as well as the anterior surface of the cornea.

In this study, comparison between the Scheimpflug camera and the TMS 5 showed a strong correlation. Guilbert and Saad [10] used US pachymetry as a reference measurement for CCT. However, Bourne and McLaren [12] believed that the most accurate pachymetry measurements can be obtained with optical pachymeters, such as the one described by Maurice and Giardini [13].

There was a significant difference in the CCT measurements between the two devices. The CCT readings with the TMS 5 system were lower than those obtained with the Scheimpflug system, with a statistically significant mean difference of 11.9 ± 12.7 μm. This difference did not increase with increasing corneal thickness as there was no correlation between the average and the absolute difference (r = 0.34), and this is also shown in [Figure 1].

Moreover, 95% LOA for the CCT measurement is broad and asymmetrical (the lower was −12.9 and the upper was 36.8), which showed poor agreement, as the direction was more toward the zero level according to the Bland-Altman plots. Therefore, we could not use a default acoustic correction factor similar to many studies of Suzuki et al. [14], Kawana et al. [15], and Bourges et al. [16], who used a 0.92 default acoustic correction factor for Orbscan (Placido Disk) measurements as there was good agreement between the hand-held US pachymetry and Orbscan measurements; thus, they could use a conversion equation, meaning that readings from the two devices can be interchangeable. In our study, we found poor agreement, with a significant difference between results, so that the readings obtained with the two devices cannot be used interchangeably. However, Hashemi and Mehravaran [17] found good agreement with the CCT measured by the TMS 5 and the Pentacam system (Scheimpflug camera) through an indirect comparison with the US readings.

A 95% LOA of the keratometric readings of the two devices in the current study showed a broader range and a higher scatter below the zero line. However, a strong positive correlation was observed between the readings of the two devices for Kf and Ks, with the correlation coefficients of 0.96 and 0.92, respectively. Savini et al. [18] compared Scheimpflug tomographers and one Placido corneal topographer and reported that there was a statistically significant difference with a relatively small 95% LOA (from −0.74 to +0.28 D) in the mean simulated K readings. This statistical difference in the Savini study and the current study is because of the difference in the principles of measurement of the two optical devices and the points obtained from the corneal surface [19].

The two optical devices in our study were comparable in keratometric astigmatism readings as there was no statistically significant difference between the mean readings of the two devices (1.4 ± 0.8 for the Scheimpflug vs. 1.6 ± 1.0 for the TMS).

Kawamorita et al. [20] reported that the combined Placido/Scheimpflug system measures the dioptric power of values between the small cone Placido topography, which measures the highest dioptric value, and the Scheimpflug system, which measures the lowest dioptric value.

In our study, anterior BFS andanterior maximum elevation readings by the Scheimpflug camera were higher than those of the TMS, with a mean difference of 0.003 ± 0.03 for the anterior BFS and 0.4 ± 2.8 for the anterior maximum elevation, but this difference was not statistically significant, meaning that the two devices are comparable in addition to being strongly correlated, with a correlation coefficient of 0.99 for the anterior BFS. Kiely et al. [21] and Guilbert and Saad [10] did not find a significant difference in these readings between the TMS 5 and the OrbscanII.

The mean readings of the two devices for posterior BFS and maximum posterior elevation were significant, with a P value of less than 0.001, in addition to a broad range of 95% LOA for these two variables.

Ha et al. [22] and Karimian et al. [23] reported that the posterior elevation readings of the Pentacam (Scheimpflug system) were lower than those of the OrbscanII (placido-scanning-slit) system.

The discrepancy in posterior elevation readings between devices is very important in screening of the corneas of patients who seek refractive surgery to avoid the complication of ectasia [24].

In our study, the repeatability of both systems was excellent. Guilbert and Saad [10] reported that the repeatability of the maximum anterior elevation and posterior elevation of the TMS 5 ICC was 0.69 and 0.67, respectively. Luo et al. [25] reported excellent repeatability of the Pentacam (Scheimpflug-based system) in all readings. This indicates the good agreement between the three successive scans of the TMS 5 in the current study.

This study focused on the accuracy of devices in the screening of corneas before refractive surgery; it was restricted to normal corneas. TMS 5 has not been compared only with the Scheimpflug system before, but it has been compared with OrbscanII (Placido disk only).

The measurements of both devices in our study were comparable and correlated, but not interchangeable, especially in pachymetry.

We studied only healthy normal corneas; thus, more researches will be needed focusing on abnormal corneas (keratoconus-corneal scar).


  Acknowledgements Top


The authors thank Dr. Heba El Weshahi, lecturer of Biostatistics and Community Medicine in the Faculty of Medicine in Alexandria University, for the statistical work of the study.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2]



 

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