• Users Online: 29
  • Print this page
  • Email this page


 
 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 34  |  Issue : 1  |  Page : 50-55

Repeatability of corneal densitometry measurements using a scheimpflug camera in healthy normal corneas


1 Noor Ophthalmology Research Center, Noor Eye Hospital; Translational Ophthalmology Research Center, Tehran University of Medical Sciences, Tehran, Iran
2 Department of Basic Sciences, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran, Iran
3 Ophthalmic Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran
4 Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
5 Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran, Iran
6 Translational Ophthalmology Research Center, Tehran University of Medical Sciences, Tehran, Iran
7 Translational Ophthalmology Research Center, Tehran University of Medical Sciences; Noor Ophthalmology Research Center, Noor Eye Hospital, Tehran, Iran

Date of Submission30-May-2021
Date of Decision17-Aug-2021
Date of Acceptance19-Aug-2021
Date of Web Publication16-Apr-2022

Correspondence Address:
Hassan Hashemi
Translational Ophthalmology Research Center, Tehran University of Medical Sciences, Tehran
Iran
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/joco.joco_173_21

Rights and Permissions
  Abstract 


Purpose: To determine the repeatability of corneal densitometry measured by the Scheimpflug imaging system.
Methods: This cross-sectional study was conducted on photorefractive keratectomy candidates. One eye of each participant underwent imaging using Pentacam HR three times, 10 min apart. The repeatability of densitometry measurements was evaluated in four concentric annuli around the corneal apex and in different corneal depths. The repeatability of the measurements was evaluated using the intraclass correlation coefficient (ICC), repeatability coefficient (RC), and coefficient of variation (CV). The difference of repeatability between layers and zones was tested by tolerance index (TI).
Results: Sixty eyes of sixty patients with a mean age of 27.76 ± 3.93 years were studied. Half of the participants were female (n = 30, 50%). ICC was above 0.9 in all corneal parts. The posterior layer and central zones showed the least variability of densitometry measurements considering the CV values. The RC was 2.06, 1.17, and 0.92 in anterior, central, and posterior layers, respectively. The RC was 0.88, 0.71, 1.51, and 4.56 in 0–2, 2–6, 6–10, and 10–12 mm circles, respectively. Only the reliability of densitometry in 10–12 mm annulus was statistically lower than the central zone (TI = 0.71).
Conclusions: Corneal densitometry measurements provided by the Pentacam had good repeatability. The repeatability of densitometry measurements decreased from the center to the periphery (with an exception for 0–2 mm and 2–6 mm) and from the posterior to the anterior of the cornea. The reliability of the 10–12 mm zone was markedly less than other zones.

Keywords: Corneal densitometry, Reliability, Scheimpflug imaging system


How to cite this article:
Pakbin M, Khabazkhoob M, Pakravan M, Fotouhi A, Jafarzadehpur E, Aghamirsalim M, Hashemi H. Repeatability of corneal densitometry measurements using a scheimpflug camera in healthy normal corneas. J Curr Ophthalmol 2022;34:50-5

How to cite this URL:
Pakbin M, Khabazkhoob M, Pakravan M, Fotouhi A, Jafarzadehpur E, Aghamirsalim M, Hashemi H. Repeatability of corneal densitometry measurements using a scheimpflug camera in healthy normal corneas. J Curr Ophthalmol [serial online] 2022 [cited 2022 May 23];34:50-5. Available from: http://www.jcurrophthalmol.org/text.asp?2022/34/1/50/343346




  Introduction Top


Corneal transparency is important for the optical function of the cornea and is considered an index of corneal health.[1] Evaluation of the corneal transparency has several applications and indications, such as diagnosis of different corneal diseases and disorders like Fuchs endothelial dystrophy[2],[3] and the healing process, and follow-up of patients undergoing refractive surgery like laser-assisted in-situ keratomileusis and photorefractive keratectomy (PRK).[4] It is also used to investigate the corneal effects of systemic diseases and drugs.[5],[6] Different methods such as slit-lamp biomicroscopy, confocal microscopy, and Scheimpflug imaging are used to evaluate corneal transparency.[7],[8],[9] The Pentacam uses Scheimpflug principles to evaluate the transparency of the cornea and crystalline lens using the densitometry factor. This device presents the corneal densitometry in different corneal diameters in the anterior, central, and posterior cornea as a numerical index.[10]

Several studies have examined the repeatability of the Pentacam measurements of anterior segment parameters. The repeatability of some parameters such as keratometry, pachymetry, and anterior chamber data in normal and keratoconus patients has been investigated in previous studies. The majority of these studies found a high intrasubjective and intersubjective repeatability of anterior segment parameters in healthy subjects.[11],[12],[13],[14],[15],[16],[17] In addition, some studies evaluated the repeatability of the lens densitometry measurements of the Pentacam and reported high repeatability in normal and cataract eyes.[18]

Only two studies investigated the repeatability of corneal densitometry; however, they focused on patients with keratoconus and included a limited number of normal eyes as a control group.[19],[20] The results of both studies showed that the repeatability of corneal densitometry was lower in keratoconus patients compared to nonkeratoconic eyes. In the Pahuja et al. study, repeatability was also lower in keratoconus patients that underwent collagen cross-linking (CXL) compared to keratoconus patients with a negative history of surgery.[19] This study recommended that the results of densitometry should be used with caution in post-CXL keratoconus patients. The other study was conducted by Kreps et al. which evaluated the repeatability of an extensive number of relevant indices with the Pentacam HR in various grades of keratoconus. They briefly deal with the repeatability of corneal densitometry and did not include the entire corneal tissue in their analysis.[20] There are no other studies on corneal densitometry repeatability in healthy eyes.

Since decreased corneal transparency is a diagnostic indicator for different corneal disorders and affects visual quality, it is very important to study the reliability of its measurements. This study was conducted to investigate the repeatability of the Pentacam measurements of corneal densitometry in normal eyes.


  Methods Top


This cross-sectional study, which was part of a clinical trial,21 was conducted on PRK candidates in the baseline phase before random allocation. The protocol of the study was approved by the Ethics Committee of Tehran University of Medical Sciences. All procedures performed in studies involving human participants were in accordance with the ethical standards of the Tehran University of Medical Sciences Research Committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. All patients signed a written informed consent (Ethics code: IR.TUMS. FARABIH.REC.1397.010).

Patients aged 21–40 years with a spherical equivalent of plano to −6.00 diopter (D), astigmatism ≤−2.00 D, and corrected distance visual acuity (CDVA) ≥20/20 were included in the study if their refractive error was stable (<0.5 D change) in the past year. Patients with any systemic diseases such as diabetes or thyroid disease, glaucoma and/or intraocular pressure ≥21 mmHg, keratoconus, corneal opacity, and dry eye; patients with a history of ocular surgery; and pregnant and lactating patients were not included in the study. Soft contact lens wearers were instructed to remove their lenses 1 week before imaging. The subjects who met the inclusion criteria underwent complete ophthalmic examinations including the measurement of uncorrected distance visual acuity, CDVA, subjective refraction, and slit-lamp biomicroscopy.

Pentacam imaging was done using the Pentacam HR (Oculus, Germany, version 1.22r. 9) in a windowless room with a light intensity of 3 lux. Repeatability was assessed in sixty eyes of sixty patients that were selected randomly. Imaging was done three times, 10 min apart, in each patient. In each stage, the patient was asked to fixate on a bright point in the middle of a blue line, blink several times, and then stop blinking until told otherwise. After completion of the imaging process, the joystick was retracted and the patient was given 10 min to rest. Then the device was set again and all the stages were repeated. Imaging was done by an experienced optometrist. Only images with a quality specification of “OK” were acceptable for analysis.

The “Cornea Densito” map shows the density of the corneal tissue in grayscale unit from 0 to 100, where 0 means maximum transparency and 100 means maximum scattering. The Cornea Densito map presents corneal densitometry in four annuli separately (central 0–2 mm, 2–6 mm, 6–10 mm, and 10–12 mm). Moreover, this map shows densitometry values according to depth in the anterior (120 μm), posterior (60 μm), and central layers (between anterior and posterior layers). Slit-lamp examination and fundoscopy were done in dilated condition after Pentacam imaging by an ophthalmologist.

Statistical analysis

The collected data were analyzed using the SPSS software version 23 (SPSS Inc., Chicago, Illinois, USA). The repeatability of corneal densitometry was investigated in different zones including the four annuli (0–2 mm, 2–6 mm, 6–10 mm, and 10–12 mm) in the anterior, posterior, and central layers as well as the entire cornea.

First, the mean and standard deviation of three readings were determined in each group. The repeated measures analysis of variance was done to compare the mean values. Considering the number of comparisons, the level of significance was reduced to 0.005 after Bonferroni correction.

The intrasession test–retest variability was also calculated. Then the within-subject standard deviation (Sw) was determined and multiplied by 2.77 to calculate the repeatability coefficient (RC). The RC is an indicator of repeated measurement error with lower values indicating better repeatability. The coefficient of variation (CV) was calculated by dividing the standard deviation by the mean value of the measurements and expressed as percentage. A lower value is more desirable with this index as well. The intraclass correlation coefficient (ICC) was calculated to assess the variance among repeated measurements. The ICC value can be between 0 and 1 which 1 indicates no difference in the variance of repeated data and shows a strong correlation.[14] The tolerance index (TI) was calculated for examining the statistically significant difference of the repeatability measurements between zones and layers. TI >0.24 (cut-off values proper to the study sample size) means that there is a statistically significant difference.[22]


  Results Top


Sixty eyes of sixty patients, of whom 30 individuals (50%) were female, were studied. The mean age of the participants was 27.76 ± 3.93 (range, 22–36 years). [Table 1] shows the characteristics of participants. The mean densitometry of three measurements is demonstrated in [Table 2]. [Table 3] shows the repeatability of corneal densitometry values in different corneal depths and concentric annuli around the apex. The ICC was above 0.9 in all corneal parts. According to the CV index, the variability of densitometry measurements increased from the center to the periphery of the cornea, and the posterior layer showed the least variability compared to the anterior and central layers.
Table 1: Subject characteristics

Click here to view
Table 2: The corneal densitometry values of three measurements and average

Click here to view
Table 3: Repeatability indices of corneal densitometry between three measurements

Click here to view


The highest and lowest repeatability of the corneal densitometry values (based on RC index) were observed in the posterior and anterior layers, respectively. However, the difference in repeatability of central and posterior layers compared with the anterior layer was not statistically significant (TI = −0.24 and TI = −0.35), respectively. According to the concentric annuli around the apex, the 2–6 mm circle had the highest repeatability (the lowest RC), followed by the 0–2 mm, 6–10 mm, and 10–12 mm. Nevertheless, only the difference of repeatability in the zone 10–12 mm compared with the central zone (0–2 mm) was statistically significant (TI = 0.71).

[Figure 1] demonstrates an example of corneal densitometry map of Pentacam.
Figure 1: Corneal densitometry output of Pentacam

Click here to view



  Discussion Top


Considering the extensive use of the Pentacam in anterior segment evaluations, the measured parameters should have high repeatability to be reliable. The Pentacam calculates corneal densitometry quantitatively using the backscattered light in a fast and noncontact process and presents it as a Cornea Densito map. The densitometry data may serve as an objective index of corneal transparency in different conditions such as assessment and follow-up of corneal opacities.[23] Since the measurements of a device should have acceptable repeatability in order to be reliable, the repeatability of the corneal densitometry values measured by the Pentacam should be evaluated. Pahuja et al. investigated the corneal densitometry values in normal, keratoconus, and postcollagen CXL groups.[19] The results showed that the repeatability of corneal densitometry was lower in keratoconus patients compared to nonkeratoconic eyes. On the other hand, it was also lower in keratoconus patients that underwent collagen CXL compared to keratoconus patients with a negative history of surgery. This study recommended that the results of densitometry should be used with caution in post-CXL keratoconus patients since the measurements may not be reliable. Kreps et al. reported that densitometry repeatability in the central 0–2 mm annulus of the anterior layer was lower in keratoconus eyes compared to controls.[20] The present study was conducted on normal subjects. The ICC was above 0.9 in all corneal depths and diameters, indicating a high correlation between densitometry values. According to the concentric annuli around the apex, the 2–6 mm circle had the highest and the 10–12 mm annulus had the lowest repeatability. Ní Dhubhghaill et al. found similar results in eight eyes.[10] The highest mean value of densitometry was in the 10–12 mm annulus in this study, and the authors attributed this finding to the normal variation in corneal diameter. Since the corneal diameter is different in different people, the densitometry values measured in corneas with a diameter of <12 mm may include part of the limbus and sclera, and considering the higher light backscatter in these parts, the value of the densitometry will be higher too. This finding was recently confirmed in a study investigating densitometry values in PRK candidates using the Pentacam. In this study, the highest densitometry value was seen in the 10–12 mm annulus followed by the 0–2 mm, 6–10 mm, and 2–6 mm annuli.[24] Pahuja et al. did not include the data of the 10–12 mm annulus in the analysis, but the best annulus in terms of repeatability in normal subjects was the 0–2 mm circle.[19] Kreps et al. only evaluated the repeatability of 0–2 mm and 2–6 mm annuli and reported better repeatability in 2–6 mm radius.[20]

In the present study, according to the corneal depth, densitometry measurements had the highest repeatability in the posterior layer compared to the central and anterior layers, which was consistent with previous studies.[10],[19] However, Pakbin et al.[24] and Asrar et al.[25] reported the lowest and highest mean densitometry values were in the posterior and anterior layers, respectively.

According to the studies, lower repeatability may be related to higher densitometry values. In other words, repeatability may be lower in corneal parts with higher densitometry values. The results of the study by Pahuja et al. also indicate the same conclusion since the repeatability index was lower in keratoconus subjects, and it was even much worse in patients with a history of collagen CXL.[19] One of the common phenomena after CXL is corneal haze, which peaks 1 month after the operation, reaches a plateau within 3 months, and then reduces markedly until 12 months.[26] Shetty et al. conducted a study in patients that underwent CXL and found that the repeatability of corneal densitometry measurements was affected by post-CXL haze, especially anterior haze in the 0–2 mm region.[27]

The present study investigated the repeatability of corneal densitometry measurements in normal eyes. The lack of similar studies in normal subjects with this sample size is a strength of this study.

This study was conducted on 20–40-year-old PRK candidates with moderate myopia, and patients with corneal diseases were not included; therefore, it does not represent the whole community and can be considered a study limitation.

In conclusion, corneal densitometry measurements in normal subjects have acceptable repeatability. The repeatability of corneal densitometry increases from the anterior layer to the posterior and from the periphery to the center. When using the data of corneal densitometry, it should be noted that the data of the 10–12 mm zone are less reliable than the data of other corneal regions; therefore, the data related to this annulus should be used with caution.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Meek KM, Knupp C. Corneal structure and transparency. Prog Retin Eye Res 2015;49:1-16.  Back to cited text no. 1
    
2.
Alnawaiseh M, Zumhagen L, Wirths G, Eveslage M, Eter N, Rosentreter A. Corneal densitometry, central corneal thickness, and corneal central-to-peripheral thickness ratio in patients with fuchs endothelial dystrophy. Cornea 2016;35:358-62.  Back to cited text no. 2
    
3.
Thériault M, Gendron SP, Brunette I, Rochette PJ, Proulx S. Function-related protein expression in fuchs endothelial corneal dystrophy cells and tissue models. Am J Pathol 2018;188:1703-12.  Back to cited text no. 3
    
4.
Poyales F, Garzón N, Mendicute J, Illarramendi I, Caro P, Jáñez O, et al. Corneal densitometry after photorefractive keratectomy, laser-assisted in situ keratomileusis, and small-incision lenticule extraction. Eye (Lond) 2017;31:1647-54.  Back to cited text no. 4
    
5.
Munjal A, Kaufman EJ. Arcus senilis (corneal arcus). In: StatPearls. Treasure Island (FL): © 2020, StatPearls Publishing LLC; 2020.  Back to cited text no. 5
    
6.
Yamagiwa Y, Haranosono Y, Nemoto S, Atsumi I, Kurata M, Kito G, et al. Characteristics of corneal phospholipidosis induced by topical ocular application of chloroquine and amiodarone in rabbits. J Toxicol Pathol 2017;30:135-43.  Back to cited text no. 6
    
7.
Fantes FE, Hanna KD, Waring GO 3rd, Pouliquen Y, Thompson KP, Savoldelli M. Wound healing after excimer laser keratomileusis (photorefractive keratectomy) in monkeys. Arch Ophthalmol 1990;108:665-75.  Back to cited text no. 7
    
8.
You JY, Botelho PJ. Corneal in vivo confocal microscopy: Clinical applications. R I Med J (2013) 2016;99:30-3.  Back to cited text no. 8
    
9.
McLaren JW, Wacker K, Kane KM, Patel SV. Measuring corneal haze by using scheimpflug photography and confocal microscopy. Invest Ophthalmol Vis Sci 2016;57:227-35.  Back to cited text no. 9
    
10.
Ní Dhubhghaill S, Rozema JJ, Jongenelen S, Ruiz Hidalgo I, Zakaria N, Tassignon MJ. Normative values for corneal densitometry analysis by scheimpflug optical assessment. Invest Ophthalmol Vis Sci 2014;55:162-8.  Back to cited text no. 10
    
11.
Mirzajani A, Asharlous A, Kianpoor P, Jafarzadehpur E, Yekta A, Khabazkhoob M, et al. Repeatability of curvature measurements in central and paracentral corneal areas of keratoconus patients using orbscan and pentacam. J Curr Ophthalmol 2019;31:382-6.  Back to cited text no. 11
    
12.
Hashemi H, Asharlous A, Aghazadeh Amiri M, Yekta A, Ramin S, Taheri A, et al. Intrasubject repeatability and interdevice agreement of anterior chamber depth measurements by orbscan and pentacam in different grades of keratoconus. Eye Contact Lens 2019;45:51-4.  Back to cited text no. 12
    
13.
Otchere H, Sorbara L. Repeatability of topographic corneal thickness in keratoconus comparing Visante™ OCT and oculus pentacam HR(®) topographer. Cont Lens Anterior Eye 2017;40:217-23.  Back to cited text no. 13
    
14.
Hashemi H, Yekta A, Khabazkhoob M. Effect of keratoconus grades on repeatability of keratometry readings: Comparison of 5 devices. J Cataract Refract Surg 2015;41:1065-72.  Back to cited text no. 14
    
15.
Schröder S, Mäurer S, Eppig T, Seitz B, Rubly K, Langenbucher A. Comparison of corneal tomography: Repeatability, precision, misalignment, mean elevation, and mean pachymetry. Curr Eye Res 2018;43:709-16.  Back to cited text no. 15
    
16.
Xu Z, Peng M, Jiang J, Yang C, Zhu W, Lu F, et al. Reliability of pentacam HR thickness maps of the entire cornea in normal, post-laser in situ keratomileusis, and keratoconus eyes. Am J Ophthalmol 2016;162:74-82.e1.  Back to cited text no. 16
    
17.
Piñero DP, Saenz González C, Alió JL. Intraobserver and interobserver repeatability of curvature and aberrometric measurements of the posterior corneal surface in normal eyes using scheimpflug photography. J Cataract Refract Surg 2009;35:113-20.  Back to cited text no. 17
    
18.
Kirkwood BJ, Hendicott PL, Read SA, Pesudovs K. Repeatability and validity of lens densitometry measured with scheimpflug imaging. J Cataract Refract Surg 2009;35:1210-5.  Back to cited text no. 18
    
19.
Pahuja N, Shetty R, Subbiah P, Nagaraja H, Nuijts RM, Jayadev C. Corneal densitometry: Repeatability in eyes with keratoconus and postcollagen cross-linking. Cornea 2016;35:833-7.  Back to cited text no. 19
    
20.
Kreps EO, Jimenez-Garcia M, Issarti I, Claerhout I, Koppen C, Rozema JJ. Repeatability of the pentacam HR in various grades of keratoconus. Am J Ophthalmol 2020;219:154-62.  Back to cited text no. 20
    
21.
Hashemi H, Pakbin M, Pakravan M, Fotouhi A, Jafarzadehpur E, Aghamirsalim M, et al. Effect of short versus long-term steroid on corneal haze after photorefractive keratectomy: A randomized, double-masked clinical trial. Am J Ophthalmol 2022;235:211-20.  Back to cited text no. 21
    
22.
Bergin C, Guber I, Hashemi K, Majo F. Tolerance and relative utility: Two proposed indices for comparing change in clinical measurement noise between different populations (repeatability) or measurement methods (agreement). Invest Ophthalmol Vis Sci 2015;56:5543-7.  Back to cited text no. 22
    
23.
Otri AM, Fares U, Al-Aqaba MA, Dua HS. Corneal densitometry as an indicator of corneal health. Ophthalmology 2012;119:501-8.  Back to cited text no. 23
    
24.
Pakbin M, Khabazkhoob M, Pakravan M, Fotouhi A, Jafarzadehpur E, Aghamirsalim M, et al. Corneal scheimpflug densitometry in photorefractive keratectomy candidates. Cornea 2020;39:1381-8.  Back to cited text no. 24
    
25.
Asrar A, Ikram B, Khan H, Asrar M. Normal values of corneal optical densitometry using pentacam scheimpflug camera. Adv Ophthalmol Vis Syst 2016;5:00142.  Back to cited text no. 25
    
26.
Greenstein SA, Fry KL, Bhatt J, Hersh PS. Natural history of corneal haze after collagen crosslinking for keratoconus and corneal ectasia: Scheimpflug and biomicroscopic analysis. J Cataract Refract Surg 2010;36:2105-14.  Back to cited text no. 26
    
27.
Shetty R, Agrawal A, Deshmukh R, Kaweri L, Rao HL, Nagaraja H, et al. Effect of post crosslinking haze on the repeatability of scheimpflug-based and slit-scanning imaging devices. Indian J Ophthalmol 2017;65:305-10.  Back to cited text no. 27
[PUBMED]  [Full text]  


    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
    Viewed245    
    Printed6    
    Emailed0    
    PDF Downloaded23    
    Comments [Add]    

Recommend this journal


[TAG2]
[TAG3]
[TAG4]