Journal of Refractive Surgery

Biomechanics 

Biomechanical Analysis of Subclinical Keratoconus With Normal Topographic, Topometric, and Tomographic Findings

Mustafa Koc, MD; Emre Aydemir, MD; Kemal Tekin, MD; Merve Inanc, MD; Pinar Kosekahya, MD; Hasan Kiziltoprak, MD

Abstract

PURPOSE:

To investigate the corneal biomechanical responses of subclinical keratoconus with normal topographic, topometric, and tomographic findings.

METHODS:

In this prospective observational study, the study group was selected from patients with clinically evident keratoconus in one eye and subclinical keratoconus with normal topographic, topometric, and tomographic findings in the fellow eye. The control group was selected from candidates for contact lens use. The biomechanical analyses were performed using the Corvis ST (Oculus Optikgeräte, Wetzlar, Germany). The following parameters were analyzed: A1 velocity, A2 velocity, A1 length, A2 length, deformation amplitude ratio, stiffness parameter at the first applanation, Corvis Biomechanical Index, and Tomographic and Biomechanical Index (TBI).

RESULTS:

The study group consisted of 21 patients (10 men and 11 women; mean age: 27.7 ± 6.9 years), and the control group consisted of 35 patients (17 men and 18 women; mean age: 26.1 ± 5.8 years). No significant differences were found between the eyes with subclinical keratoconus and normal eyes in corrected distance visual acuity and the topographic, topometric, and tomographic parameters (P > .05). Significant differences were found in the values of A2 length, A1 velocity, A2 velocity, and TBI between the subclinical keratoconus group and the control group (P < .05). In distinguishing eyes with subclinical keratoconus from normal eyes, the TBI showed the highest area under the curve (0.790; cut-off: 0.29; sensitivity: 67%; specificity: 86%) in the receiver operating characteristic analysis.

CONCLUSIONS:

Biomechanical analysis with the Corvis ST may be used as a complementary diagnostic method in detecting subclinical keratoconus.

[J Refract Surg. 2019;35(4):247–252.]

Abstract

PURPOSE:

To investigate the corneal biomechanical responses of subclinical keratoconus with normal topographic, topometric, and tomographic findings.

METHODS:

In this prospective observational study, the study group was selected from patients with clinically evident keratoconus in one eye and subclinical keratoconus with normal topographic, topometric, and tomographic findings in the fellow eye. The control group was selected from candidates for contact lens use. The biomechanical analyses were performed using the Corvis ST (Oculus Optikgeräte, Wetzlar, Germany). The following parameters were analyzed: A1 velocity, A2 velocity, A1 length, A2 length, deformation amplitude ratio, stiffness parameter at the first applanation, Corvis Biomechanical Index, and Tomographic and Biomechanical Index (TBI).

RESULTS:

The study group consisted of 21 patients (10 men and 11 women; mean age: 27.7 ± 6.9 years), and the control group consisted of 35 patients (17 men and 18 women; mean age: 26.1 ± 5.8 years). No significant differences were found between the eyes with subclinical keratoconus and normal eyes in corrected distance visual acuity and the topographic, topometric, and tomographic parameters (P > .05). Significant differences were found in the values of A2 length, A1 velocity, A2 velocity, and TBI between the subclinical keratoconus group and the control group (P < .05). In distinguishing eyes with subclinical keratoconus from normal eyes, the TBI showed the highest area under the curve (0.790; cut-off: 0.29; sensitivity: 67%; specificity: 86%) in the receiver operating characteristic analysis.

CONCLUSIONS:

Biomechanical analysis with the Corvis ST may be used as a complementary diagnostic method in detecting subclinical keratoconus.

[J Refract Surg. 2019;35(4):247–252.]

Keratoconus almost always occurs bilaterally. In most cases, the disease occurs in one eye and then affects the other eye in time. It sometimes occurs in one eye, and the fellow eye with normal findings is considered to be in the early stages of the disease (subclinical keratoconus).1 Despite the advances in the topographic and tomographic analyses of the cornea, detecting the subclinical keratoconus remains a challenge because threshold values that can definitely distinguish keratoconic from normal corneas have not been determined yet. However, recognizing subclinical keratoconus is important, especially before refractive laser surgery.2

Hypotheses suggest that the alteration of biomechanical properties of the cornea develops before the topographic and tomographic changes and that the bulging and thinning depend on the biomechanical alteration.3–5 Therefore, a biomechanical analysis that can detect keratoconus at an earlier stage is being developed. Although the alteration of biomechanical properties was demonstrated in keratoconic eyes by the Ocular Response Analyzer (ORA; Reichert Technologies, Depew, NY), the low distinctive capacity of this device prevented its widespread use.6,7 The Corvis ST (Oculus Optikgeräte, Wetzlar, Germany) was later introduced for evaluating corneal biomechanical properties using the ultra–high-speed Scheimpflug camera, and it provides more biomechanical parameters than the ORA.8

This study investigated the corneal biomechanical properties of patients with subclinical keratoconus who have no topographic, topometric, and tomographic findings.

Patients and Methods

This prospective observational study was conducted in accordance with the tenets of the Declaration of Helsinki and with the approval of the Ankara Numune Education and Research Hospital Ethical Committee. In our previous study in which we examined the medical records of 3,474 patients with keratoconus, we determined 38 patients had clinical keratoconus in one eye and no suspicious or abnormal topographic, topometric, and tomographic findings in the fellow eye.9 These patients were recalled to our cornea clinic, and a complete ophthalmologic examination including Pentacam HR (Oculus Optikgeräte) measurements was performed.

A study group was formed from these patients with clinical keratoconus in one eye and no suspicious or abnormal topographic, topometric, and tomographic findings in the fellow eye in the current Pentacam HR measurements. Clinical keratoconus was defined by characteristic keratoconus signs in the anterior axial curvature maps (asymmetric bowtie pattern and inferior or central steepening) and at least one biomicroscopic sign (eg, conical protrusion, Vogt's striae, or Fleischer ring). Subclinical keratoconus was defined by a central mean keratometry value of less than 47.20 diopters (D), an inferior–superior asymmetry for the average keratometry value of less than 1.40 D, a keratoconus percentage index (KISA%) of less than 60%, and no clinical evidence. Patients with suspicious values in the topometric and tomographic (including elevation maps and Belin-Ambrósio Enhanced Ectasia Display [BAD]) analyses of the eyes with subclinical keratoconus were excluded.9,10

Biomechanical analysis was performed with the Corvis ST (version 1.03r.1538). The following parameters measured by the Corvis ST were used: A1 velocity (A1V = speed of the corneal apex at the first applanation), A2 velocity (A2V = speed of the corneal apex at the second applanation), A1 length (A1L = length of the flattened corneal segment at the first applanation), A2 length (A2L = length of the flattened corneal segment at the second applanation), deformation amplitude ratio (DA ratio = central and peripheral largest displacement in the anterior–posterior direction ratio), stiffness parameter at the first applanation, Corvis Biomechanical Index (CBI), and Tomographic and Biomechanical Index (TBI). These parameters were described in detail in previous studies.8,11–13

The control group was randomly selected from a database of age-matched candidates for contact lens use for myopia (≤ 5.00 D) and myopic astigmatism (≤ 3.00 D) that had normal topographic, topometric, and tomographic analyses. The control group was selected from the contact lens users to form groups with similar ages and refractive errors. One eye of each control patient was randomly chosen. Patients with a history of anterior segment surgery, ocular surface problems, and topical eye drop use were excluded. The measurements were performed after rigid gas-permeable contact lenses had been stopped for at least 3 weeks and soft contact lenses had been stopped for at least 1 week.

Statistical Analysis

The study data were analyzed using the Statistical Package for Social Sciences (SPSS) software (version 22.0 for Windows; SPSS, Inc., Chicago, IL). The chi-square test was used to analyze the categorical variables. The normal distribution of the variables was tested by visual (histogram and probability graph) and analytical (Kolmogorov–Smirnov and Shapiro–Wilk tests) methods. When a parametric analysis was possible, the three groups were compared using a paired t test for the paired data or Student's t test for the un-paired data along with the Bonferroni correction. When a parametric analysis was not possible, the groups were compared using the Wilcoxon signed-rank test for the paired data and the Mann–Whitney U tests for the un-paired data. A P value of less than .05 was considered statistically significant. The receiver operating characteristic (ROC) curve was used to obtain cut-off values to differentiate the eyes with subclinical keratoconus from the normal eyes. The biomechanical parameters that showed statistically significant differences between the eyes with subclinical keratoconus and the normal eyes were included in the ROC analyses.

Results

Twenty-six of the 38 patients who were recalled came to our clinic. Five of the 26 patients were excluded from the study because of suspicious or abnormal findings in the recent Pentacam measurements. The study group was formed from the remaining 21 patients (10 men and 11 women) with a mean age of 27.7 ± 6.9 years (range: 16 to 38 years). The control group consisted of 35 patients (17 men and 18 women) with a mean age of 26.1 ± 5.8 years (range: 16 to 36 years). No statistically significant differences were found in age and gender between the study and control groups (P = .671 and .746, respectively).

Table 1 shows the corrected distance visual acuities (CDVA), spherical equivalents (SE), and topographic, topometric, and tomographic parameters of the groups. The CDVA was significantly lower and the SE significantly higher in the clinical keratoconus group than those in the subclinical keratoconus and control groups. No statistically significant difference was found in either the CDVA or the SE between the subclinical keratoconus and control groups. Statistically significant differences were observed in all measured parameters except center keratoconus index between the clinical keratoconus group and the other two groups. No significant difference was found between the subclinical keratoconus and control groups.

Comparison of the Topographic, Topometric, and Tomographic Parameters Obtained From the Pentacam HR

Table 1:

Comparison of the Topographic, Topometric, and Tomographic Parameters Obtained From the Pentacam HR

Table 2 shows the results of the biomechanical analysis of the groups. Significant differences were found in all measured parameters between the clinical keratoconus and control groups. Statistically significant differences were detected in A2L, DA ratio, CBI, and TBI values between the clinical keratoconus and subclinical keratoconus groups. Statistically significant differences were also found in the A2L, A1V, A2V, and TBI values between the subclinical keratoconus and control groups. Table 3 shows the results of the ROC curve analysis for these parameters for the eyes with subclinical keratoconus versus the control eyes. In distinguishing eyes with subclinical keratoconus from normal eyes, the TBI showed the highest area under the curve (AUC) value, sensitivity, and specificity.

Comparison of the Biomechanical Parameters Obtained From the Corvis ST

Table 2:

Comparison of the Biomechanical Parameters Obtained From the Corvis ST

Receiver Operating Characteristic Analysis for Discriminating Between Subclinical Keratoconic and Normal Eyes

Table 3:

Receiver Operating Characteristic Analysis for Discriminating Between Subclinical Keratoconic and Normal Eyes

Discussion

Tomographic and topographic changes in keratoconus are considered to develop because of biomechanical instability.3–5 According to this hypothesis, the focal biomechanical alteration causes thinning and bulging with time, and forces such as intraocular pressure and itching also contribute. This hypothesis claims that the determination of early biomechanical changes can detect keratoconus earlier than topographic and tomographic analyses.5,14 Patients with subclinical keratoconus without topographic and tomographic findings are the most appropriate population for which this claim can be assessed. The reason is that, according to global consensus, keratoconus is bilateral and subclinical keratoconus is present even if there are no topographic and tomographic findings in the fellow eye.1 Thus, in the current study, the biomechanical properties of the cornea were evaluated in patients who had clinical keratoconus in one eye and no keratoconus findings in the fellow eye.

Previous studies using the Corvis ST showed that corneas with moderate and advanced stages of keratoconus are biomechanically weak.8,11,15 However, diagnosing keratoconus with other methods is not difficult in these patients. The main problem is detecting subclinical keratoconus. Steinberg et al.12 reported one of the first studies in which patients with subclinical keratoconus were evaluated with the Corvis ST. In the current study, the first set parameters of the Corvis ST (A1V, A2V, A1L, and A2L) were used, and the values of the subclinical keratoconus and control groups were found to be similar. Although the A1L and A2L values were significantly different between the clinical keratoconus and control groups, the sensitivity and specificity were low.12 In another study, this first set parameters (DA, A1 time, A1L, A2 time, and A2V) was significantly different from the control group in patients with subclinical keratoconus with normal topography.16 However, we considered the topographic diagnostic criteria of this study to be questionable because the topography of the patients presenting with subclinical keratoconus was compatible with clinical keratoconus. Moreover, Francis et al.13 stated that the diagnostic capacity of some first set parameters could be increased by waveform analysis. In our study, we also found that A2L, A1V, and A2V were significantly different from the control group in the subclinical keratoconus group. However, we consider these parameters to be clinically not feasible because of low sensitivity and specificity.

The first-generation parameters of the Corvis ST have a low diagnostic ability, especially in patients with subclinical keratoconus. One new index is the CBI developed by Vinciguerra et al.,8 which combines dynamic corneal response parameters and Ambrósio's Relational Thickness to the Horizontal Profile (ARTh). When the cut-off value was accepted as 0.5, the CBI had a sensitivity of 94% and specificity of 100% for differentiating normal eyes from keratoconic eyes (AUC = 0.990). In another study conducted by Ambrósio et al.,11 the sensitivity and specificity of the CBI were evaluated in patients with subclinical keratoconus (very asymmetric ectasia = VAE). Ninety-four patients who had normal topographic findings, a KISA index of less than 60%, and inferior–superior asymmetry of less than 1.45 D were included in this study. The results showed that the CBI was significantly higher in the VAE group and that the sensitivity and specificity of the CBI were 68% and 82%, respectively (AUC = 0.882), with ROC analysis. The CBI sensitivity was 77%, specificity was 68%, and AUC was 0.775 in two recent studies17,18 using the diagnostic criteria similar to those of Ambrósio et al.11 In our study, the CBI value of the subclinical keratoconus group was similar to that in the control group. The different results between our study and the above mentioned studies are probably due to the difference in patient selection. Elevations and BAD analysis were not considered in the patient selection in the mentioned studies. However, the final D value was greater than 1.60 in 42.6% of the VAE group, and only 37.2% of these patients had a CBI value of greater than 0.5 in the study of Ambrósio et al.11 In the current study, stricter criteria were used in the patient selection. We showed in our previous study that only 32.7% of the VAE group had normal tomographic and topometric analyses.9 Because we did not include any patients with suspicious tomographic and topometric analysis in the current study, the final D value of all patients with VAE was lower than 1.6. Vinciguerra et al.19 published a series of cases consisting of 12 patients with a normal topometry, a final D value of less than 1.60, and a CBI value greater than 0.5. However, it should be noted that these were selected cases and cases with clinical keratoconus and normal CBI are also available.11

Ambrósio et al.11 found that using the TBI after the CBI enhances the ability of the Corvis ST to diagnose keratoconus. The TBI is a parameter produced by combining the Scheimpflug-based corneal tomographic data and the biomechanical data with a logistic regression analysis. According to this study, when the cut-off value was 0.79, the TBI distinguished patients with clinical keratoconus from those with normal eyes with 100% sensitivity and specificity (AUC = 0.996). When the cut-off value was 0.29, it was shown to have 90% sensitivity and 96% specificity in patients with VAE (AUC = 0.985).11 In our study, the cut-off value was 0.29, similar to this study. However, the TBI had a sensitivity of 67% and specificity of 86% with an AUC value of 0.790 in distinguishing eyes with subclinical keratoconus from normal eyes. The difference between our results and those of the mentioned study is probably due to the difference in patient selection as stated above. In Ambrósio et al.'s study, 42.6% of the patients had a final D value higher than 1.60 D, indicating a problem in their tomographic analysis. Because tomographic analysis is also considered in the calculation of TBI, the diagnostic capacity can be expected to be higher than that of our study. No abnormality was found in the tomographic analysis of any patient in our study.

According to our findings, biomechanical analysis with the Corvis ST may be used as an additional diagnostic method in detecting subclinical keratoconus. The specificity and sensitivity values determined in our study are not sufficient for biomechanical analysis to be used alone in the diagnosis of subclinical keratoconus and in clinical application. However, we recommend developing new parameters that specifically assess the regional biomechanical properties of the cornea with high sensitivity and specificity. We believe that the main disadvantage of the Corvis ST is that it only evaluates the single axis (horizontal axis) and the response of the entire cornea to this axis. However, the collagen structure and the sequence of the keratoconic cornea show regional differences, and the structural deterioration is most evident in the cone region.14,20 Accordingly, a localized biomechanical alteration occurs in keratoconus (especially in the early stage).5 Scarcelli et al.5 investigated corneal buttons obtained from patients who underwent deep anterior lamellar keratoplasty with confocal Brillouin microscopy and found no significant difference between the biomechanical properties of healthy cornea and the corneal region 4 mm away from the cone area. They found significant mechanical resistance loss in the cone area compared with both peripheral corneal regions and healthy cornea. Therefore, we consider that in vivo methods assessing the regional biomechanical properties of cornea in different axes, especially in the inferotemporal region where the cone is frequently localized, may be more useful in clinical practice. Another disadvantage of the Corvis ST is that despite the use of corrective parameters such as the ARTh, the corneal thickness and the effect of intraocular pressure on biomechanical measurements cannot be completely eliminated.14,21

The main limitation of our study is the low number of patients. We strictly maintained the inclusion criteria to perform the biomechanical analysis closest to ideal. Another limitation is that we did not consider intraocular pressure in the groups. Intraocular pressure may affect the biomechanical behavior of the cornea.21,22 Moreover, keratoconus is considered a bilateral disease based on a global consensus on keratoconus and ectatic diseases in our study.1 However, it should be noted that even if the probability is low, true unilateral keratoconus may be present in our patients.

Looking at the developments in the diagnosis of keratoconus, we believe that the early diagnosis of keratoconus in the future will be conducted through in vivo methods that evaluate “causalities” such as the local biomechanical alteration of the cornea and the deterioration of the collagen structure, not the methods that evaluate “outcomes” such as changes in the corneal surface and thickness. Biomechanical analysis with the Corvis ST may be one of the complementary diagnostic evaluations in detecting patients with keratoconus without any topographic and tomographic abnormalities, but it is not sufficiently sensitive to be used alone. Further prospective studies involving a larger number of patients with subclinical keratoconus with normal topographic and tomographic values are needed to determine the usefulness of Corvis ST measurements in detecting early keratoconus.

References

  1. Gomes JA, Tan D, Rapuano CJ, et al. Global consensus on keratoconus and ectatic diseases. Cornea. 2015;34:359–369. doi:10.1097/ICO.0000000000000408 [CrossRef]
  2. Randleman JB, Russell B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology. 2003;110:267–275. doi:10.1016/S0161-6420(02)01727-X [CrossRef]
  3. Vellara HR, Patel DV. Biomechanical properties of the keratoconic cornea: a review. Clin Exp Optom. 2015;98:31–38. doi:10.1111/cxo.12211 [CrossRef]
  4. Roberts CJ, Dupps WJ Jr, . Biomechanics of corneal ectasia and biomechanical treatments. J Cataract Refract Surg. 2014;40:991–998. doi:10.1016/j.jcrs.2014.04.013 [CrossRef]
  5. Scarcelli G, Besner S, Pineda R, Yun SH. Biomechanical characterization of keratoconus corneas ex vivo with Brillouin microscopy. Invest Ophthalmol Vis Sci. 2014;55:4490–4495. doi:10.1167/iovs.14-14450 [CrossRef]
  6. Hallahan KM, Sinha Roy A, Ambrósio R, Salomão M, Dupps WJ Jr, . Discriminant value of custom ocular response analyzer waveform derivatives in keratoconus. Ophthalmology. 2014;121:459–468. doi:10.1016/j.ophtha.2013.09.013 [CrossRef]
  7. Touboul D, Bénard A, Mahmoud AM, Gallois A, Colin J, Roberts CJ. Early biomechanical keratoconus pattern measured with an ocular response analyzer: curve analysis. J Cataract Refract Surg. 2011;37:2144–2150. doi:10.1016/j.jcrs.2011.06.029 [CrossRef]
  8. Vinciguerra R, Ambrósio R Jr, Elsheikh A, et al. Detection of keratoconus with a new biomechanical index. J Refract Surg. 2016;32:803–810. doi:10.3928/1081597X-20160629-01 [CrossRef]
  9. Koc M, Tekin K, Tekin MI, et al. An early finding of keratoconus: increase in corneal densitometry. Cornea. 2018;37:580–586. doi:10.1097/ICO.0000000000001537 [CrossRef]
  10. Kanellopoulos AJ, Asimellis G. Revisiting keratoconus diagnosis and progression classification based on evaluation of corneal asymmetry indices, derived from Scheimpflug imaging in keratoconic and suspect cases. Clin Ophthalmol. 2013;7:1539–1548. doi:10.2147/OPTH.S44741 [CrossRef]
  11. Ambrósio R Jr, Lopes BT, Faria-Correia F, et al. Integration of Scheimpflug-based corneal tomography and biomechanical assessments for enhancing ectasia detection. J Refract Surg. 2017;33:434–443. doi:10.3928/1081597X-20170426-02 [CrossRef]
  12. Steinberg J, Katz T, Lücke K, Frings A, Druchkiv V, Linke SJ. Screening for keratoconus with new dynamic biomechanical in vivo Scheimpflug analyses. Cornea. 2015;34:1404–1412. doi:10.1097/ICO.0000000000000598 [CrossRef]
  13. Francis M, Pahuja N, Shroff R, et al. Waveform analysis of deformation amplitude and deflection amplitude in normal, suspect, and keratoconic eyes. J Cataract Refract Surg. 2017;43:1271–1280. doi:10.1016/j.jcrs.2017.10.012 [CrossRef]
  14. Roberts CJ. Biomechanics in keratoconus. In: Adel B, ed. Textbook of Keratoconus: New Insights, 1st ed. New Delhi, India: Jaypee Brothers Medical Publishers; 2012:29–32. doi:10.5005/jp/books/11483_5 [CrossRef]
  15. Ali NQ, Patel DV, McGhee CN. Biomechanical responses of healthy and keratoconic corneas measured using a noncontact Scheimpflug-based tonometer. Invest Ophthalmol Vis Sci. 2014;55:3651–3659. doi:10.1167/iovs.13-13715 [CrossRef]
  16. Peña-García P, Peris-Martínez C, Abbouda A, Ruiz-Moreno JM. Detection of subclinical keratoconus through non-contact tonometry and the use of discriminant of biomechanical functions. J Biomech. 2016;49:353–363. doi:10.1016/j.jbiomech.2015.12.031 [CrossRef]
  17. Ferreira-Mendes J, Lopes BT, Faria-Correia F, Salomão MQ, Rodrigues-Barros S, Ambrósio R Jr, . Enhanced ectasia detection using corneal tomography and biomechanics. Am J Ophthalmol. 2019;197:7–16. doi:10.1016/j.ajo.2018.08.054 [CrossRef]
  18. Kataria P, Padmanabhan P, Gopalakrishnan A, Padmanaban V, Mahadik S, Ambrósio R Jr., Accuracy of Scheimpflug-derived corneal biomechanical and tomographic indices for detecting subclinical and mild keratectasia in a South Asian population [published online ahead of print December 7, 2018]. J Cataract Refract Surg. doi:10.1016/j.jcrs.2018.10.030 [CrossRef]
  19. Vinciguerra R, Ambrósio R Jr, Roberts CJ, Azzolini C, Vinciguerra P. Biomechanical characterization of subclinical keratoconus without topographic or tomographic abnormalities. J Refract Surg. 2017;33:399–407. doi:10.3928/1081597X-20170213-01 [CrossRef]
  20. Meek KM, Tuft SJ, Huang Y, et al. Changes in collagen orientation and distribution in keratoconus corneas. Invest Ophthalmol Vis Sci. 2005;46:1948–1956. doi:10.1167/iovs.04-1253 [CrossRef]
  21. Huseynova T, Waring GO IV, Roberts C, Krueger RR, Tomita M. Corneal biomechanics as a function of intraocular pressure and pachymetry by dynamic infrared signal and Scheimpflug imaging analysis in normal eyes. Am J Ophthalmol. 2014;157:885–893. doi:10.1016/j.ajo.2013.12.024 [CrossRef]
  22. Roberts CJ. Concepts and misconceptions in corneal biomechanics. J Cataract Refract Surg. 2014;40:862–886. doi:10.1016/j.jcrs.2014.04.019 [CrossRef]

Comparison of the Topographic, Topometric, and Tomographic Parameters Obtained From the Pentacam HR

ParameterStudy Group (n = 21)P


CK (Mean ± SD)SK (Mean ± SD)CG (n = 35) (Mean ± SD)CK vs SKSK vs CG
CDVA (logMAR)0.35 ± 0.220.0 ± 0.00.0 ± 0.0< .001a1.00c
SE (D)−4.72 ± 2.22−2.63 ± 1.62−3.25 ± 1.54< .001a.254c
K1(D)43.59 ± 4.0342.48 ± 1.4042.27 ± 1.37.017a.359c
K2(D)47.00 ± 4.0443.43 ± 1.4144.14 ± 1.04.003a.054c
Kmax(D)50.38 ± 5.8443.93 ± 1.2044.59 ± 1.10< .001a.065c
Thinnest CT (µm)468.2 ± 67.00536.4 ± 21.2544.8 ± 24.02.001a.247c
AE (µm)11.00 ± 11.492.73 ± 1.032.62 ± 1.28.010b.782c
PE (µm)38.33 ± 24.867.00 ± 2.276.03 ± 2.42< .001b.191c
Q value−0.60 ± 0.42−0.40 ± 0.08−0.35 ± 0.08.006a.08c
ISV57.40 ± 40.5617.53 ± 3.4417.25 ± 5.83.001b.865d
IVA0.57 ± 0.430.11 ± 0.050.11 ± 0.04< .001b.933c
KI1.13 ± 0.151.03 ± 0.021.02 ± 0.02.016b.086c
CKI1.02 ± 0.051.01 ± 0.001.00 ± 0.01.156b.795d
IHA24.60 ± 21.544.83 ± 4.143.94 ± 2.90.002b.394d
IHD0.08 ± 0.070.01 ± 0.000.01 ± 0.02.001b.400c
Final D6.63 ± 4.751.13 ± 0.380.88 ± 0.42.001b.057c
Average PPI2.20 ± 1.461.04 ± 0.170.97 ± 0.10.005a.079c
Maximum PPI2.76 ± 0.871.29 ± 0.141.12 ± 0.31< .001a.521c
Average ART284.6 ± 91.3533.1 ± 59.1528.2 ± 77.3< .001b.623d
Maximum ART204.9 ± 96.9405.2 ± 76.9425.5 ± 65.34< .001b.125d

Comparison of the Biomechanical Parameters Obtained From the Corvis ST

ParameterStudy Group (n = 21)P


CK (Mean ± SD)SK (Mean ± SD)CG (n = 35) (Mean ± SD)CK vs SKSK vs CG
A1L2.06 ± 0.372.15 ± 0.332.30 ± 0.22.495a.055c
A2L1.65 ± 0.401.95 ± 0.212.26 ± 0.5.015a.026c
A1V0.15 ± 0.030.14 ± 0.020.13 ± 0.01.224a.002c
A2V−0.27 ± 0.04−0.26 ± 0.03−0.23 ± 0.02.359b.003d
DA Ratio5.34 ± 0.854.51 ± 0.424.10 ± 0.34.002a.751c
SP-A167.40 ± 25.3882.48 ± 16.90102.40 ± 15.40.066b.630c
CBI0.82 ± 0.350.41 ± 0.400.23 ± 0.29.005a.090c
TBI0.96 ± 0.110.32 ± 0.250.12 ± 0.15< .001b.001d

Receiver Operating Characteristic Analysis for Discriminating Between Subclinical Keratoconic and Normal Eyes

ParameterAUCCut-off ValueSensitivitySpecificityP95% CI
A2L0.383.1930.226 to 0.539
A1V0.6390.1366%83%.0080.579 to 0.899
A2V0.215−0.2336%43%.0020.060 to 0.371
TBI0.7900.2967%86%.0010.656 to 0.925
CBI0.615.2000.429 to 0.801
Authors

From Ulucanlar Eye Training and Research Hospital, Ophthalmology Department, Ankara, Turkey (MK, EA, PK, HK); and Ercis State Hospital, Ophthalmology Department, Van, Turkey (KT, MI).

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (MK, EA, KT); data collection (MK, EA, KT, MI); analysis and interpretation of data (MK, EA, KT, MI, PK, HK); writing the manuscript (MK, EA); critical revision of the manuscript (MK, EA, KT, MI, PK, HK); statistical expertise (MK, EA, KT); administrative, technical, or material support (MK, EA); supervision (MK, EA, KT, MI, PK, HK)

Correspondence: Mustafa Koc, MD, Ulucanlar Street, Number:59, 06240 Ankara, Turkey. E-mail: drmukoc@hotmail.com

Received: June 30, 2018
Accepted: February 26, 2019

10.3928/1081597X-20190226-01

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