Correlation between Corneal Volume and Corneal Biomechanics and Corneal Volume Significance in Staging and Diagnosing Keratoconus

Purpose To investigate the relationship between corneal volume (CV) at different zones and corneal biomechanics in keratoconus (KC) along with the significance of CV in diagnosing and staging KC. Methods This prospective clinical study included 456 keratoconic eyes (Group B) and 198 normal eyes (Group A). Using the topographic KC classification method, Group B was divided into subgroups based on severity (mild, moderate, and severe). The CVs of the 3 mm, 5 mm, and 7 mm zones and biomechanical parameters were obtained by Pentacam and Corvis ST. The diagnostic utility of multirange CVs at different disease stages and severity was determined using a receiver operating characteristic (ROC) curve analysis. Results The CV of the 7-mm zone had the strongest correlation with A1V, A2T, PD, DA ratio max (2 mm), DA ratio max (1 mm), ARTh, integrated radius, SPA1, and CBI (p < 0.01). The CVs of the Group B subgroups were significantly lower than those of Group A for each diameter range (p < 0.05). There were significant differences between the severe, mild, and moderate subgroups for the 3 mm zone (p < 0.05, all). The 3 mm zone CV exhibited better diagnostic ability in each group for distinguishing KC from the normal cornea (Groups A vs. B: area under the ROC curve (AUC) = 0.926, Groups A vs. B1: AUC = 0.894, Groups A vs. B2: AUC = 0.925, Groups A vs. B3: AUC = 0.953). Conclusion The CV significantly decreased in keratoconic eyes. Progressive thinning in the 3 mm zone may be a valuable measurement for detecting and staging KC. Combining the CV examination with corneal biomechanical information may effectively enhance the ability to detect KC.


Introduction
Keratoconus (KC) is a noninfammatory form of corneal ectasia characterized by progressive thinning, steepening, and apical conic protrusion of the cornea that onsets at puberty [1].Te KC apex is displaced inferiorly, inducing irregular myopic astigmatism and causing gradual vision impairment [1,2].Te most characteristic changes in KC are those afecting the overall morphology and structure of its tissue and biomechanics, which directly refect severity.KC management depends mainly on disease stage and patient visual requirements.As the condition worsens, common treatments, such as spectacles and rigid contact lenses, have shifted to surgical procedures, and corneal grafting is the traditional treatment for advanced KC [3].KC has been reported to be the reason of 18% penetrating keratoplasty and 40% deep anterior lamellar keratoplasty [4,5].
KC detection remains an area of signifcant interest.Currently, the primary diagnostic and classifcation criteria for KC are based on anterior surface curvature data of the cornea obtained by corneal topography, which provides twodimensional imaging of the corneal surface [6].Corneal tomography is a three-dimensional imaging technique [7] that has been shown to be critical for enhancing the sensitivity and specifcity of detecting corneal ectasia compared with corneal topography [8].Pallikaris et al. [9] reported that corneal volume (CV) can be a predictive factor for the development of corneal ectasia after refractive surgery and may be considered to avoid post-laser-assisted in situ keratomileusis ectasia.Pentacam (Oculus, Dutenhofen, Germany), a relatively new three-dimensional analyzer equipped with a rotating Scheimpfug camera, allows reliable CV assessment [10].CV as a structural feature of the cornea afects corneal biomechanics and difers from corneal thickness [11].It refects corneal topographical and pachymetric alterations with a single value and may detect rare anomalies that are difcult to detect by corneal topography or/ and corneal central thickness (CCT) evaluation [12].
Corvis ST (OCULUS Optikgeräte GmbH, Wetzlar, Germany) is a noncontact biomechanical measurement device that mainly focuses on refecting comprehensive corneal biomechanical properties [13].Te updated Corvis ST software has produced unique parameters related to corneal deformation and stifness in vivo [13][14][15].Studies have demonstrated that these new parameters are highly efcient in diagnosing KC [16][17][18].CV is a structural characteristic that contributes to the biomechanical profle of the cornea.Sedaghat et al. [19] found that corneal hysteresis and corneal resistance were correlated with CV and that CV was valuable for determining patient qualifcation and may be used to predict the need for refractive surgery.In this study, we evaluated the potential correlation between CV and biomechanical parameters of keratoconic corneas provided by Corvis ST.To the best of our knowledge, this study is the frst to compare the CV values of diferent KC stages to those of normal corneas at diferent diameter areas and explore the grading and diagnostic utility of CV values for KC.

Materials and Methods
In this cross-sectional study, 395 patients with keratoconic eyes (456 eyes, Group B) and 99 participants with normal eyes (198 eyes, Group A) who visited the Ophthalmology Department of Xi'an People's Hospital (Xi'an Fourth Hospital) between January 2016 and December 2023 were recruited.All patients provided informed consent.Te study protocols were approved by the Institutional Review Board and complied with the tenets of the Declaration of Helsinki.
In order to avoid age and gender bias, the participants from Group A were matched to those from Group B for age and sex.None of the participants had a family history of KC.
Te exclusion criteria in this study included signifcant corneal scarring or associated ocular pathology, ocular allergies, nystagmus, previous ocular surgery or trauma, systemic disease, diabetes, or connective tissue disease.
All patients underwent comprehensive ophthalmic examinations, and biomechanical parameters were measured and analyzed by Corvis ST.Corvis ST is a novel biomechanical analyzer developed in a noncontact mode using a released air puf.Video footage of compression deformation was obtained using a high-speed Scheimpfug camera.Approximately 140 cross-sectional images of the cornea were recorded over a collimated air puf for 30 ms.Biomechanical parameters were obtained at the end of this process using the built-in software.
Pentacam is a commonly used corneal tomographic image analysis instrument.It uses a Scheimpfug camera to scan from the anterior surface of the cornea to the posterior surface of the lens and obtains the morphological parameters of the anterior segment by analyzing the collected data.Te CV data were obtained from the Pentacam.
All examinations were performed by trained technicians in the same examination room.Only measurements designated as "OK" quality specifcations were considered valid.

Statistical Analyses.
Data analysis was performed using the SPSS statistical software (ver.22.0 for Windows; IBM Corp., Armonk, NY, USA).Te normal distribution of the parameters was checked using the Kolmogorov-Smirnov test.Data following a normal distribution: after control for CCT, a partial correlation analysis was performed to determine the correlation between CV and biomechanical parameters; a one-way analysis of variance (ANOVA) with post-hoc Bonferroni analysis was used to compare the CV values between Groups A and B; otherwise, they were compared by the nonparametric Kruskal-Wallis test.Te Bonferroni test and post-hoc test for Kruskal-Wallis analysis were used for pairwise comparisons.A receiver operating characteristic (ROC) curve analysis was performed to determine the diagnostic accuracy of the CV values in distinguishing keratoconic eyes from normal eyes.Te best cutof points were set to be the maximum values of sensitivity (%) + specifcity (%)-1.Statistical signifcance was set at a p < 0.05.

Results
A total of 456 keratoconic eyes from 395 patients and 198 normal eyes from 99 matched control patients were included in this study.Table 1 summarizes the corneal biomechanical parameters obtained using Corvis ST.Te correlation between CV and corneal biomechanical parameters is presented in Table 2.

Journal of Ophthalmology
Tere were strong correlations between the CV and biomechanical parameters for 5 mm and 7 mm diameter ranges.Te 5 mm zone CV and 7 mm zone CV were strongly correlated with A1V, A2T, A2V, DA ratio max (2 mm), DA ratio max (1 mm), ART, integrated radius, SPA1, and CBI (p < 0.01).In addition, CV of the 7 mm zone was strongly correlated with PD, HCDA (p < 0.01).Signifcant diferences in CV for multiple ranges were detected between the subgroups of Group B (p < 0.05) (Table 3).Diferences were found between each subgroup and Group A (p < 0.05) and between the severe, mild, and moderate subgroups for the 3 mm zone (p < 0.05).
Figures 1(a)-1(d) and Tables 4-7 present the results of the KC ROC curve analysis.Te area under the curve (AUC) values ranged from good to excellent for all measured parameters.Te ROC curves and AUC values showed a strong ability to discriminate between the CVs of the 3 mm, 5 mm, and 7 mm zones.Te 3 mm zone CV had better sensitivity and specifcity for discriminating between normal and keratoconic eyes than the other zones.

Discussion
Several studies have shown a relationship between corneal resistance factor and corneal hysteresis derived from the ocular response analyzer and CV [19,23].However, it has been demonstrated that the repeatability of the ORA is low.Te Corvis ST is a novel developed tool for measuring corneal deformation in a noncontact mode by a released air puf (air-puf diameter 3.05 mm) with acceptable reliability     Journal of Ophthalmology [24,25].Due to the update of the Corvis ST software, new biomechanical parameters have been incorporated, such as the DA ratio max (2 mm), integrated radius, ART, SPA1, and CBI.Several studies have shown that these new parameters are highly efcient in diagnosing KC [16][17][18].CV refects topographical and pachymetric changes and characterizes corneal morphometric changes with a single value [12].After the control for CCT, signifcant correlations were found between most of the biometric parameters evaluated and the CVs of the 7 and 5 mm zones.Tis confrms the relevance of the biometric and volumetric profle of the cornea in the measurement of biomechanics in KC.Tis implies that, among patients with KC, decreased CV values may indicate compromised corneal deformation.Te progressive increase in corneal irregularities and a decrease in corneal CV might underlie the correlations between CV and biomechanical parameters.Te human cornea is a heterogeneous, viscoelastic biological material.Te tissue response to the application of a force depends not only on the magnitude of that force but also on the force's velocity [26].KC deformation is easier with weaker matrix collagen fbers and a thinner cornea.KC progression leads to the destruction of the corneal stroma, causing instability of the corneal biomechanical properties and weakened mechanical strength [27].

Journal of Ophthalmology
SP A1 is a parameter that refects corneal rigidity.It is defned as the ratio of the pressure loading (imposed by the air-puf) on the cornea to the displacement of the corneal apex (from the undeformed state to the frst applanation).Te SP-A1 value has been reported to be lower in thin corneas than in normal corneas [28].Molecular biology studies have showed that enzyme activation plays a key role in the degradation of the corneal stroma and in corneal thinning, thus afecting corneal stifness [8].Te CBI is an integration of several dynamic corneal response parameters measured by the Corvis ST (consists of A1V, DA ratio (2 mm), ART, SP A1, and integrated radius), refecting a comprehensive corneal biomechanical property [28].ART is a parameter of the Ambrósio relational thickness to the horizontal profle [13].Since CCT is generally assumed to be a parameter that fuctuates in parallel with CV.Te CV of the 7 mm zone had the strongest correlation with biomechanical parameters.It is consisted with the previous reports [19,23].Each corneal layer has been reported to undergo histopathological changes in KC, which are more pronounced in the central area than in the peripheral cornea.Corneas with KC have a reduction in the number of lamellae, particularly in cone development regions, without breaks in the anterior limiting lamina or scarring [29].Furthermore, it has been proposed that collagen lamellae expand in relation to the cone protrusions [30].Ectasia and thinning in KC are associated with lamellar breakage into multiple bundles of collagen fbrils and the loss of anterior lamellae.Tese structural changes may occur in addition to the lateral shifting of lamellae due to the pressure gradient over the cornea and provide a potential explanation for the central thinning of the mass, ultimately leading to a reduction in stromal thickness [31].Te CV reduction in the keratoconic eye is due to corneal thinning, which typically occurs in the central and paracentral cornea [32].In addition, sliding of the corneal collagen matrix may also contribute to CV loss in subclinical or initial-stage KC [27].Tere were signifcant diferences in CVs between Groups A and B. Additionally, there were signifcant diferences between the severe, mild, and moderate subgroups for the 3 mm zone.Tese results demonstrated that CV could be used to distinguish KC from normal eyes and indicate KC severity.
Progressive corneal thinning is a well-known indicator of KC progression.Keratometric and corneal volumetric alterations have recently been reported to be more prominent in patients with subclinical KC than in those with forme fruste KC [33].Corneal thinning and CV loss are characteristic alterations observed in eyes with subclinical KC [34].Ambrósio et al. [35] demonstrated that the percentage of increase in volume distribution was signifcantly altered in mild-to-moderate keratoconic eyes.Toprak et al. [34] calculated volumes using radius increases in steps of 0.05 mm and found that an anterior apex-centered CV value at 1.0 mm signifcantly contributed to distinguish subclinical KC from normal.Corneal thinning typically occurs in the central or paracentral cornea [32], and nipples or oval cones in the central or paracentral cornea are most common [3].In our study, the 3 mm zone CV had a good-to-excellent AUC value for discriminating between normal and mild-to-severe keratoconic eyes.
Identifcation of subclinical or mild stage of KC in patients with few clinical signs is challenging.Reports recommend using CV as an additional measurement to avoid corneal ectasia or reduce the risk of ring-segment extrusion in the implantation of Intacs [23,30,35].In this study, the 3 mm zone CV had the highest diagnostic utility and could be included as a way of classifying KC type and severity and monitoring the progression of this condition.Te combination of CV and various biomechanical parameters may provide more complete information regarding KC grading, diagnosis, and treatment from both the morphological and biomechanical perspectives.Tis study mainly discussed the diagnostic value of CV in patients with KC and did not involve the diagnosis of suspicious KC.Hence, a larger sample size and stricter data screening are needed.

Conclusions
In conclusion, among patients with KC, decreased CV values may indicate compromised corneal deformation.CV is correlated with the biomechanical properties of keratoconic eyes.Te CV of the 3 mm zone has an acceptable diagnostic accuracy for KC detection and progression.Te combination of CV and biomechanical analyses could enhance the ability to detect corneal ectasia and even to predict patients' outcomes.

Table 1 :
Corneal biomechanical parameters obtained with Corvis ST.

Table 2 :
Relationships between corneal volume (CV) and biomechanical parameters for diferent corneal diameters of Group B.

Table 3 :
Diferences in corneal volume (CV) at diferent diameter ranges by disease severity.

Table 4 :
Area under the curve (AUC) and cut-of values of corneal volume (CV) for the 3 mm, 5 mm, and 7 mm zones for distinguishing Group A (n � 198) from Group B (n � 456).

Table 5 :
Area under the curve (AUC) and cut-of values of corneal volume (CV) for the 3 mm, 5 mm, and 7 mm zones for distinguishing Group A (n � 198) from Group B1 (n � 110).

Table 6 :
Area under the curve (AUC) and cut-of values of corneal volume (CV) for the 3 mm, 5 mm, and 7 mm zones for distinguishing Group A (n � 198) from Group B2 (n � 206).

Table 7 :
Area under the curve (AUC) and cut-of values of corneal volume (CV) for the 3 mm, 5 mm, and 7 mm zones for distinguishing Group A (n � 198) from Group B3 (n � 140).