Correlations among Corneal Biomechanical Parameters, Stiffness, and Thickness Measured Using Corvis ST and Pentacam in Patients with Ocular Hypertension

Background To preliminary explore the correlations among corneal biomechanical parameters, stiffness, and thickness in patients with ocular hypertension (OHT) before and after treatment with topical antiglaucoma medications. Methods This was a retrospective study that included 35 eyes with newly diagnosed OHT. Axial length (AL), apical corneal thickness, and minimum corneal thickness were measured using Pentacam. The lengths, velocities, and times of the first and second corneal applanations (A1L, A1V, A1T, A2L, A2V, and A2T, respectively); the highest concavity radius; highest concavity peak distance (PDHC); highest concavity deformation amplitude (DAHC); highest concavity time (HCT); pachymetry (PACH); stress-strain index (SSI); stiffness parameter-A1 (SP-A1); deformation amplitude ratio (DA ratio); integrated radius (IR); Ambrosio's relational thickness horizontal (ARTh); corneal biomechanical index; noncorrected intraocular pressure (IOPnct); and biomechanically corrected IOP (bIOP) values were measured using the corneal visualization Scheimpflug technology (Corvis ST/CST). Results After 5 weeks of treatment, Goldman applanation tonometer-IOP, IOPnct, bIOP, PACH, A1T, A2V, SSI, SP-A1, and ARTh decreased, but A1V, A2T, PDHC, DAHC, DA ratio, and IR increased significantly (all p < 0.05). SP-A1 and A1T were positively associated with premedication IOP and IOP changes, whereas A1V, A2T, PDHC, and IR were negatively associated (all p < 0.05). DAHC and DA ratio had significantly negative correlations with IOP variations. PDHC was found to be positively correlated with AL (p < 0.05). A positive relationship was noted between SP-A1 and HCT before medication (p < 0.05). Conclusions SP-A1 was significantly and consistently associated with IOP. HCT might be correlated with SP-A1. SP-A1 and CST parameters could serve as potential biomarkers for evaluating OHT treatment efficacy.


Background
Glaucoma is a leading cause of blindness worldwide. Intraocular pressure (IOP) is the most important risk factor associated with the occurrence and progression of glaucoma [1]. Recently, attention toward corneal biomechanics has increased with regard to the diagnosis of glaucoma and posttreatment follow-up strategy. Alterations in the biomechanical properties of the cornea determine its deformation response to applanation, which may afect IOP and lead to the progression of glaucoma [2,3]. Several studies have suggested diferent corneal biomechanical properties as important risk factors in glaucoma. Furthermore, IOP corrected according to corneal biomechanical properties improves measurement accuracy. Te corneal visualization Scheimpfug technology (Corvis ST, CST) has been extensively employed to understand the corneal biomechanics of eyes with corneal diseases, glaucoma, or normal eyes [4,5].
Te CST is a noncontact device that can help investigate the corneal biomechanical properties in vivo [6]. Te advantages of CST are excellent noncontact measurement of IOP, central corneal thickness (CCT), and corneal biomechanical parameters simultaneously [7]. A previous study found that the long-term use of some antiglaucoma medications might have an efect on corneal biomechanical properties; these medications might exert an indirect efect on these properties by reducing the magnitude of IOP [8]. However, most studies investigate corneal biomechanics in glaucoma based on corneal hysteresis, which is not associated with corneal stifness.
Stifness parameter-A1 (SP-A1) was introduced as a novel parameter for measuring corneal stifness and rigidity using CST [9][10][11]. SP-A1 is a useful indicator of corneal resistance to deformation [12]. A higher corneal stifness is considered to refect a higher peripapillary scleral stifness and thus a greater optic nerve head vulnerability [13]. Studies indicate that stifness is a deformation parameter that could be derived by mapping a high-speed photograph series of corneal deformation responses to a calibrated air puf [10]. Initial studies have reported signifcant diferences in SP-A1 between keratoconic and normal eyes. Most previous studies on glaucoma have explored corneal hysteresis as a corneal biomechanical property. In addition, a higher corneal stifness parameter might be predictive of glaucomatous progression in glaucoma suspect eyes [13]. Tese results suggested that stifness parameter variations might be potential infuencing factors during glaucoma treatment and progression. However, few studies have been devoted to observe the dynamic corneal stifness changes and to investigate whether corneal stifness is altered with the treatment of ocular hypertension. Additionally, the correlations between corneal stifness and other corneal properties in ocular hypertension (OHT) eyes remain unclear.
In the present study, we explored the changes in and relationships among corneal biomechanical parameters, stifness, and thickness after antiglaucoma pharmacotherapy in patients with OHT. Furthermore, we aimed to assess and compare the stifness parameters with the associated factors and to evaluate the feasibility of using SP-A1 for monitoring glaucoma treatment and progression.

Methods
In this retrospective study, we reviewed the medical records of consecutive patients who underwent antiglaucoma pharmacotherapy in the glaucoma division of Henan Eye Hospital and Henan Eye Institute from December 2020 to November 2021. Tis study was conducted in compliance with the Declaration of Helsinki for research involving human participants. Institutional Review Board approval was obtained from Henan Eye Hospital [HNEECKY-2021(47)]. Informed consent was obtained from all patients.

Inclusion and Exclusion
Criteria. Tis study included 35 eyes from 22 consecutive patients who were aged >18 years and newly diagnosed with OHT. OHT was defned as having a normal visual feld (VF) with an untreated Goldman applanation tonometer-IOP (GAT-IOP) of >21 mmHg that had not previously been treated with topical or systemic medications, laser technology, or surgery [14]. Patients were excluded if they had any other ocular or neurological comorbidities or if they had previously undergone ocular treatment that could afect corneal biomechanics. Patients with secondary glaucoma caused by factors such as uveitis, trauma, steroids, or corneal scarring were excluded. Patients with hypermetropia or myopia >5.0 diopters (D) and/or astigmatism >3.0 D were excluded.

Clinical Examination.
Te measurements were performed between 9:00 AM and 4:00 PM by an experienced operator. Te measurement of best corrected visual acuity (BCVA), slit lamp examination, and fundus examination were performed by a professional glaucoma specialist. IOP was tested using a GAT before and after treatment with topical medications for 5 weeks, and the axial length (AL) in all participants was measured using partial coherence interferometry (IOLMaster, Zeiss, Germany) [15].
All patients' eyes were examined using a Pentacam imaging system (70900, OCULUS, Germany) while they fxated their eyes on a target light source. To ensure the image quality, only results that were marked as "OK" on the display specifcation window were adopted. Pachymetry (PACH) assessments and measurement of corneal deformation parameters were performed using CST (OCULUS 72100, Wetzlar, Germany). CST records the entire dynamic reaction of the cornea to a fxed air impulse. To ensure correct alignment, the patients were asked to place their heads on the chin rest such that the puf nozzle was centered on the eye.

Data Collection.
From the medical records of all patients, data on demographic characteristics, including age, sex, systemic health condition, and lens status, were collected. Clinical data on IOP, VF (Humphery, Carl Zeiss, Germany), BCVA, and corneal thickness were collected before and after treatment. CCT, apical corneal thickness (ACT), and minimum corneal thickness (MCT) were measured using Pentacam [16]. Using CST, the lengths, velocities, and times of the frst and second corneal applanations (A1L, A1V, A1T, A2L, A2V, and A2T, respectively), as well as the highest concavity radius (HCR), highest concavity peak distance (PDHC), highest concavity deformation amplitude (DAHC), and highest concavity time (HCT) were observed. Te stress-strain index (SSI) and SP-A1 were also measured. New CST parameters were considered, such as the deformation amplitude ratio (DA ratio), integrated radius (IR), and Ambrosio's relational thickness horizontal (ART). IR was the integrated area under the radius of the inversed curvature during the concave phase. ART was defned as the ratio between the thickest and thinnest point. It is a progression index that describes the increase in thickness from the thinnest point to the periphery [11]. Noncorrected IOP (IOPnct) and biomechanically corrected IOP (bIOP) were recorded using CST and considered as CST-IOP parameters.

Statistical Analysis.
Statistical analysis was performed using IBM SPSS statistics (version 19.0.0). Te Kolmogorov-Smirnov normality test was used. Clinical characteristics are presented as mean ± standard deviation (SD). Paired t-test was used to investigate the diferences in clinical parameters and changes of the frst and second corneal applanations between two time points: baseline and posttreatment. Multiple linear regression analyses were performed to determine associations between the parameters. In all analyses, p < 0.05 was considered statistically signifcant. For the clinical parameter data with p < 0.05, the statistical power analysis was performed using G * Power (version 3.1.9.7), and all parameters power >0.96, except SSI and ARTH, which were 0.757 and 0.527, respectively. For GAT-IOP, IOPnct, and bIOP, p < 0.0167 was considered signifcant in the paired t-test.

Results
A total of 35 eyes of patients with newly diagnosed OHT were included in this study. Te mean age of the participants was 28.86 ± 10.22 years, and 36.37% of the participants were women. Furthermore, 13 of the 35 patients had binocular parameters. Te mean number of anti-glaucoma medications used for treatment was 1.06 ± 0.24. None of the included patients had a history of glaucoma surgery or antiglaucoma pharmacotherapy (Table 1).
Te GAT-IOP decreased signifcantly from 25.69 ± 4.63 mmHg at baseline to 17.03 ± 3.74 mmHg after treatment (p < 0.001, Table 2). GAT-IOP and IOPnct were signifcantly higher than the corrected bIOP at baseline (p � 0.03 and p < 0.001, respectively; not shown in Table 2). However, there were no signifcant diferences in GAT-IOP and CST-IOP parameters after treatment. Tere was no signifcant change in BCVA and AL after treatment (p > 0.05, Table 2). CCT, ACT, and MCT mildly decreased (all p > 0.05) after treatment in all patients. We found that PACH serves as a proxy for CST-CCT and that it decreased signifcantly after treatment (p � 0.014, Table 2).

Corneal Biomechanical Parameters and Stifness Measured Using CST. Changes in CSTparameters in patients with
OHT after treatment are shown in Table 3. Te mean SP-A1 was 109.08 mmHg/mm (SD, 11.79 mmHg/mm) after treatment, which was signifcantly lower than the baseline level (p < 0.001, Table 3). Similarly, SSI decreased signifcantly after treatment (p � 0.010, Table 3). Moreover, A1V, A2T, PDHC, DAHC, DA ratio, and IR increased signifcantly after treatment (all p < 0.05). On the contrary, A1T, A2V, PACH, and ART were signifcantly lower at follow-up than at baseline (all p < 0.05, Tables 2 and 3). However, there were no signifcant changes in A1L, A2L, HCR, and HCT (all p ≥ 0.05). Te changes of frst and second corneal applanations (AV and AT) in the baseline compared to the changes of frst and second corneal applanations (AV and AT) in post-treatment varied signifcantly (p < 0.001, Table 3).

Association between Corneal Biomechanical Parameters and Clinical
Characteristics. Independent factor analysis showed that SP-A1 and A1T were positively correlated with pre-IOP and changes in IOP; however, A1V, A2T, PDHC, and IR were negatively correlated with pre-IOP and changes in IOP after anti-glaucoma treatment (p < 0.05). Changes in AV were positively correlated with pre-IOP and negatively correlated with changes in IOP (p<0.05). DAHC showed signifcant negative correlations with changes in IOP (p < 0.01, Table 4). Te longer the AL, the higher the PDHC (p � 0.015, Table 4). SP-A1 was positive with Pentacamderived CCT, whereas A1V and IR were negative (p < 0.001, Table 4).

Association between Corneal Stifness Parameters and the
Highest Concavity Parameters. Te highest concavity (HC) referred to the HC of the cornea in response to the air puf. Te relationships between variations in stifness parameters after treatment and HC CST-acquired values were analysed. A statistically signifcant linear relationship was noted between SP-A1 and corneal biomechanical parameters (p < 0.05). A positive linear relationship was noted between SP-A1 and HCT at baseline (p � 0.032, Table 5). Additionally, there was no signifcant correlation between SSI and HC parameters (p > 0.05, Table 5).

Discussion
A previous study reported that peripapillary stifness was higher in glaucomatous eyes than in normal eyes [17]. Moreover, eyes with normal-tension glaucoma were reported to show a lower SP-A1 than normal eyes; however, these results were confounded by a lower IOP and the use of topical medications [14]. In the current study, SP-A1 and SSI indicated that changes in corneal tissue stifness decreased signifcantly after treatment in OHT eyes. Moreover, a decrease in SP-A1 indicated that the cornea was more deformable after antiglaucoma pharmacotherapy.
A higher corneal stifness is considered to refect a higher peripapillary scleral stifness and thus a greater optic nerve head vulnerability [13]. In this study, multiple linear regression analysis showed that a higher IOP might be associated with a higher SP-A1 after treatment. Te remarkable decrease in IOP was associated with a decrease in SP-A1. Te present results suggested that SP-A1 might be very sensitive to IOP changes and that it is signifcantly and consistently associated with IOP. SP-A1 could probably provide more information about glaucoma treatment and follow-up strategy. In a previous study, a higher SP-A1 was found in eyes with a higher IOP and thicker CCT [18]. Similarly, in this study, there was a signifcant association between SP-A1 post-treatment and Pentacam-derived CCT. Tese results suggested that patients with thicker CCT might more easily achieve a higher SP-A1 post-treatment, which is a useful indicator of corneal resistance to deformation. HC described the deformability of cornea. In the current study, the lower HCT before treatment was correlated with decreasing SP-A1. Te outcomes of the present study showed that SP-A1 refects corneal stifness and is closely related to corneal properties. Te two parameters could share corroboration in evaluating treatment efcacy and predicting progression of glaucomatous optic nerve injuries.

Journal of Ophthalmology
In the present study, in addition to other CST parameters, A1V, A2T, IR, and PDHC showed a signifcant increase after treatment. Similar fndings were noted as efects of antiglaucoma medications in a previous study [8]. Serbecic et al. found A1, HC, and A2 time points showed excellent repeatability and reproducibility [19]. A previous study reported that A1V was signifcantly higher in eyes treated with prostaglandin analogues (PGAs) than in those treated without PGAs [2]. Another study reported that A1V increased signifcantly even after the frst week of antiglaucoma surgery [20]. In some studies on long-term treatment with topical medications, DAHC increased markedly after 2 years, while PDHC increased slightly [2,8]. In other studies, PDHC and CRF increased signifcantly after trabeculectomy or other antiglaucoma surgeries [21]. Partially consistent with these studies, the present study noted a signifcant increase in PDHC, DAHC, and DA ratio. Similar to previous studies [8], the present study noted that post-treatment A1V (m/s) was higher in patients with newly diagnosed OHT. Moreover, A2V decreased signifcantly whereas A2T increased after pharmacotherapy and surgery in previous studies [8,21]. Additionally, changes in A2V were reported to gradually slow     Journal of Ophthalmology 5 down over time [21]. In this study, SSI and SP-A1 showed corneal stifness changes and decreased signifcantly after pharmacotherapy. Consistent with the fndings of previous research, A1L, A2L, HCT, and HCR infrequently showed no signifcant changes after long-term pharmacotherapy or at an early stage after surgery [2,8,21,22]. Asaoka et al. found that A1T, A1V, A2V, PDHC, and DAHC were infuenced by IOP [23]. Huseynova et al. reported that IOPcc was positively correlated with A1T and A2V and negatively correlated with A2T and A1V. In the present study, the pretreatment IOP was positively associated with SP-A1 and negatively associated with A1V. A decrease in IOP was signifcantly associated with variations in corneal parameters such as A1V, PDHC, and SP-A1. Moreover, IOP showed a positive correlation with SP-A1 and a negative correlation with A1V, IR, and PDHC. Tese results suggested that a variation in IOP may lead to changes in corneal stifness and biomechanical properties in the early stage of antiglaucoma pharmacotherapy. Te velocity parameters refected corneal elasticity and were associated with the cross-linking of collagen fbrils in the cornea [24]. A decrease in IOP might enhance the elasticity and viscosity of the cornea and induce a strong pressure threshold [25]. A lower post-treatment IOP led to a smaller IR and PDHC magnitude in this study. A possible explanation for this result is that perhaps glaucoma medications, such as PGAs, activated matrix metalloproteinases (MMPs) and suppressed their tissue inhibitors to increase the keratocyte density in the corneal stroma. Tis PGA-induced corneal tissue remodeling may infuence DAHC and PDHC [26]. Te changes in DAHC at an early stage after pharmacotherapy indicate that a variation in corneal properties may occur earlier than expected. IR and ART served as new CST parameters that were rarely discussed during glaucoma treatment. In this study, IR increased signifcantly postmedication, while the higher IOP before medication induced the lower IR after treatment. ART was found to be signifcantly decreased in the present study, and a lower value indicated a thinner cornea and/or a faster thickness increase toward the periphery [11]. Tese results suggested that an IOP decrease under efective treatment may result in changes in the characteristic parameters of the cornea.
Previous studies have shown that clinical and demographic factors afect corneal dynamics. Huang et al.   Journal of Ophthalmology found that a decrease in AL following a reduction in IOP might be involved in the dynamic reaction of the cornea [21]. Tese results showed that AL is related to tissue extension, ocular rigidity [27], and biomechanical properties. In contrast to prominent AL changes after surgical treatment in a previous study [8], there were no signifcant diferences in AL after pharmacotherapy in the current study. However, in this study, a longer AL was associated with a larger PDHC after a decrease in IOP. AL suggested multiple potential factors that impact corneal biomechanics. In cornea, a stifer appearance and higher CCT are accompanied by less deformation in response to air pulse [28]. A few studies have evaluated a decrease in the changes in CCT following longterm topical pharmacotherapy [29,30]. Activation of MMPs induces the degradation of the corneal stromal extracellular matrix, which ultimately leads to a reduction in CCT [29,30]. However, some studies observed no signifcant changes in CCT during a 2-year treatment with PGA [8]. In the present study, the CCTs measured using Pentacam were stable during the short-term follow-up period. However, PACH, as a proxy of CST-CCT, showed a signifcant decrease. Te present results suggest that PACH, as a proxy of CST-CCT, might overestimate Pentacam CCT, especially when the IOP is high. Kumar et al. found the coefcient of variation (COV) of CCT measurements of Corvis were lower than those of Pentacam [31]. It was a complicated factor that could afect corneal thickness, and the exact mechanisms must be elucidated in the future. Te consistency between GAT-IOP and CST-IOP has been evaluated in patients with and without glaucoma in previous studies; some showed no signifcant diferences [32,33], while one study showed that CST-IOP was higher than GAT-IOP in patients who underwent LASIK [34]. In this study, GAT-IOP was consistent with IOPnct; however, the former decreased signifcantly after biomechanical correction before pharmacotherapy, and these results were similar to those of previous reports. No signifcant diferences were found among GAT-IOP, IOPnct, and bIOP after topical antiglaucomatous therapy.
Tese results suggest that IOP tested by CST might underestimate GAT-IOP, especially when the IOP is high [7].
Tis study has some limitations. First, the retrospective study design may have led to bias. Second, the number of patients was small. Further prospective studies with a larger number of patients and diferent types of glaucoma medications are needed.

Conclusions
In summary, signifcant decreases in corneal stifness were noted in patients with OHT after antiglaucoma therapy. SP-A1 was signifcantly and consistently associated with IOP in OHT eyes. Te corneal properties HCT and CCT were signifcantly associated with SP-A1. SP-A1 measured using CST could be a potential biomarker for evaluating the treatment efcacy and disease progression in OHT eyes. As new CST parameters, IR signifcantly increased and ART decreased post antiglaucoma therapy. IR was negatively associated with IOP and CCT. Future studies would contribute to a broader understanding of the correlations among corneal biomechanical properties, stifness, and thickness after IOP-lowering therapy.