Analysis of In Vivo Sclera Impact on the Biomechanics of Myopic Eyes

Abstract

Purpose: The purpose of this study was to evaluate the relationship between in vivo scleral thickness and biomechanical properties in eyes with myopia and to examine the association between in vivo sclera and the severity of myopia.

Methods: This prospective case series included 101 eyes from 101 patients with myopia. Corneal biomechanical parameters and anterior scleral images were acquired using the Corvis ST and anterior segment optical coherence tomography (AS-OCT), respectively. Anterior scleral thickness (AST) was measured at the scleral spur (AST0), and at 1 mm (AST1), 2 mm (AST2), and 3 mm (AST3) along the nasal (NAST) and temporal (TAST) regions. The correlations among corneal biomechanical parameters, AST, axial length (AL), and spherical equivalent refraction (SER) were analyzed after adjusting for confounding factors.

Results: The scleral spur (AST0) was thicker than the other regions (P < 0.0001). After adjusting for confounding factors, the corneal biomechanical parameters, the stiffness parameter at highest concavity (SP-HC) was significantly related to scleral thickness (P < 0.05, r_NAST0 = 0.369, r_NAST1 = 0.236, r_NAST2 = 0.217, and r_TAST0 = 0.480). SP-HC (r = 0.223) and peak distance (r = −0.259) were significantly related to SER (P < 0.05).

Conclusions: Scleral thickness, particularly at the scleral spur, is an important factor influencing in vivo biomechanics of myopic eyes. The stiffness parameter SP-HC showed the strongest correlation with in vivo AST, potentially reflecting scleral biomechanical properties. SP-HC decreased significantly with increasing myopia, suggesting that weaker scleral biomechanics may be a risk factor for myopia progression.

Translational Relevance: In vivo stiffness parameter SP-HC may indirectly quantify scleral properties and serve as a high-risk biomechanical indicator to evaluate myopia progression.

Introduction

The prevalence of myopia is rapidly increasing worldwide.¹ By 2050, it is projected to affect 4.76 billion people, approximately 50% of the global population.² The hallmark pathological change in myopia is the excessive elongation of the eye's posterior segment,³ which can lead to severe ocular complications, including myopic maculopathy, retinal detachment, and glaucoma.⁴ These conditions are major causes of acquired vision loss in adults.⁵ Studies in animal models with myopia and ex vivo human eyes have shown scleral changes, such as thinning, reduced collagen synthesis, and increased collagen degradation.⁶ It is worth noting that these alterations result in weaker scleral biomechanical properties, potentially promoting axial elongation.^6–9 However, in vivo studies on scleral biomechanics to validate these findings are limited. Understanding the relationship between myopia and in vivo scleral biomechanics is crucial for exploring its pathogenesis and developing preventive strategies for myopia-related complications. 

In vivo scleral biomechanical properties are assessed by examining scleral changes in response to intraocular pressure (IOP). Owing to the limitations of invasive protocols,⁸ there are currently no clinical methods to measure scleral biomechanics directly. Grytz et al.^10,11 proposed that scleral thickness may partially reflect scleral stiffness, as a thicker sclera contains more collagen fibers and extracellular matrix. This suggests that in vivo scleral thickness could serve as an indirect indicator of scleral biomechanical properties. In vitro studies have also shown that corneal biomechanical variables may indirectly characterize scleral biomechanics, as a stiffer sclera limits corneal motion by resisting the displacement of aqueous humor during corneal deformation.¹² Identifying specific biomechanical parameters in eyes with myopia that strongly correlate with in vivo scleral thickness could help clarify key biomechanical features in patients with myopia that warrant clinical attention. Furthermore, exploring the relationship among in vivo scleral thickness, biomechanical variables, and myopia severity could provide novel insights into how scleral biomechanics influence myopia progression and aid in identifying high-risk patients in clinical practice. Despite these implications, the relationship among in vivo scleral thickness, specific biomechanical parameters, and myopia severity remains unclear. 

This study aimed to clarify the correlation between corneal biomechanical parameters and anterior scleral thickness (AST) in eyes with myopia in vivo. Simultaneously, we further examined the relationship among biomechanical parameters, axial length (AL), and spherical equivalent refraction (SER) in eyes with myopia while accounting for potential confounding factors such as central corneal thickness (CCT), IOP, and age.¹³ The study seeks to define the impact of in vivo scleral thickness on myopic biomechanical measurements and elucidate the association between scleral biomechanics and tissue properties in relation to myopia severity. 

Methods

This prospective case series was conducted between May and December 2023, registered at ClinicalTrials.gov (NCT06109636), and approved by the Institutional Review Board of Tianjin Eye Hospital (KY2023025). All research procedures adhered to the principles of the Declaration of Helsinki, and informed consent was obtained from all participants. 

Ophthalmological Examinations and Data Collection

A total of 101 eyes from 101 subjects with myopia were included in the study. All participants underwent comprehensive ophthalmic examinations, including objective refraction, manifest refraction, IOP measurement, slit-lamp examination, and fundus examination. AL was measured using the optical non-contact Lenstar LS 900 biometer (Haag Streit, Köniz, Switzerland). Corneal morphological parameters were acquired through the anterior segment tomography (Pentacam AXL; Oculus, Wetzlar, Germany). Corneal biomechanical parameters were measured with the corneal visualization Scheimpflug technology (Corvis ST; Oculus, Wetzlar, Germany). Anterior scleral images were captured using the anterior segment swept-source optical coherence tomography (OCT; CASIA 2; Tomey, Nagoya, Japan). 

The Corvis ST uses an ultrahigh-speed Scheimpflug camera operating at 4330 frames per second to record the corneal deformation process induced by an air pulse. Fifteen sensitive biomechanical parameters, identified based on previous studies, were selected for analysis.^12,14 These parameters were exported from the Corvis ST software (version 1.6r2187), and their definitions are provided in Supplementary Table S1. 

The CASIA 2 anterior segment OCT (AS-OCT) utilizes a 1310 nm swept-source light to image the cornea and anterior segment of the eye at 50,000 A-scans per second, with a scan length of 16 mm and a depth of 13 mm. Anterior scleral images were acquired along two meridians (nasal and temporal). To ensure consistent scanning locations within and between subjects, scans were performed perpendicular to the limbus at the 3 and 9 o’clock positions to capture anterior scleral images (Fig. 1A). During the measurements, participants were instructed to gaze in the direction opposite to the quadrant being imaged.¹⁵ 

Figure 1.

 

Sequential steps involved in obtaining AST with AS-OCT. (A) Single scan perpendicular to the limbus at the 9 o’clock positions. (B) Cropped raw scan image of the AST. (C) White arrowhead indicates scleral spur. Yellow arrowheads indicate the inner and outer scleral boundaries. Red solid lines demarcate the 1-mm intervals where scleral thickness was measured.

All examinations were performed by experienced technicians, and biomechanical data were acquired from the original CSV files. Only scans rating of “OK” were analyzed. 

Measurement of AST

In the anterior scleral images, the outer boundary was identified as the hyporeflective region of the outer scleral vessels, whereas the inner boundary was defined as the line between the hyper-reflective scleral tissue and the hyporeflective ciliary body tissue. AST was measured as the vertical distance between the inner and outer boundaries (Fig. 1B).^16,17 Measurements were taken starting at the scleral spur (a slightly depressed region in the limbal area facing the anterior chamber) at distances of 0 mm (AST 0), 1 mm (AST 1), 2 mm (AST 2), and 3 mm (AST 3; Fig. 1C).^16,18 All measurements were performed using ImageJ software (National Institutes of Health, Bethesda, MD, USA) by drawing lines perpendicular to the tangents at specific points along the inner boundary.^16,18 

Participants and Inclusion Criteria

The inclusion criteria were as follows: (1) myopia, defined as at least 0.5 diopters (D) of SER; (2) best-corrected visual acuity (BCVA) of ≥ 20/20 in both eyes; and (3) evaluation by 2 experienced physicians. Only the right eye of each participant was included in this study. 

The exclusion criteria included the presence of ocular diseases other than refractive errors, a history of ocular surgery, ocular trauma, corneal scarring, pregnancy or lactation, and systemic diseases. Participants were required to discontinue wearing contact lenses before evaluation ‒ soft corneal contact lenses for at least 2 weeks and rigid contact lenses for at least 4 weeks. 

Statistical Analysis

Continuous variables are presented as mean ± standard deviation, whereas categorical variables are expressed as frequency and percentage. The Kolmogorov-Smirnov test was used to assess the distribution type of data normality. Two-factor repeated-measures analysis of variance (ANOVA) with Bonferroni post hoc tests was performed to analyze differences in AST (nasal and temporal) across the two meridians and at different eccentricities. Pearson's and Spearman's correlation analyses were applied to evaluate the relationships among CCT, IOP, age, and corneal biomechanical parameters. Partial correlation analysis was applied to examine the relationships among AST, SER, AL, and corneal biomechanical parameters while controlling for confounding factors. Scatter plots were created to display trends in the distribution of correlations. P values < 0.05 were considered to be statistically significant. All statistical analyses were conducted using SPSS version 26.0 (International Business Machines Corp., Armonk, NY, USA). 

Results

Baseline Characteristics

A total of 101 right eyes from 101 subjects with myopia were included in this study. The average age of the participants was 23.65 ± 5.18 years, with a female-to-male (F/M) ratio of 1.46. The mean SER was −5.73 ± 1.91 D (range = −10.75 to −1.38 D), and the mean AL was 26.02 ± 0.95 mm (range = 23.75 to 28.57 mm). Baseline data and AST measurements are summarized in Table 1. 

Table 1.

 

Baseline Information of Participants

 

AST Distribution

As shown in Figure 2, the AST was thickest at the scleral spur compared with other peripheral locations (P < 0.0001). Differences in thickness among meridians were observed, with the temporal side (925.27 ± 71.85 µm) being thicker than the nasal side (854.51 ± 67.37 µm; P < 0.0001). No significant differences in thickness were found between the two sides at other peripheral locations. Within the eccentric distance range of 1 to 3 mm from the scleral spur, the sclera gradually thinned; however, the average thickness of the temporal side at 3 mm (689.47 ± 74.51 µm) was greater than at 2 mm (667.34 ± 71.74 µm). 

Figure 2.

 

Boxplots of anterior scleral thickness in the nasal (red) and temporal (blue) meridian along different eccentricities from the scleral spur. “0” indicates the scleral spur and the error bar represents the standard error. *A bold asterisk (*) indicates the significant difference obtained from the two-factor ANOVA repeated measures with Bonferroni post hoc tests. ****P indicates P values < 0.0001.

Relationship Between AST and Corneal Biomechanical Parameters

Building on prior research, this study analyzed the correlation among 15 corneal biomechanical parameters provided by Corvis ST and AST in the nasal and temporal regions. Correlation analysis revealed that 6 parameters were significantly associated with CCT (P < 0.05), 11 parameters with IOP (P < 0.05), and 1 parameter with age (P < 0.05), as shown in Table 2. 

Table 2.

 

Correlation Between Confounding Factors and Corneal Biomechanical Parameters

 

After adjusting for confounding factors based on the results in Table 2, as shown in Table 3, A2V showed a significant positive correlation with nasal AST at 0 mm (NAST0; r = 0.255), 1 mm (NAST1; r = 0.217), and 2 mm (NAST2; r = 0.241, P < 0.05). Similarly, the stiffness parameter at the highest concavity (SP-HC) exhibited significant and positive correlations with NAST0 (r = 0.369), NAST1 (r = 0.236), NAST2 (r = 0.217), and temporal AST at 0 mm (TAST0; r = 0.480, P < 0.05). Scatter plots in Figures 3A to 3D visually illustrate the correlations between SP-HC and AST. 

Table 3.

 

Correlation Between AST and Corneal Biomechanical Parameters

 

Figure 3.

 

Scatter plots of the correlation between SP-HC and AST at 0, 1, 2, and 3 mm from the scleral spur in the nasal (red) and temporal (blue) meridian.

Relationship Between Biomechanical Parameters and Myopia After Controlling for Scleral Thickness

The correlation analysis among 15 corneal biomechanical parameters selected from previous research and myopia is presented in Table 4. After adjusting for confounding factors, including CCT, IOP, and age, and further controlling for AST, SP-HC (r = 0.223), peak distance (PD; r = −0.259), highest concavity deformation amplitude (HCDA; r = −0.207), and stress–strain index (SSI; r = 0.341) were significantly related to SER (P < 0.05). Additionally, whole eye movement (WEM; r = −0.226), PD (r = 0.451), and SSI (r = −0.244) were significantly related to AL (P < 0.05). 

Table 4.

 

Correlation Among SER, AL, and Corneal Biomechanical Parameters

 

Discussion

This study is the first to investigate the relationship between in vivo scleral thickness and specific corneal biomechanical parameters, revealing a significant positive correlation between SP-HC and AST in nasal and temporal regions (r_NAST0 = 0.369, r_NAST1 = 0.236, r_NAST2 = 0.217, and r_TAST0 = 0.480, P < 0.05). These findings suggest that SP-HC is a sensitive parameter for characterizing scleral tissue properties and may indirectly reflect scleral biomechanics. Moreover, scleral thickness is an important confounding factor in accurately evaluating the in vivo biomechanics of eyes with myopia. After adjusting for CCT, IOP, and age, lower SP-HC values were significantly correlated with greater myopia severity (SER, r = 0.223, P < 0.05). This suggests that the SP-HC, representing the stiffness parameter at the highest concavity, could serve as a high-risk indicator of changes in myopia severity. These findings highlight the potential role of weakened scleral biomechanics as a risk factor for myopia progression. The study provides a novel perspective for clinical monitoring and progression prediction of myopia severity through scleral biomechanics. 

We analyzed the relationship among 15 in vivo corneal biomechanical variables and AST in the nasal and temporal regions. Because CCT, IOP, and age are known to influence corneal deformation responses to air puffs, adjustments were made for these factors. The results revealed that SP-HC (r_NAST0 = 0.369, r_NAST1 = 0.236, r_NAST2 = 0.217, and r_TAST0 = 0.480) and A2V (r_NAST0 = 0.255, r_NAST1 = 0.217, and r_NAST2 = 0.241) were significantly and positively correlated with AST (P < 0.05). SP-HC, introduced by Roberts et al.,¹⁹ represents the SP-HC based on finite element modeling corrected for IOP. This parameter is defined as the ratio of load to displacement. Specifically, it uses Corvis ST's air-puff pressure on the cornea, measured by hot-wire anemometry, minus the biomechanically corrected IOP, then divided by the displacement between the first applanation and highest concavity.¹⁹ In clinical practice, higher SP-HC values are associated with smaller displacements at the maximum concavity point, indicating stronger corneal biomechanical properties. Our findings suggest that higher SP-HC values may result from a thicker sclera near the limbus, where the elevated collagen fiber and matrix contents enhance biomechanical properties and restrict corneal deformation. Ex vivo experiments demonstrated significantly higher SP-HC values after stiffening human donor sclera with 4% glutaraldehyde,¹² which indirectly supports our findings. This relationship is likely attributed to the increased resistance of stiffer scleral tissue to aqueous humor displacement during maximum corneal deformation, thereby limiting the extent of deformation.^12,20,21 Previous numerical simulation studies have emphasized the role of the sclera and aqueous humor in limiting the corneal deformation response.^22–24 The inclusion or not of the sclera in the numerical simulation of the air puff test, or the simulation of a more or less compliant behavior, can significantly alter corneal biomechanical characterizations.^22–25 Moreover, Montanino et al.²⁶ demonstrated that the distribution of aqueous humor influences corneal displacement. Based on in silico models, our study confirms these findings from an in vivo perspective, further validating their reliability. Ma et al.²⁷ hypothesized that SP-HC could serve as an in vivo parameter for assessing scleral biomechanics, but this hypothesis lacked validation through in vivo experiments. Our results suggest that SP-HC partially reflects scleral biomechanical properties, providing further evidence for its clinical applicability. Given its strong correlation with scleral thickness and ex vivo biomechanics, SP-HC may be a valuable parameter for indirectly quantifying scleral biological and biomechanical properties, particularly in the absence of direct measurement tools in clinical practice. The strategy proposed in this research offers a quick and clinically feasible method to assist with characterizing in vivo scleral biomechanics. It is also worth noting that optical coherence elastography (OCE) is a promising technique for directly estimating scleral biomechanics.^28,29 OCE allows for localized biomechanical assessment and offers precise measurements, with potential for clinical application in the near future. 

In addition, A2V exhibited a significant positive correlation with NAST at 0, 1, and 2 mm from the scleral spur. A2V represents the velocity of the corneal apex during the second applanation, with higher values indicating faster corneal rebound and a stiffer cornea. Previous studies have shown that this parameter contributes to the diagnosis of corneal ectatic diseases, evaluation of refractive surgery outcomes, and assessment of corneal cross-linking efficacy.^30,31 Our findings align with ex vivo studies, which suggested that thicker sclera or stiffer anterior chamber mounts provide firmer structural support to the cornea during air-puff deformation.³² This observation may explain why the cornea exhibits stronger corneal biomechanics, further supporting the hypothesis that scleral biological and biomechanical properties influence corneal dynamic deformation. Moreover, the A2V phase corresponds to the point of highest corneal concavity, suggesting that parameters associated with this phase may be influenced by scleral properties. Consequently, scleral factors should be considered when interpreting these parameters. Although ex vivo and in vivo studies have demonstrated an association between A2V and scleral properties, an asymmetry in its correlation with NAST and TAST, along with susceptibility to confounding factors, was observed.¹² Therefore, we recommend using A2V with caution as a clinical indicator for assessing scleral biological and biomechanical properties. 

Previous studies have highlighted IOP, CCT, and age as key factors influencing in vivo corneal biomechanical measurements.¹² Understanding the effects of these factors on corneal biomechanical parameters is essential for interpreting disease-specific changes. For instance, keratoconic eyes with higher IOP may exhibit stronger corneal biomechanics than normal eyes with lower IOP. This highlights the limitation of relying on a single biomechanical parameter without accounting for confounding factors, as it may lead to misdiagnosis. Identifying the factors that influence in vivo corneal biomechanics is therefore critical. Our study demonstrated that AST significantly impacts SP-HC and A2V, with measurements suggesting that greater AST may obscure the weakening of specific corneal biomechanical parameters in patients with myopia. Furthermore, biomechanical parameters related to the highest concavity phase were significantly affected by scleral thickness. Clinicians should carefully account for the contribution of sclera properties when interpreting corneal biomechanical parameters, especially in cases where scleral properties have been altered by treatment. For example, prostaglandin treatment for glaucoma increases scleral permeability, and surgeries modifying scleral characteristics may confound the accuracy of corneal biomechanical measurements. Understanding the influence of scleral properties on corneal dynamic deformation responses could help clinicians better extract corneal property parameters, evaluate research findings comprehensively, and interpret the clinical significance of biomechanical parameters more accurately. 

Research on myopia fundamentally involves analyzing AL and SER. This study established significant correlations among various biomechanical parameters, SER, and AL. However, prior studies on myopic biomechanics often overlooked the impact of confounding factors, such as CCT, IOP, and age.¹⁴ To minimize these effects, we further adjusted for AST. The results showed significant correlations among SP-HC (r = 0.223), PD (r = −0.259), HCDA (r = −0.207), SSI (r = 0.341), and SER (P < 0.05), as well as among PD (r = 0.451), SSI (r = −0.244), and AL (P < 0.05). Notably, SP-HC exhibited the strongest correlation with AST. After adjusting for scleral thickness, SP-HC exhibited a significant positive correlation with SER, likely because scleral thickness masks the weakening of in vivo scleral biomechanical properties in myopia. Experimental evidence has suggested that scleral biomechanics are compromised in highly myopic eyes, with myopia progression correlated with a significant reduction in the scleral collagen fiber diameter,³³ lower collagen content, and diminished proteoglycan synthesis.^34,35 These findings indirectly provide histological support for the validity of our results. Moreover, SP-HC may serve as a more sensitive parameter for detecting the weakening of in vivo scleral biomechanical properties in eyes with myopia. Combined with the strong correlation between SP-HC and scleral stiffness reported in previous ex vivo studies,¹³ our findings suggest that SP-HC could be a potential parameter for assessing in vivo scleral biomechanical properties in eyes with myopia. This emphasizes the feasibility of using corneal biomechanical measurement devices to evaluate scleral biomechanical changes in myopia. These findings provide an experimental basis for future longitudinal studies exploring the causal relationship between changes in scleral biomechanics and myopia progression. 

Furthermore, PD and HCDA are parameters measured at the point of maximum corneal concavity. PD represents the distance between the two corneal peaks at the highest concavity, whereas HCDA indicates the largest axial displacement at the corneal apex. At this stage, the cornea induces the maximum volume of aqueous humor displacement in the anterior chamber while experiencing the greatest constraint from scleral biomechanics, resulting in the strongest correlation with scleral properties. Ex vivo studies have confirmed that PD and HCDA levels significantly decrease as scleral stiffness increases,²⁰ suggesting these parameters may serve as indicators of stronger in vivo scleral biomechanics. In this study, the correlation between PD and HCDA and myopia severity suggests that as AL or refractive error increases, in vivo scleral biomechanical properties may weaken. Simultaneously, we found a significant negative correlation between WEM (r = −0.226) and AL. WEM represents the displacement of the corneal apex during maximum deformation of the entire eye, encompassing the cornea and the eyeball. These results align with previous studies^36,37 indicating that eyes with longer AL often exhibit reduced scleral collagen fiber bundles and weaker biomechanical properties. These changes result in increased eyeball compliance, making the eyeball more deformable under applied forces, which leads to reduced WEM.^36,38–40 The biomechanical parameters at the highest corneal concavity (SP-HC, PD, HCDA, and WEM) may provide more accurate characterization of scleral biomechanical properties in eyes with myopia and could emerge as novel indicators for assessing myopia progression. Incorporating SP-HC, PD, HCDA, and WEM into future predictive models of the severity and progression of myopia could enhance the accuracy of early screening and improve strategies for preventing the development and progression of myopia. 

This study had several limitations. First, owing to the measurement depth and precision limitations of the current equipment, we focused only on AST, excluding the posterior sclera. In future studies, we plan to use OCT with improved optical signal penetration to measure the posterior scleral thickness and clarify its influence on corneal biomechanical parameters in myopia. Second, the direct correlation between scleral thickness and biomechanical properties requires further investigation. Future studies should include in vivo measurements of scleral thickness and ex vivo assessments of scleral biomechanical properties in the same regions of animal eyes to establish a clearer and more direct correlation. Finally, interindividual variations in biomechanical properties are substantial, and multiple factors can influence these parameters. As this was a cross-sectional study, although the statistical correlation between biomechanical parameters and myopia progression may be valid at the population level, its direct applicability to individual patients remains uncertain. Longitudinal studies are therefore necessary to validate the feasibility of using in vivo biomechanical parameters to characterize scleral biomechanical changes associated with myopia progression. 

In conclusion, this study highlights that scleral thickness is a critical factor influencing the accurate characterization of in vivo corneal biomechanics. Additionally, SP-HC may serve as a sensitive indicator of in vivo scleral biomechanics and tissue properties. After adjusting for CCT, IOP, and age, we propose that SP-HC could be a potential in vivo biomechanical parameter to evaluate myopia progression. The weakening of in vivo scleral biomechanics may be a high-risk factor contributing to myopia progression. Future longitudinal studies are required to clarify the causal relationships between scleral biomechanical changes, which will enable their use in monitoring disease progression. 

Acknowledgments

The authors thank all the individuals who contributed to this study, as well as Editage (https://www.editage.cn) for the support in English language editing. 

Supported by the National Natural Science Foundation of China (82271118), the National Program on Key Research Project of China (2022YFC2404502), the Tianjin Key Medical Discipline (Specialty) Construction Project (TJYXZDXK-016A), the Tianjin Diversified Investment Fund for Applied Basic Research (21JCZDJC01190), and the Tianjin Health and Technology Project (TJWJ2022XK036). 

Disclosure: R. Xie, None; Y. Huo, None; Y. Li, None; Z. Qu, None; H. Zou, None; Y. Wang, None