Purpose
To map anterior ocular wall thickness across quadrants and eccentricities in a heterogeneous adult population and to evaluate the anatomical adequacy of fixed-depth suprachoroidal injection strategies currently used in clinical practice.
Design
A cross-sectional observational study.
Methods
A total of 110 eyes from 110 participants (18-75 years; +3.00 to −11.00D) were included. Swept-source anterior segment optical coherence tomography was used to measure central corneal thickness, anterior scleral thickness (AST), conjunctival-tenon capsule thickness, anterior choroidal thickness, anterior conjunctival-scleral thickness, and anterior conjunctival-scleral-choroidal thickness. Measurements were obtained in four quadrants at 0 to 5 mm posterior to the scleral spur at 1-mm intervals. Associations between thickness parameters and demographic, refractive, and anatomical factors were analyzed using mixed-effects models. The frequency of measurements below commonly used suprachoroidal injection depths (900 and 1100 µm) was determined.
Results
Anterior ocular wall thickness exhibited marked spatial variability across quadrants ( P <.001), decreasing sharply 1 to 2 mm posterior to the scleral spur before stabilizing. At 4 mm posterior to the scleral spur, all eyes exhibited AST values below 900 µm, and anterior conjunctival-scleral-choroidal thickness was below 900 µm in 63.64% of eyes. AST was independently associated with age and central corneal thickness but showed no association with spherical equivalent or axial length.
Conclusions
Anterior ocular wall thickness varies substantially by quadrant and eccentricity. While fixed-depth needles are clinically safe, understanding this anatomical variation may clarify injection mechanics and support anatomically guided approaches to optimize suprachoroidal drug delivery.
INTRODUCTION
S uprachoroidal injection (SCI) has emerged as a promising route for ocular drug delivery by enabling direct access to the potential space between the sclera and the choroid. , By delivering therapeutics into this compartment, SCI facilitates preferential targeting of chorioretinal tissues while limiting exposure to the anterior segment and vitreous cavity. Compared with conventional delivery routes, such as intravitreal or periocular injection, suprachoroidal administration offers distinct pharmacologic and anatomical advantages, including enhanced localization of drug to posterior segment tissues, improved compartmentalization, and the potential to achieve therapeutic efficacy with lower administered doses. , The recent development of microneedle-based injection systems has made reliable access to the suprachoroidal space feasible in an office-based, minimally invasive setting. Consequently, SCI has attracted increasing clinical interest and has advanced into phase 2 and 3 trials across a spectrum of genetic, inflammatory, vascular, and degenerative retinal diseases, , highlighting the importance of a comprehensive understanding of the anatomical and procedural factors underlying this delivery approach.
Intravitreal therapy revolutionized ophthalmology by enabling high intraocular concentrations of effective agents, such as antivascular endothelial growth factor therapies, while limiting systemic exposure. The growing interest in SCI reflects its potential to represent a new paradigm in ocular drug delivery. SCI may further advance this paradigm by offering enhanced targeting and compartmentalization of therapies within the posterior segment. Suprachoroidal delivery of triamcinolone acetonide has already demonstrated sustained pharmacokinetic profiles and favorable clinical outcomes, validating the feasibility of SCI. ,, These findings have catalyzed the investigation of a broader range of therapeutic modalities, including small-molecule suspensions, oncologic agents, and viral gene therapies, many of which are currently undergoing clinical evaluation. As the suprachoroidal space continues to be explored as a therapeutic platform, a rigorous understanding of the anatomical and procedural factors governing suprachoroidal access becomes essential.
Despite its growing clinical adoption, current SCI practice remains largely standardized rather than anatomically guided. Suprachoroidal access is typically achieved using microneedles with fixed free lengths, typically 900 or 1100 µm, selected empirically based on procedural experience, tactile feedback, and limited population-level considerations, rather than individualized anatomical assessment. Although prior clinical studies , suggest that these fixed depths accommodate a broad patient population, they do not account for known variability in scleral anatomy. The sclera exhibits nonuniform thickness across meridians and distances from the limbus and undergoes remodeling with aging and myopia. , Recent advances in swept-source optical coherence tomography (OCT) have enabled in vivo, high-resolution measurement of anterior scleral thickness (AST), revealing substantial interindividual and regional variation. However, most existing AST studies have focused on limited meridians or narrow refractive ranges and were not designed to inform interventional procedures such as SCI.
Because safe and effective SCI requires precise penetration of the sclera without excessive choroidal injury, a detailed understanding of AST distribution is critical. The present study was therefore designed to comprehensively map AST across four quadrants and multiple distances from the limbus (or scleral spur on OCT imaging) in a diverse adult cohort, to identify key factors influencing AST, and to evaluate the anatomical adequacy of fixed-depth SCI strategies currently used in clinical practice. By providing an anatomical framework relevant to suprachoroidal access, this study aims to inform future efforts toward more individualized and anatomically guided SCI approaches.
METHODS
study design and participants
This prospective, cross-sectional imaging study was conducted at the Sichuan Eye Medical Center (Chengdu, China) between September 2025 and January 2026. The study adhered to the tenets of the Declaration of Helsinki and was approved by the Institutional Review Board of Sichuan Eye Medical Center. Written informed consent was obtained from all participants prior to enrollment.
A total of 110 adult participants were consecutively recruited during routine ophthalmic examinations. Inclusion criteria were age between 18 and 75 years and spherical equivalent (SE) refractive error ranging from +3.00 to −11.00 diopters. Exclusion criteria included a history of ocular surgery, active ocular inflammation, scleral or uveal disease, glaucoma, intraocular pressure (IOP) >21 mm Hg, ocular surface disease (including pterygium or pinguecula), use of rigid scleral contact lenses, and inadequate image quality. For the primary analysis, one eye per participant was included to avoid intereye correlation. The right eye was randomly selected when both eyes met inclusion criteria. In a predefined subgroup (first 40 individuals), both eyes were included to assess interobserver and intereye differences and correlations.
ophthalmological examination
All participants underwent a comprehensive ophthalmologic examination on the same day as imaging. This included best-corrected visual acuity assessment, cycloplegic refraction, slit-lamp biomicroscopy, fundus examination, and tonometry using a noncontact pneumotonometer (Canon TX-10; Canon Inc). Axial length (AL) was measured using partial coherence interferometry (IOLMaster 700; Carl Zeiss Meditec). Refractive status was expressed as SE, calculated as the spherical power plus one-half of the cylindrical power.
imaging protocol
Imaging was conducted using a swept-source OCT system (TOWARD SS-OCT, Toward Medical), with a central wavelength of 1060 nm, axial resolution of 3.8 µm, transverse resolution of 10 µm, and a scanning speed of 400,000 A-scans per second. Anterior segment SS-OCT was acquired before pupil dilation by a single experienced examiner (S.C.) using a standardized protocol.
To adequately expose the anterior sclera, superior, and inferior sclera in both the temporal and nasal regions, participants adopted maximal gaze positions in four directions (temporal-superior [TS], temporal-inferior [TI], nasal-superior [NS], and nasal-inferior [NI]). Head position was stabilized using a chin rest and forehead support to minimize movement. Cross-sectional images were obtained with the anterior segment lens in line-scan mode with a 16-mm length. All examinations were performed in the same room under standardized mesopic lighting conditions with steady external fixation. No near target or accommodative stimulus was introduced during image acquisition.
oct measurements
Only OCT images with sufficient quality, defined as a signal strength intensity score greater than 8 on a 10-point scale, were included for analysis. All images were independently reviewed by two masked observers (T.C. and J.H.). For each measurement location, if the difference between the two observers’ values was within 10%, the final value used for statistical analysis was defined as the average of the two measurements. If the discrepancy exceeded 10%, the image was re-evaluated and adjudicated by a third senior examiner (X.D.), whose determination was used as the final measurement. The scleral spur was identified as the anatomical reference point for all measurements. On OCT images, the outer scleral boundary was defined by the deep episcleral vascular plexus, visualized as a thin hyporeflective layer beneath the conjunctiva-Tenon capsule, whereas the inner boundary was defined as the interface between hyperreflective scleral tissue and the relative hyporeflective ciliary body.
AST was measured manually on B-scan images as the shortest perpendicular distance between the outer and inner scleral boundaries. Measurements were obtained at 0, 1, 2, 3, 4, and 5 mm posterior to the scleral spur in each quadrant. conjunctival-tenon capsule thickness (CTT) was measured at the same locations. Anterior choroidal thickness (ACT) was defined as the perpendicular distance from the inner scleral boundary to the inner boundary of the choroid and was measured at corresponding locations when visualization permitted. Owing to anatomical and imaging constraints, ACT measurements were consistently analyzable at 3, 4, and 5 mm posterior to the scleral spur. Additionally, central corneal thickness (CCT) was measured. Based on these measurements, the combined anterior conjunctival-scleral thickness (ACST) was defined as the sum of CTT and AST, and the combined anterior conjunctival-scleral-choroidal thickness (ACSCT) was defined as the sum of CTT, AST, and ACT. All measured and derived parameters are illustrated schematically in Figure 1 .
Sequential steps involved in obtaining anterior ocular wall thickness with SS-OCT. (A) Single line scan passing through the temporal-superior scleral reflex. (B) Cropped raw B-scan image of the anterior sclera (dimension of the exported image: length of 16 mm) where blue arrowheads indicate the conjunctival episcleral vessels, the red arrowheads indicate the anterior wall of the ciliary body tissue, and the white arrowhead indicates the location of the scleral spur. (C) In the magnified image, the anterior scleral boundary is indicated by blue dashed line and posterior scleral boundaries by red dashed line. The blue, red, and white solid arrows represent CTT, AST, ACT, respectively. The composite parameters, ACST and ACSCT, are indicated by the purple and green brackets. ACSCT = anterior conjunctival-scleral-choroidal thickness; ACST = anterior conjunctival-scleral thickness; ACT = anterior choroidal thickness; AST = anterior scleral thickness; CTT = conjunctival-tenon capsule thickness.
statistical analysis
Continuous variables are presented as mean ± SD, and categorical variables are presented as frequencies and percentages. Data normality was assessed using the Kolmogorov-Smirnov test. Intraobserver and interobserver reliability were evaluated using intraclass correlation coefficients (ICC; two-way mixed-effects model, absolute agreement) and coefficients of variation (CV), with higher ICC values indicating stronger agreement. Differences in scleral thickness were analyzed using repeated-measures analysis of variance (RM-ANOVA). Mauchly’s test was used to assess the sphericity assumption, and Greenhouse–Geisser correction was applied when sphericity was violated. Bonferroni-adjusted post hoc comparisons were performed for pairwise comparisons. Associations between AST and demographic or ocular parameters, including age, gender, SE, AL, CCT, and CTT, were evaluated using Pearson correlation coefficients. Comparisons between quadrants and between eyes in the bilateral subgroup were performed using paired t tests, as appropriate.
To account for repeated measurements across quadrants and distances within the same eye, mixed-effects regression models were constructed with AST as the dependent variable. Independent variables included age, gender, SE, quadrant, and distance from the scleral spur, with eye identity included as a random effect. Stepwise multivariable linear regression analysis was performed to identify the primary determinants of AST, with the coefficient of determination ( R ²) reported. Statistical significance was defined as P <.05. All analyses were performed using SPSS software (version 21.0; SPSS Inc).
RESULTS
study population and ocular characteristics
A total of 110 eyes from 110 participants were included in the analysis. The study population comprised 54 male and 56 female subjects, with a mean age of 46.03 ± 16.31 years (range, 18-75 years). The SE ranged from + 3.00 to −11.00 diopters, with a mean of −2.34 ± 3.26D. AL, available in all subjects, ranged from 22.24 to 31.63 mm. All eyes were phakic, and none had a history of ocular surgery, inflammatory eye disease, glaucoma or scleral pathology. Image quality was sufficient for reliable delineation of the anterior scleral and anterior choroidal boundaries in all predefined quadrants and measurement locations. Baseline demographic and ocular characteristics are summarized in Table 1 .
TABLE 1
Baseline Demographic and Ocular Characteristics of the Study Population.
| Variable | Statistics ( N = 110) |
|---|---|
| Demographics | |
| Age (y) | 46.03 ± 16.31 |
| Gender (male/female) | 54/56 |
| Ocular parameters | |
| Spherical equivalent (D) | −2.34 ± 3.26 |
| Axial length (mm) | 24.85 ± 2.09 |
| Central corneal thickness (µm) | 537.5 ± 35.3 |
Data are presented as mean ± standard deviation.
The reliability and reproducibility of AST measurements were excellent. Intraobserver repeatability, assessed using three repeated measurements by the same observer, showed a high ICC of 0.96 (95% CI, 0.92-0.98; P <.001) with a mean CV of 1.69% (range, 0.69%−2.94%). Interobserver agreement between two independent examiners was similarly high, with an ICC of 0.981 (95% CI, 0.961-0.991; P <.001) and a mean CV of 2.14% (range, 1.28%−3.84%), confirming robust measurement consistency.
To justify the inclusion of a single eye for subsequent analyses, intereye comparisons were performed in the first 40 participants using superior temporal quadrant measurements. Strong correlations were observed between right and left eyes for AST at the scleral spur (AST0; r = 0.804, P <.001), combined conjunctival-scleral thickness at the scleral spur (ACST0; r = 0.844, P <.001), and combined conjunctival-scleral-choroidal thickness 3 mm from the scleral spur (ACSCT3; r = 0.868, P <.001). No significant intereye differences were detected for any measured parameter (all P >.05). Mean ± SD values for right vs left eyes were 651.1 ± 56.2 µm vs 657.2 ± 54.5 µm for AST0, 881.6 ± 90.8 µm vs 892.6 ± 90.3 µm for ACST0, and 910.2 ± 109.4 µm vs 923.5 ± 114.7 µm for ACSCT3, respectively ( Table 2 ). Based on these findings, the right eye was selected for all subsequent analyses.
TABLE 2
Intereye Comparison of Superior Temporal Quadrant Thickness Measurements in 40 Subjects.
| Temporal-Superior Quadrant |
Right Eyes
( N = 40) |
Left Eyes
( N = 40) |
Correlation(
r
)
P Value |
P Value Difference |
|---|---|---|---|---|
|
AST0 (µm)
Range |
651.1 ± 56.2
(532-756) |
657.2 ± 54.5
(544-762) |
0.804
<.001 |
.272 |
|
ACST0 (µm)
Range |
881.6 ± 90.8
(671-1076) |
892.6 ± 90.3
(657-1061) |
0.844
<.001 |
.175 |
|
ACSCT3 (µm)
Range |
910.2 ± 109.4
(752-1174) |
923.5 ± 114.7
(744-1223) |
0.868
<.001 |
.154 |
Data are presented as mean ± standard deviation.
ACSCT3 = anterior conjunctival-scleral-choroidal thickness 3 mm from the scleral spur; ACST0 = anterior conjunctival-scleral thickness at the scleral spur; AST0 = anterior scleral thickness at the scleral spur.
quadrant-dependent distribution of anterior ocular wall thickness
Because selection of the optimal quadrant and injection distance is critical for safe and effective SCI, a detailed understanding of scleral, conjunctival, and choroidal thickness across quadrants and eccentricities is required. To facilitate clinically relevant comparisons, measurements were initially anchored to a representative anatomical location for standardized comparison across subjects, the temporal-superior quadrant approximately 4 mm posterior to the limbus. Importantly, because current guidelines allow SCI in any eligible quadrant, we subsequently mapped tissue thicknesses across all four quadrants. , Accordingly, tissue thicknesses were first compared across quadrants at a standardized location 4 mm posterior to the limbus, using representative midquadrant positions (corresponding to the 1:30, 4:30, 8:30, and 10:30 clock-hour positions). In addition, to define the optimal injection distance within one quadrant, scleral thickness was further assessed at multiple locations ranging from 0 to 5 mm posterior to the scleral spur. This approach enabled systematic evaluation of both quadrant-dependent and distance-dependent anatomical variations relevant to SCI site selection.
AST, ACST, ACSCT were measured, which demonstrated a nonuniform spatial distribution across quadrants. Detailed quantitative data are summarized in Table 3 . Overall, the NI quadrant AST exhibited the greatest thickness, followed by the TI and TS quadrants, whereas the NS quadrant consistently showed the thinnest measurements. This distribution was evident not only for AST, but also for the ACST and ACSCT ( Figure 2 ).
TABLE 3
Quadrant Comparison of Mean Thickness of Anterior Ocular Wall Thickness at 4 mm Posterior to the Scleral Spur ( N = 110).
| AST (µm) | ACST (µm) | ACSCT (µm) | |
|---|---|---|---|
| NI | 594.1 ± 89.2 | 842.0 ± 86.6 | 940.9 ± 98.6 |
| TI | 551.7 ± 62.7 | 812. 8 ± 71.7 | 921.8 ± 86.9 |
| TS | 489.6 ± 65.1 | 763.5 ± 82.4 | 869.7 ± 102.7 |
| NS | 488.6 ± 71.7 | 719.7 ± 85.1 | 820.9 ± 95.6 |
| RM-ANOVA a (quadrant) | F (2.743, 279.821) = 108.665, P <.001 | F (2.579, 268.211) = 96.847, P <.001 | F (2.529, 262.969) = 81.486, P <.001 |
Data are presented as mean ± standard deviation.
ACSCT = anterior conjunctival-scleral-choroidal thickness; ACST = anterior conjunctival-scleral thickness; AST = anterior scleral thickness; NI = nasal-inferior; NS = nasal-superior; TI = temporal-inferior; TS = temporal-superior.
Distribution across quadrants of AST, ACST, ACSCT at 4 mm posterior to the limbus. ACSCT = anterior conjunctival-scleral-choroidal thickness; ACST = anterior conjunctival-scleral thickness; AST = anterior scleral thickness; NI = nasal-inferior; NS = nasal-superior; TI = temporal-inferior; TS = temporal-superior.
distance-dependent distribution of anterior ocular wall thickness
Detailed thickness measurements by distance from the scleral spur are summarized in Tables 4 , 5 , and illustrated in Figure 3 . A total of 110 eyes were included in the analysis.
TABLE 4
Thickness Measurements of AST, CTT, ACT by Distance From the Scleral Spur
| Distance (mm) | AST (µm) | CTT (µm) | ACT (µm) |
|---|---|---|---|
| 0 | 641.6 ± 79.1 | 274.1 ± 45.8 | – |
| 1 | 490.4 ± 52.8 | 271.8 ± 52.3 | – |
| 2 | 480.8 ± 54.3 | 270.8 ± 55.9 | – |
| 3 | 494.1 ± 60.6 | 264.4 ± 51.7 | 176.0 ± 84.0 |
| 4 | 489.6 ± 65.1 | 273.9 ± 57.9 | 106.2 ± 50.6 |
| 5 | 493.7 ± 73.7 | 297.7 ± 68.8 | 76.9 ± 33.2 |
| RM-ANOVA a (distance) | F (2.590, 282.493) = 234.553, P <.001 | F (2.462, 268.377) = 11.265, P <.001 | F (1.271, 138.516) = 192.132, P <.001 |
Data are presented as mean ± standard deviation.
ACT = Anterior choroidal thickness; AST = anterior scleral thickness; CTT = conjunctival-tenon capsule thickness.
TABLE 5
Thickness Measurements of ACST, ACSCT by Distance From the Scleral Spur.
| Distance (mm) | ACST (µm) | ACSCT (µm) |
|---|---|---|
| 0 | 915.7 ± 84.0 | – |
| 1 | 762.2 ± 66. 6 | – |
| 2 | 751.7 ± 72.4 | – |
| 3 | 758.5 ± 77.6 | 934.5 ± 124.2 |
| 4 | 763.5 ± 82.4 | 869.7 ± 102.7 |
| 5 | 791.4 ± 88.4 | 868.3 ± 95.5 |
| RM-ANOVA a (distance) | F (2.062, 224.747) = 194.565, P <.001 | F (1.271, 138.516) = 192.132, P <.001 |
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