Abstract
Background
To assess the characteristics of the ciliary sulcus in patients with myopia with different axial lengths using ultrasound biomicroscopy (UBM).
Methods
A total of 108 eyes from 54 patients were recruited between January and March 2024. Patients were matched for age, cylinder and white-to-white (WTW) ratio, and further divided into a control axial length (AL) group (CON, AL < 26.5 mm, 54 eyes) and a long AL group (L-AL, AL ≥ 26.5 mm, 54 eyes). Anterior and posterior chamber parameters were examined based on UBM. Anterior chamber parameters included sulcus-to-sulcus distance (STS) in the horizontal and vertical orientation, lens anterior surface to ciliary sulcus distance (STSL), anterior segment length (ASL) and anterior chamber depth (ACD). Posterior chamber parameters included ciliary process length (CPL), trabecular–ciliary process distance, trabecular–ciliary angle (TCA), iris–ciliary angle (ICA) and maximum ciliary body thickness in four spatial directions.
Results
The anterior chamber parameters, including STS, STSL and ASL, were significantly lower in the horizontal direction than those in the vertical direction in both groups, whereas STS was significantly lower in the CON group than in the L-AL group. Posterior chamber parameters, including CPL, TCA and ICA, showed significant spatial differences between the two groups. Furthermore, STSL correlated significantly with ACD, mean keratometry and WTW.
Conclusions
Significant spatial differences in ciliary sulcus and ciliary body morphology were observed in patients with axial myopia. In ICL size selection and vault prediction, STS and STSL are potential indicators in preoperative ICL assessment.
1
Introduction
Myopia is a worldwide public health concern. Implantable collamer lens (ICL) implantation has been a common surgical procedure that is now widely performed for myopia correction, particularly for high myopia. In this procedure, determining ICL size is critical for maintaining a safe vault with an ideal prognosis. Anterior segment parameters are major predictors of ICL size. For example, the size of the STAAR ICL was calculated according to the white-to-white distance (WTW) and the anterior chamber depth (ACD). The NK formula reported by Nakaruma et al. further introduces anterior chamber parameters, such as the crystalline lens rise (CLR), into ICL size selection. As the ICL is a posterior chamber intraocular lens (IOL) fixed in the ciliary sulcus, the structure of the posterior chamber theoretically plays a non-negligible role in ICL size selection and postoperative vault prediction.
The application of ultrasound biomicroscopy (UBM) visualizes the posterior chamber structure and obtains objective quantitative measurements of relevant parameters, including the ciliary body and sulcus. It has been reported that the size and position of the ciliary sulcus as well as the morphology and position of the ciliary body could affect the vault. Thus, the anterior and posterior chamber parameters not only provide important information about the anatomy of the eyes but are also crucial for the preoperative assessment for ICL selection. However, the posterior chamber structures of patients with myopia with different axial lengths (ALs) have not been studied to date, particularly with respect to the size and position of the ciliary sulcus and the morphology and position of the ciliary body in different quadrants. Given their significance in ICL size selection, the evaluation of their characteristics and correlations is essential.
In this study, we comprehensively assessed the anterior segment parameters (e.g. ACD) and the posterior chamber parameters (e.g. the size and position of the ciliary sulcus and the morphology and position of the ciliary body) in different quadrants in a population with different ALs using UBM and further analyzed the factors influencing these parameters.
2
Methods
2.1
Ethics statements
This study received ethical approval from the Ethics Committee of the Affiliated Eye Hospital, Jiangxi Medical College, Nanchang University (No. 2024011). All procedures were conducted in accordance with the principles set out in the Declaration of Helsinki. All participants voluntarily signed an informed consent form.
2.2
Participants
Patients with myopia were enrolled in the Affiliated Eye Hospital, Jiangxi Medical College, Nanchang University, between January and March 2024. The final sample size was 108 eyes (54 patients). These were further divided into the control group (CON, 54 eyes) and the long AL group (L-AL, 54 eyes) according to their AL. The inclusion criteria for the CON group were as follows: age ≥ 18 years, cylinder ≤ –5.0 diopters (D) and AL < 26.5 mm. The exclusion criteria were as follows: any risk or diagnosis of keratoconus, a history of ocular surgery and the presence of any systemic or ophthalmic diseases, except ametropia. The inclusion and exclusion criteria for the L-AL group were the same as for the CON group, except for AL ≥ 26.5 mm.
2.3
Ocular examination
All patients underwent comprehensive ocular examinations, including a review of their medical history, slit-lamp examination, intraocular pressure (IOP) measurement, subjective refraction, AL measurement, WTW measurement, corneal tomography and UBM. IOP was measured through Goldmann applanation tonometry. AL and WTW were measured using IOLMaster 700 (Carl Zeiss Meditec AG, Jena, Germany). Corneal tomography was performed using a Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany). UBM was performed at 35 MHz (SW-3200L, Tianjin Suowei Electronic Technology Co. Ltd., China).
2.4
UBM
An experienced examiner (CW) performed UBM on all patients. Patients were positioned supine, and the procedure was performed in a dark room. After surface anesthesia, the eyelids were separated using an eye cup filled with methylcellulose as a coupling agent. Patients were instructed to direct their gaze toward a target located on the ceiling with the contralateral eye in a relaxed state, maintaining fixation and standardizing accommodation. The UBM probe was positioned in a perpendicular orientation to the ocular structures in the region of the eye margin to be examined. Each eye was scanned radially through the center of the pupil to obtain images at the 3, 6, 9 and 12 o’clock positions, representing the four quadrants ( Fig. 1 ). Radial scans were performed using the typical ciliary processes to demonstrate their relationship with the posterior part of the iris. After obtaining clear images, the parameters below were measured by Image J software (National Institute of Health, US).
2.4.1
Anterior chamber parameters (horizontal and vertical position)
STS: sulcus-to-sulcus distance.
STSL: the distance between the ciliary sulcus and the anterior surface of the crystalline lens.
ACD: anterior chamber depth.
ASL: anterior segment length, referring to the sum of the ACD and the lenticule thickness.
2.4.2
Posterior chamber parameters (four points at 3, 6, 9 and 12 o’clock)
CPL: ciliary process length; the distance between the iris–ciliary junction and the innermost point of the ciliary process.
TCPD: trabecular–ciliary process distance; the distance between the 500-μm extension of the corneal endothelium anterior and the scleral spur to the ciliary process.
TCA: trabecular–ciliary angle; the angle measured with the scleral spur designated as the apex, and the corneal endothelium and anterior surface of the ciliary body designated as the arms or the angle.
ICA: iris–ciliary angle; the angle between the posterior surface of the iris and the anterior surface of the ciliary body.
CBTmax: the maximum ciliary body thickness; the distance between the inner scleral wall or its extension and the innermost point of the ciliary body.
2.5
Statistical analysis
The statistical analyses were performed using IBM SPSS Statistics (version 26.0; IBM Corp., Armonk, NY, US). The normality of the data distribution was verified for the selection of appropriate statistical methods through Kolmogorov–Smirnov test. Independent t tests were performed for data conforming to normality and Mann-Whitney tests were performed for data of non-normality. Multiple linear regression with Bonferroni adjustment revealed the correlation outcomes. P < 0.05 was considered as indicating statistical significance.
3
Results
3.1
Baseline information
A total of 108 eyes were enrolled in the two groups (54 eyes in the CON and 54 eyes in the L-AL groups). Patient information is summarized in Table 1 . The baseline information, including age, cylinder and WTW. were matched between the two groups. The sphere, spherical equivalent (SE) and AL of the CON group were significantly lower, whereas the mean keratometry (KM) of the CON group was significantly higher than these values in the L-AL group.
| CON (54 eyes) | L-AL (54 eyes) | P value | |
|---|---|---|---|
| Gender (M:F) | 10:44 | 20:34 | |
| Age (y) | 24.50 ± 2.75 | 25.7 ± 5.50 | 0.587 * |
| Sphere (D) | –5.82 ± 2.64 | –8.36 ± 2.03 | ＜0.001 * |
| Cylinder (D) | –1.05 ± 1.00 | –1.20 ± 1.18 | 0.531 * |
| SE (D) | –6.34 ± 2.62 | –8.96 ± 2.08 | ＜0.001 |
| AL (mm) | 25.51 ± 0.78 | 27.35 ± 0.73 | ＜0.001 |
| KM (mm) | 44.31 ± 1.09 | 43.21 ± 1.11 | ＜0.001 |
| WTW (mm) | 12.04 ± 0.44 | 12.21 ± 0.49 | 0.063 |
3.2
Anterior segment parameters analysis
Intragroup analysis showed that the STS, STSL and ASL were significantly lower in the horizontal position than in the vertical position in both groups, whereas no significant differences in ACD-L were observed in either CON or L-AL group ( P = 0.834 and 0.799 respectively). As shown in Fig. 2 , the intergroup analysis revealed that the STS was significantly lower in the CON group in both the horizontal and vertical orientations. No statistically significant differences were observed in other parameters.
3.3
Posterior segment parameters analysis
Intragroup analysis showed significant spatial differences in the CPL, TCA and ICA in all four directions within both groups. The CBTmax of the CON group showed a four-point spatial difference. The TCPD in the CON group was lower than that in the L-AL group at 3, 6, and 12 o’clock. No significant differences between the groups were noted in the remaining parameters ( Table 2 ).
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