In recent years, various new techniques have been developed to measure anterior parameters, including AS-OCT, Orbscan (Bausch & Lomb, Munich, Germany), and UBM. In our study, a non-contact anterior segment imaging method of the Pentacam Scheimplug system was used to evaluate the anterior segment change after SB. The Pentacam Scheimpflug camera was shown to measure the anterior segment with high reproducibility and repeatability in early studies [10]. Although UBM can provide reproducible and precise images of ACA anatomy, this requires a water bath on the eye and for the patient to lie in a supine position. Consequently, the applanation using the hand-held contact ultrasound probe leads to an increased risk for infection after surgery [11]. AS-OCT and Orbscan, similarly to Pentacam, are non-contact and have the same accuracy and rapidity to evaluate the anterior segment. By contrast, Pentacam does not require extensive experience for image acquisition, while AS-OCT always causes an image degradation due to a motion artifact secondary to the patient’s microsaccades [12]. Additionally, the Orbscan device requires clear reflections from the epithelial and endothelial corneal surfaces to obtain precise data, which might be difficult after the surgery [13].
The ACV is calculated by an integral calculus of Pentacam as a solid bounded by the posterior surface of the cornea (12.0 mm around the corneal vertex), the iris, and the lens [14]. Because Pentacam generates a 3D virtual model of the anterior segment, the ACV is currently the unique parameter which other anterior segment devices cannot obtain, and it was reported to be the most clinically important parameter to assess the status of the closure angle [15]. Our study showed that while the ACV was decreased at 1 day after surgery, it returned to the preoperative value by 1 week, and remained so up to 3 months. Araie et al. [16] demonstrated that the ACV fell significantly after SB surgery as calculated by topical fluorometric examination. The reason why the ACV decreased may be related to the rotation and supraciliary effusion of the ciliary body caused by the buckle and encircling, while the rapid recovery of the ACV after surgery may be due to the alleviation of these.
The anterior chamber depth (ACD) is the anterior-posterior distance from the corneal endothelium to the lens apex. Fleur et al. [6] evaluated the ACD by AS-OCT and found a significant decrease until 9 months after SB surgery. Similarly, the ACD decrease after scleral encircling has been attributed to the anterior rotation of the ciliary body associated with surgically-induced ciliary congestion and subsequent forward shift of the lens-iris diaphragm [17]. However, in our study, there was no significant decrease of the CACD in the operated eyes.
Another important index, the ACA in the horizontal sections, has been previously calculated, and revealed that after surgery it became narrower than preoperatively [18]. However, in our study, only the ACA at the buckle quadrant was influenced by scleral surgery, and surprisingly the ACA in the buckle quadrant site was wider than the preoperative value until 1 month postoperatively.
Other studies had demonstrated that the ACV, ACD, and ACA were decreased after SB surgery by AS-OCT and the common factor may refer to an additional encircling band. This encircling element may cause ciliary body edema with forward rotation and a shift of the lens–iris diaphragm [8, 19]. Subsequently, the chamber anglestarts to reduce and the IOP increases spontaneously. However, in contrast with previous studies, the value of the CACD and mean ACA were not reduced significantly, while the ACV was only decreased transiently after the surgery in our study. This may have resulted from the contraction of the encircling band being performed according to the eye axis of each patient, which ensured a suitable extent of encircling without over-tightening, such that the ciliary body was only slightly affected [20]. Moreover, drainage of the SRF (50.0%) and anterior chamber paracentesis (46.4%) could also maintain a stable IOP during the surgery. As is well established, the degree of the postoperative ciliary body edema can be alleviated with a stable IOP. Moreover, compared with other measurement tools, Pentacam has the advantage that it can create a 3D-simulated image of the anterior morphology with the 360-degree rotation measurement and locate the corneal vertex automatically for accurate CACD calculation. It has been demonstrated that an increased CCT after SB as evaluated by AS-OCT and Orbscan II could subside by 2 to 6 months postoperatively [21]. In our study, it returned to preoperative levels within 1 week, which might also be due to the rapid recovery of the anterior chamber condition. Another factor from the present study is that the increase of the ACA in the buckle quadrants may mainly reflect the scleral deformation. First, vitreous shrinkage and mechanical traction as well as ciliochoroidal detachment and zonular relaxation that rotate the iris posteriorly lead to the phenomenon of iris backbowing, as discovered in the so-called iris retraction syndrome (IRS), which may reveal the mechanism of a wider chamber angle after SB surgery [22]. Second, on the basis of the mechanism of IRS, we speculate that chronic inflammation after surgery leads to decreased aqueous production by the ciliary body and increased leakage from the iris vasculature. These processes, together with retrograde flow through the trabecular meshwork in the setting of hypotony, cause the pressure in the anterior chamber to rise above that in the posterior chamber, resulting in posterior bowing of the iris [23]. Third, Zhou et al. reported that use of atropine could increase the ACA measured by Pentacam, which is similar to our study [24]. Moreover, Fernández-Vigo et al. [25] showed that Pentacam tends to overestimate the measurements of narrow angles and underestimate that of open angles in comparison with OCT measurements.
There are several limitations in our study. First, the sample size of 28 eyes is relatively small. A larger population may be helpful to validate the findings. Second, all measurements were performed in eyes with a dilated pupil, which would lead to an altered angle configuration compared with other studies. Third, all patients included in this study were operated on by the same method. Other SB surgeries, such as those without encircling or radial buckling, might yield different results. Finally, Scheimpflug photography has the limitation that it cannot fully visualize the entire angle. This leads to the location of the angle recess being estimated, and the extrapolation of data by the Pentacam software to calculate the ACV [9]. A comparative study of Pentacam and other devices, including in AS-OCT, Orbscan, UBM, would help to elucidate the nature of anterior chamber changes.