The orientation of the SS is crucial in angle assessment, as it offers a reference point of discerning trabecular meshwork and serves as a landmark for quantitative measurements such as AOD500/750 and the LTM[5]. Our study demonstrated impressive identification ability of the SS at nasal and temporal quadrants using SS-OCT compared with SD-OCT. Since the central wavelength of SS-OCT is 1310nm, it is endowed with a powerful penetrability (6mm in depth) to detect deeper structures. Satisfactory visualization of the SS using SS-OCT was also achieved by the study of Cumba et al.[9]. Additionally, they reported good interobserver reproducibility of SS-OCT for SS identification, especially in temporal (87%) and nasal (81%) quadrants. Similarly, time-domain OCT with an equal central wavelength also presents an advantage of detecting the SS. In the study by Leung et al.[2] SS can be seen in 98.0% (nasal) and 85.7% (temporal) of the subjects using slit-lamp OCT and in 98.0% (nasal) and 96.0% (temporal) of the subjects using Visante OCT. On the other hand, considering the employment of a superluminescent diode laser wavelength of 840nm, SD-OCT falls short of clear visibility of deep tissue structures such as the SS. However, it is possible for SD-OCT to identify more superficial structures such as the SL, as we found in our study. Due to this feature, new methods of angle quantification with reference to the SL was proposed by Cheung et al.[10].
Opinions divide when it comes to the identification of the SC. Usui et al.[11] disagreed with Asrani et al.[12] concerning the morphology and location of the SC. The latter claimed that SC was an arched-shape black space that was located two thirds of the corneal thickness from the corneal surface at the limbus. Conversely, Usui et al. made an argument that analysis of OCT images of the angle structure was easily interfered with the coexistence of the cornea, sclera, SC, and trabecular meshwork which have different reflection and polarization properties [11]. According to their criteria of SC identification, 60.0% (nasal) and 63.3% (temporal) of the SC in subjects’ right eyes were completely observable. The same statistics for the fellow eyes were 90.0% (nasal) and 66.7% (temporal). We basically agreed their definition of the SC’s morphology, whereas, their data were clearly inconsistent with the conclusion we drew. We attributed this disagreement to the different age range of tested subjects. Participants of our study were all above 50 years old whereas the subjects’ age ranging from 29 to 81 in Usui’s study. The transparency of cornea and sclera decreases as we age. For example, the development of pinguecula and pterygium could significantly interfere the visibility of underlying structures. (Fig. 3) In our study, there were no significant differences in discerning capability of the SC between SS-OCT and SD-OCT both at nasal and temporal quadrants (p=0.2810, p=1.0000). Although, with a shorter wavelength, it should be more difficult to view the angle recess. However, Aung T et al. [13] reported good visualization of angle structures including the SC by SD-OCT with certain image processing. Considering the disparities among studies mentioned above, we hold that further studies should be dedicated to standardize the identification and measurement of the SC.
Our study demonstrated significant difference in the measurements of AOD500/750 between the two devices. For both OCTs, the temporal data were larger than the nasal ones. And for both quadrants, the measurements were larger using SD-OCT. The former finding could be supported by many other published results. Using slit lamp OCT and Visante OCT, Leung et al. reported that the nasal AOD500 were 534±234μm / 527 ±249μm, while the temporal AOD500 were 628±254μm /572±275μm [2]. Similar conclusions were also drawn by Pettersson et al. who measured the ACA in four meridians (0°, 94°, 180°, 274°) with the Sirius Scheimpflug camera and found the mean nasal angle was 40.895±6.908 degrees while the temporal 47.531±5.578 degrees[14]. As to the latter conclusion, previous studies have been concentrated on the agreements between different instruments in angle quantification. Radhakrishnan et al. [15] showed that TD-OCT was similar to UBM in quantitative measurements of the angle such as AOD500 and Trabecular-iris space area (TISA) 500. Pan et al. [16] and Akil et al. [17] demonstrated in their study that SD-OCT was able to give consistent Schwalbe’s line-based angle metrics. However, the studies on SS-OCT were relatively limited. In our study, although good correlation of the results between SS-OCT and SD-OCT was found, the analysis of Bland-Altman plots (Fig. 4 and Fig. 5) revealed that the two devices had poor agreements. The spans of 95% limits of agreement for the nasal/temporal LTM, AOD500, AOD750 and LSC between these two devices were 411.1/54.2μm, 270.3/391.7μm, 308.8/532.9μm and 140.7/209.7μm, respectively. Considering different types of OCTs were compared in this study, some plausible postulations might serve to explain the differences. Firstly, it should be noted that the refraction of light at the anterior and posterior surface of the cornea leads to the distortion of angle measurements. Both OCT devices adopted a “dewarping” algorithm for the correction of these distortion, so the difference in algorithm (e.g. refractive indexes) should be considered. Secondly, since both devices use external target lights, the difference in illumination might contribute to the phenomenon. Unfortunately, after consulting with the manufacturers’ representatives, we still couldn’t get the exact illumination for the two instruments. Additionally, the distance between the light source and the tested eye could induce disparities in accommodation state, which could affect the lens position and the pupil size.
There were some limitations to this study. Firstly, the participants were all healthy subjects with normal angle conditions. The discerning ability of angle structures under ocular pathologies by different OCT devices was beyond our concern, which confined the extension of the conclusions. Secondly, the subjects were all above 50 years old. It remains to be verified whether the same conclusions can be drawn from younger populations.
Since their first introduction to the assessment of angle structures, different generations of AS-OCTs are now commercially available. Preceding studies have been conducted focusing on the measurement of the ACA. The current study comprising a relatively large number of consecutive patients and offered a new perspective about the value of SD-OCT and SS-OCT when it comes to angle evaluation.