## 4 Discussion
In addition, EMV-OA/OAV can represent the relative degree of compression in the orbital apex, which can effectively compensate for variations in sex and age. EMV-OA/OAV \(\geq\)28.75% can be used as a reference parameter in detecting DON. It explains the deep orbital decompression in orbital reconstructive surgery [24]. It may be worthwhile in the future to conduct a longitudinal study to explore changes in the orbital apex before and after surgical orbital decompression. Some authors found spontaneous orbital decompression by increased orbital volume to offset the enlarged intraorbital EM and fat tissue [23; 25]. Although our study showed no statistical difference in OV, it was found to be larger in patients with DON than in non-DON patients. This probably shows that spontaneous expansion of the orbits does not effectively relieve the increased intraorbital pressure in the late stage. Therefore, damage to the optic nerve happens. To further investigate the compression that causes DON in TED patients, we measured the AMR-ON and ALR-ON parameters. In TED-NM patients, the angles always exist both in the medial and lateral rectus, which is consistent with clinical findings, since there was no enlarged extraocular muscle. On the other hand, when comparing DON and non-DON patients, a significantly different distribution of the AMR-ON and ALR-ON was found. Overall, AMR-ON was mostly zero in DON patients, while ALR-ON tended to be larger in non-DON than in DON patients. This is consistent with [PERSON] et al. 's finding that MR size is the only variable significantly associated with DON [26]. It is also the reason why we chose to measure angles of MR and LR to ON. It is best to explain the greater impact of MR on DON development in terms of its close interconnectedness to the optic nerve at the orbital apex [27]. It can also explain why medial wall decompression in orbital reconstructive surgeries seems to be a very effective procedure in DON patients [28; 29]. The smaller and disappearing angle between recti and ON may also have implications for operative techniques in surgical decompression. A longitudinal study using this geometric analysis is expected, in order to visualize the changes after the surgical expansion of the orbits. Our research is limited to a relatively small sample size localized in a particular hospital system. When performing CT examination, we should be careful that the time of examination is limited per year due to its radiation. Moreover, imaging features are not a stand-alone indicator for the diagnosis and treatment of TED. The indication for surgery should be based mainly on the clinical features. Furthermore, for a better imaging evaluation of TED patients, magnetic resonance imaging (MRI) examination should be included. In conclusion, geometric and volumetric measurements using CT scans are supplemental quantitative methods in the image-based diagnosis of TED combined with clinical ophthalmological diagnosis. Conceptualization, R.L. ; data curation, Y.B. and H.M.; formal analysis, Z.Z. ; methodology, Z.Z. ; project administration, Y.B. and Y.W. ; software, Y.B. and Z.Z. ; supervision, R.L. and G.L. ; validation, P.Y. ; writing--original draft, Y.B. and Z.Z. ; writing--review & editing, R.L. and C.L. All authors have read and agreed to the published version of the manuscript. This work was financially supported by the Natural Science Foundation of Guangdong Province, China (General Program, No. 2019A1515010361) and High-level Hospital Construction Project (303010406). The funding organizations had no role in the design or conduct of this research. Ethical approval and patient consent were obtained before surgery and the procedures adhered to the tenets of the 1964 Declaration of Helsinki. Protocols were approved by the Institutional Ethics Committee (2020 KYJ199, Medical Ethics Committee, Zhongshan Ophthalmic Center, Guangzhou, Guangdong, China). For images that reveal personal identity, informed consent to publish was obtained from legal guardians of participants aged under 18 years.