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Orbit definition4/9/2023 Using abnormal cases with orbital pathology would not have been appropriate in this study as the pathology could change the structure of the orbit. The information is not sensitive in nature, and the data are derived from the clinically indicated CT Imaging already performed.ĬT was performed for non-ophthalmological medical indications without any connection to our research (Table 2). Waiver of informed consent was permitted by EMC Ethics Committee review board as the research was retrospective and anonymous presenting no threat to the rights and welfare of research subjects. Methods were carried out in accordance with ICH-GCP and Ministry of Health guidelines, approved by the EMC Ethics Committee review board. We confirm that study number for this study was 0045-11EMC. Data included age, gender, race, and indication for CT.Īll experimental protocol were approved by the EMC Ethics Committee review board. In this way we were able to define where the surgical apex is located.Ī retrospective analysis of CT images, measuring right retrobulbar volume was carried out in a random sample of 100 patients without orbital pathology, from the database of CT scans performed at Emek Medical Center (EMC) between 2010–2013. In this study we used CT images of normal orbits and determined the area where volume depletion changes significantly as we move deeper into the orbital cavity. Defining where this critical area begins, as we go deeper into the orbit, might also help in anticipating difficulties in orbital biopsy. Decompression surgery of the orbit in Thyroid Associated Eye Disease due to optic nerve compression should theoretically address this area in particular. For example, an expanding sub-periosteal abscess when it reaches this area should theoretically pose a greater risk of optic nerve compression demanding urgent surgical attention. This conceptual space, "the surgical apex" has not been scientifically defined but is accepted as the posterior area of the orbit where the most significant decrease in volume occurs 11, 12, 13. Most experienced orbital surgeons understand the concept of the orbital apex as a crowded and complex space demanding special attention. Sub-periosteal abscess, Thyroid associated eye disease, space occupying lesions, or trauma with bleeding can cause optic nerve compression in this area due to crowding and these problems often need surgical intervention 2, 7, 8, 9, 10. As we move more posteriorly towards the apex, the area becomes more confined and thus more difficult to approach surgically 6. Diseases affecting the orbit commonly involve retrobulbar area which contains a complex of muscles, nerves and blood vessels 3, 4, 5. The shape of the orbit is a pyramid with an anterior open base and a posterior apex 1, 2. This definition could be routinely utilized by ophthalmologists and neuroradiologists when evaluating masses affecting the orbit. It is consequently the area of higher risk for optic nerve compression. We defined the surgical apex as the posterior 3/5 of the retro-bulbar orbital space. The V3 segment (the posterior 3/5 of the retrobulbar space volume) is the location where decrease in orbital volume impacts, and measured ratios are statistically significant. The most remarkable difference in ratio was between V4 and V5 (mean 5.67 ± 1.66 with p < .0001). The mean numeric value of measured orbital volumes was compared. The volumes of all 5 segments were compared and the most significant area of volume depletion was established. The measured length between these two points was divided into five equal segments, V1-V5. The first landmark being the point of exit of the optic nerve from the eye and the second landmark the optic nerve's point of exit from the orbit. Volume of the retrobulbar space was measured between two recognizable landmarks. A retrospective analysis using PACS program processing, measured the right retrobulbar space volume changes in 100 randomly selected cases without orbital pathology where CT was performed for non-ophthalmic indications. We sought to determine the surgical apex area specifically where the volume ratio decreases significantly impacting on the optic nerve. And indeed a bijection $\pi\colon X\to X$ (which just says that $\pi\in S_7$ as well) that respects these orbits suitably, such as $\pi(1)=7$, $\pi(3)=6$, $\pi(2)=5$, $\pi(6)=4$, $\pi(5)=3$, $\pi(4)=2$, $\pi(7)=1$, shows us that $\sigma,\tau$ are conjugate: $\tau\circ \pi=\pi\circ\sigma$.The orbital apex is an undefined but well understood concept of Orbital Surgeons. You will notice that $\tau=(7\ 6)(5\ 4\ 3)$ has exactly the same orbit lengths as $\sigma$. The symmetric group $S_n$ is defined via its action on the set $X=\$).
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