engleski

Development of the osseous part of the visual system

Introduction: The function of the orbit is to protect and support the eye. It is a four-sided pyramidal cavity of the skull. Seven bones conjoin to form the orbital structure. The orbits are aligned so that the medial walls are parallel and the lateral walls are vertical. The arc from the medial to lateral wall in each orbit is 45°. The ordinary dimensions of the adult orbit are: height of orbital margin – 40 mm, width of orbital margin – 35 mm. The optic canal is one of the paired openings in the sphenoid bone that transmits the optic nerve, opthtalmic artery and central retinal vein. It connects the anterior cranial fossa and the orbit. There have been several attempts to clarify the exact anatomy and variations of the optic canal, since procedures such as endoscopic optic nerve decompression have variable success rates. The orbital apex is a congested area and orbital surgeons should have an exact knowledge of the relations and sizes of each structure. For a better understanding of those structures in adults, it is crucial to investigate their exact fetal evolution. Our aim was to investigate, for the first time, the dynamic of growth and development of those structures on fetal skulls. Material and methods: In this experiment, we used 12 fetal frontal and sphenoid bones from bone collection of the Department of Anatomy at the University of Zagreb. Ethical approval was obtained from the Research Ethics Committee of the University of Zagreb. The bones were analyzed and following groups were established (based on the length of fetus): (1) 300 ± 20 mm, (2) 400 ± 20 mm and (3) 500 ± 20 mm, four skulls per group. The diameters of optic canals and orbits from frontomaxillar suture to frontozygomatic suture were measured by microCT. Results: The average diameter of the optic canal in the 300 mm fetus was 1570 µm, in 400 mm fetus 2436 µm and in the 500 mm fetus 3812 µm. Standard deviations in all groups are around 50 µm, which represents 2% of the mean value. There was a slight difference in fetus weight among the groups. In the first group, the optic canal diameters were practically the same (±35 µm), while their weight varied substantially. The same trend occurred in the second group. In the third group, the lightest fetus had the largest optic canal. In the same period, the orbit diameter enlarged from 13290 µm to 19657 µm, showing strict linear enlargement. We also calculated the ratio of the orbit diameter and optic canal diameter: it was 8, 5 in the first group, and 7 and 5 in the second and the third group respectively. Conclusion: The development of both the optic canal and the orbit varies during fetal periods ; there are representative differences between intervals. The ratio of those diameters in different time periods is significant and shows that orbit grows faster in the earlier fetal period, while the optic canal enlarges later. We also show that the diameters of those structures correlate better with fetus length then weight.

optical canal; orbit

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