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n.1.(Anat.) The middle of the posterior, or dorsal, margin of the great foramen of the skull.
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Well-known examples of craniometric points used, included the pterion, asterion, vertex of the skull, the lambda stephanion glabella and the opisthion.
No Anatomical landmarks definitions 1 Minimum frontal breadth Distance between the two frontotemporale (ft) 2 Biorbital breadth Distance between right and left ectoconchion (ec) 3 Bizygomatic diameter Distance between most lateral points on the zygomatic arches 4 Nasal breadth Maximum breadth of the nasal aperture 5 Orbital height Distance between the superior and inferior orbital margins 6 Foramen magnum breadth Distance between the lateral margins of foramen magnum at the points of greatest lateral curvature 7 Foramen magnum length Distance from basion (ba) to opisthion (o) 8 Foramen ovale Distance between left and right of foramen ovale 9 Foramen spinosum Distance between left and right of foramen spinosum Table 4: Dimensional measurements of dry skull and CAD models.
Degree of tonsillar ectopia has been considered the most important criterion for the diagnosis of Chiari I malformation, tonsillar ectopia of >6mm (Below the nasion - opisthion line) between the ages of 5 and 15 years and tonsillar ectopia of >5mm outside the 5-15 year age group is considered pathologic and is associated with increased incidence of clinical symptoms.
Opisthion (Op): The mid-point on the posterior margin of the foramen magnum is termed the opisthion.