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| Parameter | Reference point |
|---|---|
| Screw insertion point1 | A |
| Apex of odontoid | B |
| Posterior-inferior point of the body of axis | C |
| Posterior odontoid tip | D |
| Point at which imaginary lines extending through AB and CD meet to form an angle | E |
| Screw projection back angle at point D | ADC |
| Screw projection back angle at point E | AEC |
