Atlanto-occipital

Fig. 20.8. The measurements used in assessment for atlanto-occipital dissociation. B, tip of basion C, spinolaminar junction of atlas A, posterior aspect of the anterior arch of atlas 0, posterior lip of foramen magnum D, tip of dens. Powers ratio BC/OA should normally be less than 1. The distance BD should normally be less than 12.5 mm...

Side-bending and rotation of the occipitoatlantal joint always occur In opposite directions, in part because of the position of the lateral atlanto-occipital ligament. When the occiput... 

It is generally preferable to keep the width of the approach no more than 10 mm from the midline in order not to interfere with the atlanto-occipital joints while resecting the atlas itself (see below). 

If symptoms of interruption of the vertebro-basilar circulation are present e.g., in cases of Jefferson fracture) vertebral angiography is mandatory. In severe subluxation of the atlanto-occipital joint damage to the vertebral artery is rather common. 

Fig. 7. Resection of the anterior arch of the atlas in a curved line to avoid injury to the atlanto-occipital or atlanto-axial joints...

During these manipulations it is of vital importance to know the exact dimensions of the structures seen under the microscope. The average size of some important bony structures in millimetres is given. We recommend the use of a small ruler or an instrument with precisely calibrated dimensions which when placed in the operative field aids precise measurement e.g., the width of a bony resection or the depth of a cavity). Resection of the anterior arch of the atlas should not be carried further than 8-10 mm from the midline because of the proximity of the atlanto-occipital joints. 

The resection surface should be curved with its lower part being more medially (Fig. 7), for although the medial border of the atlanto-occipital joint is about 10 mm from the midline, the atlanto-axial joint can already be inadvertantly entered at 8 mm from the midline. 

---