"The Rule of Spence is a radiologic method to evaluate the likelihood of injury to the transverse atlantal ligament (TAL) on an open mouth AP (“peg”) radiograph. As originally framed, if the combined projection of the lateral masses of the atlas is more than 6.9 mm beyond the lateral masses of the axis, an injury to transverse ligament is likely. However the original rule was derived from cadaveric studies, and not on radiographs. It has since been shown that using a threshold of >8.1 mm is more accurate once radiographic magnification error is accounted for 3."