The accessory atlantoaxial ligament.

Objective: The stability of the joints connecting the cranium to the upper cervical spine is of vital importance. The ligaments of this region, for the most part, have been thoroughly investigated, with the exception of the accessory atlantoaxial ligament.
Methods: Ten cadaveric specimens were examined to observe the anatomy of this ligament.
Results: This ligament was found in all specimens, and in each, it not only connected the atlas to the axis but also continued cephalically to the occipital bone. The approximate dimensions of this structure were 3 cm x 5 mm. Functionally, this ligament became maximally taut with a rotation of the head of 5 to 8 degrees. Laxity was observed with cervical extension, and maximal tautness was seen at 5 to 10 degrees of cervical flexion.
Conclusion: The accessory atlantoaxial ligament seems to participate in craniocervical stability and perhaps should be renamed the accessory alar ligament or accessory atlantoaxialoccipital ligament; both of these terms better denote its anatomic characteristics. Perhaps in the future, better magnetic resonance imaging techniques and machines will be able to identify this structure so as to appreciate its integrity after upper cervical spine trauma.