Magnetic Resonance Imaging of the Craniocervical Junction at 3-T: Observation of the Accessory Atlantoaxial Ligaments

OBJECTIVE: Instability of the craniocervical junction can cause neurological sequelae or pain. Stability of this region depends on extensive ligamentous support structures, most of which are well studied by magnetic resonance imaging (MRI) scanning. Although the contribution of the accessory atlantoaxial ligament to rotational stability has been described, this ligament has not been identified by imaging. Therefore, we investigated the imaging characteristics of this ligament and its relationship to the ligamentous complex of the craniocervical junction using the high resolution offered by 3-T MRI scans. METHODS: Ten healthy volunteers underwent MRI scanning at 3-T to determine the normal anatomy of this ligament. RESULTS: The atlantoaxial (C1–C2) segment of the ligament was identified in all 10 subjects bilaterally and symmetrically. Its mean dimensions were 2.8 × 1.8 mm. In four out of 10 subjects, the occipitoatlantal (C0–C1) segment was observed. At this level, the mean dimensions of the ligament were 1.6 × 1.2 mm on the right and 1.8 × 1.4 mm on the left. Its size varied between the right and left sides. CONCLUSION: The accessory atlantoaxial ligament can be visualized using high-resolution MRI scans at 3-T. The ligament was most consistent and robust at C1–C2. The ligament may, therefore, contribute to rotational stability at this level. Future studies will determine the biomechanical importance of this ligament, especially in the setting of trauma.

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 ×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.