[Anatomical study of the ligaments in the occipito-atlantoaxial complex]

To investigate the role of the ligamentous complex in the stability of the atlanto-axial joint, the macroscopic anatomy of the occipito-atlanto-axial complex was studied in 44 cadavers involving the cruciate ligament of atlas, alar ligament, and the anterior atlanto-axial ligament (AADL). The transverse ligament (TL) had two layers, one was membranous tissue and the other was ligamentous. The mean length of the TL was 19.7 mm (range 14-25 mm). The TL was significantly longer in males than in females. The diameter of the TL was thinnest at the centre (mean 2.0 mm, range 1-5 mm). The thicker ligaments were significantly larger in diameter. The alar ligament (AL) was dissected and examined in all cadavers. The direction of the AL ran postero-laterally. This ligament was divided into five types according to the fiber orientation. Type 1 was separated type in its origin at the dens. Type 2 was partially connected type. Type 3 was totally connected with a covered dens tip. Type 4 was totally connected but did not cover the dens tip. Type 5 was a combined type of Type 3 covering Type 1. The fiber direction of the AL was divided into three types; caudocranial type, horizontal type, and craniocaudal type. The AADL was examined in 24 specimens (60%). This ligament existed bilaterally in 18 specimens (45%) and hemilaterally in 6 (15%). The right side only was examined in 2 specimens (5%) and the left side only in 4 specimens (10%). In 6 specimens, the orientations of this ligament was identified and the shape was found to be thin. From these results, it was considered that not only TL, but also the AL and AADLs played an important role in stabilizing the atlanto-axial complex.