Anomalous vertebral artery not passing through the transverse foramen of the atlas

Ssually the vertebral artery （VA） arises from the first part of the subclavian artery and enters the transverse foramen of the C-6 vertebra （first segment） and then ascends through the transverse foramen from C-6 to the atlas （second segment）. In its third segment, the artery emerges on the superior surface of the atlas and curves horizontally over the lateral and posterior surfaces of the superior articular process of C-1 and then passes medially in front of the posterior atlanto-occipital membrane. In its fourth segment, the artery pierces the dura mater and arachnoid between the occipital bone and atlas then enters the cranial cavity through the foramen magnum. To identify and preserve the VA is vital in the surgery of the craniaocervical and cervical region. However, the variations of the VA complicate the procedures of VA identification. It is valuable to know the course of VA preoperatively. Some anatomical variations of the VA occur due to its abnormal development process.

Anatomic and radiological study on posterior pedicle screw fixation in the atlantoaxial vertebrae of children

Objective： To investigate the feasibility of posterior fixation with 3.5-mm pedicle screws in the atlantoaxial vertebrae of children. Methods： In this study, atlantoaxial vertebrae specimens were obtained from 10 cadavers of children aged 6-8 years. We measured the height and width of the C1 pedicle and the midportion of C1 lateral mass; the width of C1 posterior arch under the vertebral artery groove and the height of the external and internal one-third of this part; the external, internal height and the superior, middle, inferior width of the C2 pedicle （transverse foramen）. Furthermore, computed tomography （CT） axial scan was performed on 20 agematched volunteers to obtain relative data of their atlantoaxial vertebrae. We measured the length and width of the C1 and C2 pedicles in the atlantoaxial cross-sectional plane. On CT workstation, we also measured the angles between the longitudinal axes of the atlantoaxial pedicles and the midsagittal plane. Results： For the cadaveric specimen group, the height and width of the C1 pedicle were （5.26±0.44） mm and （6.26±0.75） mm respectively. The height of the medial one-third of the Ct posterior arch under the vertebral artery groove was （4.07±0.24） mm. The external, internal height and superior, middle, inferior width of the C2 pedicle was （6.86±0.48） mm, （6.67±0.49） mm, （6.63 ±0.61） mm, （5.41±0.39） mm and （3.71±0.30） mm, respectively. For the volunteer group measured by CT scan, the height and width of the C1 pedicle were （5.47 ±0.34） mm and （6.63±0.54） mm respectively, while （6.59±0.51） mm and （5.13 ±0.42） mm of the C2 pedicle. The angles between the atlas, axis pedicles and the midsagittal plane were （9.60±1.32）° and （27.80±2.22）° respectively. Conclusion： It is feasible to place a 3.5-mm pedicle screw in the C1 and C2 pedicles of children aged 6-8 years old.