Axial superior facet slope may determine anterior or posterior atlantoaxial displacement secondary to os odontoideum and compensatory mechanisms of the atlantooccipital joint and subaxial cervical spine.

Objective: To introduce novel parameters in determining directions of os odontoideum (OO) with atlantoaxial displacement (AAD) and compensations of cervical sagittal alignment after displacement.

Methods: Analysis was performed on 96 cases receiving surgeries for upper cervical myelopathy caused by OO with AAD from 2011 to 2021. Twenty-four patients were included in the OO group and divided into the OO-anterior displacement (AD) group and the OO-posterior displacement (PD) group by displacement.

Pelvic incidence measurement using a computed tomography data-based three-dimensional pelvic model.

Objectives: To introduce a new method of pelvic incidence (PI) measurement based on three-dimensional (3D) pelvic models reconstructed from CT images and to report the normal distribution of PI in normal pelvic anatomy.

Methods: CT images of 320 subjects with normal pelvic anatomy who visited the Radiology Department between 2006 and 2017 were retrospectively selected and saved in Digital Imaging and Communications in Medicine (DICOM) format. A computerized method was employed to determine the bony landmarks required for the measurement of PI.

Pelvic incidence variation among individuals: functional influence versus genetic determinism.

Pelvic incidence has become one of the most important sagittal parameters in spinal surgery. Despite its great importance, pelvic incidence can vary from 33° to 85° in the normal population. The reasons for this great variability in pelvic incidence remain unexplored.