Morphobiomechanical analysis of atlanto-occipital fusion The presence of bony deformities in atlanto-occipital fusion is an important cause of changes in the biomechanical characteristics of the craniocervical junction region, yet detailed geometric morphological evidence has not been available. Some authors have made a model of atlanto-occipital fusion by fixing the atlanto-occipital bones of normal cadaveric specimens to study the biomechanical changes, but this model is still very different from the actual situation and cannot reflect any bone and joint deformities other than the loss of atlanto-occipital joint function. In this group, an in vivo model was used to obtain the patient’s craniocervical junction spiral CT scan data, and then a 3D visualization technique was used to measure the 3D geometric angles of the articular surfaces. The measurement reference was made using reconstructed tomographic planes based on a 3D coordinate system with mutually perpendicular axial, coronal and sagittal orientations, where the axial plane is parallel to the FH plane. the FH plane was first determined by anthropological researchers and is considered to be parallel to the ground, and using this plane as a reference to observe the tilt angle of the subatlantoaxial articular surface can better reflect the mechanical characteristics of the patient’s head-bearing atlantoaxial joint in an upright position. The occipito-atlanto-axial structure is a special functional unit of the human spine, which is responsible for most of the motor functions of the head, with the atlanto-occipital joint mainly responsible for the flexion and extension of the head, and the atlanto-axial joint mainly responsible for the rotation of the head. In the normal anatomical form of the occipito-atlanto-axial structure, the atlanto-axial vertebrae are located between the occipital condyles and the pivot vertebrae, acting as a cushion to cushion the load on the joints during various complex movements of the head, and are the hub of the complex motor functions burdened by the craniocervical junction (upper cervical vertebrae). With atlanto-occipital fusion, the loss of the cushioning function of the atlanto-axial spine will result in increased stress on the joints and ligaments of the craniocervical junction. Therefore, it is commonly believed that fatigue occurs as a result of increased ligament loading during atlanto-occipital fusion, which eventually leads to ligament stretching deformation and inability to maintain joint alignment relationships and dislocation. However, our anatomical observations confirm that the atlanto-occipital fusion is associated with an angular change in the articular surface of the lateral atlanto-axial joint, such as anterior and lateral tilt, and that this change in geometry will cause changes in the static and dynamic characteristics between the atlanto-axial vertebrae, which may be a more direct cause of ligamentous stretch deformation and joint dislocation. The results show that the majority of patients with atlanto-occipital fusion show a change in the subatlantoaxial articular surface with a forward tilt, with a corresponding difference in the degree of anterior tilt of the articular surface on both sides, consistent with the degree and type of atlantoaxial subluxation. This change in the angle of the articular surfaces will result in a change in the balance of forces on the cervical spine supporting the head. As a result, the gravitational force of the head and the flexion and extension of the head during daily activities cause the cranium to slip forward and downward relative to the position of the cardinal spine, and this subluxation is not only a misalignment in the anterior-posterior direction, but also a displacement in the longitudinal direction, which is known as cranial settling. In both cases, there was actual atlanto-occipital fusion, as shown in the CT sagittal and coronal images in this paper. Goel et al. reported a similar case without occipito-cervical fusion, which Goel hypothesized to be due to a congenital malformation that tilted the atlantoaxial joint and caused the atlantoaxial spine to “slip” above the pivot, resulting in atlantoaxial subluxation and cranial settling. This caused atlantoaxial subluxation and cranial base entrapment. We have also found similar atlanto-occipital fusion patients with further cranial subsidence or anterior-posterior dislocation after dentatectomy and internal fixation with occipital-cervical implants and cables. The present set of results supports Goel’s inference. It is questionable whether the morphological changes in the bone and joint, such as changes in the angle of inclination of the inferior atlantoaxial articular surface, originate exclusively from congenital developmental abnormalities. According to Wolff’s law, bone remains adapted to the mechanical forces acting on it during growth. There is evidence that changes in the stresses applied to the articular surfaces of the spondylolisthesis can lead to changes in the morphology of the joint. It is therefore reasonable to speculate that the change in geometry of the craniocervical junction deformity joint may be due, at least in part, to the change in stress-strain at the craniocervical junction after atlanto-occipital fusion, which leads to deformation during bone growth, further aggravation of the stress change by deformation of the articular surface, and thus a vicious cycle. Based on a review of the literature and our case review, it is difficult to see patients in the infant and toddler groups clinically, suggesting that factors contributing to progression such as deformity and dislocation may progressively worsen with growth and development, evidence that indirectly supports our speculation. Although we do not yet know the exact cause of the undeformed joint, at least the likelihood of atlantoaxial dislocation is significantly reduced in the absence of a vicious cycle of stress-strain. Atlanto-occipital fusion with atlanto-axial dislocation resulting in compression of the dentate process by the medulla oblongata or medulla oblongata junction is a characteristic pathological change in this disease, and therefore its treatment generally involves two procedures, namely ventral decompression of the dentate process by excision and internal fixation by occipitocervical fusion. The present results suggest that the atlanto-occipital fusion presents an anteriorly inclined geometric morphology of the inferior atlantoaxial articular surface, which raises new questions about the biomechanical stability of the cable-only fixation approach. However, in patients with congenital craniocervical junction deformities that include atlanto-occipital fusion, the deformed shape of the atlanto-axial spine leads to uncertain anatomical features and more complex variants in the course of the vertebral artery, posing a greater challenge to the safety of the screw technique. This poses a greater challenge to the safety of the screw technique. Nevertheless, several authors have successfully used the screw technique in congenital craniocervical junction deformities, and we have begun the initial application of the screw technique.Goel et al. performed intraoperative repositioning of a fixed atlantoaxial subluxation in a group of patients, exposing the lateral mass joint and then inserting a custom mesh titanium disc followed by C1-2 nail bar fixation, with satisfactory results, and some of the cases in their reported series were congenital atlanto-occipital fusions. Based on the geometric characteristics of the subatlantoaxial articular surface during atlanto-occipital fusion and the tendency for cranial subsidence in such cases, a more stable internal fixation is necessary to obtain a more stable biomechanical result in patients with atlanto-occipital fusion. In conclusion, it is a common change that the weight-bearing lateral block joints between the atlanto-occipital spine in patients with atlanto-occipital fusion show varying degrees of anterior tilt and asymmetry, and it is possible that the biomechanical changes in the joints due to this geometric change are the direct cause of atlanto-occipital dislocation. The analysis of the tilt angle of the joint surface is important for the judgment of the condition, the grasp of the surgical indications and the selection of the internal fixation method.