In recent years, there is a towering up tendency in the incidence rate of tumor, tuberculosis, and trauma in craniovertebral junction, many scholars have reported the dangerousness and challenge of these diseases[10, 11]. And en bloc resection is a popular operation method for upper cervical reconstruction, which has been reported achieved desired clinical efficacy[6, 12, 13]. Similarly, clival screw and plate fixation was also reported as a feasible technique for craniovertebral reconstruction in anatomy. However, both two surgical method have some disadvantages. On the one hand, there were some complications and risks of aforementioned methods including (1) the inner structure of clivus was the epencephalon and foramen magnum which contains medulla oblongata and it maybe injury when inserted the clival screw. Invade into cranial cavity is a fatal risk, (2) Duding the posterior approach exposure, the vertebral, venous sinus and plexus around the upper cervical spine which maybe cause bleeding once injured. On the other hand, many important anatomical structures such as vertebral artery, C2 nerve root and posterior cervical muscle will be damaged during exposure in posterior approach which may result neck pain[16, 17]. And posterior pedicle screws maybe infeasible in some patients with anatomic variation such as narrow C2 isthmus and absence of bone structure[18, 19].
As a consequence, our group was intended to design an alternative operation method, which can prevent from aforementioned difficult and complications. Bosco et al reported a morphometric evaluation and anatomical parameters of occipital condyle, and shows that the average occipital condyle length, width, anterior height and posterior height is 18.8 ± 2.3 mm, 10.3 ± 1.5 mm, 13.2 ± 2.2 mm and 8.5 ± 1.6 mm, respectively. Moreover, the occipital condyle can safely contain a screw without hypoglossal canal invaded. Similarly, our group have reported the same outcome in a CTA based study. As for cadaveric specimen evaluation, Yu et al reported that a total of 40 4-mm posterior occipital condyle screws successfully inserted into twenty (40 occipital condyles) cadaveric specimens. And no screw invaded into hypoglossal canal verified by postoperative CT scan. According to those study, occipital condyle could be a feasible bone structure for craniovertebral junction reconstruction in anterior approach. However, this novel surgical method shows feasible but vital surrounding structures need take into account. The cephalad and medially of occipital condyle is hypoglossal canal which contains hypoglossal nerve, and foramen magnum. So, in the sagittal plane, if the anterior occipital condyle screw placed with a large inclination angle, it may invade hypoglossal canal and damage hypoglossal nerve, on the contrary, if the screw inserted too caudally or medially, it may violate vertebral artery or foramen magnum which may injury to spinal cord and medulla oblongata[24, 25].
In this study, we enrolled 40 healthy volunteers’ craniovertebral junction CT data and generated 3D models using Mimics 19.0 software successfully. This digital anatomy and cadaveric study of craniovertebral junction confirmed the anatomical feasibility of anterior occipital condyle screw and plate fixation and primarily verified the entry point and optimal trajectory of AOCS. According to our result, the optimal trajectory for AOCS is inserted with an inclination angle of 5.9° ± 3.4° on sagittal plane and diverge angle is 26.7° ± 6.0° on axial plane, and the average screw length is 21.6 ± 1.2 mm. Researchers reported that screw length longer than 18 mm can have enough pull-out strength. As for simulation in cadaveric specimens, then all anterior occipital condyle screws were inserted assisted with plate successfully without hypoglossal canal injury or cortex broken. All specimens were performed CT scan and generated 3D models using Mimics 19.0 software, after that parameters of anterior occipital condyle screw were measured and compared in specimen’s models and specimens. There was no significant difference in inclination angle, diverge angle and screw length between two group (P > 0.05). This shown that the anterior occipital condyle screw can be safely placed assisted with plate, and using plate can enhance the accuracy and decrease the risk of vital surrounding anatomical structure damage.
We should pay attention to the hypoglossal canal during anterior occipital condyle screw fixation, because hypoglossal canal which contains hypoglossal nerve and venous plexus passes from intracranial to the anteromedial upper part of occipital condyle, its location determined the safe area of screw placement. Usually, the hypoglossal canal located in the anterior medial of occipital but some patients’ hypoglossal canal located posterior medial of occipital condyle. As a result, we should analyze the CT scan every single patient and justify the inclination angle when hypoglossal canal located in the posterior medial of occipital condyle. Therefore, correct preoperative diagnosis and radiological outcome analysis should be taken before perform anterior occipital condyle screw fixation.
The primary indication of anterior occipital condyle screw fixation shown as follow: (1) Anterior craniovertebral junction reconstruction; (2) upper cervical spine abnormal such as C2 isthmus narrow; (3) occipital condyle fracture; (4) patients suffer from failure of posterior upper cervical spine fixation who can’t perform a second operation. However, patients with occipital condyle or vertebral artery deformity should be considerate as the contraindication.