Objective To investigate the safety of the occipital condylar screw with vertical position and evaluate the selection strategy of the posterior approach of the posterior occipital condylar screw in Chinese people.Methods The clinical imaging data of 60 outpatients from September 2013 to September 2015 were retrospectively analyzed,36 male and 24 female,the average age was 41.6±9.2 (range from 25-58),Excluded occipitocervical injury,tumor and deformity patients.We built a three-dimensional digital model and simulated placing screw by utilizing CT data on Mimics software,after that we took the occipital condyle posterior medial and lateral midpoint as the entry point,then made 2 points equidistantly to the midpoint in vertical direction.We put 3.5 mm diameter virtual screws in 4 different conditions:largest cranial angle,smallest cranial angle,longest screw path and shortest screw path.Then we assessed the anatomical relationship between the screw and the hypoglossal canal or the atlanto-occipital joint by a three-dimensional window and measured the cranial angle,medial angle and length of screw path,then calculated the safety angle of the cranial angle,the successful rate of setting screw,and compared the safety of different screw points by 3-Matic software.Results 120 occipital condyles were obtained from the CT data of 60 patients by Mimics software.There was no significant difference in the data of the cranial angle,medial angle,safety range and length between both left and right sides.The obtained safe cranial angle of each point respectively was 20.9°±6.0° (lowest point),17.0°±6.2° (middle point),and 11.6°±7.1°(top point),obviously the largest angle was in the lower point and the smallest was in the top point.The difference was statistically significant.We then acquired the successful rates of different cranial angle of each point,the highest successful rate was 99.17％,96.67％,74.17％ in lowest,middle and top point when cranial angle were 3°or 4°,3°and 0°respectively.The successful rates of lower point and niddle point were significantly higher than the top point,and the difference was statistically significant.The medial angle parameters obtained were 34.41°±2.59°on left and 34.06°±2.44°on right,and there was no significant difference.The length parameters of the longest screw path acquired were 23.09± 1.47 mm,22.84± 1.40 mm and 23.15± 1.45 mm at top,middle and lowest entry point.The average value of shortest screw path of each point was 21 mm,and there was no significant difference among every entry point.Conclusion Among the occipital condyle posterior screw entering points,selecting the lower point can improve the success rate and safety;the change of nail enter point in the vertical direction has little effect on the length of the nail.We can increase the safety and reduce the risk of occipital condylar screw placement as far as possible through the three-dimensional digital technology.

Objective To explore the biomechanical properties of posterior occipital condyle screws compared with common occipitocervical fusion internal fixation and it's impacts upon stress of hypoglossal canals.Methods Finite element models based on the occipitocervical CT data of one 28-year-old male healthy volunteer were built,including normal model,instability model,internal fixation model by occipital condyle screws,internal fixation model by occipital plate screws,and internal fixation model by transarticular screws.Fifty N gravity and 1.5 N · m torque were exerted upon the surface of occipital bone so that the models could perform lateral bending,flexion,extension,and rotational motions.The motion range and stress distribution of internal fixation were compared under varying conditions among different occipitocervical fusion models.In addition,the impact of occipital condyle screw upon hypoglossal canals was examined.Results Compared with instability model,the motion range in the internal fixation model by occipital condyle screws declined by 96.8％,95.6％,95.0％ and 98.5％ respectively in lateral bending,flexion,extension and rotation.In the internal fixation by occipital plate screws,the motion range decreased by 96.3％,95.7％,98.4％ and 99.6％ respectively.In the internal fixation by transarticular screws,the motion range exhibited a decline of 95.7％,94.0％,94.3％ and 98.9％,respectively.The stress peaks in the occipital condyle screw were 192.4 MPa,201.6 MPa and 187.6 MPa under lateral bending,flexion,and rotation conditions,respectively.The stress peaks in the occipital plate screw were 279.6 MPa,213.7 MPa,and 154.1 MPa,respectively.The stress peaks in the transarticular screw were 232.4 MPa,220.9 MPa,and 224.5 MPa,respectively.The stress impact peak of occipital condyle screw on the hypoglossal canals wall was 12.96 MPa,and the content deformationunder the hypoglossal canal was 0.64％.Conclusions The occipital condyle screw internal fixation has similar stability with common occipitocervical fusion fixations.The occipital condyle screw has more uniform stress distribution and less effect on the hypoglossal canals,and hence is safe and reliable as anchor point on the cranial side in occipitocervical fusion.

Objective:To investigate the safety and feasibility of the occipital condyle screw and evaluate the safepath parameters for the occipital condyle screw.Methods:Data of 64 patients with upper cervical computed tomographic angiograms from September 2016 to September 2018 were retrospectively collected. Excluded occipito-cervical injury, tumor, and vertebral artery course variation. Mimics software was used to reconstruct the occiput, atlas and vertebral artery. Three candidate entry points were placed for each occipital condyle, the midpoint of posterior of occipital condyle as middle entry point, and the medial and lateral entry points were located 3 mm medial and lateral to the middle entry point. The vertebral artery-occipital bone distance (VOD) of each entry point were measured on sagittal plane, and the minimum feasible value was determined to be 4mm. After that 3.5 mm diameter virtual screw was inserted into each candidate entry point with VOD>4 mm, each screw with maximum and minimum cranial angulation was combined with appropriate medial angulation to get the maximum screw length. Then, the screw placement parameters were measured by 3-Matic, and the safe range of cranial angulation and the success rate of screw placement were calculated.Results:The VOD of medial and middle entry point were 8.07±2.13 mm and 7.70±2.19 mm respectively, and the feasibility rate of screw placement of those entry point were 97.7% and 96.1%, respectively. There were significant differences inVOD and feasibility rate of screw placement between medial and middle entry point. The VOD of lateral entry point was 5.63±1.66 mm, and the feasibility rate was only 78.9%, which was significantly lower than that of medial and middle entry point. The lateral entry point could obtain a larger medial angulation, which was supplemented by a longer screw length. The medial angulation and length of screw gradually decreased with the inward movement of the entry point. There were significant differences in medial angulation and screw length among groups. The safe range of cranial angulation of medial, middle and lateral entry points were 8.17°±2.55°, 12.58°±4.23° and 12.09°±3.83°, respectively, and the difference were statistically significant. Among the screw entry point that could accommodate screw fixation, the maximum screw placement success rate can be obtained by adding 5° cranial angulation to the lateral and middle entry point, which were 98.02% and 98.37%, respectively,while 100% success rate of screw placement could be obtained at the medial entry point at 3° cranial angulation.Conclusion:In the selection of the entry point in the horizontal direction, middle and medial entry points have higher success rate of screw placement and wider safe range of cranial angulation because of less affection of horizontal segment of the vertebral artery. However, the screw length of medial entry point is much shorter than middle and lateral entry point. As a result, the middle entry point may be an optimal entry point for the occipital condyle screw.