Analysis of the safepath parameters for the occipital condyle screw: a computer simulation study / 中华骨科杂志

ABSTRACT

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.