Optimal Trajectory for the Occipital Condylar Screw

Abstract

Study Design. Evaluation by simulation of screw insertion using fine-cut computed tomographic (CT) scans and screw trajectory software.Objective. To evaluate the feasibility and optimal trajectory of the occipital condylar screw.Summary of Background Data. To the best of our knowledge, no large series examining the feasibility and optimal trajectory of occipital condylar screws have been published.Methods. We simulated unicortical placement of a 4 x 18-mm screw using 1-mm sliced CT scans and 3-dimensional screw trajectory software in 314 occipital condyles of 157 patients. With the screw tip directed toward a point just below the tip of the basion on lateral fluoroscopy, 3 entry points were compared in view of success rate of screw placement, safe range of medial angulation in an axial plane, and maximum screw length.Results. The lateral entry point had the highest success rate (93.0%) of screw placement, mean safe range of the medial angulation (10.9 degrees), and maximum screw length (20.7 mm), followed by the middle (92.0%, 10.8 degrees, and 20.6 mm, respectively) and medial (74.2%, 7.7 degrees, and 19.7 mm, respectively) entry points. While the lateral and the middle entry points did not have any statistically significant difference in the 3 parameters, they were significantly better than the medial entry point. All 3 entry points had highly variable ranges of safe medial angulation, making it difficult to provide a single recommended value.Conclusion. Screw placement is feasible in up to 93% of the occipital condyles. The lateral and the middle entry points are significantly better than the medial entry point. Selection between the middle and the lateral entry points should be individualized taking into account local anatomic variation. Because the medial angulation of these screws is highly variable, preoperative 3-dimensional CT evaluation and possibly even intraoperative navigation may be required.