Imaging study of transoral anterior occipital condylar screw placement

English Keywords:Occipital condyle  Screw fixation  Transoral  CT

English Abstract:

  【Abstract】 Objectives: To investigate the feasibility and technical parameters of transoral anterior occipital condylar screw placement. Methods: Three dimensional reconstruction was performed in 80 occipital condyles of 40 cases by using 1mm sliced CT scans and MIMICS software. There were 17 females and 23 males with an average age of 38.7(range, 19-63). All of them had craniovertebral region intact. Infection and tumor were excluded in this study. 3 entry points(medial, middle and lateral) were selected. The middle entry point located at the cross junction of the foramen magnum and midline of atlas lateral mass. The medial and lateral entry points were located at 5mm medial and lateral to middle point respectively. Screw placement(3.5mm in diameter) was simulated at these three entry points. The end of the screw was located at the level of the junction of the occipital condyle and the rim of the foraman magnum. The screw length and safe range of extroversion among 3 entry points were compared. For the entry points available in clinical practice, the rate of successful screw placement for each degree of extroversion was compared. Results: The lateral entry point had the highest successful rate(100.0%) of screw placement with screw length of 13.5-21.6mm, minimum extroversion angle of (-19.5°)-5.6°, maximum extroversion angle of (-8.1°)-24.9°, and followed by the middle entry point of 88.6%, 16.3-24.2mm, (-4.1°)-29.7°, 12.6°-34.2°, respectively and medial entry point of 72.5%, 20.4-27.0mm, 16.3°-40.3°, 27.2°-44.8°, respectively. The three parameters for three entry points were significant different(P<0.01). It was difficult to recommend a general accepted extroversion angle. For the middle point, 20° extroversion angle had the highest successful rate of 68.75%. For the lateral entry point, paralleling to sagittal plane resulted in the successful screw placement in 80% of the cases. Conclusions: It is feasible to place transoral occipital condylar screw. The lateral and middle entry points are superior than the medial entry point. Determination of entry point and screw orientation must depend on preoperative 3-dimensional CT scan.