The pedicle screw is one of the most common medical devices used in spinal surgery. Although there are well-established insertion points based on anatomical landmarks, such as the mammillary process and the transverse process, morphological data on the relationship between the accessory process and the pedicle are still scarce. To clarify this relationship, we recruited 50 cases of hernia of lumbar intervertebral disc, diagnosed using three-dimensional computed tomography of the lumbar vertebrae. We identified the pedicle isthmus in a transverse plane parallel to the upper endplate and measured the angles and distances from the tip of the accessory process to the intersection points at the medial or lateral surface, or at the midpoint between the two intersection points. In a sagittal plane showing the pedicle isthmus, we measured the wedging angle of the vertebral body as well as the angle from the tip of accessory process to the posterior edge of the upper endplate of vertebral body, or to the lower end of the pedicle root. We found that from the tip of the accessory process passing through the pedicle isthmus, a line should be directed 20 (± 6.6) degrees medially in the transverse plane and 5 (± 4.3) degrees cranially in the sagittal plane. This distance from the tip of the accessory process to the isthmus was 1.5 (± 0.3) cm. Our study provides a new anatomical basis for the use of the accessory process as a landmark for insertion of the pedicle screw.
Keywords: Accessory process; Computed tomography; Lumbar spine; Pedicle screw.