Identification of the Entry Point

The midline incision is made in the lumbar spine. Subperiosteal dissection is made to split muscles from the spinous processes, laminae, and facets. On both sides, dissection is carried out to the level of the transverse processes in the region of the intended fusion. The surgeon should remove the musculature and the periosteum meticulously in the region of the segment to be fused. After the muscle is dissected, the fat tissue is seen from the base of the transverse process (lateral fat pad) and under the isthmus (medial fat pad). The superior articular process has two prominent bony ridges, the mamillary process and the accessory process. The mamillary process is located at the inferior aspect of the facet joint and is more laterally located at the lower spinal levels. A tendinous membrane (the mamilloaccessory ligament) connects the bony tips of the mamillary and accessory processes.³ The ligament is embedded medially and laterally in fat pads. The pedicle is located underneath the floor of the mamilloaccessory notch (Fig. 39-1). The ligament can be considered to be the lateral boundary of the pedicle.

Fig. 39-1 Anatomical structures around the entry point.

The entry point for the lumbar pedicle screw can be decided by other references. The entry point is usually determined as the intersection point between two lines. A bisecting line of the transverse process is commonly used as the horizontal line. The vertical line is variable according to the surgeons. Roy-Camille’s screw entrance point is situated on the vertical line given by the articular process 1 mm under the facet joint (Fig. 39-2, point a). In Margerl’s method, the vertical line touches the lateral border of the superior articular process (Fig. 39-2, point b). Some surgeons use the more lateral trajectory to avoid facet joint injury (Fig. 39-2, point c).

Fig. 39-2 Identification of the entry point. A, Posterior view of entry points a, b, and c. B, Axial view of entry points a, b, and c.

In most cases, the bisecting line of the transverse process coincides with the center of the pedicle. However, mild variations may be found depending on the level of the lumbar spine.⁴ The center of the cross-section of the pedicle is reported to be 3–4 mm higher than the bisecting line of the transverse process at L1, 2–3 mm higher at L2, and 0.5–1 mm higher at L3. At L4 and L5, the center of the pedicle is shown to be slightly lower than the bisecting line by 0.5–1.5 mm (Fig. 39-3).

Fig. 39-3 The location of the center of the pedicle with reference to the transverse process.

Preparation and Screw Insertion

The medial and lateral fat pads are dissected bilaterally to expose the mamilloaccessory ligament. The pedicle is opened by grasping and rotating the bony ridge between the accessory process and the mamillary process with a rongeur (Fig. 39-4). After the pedicle is opened, marking wires (Steinmann pin) are inserted. The insertion angle of the Steinmann pin is 10–20 degrees convergent toward the sagittal plane (see Fig. 39-2). The point of entry is in the central axis of the pedicular tube, indicated by the intersection of the two lines. The vertical line touches the lateral border of the superior articular process. The horizontal line bisects the base of the transverse process. An x-ray is made to determine the exact position of the drill channels in the lateral and anteroposterior projections. In slightly oblique fluoroscopic images, the pedicle and Steinmann pin appear as an oval structure with a central dot (Fig. 39-5).

Fig. 39-4 Mamilloaccessory ligament resection for pedicular opening.

Fig. 39-5 The Steinmann pin is located in the center of the pedicle on the oblique view.

After all pins are placed into the vertebral bodies to create paths for the pedicle screws, the Steinmann pins are removed, and the superficial 5–10 mm area of each track is enlarged with a drill. Next, the tapping screw is inserted into the pedicles for subsequent insertion of the screws. The depth of the drilled channel can be established with the depth gauge. Pedicle screw diameter and length are pre-selected on the basis of the computed tomography (CT) images of each vertebra. The screws are placed into the prepared holes with the same trajectory as that of the Steinmann pins. Screw purchase is obtained by advancing the screw with a screwdriver to a depth of 70–80% of the vertebral body. The anterior cortex of the vertebral body should not be penetrated to prevent injury to vascular and visceral structures in the retroperitoneum.

Anatomy of the Lumbar Spine Pedicles⁵

The transverse diameter of the pedicle is at its minimum at L1 (6–9 mm): it increases at lower levels and is at its maximum at the L5 level (12–18 mm). The diameter of the pedicle screw is determined to be 70–80% of the pedicle diameter. The axial length is at its minimum at L1 (47–53 mm) and at its maximum at the L3 level (49–56 mm), maintaining the stationary value to L5. The axial angle is at its minimum at L1 (8–15 degrees), increases at lower levels, and is at its maximum at the L5 level (25–35 degrees). The axial angles of the pedicles are reported to be significantly smaller in degenerative stenosis patients, as compared with a healthy control group, although no disease-related differences are recognized in the transverse diameter and axial length of the pedicles⁶. The axis of the pedicles in the coronal plane changes as the lumbar column descends; the axis of the pedicle of L5 is more horizontal and oblique than that of L1 (Fig. 39-6). These changes are correlated with changes in the course of the lumbar nerve because the lumbar rami run very close to the pedicle, as does the dorsal ganglion.⁶ Neural structures around the pedicle have three courses: ventral and dorsal nerve roots in the spinal canal, spinal ganglion in the foramen, and the spinal nerve in the extraforaminal space.

Fig. 39-6 Posterior aspect of an anatomical specimen after posterior element removal, showing the lumbar plexus. Dashed line illustrates the three courses of the lumbar nerves.