The pedicle forms a cylindrical bone bridge between the dorsal spinal elements and the vertebral body. The pedicle has a strong cortical shell with a central core of cancellous bone. Pedicle size and angulation vary significantly throughout the spinal column. In general, the transverse width of the pedicle is less than the pedicle height (Fig. 12.1a). The exception is the L5 pedicle which often has a width that is greater than its height. Pedicles between T10 and L1 generally have a transverse width of at least 7 mm, while pedicles below L1 generally have a transverse width of 8 mm or more. Due to the variability between patients, the best strategy for choosing the ideal implant size is to measure the specific pedicle dimensions from the preoperative imaging studies.

Medial angulation of the pedicles increases as one descends caudally from the thoracolumbar junction through the lumbosacral region (Fig. 12.1b). The nerve roots course along the medial aspect of the pedicle and occupy the rostral portion of the neural foramen. Therefore, violation of the medial or caudal pedicle cortices places the adjacent nerve root at the greatest risk of injury [5, 6]. The precise angulation of the pedicles can best be determined by measuring this parameter on the preoperative imaging studies.

In the thoracic spine, the relationship between the transverse process and the central axis of the pedicles differs compared to the lumbar region and varies by region within the thoracic spine. The pedicle is localized along the cranial portion of the transverse process in the upper thoracic region but is closer to the mid-transverse process in the mid and lower thoracic region. Due to variation between patients, fluoroscopic guidance with the true anterior-posterior (AP) view is helpful to define the exact pedicle location during surgery. The rib head lies along the lateral margin of the thoracic pedicles and adds to the bony corridor available for screw fixation. Both pedicle angulation and vertebral body depth must be considered more carefully in the upper thoracic region, where shorter implants are generally required [5, 6]. The great vessels are at particular risk in the thoracic region if a pedicle screw is misplaced either anteriorly or in a lateral direction on the left side where the aorta may lie along the lateral margin of the pedicle-rib complex [5, 6].

The conventional entry site for pedicle screw placement in the lumbar spine is at the junction of the lateral facet (superior articular process) and mid-portion of the transverse process [5, 6]. The pars interarticularis is generally located at the medial boarder of the pedicle at the L1 to L4 levels and is at the level of the mid-pedicle at L5. Facet hypertrophy may lead to overgrowth of the superior-lateral facet joint which may overlie the pedicle starting point in many cases, particularly in the lower lumbar region. Fortunately, the true AP fluoroscopic view will precisely localize the pedicle and guide the surgeon to the correct starting point. The medial angulation of the pedicles increases from the L1 level (where it is minimal) to the L5 level (where it is generally 15° or more). In some cases, the medial angulation of the pedicles at the L5 level will make the true AP view hard to interpret; in these cases the en face view is helpful to define the pedicle boundaries.

Preoperative planning begins by careful analysis of the imaging studies to define the sites for implant placement along with the dimensions and angulation of the specific pedicles to be instrumented. The strategy for surgical incisions should be considered in light of all the surgical goals for the procedure including the need for neurologic decompression and/or posterior element fusion. In some cases, a single skin incision may be used to access separate fascial incisions that can be used to access different regions of the vertebral column [1, 7].

When performing a minimally invasive surgical approach, the surgeon must obtain good quality fluoroscopic imaging of the vertebral column. The initial procedural step is to position the patient prone on a radiolucent spinal table or frame. The patient should be “squared up” or positioned to reduce trunk rotation. Next, the location of the surgical incisions should be demarcated on the skin using fluoroscopic guidance.

Prior to making any surgical incisions, C-arm images should be obtained and analyzed to verify that the quality of imaging is sufficient and that the pedicles are able to be clearly visualized on properly aligned fluoroscopic images. Severe osteopenia, morbid obesity, or intra-abdominal contrast may preclude adequate visualization of the bony landmarks and prevent safe implantation of percutaneous pedicle screws. In this situation, an alternative surgical technique should be utilized.

The key fluoroscopic views used during the placement of percutaneous thoracolumbar pedicle screws are the true AP view, the true lateral view, and the en face view (Fig. 12.2a–c). Proper alignment of the C-arm is a critical step with each of these fluoroscopic views. A properly aligned true AP image will demonstrate a “flat” superior end plate (only one superior end plate shadow should be seen) [Fig. 12.2a]. The pedicles should be localized just caudal to the superior end plate, and the spinous process should be centered between the pedicles. On the true lateral fluoroscopic image, the superior end plate should again appear “flat.” The pedicles should be superimposed. The surgeon should also analyze the posterior cortex of the vertebral body to be sure that there is no malrotation (only a single shadow should be seen) (Fig. 12.2b). Any malrotation should be corrected prior to proceeding. The en face view is obtained by starting with the true AP view and then rotating the C-arm until the fluoroscopic beam is in line with the pedicle axis (Fig. 12.2c). When the C-arm is aligned with the pedicle axis, the greatest medial-lateral width will be seen, and the medial boarder of the superior articular process will generally align along the medial boarder of the pedicle. When targeting a pedicle with the en face view, the middle of the pedicle (not the lateral wall of the pedicle as in the AP view) is targeted. In all cases, it is important that the region of the vertebra that is being targeted is localized in the mid-portion of the fluoroscopic image to ensure that the parallax phenomenon does not lead to misinterpretation of the image.