Transpedicular Instrumentation of the Lumbar Spine

Introduction and Patient Selection

Transpedicular screw fixation is commonly used as a means to treat spinal instability resulting from traumatic, neoplastic, congenital, or degenerative processes. Several techniques are used to place lumbar pedicle screws. Freehand techniques, image-guided techniques, and percutaneous techniques have all been described. Regardless of technique, the safe placement of pedicle screws is dependent on an understanding of the regional anatomy of the lumbar pedicle. This chapter reviews several techniques for the placement of lumbar pedicle screws with an emphasis on avoidance of complications.

52.2 Preoperative Preparation

Preparation for placement of posterior transpedicular instrumentation generally requires that, at a minimum, a set of high-quality radiographs be available. It is also generally advisable to have available an axial plane imaging study, preferably a computed tomographic (CT) scan, to evaluate the size and morphology of the pedicles. Specialized insertion techniques, such as placement using computer-guided frameless stereotaxy or robotic assistance, may require that additional studies be performed.

52.3 Operative Procedure

Transpedicular instrumentation requires the surgeon to determine the correct entrance point to the pedicle and to determine the correct sagittal and axial angle for screw placement. The dorsal aspect of the lumbar pedicle is localized using the junction of two lines. The first line is a straight rostrocaudal line drawn along the lateral border of the superior articular facet. The second line is a transverse line through the center of the transverse process ( ▶ Fig. 52.1). The lateral aspect of the pedicle may be palpated with a dissector placed over the rostral border of the transverse process; when practical, the medial aspect of the pedicle may be exposed by laminectomy or laminotomy. The screw entrance site (usually at the caudal third of the superior articular facet) is decorticated with a drill or rongeurs. The decortication is continued until the cancellous bone of the pedicle is visualized (“red eye” of the pedicle). The transverse pedicle angle (lateral to medial angulation of the pedicle in the axial plane) increases from near 0 degrees (straight dorsal–ventral) at L1 to nearly 30 degrees (dorsolateral to ventromedial) at L5. The sagittal angle of the pedicle also varies somewhat, but in a narrower range (5 degrees craniocaudal at L1 to 15 degrees at L5) ( ▶ Fig. 52.2). There is substantial variability in these angles, and pathological conditions can cause further deviance from the norm. Therefore, careful study of preoperative and intraoperative imaging studies is essential for the safe placement of transpedicular instrumentation.

Fig. 52.1 The pedicle insertion site is found at the intersection between a rostrocaudal line drawn along the lateral border of the superior articulating process of the facet and a transverse line drawn along the midpoint of the transverse process. The black dot in this illustration demonstrates the pedicle entrance site.

Fig. 52.2 (a) Axial computed tomographic (CT) image through the L2 vertebral body. The black line represents a perpendicular line through the vertebral body. Note the relative straight-ahead orientation of the pedicles at this level. (b) Axial CT through the L5 vertebral body. The black line represents a perpendicular line through the vertebral body. Note the substantial lateral-to-medial angulation of the pedicles at this level.

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