Rhee_Ch030.indd

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Operative Techniques in Spine Surgery

■ Screw lengths are then measured by marking the depth on the ballpoint probe. Common screw sizes are 7 to 8 mm in diam- eter and 70 to 80 mm in length. ■ The screw path is tapped using a handheld tap, and an appropri- ate length screw is inserted. ■ Finally, the screw must then be attached to the longitudinal rods of the main spinal construct by using a modular connector system. The connectors are tunneled anterior to the paraspinal muscles ( TECH FIG 3E,F ).

■ Pelvic radiographs or C-arm fluoroscopic images may be taken to confirm placement of the instrumentation. The trajectory and starting point of the screw should be assessed. ■ For the iliac screw, the starting point should be over the PSIS and directed toward the AIIS, proximal to the sciatic notch. ■ For the S2AI screw, the starting point is midway between S1 and S2 sacral foramina and directed toward the AIIS, using an anteroposterior (AP) view will ensure the cepha- lad/caudal position; a tear drop view will ensure no breach to the medial or lateral cortices of the ilium. ■ An awl is used to breach the dorsal cortex over the starting point, located in line with the lateral edge of the S1 foramen and mid- way between S1 and S2 dorsal foramina ( TECH FIG 5A ). ■ The most common S2AI trajectory averages 40 degrees of lateral angulation in the transverse plane and 40 degrees of caudal angulation in the sagittal planes directed toward the AIIS ( TECH FIG 5B,C ). ■ A 2.5-mm pelvic drill bit (extended length) is used to tap drill through the sacral ala, the SI joint, and into the ilium. This dis- tance is roughly 30 to 45 mm in most patients ( TECH FIG 5D ). ■ At this point, the drill bit is removed and replaced with a 3.2-mm drill bit in order to reduce the risk of breakage in harder bone and is advanced for a total depth of approximately 80 to 90 mm. ■ Once the drill has reached the ilium, the drill itself is removed. A depth gauge is then inserted into the hole in order to determine the length of the screw. ■ Next, a 1.45-mm guidewire mounted on a handheld driver is advanced for an additional 10 to 20 mm to seat the guidewire in bone ( TECH FIG 6A ). Angulation continues to be directed down toward the ASIS. ■ Placement can be confirmed with intraoperative fluoroscopy. ■ The hole is then manually tapped over the guidewire using a cannulated tap ( TECH FIG 6B ), and an appropriate length screw is placed ( TECH FIG 6C ). Screw sizes of 8 to 10 mm by 80 to 100 mm are most commonly used in adults. ■ Because the screws are in line with the rest of the spinal con- struct, no additional cross-connectors are required for assembly ( TECH FIG 6D,E ).

T E C H N I Q U E S

■ Open S2 Alar Iliac Technique

■ The placement of the S2AI screw should take place only after the other points of fixation, including the S1 screws, have been secured. ■ The position of the S1 and S2 dorsal foramina are identified using a Woodson elevator ( TECH FIG 4 ).

TECH FIG 4 ● Standard midline exposure. Probes mark the lo- cation of the S1 and S2 dorsal sacral foramina at points A and B , respectively.

Starting point of S2A1 screw

B C TECH FIG 5 ● A. Starting point for the S2AI screw. B,C. Sagittal and coronal views, respectively, of the final trajectory of the S2AI screw. (continued) A