Rhee_Ch030.indd

310

Operative Techniques in Spine Surgery

■ Sacral Tricortical Pedicle Screws ■ The iliac screw and S2AI methods for sacropelvic fixation begin with placement of sacral (S1) screws, which should be com- pleted prior to placement of the pelvic fixation. ■ Sacral pedicle screws can be placed through either two or three (through the sacral promontory) cortices, but tricortical screws have been shown to have twice the insertional torque of bicorti- cal screws, and this is the preferred technique. 16 ■ An awl is used to breach the dorsal cortex of the sacrum at the starting point 1 cm proximal and immediately lateral to the S1 sacral foramen ( TECH FIG 2A ). ■ A slightly curved large pedicle finder (gearshift type) is used to sound the cancellous bone. The path should be directed 30 to 40 degrees anteromedially and 15 degrees cephalad toward the anterior tip of the sacral promontory ( TECH FIG 2B ).

■ The direction toward the anterior tip of the promontory can be estimated from preoperative plain radiographs and confirmed intraoperatively with lateral radiograph or by fluoroscopy prior to placement of the screw ( TECH FIG 2C ). ■ The pilot hole is tapped using a tap size 1 mm less than the screw to be inserted, typically a 6-mm tap is used and a 7-mm screw is placed ( TECH FIG 2D ). ■ All five boundaries of the screw hole are sounded using a ball- point probe in order to verify that the bony cortex has not been breached. ■ Screw length is then measured using a ballpoint depth gauge. Screws are typically placed with a bicortical purchase. ■ The screws are then placed under direct visualization.

T E C H N I Q U E S

Starting point

Trajectory

A

B

C

TECH FIG 2 ● Starting point ( A ) and trajectory ( B ) for the tricortical S1 screws. C. Intraoperative lateral radio- graph showing the trajectory of the pedicle finder, which is 30 to 40 degrees anteromedially and 15 degrees cephalad toward the anterior tip of the sacral promontory. D. Placement of the S1 tricortical screw. The pilot hole is tapped using a tap size 1 mm less than the screw to be inserted, the depth is measured using a ballpoint depth gauge, and an appropriate length screw is placed along the path obtaining a bicortical purchase.

D

■ The Iliac Screw Technique

■ The PSIS is palpated, and the subcutaneous tissue is dissected off the lumbosacral fascia bilaterally toward the PSIS using Cobb elevators and Bovie electrocautery ( TECH FIG 3A ). ■ A longitudinal or oblique incision is made in the fascia overlying the PSIS. ■ The incision is extended both caudally and cephalad along the ilium with respect to the PSIS. ■ A rongeur or burr is used to breach the cortex overlying the PSIS, approximately 1 cm from the distal ilium. The amount of bone

■ We recommend that the iliac screw be placed only after other points of fixation, including the S1 screws, have already been secured. ■ When placing the iliac screws using the S2AI technique, insert- ing the proximal screws, especially the S1 pedicle screws, helps in guiding the surgeon in fine-tuning the lateral/medial starting point for the S2AI screws so as to end with a straight in-line anchor at the lumbosacral junction facilitating rod placement.