Rhee_Ch030.indd

C H A P T E R 30

Sacropelvic Fixation Techniques

Floreana A. Naef and Khaled M. Kebaish

DEFINITION

■ Numerous critical structures—including the internal iliac artery and vein, middle sacral artery and vein, sympa- thetic chain, lumbosacral trunk, and sigmoid colon—lie directly on the sacrum at some point and could potentially be injured by the instrumentation used in sacropelvic fu- sions ( FIG 2 ). 19 ■ The ilium is the most superior of the three bones that make up the os coxa. ■ In adolescents, the ilium is connected to the pubis and the ischium through the triradiate cartilage. Fusion of this car- tilage completes between 13 and 16 years of age in most patients. ■ In thin patients, the posterior superior iliac spine (PSIS) is marked by an overlying dimple in the skin. A transverse line drawn between these two dimples crosses the sacrum at the level of S2. ■ The structures of the greater sciatic foramen are at risk of damage during instrumentation of the pelvis. 11

■ Sacropelvic fixation is a term used to describe instrumenta- tion into the sacrum and pelvis. ■ The most common indication is a long spinal fusion to the sacrum. Other indications include high-grade spondylolis- thesis, flat back syndrome requiring corrective osteotomy, and correction of pelvic obliquity. ■ The purpose is to provide secure distal fixation points that resist the strong flexion moments and cantilever forces pres- ent at the lumbosacral junction. ■ Multiple techniques are used, including the Galveston rod, the iliac screws, and the S2 alar iliac (S2AI) technique.

The Ilium

ANATOMY

■ A clear understanding of the anatomy of the sacrum and pelvis is crucial to the safe and accurate placement of sacral and pelvic instrumentation. Familiarity with the anatomy of the sacrum, ilium, and sacroiliac (SI) joint is of particular importance.

The Sacrum

■ The sacrum lies at the junction between the mobile and fixed portions of the spine and functions as a keystone that unites the two hemipelvises. ■ The sacral vertebrae are fused, and the transverse processes merge into the expanded lateral sacral ala. ■ The majority of the bone in the sacrum has a cancellous osseous structure. 26 The trabecular density is greatest in the pedicle and body of the vertebrae and least in the sacral ala. 26 Therefore, sacral pedicle screws are best directed toward the midline. 20 ■ The sacrum does not contain a true pedicle, but rather, a con- fluence of cancellous bone between the sacral body and the ala. Compared to pedicles in the mobile spine, this area is ca- pacious. The S1 pedicle has a mean length of 46.9 3.3 mm in women and 49.7 3.7 mm in men and is angled roughly 40 degrees from the midline ( FIG 1 ). 29

Internal iliac artery and vein

Obturator n.

Colon

FIG 1 ● Cross-section of sacrum. The bone density is greatest in the pedicle and body of the vertebrae ( A ) and lowest in the ala ( B ). Arrow marks the location of S1 pedicle.

FIG 2 ● Important anatomic structures overlying the sacrum.

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