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Chapter 30 Sacropelvic Fixation Techniques

Supraspinous ligament

Ventral sacroiliac ligament

Greater sciatic notch

Iliac fossa

Anterior superior iliac spine

Dorsal sacroiliac ligament

Acetabulum

Sacrotuberous ligament

Obturator foramen

Ischial tuberosity Pubis symphysis

Anterior inferior iliac spine

Lesser sciatic notch

Sacrospinous ligament

FIG 3 ● Ligamentous support and bony anatomy of the pelvis.

The Sacroiliac Joint

and torsional forces from the spine above. These forces cause motion at the junction that may increase the risk of pseudarthrosis. 4,7 ■ Furthermore, the density of bone in the sacrum is poor, and obtaining adequate fixation is a particular challenge. 10 ■ McCord et al 18 introduced the concept of the lumbosacral pivot point ( FIG 4 ), which is defined as the middle of the os- teoligamentous column at the junction between L5 and S1. ■ The farther that the pelvic implant progresses anterior to this point, the more stable the construct. Furthermore, in- strumentation that crosses the SI joint is not effective unless it passes anterior to this pivot point. 18 ■ O’Brien et al 24 introduced the concept of three zones of sa- cropelvic fixation ( FIG 5 ). Fixation in zone 3 has the highest biomechanical strength and allows placement of the instru- mentation farthest anterior to the pivot point.

■ The SI joint is an L-shaped synovial joint with an irregularly contoured surface that interlocks in order to resist move- ment. The joint functions to transfer axial load from the torso onto the hemipelvises. ■ The joint is stabilized by the anterior SI ligament, interosse- ous SI ligament, and the posterior SI ligament ( FIG 3 ). ■ Fusions across the lumbosacral junction are a particular challenge for spine surgeons and this location has a high incidence of pseudarthrosis. 11 ■ Substantial biomechanical forces are concentrated at the lumbosacral junction. The fusion mass above the sacrum acts as a long lever arm that transmits flexion, extension, BIOMECHANICS

Pivot point

S2 screw

S1 screw

Iliac screws

Axis of M.C.

Pivot point

Lever arm

S1 pedicle screw

S2 pedicle screw

B

Iliac screw

A

FIG 4 ● Lumbosacral pivot point. Lateral ( A ) and axial ( B ) views of the pivot point.