Axial loading of the spine causes a ventral rotational tendency of the sacrum at the sacroiliac joint. The center of rotation falls near the center of the S2 vertebral body. The sacrospinous and sacrotuberous ligaments resist such rotation and transfer the pivot point (center of rotation) to the first mobile segment, L5-S1. The authors found maximum load to failure with constructs having a purchase between the iliac cortices down into the superior acetabular bone.¹

As spinal constructs extend proximally, the lever effect becomes increasingly stronger requiring additional distal fixation. Pelvic and sacropelvic fixations have gained popularity over the last two decades. Many techniques have been described in the literature with varying degrees of difficulties and biomechanical properties.²,³ A major determinant of the mechanical strength of the iliac or sacroiliac fixation is the relationship of the tip of the distal fixation to the center of rotation that is located at the posterior third of the L5-S1 disk. The longer the screw, the more anterior it is and the better the fixation is.¹ O’Brien et al.⁴ divided the sacropelvis into three Zones. Zone I is comprised of the S1 body and cephalad margin of sacral alar, Zone II extends from the inferior margins of the sacral alar, S2 to the tip of the coccyx, and Zone III includes the ilium. The fixation strength improves from Zone I to Zone III.

Pelvic instrumentation gained popularity following the description of the Galveston technique comprising fixation points between the inner and outer tables of the iliac bone.⁵,⁶

Disadvantages of the traditional iliac bolt usage have been observed over the past decades and include painful hardware prominence, wound dehiscence, pseudarthrosis, iliac screw loosening, hardware breakage, alignment difficulty with proximal lumbar hardware, and the need for offset connector use.⁷ Alternative techniques, such as S2-alar-iliac (SAI) screws and minimally invasive iliac screw placement have been introduced to minimize the complications associated with the conventional pelvic fixation, namely soft tissue dehiscence, hardware prominence, and hardware alignment.⁸, ⁹, ¹⁰, ¹¹

This chapter will describe a percutaneous iliac fixation technique developed by the authors. The percutaneous iliac screw fixation is preferred over the S2 or S2Alar screws since the advocated use of connecting iliac rod adds another degree of freedom between the long segment construct and the iliac fixation which can be challenging in alternative sacropelvic fixation options.