Historical Overview

Fusions from the spine to the pelvis have historically been difficult to achieve because of the high stresses produced at this pivot point between the trunk and lower body.2 The evolution of surgical techniques for SPF largely reflect attempts to prevent implant failures, pseudarthrosis, and truncal imbalance. Before the era of Harrington instrumentation, SPFs were done with autologous bone grafting and body casting, with resultant pseudarthrosis in up to 83% of cases.2–4 In the 1970s and 1980s, Harrington distraction instrumentation was used with sacral hooks or a transiliac bar in an attempt to obtain SPF. The distraction technique frequently led to a loss of lumbar lordosis (flat back) in up to 49% of cases and pseudarthrosis in 20 to 40% of cases ( Fig. 23.1 ).5 The modified Luque technique for SPF with sub-laminar wires and a contoured L-rod driven into the ilium for SPF6,7 overcame the problem of inducing flat back by distraction, and introduced segmental fixation; however, the wire and pelvic anchors used in this technique lacked sufficient bending and torsional rigidity to consistently overcome scoliosis and pelvic obliquity.8 Therefore, early techniques of fixation to the pelvis had low rates of successful fusion and the restoration of global balance.

The SPF techniques developed by Dubousset and Cotrel, Jackson, and Allen and Fergusen are the foundations of modern iliac fixation ( Fig. 23.2 ).9–12 Dubousset and Cotrel utilized iliosacral screws directed across the posterior part of the sacroiliac (SI) joint and into the S1 centrum for distal fixation, and coupled these screws to rods on either side of the midline of the spine with subsequent cross-linking to provide further rigidity.12 Jackson’s technique placed rods within the sacral ala and used the posterior sacral cortex and overhanging posterior iliac wings as a buttress to neutralize bending stress.9 The insertion point of the rod was caudal to S1 so that the rod could be coupled to S1 pedicle screws. The Jackson technique is technically demanding and more susceptible to bending failure than coupling of the rod to iliac screws or to parts of sacral screw.13 The Galveston technique, introduced by Allen and Ferguson, uses bilateral L-shaped rods with the horizontal part of the “L” driven into the iliac wings for pelvic fixation, whereas the vertical component provides the spinal fixation.11 Variations of this technique, using rods or screws passed between the iliac tables, have been used extensively for SPF with long fusions in neuromuscular and adult deformities.1 McCarthy and colleagues developed an S-shaped rod positioned with the short arm of the “S”-rod placed over the sacral ala from posterior to anterior and coming to rest on the anterior ala, whereas the long arm of the “S” ran along the spinal axis. This S-rod was specifically designed to resist flexion in patients with neuromuscular conditions, particularly meningomyelocele associated with lumbar or thoracolumbar kyphosis.10

The specific aspects of these older SPF techniques that persist today include multiple points of fixation (segmental fixation), chiefly with iliac and sacral screw-anchored rods, augmented anterior column support, and cross-linking of rods to increase rigidity of the construct. Compression forces and restoration of coronal and sagittal truncal balance are recognized as crucial for minimizing complications with such techniques. Nevertheless, despite the use of modern techniques, fixation to the pelvis remains a challenge. As will be discussed, even in the most recent series, rates of pseudarthrosis and complication remain above 20%.14–18 As a starting point, the anatomy and biomechanics of lumbosacral fixation will be considered.

Anatomy

The keystone configuration of the sacrum between the iliac wings provides stability in the face of extremely high stresses as weight is transferred from the spine to the pelvis and lower limbs ( Fig. 23.3 ). In the sagittal plane, the C7 plumbline falls near the posterior aspect of the L5–S1 disc, defining it as the instantaneous axis of rotation (IAR) for flexion and extension in the lumbosacral spine. The L5–S1 disc is the most vertical intervertebral segment, with the superior endplate of S1 tilted an average of 40 degrees to the horizontal. Consequently, the lumbosacral junction is subject to an enormous amount of stress including bending, shear, and rotational stresses. Consequently, it is logical that the L5–S1 intervertebral segment and lumbosacral junction are not only the areas of the spine most commonly developing degenerative changes, but also the hardest areas to fuse.

The SI joint allows relief of stress from forces coming across the upper three sacral segments and the ilium. Strong ligamentous connections between the spine, sacrum, and ilium include the iliolumbar ligaments, sacroiliac ligaments (posterior and anterior sacroiliac ligaments), and sacrospinous and sacrotuberous ligaments. The SI joint is stabilized anteriorly by the ventral SI ligaments and the sacrospinous ligament and posteriorly by the sacrotuberous and dorsal SI ligaments. The dorsal portion of the SI joint is fibrocartilaginous and is the preferred site into which to pass iliosacral screws ( Fig. 23.4 and Fig. 23.2A ).19 An iliosacral screw typically enters the outer table of the ilium at 1 cm below the iliac crest and in line with the S1 superior articular facet, and is directed at a 45-degree angle to the sagittal plane, exiting in the iliosacral space posterior to the SI joint ( Fig. 23.2A ). The screw then proceeds into the S1 pedicle, parallel to the S1 end-plate and toward the promontory.1

The sacrum is initially composed of five vertebrae, which fuse together as a solid bone in adults ( Fig. 23.5 ). Caudally, the sacrum articulates with the coccyx, which has from three to five segments. The sacrum is thickest in the midsagittal plane at S1, where it averages 50 mm thick, but tapers to a thickness of 20 to 30 mm at S3. The sacral canal is continuous with the lumbar canal and is covered by the lamina. The laminae typically join in the midline to form the spinous processes; however, ~10% of patients have a bifid S1 or S2.

The sacrum has five posterior crests ( Fig. 23.5 ). The median sacral crest is the midline ridge of the spinous processes, which extends down to the sacral hiatus and the termination of the dural sac at S3. The rudimentary facets form the two intermediate crests or ridges with four neuro-foramina located just lateral to them. Dorsal sensory rami with vessels penetrate the dorsal neuroforamina of the sacrum. There are also two lateral sacral crests, located lateral to the neuroforamina and medial to the ilia, which are the equivalent of transverse processes fused together.

The sacral alae are bound by the intermediate and lateral sacral crests and are a key location for fixation. The upper sacral alae are the structures over which McCarthy rods are passed ( Fig. 23.2b ) and where Harrington sacral hooks were inserted, and are used for bone grafting up to the lumbar transverse processes. Also, Jackson’s intrasacral rod technique for SPF involves having the lower portions of the rods inserted into the sacral ala below the S1 pedicle and lateral to the S1 dorsal foramina, with the rods directed in-feriorly, laterally, and anteriorly toward the SI joint for a distance of ~30 mm ( Fig. 23.2a ).9

Fixation techniques for SPF often involve penetration of the anterior cortex of the sacrum and in some cases anterior approaches to the lumbosacral area are used for fusion. Therefore, the neurovascular anatomy of the lumbosacral region must be taken into account for both implant placement and surgical approaches to lumbosacral fusion ( Fig. 23.6 ). The dominant motor–sensory nerve roots in this area exit through the anterior neuroforamina to form the lumbosacral plexus, with portions of L4, L5, and S1 nerve roots passing inferiorly and laterally across the anterior sacral alae. Typically, the bifurcation of the aorta and vena cava into the common iliac vessels occurs anterior to L4 or at the L4–L5 disc, whereas the bifurcation of the common iliac vessels into the internal and external iliac vessels is located over the lateral edge of S1, near the anterior SI joint.20–22 The internal iliac vessels, after arising from the common iliac vessels, lie directly anterior to the ala. The middle sacral artery comes directly off the aorta to lie on the mid sacrum. Also over the midsacrum is the presacral parasympathetic plexus, which is important for sexual function. Damage to this plexus can result in male impotence and retrograde ejaculation.

Given this description of the anterior lumbosacral anatomy, one can see that the safest projection of screws and safest surgical approaches to lumbosacral fusion lie in the midline. For example, S1 pedicle screws should be directed toward the sacral promontory and midline approaches to L5–S1 are preferentially made between the common iliac vessels. If laterally directed screws are used (e.g., alar screws), they should be directed toward the anterior SI joint and not penetrate excessively through the anterior cortex, to avoid injury to the internal iliac veins and roots of the lumbosacral plexus.23 Blunt self-tapping screws are preferred.

Another aspect of spinopelvic anatomy to keep in mind when approaching the anterior lumbosacral spine is that the left common iliac vein is medial to the artery, whereas the right common iliac artery is medial to the right common iliac vein. Theoretically, a right-of-midline approach to the L5–S1 disc is recommended for keeping away from the left common iliac vein.22 However, the left paramedian retroperitoneal approach allows both midline access (between the vessels for L5–S1) and a left lateral approach to the L4–L5 disc through ligation of the L4 and L5 lumbar vessels. The surgeon must be aware that variations of this vascular anatomy exist.21

Osteology

The shape and bone quality of the lumbosacral region provide challenges for secure fixation. The lumbar facets are large and sagittally oriented, although in cases of dysplasia the L5–S1 facet can be more horizontal. Under normal circumstances, the L5–S1 facet is quite large and facet screws were one of the first forms of local internal fixation used in the spine.1,2,24 The screws are inserted at the base of the spinous process and directed laterally across the facet and into the base of the transverse process or ala. This technique has only been tested for short lumbosacral fusions, and is not strong enough as a main fixation method in the long constructs related to scoliosis and SPF.24

The junction of the superior S1 endplate with the anterior S1 cortex forms the sacral promontory and is the site of greatest bone density and the strongest fixation point in the sacrum.25–27 Bone density in the midsacral ala is 30% less than in the S1 body, and within the substance of the ala there is a particular bone void that makes strong fixation at this location unlikely.28,29 The best bone in the ala is found at the intersection of the lateral ala and anterior sacral trabecule, anterolateral sacral cortex, and the anterior SI joint ( Fig. 23.7 ). Lateral alar screws should therefore be directed to this bone.

For the average adult, the bone dimensions of the L4 and L5 pedicles are ~10 mm by 10 mm in the coronal plane, with L5 being wider than L4. The transverse angle increases caudally, being ~20 degrees for L4 and 30 degrees for L5 and S1.30 The S1 pedicle is quite large, lying between the intermediate crest (neural canal) and lateral crest (the SI joint) in the region superior to the S1 neuroforamen. Two screw trajectories can be used in the region of the S1 pedicle: one medial and toward the promontory, and one lateral and toward the anterolateral cortex of the ala, as described above ( Fig. 23.7 ). Because the S1 pedicles are large, pedicle screws will not necessarily achieve ideal purchase in the cortical bone of the pedicle. The medial S1 pedicle–screw trajectory enters lateral to the S1 articular process and is directed medially (by 20 to 30 degrees) and upward (by 10 to 20 degrees) toward the promontory, to engage the cortex and achieve tricortical purchase. The screws engage the junction of both the anterior cortex of the promontory and the S1 endplate ( Fig. 23.4 ). The medial trajectory at S1 maximizes screw length, allows the triangulation of implants, and provides stronger fixation than the lateral S1 screw trajectory ( Fig. 23.7 , 23.8 ).27,30,31,32,34,35

The lateral trajectory involves an alar screw inserted over the S1 pedicle area and directed 30 to 40 degrees laterally, toward the junction of the ala with the SI joint in a dorsal-to-ventral direction ( Fig. 23.7 ). As discussed, penetration of the ala endangers the lumbosacral plexus (L4 and L5 nerve roots) and the internal iliac veins, and should be done cautiously and with blunt screws.20,23 The laterally directed screws are not as strong as those directed into the promontory, but triangulating these two screws will provide stronger fixation than using either one alone.33

The S2 pedicle is bounded superiorly and inferiorly by the S1 and S2 neuroforamina, medially by the spinal canal and the intermediate crest, and laterally by the lateral crest ( Fig. 23.5 ). Because of the tapered shape of the sacrum, possible screw lengths for sacral fixation decrease rapidly from S1 to S3. Although 40- to 50-mm screws can often be placed in S1 when a medial trajectory is utilized, screws in S2 and S3 may be limited to a length of 25 to 30 mm. As described earlier, sacral bone density is greatest along surfaces where cortical and cancellous bone merge.28 S2 screws are relatively safe, although penetrating the anterior cortex excessively, particularly on the left, has the potential to damage the sigmoid colon.23

The ilium of the pelvis extends posterior to the sacrum as the posterior-superior iliac spine (PSIS), and overhangs the SI joint medially ( Fig. 23.4 and Fig. 23.7 ). A dorsal midline incision is often used to expose the iliac crest, PSIS, and sciatic notch for instrumentation or bone grafting. Subperiosteal exposure of the outer ilium will help avoid injury to the superior gluteal nerve and vessel, located around the sciatic notch. The PSIS of the ilium has a thickness of up to 25 mm and is located inferior to the origin of the dorsal S1 pedicle, with the result that iliac screws will be caudal to the S1 pedicle screw. The PSIS serves as the entry point for the iliac screws, which are directed toward the anterior inferior iliac spine, tracking to within 1.5 cm above the sciatic notch, to capitalize on the thick, trabeculated bone in this region of the ilium ( Fig. 23.4 ). When this track is followed, the screws can be up to 10 mm in diameter and 15 cm in length for adults, and at least 7 mm in diameter and 8 cm in length for most adolescents.34,35 Screw fixation in this iliac column is more secure than is the smooth Galveston-rod technique,35 but crossing the SI joint can lead to pain. As will be discussed, the mechanical advantages of this approach outweigh its disadvantages.