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Complex Lumbosacropelvic Fixation Techniques
Summary of Key Points
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Diseases of the sacrum and lumbosacral junction lead to clinically complex problems for surgical treatment and biomechanical stabilization.
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The lumbosacral pivot point, the axis of rotation at the lumbosacral junction, is marked by the intersection of the middle osteoligamentous column and the lumbosacral (L5‒S1) disc. In constructs that cross the sacroiliac joint, only those devices that pass ventral to the lumbosacral pivot point provide appreciable biomechanical fixation.
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Sublaminar devices, S1 or S2 pedicle (medially) or alar (laterally) screw fixation, iliac screw fixation, or fixation across the sacroiliac joint can be performed to produce stabilization or reconstruction. Cross-linking the longitudinal members of fixation creates a triangulation effect that enhances the rigidity of the construct, pullout resistance, and torsional stability.
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Longer-segment cases (e.g., scoliosis, postsacrectomy reconstruction, and multisegmental lumbosacral fusion), osteoporotic bone, osteoarthritis of the hips, prior surgery, smoking, obesity, osteoarthritis, and poor technique in instrumentation placement can decrease fusion rates.
Diseases of the sacrum and lumbosacral junction (LSJ) may lead to clinically complex problems for surgical treatment and biomechanical stabilization. Trauma, tumor, infection, degenerative disease, and scoliosis (congenital or acquired) are among the common entities affecting the sacrum and LSJ. Neoplasms of this area (e.g., chordoma) and infections that result in erosion of the LSJ often are especially challenging, as their treatment may require sacrum resection and pelvic ring destabilization. These bony landmarks provide the main points of fixation for lumbosacral constructs, as well as for longer thoracolumbosacral constructs, such as those required for the treatment of significant spinal deformities.
Anatomic and Biomechanical Considerations
The LSJ is a unique spinal level in several respects. Kinematically, the LSJ shares some similarities to the thoracic spine. Similar to the thoracic spine, motion at the LSJ occurs largely in the sagittal axis, and in some respects its physiological movements can be thought of as an extreme version of those seen at the thoracic levels. Notably, the LSJ has the largest range of flexion-extension motion of any thoracic or lumbar level, averaging 17 degrees of total movement, and it has the most limited range of motion in terms of rotation and lateral bending, averaging 1 and 3 degrees, respectively. Anatomically, the LSJ has three unique features that must be considered when designing a biomechanical construct. First, the lumbosacral intervertebral disc (L5‒S1) is invariably the steepest disc with respect to the true horizontal, creating the highest shear forces in the thoracolumbar spine ( Fig. 133.1A ). , Second, the vertebral pedicles are most obliquely oriented at the LSJ, which requires more extensive muscular dissection to expose entry points for screw placement. Last, the LSJ is bounded laterally by the ilia, making it more difficult for the operating surgeon to lateralize instruments for pedicle screw placement. In addition to these considerations, 7% to 19% of the population is known to have aberrant lumbosacral anatomy. Specifically, up to 7% of the population and 15% of patients with low back pain possess a sixth lumbar vertebra. , This can result in incorrect intraoperative localization and should be considered in all patients being evaluated for lumbosacral surgery.
Sacrum
The posterior aspect of the sacrum is convex with a triangular shape, and the lateral sacral wings are covered by the iliac wings. The sacrum is formed from five fused vertebrae in which the specially adapted and large transverse processes merge into thick lateral masses, the alae. The sacral spinal canal has four pairs of dorsal and ventral foramina. The subdural and subarachnoid spaces terminate as the thecal sac tapers at the caudal margin of S2. The filum terminale internum is an extension of the pia arachnoid of the conus medullaris, extending from the tip of the conus to the end of the subdural space. At the termination of the subdural space, the thecal sac tapers to invest the filum terminale internum and form the filum terminale externum. The filum terminale externum extends to the end of the sacral canal and attaches to the rostral portion of the coccyx.
Structures Adjacent to the Sacrum
For the safe placement and attachment of instrumentation constructs in the lumbosacropelvic region, a thorough knowledge of the anatomic relationships of the neural, vascular, and visceral structures in the region is important. The common iliac arteries begin at the aortic bifurcation (typically at L4 level) and pass along the lateral surface of the L5 vertebral body. They then bifurcate at the level of the LSJ, giving rise to the internal and external iliac arteries. The iliac arteries lie ventral and lateral to the iliac veins and therefore do not actually make contact with the spine. The internal and external iliac arteries pass ventral to the sacral alae. The internal iliac arteries pass close to the bony surface of the ala, whereas the external iliac arteries are separated from the bony surface by the psoas muscles. The lumbosacral trunk is formed by the ventral branches of the L4 and L5 nerve roots. It is joined by the sacral nerves located on the ventral surface of the alae between the iliac veins and the sacroiliac joint (SIJ). The sigmoid colon is also found in approximation to the ventral surface of the sacrum. It loses its mesentery and becomes far less mobile as it reaches the ventral aspect of the S3 vertebral body and becomes the rectum.
Sacroiliac Joint
The SIJ is formed by the interdigitating surfaces of the sacral alae and the iliac bones. It is predominantly a fibrocartilaginous amphiarthrodial (no synovial capsule) joint. There is a small diarthrodial (synovial capsule present) portion located at the ventral aspect of the SIJ. The interdigitation and matching contours of the iliac and sacral alar surfaces create an interlocking mechanism to help stabilize the joint. The wedge-like shape of the sacrum helps stabilize the SIJ and transfers loads from the spine to the pelvis (see Fig. 133.1B ).
The SIJ is essentially an immobile joint that functions as a shock absorber for the spine. In studies on fresh cadavers, there was minimal motion in pediatric specimens, and none in adults. Another cadaveric study demonstrated that, in adults older than 50 years of age, autofusion of the joint is observed in 75% of specimens.
The major biomechanical function of the pelvis is that of transferring loads from the SIJ to the hip joints. The stable transfer of these loads is dependent on the ligaments connecting the lumbar vertebrae and the sacrum to the pelvis. The ligamentous structures spanning the SIJ include the interosseous, dorsal, and ventral sacroiliac ligaments ( Fig. 133.2 ). The interosseous, ventral sacroiliac, and dorsal sacroiliac complex provides the major stabilization for the SIJ. Most importantly, by providing a compressive force on the SIJ, they allow for transfer of axial loading forces from the spine to the ilia and hips bilaterally while also restricting movement at the SIJ.
The iliolumbar ligament passes from the transverse process of the L5 vertebra to the iliac crests. A less substantial part of the ligament may span to the transverse process of L4 as well. The position of this ligament allows a wide range of motion in flexion and extension across the LSJ, but it severely restricts lateral bending and axial rotation.
The force vector of axial load from the spine is located ventral to the SIJ. This causes a ventral rotational tendency of the sacrum at the level of the SIJ. The center point of this rotational vector is located near the center of the S2 vertebral body ( Fig. 133.3 ). The sacrospinous and sacrotuberous ligaments pass from the lower sacrum to the ischial bones. The position of these ligaments creates a long moment arm through which they are able to resist sacral rotation and are thereby able to maintain the lordotic lumbosacral posture despite the gravitational sagittal plane vector.
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