Anatomic and Biomechanical Considerations

The LSJ is a unique spinal level in several respects.¹ In the sagittal or flexion-extension axis, it has the largest range of motion of any thoracic or lumbar level, averaging 17 degrees of total movement. In the axial plane and during rotation and lateral (coronal plane) bending, the LSJ has the most limited range of motion of any spinal level, averaging 1 degree of rotation and 3 degrees of bending, respectively.² Because of the normal lordotic curvature of the lumbar spine, the slope of the lumbosacral intervertebral disc (L5-S1) is usually the steepest of any disc, with respect to the true horizontal. The summation of spinal load vectors results in exposure of the lumbosacral disc to the largest loads encountered throughout the spine. The large loads carried and the angular position of the disc at the LSJ produce unique load-bearing characteristics, including the highest level of translational shear force in the entire spine (Fig. 152-1).^1,^3,⁴

FIGURE 152-1 The L5-S1 disc space is the most vertically oriented of any disc space in the spine (arrow depicts approximate angle formed by the disc space). This predisposes the L5-S1 level to unique load-bearing characteristics.

Sacrum

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 lumbosacralpelvic 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 (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 iliacs 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 wedgelike shape of the sacrum helps stabilize the SIJ and serves to transfer loads from the spine to the pelvis (Fig. 152-2).

FIGURE 152-2 The keystone configuration of the pelvis allows the transfer of weight from the spine to the pelvis and ultimately to the lower extremities.

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.⁵ Another cadaveric study has demonstrated that in adults older than 50 years of age, autofusion of the joint is observed in 75% of specimens.⁶

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. 152-3). The interosseous, sacroiliac, and dorsal sacroiliac complex provides the major stabilization for the SIJ.

FIGURE 152-3 Dorsal (A) and ventral (B) views of the major ligamentous attachments of the sacroiliac joint.

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. 152-4). 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.

FIGURE 152-4 The center point of the rotational vector is located near the center of the S2 vertebral body. The diagram depicts the rotational tendency of the sacrum about this point. It is through the long lever arms created by the sacrospinous and sacrotuberous ligaments that this rotational tendency is counteracted and the sacrum is stabilized.

Muscular Interactions

The musculature of the lumbosacralpelvic region acts on the spine in a complex multidirectional fashion. The muscles and the weight of the upper body act in many instances via long moment arms and may place substantial forces on the spine. An example of such action is the force exerted on the spine by the rectus abdominis musculature. These muscles act by a moment arm extending from the pubic symphysis to the sternum, producing a spinal vector toward kyphosis. A pendulous abdomen does the same in providing a constant exaggerated spinal load in the upright position and to some extent during sitting.

In the resection of sacral tumors, stability of the sacropelvic region can be jeopardized because portions of the sacrum and possibly the SIJ are removed. Resection of the caudal portion of the sacrum up to the S1-2 interspace and removal of up to one third of the SIJ can be performed with only a 30% loss of weight-bearing capacity. The lower half of the S1 body and up to one half of the SIJ can be resected with a 50% loss of weight-bearing stability.⁷ Preservation of 50% of weight-bearing capacity is adequate for early ambulation in the postoperative period, and further stabilization is not likely to be necessary. In general, in cases of tumor or other destructive lesions of the sacral (e.g., infection), the bilateral alae should be evaluated. If one ala or more than 50% of bilateral alae are destroyed, the patient will require lumbopelvic fixation.

Indications for Lumbosacralpelvic Fixation

In short-segment cases and in the absence of osteopenia, sacral fixation with a single pair of bone screws is adequate. In longer-segment cases (e.g., scoliosis, postsacrectomy reconstruction, and multisegmental lumbosacral fusion) or with osteoporotic bone, more substantial segmental fixation is required to achieve rigidity. In addition, when high-grade lumbosacral spondylolistheses (grades III and above) are reduced, standard sacral screws may be inadequate and lead to loosening or sacral fracture. Rigidity is a crucial element in these constructs because fusion rates are directly related to use of rigid instrumentation, and better outcomes clearly correlate with the acquisition of a solid fusion.⁸^–¹⁵ If a long instrumentation construct is placed, the sacral attachment is usually subjected to large cantilevered forces that may lead to screw pull-out (Fig. 152-5). Additional points of sacral or sacropelvic fixation may prevent complications in such cases.

FIGURE 152-5 As the dorsal construct becomes longer, the potential cantilevered forces on the sacral attachment are increased (dashed lines). This predisposes to sacral screw pull-out. Additional points of fixation may thus be required (shaded screw).

Instrumentation may be used in compression or distraction to reduce deformity. Distraction, in particular, may place a substantial stress on the implants, in addition to the physiologic loads that will be exerted by the daily activities and movements of the patient. This stress constitutes implant preload. Instrumentation that will bear a significant preload may require either further sacral fixation points or attachments that cross the SIJ. If the preload is not symmetrically distributed, as in the case of scoliosis correction, the additional instrumentation does not necessarily need to be placed bilaterally but should be included on the side that will bear the larger load. If significant pelvic obliquity is present, as occurs commonly with scoliosis of neuromuscular origin, the construct should cross the SIJ in most instances and should be symmetrical.

The SIJ is autofused in many adults older than 50 years of age. Long-term follow-up study of patients with instrumentation constructs crossing the SIJ has demonstrated no adverse effects relating to the presence of the implants.¹⁶ Therefore, if it is necessary for additional security of fixation in the lumbosacropelvic region, placement of instrumentation across the SIJ is a rational approach for providing spinal stability.

Lumbosacral Pivot Point

In a study of the biomechanics of sacropelvic fixation, McCord et al. described the concept of the lumbosacral pivot point.¹⁷ This is the axis of rotation at the lumbosacral junction. During flexion, the portions of L5 and the sacrum that are ventral to this pivot move toward one another. Likewise, the portions of L5 and the sacrum located dorsal to this pivot point will move apart during flexion (Fig. 152-6). Anatomically, the lumbosacral pivot point is marked by the intersection of the middle osteoligamentous column and the lumbosacral (L5-S1) disc. In constructs that cross the SIJ, only those devices that pass ventral to this point provide a significant biomechanical advantage regarding rigidity of fixation.

FIGURE 152-6 As the lumbosacral joint is flexed, those points located ventral to the pivot point (dot) move toward each other, whereas those dorsal to it separate.

Complex Techniques of Sacral Fixation

Many lumbosacral fusions can be adequately immobilized with placement of bone screws into the sacral pedicles. These screws, however, obtain their thread purchase in the broad cancellous channel of the sacral pedicle. Therefore, bone screws in the sacral pedicles are subject to failure because of the relative porosity of the sacrum, the manner in which stress tends to be concentrated at the termini of a fusion construct, and the large flexion moments to which these constructs are subjected.^18,¹⁹ Sacral screws may fail by pull-out or by fracture.¹⁸ In cases in which it is believed that the use of a single pair of bone screws may not be adequate for stabilization, the use of more complex techniques is warranted.

With regard to injuring structures ventral to the sacrum, cadaveric studies have shown that the widest margin of safety is found at the medial safe zone (Fig. 152-7).²⁰ Therefore, placing the screws in a medial or toed-in direction is preferred at the S1 or promontory level. Some authors have advocated bicortical purchase of sacral screws to enhance pull-out resistance, which affords some pull-out strength advantage, although this involves additional risk.²⁰^–²⁵ Zindrick et al. found that bicortical purchase with a 6.5-mm diameter screw resulted in an increase in pull-out strength of about 30%.²⁵ Penetrating an excessive distance beyond the ventral cortex carries the risks of neurologic deficit, chronic pain from lumbosacral trunk injury, sympathetic chain injury, peritonitis, sepsis, and hemorrhage,^20,²⁴ although these risks are minimal if the screw penetrates 1 cm or less.