Complex Lumbosacropelvic Fixation Techniques

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Chapter 152 Complex Lumbosacropelvic Fixation Techniques

Diseases of the sacrum and lumbosacral junction (LSJ) lead to clinically complex problems for surgical treatment and biomechanical stabilization. Trauma, infection, degenerative disease, and scoliosis (congenital or acquired) are among the common entities affecting the sacrum and LSJ. Although less common, neoplasms of this area often are especially challenging for postresection reconstruction. The sacrum and dorsal pelvis are also important points of fixation in the treatment of similar disorders at higher spinal regions in which long instrumentation constructs are required.

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

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).

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.

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.

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.815 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.

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.

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.2025 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.

Fixation with Sublaminar Devices

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