11.4.5\ Dynamic Rods

11.4.5.1\ Discussion

Dynamic posterior stabilization with pedicle fixation, such as Dynesys, consists of a semirigid fixation system that allows minimal movement between two segmental pedicle screws compared to a rigid metal rod and is used to treat lumbar spinal stenosis and degenerative spondylosis. Dynesys comprises and employs two titanium

a

pedicle screws at each treated level. The screws at adjacent levels are connected by rods comprised of radiolucent polyethylene terephthalate cord surrounded by a polycarbonate urethane spacer, which appears as a two concentric rings, slightly more hyperattenuating centrally (Fig. 11.89). Complications include screw loosening, screw breakage, and degeneration in the adjacent levels in up to approximately 50% of cases.

b

c

e

Screw: Anchors the system to the spine through the pedicles

Fig. 11.89  (continued)

d

11.5\ Failed Back Surgery

Syndrome and Related Spine

Surgery Complications

11.5.1\ Overview

Failed back surgery syndrome (FBSS) is a clinical entity that describes the persistence of lumbosacral pain following surgical intervention. Etiologies include structural abnormalities in the back, psychosocial influences, or a combination of these. Imaging plays an important role in eval-

uating patients with FBSS. In particular, a comprehensive and systematic assessment of the postoperative spine includes a review of the neural and vascular structures, including the neural foramina, thecal sac, spinal cord and cauda equina, hardware, and adjacent structures such as the major abdominal vessels, psoas musculature, posterior mediastinum, and prevertebral soft tissues. Some of the causes of FBSS and related complications of spine surgery in general that can be identified on imaging are summarized in Fig. 11.90 and in the following sections.

11.5.2\ Hardware Malpositioning

and Migration

11.5.2.1\ Discussion

The incidence of malpositioned pedicle screws ranges from 4% to 16%. Medial ­malpositioning of the screw can result in spinal cord or nerve root injury, while screws that extend too far beyond the vertebral body can injure the great vessels (Figs. 11.49 and 11.91). However, clinically significant sequelae of screw malpositioning, such as dissection, rupture, or pseudoaneurysm formation, are rare. CT with multiplanar reformats is useful for initial evaluation of screw position. CTA can be used to evaluate for significant vascular­ injury, and MRI or CT myelogram can help assess for nerve root compression.

a

Postoperative spinal hardware displacement can be a significant complications that can produce pain and new neurological symptoms. Retropulsion of interbody fusion devices and grafts most commonly occurs at L5–S1 and is associated with a wide disc space and multilevel fusion surgery. However, these devices can also become displaced anteriorly, especially if there has been a disruption of the anterior longitudinal ligament. Radiographs or CT can often adequately demonstrate the displacement of these materials into the spinal canal (Fig. 11.92).

Spinal rods can potentially become dissociated from the screws and migrate out of position. The rods have been reported to migrate into the spinal canal, retroperitoneum, and lower extremities. Radiographs can be used to screen for rod displacement (Fig. 11.93).

b

Fig. 11.92  Interbody fusion device retropulsion. The patient is status post anterior-posterior lumbar fusion arthrodesis at L4–L5 and L5–S1 with placement of biomechanical prosthetic interbody fusion device (Pioneer

bullet-tip). Axial (a) and sagittal (b) images of the lumbar spine show retropulsion of the interbody box prosthesis into the spinal canal (arrow)