11.3.8\ Posterior Fusion

11.3.8.1\ Discussion

Most thoracic and lumbar spine fixation is performed via a posterior approach, most commonly using rods and pedicle screws. Pedicle screws attach posteriorly to rods or plates via clamps or bolts and have shallow cancellous threads that pass through the pedicle and into the vertebral body. The screw should enter 50–80% of the vertebral body and be parallel to the endplates with at least 2 mm of separation. The screws can produce considerable beam-hardening artifacts on CT, which can be minimized through the use of metal artifact reduction software (Fig. 11.47). On MRI, the pedicle screws can produce variable degrees of metal susceptibility artifact that can obscure adjacent structures, which is more pronounced at higher magnetic field strength (Fig. 11.48). Imaging via CT is sometimes performed in order to assess whether the screws are malpositioned (Fig. 11.49). Medial malpositioning is a potentially devastating complication that can result in spinal cord or nerve injury. Laterally malpositioned screws can injure exiting nerve roots. Pedicle screws can also potentially cause vascular injury, such as the aorta or inferior vena

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cava, if they are too long and exit the anterior vertebral body. Although transdiscal screws are sometimes used for fixation in scoliosis surgery, penetration into the disc space is generally avoided. Another option for securing rods is through lateral mass screws, which are situated between the superior and inferior articular processes, thereby lowering the likelihood of the types of malpositioning associated with pedicle screws (Fig. 11.50).

Instead of screws, rods can also be secured to the vertebrae via sublaminar wires or cables (Fig. 11.51). Sublaminar wires or cables pass around the lamina and rods and are twisted or clamped at their ends. Alternatively, laminar or sublaminar hooks can be used for compression or distraction. Hooks that pass below the lamina are termed up-going, while those that pass above the lamina are termed down-going (Fig. 11.52). These two configurations are usually applied simultaneously for optimal stability. The hooks are connected to the rods via screws, bolts, or washers. Facet screw fixation is an alternative to pedicle screw fixation whereby the articular facets are fused. The screws are not attached to rods but may be used in conjunction with interbody fusion or anterior plating (Fig. 11.53).

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Fig. 11.47  Metal artifact reduction software. Axial CT image (a) shows extensive metal artifact associated with the surgical hardware. The corresponding axial CT

image with metal artifact reduction software (b) shows much less artifact

Fig. 11.48  Spine hardware artifacts at 3 T versus 1.5 T. Sagittal T2-weighted MRI performed on a 3 T scanner (a) shows extensive susceptibility artifact related to

the pedicle screws, which obscures the surrounding anatomy. Sagittal T2-weighted MRI performed on a 1.5 T scanner (b) shows much less artifact from the hardware

Fig. 11.49  Malpositioned screws. Coronal CT image (a) shows a screw that breaches the superior endplate and enters the intervertebral disc space. Axial CT image (b) in another patient shows medial malposition of the right tho-

racic pedicle screw into the spinal canal (arrow), resulting in paraplegia. Axial CT images (c and d) in another patient show screws impinging upon the aorta, which contains an endograft and impinging upon the trachea

Fig. 11.52  Pedicle hooks. Sagittal CT image shows upgoing and down-going hooks secured to the laminae and attached to a rod

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Fig. 11.53  Facet screws. Frontal radiograph (a) and axial CT image (b) demonstrate bilateral L5–S1 facet screws. Anterior fusion was also performed