Posterior Scoliosis Correction: Pedicle Screws

Pedicle Screws

Se-Il Suk and Won-Joong Kim

Goals of Surgical Treatment

1. Halt the progression of the deformity.

2. Correct existing deformities.

3. Restore three-dimensional balance of the trunk and the spinal column, with minimum sacrifice of motion segments.

Diagnosis

Scoliosis is defined as a three-dimensional deformity of the vertebral column characterized by lateral deviation in the frontal plane with derangement in the sagittal plane (hypokyphosis, hyperkyphosis, hyperlordosis, or hypolordosis) and torsion in the horizontal plane. Depending on the etiology of the deformity, it is classified as idiopathic, congenital, neuromuscular, or neurofibromatosis, or of miscellaneous etiologies. Specific diagnosis is made by appropriate physical examination and radiologic studies including magnetic resonance imaging (MRI) of the central nervous system.

Indications for Surgery

1. Deformities with potential of adulthood progression

2. Idiopathic scoliosis

a Thoracic curve > 40 degrees

b Lumbar or thoracolumbar curve > 35 degrees

3. Congenital scoliosis

a Documented progression of more than 5 degrees on serial follow-up

b Deformities with known natural history of inevitable progression (e.g., unilateral unsegmented bar with contralateral hemivertebra, multiple unincarcerated unilateral hemiverterbae)

4. Progressive deformities from neuromuscular disorders, neurofibromatosis, and other etiologies

5. Significant derangement of spinal and trunk balance in coronal and sagittal planes

6. Significant thoracic hypokyphosis associated with coronal plane deformity

7. Poor trunk cosmesis

Contraindications

There is no absolute contraindication. Relative contraindications may include severe pedicle hypoplasia (e.g., neurofibromatosis); previous lengthy laminectomies; and anticipated future posterior surgeries (e.g., partially resected spinal cord tumors).

Advantages of Posterior Pedicle Screw Fixation for Scoliosis

1. Improved three-dimensional deformity correction and maintenance

2. Improvement of thoracic hypokyphosis

3. Shorter fusion, preserving additional motion segments

4. Easier surgery design (do not need complex instrumentation patterns)

Disadvantages

1. Potential neurovascular complications related to screw misplacement

2. Overcorrection of the instrumented curve

Procedure

Determination of fusion levels: Fusion levels are determined using 14” × 35” standing anteroposterior (AP) and lateral radiographs and appropriate side bending studies (Fig. 33–1). Single thoracic curves that do not extend into the lower lumbar spine (King types II, III, and IV): Selective thoracic fusion from one level above the upper end vertebra to the distal neutral vertebra on the standing radiograph.

Double thoracic curve (King type V): Fuse both the upper and the lower thoracic curve. When the lower curve do not extend into the lower lumbar spine, fuse to the distal neutral vertebra of the lower curve as in the single thoracic curve.

Structural curves involving the thoracolumbar or lumbar region (King type I, thoracolumbar, and lumbar curves): All structural curves are fused. Distal fusion to the bending stable vertebra that centralizes on sacrum and bisected by the center sacral line. If the bending stable vertebra does not derotate to less than Nash-Moe grade II on side bending, go down one level caudally. In essence, the distal fusion level for the pedicle screw fixation is identical to that of an anterior instrumentation.

Intraoperative neurologic monitoring: Neurologic complications related to pedicle screw instrumentation for scoliosis is extremely rare. However, intraoperative neurologic monitoring [e.g., somatosensory evoked potential (SSEP), multimodality evoked potential (MEP)] is a valuable assistance in making sure that everything is going fine.

Incision and exposure: A standard posterior midline incision is made from the upper end of the spinous process two levels above the uppermost pedicle instrumented to the lower end of the lamina of the lowest instrumented vertebra. The proximal incision should be long enough to allow convergence of the pedicle screws in the uppermost vertebra. The spine is exposed in the standard fashion, staying strictly subperiosteal to reduce bleeding. In the thoracic and the lumbar spine, the vertebra instrumented is exposed to the tip of the transverse processes bilaterally. In the course of the exposure, care should be taken not to disturb the facets adjacent to the uppermost pedicles instrumented, as damage to the facets may result in instability and precocious degenerative change.

Facetectomy: The facets included in the fusion are destroyed by inferior facetectomy and removal of the articular cartilages to promote intraarticular arthrodesis.

Determination of pedicle entry sites: Presumed pedicle entry points are decorticated with a rongeur to facilitate the insertion of the guide pins. In the thoracic spine, the presumed pedicle entry point is at the junction of the superior margin of the transverse process and the lamina. In the lumbar spine, the point is at the junction of the line drawn through the middle of the transverse process and the lateral margin of the facet joint (Fig. 33–2

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