D Thoracic and lumbar spinal instrumentation and fusion
Anterolateral, posterior, or combined anteroposterior approaches can be used to treat pathologic processes of the thoracic and lumbar spine. Spinal instrumentation refers to implanted metal rods affixed to the spine to correct and internally splint a deformed spine. Originally designed for scoliosis, posterior spinal instrumentation is commonly performed simultaneously with spinal fusion for a variety of diagnoses, including fracture, tumor, degenerative changes, and developmental spinal deformity. The original Harrington rod is the simplest and still considered by many to be the standard. Other procedures, such as segmental spinal instrumentation, can distribute correctional forces by sublaminar wiring (Luque) or by hook or screw (Cotrel-Dubousset) procedures that apply multilevel corrective forces on the rods. Bone chips from the posterior iliac crest are placed over the site of fusion. Harrington rodding or similar extensive spinal instrumentation procedures to correct spinal column deformities put the spinal cord at risk for ischemia secondary to mechanical compression of its blood supply. This complication has been mitigated with methods to assess spinal cord function intraoperatively. These include intraoperative testing of neurologic function (wake-up test) and SSEP monitoring. Wake-up testing requires an informed cooperative patient and a practice trial of patient responses.
The anterior approach may use the Dwyer screw and cable apparatus or Zielke rod. It offers a limited fusion area and less blood loss and can correct significant lordosis. There is a greater risk of damage to the spinal cord compared with the Harrington rod procedure. The spinal cord can be damaged from the vertebral body screw, especially in the smaller thoracic vertebral bodies and larger number of segmental spinal arteries that require ligation. The patient is positioned laterally, and a transthoracic or retroperitoneal approach is used. The potential for respiratory compromise is significant when using the thoracoabdominal approach. Surgery above the level of T8 requires a double-lumen ETT to collapse the lung on the operative side. The procedure may require the removal of the tenth or sixth rib (or both) and diaphragmatic manipulation.
An anteroposterior fusion may be required for patients with unstable spines. Usually, the anterior procedure is performed first followed by posterior instrumentation after 1 to 2 weeks. Immediate posterior fusion is possible when the area to be fused is small and the primary curvature is below the diaphragm. The anteroposterior approach necessitates an intraoperative position change. Anesthetic considerations are similar to those required for posterior instrumentation. Anesthetic concerns for thoracic and lumbar spine procedures are positioning, replacing blood and fluid losses, maintaining spinal cord integrity, preventing venous air embolism, and avoiding hypothermia. The wake-up test and SSEPs are frequently used.
2. Preoperative assessment
Patients requiring spinal reconstruction usually have either idiopathic or acquired scoliosis. Scoliosis is a deformity of the spine resulting in curvature and rotation of the vertebrae, as well as an associated deformity of the rib cage. Scoliosis can be classified as idiopathic, neuromuscular, myopathic, congenital, or trauma or tumor related or as part of mesenchymal disorders. Most cases are idiopathic, with a male-to-female ratio of 1:4. Surgery is indicated when the curvature is severe (the Cobb angle is greater than 50 degrees or rapidly progressing). Spinal instability requiring surgery may also result from trauma, cancer, or infection. Patients with scoliosis need careful preoperative evaluation of their cardiac, pulmonary, neuromuscular, and renal systems because associated anomalies occur frequently.
a) History and physical examination
(1) Cardiovascular: Patients have an increased incidence of congestive heart disease mitral valve prolapse, right ventricular hypertrophy, pulmonary hypertension, and cor pulmonale. Pulmonary vascular resistance is increased independent of the severity of scoliosis.
(2) Respiratory: Respiratory impairment is proportional to the angle of lateral curvature. Respiratory involvement is more likely when the Cobb angle is greater than 65 degrees. There may be a decreased total lung capacity and vital capacity (restrictive pattern). Ventilation–perfusion mismatch and alveolar hypoventilation may result in hypoxemia. If vital capacity is less than 40% of predicted, postoperative ventilation usually is required. Patients with neuromuscular disease may also have impaired protective airway mechanisms and weakness of respiratory musculature, making them prone to aspiration and respiratory failure.
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