This retrospective study included 10 patients with severe rigid thoracolumbar spine deformity who underwent PEO at T12 or L1 between July 2016 and June 2017. Deformity was assessed both clinically and radiographically as part of the preoperative planning. Diagnosis of thoracolumbar spine deformity was made by human grid analysis, X-rays after bending or traction, 3D CT and 3D printing models. Patient complaints included serious waist deformity, lower back pain and increased lower limb weakness and leg numbness after activity. All patients underwent a one-stage osteotomy instead of preoperative traction. Patients with spinal cord or nerve root injury or other serious respiratory complications were excluded.
Assessment of deformity
Using standard Cobb’s method on standing lateral radiograph of the whole spine, the global thoracolumbar kyphosis or scoliosis was measured from the upper endplate of T1 to the lower endplate of L5. Severe rigid spinal deformity was defined as having curve angles more than 80°, with flexibility less than 25% by X-rays after bending or traction, exclusion patients with spinal cord or nerve root injury or have other serious respiratory complications. Radiographic parameters were evaluated, and clinical records were reviewed. The osteotomy location was usually chosen as the vertebra that contributed most to the deformity, according to the apex of the deformity. Such a measurement not only represents the severity of the overall deformity, but also determines the amount of correction to be achieved by spinal osteotomy.
The surgical procedure
The patient was placed prone over the table, which was in a flat position throughout the operation. The location of the upper and lower end vertebrae was determined by mobile C-arm X-ray imaging before surgery. The vertebral column from the upper to the lower end vertebrae was prepared and draped. A straight vertical midline incision was made over the spinous processes and was extended to the upper and lower end vertebrae. If the deformed vertebra had a rotation, an incision was made on the convex side, deviating approximately 1-2 cm from the centerline. The paraspinal muscles were dissected subperiosteally from the spinous processes and laminae, and then retracted laterally. After a careful dissection of the area around the facet joints, a large spinal retractor was applied. By spreading the spinal retractor and detachment of muscles around the facet joints, a wider exposure was obtained. The spinal retractor could reduce soft tissue damage and infection, and avoid long-term traction. An intraoperative radiograph with guide pins was obtained for accurate localization of the deformity and determination of the level and area for osteotomy. Pedicle screws were then inserted into the segments from the upper end vertebra to the lower end vertebra using a free-hand technique at all levels planned prior to surgery. The osteotomy site was usually chosen as the vertebra that contributed most to the deformity, according to the apex of the deformity.
Usually the spine was stabilized with a short bent rod in situ adjacent to the resected area to avoid coronal and sagittal plane translation during the reduction maneuver. The first unilateral rod was temporarily fixed on the concavity side bend to maintain spinal stability after PEO. A complete laminectomy and facetectomy was performed to expose the spinal cord. Usually, the spinal cord was located in the concave curve side, sometimes slightly located in the convex curve side. If the spinal cord was located in the convex curve side, greater caution should be exercised because of neurological complications due to high tension of the spinal cord. In some cases, the spinal cord was as tight as a cord with the diameter of only one-third of a normal spinal cord. Any slight maneuver would make the action potentials to decline sharply by over 50%, or even disappear. Timely identification and prompt intervention must be performed, including enlarging the resected area to reduce the abrupt turning tendency of the spinal cord.
In our experience, we removed two levels of nerve roots of the thoracolumbar spinal cord for PEO, if necessary. These procedures allow circum-spinal decompression of the spinal cord. One of the critical pitfalls in this step was the careless mistake of pulling out the preserved spinal nerve roots at the corresponding level. This was quite dangerous for the spinal cord, which was already compressed at the apex of the angular kyphotic deformity, because pulling out the nerve root increased the pressure of the spinal cord at the apex. When the initial PEO was carried out, we did not pay special attention to the L1 nerve roots. Due to position variation, we made the careless mistake of regarding L1 as T12 nerve roots and damaged L1, which was confirmed by lower limb EMG after surgery. Thereafter, we attempted to use a nerve stripper to separate and release the L1 nerve roots under somatosensory-evoked potential (SEP) and motor-evoked potential (MEP) monitoring, ultimately leaving the L1 nerve roots slack and floating in the gap.
For PEO, the pedicle of the vertebral arch, 2/3 of the posterior vertebra, the bilateral walls of the vertebra and the posterior wall of the vertebra (5 mm to endplate) were carefully removed using an osteotome, curette, rongeur and ultrasonic osteotome (Figure 1). The parallel endplate osteotomy area had two situations: a single vertebral osteotomy if the angle of the curve was less than 90°, or a multiple vertebral osteotomy if the angle of the curve was greater than 90°. If the spinal cord was compressed at the apex of the angular deformity, the lesion compressing the spinal cord was also drilled out using an L-shaped bone separator under direct vision from the lateral direction. A thin stripper was used to confirm whether the soft tissue, such as the posterior longitudinal ligament, attached to the dural sac was soft enough.
The osteotomy was performed carefully to avoid over-penetration of the anterior vertebral body cortex or anterior intervertebral disc for the purpose of preventing injury to the major vessels in front of the vertebral body. Then, we inserted another precontoured correction rod on the convex side to exchange the rods, 30 degrees per correction. It was important in this step to keep an adequate compression force on the concave rod while its adjunct screws on the cephalic side were slightly released until the concave rod and screws were tightened one by one. In situ rod bending on the concave side should never be performed because it is very dangerous procedure to the naked spinal cord, and applying too much torsion to the pedicle screws could easily cause screw loosening and rod bender stick out and injure the spinal cord. After repeated compressions and shuttling segmental transient rods, finally, we placed the terminal fixation rods after main correction was achieved. Then, segmental derotation, compression, and distraction on the secondary curves were performed to achieve final correction. During the entire correction procedure, the dural sac was closely observed to avoid migration in any direction, and tension of the spinal cord was assessed by observation and frequent palpation. Adequate and quick adjustments were needed to ensure that spinal cord tension did not exceed the initial state under distraction, and to prevent excessive kinking of the dural sac after spinal shortening. Kawahara et al. confirmed that the spine that was shortened within one-third of the height of the vertebrae would not lead to a functional change of the spinal cord. During osteotomy, we had a maximum spinal shortening of 5 centimeters, there were no warning signs by SEP and MEP. After completion of resection and deformity correction, any residual gap was filled with resected vertebral body bone morsels. Finally, we checked that the spinal cord was thoroughly smooth. We gave patients an autologous blood transfusion that was recycled, or allogeneic blood transfusion if the volume of bleeding was high.
Intraoperative monitoring technique and postoperative follow-up
We monitored the somatosensory-evoked potential (SEP) and motor-evoked potential (MEP) to effectively monitor the spinal cord and nerve roots under the supervision of an experienced neurophysiologic physician throughout the PEO procedure, and an additional wake-up test was performed after finishing the correction step at the end of the surgery to ensure the neurological status. Intraoperative and postoperative complications were recorded. For patients with L1 nerve roots injury, continuous electromyography (EMG) monitoring was necessary. In an attempt to validate these patients’ clinical outcome, 10 patients were also asked to respond to the SF-36 quality of life questionnaire via telephone interview after a year of follow-up.