Patients
This retrospective study enrolled patients with severe rigid spinal deformities who underwent PEO between July 2016 and December 2018. Spinal deformities were diagnosed by human grid analysis, roentgenography after bending or traction, three dimensional (3D) computed tomography (CT) and 3D printing models. The inclusion criteria were 1) a spinal scoliosis and kyphosis angle more than 80°; 2) flexibility less than 25%; 3) receipt of one-stage posterior-only PEO and correction surgery; 4) a minimum 2-year postoperative follow-up. We excluded patients with spinal cord or nerve root injury or other serious respiratory complications before surgery.
The study protocol was approved by the institutional review board of the authors’ affiliated institution. Patient data were anonymized in the paper.
Assessment of deformity
Instrumentation levels were determined according to the Cobb angle and flexibility of the main curve. Severe rigid spinal deformities were defined as having curve angles more than 80°, with flexibility less than 25% by X-rays after bending or traction[14]. The site of osteotomy was usually chosen as the vertebra that contributed most to the deformity according to the apex of the deformity.
Surgical technique
All surgeries were performed as a single-stage procedure by a single surgeon (the corresponding author, SXD) through a posterior incision. The surgical procedure has been described in our previous paper[15].
The patient was placed in the prone position on the operating table, on chest rolls. A single midline posterior longitudinal incision was made to expose the area and previously determined levels. Paraspinal muscles and all soft tissues were stripped subperiosteally from the bone laterally to the tips of the transverse processes to allow the rigid spine to become more flexible. Then, the pleural and paravertebral vessels were bluntly dissected. 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 inserted in the cephalic and caudal aspects of the vertebrae identified for resection using a free-hand technique at all levels planned prior to surgery. It should be noted that abnormal pedicle development, including absence of pedicles, causes more difficulties in establishing the screw trajectory, and that screw insertion is time consuming[16]. Usually, the spine is stabilized with a short bent rod in situ adjacent to the resected area to avoid coronal and sagittal plane translation during the reduction maneuver. A complete laminectomy and facetectomy was performed to expose the spinal cord. In the thoracic spine, the rib heads were removed to allow complete resection of the lateral wall of the vertebral body and to allow untethered motion of the vertebral column. Usually, the spinal cord is located in the concave curve side, but sometimes slightly located in the convex curve side. If the spinal cord is located in the convex curve side, we need to be more careful to avoid cause neurological complications due to high tension of the spinal cord. For example, some patients have no dural sac in the spinal cord, and in other cases the spinal cord is 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 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.
The spinal nerves were carefully dissected and preserved, but if they obstructed the osteotomy, one level of spinal nerve roots of the thorax on the convex curve side was usually resected. For PEO, the pedicle of the vertebral arch, 4/5 of the posterior vertebra, the bilateral walls of the vertebra and the posterior wall of the vertebra (5 mm to endplate) (Figure 2A to 2D) were carefully removed using an osteotome, curette, rongeur and ultrasonic osteotome. The apex area of PEO was planned in which the anterior 1/5 of the vertebral body was preserved during osteotomy. Compression over the resected area and shortening of the spine were performed to reduce tension on the spinal cord (Figure 2E and 2F). The PEO area had two situations: 1) a single vertebral osteotomy, if the angle of the curve was less than 100° (Figure 3A to 3C), and 2) a multiple vertebral osteotomy, if the angle of the curve greater than 100° (Figure 3D to 3I).
The osteotomy was performed carefully to avoid over-penetration of the anterior vertebral body cortex or anterior intervertebral disc, for the purpose of providing a hinge point to avoid coronal and sagittal plane translation, and also to prevent injury to the major vessels in front of the vertebral body. Then, we inserted another pre-contoured correction rodon on the convex side to exchange the rods, 30° 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 to stick out and injure the spinal cord. Therefore, we did not use the bent bar in PEO. After repeated compression and shuttling the segmental transient rod, we finally placed the terminal fixation rods after the main correction was achieved. The temporary rods should be exchanged with new rods because their mechanical integrity may be impaired by the bend. 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 does 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 shortened within one-third of the height of the vertebrae would not lead to a functional change of the spinal cord[17], but we did not worry about the excessive ruga of the dural sac. Spinal stability was always carefully maintained by the pedicle screw-rod system to prevent sudden migration of the spinal cord due to unstable instrumentation. We placed the terminal fixation rods after the main correction was achieved. After completion of resection and deformity correction, we filled any residual gap with resected vertebral body bone chips[18].
We monitored 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.
PEO grade
Grade I: For patients with a scoliosis and kyphosis angle less than 80°, we recommend osteotomy with single vertebra by PEO (Figure 4A). The osteotomy angle can reach 50°-60°, and the correction rate can reach 80%-85% by the previous cases data. Grade II: For patients with a scoliosis and kyphosis angle between 80° and 100°, we recommend osteotomy with a single vertebra and the intervertebral disc by PEO (Figure 4B). The osteotomy angle can reach 70°-85°, and the correction rate can reach 70%-85%. Grade III: For patients with a scoliosis and kyphosis angle between 101° and 120°, we recommend osteotomy with two vertebrae and the intervertebral disc by PEO (Figure 4C). The osteotomy angle can reach 80°-100°, and the correction rate can reach 70%-85%. Grade IV: For patients with a scoliosis and kyphosis angle more than 120°, we recommend osteotomy with two or more vertebrae and the intervertebral disc by PEO (Figure 4D). The osteotomy angle can reach 100°-120°, and the correction rate can reach 70%-75%. According to the severity of spinal cord folds, blood supply, tolerance of spinal cord twists, electrophysiological monitoring or wake-up experiments to determine whether to add the titanium mesh implants for the front support of the vertebral, we had a maximum spinal shortening of 5 centimeters (Figure 5A and 5B). For some patients with vertebral malformation, a more flexible osteotomy grade is adopted.