Thoracic and Lumbar Spinal Anatomy
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Degenerative arthropathy, trauma and congenital anomalies, as well as focal abnormalities such as facet overgrowth and disk herniations render each patient unique, and these abnormalities can impact surgical approach. The goal of this chapter is to discuss anatomic considerations that impact surgical planning and to provide a framework for thinking about patient-specific anatomy when approaching the thoracic and lumbar spines.
KeywordsThoracic spine Lumbar spine Degenerative arthropathy Intraoperative localization
The angulation of the thoracic pedicles further changes, with more caudally oriented trajectories in the upper thoracic spine. Thoracic instrumentation can be challenging due to this variability, and navigation or fluoroscopy-based techniques can assist in planning thoracic pedicle screw trajectories.
The thoracic spine is more rigid than the cervical or lumbar spine, as it is fixed to the sternum via the ribs, limiting the range of motion in the thoracic spine. The cervicothoracic junction and thoracolumbar junctions are more mobile points of transition and are thus more likely to succumb to traumatic pathology. Degenerative pathologies of thoracic spine include sagittal kyphotic deformity, typically secondary to progressive compression fractures, and coronal scoliosis (Fig. 1). Adolescent scoliosis is a common childhood disorder affecting thoracic spine alignment, and in adult patients, iatrogenic or degenerative scoliosis with coronal curvature may affect thoracic spine alignment.
Intraoperative localization in the thoracic spine can be challenging if the lesion is not readily identifiable on standard radiographic studies or fluoroscopy. Unlike cervical and lumbar spine localization, there is no distinct body (i.e., sacral endplate or dens) available as a reference for counting. Preoperative thoracic and lumbar x-rays may be helpful to determine the true number of ribbed and non-ribbed vertebrae. This may help to correlate with the MRI if the patient has a transitional S1 that may be lumbarized or hypoplastic or an abnormal number of ribbed vertebrae.
Preoperative CT or MRI scans for localization may be obtained prior to surgery incorporating a reference body (i.e., sacral endplate, dens, or other identifiable structure). A radiologist can provide labeling of the localization scan to confirm the precise thoracic body affected. This allows congenital anomalies, such as sacralized lumbar vertebrae, or an abnormal number of rib-bearing vertebrae to be identified. These studies can be correlated with preoperative plain films to reduce the likelihood of wrong-level surgery.
Intraoperative anterior-posterior and lateral fluoroscopic images may be taken to localize the surgical level. Live intraoperative fluoroscopy may also be used for level confirmation, typically by counting up from the sacrum. Intraoperative 3-D imaging, if available, may also help to provide more definitive surgical localization. Prior instrumentation, kyphoplasty cement, or unique fractures may further help to confirm the target level. It is important to note that technical and patient factors (obesity, surgical position, non-radiolucent OR tables, and others) may interfere with the correct interpretation of these studies. In these instances, consultation with a radiologist should be undertaken.
Posterior and posterolateral approaches to the thoracic spine provide limited access to the posterior vertebral body for debulking of metastatic tumors and decompression of fracture fragments. Following laminectomy, unilateral or bilateral pedicles can be resected via careful drilling to access the ventral vertebral body. In the thoracic spine below T2, nerve roots do not provide significant motor contributions, and these roots may be sacrificed lateral to the dorsal root ganglion to further expand the exposure and improve access to the ventral disk space. Nerve root avulsion or compression should be avoided as this may result in postoperative radicular pain.
For calcified disk herniations, pedicle resection can be an effective way to access the disk space. If the pedicle is sacrificed for access, unilateral or bilateral posterior fusion may be required to limit segmental motion and collapse. Posterior fusion of the thoracic spine typically involves placement of pedicle screws, and preoperative evaluation should take into account pedicle length and width. Medialized screws in the thoracic spine result in cord compression, while lateralized screw trajectories can incorporate rib or injure thoracic viscera. Intraoperative navigation is a useful tool to decide optimal screw trajectory.
Ventral disk herniations in the thoracic spine may be calcified.
Thoracic spine localization requires careful preoperative planning.
Unilateral nerve roots T2-12 can be sacrificed to improve surgical exposure in posterior and posterolateral approaches.
Intraoperative localization for lumbar spine surgery is typically achieved with lateral radiographs, and the levels are identified by counting from the L5 to S1 disk space. In some patients the fifth lumbar vertebrae may be sacralized, meaning its orientation mimics a typical S1 caudal orientation. This anatomic variant should be identified prior to surgery, as it affects intraoperative localization. Spina bifida occulta may be recognized in patients undergoing evaluation for other spinal issues, and laminar defects may be identified prior to surgery. Another congenital anomaly in the lumbar spine is a pars defect. In this variant, the pars interarticularis (the bony bridge between the superior and inferior articulating facets) fails to develop. The pars interarticularis resists the vector of anterolisthesis, and when the pars is compromised, patients may be at increased risk of developing progressive spondylolisthesis (Fig. 6).
The pelvic incidence is measured as the angle between a line drawn perpendicular to the center of the S1 endplate and a second line from the center of the S1 endplate to the center of the femoral heads. A patient’s lumbar lordosis should be comparable (within 10°) to their pelvic incidence; otherwise an iatrogenic “flat-back” deformity of the lumbar spine may occur.
Anterior approaches to the lumbar spine are typically achieved with the help of an access general surgeon or vascular surgeon. The anterior lumbar interbody fusion (ALIF) involves removal of the intervertebral disk, placement of a disk replacement, and securing the disk replacement with an anterior plate and screws (Phan et al. 2017). This approach is complicated by the presence of the lumbosacral plexus (L5-S1) and the iliac bifurcation (L4-5). Anterior approaches may be utilized for patients with failed posterior fusion or in patients with severe deformity requiring anterior and posterior instrumentation to facilitate strength and reduce the risk of failure. Anterior lumbar approaches can be challenging in obese patients if the abdominal girth exceeds the length of surgical instruments. Further, retrograde ejaculation is a reported complication in men at a rate of 7.4–9.8% following manipulation of the lumbosacral plexus in ALIFs, thus compromising male fertility (Lindley et al. 2012).
Lumbar degenerative arthropathy can obscure the laminar edge on initial dissection.
Sacralized lumbar vertebra can complicate intraoperative localization.
Anterior lumbar fusions may be challenging in obese patients and carry the risk of retrograde ejaculation in male patients.
Careful patient examination and review of available preoperative imaging is crucial for success in spine surgery. CT scans provide key information regarding calcifications, and MRI scans are necessary to identify disk herniations. Prior to entering the operating suite, technical aspects of the surgery should be decided, including the type of surgical instrumentation if indication. Patient anatomical anomalies, such as sacralized vertebrae, abnormal rib-bearing vertebrae, osteophytes, and overgrown facets, should also be reviewed in detail. Upright or standing radiographs can complete the picture, as they may highlight loss of lordosis or spondylolisthesis. While knowledge of general spinal anatomy is crucial to form the foundation of spine surgery, patient-specific details must be considered to ensure the optimal outcome.
- Xu R, Garces-Ambrossi GL, McGirt MJ, Witham TF, Wolinsky JP, Bydon A, Gokaslan ZL, Sciubba DM (2009) Thoracic vertebrectomy and spinal reconstruction via anterior, posterior, or combined approaches: clinical outcomes in 91 consecutive patients with metastatic spinal tumors. J Neurosurg Spine 11:272–284CrossRefPubMedCentralGoogle Scholar