Posterior Thoracic Instrumentation/Fusion



Posterior Thoracic Instrumentation/Fusion


Dheera Ananthakrishnan

Andrew H. Milby





Indications for Thoracic Fusion

This chapter focuses on deformity, but there are a variety of indications for thoracic fusion, such as:



  • Deformity


  • Tumor


  • Degenerative disease (stenosis)


  • Disk herniation


  • Trauma


Radiologic Assessment



  • Carefully assess preoperative imaging for the presence of dural ectasia, sclerotic or atretic pedicles.


  • Includes plain x-rays (standing, supine, bending films, and traction films), magnetic resonance imaging (evaluation of the neuraxis and the disks and facets), and computed tomography (CT) scan (bony anatomy of individual vertebrae, measurement of pedicles, and measurement of pelvic parameters)



  • Measure Cobb angles to determine proximal and distal levels of construct and desired correction to maintain global balance.


  • Consider sagittal vertical axis and pelvic parameters when assessing need for sagittal deformity correction.


  • Measure approximate lengths and sizes of planned instrumentation.


  • Review preoperative plain films carefully to establish vertebral level nomenclature for intraoperative localization; careful assessment of scans is imperative to ensure correct levels.




Positioning



  • May position prone with Mayfield head holder if crossing cervicothoracic junction or on the prone Jackson table for thoracolumbar exposure


  • Care should be taken to adjust the hip and chest pads to assist with correction. Proper positioning preoperatively can facilitate deformity correction and fracture reduction. Extra blankets can be placed under the patient’s thighs to facilitate as much natural lordosis as possible. Extra hip pads and/or foam padding can facilitate correction of a lumbar scoliosis. Extra chest pad can be used to facilitate neutral positioning of the cervical spine when head is in ProneView holder.


  • Ensure posterior iliac crest is prepped and draped if harvesting autograft through a separate incision. If the lumbar spine is being included in the surgery, often the same skin incision can be used with a separate fascial incision over the iliac crest.


Anesthesia/Neuromonitoring Concerns



  • Be sure to discuss the anesthesia plan for the case well in advance, to facilitate a multidisciplinary approach to the management of estimated blood loss (EBL) and perioperative pain control.


  • This plan should include the use of medical management of hemorrhage (for deformity cases, high-dose tranexamic acid bolus and infusion, DDAVP [1-deamino-8-D-arginine vasopressin] administration prior to surgery).


  • Multimodal anesthesia as needed to include ketamine, intravenous acetaminophen, nonsteroidal anti-inflammatory drugs, long-acting narcotics such as methadone if indicated


  • In addition, the incidence of postoperative ileus in deformity patients can be quite high, so use of intestinal motility agents and gastric tubes should be considered.


  • Blood pressure management should also be discussed: mean arterial pressure goal more than 80 mm Hg in the setting of myelopathy or during planned deformity correction; possible use of hypotensive anesthesia in the proper patient (adolescent idiopathic scoliosis) for minimizing EBL during exposure.


  • Total intravenous anesthesia in the setting of MEP monitoring: the timing of MEPs should be discussed with the anesthesia and neuromonitoring teams prior to surgery. If neurology is intact and not in danger, we obtain MEPs after the flip to the prone position, then allow for administration of paralytic to relax the patient during exposure, which decreases EBL. The paralytic should work off by the time screws are ready to be inserted; if not, the anesthesia team can administer a reversal agent.


Localization of Incision



  • Midline incision, either curvilinear over spinous processes or straight approximating midline at surgeon’s discretion


  • Ensure same counting nomenclature is used between preoperative and intraoperative imaging for localization.



  • For a long deformity correction, anatomic landmarks can be used; else, preoperative plain films or C-arm can be used, depending on the indication for surgery.


  • Take care to check levels repeatedly, as it is not difficult to be at the wrong level in the thoracic spine. If in doubt, check with a different radiographic method.


Approach



  • Standard posterior midline approach is utilized.



    • Minimize tension to avoid avulsing muscular attachments and small vessels.


    • Minimize retractor repositioning so as not to create additional bleeding points.


  • Continue dissection laterally over transverse process of proximal instrumented vertebra (PIV).



    • Ensure that interspinous ligaments and facet capsules remain intact between PIV and cranial-adjacent level to minimize risk of proximal junctional kyphosis.


    • Clean the laminae and facets of the levels to be fused of all investing soft tissue using Cobb elevators as well as curettes and electrocautery. This can be a time-consuming step, but is of paramount importance in identification of anatomic landmarks (particularly in deformity) and to facilitate fusion. Even the intercalary spinous processes should be cleaned, as they will likely be removed and used for local autograft, which should all be cleaned of soft tissue prior to bone grafting.


  • Define anatomic landmarks for freehand pedicle screw insertion at each level:



    • Bony crest along cranial aspect of transverse process


    • Lateral border of cranial-adjacent facet


    • Medial border of facet/lateral edge of canal


Retractor Placement



  • May use a combination of short and long cerebellar and/or Weitlaner retractors to achieve exposure while minimizing need for retractor repositioning during instrumentation


  • In a long deformity correction, consider releasing tension on the retractors to minimize paraspinal muscle necrosis. Also consider packing off portions of the wound with sponges to minimize blood loss.


Decompression Techniques



  • If placing freehand thoracic pedicle screws in the setting of severe deformity, consider use of multiple laminotomies to assist in direct palpation of the pedicles from within the canal to guide screw placement.


Bony Resection Techniques (Figures 13-3 and 13-4)






Figure 13-3 ▪ Facetectomy posterior column osteotomy: extent of bony resection.







Figure 13-4 ▪ Closure of posterior column osteotomy.

Oct 13, 2019 | Posted by in ORTHOPEDIC | Comments Off on Posterior Thoracic Instrumentation/Fusion

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