Preoperative and Postoperative CT Evaluation of Structures at Risk with ASF

· (a – Medtronic Sofamor Danek; c – Medtronic Sofamor Danek)

Introduction: With the increasing popularity of anterior spinal fusion (ASF) for AIS, there has also been a rising concern over the proximity of the thoracic aorta (TA) to the screw tips and the possibility of vessel wall erosion over time.

Purpose: This preoperative and postoperative CT study attempts to define the relative position of the TA, and other vital structures in deformity patients, to the spine (preoperatively), as well as to the projected instrumentation (postoperatively) by level and curve magnitude.

Methods: 20 consecutive pts (17F,3M) with an ave. age of 14.5 yrs (range 12.4-18.5) with AIS and a right main thoracic/ Lenke 1 curve, ave. 55.2° (range 50-66°, ave. apex T8), underwent preoperative and postoperative CT scans as part of their planned ASF. All images were analyzed for proximity (distance from the mid-vertebral body) and position to (as defined relative to the center of the vertebral body in the axial plane) the spine preoperatively and the projecting screw tip postoperatively. These were compared to 10 age-matched non-deformity thoracic CT scans to assess the relative position of the thoracic aorta to the vertebral bodies by level. Preoperative and postoperative plain radiographs were also analyzed for curve magnitude, correction and fusion levels to assess the possible effect of these variables on thoracic aorta proximity.

Results: Postoperative curve magnitude averaged 26.9° (range 17-40°, 51% correction) with an ave. follow-up of 4.1 yrs (range 3.2-7.0) analyzing 151 screws (7.5 levels/pt). The trachea/main bronchi, esophagus and pleura were not found to be at risk. Screw to spinal canal distance averaged 5.3mm (range 3.5-8.2mm), and +4.5° (range -11° to +15°) from the coronal axis. Screw tip extrusion (distance beyond far cortex) averaged 2.8mm (0-5mm). Spine/screw tip to aorta distance was as follows:

*denotes p<0 .05

Additionally, 23/151(15%) of screws were thought to be adjacent to the TA with 4/60 (7%) proximal screws were judged to be juxtaposed to the aorta, while 6/40 (15%) periapical screws and 13/51 (26%) distal screws were juxtaposed.(p<0 .05) There were no screws compressing (indenting) the aorta and complications.

Conclusions: The course of the thoracic aorta may vary in individuals, and in deformity; however, it generally moves from a relatively anterolateral position proximally, to posteromedial position at the apex, and then to a more anterior position distally. Consequently, the aorta moves closer to the screw tips both at the apex and distally, while the distal screws are more frequently juxtaposed to the descending aorta. 96

· If noted, the author indicates something of value received. The codes are identified as: a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options.