Volumetric Spinal Canal Intrusion

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Abstract
A computer-aided design analysis. To introduce the concept of volumetric spinal canal intrusion and report the relative intrusion volumes for thoracic pedicle screws compared to thoracic laminar and pedicle hooks. Thoracic pedicle screws are being used more frequently; however, there is concern about neurologic risk from medial misplacement. The accepted alternative to screws is hooks. Laminar and pedicle hooks also have significant obligatory spinal canal intrusion. To date, there have been no comparison studies. Volumetric analysis of canal intrusion of pedicle screws and hooks was performed by computer-aided design CAM. All implants were of a single product line by a single manufacturer (CD Horizon M8, Medtronic Sofamor Danek). Intrusion of pedicle screws with medial positioning was analyzed in 0.5-mm increments, including a calculation of the "screw shadow," representing additional space not available for the spinal cord between screw threads and lateral to a medially positioned screw with intrusion greater than the screw radius. The length of screw intrusion was determined from postoperative CT scans in patients with thoracic pedicle screw instrumentation. All hook styles were analyzed. The volume of the footplate in line with the dorsal surface of the footplate was considered the intruding volume for laminar hooks, with increasing offset in 0.25-mm increments to represent imperfect fit. Half of the volume of the footplate was considered to be the intruding volume for pedicle hooks since a properly positioned pedicle hook straddles the pedicle. Volumetric intrusion for a 4.5-mm screw ranged from 2.2 mm3 (0.5 mm medial perforation) to 83.4 mm3 (3.0 mm perforation). For a 5.5-mm screw, intrusion volume range was from 1.3 mm3 to 83.2 mm3. Accounting for the "screw shadow," the volumetric intrusion was 9.83 mm3 to 116.3 mm3 and 10.88 mm3 to 134.89 mm3, respectively. Hook volumetric intrusion ranged from 21.15 mm3 for a pediatric narrow-blade ramped pedicle hook to 113.9 mm3 for a wide-blade laminar hook with 1.0 mm of step-off. A 4.5-mm or 5.5-mm thoracic pedicle screw must have a medial perforation of >or=1.5 mm to have the same volumetric spinal canal intrusion as a pediatric narrow-blade pedicle hook, the smallest hook footplate. Further, the medial violation must be >3 mm to approach the same volumetric intrusion as the largest hook. Accounting for the "screw shadow," a thoracic pedicle screw must have a medial perforation of >2 mm to approach the same intrusion volume as a standard pedicle hook. In the absence of direct neural injury, this explains the clinical finding of medial perforation of up to 4 mm without neurologic compromise.