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Pedicle Screw Misplacement In Apical and End Vertebrae: A CT-Based Review of 285 Pediatric Patients

Saturday, October 20, 2012: 1:35 PM
Melrose (Hilton Riverside)
Terry Amaral1, Adam Wollowick1, Beverly Thornhill2, Meredith Steinman1, Jonathan J. Horn1, Elliot Harmon1, Preethi M. Kulkarni1, Yungtai Lo3 and Vishal Sarwahi1, (1)Orthopaedic Surgery, Montefiore Medical Center and the Albert Einstein College of Medicine, Bronx, NY, (2)Radiology, Montefiore Medical Center and the Albert Einstein College of Medicine, Bronx, NY, (3)Epidemiology, Albert Einstein College of Medicine, Bronx, NY

Purpose: It is largely believed that the concavity of the curve apex is the most difficult part of the spine to instrument because the pedicles are the smallest and most deformed. However, technical difficulties may also affect the accuracy of instrumentation and may lead to screw misplacement in other areas. The purpose of this study is to determine if foundation screws are more often malpositioned than screws at the curve apex. 

Methods: CT scans of 285 pediatric patients with spinal deformity were reviewed for pedicle morphology and screw placement in the upper and lower instrumented vertebrae and the apical vertebra of the major and minor curves. Pedicle morphology was classified according to our previously described CT-based classification system. Screws of concern were defined as those adjacent to or in contact with blood vessels, pleura, esophagus, diaphragm, or trachea. Chi square analysis and logistic regression were used to compare the incidence of abnormal pedicles and screw placement between foundation and apical vertebrae. 

Results: 1216 vertebrae were studied, including: 346 at the apex of major curves, 180 at the apex of minor curves, 346 at the L.I.V., and 344 at the U.I.V. There was a significant difference in the incidence of abnormal pedicles and in the accuracy of screw placement among the groups (p<0.001). U.I.V. had the highest percentage of abnormal pedicles and the highest rate of screw misplacement. Fisher's exact test showed significantly more screws of concern in the U.I.V. than in the other groups (p=0.0335). Logistic regression demonstrated a significantly higher risk of screw misplacement in upper instrumented vertebrae than in apical vertebrae (p=0.023). 

Conclusion: Foundation vertebrae had more pedicle screw placement error than apical vertebrae. Of note, significantly fewer caudal foundation screws were malpositioned than cephalad foundation screws. Caudal screws were in the lumbar spine, which has larger pedicles and thus is more easily visualized. Cephalad pedicles were in the upper thoracic spine with kyphosis and soft tissue constraints making visualization difficult. Apical vertebrae had more ease in visualization and accessibility, thus less placement error.