Anatomic considerations of anterior instrumentation of the thoracic spine

Forty-seven dry thoracic specimens from T-3 to T-12 (470 thoracic vertebrae) were used to measure the dimensions of the vertebral body of the thoracic spine and to determine the relationship of the posterior angulation of screw placement to the spinal canal from different entrance points. Statistically significant differences in dimensions of male and female specimens were found in the anterior vertebral body height and all of the angular measurements. The average maximum posterior angle relative to the frontal plane from T-3 to T-12 for both sexes ranged from 11 degrees to 14 degrees at the initial point (the level of the most anterior edge of the upper costal facet), from 20 degrees to 23 degrees at the point of 5 mm anterior to the initial point, and from 30 degrees to 34 degrees at the point of 10 mm anterior to the initial point. This study suggests that there is considerable risk of violating the spinal canal if the screws are inadvertently angled posteriorly. The authors recommend insertion of screws in the anterior or middle part of the lateral aspect of the vertebral body. The screws should be directed perpendicular to the lateral plane of the vertebral body. A posteriorly placed screw should be directed anteriorly.