Regional differences in anatomical landmarks for placing anterior instrumentation of the thoracic spine in both normal patients and patients with adolescent idiopathic scoliosis

Study Design. A retrospective analysis of patients who had magnetic resonance imaging (MRI) of the thoracic spine, comparing those with a normal straight spine and those with a right thoracic adolescent idiopathic scoliosis (AIS). Objective. To analyze the position of the rib head with respect to the spinal canal and vertebral body in normal patients and those with right thoracic AIS using MRI. Summary of Background Data. When placing anterior vertebral body screws in thoracic AIS, the most cephalad screws are most at risk for loosening because of smaller vertebral body size and the position of the rib heads, which may obscure more of the vertebral bodies. To our knowledge, there are no studies defining the relationship of the rib head to the vertebral anatomy in thoracic AIS. Methods. Transverse MRIs of the vertebral bodies from the 4th thoracic (T4) vertebra to the 12th thoracic (T12) vertebra in normal patients (n = 21) and patients with AIS (n = 21) group were analyzed regarding the following parameters: (1) percent vertebra obscured by rib head (i.e., the percent of the sagittal plane vertebral body length obscured by the overlapping rib head); (2) posterior safe angle, defined as the most posterior angle a screw can be placed, which avoids the spinal canal; and (3) anterior safe angle, defined as the most anteriorly directed screw trajectory that safely obtained good screw purchase. Results. In both the normal and AIS groups, the percent vertebra obscured by rib head significantly decreased from T4 (30% in normal group and 34.7% in AIS group) to T12 (-0.4% in normal group and 3.5% in AIS group) (P < 0.05). The rib head was positioned more anterior to the vertebral body in the cephalad-thoracic spine when compared to a more posterior position in the caudal thoracic spine. In each group, the posterior safe angle significantly decreased from T4 (23° in normal group and 20.8° in AIS group) to T12 (-0.9° in normal group and 2.1° in AIS group) (P < 0.05), while the anterior safe angle significantly increased from T4 (27.5° in normal group and 26.6° in AIS group) to T12 (38.3° in normal group and 38.5° in AIS group) (P < 0.05). Conclusions. It is important to understand the relationship of the rib head to the vertebral body to provide-excellent screw purchase within the vertebral body without risking penetration into the spinal canal. In both normal and AIS groups, the relationship of the rib head to the vertebral body and spinal canal changes so that the rib head is positioned more anteriorly in the cephalad-thoracic spine and more posteriorly in the caudal thoracic spine. When placing anterior thoracic screws, at the cephalad-thoracic spine (T4, T5, T6, and T7), removal of rib heads is recommended to allow for good screw purchase. However, at the caudal thoracic spine (T10-T12), staying anterior to the rib head is important to avoid penetration into the spinal canal.