Morphometric analysis of the thoracic spine using CT and its association with upper instrumented vertebra fracture in adult spinal deformity surgery.

Abstract

PURPOSE: Retrospective observational cohort study. In Adult spinal deformity (ASD) surgery, the upper instrumented vertebra (UIV) is often located within the thoracic spine. However detailed morphometric evaluation is limited. In particular, narrow pedicles may increase the risk of UIV fracture (UIVF). This study aimed to evaluate morphometry in thoracic spine and the incidence of UIVF. METHODS: This study included ASD patients using pedicle screws with a diameter of 5.5 mm at the UIV. Preoperative CT scans were used to measure vertebral body dimensions and pedicle diameters at each thoracic level. The incidence of UIVF within two years postoperatively was evaluated. RESULTS: A total of 174 patients were included, comprising 154 females and 20 males. The transverse diameter of the pedicle was smallest at the T4 level in both sexes (3.5 mm in females and 4.5 mm in males). Pedicle diameters increased progressively toward the thoracolumbar junction. The incidence of UIVF varied across thoracic levels, with higher rates observed at T4 (42%), T5 (62%), and T6 (50%). The incidence decreased at lower thoracic levels, such as T8 (18%), T9 (25%), and T10 (24%). The logistic regression analysis showed that selecting T4, T5, or T6 as the UIV was associated with an odds ratio of 4.6 for UIVF.　Moreover, UIVF was the most common mechanical complications in this study (30%). CONCLUSION: The transverse pedicle diameter was narrowest around the T4, and the incidence of UIVF was notably higher at T4, T5, and T6. These levels may not be ideal for screw fixation.