Standardized posterior pelvic imaging: use of CT inlet and CT outlet for evaluation and management of pelvic ring injuries.

OBJECTIVES: The null hypothesis of this study states that routine axial computed tomography (CT) images are obtained at a consistent and reproducible orientation relative to the sacrum. The secondary null hypothesis states that there is no difference in the measurement of the safe zone for placement of iliosacral screws when using routine axial CT images and standardized reconstructions in defined planes perpendicular and parallel to the sacrum.

DESIGN: Retrospective review.

SETTING: University Level 1 Trauma Center.

PATIENTS: Sixty-eight consecutive trauma patients evaluated with routine pelvic CT, without pelvic ring injury.

INTERVENTION: Retrospective radiographic review and measurement.

METHODS: Sixty-eight consecutive adult patients with routine axial pelvic CT scans, without injury to the pelvic ring, and obtained as part of a trauma evaluation were retrospectively identified. The orientation of the axial slices relative to the sacrum was measured for each patient and compared. The maximal cross-sectional distance at the smallest section of the sacral ala (safe zone) was measured using the routine axial CT images, and these measurements were compared with similar measurements taken on standardized images perpendicular (CT inlet) and parallel (CT outlet) to the body of the sacrum. Additional data referencing the orientation of multiple sacral radiographic landmarks were also collected.

RESULTS: The orientation of routine axial CT image planes relative to the sacrum spanned a wide range. The angle between the routine axial CT plane and the sacrum varied from 43.5 to 82.0 degrees (SD = 9 degrees). Significant differences were found in measured safe zones of routine axial CT images compared with standardized CT inlet and CT outlet images. Compared with CT inlet images, routine axial CT images underestimated safe zones for transverse sacral screws at both S1 (P < 0.01) and S2 (P < 0.01). When compared with CT outlet images, routine axial CT images overestimated safe zones for oblique sacroiliac screws (P < 0.01) and underestimated the safe zone for S2 transverse sacral style screws (P < 0.01). No significant differences in measured variables were found between genders and sacral morphology.

CONCLUSIONS: Our null hypotheses were rejected: routine axial CT images were found to be at widely ranging orientations relative to the sacrum, and standardized CT images (CT inlet and CT outlet) demonstrated statistically significant differences in measurements of safe zones compared with routine axial CT images. Furthermore, the CT inlet and CT outlet views provide additional information regarding sacral landmarks that could be useful for preoperative planning.