Introduction: Sagittal alignment is a key consideration in operative planning. Upright films for assessment of pelvic incidence(PI) and lumbar lordosis(LL) are not tolerated in all populations(i.e. trauma patients). Our study aims to determine the accuracy of PI and LL measured on supine CT compared with standing X-rays.

Methods: Consecutive patients with both CT and standing X-rays suitable for measurements of PI(n=30) and LL(n=33) were collected over two years. Two independent physicians measured PI and LL on CT and X-ray to account for inter-observer reliability. Remote, repeat measurements were performed to determine intra-observer reliability. X-ray measurements of PI and LL were performed in standard fashion. CT measurements of lumbar lordosis were performed by measuring the angle between the superior endplates of S1 and L1 in the midsagittal cut. PI was measured first by determining the midpoint of the line between the femoral heads in the midsagittal plane, then drawing a line from that point and the midpoint of the S1 endplate. The angle measured between that line and one perpendicular to the S1 endplate determined PI. Paired t-tests were run to compare modalities as well as intra- and inter-observer values.

Results: The mean PI for 30 patients was 57.8°(+/-10.4°) using X-rays and 53.3°(+/-11.5°) on CT. Pearson’s coefficients were .67 for inter-observer X-ray, .89 for inter-observer CT, .81 for intra-observer X-ray, and .95 for intra-observer CT. Mean LL for 33 patients was calculated at 53.1°(+/-11.4°) with X-ray and 52.2°(+/-11.4°) on CT. Correlation coefficients were .76 for inter-observer X-ray, .86 for inter-observer CT, .91 for intra-observer X-ray, and .97 for intra-observer CT. Averaged CT measurements had correlation coefficients with averaged X-rays of .94 for PI and .75 for LL.

Conclusions: Supine CT correlates well with standing scoliosis X-rays for measurements of PI and LL.

Patient Care: Preoperative consideration must be given to spinopelvic parameters to avoid iatrogenic sagittal deformity. Standing X-rays, the preferred modality for assessing pelvic incidence and lumbar lordosis, are not always obtainable. Trauma patients in particular are frequently unable to withstand the rigors of upright films, but routinely undergo CT of the abdomen and pelvis. Accurate assessment of pelvic incidence and lumbar lordosis on supine CT will allow a more complete consideration of these patients’ sagittal balance, less iatrogenic deformity, and better outcomes.

Learning Objectives: By the conclusion of this session, participants should be able to: 1)Describe a method for determining Pelvic Incidence and Lumbar lordosis on CT scans. 2)Compare the validity of CT-obtained measurements with those obtained from standing X-rays. 3)Discuss the reliability of CT-obtained measures of PI and LL compared with the reliability of intra- and inter-observer measurements obtained from standing X-rays.

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