Sacrum pubic incidence and sacrum pubic posterior angle: two morphologic radiological parameters in assessing pelvic sagittal alignment in human adults.

PURPOSE: The morphology and position of pelvis are critical in regulating the biomechanical organization of spine-pelvis-leg in the sagittal plane. Several radiological parameters have been developed to present the sagittal morphology of the pelvis such as pelvic incidence (PI) and Jackson's angle (PRS1). In addition, the femoral sacral posterior angle (FSPA) was developed for patients with a dome-shaped deformity in the upper plate of the sacrum. The identification of hip axis, which was represented by the line connecting the centers of femoral heads in normal subjects, was important for these parameters measurement. However, in subjects with fused hip joint or deformed femoral heads, the accurate localization of hip axis become imprecise. Herein, the upper edge of the pubic symphysis, which is easy to identify on the lateral X-ray film, was selected as an alternative landmark of the hip axis, and two morphologic parameters, the sacrum pubic incidence (SPI) and sacrum pubic posterior angle (SPPA), were proposed accordingly. The present study aimed to understand the reliability of these two parameters and their value in predicting PI, PRS1 and FSPA.
METHODS: Upright standing spine and pelvis radiographs of 60 normal adults (30 male and 30 female) with an average age of 38.5 years were obtained. Two independent observers then measured the following radiological parameters on the films: PI, PRS1, FSPA, SPI, SPPA, sacral slope, pelvic tilt and lumbar lordosis. The SPI is the angle between the line perpendicular to the superior plate of the first sacral vertebra at its midpoint and the line connecting this point to the upper edge of the pubic symphysis, while the SPPA is the angle between the line extending from the posterior upper edge of the sacrum to the upper edge of the pubic symphysis and the posterior side of the first sacral vertebral body. The intra-observer and inter-observer reliabilities of the parameters were analyzed using intraclass correlations. The correlations between parameters were analyzed by Pearson's correlation coefficients. Regression analysis was carried out to establish formulas to predict the values of PI, PRS1 and FSPA using the SPI and SPPA. A p < 0.05 was considered statistically significant.
RESULTS: The SPI was 64.4° ± 9.5° and 68.3° ± 9.4°, and the SPPA was 77.7° ± 7.5° and 78.7° ± 9.4° in males and females, respectively. These radiological parameters showed excellent intra- and inter-observer reliabilities, with an intraclass correlation >0.8. No gender differences were identified in these morphologic and positional radiological parameters. The SPI demonstrated strong correlation with PI and PRS1 (R (2) > 0.9, p < 0.001). In addition, strong correlation was also found between SPPA and FSPA (R (2) > 0.9, p < 0.001). Furthermore, both SPPA and FSPA showed close correlations with the other morphologic and positional parameters. Linear regression analysis established equations to predict PI and PRS1 using SPI and to predict FSPA by SPPA with significantly high reliability.
CONCLUSIONS: Both SPI and SPPA are reliable parameters for determining the morphology of the pelvis. The SPI is precise in predicting PI and PRS1, while SPPA is reliable in predicting FSPA. The SPI and SPPA will allow further study on lateral spinal-pelvic alignment in patients with hip joint abnormalities.