Pelvic Incidence Measurement with Supine Magnetic Resonance Imaging: A Validity and Reliability Study. and Reliability Study

Objective: Classically, pelvic incidence (PI) and other spinopelvic sagittal parameters are measured using plain x-ray obtained with the patient standing. However, it is difficult to obtain a perfect mid-sagittal appearance of the sacral endplate and superimposition of both femoral heads from a plain x-ray. Overlapping of the iliac wings also could obscure the appearance of the sacral endplate. Recent studies showed that MRI was more reliable than x-ray for evaluating some spinal sagittal parameters. To our knowledge, measurements of spinopelvic sagittal parameters using supine MRI have not been reported previously. We assessed the validity and reliability of measurements of spinopelvic sagittal parameters from standing lateral x-rays and supine magnetic resonance imaging (MRI). Methods: We recruited 26 asymptomatic volunteers for this study. Standing lateral lumbosacral radiographs, including femoral heads and spinopelvic MRI images with coronal images of the femoral heads were performed. The anatomic reference point required to measure PI was found on coronal MRI images and transferred to the midline sagittal MRI using the bladder wall as a second reference point. PI, sacral slope (SS), and pelvic tilt (PT) were measured on x-ray and MRI images. Validity and reliability of results also were tested. Results: Of 14 males and 12 females (average age, 31.30), PI was obtained from x-ray and MRI in 52. +/- 6.89 and 51.42 +/- 6.43, respectively. From standing x-ray to supine MRI, PT decreased by 3.16(?)degrees, while SS increased 2.5?degrees. A paired t-test showed a significant difference between PT values from x-ray and MRI. The correlation was highest between the x-ray and MRI measurements of PI, PT, and SS, respectively. Intraobserver and interobserver reliabilities were between 0.88 and 0.96 on x-ray and MRI. All reliabilities were excellent, although MRI values were higher. Conclusion: MRI was more reliable in the measurement of spinopelvic parameters than classic standing x-ray examination. Higher reliability and being radiation-free could make MRI a good alternative to standing x-ray.