A Novel, Easy-to-Measure Radiographic Parameter to Assess Spinopelvic Malalignment: The Pelvic Inclination Angle

BACKGROUND: Pelvic tilt (PT) is an important sagittal parameter to be restored to the normal range in corrective surgery for spinopelvic malalignment. However, the normal value of PT varies among patients. With the introduction of the pelvic inclination angle (PIA), which is the angle subtended by the vertical axis and the line connecting the anterior pubic tubercle and the anterior superior iliac spine, we aimed to determine whether the PIA could reflect the symptom severity and whether normal PIA values exist. METHODS: The study cohort consisted of patients with spinopelvic malalignment (patient group) and normal healthy adults (normal group). In the patient group, correlation analyses were performed to investigate the relationship between the PIA and other sagittal parameters and between the sagittal parameters and patient-reported outcome measures. In the normal group, correlation analysis was performed to assess the relationship between pelvic incidence (PI) and other sagittal parameters. The radiographic parameters were compared according to PI categories. RESULTS: There were 162 patients in the patient group, with a mean age of 71.1 years, and 108 in the normal group, with a mean age of 32.1 years. In the patient group, the PIA strongly correlated with the conventional parameters such as PT, PI-lumbar lordosis mismatch, and T1-pelvic angle. The PIA had weak to moderate correlations with all patient-reported outcome measures, of which the correlation coefficients were similar to or greater than those of other sagittal parameters with patient-reported outcome measures. In the normal group, PI showed moderate to strong correlations with all conventional sagittal parameters except for the PIA, which correlated very weakly with PI. Unlike the other sagittal parameters, the PIA did not significantly differ among the PI categories. CONCLUSIONS: The PIA reflected the symptom severity and had a normal value independent of PI. As an alternative to PT, the PIA can be a clinically useful parameter in evaluating and managing patients with spinopelvic malalignment. CLINICAL RELEVANCE: As the PIA is independent of PI, it will be more useful in differentiating between patients with normal spinopelvic alignment and those with spinopelvic malalignment and in reflecting the clinical symptoms of patients with spinopelvic malalignment. In addition, by providing a cutoff value of 13.6°, the PIA can be used as a surgical target or to predict postoperative outcomes.