Radiographic study of peak velocity of pelvic incidence in adolescent idiopathic scoliosis

Background: Pelvic incidence (PI), a parameter related to the ideal spinopelvic alignment, is a morphological parameter that is usually considered fixed, but the PI's growth during adolescence has been reported. We investigated the peak PI velocity during adolescence and describe the relationship between increasing PI and changes in the morphology of the pelvis and sacrum. Methods: We measured standing height (SH) and radiological anatomical parameters including pelvic height (PH), pelvic width (PW), sacral width (SW), femoral head-sacrum (FH-S), sacrum-coccyx (S-C) length, and S-C distance at each follow-up of 76 adolescent idiopathic scoliosis (AIS) patients. Delta Parameter was the difference between the next measurement and the previous one. Growth velocity was Delta Parameter divided by time interval. All Delta Parameters were compared between different Risser stages using repeated measures analysis of variance (ANOVA). The Pearson coefficients of correlation were calculated to assess the relationships between PI and Delta Parameters. Results: PI reached peak growth with a 1.6 degrees/year growth in females and 1.8 degrees/year in males at Risser stage 1. PI tended to grow rapidly with Risser 0 and closed triradiate cartilage (female: 1.3 degrees/year and male: 1.4 degrees/year) and to slow down at Risser 2 (female: 1.2 degrees/year and male: 1.3 degrees/year). Delta PI strongly correlated with Delta FH-S (R>0.508, P<0.05) and also correlated with Delta SH, Delta PH, Delta PW, Delta SW, and Delta S-C length (R>0.192, P<0.05) but not correlated with Delta S-C distance and Delta S-C ratio. Conclusions: In patients with AIS, the peak PI velocity is at Risser 1, and it is still increasing at Risser 5. Our result suggested that the growth of the PI may be associated with SH and changing pelvic morphology during skeletal growth of adolescence.