Introduction: Pelvic incidence (PI) is a sagittal anatomical parameter of the pelvis and is known to remain unchanged. However, recent studies found PI increase with age. No studies have been conducted evaluating longitudinal changes in PI. Our objective of this study was to evaluate the longitudinal change of PI and related demographic and radiological factors.

Methods: We analyzed 84 individuals who did not undergone spinal instrumentation and were followed more than 13 years in our institute (the mean duration of follow-up length was 14.2 ± 0.8 years). Standing lumbar radiographs were taken at the initial visit and at the final follow-up. PI, pelvic tilt (PT), sacral slope (SS), L1-S1 lumbar lordosis (LL), L4-S1 lower lumbar lordosis (LLL), anteroposterior length of S1 upper endplate (S1UE), and pelvic thickness (PTH) were measured on both radiographs. Pelvic height (PH) and anteroposterior pelvic length (PA) were also measured (Figure 1). All baseline and follow-up parameters were compared using the paired t-test. Study subjects were divided into PI change group (PI increase more than 5° during follow-up, n=21) and the control group (n=63) and the reference parameters were compared between groups using the t-test. Among PI change group, changes in PTH, PH and PA (normalized by S1UE) were analyzed.

Results: The mean age at the initial visit was 49.9 ± 14.6 years (range: 19-82 years). There were 27 men and 57 women. Baseline PI, PT, SS, LL, LLL, S1, and PTH were 50.5 ± 10.0°, 15.7 ± 7.7°, 34.7 ± 10.7°, 45.4 ± 13.2°, 33.9 ± 8.5°, 41.4 ± 3.8mm, and 132.0±108mm, respectively. PI and LLL were not significantly different between initial and follow-up images. PT increased, LL decreased during follow-up (all P<0.05). Between PI change group and the control group, age and gender were not significantly different. In PI change group, the baseline LL (40.3 ± 15.0° vs. 47.0 ± 12.3°, P=0.041) and the LLL (30.8 ± 7.8° vs. 35.0 ± 8.5°, P=0.046) were lower. Among the PI change group, PA increased (2.4 ± 0.4 to 2.6 ± 0.4, P<0.001) and PH decreased (2.2 ± 0.4 to 1.8 ± 0.5, P<0.001) without significant changes in PTH (3.3 ± 0.4 to 3.2 ± 0.3, P=0.500)

Discussion: PI did not change significantly changed in a minimum follow-up of 13 years. However, PI increased more than 5 degrees in 25% of cases. There was no difference in age between the group with and without PI, and there was a difference in LL and LLL. This implies that sagittal imbalance has more influence on PI change than age itself. In the cases where the PI was changed, the distance between the sacrum center and the femoral head center did not change, but changed in the form of increasing PA and decreasing PH. A larger number of study on PI changes are additionally needed to confirm to change of PI, and biomechnical study on PI changes are also needed to understand the change of PI.   

Figure 1. Anteroposterior length of pelvis and pelvic height