Differences in pelvic tilt and spinal alignment according to age in patients with hip dysplasia

INTRODUCTION: We aimed to establish age-related characteristics of posture-dependent pelvic tilt and standing spinopelvic alignment in patients with symptomatic hip dysplasia (HD) from adolescence to middle age. MATERIALS AND METHODS: A total of 101 patients with HD indicated for eccentric rotational acetabular osteotomy (ERAO) between 2016 and 2024 were included. Pelvic tilt in supine and standing positions was quantified using the anterior pelvic plane (APP) angle estimated from computed tomography-based 3D pelvic models matched to supine and standing anteroposterior hip radiographs. The change from supine to standing was calculated as standing minus supine; negative values indicated a posterior change. Three-dimensional acetabular coverage during postural transition was compared across age groups. Standing radiographic hip and spinopelvic parameters were also examined using multivariable regression analysis to identify factors associated with age. RESULTS: Age was correlated with the supine-to-standing change in the APP angle (r = 0.47, p < 0.001). The change shifted closer to zero with increasing age, with younger patients showing a larger posterior change compared with older patients. Patients in their teens showed a more pronounced decrease in anterosuperior acetabular coverage at the 2 o'clock position compared with those in their 40 s (p = 0.017) and 50 s (p = 0.040). Furthermore, anterior wall index in standing (β = 0.26, p = 0.007), sagittal vertical axis (β = 0.24, p = 0.015), and sacral slope (β = 0.24, p = 0.014) were identified as independent factors associated with age. CONCLUSIONS: Younger patients with HD showed a larger posterior change in pelvic tilt during postural transition compared with middle-aged patients. These findings support preoperative assessment of functional acetabular coverage in the standing position, particularly in the anterosuperior region in younger patients, when planning ERAO. Longitudinal studies are needed to determine whether maintaining anterior pelvic tilt during standing contributes to preservation of joint integrity into middle adulthood.