Radiographic Assessment of Sagittal Spinopelvic Alignment and Coccygeal Morphology and Mobility in Sitting and Standing Positions in Coccydynia

Purpose Coccygeal morphology has been described in coccydynia, but dynamic assessments have been limited to standing or supine positions. As coccydynia is posture-related and aggravated by sitting, evaluating functional postures is essential but lacking. Not all individuals with similar morphology or risk factors develop symptoms, suggesting that posture-dependent biomechanical differences may contribute to pain. The coccyx participates in spinopelvic motion during sitting, and alterations in this dynamic alignment may influence symptom onset. Therefore, this study aimed to investigate posture-dependent changes in spinopelvic and coccygeal parameters among different coccygeal morphologic types and to explore their potential association with coccydynia. Methods Lateral radiographs were obtained in standing and sitting positions for 50 patients with coccydynia. Spinopelvic (thoracic kyphosis [TK], thoracolumbar kyphosis [TLK], lumbar lordosis [LL], thoracopelvic angle [TPA-T1PA], anterior pelvic plane angle [APPA], sagittal vertical axis [SVA], sacral slope [SS], pelvic incidence [PI], pelvic tilt [PT], spinosacral angle [SSA], and sacral kyphosis [SK]) and coccygeal (sacrococcygeal [SCA] and intercoccygeal [ICA] angles, coccygeal [CH], sacral [SH], and sacrococcygeal height [SCH]) parameters were measured and compared. Coccygeal morphology (segments, fusion, mobility) was evaluated and classified using the modified Postacchini-Massobrio system. Logistic regression identified predictors of pain, and analyses by morphology and low back pain (LBP) status examined associations with posture-related adaptability. Results Significant postural changes were observed in spinopelvic (TK, TLK, LL, TPA-T1PA, SVA, APPA, SS, PT, SSA) and coccygeal (SCA, CH) parameters (p < 0.05). Logistic regression identified ICA, TLK, and APPA as pain predictors: ICA increased odds by 9% (p = 0.028), TLK reduced odds by 13% (p = 0.038), and APPA increased odds by 19% (p = 0.002). Types 1 and 2 showed the greatest postural variability, while Types 4 and 5 were more rigid. SK showed a significant interaction with LBP (p = 0.04, η²=0.01), and ICA was associated with morphological type (p = 0.01, η²=0.05). Conclusion Sitting posture significantly alters sagittal spinopelvic alignment in coccydynia. Coccygeal morphology and mobility appear to influence these postural adaptations and pain severity. Patients with coccydynia demonstrated significant changes in spinopelvic parameters, particularly LL, SS, PT, and SVA, with ICA emerging as a morphology-related marker. Dynamic assessment in functional postures, particularly sitting, may provide valuable insights for diagnosis and rehabilitation planning in patients with coccydynia.