Cartilage formation in the pelvic skeleton during the embryonic and early-fetal period

The pelvic skeleton is formed via endochondral ossification. However, it is not known how the normal cartilage is formed before ossification occurs. Furthermore, the overall timeline of cartilage formation and the morphology of the cartilage in the pelvis are unclear. In this study, cartilage formation in the pelvic skeletons of 25 human fetuses (crown-rump length [CRL] = 11.9-75.0 mm) was observed using phase-contrast computed tomography and 7T magnetic resonance imaging. The chondrification center of the ilium, ischium, and pubis first appeared simultaneously at Carnegie stage (CS) 18, was located around the acetabulum, and grew radially in the later stage. The iliac crest formed at CS20 while the iliac body's central part remained chondrified. The iliac body formed a discoid at CS22. The growth rate was greater in the ilium than in the sacrum-coccyx, pubis, and ischium. Connection and articulation formed in a limited period, while the sacroiliac joint formed at CS21. The articulation of the pubic symphysis, connection of the articular column in the sacrum, and Y-shape connection of the three parts of the hip bones to the acetabulum were observed at CS23; the connection of the ischium and pubic ramus was observed at the early-fetal stage. Furthermore, the degree of connection at the center of the sacrum varied among samples. Most of the pelvimetry data showed a high correlation with CRL. The transverse and antero-posterior lengths of the pelvic inlet of the lesser pelvis varied among samples (R2 = 0.11). The subpubic angle also varied (65-90°) and was not correlated with CRL (R2 = 0.22). Moreover, cartilaginous structure formation appeared to influence bone structure. This study provides valuable information regarding the morphogenesis of the pelvic structure.