Dear Dr. Sugimoto-san,
we have carefully examined the photographs you sent recently to Professor Borkhvardt with a CT image of the abnormal bone in the sacro-pelvic region of a 6 year old boy.
Indeed, I think that your sugestion about the sacral rib might be correct. Normally, mammals, including humans have very small ribs adjacent to the sacral transverse processes, which fuse very soon in ontogenesis. However, if something wrong is happening they may develop as a separate bone and expand much along the corresponding myoseptum.
Several cases have been reported of such an anomaly in the past. I attach examples of case reports of such a kind.
Most reports say there is a narrow protuberance from the sacrum, which is usually interpreted as a rib, or a digit (sacral or pelvic). However, I met also an interpretation of this structure as coccygeal duplication.
When I look at your image, I also think that there might be an alternative explanation as for example, an underdeveloped duplicated pelvis from one side. E.g., when the limb bud develops, it could be initially duplicated, but later in development only the larger counterpart developed into a normal limb, while the smaller duplicated limb bud gave rise only to a small portion of the second ilium, which remains within a normal pelvis. This alternative arises from the fact, that at your image the "sacral rib" does not look much like a real rib or a digit, it is rather wide and short. Otherwise, this anomaly should have a spectrum of manifestations. I would then suggest to examine carefully the literature to decide, which interpretation is most close to your particular case.
An interesting thing, for me at least, is that in any case, this strange anomaly is most usually, unilateral. One could suggest that some genes like Pitx1, which is responsible for pelvis development is involved, since when this gene is disrupted it may cause a unilateral underdeveloped pelvis. However, as far as I know sacrum anomalies, additional sacral ribs (in lizards) or sacral transverse processes (in amphibians) are also very often appears on one side only. It would be ineteresting to learn, which side is preferential....
As for the correct diagnosis, the literature says, it might be necessary to learn, whether there is a degeneration of pelvic muscles around the "sacral rib" to be certain, that the case reported is this particular anomaly. But as you may learn from our paper with Prof Borkhvardt, the muscle degeneration may be a prerequisite of pelvis formation, at least in reptiles. I am not sure about mammals.
Please, do not hesitate to contact me again, if you have any further questions or if I could be of any help with this interesting case.
I wish your patient no pain and good health.
...by the way, do you know, is this abnormal bone connects to the ischium (I can not understand this from the image)?
Dr. Yegor B. Malashichev, Associate Professor/Dozent
Department of Embryology & Department of Vertebrate Zoology,
St. Petersburg State University, Universitetskaya nab., 7/9,
St. Petersburg, 199034, Russia.
Phone: +7(812)328-9453; -9689; Fax: +7(812)328-9569
２）Underdeveloped duplicated pelvisという考えもある．
そうなると，sacral ribというよりは，duplicated pelvisと