The coccyx when nortually placed is rarely fractured, on account of its great mobility and small size. It always happens by di rect violence. When ankylosed, it is more frequently broken, and instances of this have been before mentioned, in relation to the obstruction of the outlet in parturition which it occasions. It is diagnosed by the mo bility and grating of the fragments, and by the pain caused by the action of the great glutei muscles. Dislocation of the coccyx is said to have occurred backward in difficult labours, and to have been followed by abscess, but these cases have been most probably fractures like those just mentioned.
Fractures of the innominate bone generally occur on one side only, where the greatest part of the force has fallen, but sometimes on both. They are most frequently found in the ilium—which is most exposed, but often impli cate both the ischium and pubes. They may be confined to one part of the bone ; in which case they are found chiefly about the iliac crest and wing. Boyer relates a case in wkich the inferior anterior iliac spine was broken off by the kick of a horse. Cases are not un frequently seen where the anterior superior iliac spine and a portion of the crest are broken off. Sometimes they are comminuted in, and radiate from, the cotyloid cavity, such fractures generally resulting from direct vio lence against the lateral pelvic arch, and act ing on the head of the femur so as to drive it inwards through the pelvic wall.
Fractures of the pelvis, like those of the spinal column, are seldom present without dis kcation also of the sacro-iliac or pubic joints. This results from the circular arrangement of its bones, and from the laws of its mechanism, explained in the first section of this article. Thus, when force is applied so as to compress the pelvic circle from before backward (as commonly the cause of these fractures is such compression by the wheels of a loaded cart or other vehicle), then the cotylo-sacral arch yields inwards at its haunches,—the sacro-iliac joints, the anterior ligaments of which are torn, and the articular surfaces separated in front. At the same time the pubic tie yields either at the symphysis or in the superior rami.
The innominate bone may be entirely dis located upward or backward, generally on one side only. A case is related by Cloquet, in which this was the case on both sides, the pubic symphysis, at the same time, separated + an inch, the pubis and ischio-pubic ramus were broken, and the bladder ruptured. Another case, where the left innominate bone was dis placed upwards, was treated successfully by Chaussier.
The ligaments and fibro-cartilage of the sym physis pubis are usually torn, the latter ge nerally carrying with it a portion of one of the bones ; or the superior ramus is broken at its weakest part, above the obturator fora men, or it separates from the other innorninate pieces. In most cases the ischio-pubic ramus of the same side also yields at or near the point of suture, which is its thinnest and weakest part, and the obturator foramen is again penetrated. This results from the ope ration of the force upon the unsupported ischio-sacral arch and its tie, after the other has yielded. A preparation exhibiting such a fracture is found in the King's College Mu seum. In this case the superior pubic ramus has separated in the acetabulum from the other pieces of the innominatum in the line of the Y-shaped suture, while the fracture through the ischio-pubic ramus extends up. wards, along the side, and into the centre of the pubic symphysis (fig. 126.).
This fracture, accompanied by that of the superior pubic ramus, may also happen from a fall from a great height upon the breech, on one side or both, as the force happens to fall. The great strength of the body of the ischium renders a fracture there of less frequent oc currence than in the other pieces of the innominatam.
Fractures of the innominate bones seldom occur without displacenzent, produced usual ly— not by subsequent muscular action, which is kept in check by their balanc2 of power, and by the extensive ligamentous and fascial at tachments and the opposing fractured sur faces,—but by the original direct violence. By such displacement of comnainuted and spicular fragments in the true pelvis, the bladder and urethra, particularly if the former be distended at the time of the accident, often suffer great laceration, which may even extend to the peritoneal investments and open the cavity of the abdomen. Such extensive injuries are invariably followed by extravasa tion of the urine into the pelvic areolar struc tures or peritoneal sac ; and violent peritonitis carries off the patient, even if he survive the first shock of such a formidable accident. The external soft parts, also, generally suffer greatly from the violence, and from the great extravasation of blood which usually takes place from the torn vessels. Gangrene may, in these cases, succeed to a great extent, and destroy the patient. It is these injuries, and their consequences to the soft parts and in ternal structures, that render fracture of the pelvis, like those of the cranium, so dangerous and fatal to life.