Pelvic Fractures and Dislocations

The pelvis is originally made up of fifteen bones; three in either os innominatum, five in the sacrum and four in the coccyx. When ossification is complete we have but four, including the coccyx which is attached to the lower end of the sacrum. The pelvis has fifty-five centers of ossification, some of which are of surgical importance. The os innominatum has three primary centers, one for each of the three bones entering into its formation. The ilium, ischium and pubis meet in the acetabulum, and prior to the fusion of these three bones they are separated by a Y-shaped cartilage. A blow on the trochanter therefore, may break the os innominatum into its three anatomical parts, the injury resembling the epiphyseal separations seen elsewhere in the long bones. The entire crest of the ilium may be separated as an epiphysis, or its anterior portion may be pulled away by muscular action prior to the twenty-fourth year, at which time it joins the rest of the ilium. The epiphysis of the anterior inferior iliac spine has been avulsed by the action of the straight head of the rectus femoris, and by traction transmitted to it through the Y-ligament of Bigelow. The development of the sacrum is also of interest in this respect; the lateral surfaces of the bone are ossified by plates which are in reality epiphyses. Fracture in this region will find the plane of least resistance, often following the epiphyseal cartilages rather than the articulation itself. Fracture of the incompletely ossified sacrum follows the cartilage rather than the bone. The os innominata transmit the weight of the body from the spine to the femora in the erect posture, or to the tubera ischiorum in the sitting position. The sacrum, together with that portion of the os innominatum behind the acetabula, forms an arch for the transmission of weight to the femora; another arch with the tubera ischiorum as the base transmits the weight from the spine to the object on which the person sits. The anterior portion of the pelvis is composed of the pubes, and the rami which connect them above and below the thyroid foramina with the remainder of the pelvis. This part of the pelvis is composed of two counter-arches for the support of the body, one lighter than the posterior and more frequently fractured. Numerous muscles are attached to the surface of the pelvis, both internally and externally, and these muscles each have a displacing tendency when fracture is present. Their origins and insertions should be understood in order that we may appreciate their influence on deformity. When antero-posterior force is applied the anterior portion of the pelvis gives way first, and then, if the force continues to act, the lateral portions of the pelvis are forced apart and there is likely to be injury in the region of the sacro-iliac joints. Transverse violence breaks the counter-arches forcing them forward, after which the main arches give way and we have fracture of the lateral portions of the pelvis or in the region of the sacro-iliac synchondrosis. In the first instance fracture of the anterior portion is by direct violence, while the second is by indirect violence. The action of violence on the pelvis might be likened to the mechanism of fracture of the skull, which results in fracture 'by bending' and fracture 'by bursting'. Isolated fracture of the posterior portion of the pelvis only occurs in the presence of well localized and severe trauma. Numerous muscles are attached to the surface of the pelvis, both internally and externally, and these muscles each have a displacing tendency when fracture is present. Their origins and insertions should be understood in order that we may appreciate their influence on deformity. To the crest of the ilium we have the erector spines, the latissimus dorsi, quadratus lumborum, external oblique, internal oblique and transversus abdominalis, tensor vaginae femoris and sartorius. These muscles, with the exception of the last two, pull in an upward, or upward and inward direction, while the last two pull downward toward the thigh. The external surface of the ilium affords attachment to the three gluteal muscles, while the inner surface, above the brim of the true pelvis, is taken up by the origin of the iliacus. The pubis affords attachment to the conjoined tendon of the internal oblique and transversalis, the tendon of the external oblique, rectus and pyramidalis. All the muscles of this group pull upward, or upward and outward. The anterior surface of the pubis and the ischiopubic rami afford attachment to the adductor muscles; the tuber ischii to the semimembranosus, semitendinosus and the biceps, all of which have a downward pull. When the fragment is free the tendency is toward displacement in the line of pull of the muscle attached. The vascular and nervous structures passing under Poupart's ligament lie close to the ascending ramus of the pubis and may be injured by displaced fragments. We have the internal pudic artery running on the inner surface of the ischiopubic rami where it may be injured when this portion of the pelvis is fractured. The structures passing through the greater and lesser sciatic foramina are also to be considered in fracture of the posterior portion of the pelvis. The bladder lies behind the pubes and is more or less attached to the bones by fibrous tissue; it may be injured by compression when the viscus is full, by the penetration of fragments or by the pull of the connective tissue when portions of the bone are displaced. The two layers of the triangular ligament, composed of heavy fibrous tissue, are firmly attached to the ischiopubic rami which form the subpubic arch. When this arch -is fractured the ligament is usually torn and since the urethra passes through the ligament it may also suffer injury. The rectum is situated in the posterior portion of the pelvis, follows the curve of the anterior surface of the sacrum and coccyx, and in this protected position is rarely injured. The nerves passing through the sacrum and in close relation to the coccyx may be injured when this region of the pelvis is fractured.<Callout type=