The sacro-iliac articulation is a synovial joint between the auricular surfaces of the sacrum and ilium. The articular surface of each bone is covered with a plate of cartilage, which is thicker on the sacrum than on the ilium. Owing to the fact that the sacro-iliac joint interrupts the line of weight transmission from the vertebral column to the lower limbs, it is important; in view of the necessity for stability, that the range of :movement should be restricted. In the adult male a large number of short but strong bundles of fibers enter into the constitution of the sacro-iliac ligaments and, as a result, only a very small amount of antero-posterior rotatory movement is possible. in the female, after puberty, the range is much greater and it is increased temporarily in the later months of pregnancy.* The ligaments of the joints are: Anterior sacro-iliac, Interosseous sacro-iliac, and Posterior sacro-iliac.
The anterior sacro-iliac ligament (fig. 545) covers the anterior and inferior surfaces of the joint and consists of numerous thin bands. The superior fibers of the ligament connect the ala of the sacrum to the adjoining part of the iliac fossa; the inferior fibers are placed below the arcuate line, and unite the lateral parts of the three upper sacral vertebrae to the pre-auricular sulcus and adjacent part of the ilium.
The interosseous sacro-iliac ligament is very strong, and forms the chief bond of union between the two bones. It fills the irregular space immediately. above and behind the joint-cavity (figs, 548, 549) and is covered by the posterior sacroiliac ligaments. It consists of bundles of short fibers which connect the iliac and sacral tuberosities.
The posterior sacro-iliac ligament is oblique in direction; it connects the posterior superior iliac spine to the upper four transverse tubercles of the sacrum. Its lower fibers are long and partly blended with the upper end of the sacrotuberous ligament. Its lower fibers are short and nearly horizontal.
The ilium is connected to the fifth lumbar vertebra by the iliolumbar ligament and the sacrum to the ischium by the sacrotuberous and sacrospinous ligaments.
The iliolumbar ligament (fig. 545) is attached above to the lower and front part of the transverse process of the fifth lumbar vertebra, and occasionally has an additional, weak attachment to the transverse process of the fourth.
It radiates as it passes laterally and is attached by two main hands to the pelvis. The lower band runs to the ala of the ilium and the base of the sacrum, blending with the anterior sacro-iliac ligament: the upper, which gives partial origin to the quadratus lumborum muscle, is attached to the crest of the ilium immediately in front of the sacro-iliac joint and is continuous above with the lumbar fascia.
The sacrotuberous ligament (figs. 545; 546) is placed at the lower and posterior part of the pelvis. It is attached by a broad base to the posterior iliac spine (where it is partly blended with the posterior sacro-iliac ligament), to the third, fourth and fifth transverse tubercles of the sacrum and to the lateral margin of the lower part of the sacrum and upper part of the coccyx. Its fibers run obliquely downwards and laterally, and converge to form a thick, narrow band; this band widens out below and is fixed to the medial margin of the ischial tuberosity, and is continued along the ramus of the ischium under the name of the faldform process, the free concave edge of which gives attachment to the fascia of the obturator internus muscle. On its posterior surface the ligament gives origin to the lowest fibers of the gluteus maximus, and. some of the superficial fibers of its lower part are continued into the tendon of origin of the long head of the biceps femoris. The ligament is pierced by the coccygeal branches of the inferior gluteal artery, by the perforating cutaneous nerve and by minute filaments of the coccygeal plexus.
These two ligaments convert the sciatic notches into foramina. The greater sciatic foramen is bounded, in front and above, by the greater sciatic notch; behind by the sacrotuberous ligament; and below by the sacrospinous ligament and the spine of the ischiun. It is partially filled up, in the recent state, by the piriformis muscle which emerges from the pelvis through it. Above this muscle, the superior gluteal vessels and nerve pass out of the pelvis; and below it, the inferior gluteal vessels and nerve, the internal pudendal vessels and nerve, the sciatic and the posterior femoral cutaneous nerves, and the nerves to the obturator internus and quadratus femoris make their exit from the pelvis. The lesser sciatic foramen is bounded, in front, by the body of the ischium : above, by the body of the ischium and the sacrospinous ligament; behind, by the sacrotuberous ligament. It transmits the tendon of the obturator internus, the nerve to this muscle, and the internal pudendal vessels and nerve.
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