The articulations of the pelvis group may be subdivided into:

• The sacro-iliac articulation.
• The sacro-coccygeal articulation.
• The intercoccygeal articulation.
• The symphysis pubis articulation.

The Sacro-iliac Articulation and Sacro-sciatic Ligaments

Class. - Diarthrosis. Subdivision. - Arthrodia.

It is now generally admitted that the sacro-iliac joint is a diarthrosis, the articular surface of each bone being covered with a layer of cartilage, whilst the cavity of the joint is a narrow cleft and the capsule is extremely thick posteriorly. The cartilage on the sacrum is much thicker than that on the ilium and the cartilages are sometimes bound together here and there by fibrous strands. The different character of the joint in the two sexes should be noted. Briefly, the female joint has strong ligamentous bonds with but little bony apposition, while the male joint gains its strength by virtue of extensive areas of bony contact and a slighter development of ligaments. This difference is, of course, a physiological one; for some laxity of the joint is demanded during pregnancy and labour. The bones which enter into the joint are the sacrum and ilium, and they are bound together by the following ligaments:

• Anterior sacro-iliac.
• Superior sacro-iliac.
• Posterior sacro-iliac.
• Inferior sacro-iliac.
• Interosseous.

The anterior sacro-iliac ligament consists of well-marked glistening fibers which pass above into the superior, and below into the inferior, ligaments. It extends from the first three bones of the sacrum to the ilium between the brim of the pelvis minor and the great sciatic notch, blending with the periosteum of the sacrum and ilium as it passes away from the united edges of the bones.

The superior sacro-iliac ligament extends across the upper margins of the joint, from the ala of the sacrum to the iliac fossa, being well marked along the brim of the pelvis, where it is thickened by some closely packed fibers. Behind, it is far stronger, especially beneath the transverse process of the fifth lumbar vertebra. This ligament is connected with the strong sacro-lumbar ligament, which spreads lateralward and forward over the joint to reach the iliac fossa and terminal line. By some authors, it is described as a part of the ilio-lumbar ligament.

The posterior sacro-iliac ligament is extremely strong and consists essentially of two sets of fibers, deep and superficial. The deep fibers (short posterior sacro-iliac ligament) pass downward and medialward from the rough area of the

 ilium behind the auricular surface to the back of the lateral mass of the sacrum, both lateral to and between the upper foramina and to the upper sacral articular process, and the area between it and the first sacral foramen. The deepest fibers of this group constitute the so-called interosseous ligament. The more superficial fibers (long posterior sacro-iliac ligament) are oblique or vertical, and pass from the posterior superior iliac spine to the second, third, and fourth tubercles on the back of the sacrum, a more or less well-defined band which goes to the third and fourth sacral tubercles being called sometimes the oblique sacro-iliac band and sometimes the long straight band.

The inferior sacro-iliac ligament is covered behind by the upper end of the sacro-tuberous ligament; it consists of strong fibers extending from the lateral border of the sacrum below the articular facet to the posterior iliac spines; some of the fibers are attached to the deep surface of the ilium and join the interosseous ligament.

The interosseous ligament is the strongest of all, and consists of fibers of different lengths passing in various directions between the two bones. Immediately above the interspinous notch of the ilium the fibers of this ligament are very strong, and form an open network, in the interstices of which is a quantity of fat in which the articular vessels ramify.

The ear-shaped cartilaginous plate, which unites the bones firmly, is accurately applied to the auricular surfaces of the sacrum and ilium. It is about one-twelfth of an inch (2 mm.) thick in the center, but becomes thinner toward the edges. Though closely adherent to the bones, it tears away from one entirely, or from both partially, on the application of violence, sometimes breaking irregularly so that the greater portion remains connected with one bone, leaving the other bone rough and bare. It is usually one mass, and is only occasionally formed of two plates with a synovial cavity between them.

Because of the occasional presence of a more or less extensive synovial cavity within the fibro-cartilage, and also of a synovial lining to the ligaments passing in front and behind the articulation, the term 'diarthro-amphiarthrosis' has been given to this joint, and also to the symphysis pubis. Testut mentions certain folds of synovial membrane filling up gaps which here and there occur at the margin of the fibro-cartilage but they are not usually seen.

The sacro-tuberous (great sciatic) ligament is attached above to the posterior extremity of the crest of the ilium and the lateral aspect of the posterior iliac spines. From this attachment, some of its fibers pass downward and backward to be attached to the lateral borders and posterior surfaces of the lower three sacral vertebrae and upper two segments of the coccyx; while others, after passing for a certain distance backward, curve forward and downward to the ischium, forming the anterior free margin of the ligament where it limits posteriorly the sciatic foramina. These fibers are joined by others which arise from the posterior surfaces of the lower three sacral vertebrae and upper pieces of the coccyx. At the ischium, it is fixed to the medial border of the tuberosity, and sends a thin sharp process upward along the ramus of the ischium which is called the falciform process, and is a prolongation of the posterior edge of the ligament.

A great many fibers pass on directly into the tendon of the biceps muscle, so that traction on this muscle braces up the whole ligament, and the coccyx is thus made to move on the sacrum. The ligament may not unfairly be described as a tendinous expansion of the muscle, whereby its action is extended and a more advantageous leverage given. It is broad and flat at its attached ends, but narrower and thicker in the center, looking like two triangular expansions joined by a flat band, the larger triangle being at the ilium, and the smaller at the ischium The fibers of the ligament are twisted upon its axis at the narrow part, so that some of the superior fibers pass to the lower border.

The posterior surface gives origin to the gluteus maximus muscle, and on it ramify the loop; from the posterior branches of the sacral nerves; its anterior surface is closely connected at its origin, with the sacro-spinous ligament, and some fibers of the piriformis muscle arise from its below the obturator internus passes out of the pelvis under its cover, and the internal pudic vessels and nerve pass in. At the ilium, its posterior edge is continuous with the vertebra, aponeurosis; while to the anterior edge is attached, the thick fascia covering the gluteus medius. The obturator fascia is attached to its falciform edge. It is pierced by the coccygeal branches of the inferior gluteal {sciatic) artery and the inferior clunial (perforating cutaneous) nerve from the second and third sacral.

The sacro-spinous (small sciatic) ligament is triangular and thin, springing by a broad base from the lateral border of the sacrum and coccyx, from the front of the sacrum both above and below the level of the fourth sacral foramen, and from the coccyx nearly as far as its tip. By its apex, it is attached to the front surface and the borders of the ischial spine as far outward as its base. Its fibers decussate so that the lower ones at the coccyx become the highest at the ischial spine; muscular fibers are often seen intermingled with the ligamentous.

The sacro-spinous ligament is situated in front of the sacro-tuberous ligament, with which it is closely connected at the sacrum, and separates the greater from the lesser sciatic foramen.

Its front surface gives attachment to the coccygeus muscle, which overlies it. Behind, it is connected with, and hidden by, the sacro-tuberous ligament, so that only the lateral inch or less (2 cm.) and a small part of its attachment to the coccyx can be seen; the internal pudic nerve also passes over the posterior surface.

The arterial supply of the sacro-iliac joint comes from the superior gluteal, ilio-lumbar, and lateral sacral.

The nerve-supply is from the superior gluteal, sacral plexus, and external twigs of the posterior divisions of the first and second sacral nerves.

Movements. - Recent investigations have shown that in spite of the interlocking of the articular surfaces and the strong ligaments connecting the bones together a slight amount of movement, both a gliding and rotatory, does occur at the sacro-iliac joint. The gliding movement is both up and down, and forward and backward, and the latter is associated with a slight rotation round a transverse axis which passes through the upper tubercles on the back of the sacrum. The movement is but small in extent, nevertheless as the base of the sacrum moves downward and forward the conjugate (antero-posterior) diameter of the pelvic inlet is diminished and at the same time, as the coccyx moves up and back, the conjugate diameter of the outlet is increased. This rotatory movement is limited principally by the sacro-sciatic (sacro-tuberous and sacro-spinous) ligaments which prevent any extensive upward and backward movement of the coccyx and lower part of the sacrum.

Downward displacement of the sacrum when the body is in the sitting posture is prevented not only by the surrounding ligaments, but also by the wedge-like character of the sacrum, which is broader above than below. Downward and forward displacement of the sacrum in the erect posture is prevented by the ligaments and more particularly by the posterior sacro-iliac bands, while backward displacement would be hindered by the breadth of the anterior as contrasted with the posterior part of the sacrum as well as by the anterior ligaments.

Relations. - The sacro-iliac joint is in relation above with psoas and iliacus. In front, it is in relation at its upper part with the hypogastric vessels and obturator nerve, and at its lower part with the piriformis muscle.