Lumbo-Sacral Joint

This takes place between the fifth lumbar vertebra and the sacrum, and is similar to the other intervertebral joints. It has, however, an accessory ligament, the lateral lumbosacral ligament, which is attached to the front of the transverse process of the fifth lumbar vertebra and the front of the sacrum close to the sacro-iliac joint.

Sacro-Iliac Joint

Sacro-Iliac Joint between the articulating portion of the wing of the ilium and the similar auricular surface on the sacrum. The joint surfaces are very closely applied to one another, so that movement is limited, as great stability is required rather than free movement. The joints are just movable enough to prevent absolute rigidity, and their roughened surfaces prevent gliding to any extent.

Ligaments

A capsular ligament surrounds the joint, which is strengthened by transverse fibres dorsally and ventrally.

The synovial membrane is rudimentary.

Accessory Ligaments: The Ilio-Lumbar ligament extends from the tip of the transverse process of the fifth lumbar vertebra to the iliac crest just behind its highest point. It is in reality a thickened portion of the lumbar fascia.

The Great Sciatic Ligament is triangular in shape. It is attached to the posterior inferior spine of the ilium, the tuberosity of the ischium, and to the posterior aspects of the lower part of the sacrum. It fills the gap between the innominate bone and sacrum, and, by completing the great and small sciatic notches, forms the great and small sacro-sciatic foramina.

The Small Sciatic Ligament is also triangular; it lies ventrally to the great sciatic ligament, and is attached by its base to the last two segments of the sacrum, and by its apex to the spine of the ischium.

The Symphysis Pubis is an immovable joint between the two pubic bones. The articular surfaces are covered with cartilage as usual, and have, in addition, an interarticular disc of cartilage welding them firmly together. There are ligaments all round the joint, the most important being the subpubic ligament, which is in the arch of the pubis and separated from the triangular ligament of the perineum by a small interval. The subpubic ligament is attached to the interarticular ligament and the descending rami of the pubic bones. There is no synovial membrane.

The Triangular Ligament of the perineum helps to complete the pelvic walls, and stretches across between the descending rami of the pubic bones. It occupies a horizontal position, and has superior aud inferior surfaces. Posteriorly it fuses with the deep fascia of the perineum.

The Obturator Membrane fills the obturator foramen, leaving superiorly a small canal, through which vessels and nerves pass.

There are no muscles acting on these joints in the ordinary way, the joints being for the sole purpose of preventing absolute rigidity, so that the pelvis will "give" to the pull of the muscles as required.