The sacrum also plays an significant role in the innervation of the pelvis. The pelvis is mainly innervated buy the sacral and coccygeal nerves and by the pelvic part of the autonomic nervous system. The piriformis muscle pads the posterior wall of the pelvis and forms a bed for the sacral and coccygeal nerve plexuses. On the anterior surface of the piriformis muscle is the sacral plexus. This is formed by the lumbosacral trunk, the ventral rami of the first three sacral nerves and the descending part of the fourth sacral nerve. The main nerves of the sacral plexus lie external to the parietal pevix dascia. All branches of the sacral plexus leave the pelcis through the greater sciatic foramen, except for the nerve to the piriformis muscle (S2), the perforating cutaneous nerve supply (S2,S3) and those to the pelvis diaphragm. From this plexus also arise the sciatic nerve, the pudenal nerve and both the superior and inferior gluteal nerves. All these supply muscles of the pelvis and also those in the lower limb and therefore infer great clinical importance upon the sacrum. Injuries to the sacral plexus are uncommon but may arise due to compression following pelvic tumours causing pain in the lower limb and especially during child birth as the head of the fetus may compress the plexus giving aching pains in the lower limb.
There are numerous blood vessels and supplies that pass through and around the sacrum that give it heightened importance in the pelvis. The lateral sacral arteries pass medially and descend anterior to the sacral ventral rami, giving off spinal branches that pass through the pelvic sacral foramina and supply the spinal meninges and the roots of the sacral nerves. Some branches of the lateral sacral arteries pass from the sacral canal through the dorsal foramina to supply the muscles and skin overlying the sacrum. The median sacral artery is a small unpaired artery that arises from the posterior surface of the abdominal aorta, just superior to its bifurcation and runs anterior to the body of the sacrum to end in a series of anastomoses that form the coccygeal body. Accompanying the medial sacral artery is the medial sacral vein which ends by joining the left common iliac vein.
The greatest importance that the sacrum gives to the pelvis is by means of the sacro-iliac articulations. The sacro-iliac articulations are normally immovable, They may become physiologically movable, in the pregnant woman, in order to facilitate the birth of the child, i.e., they exhibit functional adaptation. Following the act of parturition they normally become immobile, i.e., exhibit functional adaptation to weight carrying. Failure of either of these forms of adaptation is an abnormality. In case the articulations do not relax in the parturient woman, the whole process of adapting the birth canal and its contents, is exhibited by the head of the child. Normally the bony birth canal and the child's head mutually undergo adaptive changes. In case these articulations do not regain comparative immobility, following parturition, a condition of instability will exist, which will express itself in a disturbance of the statics of the body. Balancing and weight-carrying functions will be injured. Dissection of sacro-iliac joints discloses the existence of the same structures found in other joints, i.e., bone, cartilage, synovial membrane and ligaments. The fact that these structures do exist in the sacroiliac articulations, naturally classifies these joints as having possible mobility. These joints serve to absorb shocks transmitted through the legs to the pelvic girdle. The slight movement, normally possible in them, subjects them to much the same conditions which serve to injure other joints. The sacroiliac articulations are inherently weak, so far as any bony interlocking is concerned. Their stability is a matter of ligamentous strength. The sacrum is wedge-shaped from above downward and from anterior to posterior. The anterior surface, being broader than its posterior, does not serve well to offer resistance to the superincumbent weight of the spine. The sacrum articulates by its auricular surfaces with those of the ilia. The articulating surfaces of both bones are covered with cartilage. The joints are surrounded by capsular ligaments and contain synovial sacs. The opposing auricular surfaces are reciprocally, slightly, uneven but not enough so to sustain any weight without ligaments. The structure of the sacro-iliac synchondroses indicates that movement is possible and, in fact, probable. The primary object of the movement is to produce elasticity in the pelvic girdle and interrupt shocks which would be transmitted from the legs to the trunk.
In conclusion, we can see how the sacrum positively contributes to pelvis by being the strongest bone, providing a comprehensive nerve supply, having important articulations with the pelvis, being the site of muscle attachments of the pelvic floor, providing shape to the pelvis by means of the sacral curve and it’s association with the blood vessels in the area. However, despite all these essential roles, the sacrum is not mobile and it doesn’t offer any support when either sitting or standing. In light of all that has been said about the sacrum, I think that it plays an important role in the pelvis and can be considered it’s keystone.