The sacrum is a strong, triangular bone forming the back part of the pelvis and lowest part of the spine, to which the coccyx or pointed tail-end of the spine is attached below. The sacrum is joined on each side to the hipbones, and above it is joined to the last, 5th, lumbar vertebrata.
The sacrum really consists of segments or vertebrae which are fused together. The bone does not become completely fused and solidified before adolescence.
The sacrum has five openings on each side, through which nerves from the spinal cord pass.These nerves join other nerves coming out from between the lumbar vertebrae and together form a plexus (lumbosacral plexus) of nerves which furnish the nerve supply of the lowest part of the trunk and of the lower limbs.
The well-known sciatic nerve in the back of the thigh emanates from this nerve plexus. The sciatic nerve is easily affected by neuritis and neuralgia because it is frequently exposed to cold and wet.
The sacroiliac joints are the ones which give rise to the familiar sacroiliac disease or lower back pain when these joints are strained or affected by arthritis