The pelvic is a basin. If you like to elk hunt, and you stand on a ridge in the high country of the Rockies, you can look into a basin full of elk, yet can't see them until they move!
The pelvis consists of three major bones; two called the ilium and the central bone called the sacrum. Working together the three large bones provide a container for the internal organs and viscera. The core muscle, the psoas, moves through the pelvic basin creating a dynamic and supple support for the internal organs. The iliacus muscle, sharing a tendon with the psoas at the lesser trochanter, fans up and opens, lining the inside of each ilium. It is the iliacus muscle that helps shape the inner bowl of the basin. The muscles attaching from the sacrum to the femurs and from the sacrum to the front of the pelvic basin (the pubis) form the bottom diaphragm of the free-swinging pelvic basin.
The sacrum, in the center of the pelvic, is the keystone and the base of the spinal column, plays the integrating role in all three systems. Suspended between the two iliums it is the sacrum's igniting fluid motion of flexion and extension that brings the two openings (mouth and anus) together.
The sacroiliac joints (SI Joints) are not skeletally articulated, but consist of a web of ligaments that weave the three bones together. As ligaments they are strong, maintaining "range-of-motion" within the joint, have limited rotational ability and do not respond positively to torquing! Only during the birthing process for females as a result of specific hormones released into the bloodstream, do the SI joint ligaments intentionally soften and become pliable in preparation for forming the birth canal.
Major proprioceptive neuro-receptors, responsible for coordinating alignment, balance and orientation, are imbedded within the SI joint ligaments. Sensitive and responsive to our continually changing relationship with the earth's gravitational forces, these neuro-receptors constantly register and respond to physical and mental, and emotional information.
Proprioceptors are electrical in nature. When stressed they can become overloaded. Just as an electrical fuse blows when power surges, so the human electrical system can short circuit when overwhelmed! What is commonly referred to as 'blowing out' the SI joints, is the result of excessive stress on the perceptive pelvic attachments.
Stressed pelvic proprioceptors do not fire properly and "cannot" send valid information through the spine to the brain via the central nervous system. No matter what the origin, stress has the power to pull the pelvic keystone out of its integral position. Cranial receptors located in the head/neck take over for the insufficiency of pelvic receptors by backing-up the pelvic proprioceptive system.
It's interesting that grinding teeth is a proprioceptive method for regaining balance, coordination and positioning while maintaining the essential flow of synovial fluid throughout the central nervous system. Any tension within the pelvis will be reflected in the jaw and visa versa. Dental surgery, orthodontic apparatuses and accidents affecting the face, jaw and head will alter the coherent relationship between cranium and sacrum.