The sacroiliac joint is the articular connection between the sacrum and the ilium. It connects the spine with the pelvic girdle[^1].
Anatomy
The SI joint is an amphiarthrosis. This means that it is a true joint with a joint space and capsule, but its mobility is severely limited by a tight ligamentous apparatus.
Biomechanics
Although mobility is minimal (approx. 2° to 4°), two specific tilting movements of the sacrum are defined:
Nutation: The base of the sacrum tilts forward and downward, the apex backward and upward. The pelvic inlet becomes narrower, the outlet wider.
Counter-nutation: The base of the sacrum tilts backward and upward. The pelvic inlet becomes wider, the outlet narrower.
Figure
