Form and Force Closure of the Sacroiliac Joints.

The principles of form and force closure were introduced to describe the complex mechanism of sacroiliac joint (SIJ) stability. Form closure refers to a theoretical stable state of a joint with close fitting articular surfaces, where no extra forces are needed to maintain the stable state of the system during loading and unloading situations. If the sacrum would fit in the pelvis with perfect form closure, no lateral compressional forces would be needed to maintain stability. However, such a construction would make mobility practically impossible. Force closure is the theoretical state where lateral force and friction resulting in joint compression, are required for the joint to withstand a vertical load. Structures that contribute to SIJ stability via "form closure" include (1) the configuration of the interfacing joint surfaces, along with dorsocranial "wedging" of the sacrum into the ilia; (2) the corresponding ridges and grooves of the articular surfaces of the SIJs and resultant high coefficient of friction; and (3) the integrity of the binding ligaments, which are among the strongest in the body. Shearing forces absorbed in the SIJ occur because of a combination of person-specific anatomical features. This results in unique form and force closure situations that provide effective and tailored joint accommodation that balance both friction and compression in the joint. Force closure occurs because of altered joint reaction force via taut ligaments, fascia, muscles, and the ground reaction force they are reacting to at the moment. In the ideal state, force closure creates a perpendicular compressive reaction force to the SIJ to adapt and overcome the forces of gravity. In order for force closure to be effective, sacral nutation must occur and is considered to be anticipatory for joint loading. Sacral nutation results in tensing all of the dorsal SIJ ligaments (interosseous, dorsal sacroiliac) with the exception of one, the long dorsal ligament (LDL). This prepares the pelvis to absorb and increase in load. As a result, the posterior ilium are pressed together causing an increase in SIJ compression. This review will discuss the importance of understanding form and force closure principles because they are related to understanding the relationship of anatomy and function of the SIJ.