Form closure and Force closure of a joint are concepts that describe how a joint is both “held” together passively and how it is stabilized actively. The easiest example to illustrate this is the SI (sacroiliac) joint because both concepts apply, in varying degrees, depending on the individual.
SI joint dysfunction can not only lower back pain, but also groin and thigh pain. For a looong time, it was thought that SI joints didn’t move, and until this day I still run into folks who believe this But we know that SI joints are in fact, capable of movement, and that movement is very necessary to efficiently transmit forces through the pelvis. They also act as shock absorbers between the lower limbs and spine, and act as a proprioceptive feedback mechanism for coordinated movement and control between trunk and lower limbs. Kind of important
Form closure is a feature of the anatomy of the SI joints; the sacrum and the ilium each have one flat surface and one ridged surface which interlock together, promoting stability. The SI joint also depends on its ligament complex for the vast majority of its stability; but on the flips side because ligaments have a relatively poor blood supply, they do not heal well; therefore, an SIJ sprain usually results in a chronic hypermobile joint if left untreated. Force closure involves the structures of your body that hold the bones of the SI joint together; these include the muscles that cross the SI joint (the big ones are of the posterior sling and anterior sling system, when they work with each other) and to some extent the ligaments. Ligaments are kind of a gray area—they are non-contractile tissue so they aren’t an active controller of movement, but they do give varying levels of support to joints.
When you compare the two, form closure on its own is not my idea of stability in a being that moves. That has wear and tear written all over it, so it is ideal to work on the dynamic stability that your system can provide, with proper training. And that’s what’s coming up #physiodocwellness