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Form and force closure in the bony pelvis

This is an excerpt from The Pelvic Diaphragm by Julie Hammond.

Form and Force Closure
The concepts of form closure and force closure were introduced by Vleeming (1990). Form closure refers to the joint’s stability through its anatomy. However, if there was a perfect congruency between these surfaces, mobility and childbirth would not be possible. Form closure is created by the wedging of the sacrum into the ilia, the ridges and grooves on the articular surfaces, and the dense ligaments of the SIJ. Force closure is the extra force needed to stabilize the joint via muscles, fascia, and ligaments, which generate compressional forces (Vleeming and Scheunke 2019). Form and force closure together provide joint stability (figure 3.8). We can describe sacroiliac movement as the movement of the sacrum on the innominate bone; we can also have iliosacral movement, which is the movement of the innominate bone on the sacrum (Gibbons 2017).

SIJ movement is classified as nutation and counternutation. Nutation of the sacrum is essential for force closure of the pelvis (Vleeming et al. 2012). Nutation is forward nodding of the sacrum. The base of the sacrum rotates anteriorly and inferiorly in relation to the innominate bone, which is seen as anticipation of joint loading (Vleeming and Scheunke 2019).

In counternutation, the base rotates posteriorly and superiorly (Gibbons 2017; Zlomislic and Garfin 2019). Most of the SIJ ligaments, other than the long dorsal sacroiliac ligament, are tightened in nutation; the posterior iliac bones are drawn together, increasing the compressive forces across the SIJ in preparation for increased loading (Steinke et al. 2010). The long dorsal sacroiliac ligament limits counternutation; this position is said to be the least stable owing to the laxity of the other ligaments. The sacrum nutates in sitting or standing and is counternutated in supine lying (Vleeming et al. 2012).

Force closure is created by muscles, fascia, and ligaments. The sacrotuberous ligament is involved in force closure of the SIJ. It can be tensioned by nutation of the sacrum relative to the innominate bone or posterior rotation of the innominate relative to the sacrum. Various muscles that connect to it can also tension it, such as piriformis, biceps femoris, gluteus maximus, and multifidus (Gibbons 2017).

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