Sacroiliac Joint Force and Form Closure

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Form Closure

Nutation of the sacrum is necessary during force closure and according to Vleeming et al (2012) this movement can be regarded as anticipation for joint loading. Nutation causes most of the SIJ ligaments to tighten and therefore helps to prepare the pelvis for loading. Nutation also causes the posterior parts of the iliac bones to press together causing increased compression of the SIJ.

Force Closure

Although form closure provides stability to the SIJ, for mobility to occur further joint compression and stabilisation is required to withstand a vertical load. (Pool-Goudzwaard et al 1998; Vleeming et al 2012). Force closure is the term used to describe the other forces acting across the joint to create stability (Takasaki et al 2009). This force is generated by structures with a fibre direction perpendicular to the sacroiliac joint and is adjustable according to the loading situation (Pool-Goudzwaard et al 1998). Muscles, ligaments and the thoracolumbar facia all contribute to force close. (Pool-Goudzwaard et al 1998; Vleeming et al 2012; Liebenson, 2004; Arumugam et al 2012; Cohen, 2005; Harrison, Harrison and Troyanovich, 1997; van Wingerden et al, 2004) Force closer is particularly important during activities such as walking when unilateral loading of the legs creates shear forces. (Pool-Goudzwaard et al 1998; Liebenson, 2004).

Force closure creates greater friction and therefore increased form closure and what is called “self-bracing” or “self-locking” of the joint. (Vleeming et al 2012; Takasaki et al, 2009; Vleeming et al, 1990a; Vleeming et al, 1990b; Snijders et al 1993; Arumugam et al 2012) According to Vleeming et al (2012) force closure reduces the joint’s ‘neutral zone’ thereby facilitating stabilisation.

Muscles involved in self-locking mechanism

Table 2. shows three muscle slings that contribute to force closure of the SIJ, the longitudinal, posterior oblique and anterior oblique slings.

Table 2.

Name of Sling:	Components of sling:	Action on SIJ:
Longitudinal	• Multifidus attaching to the sacrum • Deep layer of thoracolumbar facia • Long head of biceps attaching to the sacrotuberous ligament	• Contraction of the sacral part of the multifidus causes the SIJ to nutate thereby increasing tension in the interosseous and short dorsal ligaments and creating increased force closure of the SIJ. The iliac connections of this muscle along with the erector spinae muscle also pull the posterior sides of the iliac bones toward each other, constraing nutation. • These muscles cause the thoracolumbar fascia to inflate increasing force closure. • Contraction of the erector spinae muscle and the long head of the biceps femoris can help to increase force close due to their anatomical connections with the sacrotuberous. The functions of this ligament have already been described (see Table 1).
Posterior Oblique	• Latissimus dorsi and contralateral • Gluteus maximus	• These muscles work as synergists to directly stabilise the SIJ. • Force closure can be increased indirectly due to the anatomical connections of the gluteus maximus and the thoracolumbar facia with the sacrotuberous ligament.
Anterior Oblique	• External oblique • Internal oblique • Transverse abdominus	• These muscles connect via the rectus sheath and help to increase force closure.

Pool-Goudzwaard et al 1998; Vleeming et al 2012; Liebenson, 2004; Cohen, 2005; van Wingerden et al, 2004; Pel et al 2008)

Other muscles affecting the SIJ

Deep muscles including the transverse abdominis, internal oblique, multifidus, the diaphragm and the pelvic floor muscles all exhibit anticipatory stabilising contractions prior to large movements. These deep muscles are closer to the centres of rotation of the spinal and the SIJ and are therefore able to exert a greater compressive force on the SIJ. ( Vleeming et al 2012; Adams and Dolan, 2007)

The Thoracolumbar Fascia

References

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