Anatomy · functional reading
Sacroiliac joint
The sacroiliac joint connects the sacrum to the iliac bone on each side of the pelvis. A true joint, very stable and with a very small range of motion, it transmits and distributes loads between the trunk and the lower limbs.
Scientific basis
Each sacroiliac joint is a true synovial joint, combining irregular articular surfaces with a dense ligamentous apparatus. Its movements are small in amplitude, on the order of a few degrees, and three-dimensional: forward tilting of the sacrum (nutation) and backward tilting (counternutation), combined with movements of the iliac wings in the other planes, around centers of rotation that shift.
Rather than a rigid keystone, the sacrum works as a pressure distributor suspended from the iliac wings by powerful ligaments: the sacroiliac joints dissipate loads more than they bear them (Viel, 1989). Their stability depends both on the shape of the articular surfaces and on the forces exerted by the muscles, fascia and ligaments that compress them.
The sacroiliac joint is a possible source of low back and buttock pain; its diagnosis relies on clinical examination and, where needed, on diagnostic injections.
Blue Portance / SBNFA™ reading
In the SBNFA™, the sacroiliac joints are read through their sacroiliac micro-rotation: a micro-movement that combines nutation, counternutation and lateral tilting. These micro-movements accompany the mobility of the pelvis in the horizontal plane and take part in the fine adjustments and compensations of the lumbopelvic complex. Sacroiliac mobility is therefore a micro-rotation of very small amplitude, on the order of a few degrees: this is not a doctrine of large sacroiliac mobility.
These compensations allow stress redistribution; in static sitting, the possibilities for redistribution are limited. Conversely, when the mobility of the lumbar spine decreases, the sacroiliac joints may be placed under greater demand (Kouyoumdjian, 2023).
Contribution to the analysis of sitting
In the seated position, the distribution of support between the two sit bones and the two thighs changes the loads transmitted to the pelvis. A seat can be analyzed according to the symmetry or asymmetry of the support it imposes and according to the freedom it leaves to vary that support.
Not to be confused with
Low back pain, pain in the lower back that may have multiple origins; cluneal neuralgia, involvement of the small nerves of the buttock; hip joint, which connects the pelvis to the femur.
Related terms
Sacrotuberous ligament · Low back pain · Cluneal neuralgia · Sacral nutation and counternutation · Pelvic mobility · Hip mobility · Postural compensation
Sources and scope
- Vleeming et al. (2012): the sacroiliac joint, anatomy, function and clinical implications (Journal of Anatomy)
- Viel (1989): biomechanics of the sacroiliac joints (Annales de kinésithérapie) (in French)
- Kouyoumdjian (2023): involvement of the hip-spine relationship in hip arthroplasty surgery (Revue de chirurgie orthopédique et traumatologique) (in French)
Established anatomy and biomechanics: small, complex and three-dimensional mobility. The notion of sacroiliac micro-rotation and its role in stress redistribution belong to the SBNFA™ reading. Pain in the sacroiliac region is not enough to attribute its origin to this joint.
