The sacroiliac joint: an overview of its anatomy, function and potential clinical implications.

The sacroiliac joint (SIJ) extends over sacral segments S1 to S3; the articular surfaces lie at an angle to the sagittal plane and have an auricular (C- or L-shaped) form. In general, the sacral part can be described as concave – although an intra-articular bony tubercle is common – and the iliac part as convex. The articular surfaces have characteristic interdigitating grooves and ridges, but only the most upper (or ventral) part of the SIJ is fibrous in nature, the rest of the joint is synovial.

The wedge-shaped form of the sacrum prevents the SIJs from shearing. Nevertheless, some articular motion is desirable, since the SIJs play a deciding role in load transfer between spine and legs – therefore, the different bony structures and body segments are functionally interrelated by ligamentous, fascial and muscular connections.

Locally, the long dorsal ligament, sacrotuberous ligament, sacrospinal ligament and interosseous ligaments help to improve force-closure. Furthermore, the superficial lamina of the posterior layer of the thoracolumbar fascia continues in the fascia glutea and goes on to merge with gluteus maximus muscle fibers thereby establishing a controlling unit consisting of two functionally interdependent forces.

The sacrotuberous ligament and ischial tuberosity are the center in a similar connection: distally, they are the origin site of the biceps femoris, semitendinosus and semimembranosus, while proximally, the gluteus maximus, piriformis and lumbar multifidus insert onto them. In this way, the trunk is functionally linked up to the lower limbs and load transfers are possible. > From: Vleeming et al., J Anat 221 (2012) 537-567. All rights reserved to The Authors.

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