Dinesh P. Thawrani, MD, FACS; Steven S. Agabegi, MD; Ferhan Asghar, MD

Disclosures

J Am Acad Orthop Surg. 2019;27(3):85-93. 

In This Article

Anatomy

Bony and Ligamentous Anatomy

The SIJ is a true diarthrodial synovial joint that serves as a connecting link between the axial and appendicular skeleton. The anterior third of the interface between the sacrum and ilium is a true synovial joint, covered by hyaline cartilage that provides a gliding surface between the bones. The rest of the joint has fibrocartilage and is composed of an intricate set of ligamentous connections, which give inherent stability to the joint. The posterior capsule is rudimentary and is almost replaced by extensive ligamentous structures. Overall, the articular surfaces are rough and have many ridges and depressions that minimize movement and enhance stability. The coarse nature of the SIJ is considered to be an adaptation to stress across this vertically oriented joint located in axial spine, and therefore, it varies among individuals and changes with age.[1]

Innervations

The lateral branches of L4-S3 dorsal rami supply the posterior SIJ, whereas some studies suggest contribution from L3 to S4 dorsal rami. The anterior joint is innervated by L2-S2.[1] The SIJ and lumbosacral plexus are in close proximity. Various patterns of extravasation of the fluid from SIJ to the nearby neural structures have been demonstrated: mainly posterior extravasation into the dorsal sacral foramina, superior recess extravasation at the sacral alar level to the L5 epiradicular sheath, and ventral extravasation to the lumbosacral plexus. Because of the insufficient capsular envelope around the SIJ the inflammatory pain mediators can leak through any of these pathways into the neural structures. This mechanism may explain the radicular leg pain (sciatica) that is commonly seen with SIJ pathology.[2]

Function and Biomechanics

Ligaments around and within the joint prevent separation of the joint and provide movement of the pelvis along the various axes of the sacrum. The SIJ rotates about all three axes, although the movements are very small. Several cadaveric studies concluded that sagittal plane motion often ranges between 1° and 4° and translation between 0.5 and 2 mm. Male patients tend to have translational motion, whereas female patients have more rotational motion.[2,3] In women, the ligaments are weaker, allowing the mobility necessary for parturition. Walker[3] averaged reports from various studies and found a wide range of motion in the SIJ, mean rotation ranged between 1 and 12 mm, and mean translation ranged between 3 and 16 mm, with the caveat that measurements differed based on patient position. Interestingly, a roentgen stereophotogrammetric analysis conducted by Sturesson et al[4] found no differences in either rotational or translational movements between symptomatic and asymptomatic joints.

The SIJs are the connecting link between the axial skeleton and lower extremities. The literature is unclear whether the primary function of the SIJ is supportive or to provide mobility. However, with the available literature on SIJ anatomy (intra-articular band of ligaments and surrounding thick ligaments) and motion (limited motion in all planes), it is reasonable to conclude that the main function of this joint is stability, for transmission and dissipation of truncal load to the lower extremities. Biomechanical studies found that because of their distinct anatomy and location, the SIJ can only withstand half the torsion and 1/20th of the axial compression load compared with lumbar spine, which may strain and injure the weaker anterior joint capsule.[1]

The SIJ has several unique anatomic characteristics that may render it vulnerable to unusual stress and strain. The SIJ is the largest axial joint in the body with an average surface area of 17.5 cm2. Only the anterior third of the interface between the sacrum and ilium is a true synovial joint, and the rest is composed of an intricate set of ligamentous connections functioning as a connecting band limiting motion in all planes of movement. The posterior capsule of the SIJ is absent or rudimentary. The joint is vertically oriented causing more shearing forces across the joint, and these forces are concentrated in the limited anterior synovial area. SIJ mobility is affected by the action of several muscles such as the gluteus maximus, piriformis, biceps femoris, and thoracolumbar fascia, as all of these are functionally attached to the SIJ. The SIJ is in very close proximity to the lumbosacral plexus.[5,6] The sacral articular cartilage of the joint is twice as thick as the iliac cartilage, whereas the subchondral iliac bone end-plate is 50% thicker than that of the sacrum. This finding may explain why degenerative changes on the sacral side usually lag 10 to 20 years behind those affecting the iliac surface.[13] In addition, the natural history of capsular structures of the SIJ is conversion into markedly rigid collagenous and fibrous ankylosis, leading to restricted motion by the sixth decade and inevitable erosion and plaque formation by the eighth decade of life.[7,8]

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