I'm pretty sure I see a space when I oblique. Anyway I can't tell if the result if from capsule/fascial insertion injection or what.
Only the inferior third of the joint is synovial.
ANESTH ANALG REVIEW ARTICLE COHEN 1441
2005;101:1440–53
Anatomy
The sacroiliac (SI) joint is the largest axial joint in the
body, with an average surface area of 17.5 cm 2 (1).
There is wide variability in the adult SI joint, encom-
passing size, shape, and surface contour. Large dis-
parities may even exist within the same individual
(2,3). The SI joint is most often characterized as a large,
auricular-shaped, diarthrodial synovial joint. In real-
ity, only the anterior third of the interface between the
sacrum and ilium is a true synovial joint; the rest of the
junction is comprised of an intricate set of ligamentous
connections. Because of an absent or rudimentary pos-
terior capsule, the SI ligamentous structure is more
extensive dorsally, functioning as a connecting band
between the sacrum and ilia (4).
J Am Acad Orthop Surg 2004;12:255-265
Anatomy
ThesacroiliacjointbecomesC-shaped
by adulthood. The sacrum is wedged
between the ilia. It is the largest ax-
ial joint in the human body, with a
surface area of approximately 17.5
cm 2 . It is considered to be a synovial
joint even though 75% of its superior
jointsurfaceisnotsynovial
www.sijoint.com
Sacroiliac Joint Structure
The anterior one-third of the sacroiliac joint is a diarthrodial (synovial) joint, with a hyaline cartilage surface. The remainder of the joint is comprised of a complex network of ligaments, more extensive dorsally than ventrally, which limit joint motion to only a few degrees. The joint surface is auricular, or ear-shaped, with an average surface area of 17.5 cm2. 4
The articular cartilage is thicker and convex on the sacral side, and thinner and concave on the ilial side. In childhood, the articular surfaces are smooth. In the second to third decades, the joint surfaces start to corrugate and roughen, with congruent grooves and ridges forming. These surface irregularities continue to deepen in the fourth decade, leading to decreased joint mobility, with progressive loss of articular cartilage and continued degeneration from the fourth decade on.
Although cartilage is thought to be aneural and avascular, recent studies have verified the presence of substance P and CGRP immunoreactive superficial nerve fibers sensitive for pain within the iliac and sacral cartilage, anterior joint capsule, interosseus ligament, as well as in the subchondral bone of sacroiliac joints in human cadavers. These findings support the concept that the intra-articular structures of the sacroiliac joint may give rise to nociceptive signals, though whether these findings are related to osteoarthritic changes and/or normal aging has not been established.5,6
