Systemic diseases can have osteoarticular manifestations. A classic set of diseases that can manifest in the region of the sacroiliac joint are the HLA-B27 spondyloarthropathies, which include ankylosing spondylitis, reactive arthritis, and psoriasis.
Part of the radiologic evaluation for these diseases can include studies of the sacroiliac (SI) joint. Conventional radiographs are typically obtained, but in some cases CT and MRI are also used. Ironically, the hardest one of these three exams to interpret accurately is the low tech X-ray. The non-tomographic nature of x-ray, overlapping bowel gas, and the curving nature of the SI joint all confound interpretation.
In this post, we will try to clarify the standard AP view of the pelvis, which visualizes the SI joint. The shape of the SI joint is worth noting:
This is an oblique view of the pelvis skeleton with the sacrum removed, viewed from the inside looking out. It depicts the origin of the iliacus muscle along the ilium (red outline) anteriorly. The articulation of the sacrum with the ilium is depicted in orange, outlined by the red arrows. (The blue arrows point out non-anatomic raised pins that hold the sacrum in place, and these pins should be ignored).
Note how the SI joint has a saddle-shape, with the anterior portion of the joint extending more superiorly than the posterior portion of the joint.
This photograph of the pelvis, with the sacrum removed, depicts the iliac side of the sacroiliac joint articulation (region outlined by the red arrows):
A series of CT images, posterior to anterior, show that the inferior edge of the posterior aspect of the SI joint (second frame) is just medial to the inferior edge of the anterior aspect of the joint (last frame):
Red arrows = posterior aspect of joint. Green arrows = anterior aspect of joint. In the area above the yellow arrow, we visualize only the anterosuperior aspect of the joint.
Armed with this knowledge, we can better evaluate the SI joint on x-ray, but this remains a challenging endeavor. Ultimately, some patients will need CT or MRI to better evaluate the complex anatomy in this region.