Spring, 2014


  • The Sacral side of the SIJ is lined with hyaline cartilage
  • The Iliac side of the SIJ is lined with fibrocartilage
  • These cartilage interfaces are uniform throughout the SIJ (proximal to distal)
  • The inter-osseous SIJ ligament occupies the joint space… entirely.

Anatomically, the SIJ is a synovial joint from “top to bottom”. There is no variance of cartilage interfaces from proximal to distal, but rather it does exist medial (hyaline) to lateral (fibrocartilage)

SIJ Imaging: What’s normal ?

Typically, SIJ imaging/injections are performed to diagnose and treat sacro-ilitis.
The joint is highly innervated with pain receptors.
As in many instances of MSK imaging, there is limited insonation deep into an inter-osseous or intra-articular space. Clinical correlation of pain to depth of hyper-echoic signal extending into the SIJ on ultrasound and active SIJ inflammation has been demonstrated. Increased vascularity… inflammation is displayed as hyper-echoic signal extending deep into the joint space. This is easily visualized with a short axis/transverse image of the SIJ. It is best performed with a low frequency, convex array transducer.
Arthritis Care &ResearchVolume 61, Issue 7, pages 909-916, 29 JUN 2009
Active SIJ inflammation was identified sonographically with a 15-18mm measured depth of hyper-echoic signal into the joint.
Further consideration of the grayscale appearance of the Long Posterior SI Joint Ligament can add to valuable information about chronicity and fibrosis in the SIJ region.

SIJ Injections: Proximal or Distal ?
Anatomically the SIJ is consistent and uniform relative to the cartilage linings throughout the articulation. There is no distinction of a “synovial portion” distally and a “fibrotic portion” proximally.
Using the PSIS (posterior superior iliac spine) as the lateral osseous/bony landmark for imaging and injections, places the probe essentially in the middle of the SIJ.
It is neither proximal nor distal.
Ultrasound guided injections are performed with a prone patient. Short axis/transverse probe. Utilizing the PSIS, Sacral base, and Median Sacral Crest as landmarks for standardization. Needle advancement is always “medial to lateral”in-plane.

The SIJ can be imaged sonographically to evaluate for active inflammation, and also for chronic peri-articular signs of fibrosis. Anatomically, there does not appear to be a distal “synovial portion” of the SIJ. Utilizing the PSIS as a lateral landmark with an in-plane, medial to lateral needle advance places injectate in the mid-region of this joint highly susceptible to producing pain.