Osteitis: Active Inflammation in the Sacroiliac Joints

Inflammatory Arthritis

Artists depiction of active inflammation at the sacroiliac joints.
Artists depiction of active inflammation at the sacroiliac joints. SCIEPRO/Science Photo Library/Getty Images

Osteitis refers to inflammation of the bone. When signs of osteitis are found in and around the sacroiliac joints, it may indicate active inflammation that is associated with ankylosing spondylitis

Osteitis may also be caused by infection, degenerative changes in your spine or trauma. It generally leads to edema, or swelling, of bone marrow. Symptoms of osteitis include most of the typical symptoms of inflammation, including swelling, redness, and pain.

(Two other symptoms of bone edema are tenderness and dull aching.)

Osteitis at the Sacroiliac Joints

Osteitis is one of the bone changes found in the sacroiliac joints in cases of inflammatory arthritis disease such as spondylitis. (It tends to be present on MRIs of sacroiliitis patients in the early stages.) For this reason, osteitis is considered to be an active inflammatory lesion.

For people with spondylitis, an MRI using one of the several specialized techniques is a sensitive test to find signs of osteitis and other active inflammatory lesions.

Early Osteitis

According to sacroiliitis experts Hermann and Bollow, early osteitis around the articulating surfaces (articulating surfaces is a term that refers to the surfaces of bones of a joint that fit together, and between which movement occurs) of the sacroiliac joint indicates inflammation of the bone marrow in the hip bone in that area.

 

The sacroiliac has two articulating surfaces - the lateral sacrum on one side and the medial ilium, or hip bone, on the other. Note that Hermann and Bollow say that using MRI, osteitis (in spondyloarthritis patients with sacroiliitis) is predominantly on the ilium or hip side - and not the sacrum side.

Hermann and Bollow explain that this MRI finding means inflammation in the fibrocartilage located between the bones of the sacroiliac joint has extended to the bone marrow of the hip, and that the degree of inflammation determines the size of the bone marrow area that is affected.

The authors go on to say that osteitis is the only sign on an MRI that definitively indicates sacroiliitis, according to the parameters set forth by the ASAS group (an acronym that stands for the Assessments in Ankylosing Spondylitis group.)  

This is especially true when the sacroiliitis signs are seen in two different views, they say.

Later, or Progressed Osteitis

As the disease of sacroiliitis progresses, Hermann and Bollow say that areas of osteitis transform into fatty tissue deposits that are (again) located around the sacroiliac joint.  This is but one of the unique changes that show up on MRIs in later stage sacroiliitis, they say.  Some of the others changes include erosions, subchondral sclerosis, transarticular bone bridges, and bone buds.

 These later stage changes can lead to complete ankylosis (fusing) of the sacroiliac joints.  When doctors see such fusing on an MRI, they call it a "phantom joint."

Source:

Hermann, K. - G. A., Bollow, M., Magnetic Resonance Imaging of Sacroiliitis in Patients with Spondyloarthritis: Correlation with Anatomy and Histology Fortschr Röntgenstr 2014; 186(3): 230-237 Georg Thieme Verlag KG Stuttgart · New York https://www.thieme-connect.com/products/ejournals/html/10.1055/s-0033-1350411

Hermann, K. - G. A., Bollow, M., Magnetic Resonance Imaging of Sacroiliitis in Patients with Spondyloarthritis: Correlation with Anatomy and Histology Fortschr Röntgenstr 2014; 186(3): 230-237 Georg Thieme Verlag KG Stuttgart · New York https://www.thieme-connect.com/products/ejournals/html/10.1055/s-0033-1350411

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