Fat metaplasia

Last revised by Daniel J Bell on 8 Feb 2021

Fat metaplasia refers to the replacement or infiltration of subchondral bone and bone marrow with tissue exhibiting fat signal on MRI.


The nature of this tissue histologically is not well-known and does not necessarily correspond to the simple replacement of red marrow by yellow.

Radiographic features

Fat metaplasia has been observed as a relatively common senescent and degenerative change throughout the skeleton but has gained special interest as a sign of inflammatory joint disease, most prominently with reference to ankylosing spondylitis.

Another well known example of fat metaplasia is the fat signal observed in vertebral endplates exhibiting Modic type II endplate changes.

Periarticular fat metaplasia of the sacroiliac joints in a band-like or semilunar pattern (mirroring the distribution of edema seen on fluid-sensitive sequences in early osteitis) has been described as a relatively specific sign of ankylosing spondylitis representing a disease stage between erosion and ankylosis. Some groups have found sacroiliac joint fat metaplasia to be a biomarker of patients at greater risk of radiographic progression of the disease. 

ADVERTISEMENT: Supporters see fewer/no ads

Updating… Please wait.

 Unable to process the form. Check for errors and try again.

 Thank you for updating your details.