Abstract: Objective To evaluate the diagnostic value of pelvis bone marrow fat depositions (BMFD) displayed by magnetic
resonance imaging (MRI) in patients with ankylosing spondylitis (AS). Methods Eighty-eight subjects undergoing pelvic MRI
examinations were enrolled in this study, including 44 with clinically confirmed AS (39 male and 5 female patients with a
mean age of 26.41±8.09 years) and 44 control subjects without AS (37 male and 7 female subjects with a mean age of 29.32±7.31
years). The incidence of BMFD in the bilateral sacroiliac (SI) joints and acetabulums were compared between the two groups.
The distribution features of BMFD of the periarticular cancellous bone marrow in the pelvis and in other regions of the pelvis
were analyzed for the AS patients, and the incidence of BMFD was determined in different stages of sacroiliitis and hip
arthritis. Results The incidence of BMFD in the SI joints and acetabulums was significantly higher in the AS patients than in
the control subjects (P<0.01); The incidence of BMFD was significantly higher in the periarticular cancellous bone marrow than
in the other positions of pelvis (P<0.01). The incidence of BMFD ranged from 40.0% to 45.9% in early stages of sacroiliitis,
significantly lower than the incidence in later stages (58.3%-73.1% , P<0.01); the incidence showed no difference between
different stages of hip arthritis (P>0.01). Conclusions AS patients have a higher incidence of BMFD in the pelvis than control
subjects. BMFD is distributed mainly under the articular surface, seen throughout the stages of AS, indicating that BMFD is an
important pathological change of the bone marrow in AS to potentially allow early diagnosis of AS.