PATHOGENESIS
Primarily affects SI joints and axial spine
Subchondral osteitis, marked proliferation of capillaries and fibrous tissue
Cartilage metaplasia and ossification
Mast cells present, often in contact with lymphocytes
Degranulation of mast cells
Ultimate fibrous and bony ankylosis
Enethsopathy (inflammation at tendon, ligament, articular capsule insertions)
Endochondral ossification causes bony ankylosis
SKELETAL MANIFESTATIONS
New York criteria
Spinal pseudarthrosis associated with pain
Shoulder findings include limitation of scapulothoracic motion, SC or AC joint inflammation, GH joint ? ROM
Heterotopic ossification after THA
Limited chest expansion (most specific evidence of disease), lumbar excursion
3 patterns of spinal fx through the diffuse ossifcation (even without hx of trauma)
Vertical compression fx
Transverse shear fx
Stress fx associated with pseudarthrosis
Cauda equina syndrome may be due to thecal diverticulosis, arachnoiditis, demyelination, or post-irradion ischemia
Muscle atrophy
EXTRASKELETAL MANIFESTATIONS
Active iritis (anterior uveitis), typically unilateral in onset, ? iris scarring ? posterior synechiae ? secondary glaucoma ? cataract formation
Focal inflammation at aortic root, ring, and ascending aorta
Perivascular infiltration of vasa vasorum with lymphocytes and plasma cells ??scarring and elastic tissue weakening ? aortic ring dilitation and aortic valve incompetence
Chronic infiltrative and fibrotic changes in upper lung fields (ddx: TB, amyloidosis)
Mild disease associated with ? BMD, advanced disease with ? BMD, lumbar spine
+ HLA-B27 antigen
TREATMENT
Closing-wedge posterior osteotomy of lumbar spine described
? risk of stiffness and HO after TKA
? fiberoptic intubation if indicated
Corticosteroid injection into SI joint (? CT guidance)
