1.  What are the findings?
The most striking finding is the presence of sclerosis and erosions on the iliac side of the sacroiliac joints bilaterally.  This is consistent with sacroiliitis.  The findings appear bilateral and symmetric. 

2.  What is your differential diagnosis?
The classic differential diagnosis for bilateral sacroiliitis includes:
  • ankylosing spondylitis
  • enteropathic arthritis secondary to inflammatory bowel disease such as ulcerative colitis, Crohn's disease, Whipple's disease.  This is also sometimes referred to as secondary ankylosing spondylitis
  • psoriatic arthritis
  • osteocondensans ilii - more common in young, multiparous women
  • hyperparathyroidism - sclerosis but not true erosion but subchondral bone resorption and resulting joint space widening
  • degenerative arthritis - sclerosis is common however the erosive changes are not associated with degenerative disease
  • Reiter's syndrome - bilateral asymmetric pattern is more common
  • gout - crystalline deposition within the joint produces large erosions surrounded marked sclerosis

3.  What laboratory test may be performed?
In a young male such as this patient one of the primary diagnostic considerations is ankylosing spondylitis (AS).  Although not specific for AS, HLA-B27 is commonly positive, seen in up to 90% of AS patients.

4.  Does this represent an early, middle, or late stage process?
Essentially all patients with AS will develop sacroiliitis and it is most commonly the earliest radiographic finding of the disease process.  The sacroiliitis primarily involves the iliac side of the joint and may progress to complete fusion of the joints.  The disease process also affects the spine, presenting early as a vertebral body corner erosion producing a shiny corner sign and progressing to non marginal syndesmophytes which fuse the vertebral segments.  Complete fusion of the spine produces an appearance referred to bamboo spine.  Facet fusion and ossification of the interspinous ligaments produces the trolley-track and dagger radiographic signs of AS.

As noted, the disease may progress to complete fusion of the spine and sacroiliac joints.  Patients may develop a forward body posture as the spinal fusion progresses.  The patients are at a greater risk for spinal fracture.