One hundred and one consecutive patients with ankylosing spondylitis (males 85%) were reviewed by radiographic evaluation after a mean disease duration of 21.2 years (mean age at onset 22.9 years). Osteitis pubis occurred in seven cases of varied duration and severity of disease. Hip involvement (grade 2-4) occurred in 19%. Their mean age of onset (19.6 years) was younger than for those with normal hips (25.1 years; p = 0.003). Lone sacroiliitis (grade 3 and 4) without ascending spinal disease occurred in 11 patients. The mean age at onset for those with lone sacroiliitis and marked spinal disease was similar (23.3 and 24.9 years respectively; NS). By contrast, disease duration was 13.3 and 28.4 years respectively (p = 0.0007). Five patients (three women) had severe pelvic and cervical spine disease with normal lumbar spine.
In conclusion: 1. Osteitis pubis occurs as an unusual, random event. 2. Hip involvement is an expression of young age at onset. 3. Lone sacroiliitis is uncommon and progression to ascending spinal disease is primarily a function of disease duration. 4. Skip lesions occur rarely and particularly in women. 5. Sacroiliitis and ascending spinal disease are part of the same spectrum.