The aim of this study was to review clinical and radiological signs of melorheostosis in a large series of cases. Family history, patient history, clinical data and radiological features of 23 consecutive cases of melorheostosis were investigated. Criteria for establishing the diagnosis "melorheostosis" were defined. Sixteen patients (mean age 34 years, equal ratio between genders) had chronic pain in the affected limb(s) and/or subcutaneous fibrosis and/or various skin lesions. Number of involved bones: one bone (n = 10); two bones (n = 4); three or more bones (n = 9). Anatomic distribution: upper extremity (n = 5); lower extremity (n = 16); upper and lower extremity (n = 1); sacrum (n = 1). Radiologic pattern: osteoma-like (n = 7); classic candle wax appearance (n = 5); myositis ossificans-like (n = 1); osteopathia striata-like (n = 6); mixed pattern (n = 4). Patterns different from the appearance formerly judged to be "classic" prevail. The standard concept of disease manifestation has to be adjusted. Pathogenesis remains unclear. The classic theory claims the presence of an early embryonic infection of a sensory nerve inducing changes in the respective sclerotome, but we propose the concept of mosaicism as a better explanation for the sporadic occurrence, the asymmetric "segmental" pattern with variable extent of involvement and equal gender ratio of the disease.

Bone scintigraphy is the imaging modality of choice for the diagnosis of skeletal involvement in PAO. Nuclear medicine reveals unexpected locations and shows the typical pattern of focal hot spots of the spine, sacroiliac joints and/or appendicular skeleton in the large majority of cases in combination with a bullhead-like tracer uptake of the sternocostoclavicular region. The bullhead sign is the typical and highly specific scintigraphic manifestation of SCCH and PAO in radionuclide bone scans and helps to avoid unnecessary biopsies.

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Of 88 selected patients with possible ankylosing spondylitis (AS) 54 (61%) participated in two phases of a 10 years' follow-up study. Thirty-two (59%) developed definite AS according to the New York criteria, 10 (19%) had possible/undifferentiated seronegative spondylarthropathy (SSA) and 12 patients had other diagnoses. Only 3 (9%) of 35 patients with sacroiliitis did not fulfill the New York criteria for definite AS until the last examination. Sacroiliitis and radiological spinal signs of AS appeared rather late above a mean age of 40 years and after a mean disease duration of more than 10 years. After 18 years mean disease duration 25 (78%) of 32 AS patients had good or sufficient functional capacity indicating an overall good functional prognosis. HLA B27 typing proved to be useful in patients with possible early AS: 29 (71%) of 41 B27 positive and 3 (23%) of 13 B27 negative patients developed definite AS (p less than 0.005). A combination of the B27 test with data of the history, clinical, laboratory, and radiological examination proposed as early diagnostic criteria detected patients with the outcome diagnosis of definite AS with even higher significance (p less than 0.001). These criteria were also useful in the identification of patients with possible or undifferentiated SSA. The recently recognized entity of undifferentiated SSA should only be diagnosed after long term follow-up.

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