Paget's disease of the pelvis

The X-ray revealed marked prominence of the bone trabeculae of the right ilium, ischium and pubic bones with cortical thickening, coarse trabeculae and marked sclerosis, a thickening of the pelvic rim (the "rim sign") and involvement of the right coxo-femoral articulation with narrowing of the hip joint and medial migration of the femoral head. There were also arthrosic changes of the pubis.

The most probable diagnosis is Pagetic involvement of the pelvic bones.

Pathologically, the disease occurs in three phases. Initially, bone is resorbed secondary to increased osteoclastic activity. Osteoblasts then form abnormal new bone. The primitive woven bone production is disorganised, contains increased vascularity and connective tissue, and does not contain Haversian systems. These two phases are repeated until the osteoclastic activity subsides. The final, or inactive phase is reached when there is a cessation of osteoblastic activity. The disease usually involves more than one bone although 10-35% of patients have monostotic disease.

The three distinct radiographic phases (lytic, mixed and sclerotic) correspond to the pathological phases. In the lytic phase, most commonly recognised in the skull as osteoporosis circumscripta and in long bones, a purely lucent defect is noted. As the disease progresses, through the mixed, to the inactive sclerotic phase, new pagetoid bone enlarges the bone predominantly through cortical apposition. Coarse and thickened trabeculae are also noted and in the skull a "cotton wool" appearance may be seen. In the inactive or sclerotic phase, a diffuse increase in bone density as well as bone enlargement is noted.

The pelvis is involved in about 60-70% of cases, usually in the combined phase. Involvement may be localised to a small area, may involve one side of the pelvis or may be a generalised process. -X-rays generally demonstrate sclerosis, particularly along the iliopubic and ischial lines, a coarsening of the trabecular pattern, thickening of the pelvic rim (also called the "rim sign"), and narrowing of the hip joint. Patchy areas of lucency and sclerosis may be seen. Protrusio acetabuli is a common late finding.

Most difficulty in diagnosis will occur in the mixed phase, when patchy areas of sclerosis are present with minimal or no bone enlargement. The main differential diagnoses are metastasis to the bone; fibrous dysplasia, if the facial bones are involved; myelofibrosis; fluorosis; renal osteodystrophy; and haemolytic anemias. The clinical context and biological and haematological tests lead usually to the correct diagnosis of Paget's disease.