A 36 year old gentleman presented to the Metabolic Bone Disease Clinic with a progressive history of thoracic and lower limb pain. He had originally been seen by the podiatrists with worsening foot pain for which no cause had been found. Initial investigation revealed a hypophosphataemic osteomalacia and a bone scan demonstrated multiple abnormalities suggesting old fractures.

Investigations were performed to establish the cause of the osteomalacia and we discuss the differential diagnosis and the progression towards a diagnosis based on the results of these tests. The most useful investigation in this case was an octreotide scan which indicated the presence of an endocrine tumour in the medial femoral condyle of the right knee.

Plain x-rays revealed no clear bony abnormality in the area of increased uptake on the octreotide scan. The lesion was therefore localised with an MRI scan.

This subsequently demonstrated the exact location of the lesion and in image guided biopsy was performed in theatre. This confirmed the presence of a benign Phosphaturic Mesenchymal tumour. This rare tumour is usually found in soft tissues and this case is atypical given that the lesion was wholly within the femoral condyle.

Despite the benign appearance of the tumour cells there were some areas of locally invasive growth and excision rather than curettage of the tumour was recommended. It was possible to preserve both the bulk of the femoral condyle and the articular surface although the knee was protected with a hinged brace for six weeks following surgery.

Follow up biochemistry results demonstrate that the serum phosphate and alkaline phosphatase are returning to normal. Symptomatically the patient is much improved.