Popliteal pseudoaneurysm secondary to multiple hereditary exostoses.

A 20-year-old man with multiple hereditary exostoses presented with a pulsatile swelling in the posterior-medial distal thigh. He gave a history that 5 weeks previously, after squatting down then standing, he experienced a sharp pain behind his right knee. Magnetic resonance (MR) imaging demonstrated a large haematoma associated with the popliteal artery. Ultrasound imaging visualized a 9-cm popliteal artery pseudoaneurysm with flow in the sac. Computed tomography (CT) prior to emergency open surgery further characterized the arterial injury (Fig. 1). A small hole in the posteriorlateral popliteal artery was simply closed with a single suture and the pseudoaneurysm and exostosis excised (Figs 2,3). At follow-up, no arterial compromise was evident. Elective excision of the contralateral exostosis is planned. Multiple hereditary exostoses is a rare genetic autosomaldominant benign bone disorder with a male predominance affecting somewhere between 1:50 000 and 1:100 000 in Western populations. Mutations have been identified in the EXT family of genes with about two-thirds of patients displaying a mutation in the tumour-suppressor EXT1 gene on chromosome 8q23-q24 and the remaining third the EXT2 gene on chromosome 11p11-p12. The condition leads to cartilage-capped tumours (osteochondroma) growing outward from metaphyses of long bones, growth plates or from the surface of flat bones throughout the body. Patients typically have multiple osteochondroma often arising from long bones around the knee, and in the majority of cases they are usually asymptomatic. Malignant degeneration occurs rarely in solitary osteochondromas but where multiple osteochondromas are present the lifetime risk is estimated between 1% and 5%. Histopathological assessment of malignant change can be challenging but the presence of a cartilaginous cap >1.5 cm is suspicious of secondary peripheral chondrosarcoma. Indeed, for low-grade chondrosarcomas, the radiological depth of the cartilaginous cap is a more reliable marker of malignant change than cellular features on histology. Thus, standardized radiological reporting has been devised to aid diagnosis