Musculoskeletal case of the day. Pseudoaneurysm of the left superficial femoral artery.

Abstract

About 6 months before presentation, this 62-year-old man noticed diffuse swelling and progressive loss of motion of the left shoulder and upper arm. About 4 months before presentation, he identified a nontender lump in the left shoulder. Physical examination revealed a large firm mass that extended into the root of the neck, medially to the chest wall, and inferiorly to the supracondylar region of the left elbow. Circulation, sensation, and function in the left hand were normal. Twenty years earlier, he had undergone biopsy for a left shoulder mass, which was diagnosed as an osteochondroma. The patient’s history did not reveal whether the mass had been completely surgically excised. Radiologic examination of the left humerus and shoulder (Fig. 1A) revealed a massive, amorphous, irregularly calcified, and ossified mass replacing and infiltrating the shoulder and arm as well as encircling the humerus. An anterior mTc methylene diphosphonate (MDP) scintigram (Fig. 1 B) showed markedly increased tracer accumulation in the mass in the left upper extremity. Intramedullary invasion, cortical destruction, and infiltration of adjacent structures by the juxtacortical humeral mass were shown by CT (Fig. 1 C). The extent of the tumor was delineated by MA examination. The Ti -weighted coronal MR images (Fig. i D) showed intermediate-signal tumor replacing the marrow of the left proximal humerus, enveloping the humerus, and infiltrating adjacent structures. The superior, medial, and inferior extent of the tumor was readily identified as bright signal infiltrating adjacent tissues on the T2-weighted MR coronal images (Fig. i E). Histologic examination revealed a parosteal osteosarcoma. Parosteal osteosarcoma is a rare, slow-growing, malignant, bone-forming neoplasm that arises from the surface of long bones. It has a better prognosis than conventional central osteosarcomas and other surface (juxtacortical) osteosarcomas [i -3]. Patients are generally affected in the second to fifth decades of life. They usually have gradual onset of pain, swelling, and a palpable mass [1 , 4]. The indolent behavior of the tumor delays diagnosis. As illustrated in this case, the tumor may attain enormous size before the patient seeks clinical evaluation [i , 4]. In addition, radiologic and histologic misdiagnosis as benign osteochondroma [2] may result in inadequate surgical excision of the tumor-a distinct possibility in this patient. The lesion most often arises from the metaphysis of a tubular bone, occasionally from the epiphysis, and rarely from the diaphysis [1 , 4]. The most common location is the posterior aspect of the distal femur, followed by the proximal humerus, the tibia, fibula, and ulna [i , 4]. This malignant neoplasm grows circumferentially along the cortex of the involved bone, eventually enveloping the bone shaft. Cortical destruction and neoplastic invasion of the medullary cavity are uncommon except in cases of prior treatment or a long history of growth, as illustrated in this case [3-5]. On radiographs, early parosteal osteosarcomas are radiodense, lobulated, or oval masses with a broad base of attachment to the external cortex. A thin, radiolucent cleavage plane between the tumor and underlying bone is characteristic but not a constant finding [i , 3-5]. Progressive growth of the tumor may obliterate this plane. This tumor ossifies from the base of the lesion to its periphery in contradistinction to myositis ossificans, which initially ossifies from the periphery [1 , 2, 4]. The differential diagnosis of a less advanced case of parosteal osteosarcoma includes osteochondroma, myositis ossificans, central osteosarcoma, extraosseous osteosarcoma, ossifying or calcifying hematoma, and exuberant callus [4, 5]. CT and MR studies assist in delineating the extent of the process and in preoperative planning [4, 5]. Margaret A. Stull Mark Glass-Royal