Nora Lesion (Bizarre Parosteal Osteochondromatous Proliferation): An Ultrasound Diagnosis with Magnetic Resonance Imaging Correlation.

Advanced cross-sectional imaging techniques are firmly established as a means of evaluating musculoskeletal disease, and ultrasound (US) is increasingly being used for the assessment of a diversity of tendon, joint, and soft-tissue abnormalities. A benign condition - bizarre parosteal osteochondromatous proliferation (BPOP) - arises from the periosteum, typically in the small bones of the hands and feet, and grows as a surface bone lesion in the surrounding soft tissue. Proliferations can become symptomatic, exercising mass effect on adjacent structures that may require operative management. As a bone-forming process, BPOP may occasionally assume worrisome histologic features that mimic sarcoma, and a pronounced tendency to recur after primary excision. A solitary mass was growing in the middle finger of a young woman that curtailed proper hand function. With US, a partially ossified formation was revealed in the proximal phalanx situated on the outer surface of the bone. There was faint acoustic shadowing distal to the lesion, and a hypoechoic halo was seen covering part of the abnormal tissue growth. Importantly, the lesion caused significant limitation of motion of the finger, on the dynamic flexion US images with the displacement of the flexor tendon and compression of a digital nerve. To restore the range of motion in the finger, surgical excision of the juxtacortical mass was performed and histology yielded a diagnosis of BPOP. We describe the US features of digital BPOP, which were found to correspond closely to those of computed tomography and magnetic resonance imaging.