The “Fallen Fragment Sign” in the Diagnosis of Unicameral Bone Cysts

Abstract

Asolitary unicameral bone cyst is a relatively rare skeletal lesion. It consists of a fluid-filled intramedullary cavity,2 or 3 cm in diameter, which is lined by a thin layer of connective tissue. The surrounding cortex is preserved, but markedly attenuated (8). Although the lesion has been encountered in the pelvis, rib cage, calcaneus, and metatarsal, it is usually located in a long bone. The most common sites are the proximal humerus or femur where two-thirds of all reported cases have been discovered (9, 11). It evolves during childhood and usually produces no symptoms until pathological fracture occurs. Since it is clinically occult and causes no physical signs or biochemical aberrations, it can be detected only by radiological means. Thus, it is usually the radiologist who is first aware of its existence, and it is his role to distinguish it from other skeletal lesions on the basis of its roentgen characteristics. Although Virchow described a bone defect with the pathological features of a unicameral bone cyst ninety years ago, the lesion was first recognized as a distinct pathological entity in 1906 by Mikulicz who felt that it was caused by a local disturbance in bone growth and development (9, 10). More recently, the unique pathological characteristics of the lesion were more sharply defined by Jaffe and Lichtenstein (9). They described two forms: (a) the “active” cyst, which still occupies the site of its origin in the metaphysis, immediately adjacent to the epiphyseal plate, and (b) the "latent" cyst which is diaphyseal in location. The pathologist usually has no difficulty in distinguishing the solitary unicameral cyst from other entities on the basis of its gross structural characteristics. Skeletal echinococcosis produces cystic changes, but the solitary unicameral cyst is the only true cyst of bone. Occasionally certain firm, cellular lesions of bone, such as osteitis fibrosa cystica, may undergo partial cystic degeneration. This type of alteration is neither extensive nor common, however, and when it occurs the bulk of the lesion usually retains its firm texture. It is more difficult for the radiologist to identify the unicameral cyst than it is for the pathologist, as he sees only the image of the gross pathological changes produced by the lesion. The unicameral bone cyst appears as a unicentric, intramedullary radiolucent defect surrounded by a thin, but intact cortex. However, many of these roentgenographic characteristics are shared by a number of other entities such as fibrous dysplasia, benign cortical defect, nonossifying fibroma, aneurysmal bone cyst, and enchondroma. Although these other lesions are firm and noncystic, this important feature is not manifest radio-logically, for the roentgen ray cannot distinguish between their solid, fleshy core and the liquid contents of the cyst.