Radiography

The identification of osteopenia on a radiograph requires a significant, 30% or greater, loss of bone mineral. There are two general types of bone loss, and the differentiation between them determines whether osteopenia/osteoporosis is in the differential. Regional bone loss may be seen in post-traumatic events, such as fracture, transient osteopenia, and reflex sympathetic dystrophy. Generalized osteopenia is either involutional or senile. Osteopenia is also seen with other metabolic disease such as osteomalacia, hyperparathyroidism, hyperthyroidism, drug-induced bone loss, and osteogenesis imperfecta.

Bone loss is manifested on radiographs differently depending on the disease process and the ratio of cortical to trabecular bone. Trabecular bone is remodeled four to eight times faster than cortical bone, and in osteopenia/osteoporosis the vertebral (especially thoracic) bodies are the first bones to demonstrate demineralization. The appearance of a picture frame vertebral body, density of intervertebral disc equal to or greater than the vertebral body, and prominent vertical trabeculae all are radiographic signs of bone loss. Vertical trabeculae become prominent when secondary and tertiary trabeculae are resorbed with resultant hypertrophy of the vertical primary trabeculae. Osteopenia can be identified in any bone. When cortical bone loss is present, the osteopenia is more severe.

A radiologic technique known as the second metacarpal index can be used to determine cortical bone loss. The width of the midpoint of the second metacarpal is measured, and the width of the combined cortex is determined at the same level. If cortical combined thickness is less than 50% of the width of the diaphysis, then the bone is osteopenic.

Film-based radiographs are being replaced by digital images. Because of window and leveling capabilities that digital imaging workstations provide, it is possible to make normally mineralized bone look osteopenic. Therefore, use of more reliable objective criteria than visual appearance of bone should be used to confirm demineralization.

Low-impact fractures that can be attributed to osteopenia are a significant finding and allow the diagnosis of osteoporosis without quantitative assessment of bone mass. The presence of a vertebral fracture on a radiographic image is termed prevalent if noted on the first film and incident if a new finding on subsequent radiographs. Vertebral fractures are classified as mild, 20% to 25% loss of height; moderate, 25% to 40% loss of height; or severe, greater than 40% loss of height. Height loss can be anterior, concave, or crush. Other traditionally associated fractures are the radius and the hip (see Plate 3-31). Fragility fractures are defined as a fall from standing height of the individual or less. Insufficiency fractures of the sacral ala, pubic ring, and ribs may also be seen in osteopenic patients. Fractures may be occult and require additional imaging for detection.