Interpretation of bond radiographs: some hazards for the unwary

Four aspects of normal radiographic anatomy are discussed in relation to the diagnosis of bone pathology. Where two bones are superimposed the observer may perceive a phantom radiolucent line parallel and close to one of the bones. The radiolucent line is a Mach band and is an illusion which may simulate a fracture. Nutrient canals may also simulate fractures. Although subject to typical site, number and direction, exceptions are frequent. They are commonly seen in the bones which participate in the elbow and knee joints of the dog and cat, and are most likely to be misinterpreted in the tibia and humerus. The thickness of cortical bone is not easily predicted from first principles and is of similar dimensions in both toy dogs and giant breeds. It may prompt a diagnosis of osteoporosis in the latter. The normal appearance of cancellous bone varies from species to species. In the cat and wheep the spongiosa is coarse and sparing and may falsely tempt a diagnosis of osteoporosis. The trabeculae are delicate in the pig and abundant in the horse. To aid correct interpretation it is recommended that a comprehensive library of normal films should be collected and used for empirical comparison where doubt exists.