Navicular Bone Location in Radiographs and Solar Scintigrams

The anatomical location of the navicular bone region in the solar projection scintigram has not been clearly established and it is often not possible to define the navicular bone in solar projection bone phase scintigrams. This is due to the relatively poor anatomical detail of scintigrams. In contrast, skeletal radiology has high spatial and contrast resolutions and skeletal structures can be well defined, however radiology does not provide the functional information that scintigraphy does. Techniques using quantitative analysis of scintigrams, particularly the navicular bone region in the solar scintigram, seem to be becoming more popular. For these techniques to be reliable, correct positioning of regions of interest or profiles is essential, and this requires accurate location of anatomical structures. Superimposition of the proximal interphalangeal joint region on the navicular bone region in the solar projection scintigram is a recognised problem when evaluating the palmar regions of the foot for abnormal areas of radiopharmaceutical uptake, and this superimposition could result in incorrect interpretation of the radiopharmaceutical uptake in a navicular bone region of interest. It was hypothesised that landmarks in the solar projection scintigram could be used to locate the navicular bone region and that there would be the least superimposition of the proximal interphalangeal joint region on the navicular bone region when the interphalangeal joints were flexed. Measurements were taken from radiographs of specimen horse legs to determine if the navicular bone moved relative to the distal phalanx when the leg position was changed. Using both specimen horse legs and live horses radiographs were examined to investigate the navicular bone position relative to landmarks on the distal phalanx. Radiographs of specimen horse legs were taken with the legs positioned in maximum interphalangeal joint flexion, maximum interphalangeal joint extension and a fixed angle (50o), and the degree of overlap of the proximal interphalangeal joint region and the navicular bone region was measured. Leg positioning did not change the position of the navicular bone relative to the distal phalanx in a dorsal to palmar plane. Very little variation existed between individuals in: the ratio of the distance from the dorsodistal margin of the distal phalanx to the dorsal margin of the navicular bone (P3-Navicular) compared to the dorsopalmar length of the navicular bone (Navicular length), the ratio between the maximum lateral width (P3 width) of the distal phalanx and P3-Navicular, and the ratio between P3 width and the maximum lateral width of the navicular bone (Navicular width). In contrast, large variation existed in the ratio of the distance from the palmar aspect of the palmar processes to the palmar aspect of the navicular bone (Navicular-PP), compared to Navicular length. Positioning the pastern in maximum flexion resulted in the least overlap of the proximal interphalangeal joint and navicular regions. It is recommended that the phalanges are flexed and the sole is placed flat on a raised horizontal gamma camera when the solar projection is acquired. The measurement ratios of the distal phalanx and the navicular bone were applied to a solar projection scintigram to predict navicular bone position and position a navicular bone region of interest using anatomical landmarks that could be distinctly identified in the scintigram.