Radiography can help identify the site of lameness and provide information about the stage to which the pathology has progressed. This helps to determine the most advantageous treatment. When pathologic changes are seen in the region of the distal interphalangeal joint, tissue damage is often rapid and severe.
Before a radiograph is taken, the interdigital space and both claws should be cleansed thoroughly, and both claws lightly trimmed. If this is not done, false images or shadows may mask abnormalities present in the claws. The digits can be viewed radiographically using four angles or projections.
In the dorsopalmar/plantar projection, the image produced shows all of the major bones and joints without overlap. This view allows diagnosis of many diseases of the bovine foot.
In the oblique projection, the plate is positioned beneath the claw, and the head of the machine is placed dorsad to the digits and rotated backward at a 45° angle. Because cattle have two digits that overlap one another when viewed radiographically from the side, an abnormality may be unclear or obscured. The oblique view allows the digit to be viewed from such an angle that one claw appears to be behind the other, which gives a much clearer picture than can be obtained when the digits are superimposed. Because each digit is projected differently on an oblique view, it is best to compare two radiographs of oblique views taken at comparable but opposite angles.
The lateromedial or mediolateral projection is generally of much less value than the oblique view. However, because positioning is relatively easy, this view is useful to evaluate fractures, fracture repairs, and luxations.
In the axial projection, a lateromedial or mediolateral view of a single claw is accomplished by placing a nonscreen film (eg, a paper “cassette”) between the digits. This view produces a good image of the affected distal phalanx and, if interdigital soft-tissue swelling is not too great, the distal interphalangeal joint.
A number of factors should be considered in radiographic analysis and interpretation. Age differences can be seen radiographically as differences in skeletal development. In calves, physes are present in the distal metacarpus and metatarsus and at the proximal ends of the proximal and middle phalanges. In a very young calf, the distal phalanges may be incompletely ossified so that the bones appear small, and their distal ends are rounded and indistinct. The subchondral bone may appear indistinct and finely irregular; this should not be mistaken for the subchondral bone lysis seen in septic arthropathy.
Diseases stimulating periosteal new bone in cattle (such as corkscrew claw and postrecovery septic arthritis) can cause marked changes in bone contour and increased bone opacity.
Slight bony changes at articular margins and musculotendinous attachments are commonly seen on radiographs of older cattle. Roughening of the distal surface of the distal digit is a normal sign of aging. Changes that occur during the normal aging process should not be confused with active bony changes.
Reactive new bone (osteophyte, enthesiophyte, or exostosis) that has been present for some time has a distinct border and a rough outline, and the opacity is normally even. Active new bone has an indistinct border and a rough outline, and the opacity is uneven.
Diffuse loss of bone opacity occurs in subacute laminitis, nutritional bone disease, and after limb immobilization. Focal or localized loss of bone opacity occurs in bone infection (osteomyelitis) or inflammation (osteitis), early fracture healing, and with defects in endochondral ossification (osteochondrosis).
Increase in joint width is caused by the presence of increased fluid in the joint. However, this is less evident if the animal is bearing weight at the time the radiograph is taken. To confirm that the joint is in fact wider than normal, it may be compared with the contralateral joint.
Indistinctness and loss of opacity of the subchondral bone are often associated with joint infections. Loss of opacity is often irregular. For this reason, a single radiograph is unlikely to detect this pathology; therefore, several radiographs taken from different angles are usually advised. Comparison between suspect and known normal joints is recommended. Subchondral bone may be indistinct in a young animal.
Radiography is important to evaluate the progress of a fracture repair. A radiograph taken immediately after a fracture has been realigned is the basis for future evaluations, and subsequent radiographs are essential if nonunion or bone infection is suspected.
Loss of bone opacity is difficult to recognize with certainty in metabolic and nutritional diseases. Because all of the bones in the body may be equally affected, it is not helpful to compare one bone with another. In an adult animal, cancellous regions in the bone ends may become coarser or “granular” in appearance as smaller bone trabeculae are resorbed. In the diaphysis of a normal bone in both immature and adult animals, the cortex is thickest at midshaft and becomes thinner toward both ends. If the cortex at midshaft approaches the thinness of the proximal and distal diaphyses, generalized osteopenia must be suspected.
Soft-tissue swelling can be demonstrated on radiographs only in the early stages of a septic disease. The characteristics of a soft-tissue swelling may indicate the location of a lesion and the tissues involved, muscle or tendon disease, cellulitis or edema, or dark gas shadows (eg, a sinus or a cap of an abscess).