The equine foot is arguably one of the most x-rayed regions and most scrutinized. Numerous studies describe pathologic changes in the foot and association with lameness Imaging the equine foot most often starts with radiographs. Indications to radiographic the foot include lameness that localized to the region, farriery, regions wounds/lacerations, draining tracts, guided needle placement or interoperative imaging.
Foot preparation is paramount to obtaining quality, diagnostic radiographic images of the equine foot. The shoe should be removed, and the foot should be cleaned—preferably with a wire brush. Debris in the hair should be also be removed. The sulci of the frog should be thoroughly cleaned. The sulci should be packed with a soft deformable substance such as Play-Doh. Only the sulcus should be packed to eliminate gas artifact, not the entire sole.
The conventional radiographic projections of the foot include a lateromedial (LM), dorsopalmar (DP), dorso-proximal-palmarodistal oblique (D60Pr-PaDiO), and palmaroproximal-palmarodistal oblique (Pa45-55Pr-PaDiO) or skyline view. The DP view can be horizontal or angled 30° dorsoproximal-palmarodistal. Changing the angle will highlight different regions of the articular surfaces of the distal interphalangeal joint. If suspecting pathologic change in the wings of the distal phalanx or fossa of the collateral ligaments, additional oblique views can be used to highlight these regions. The lateral 45° proximal-mediodistal oblique. Oblique projections (including opposite projection) to highlight the palmar processes. Fractures of the palmar processes can be radiographically occult with standard views and may require oblique imaging and even time (14 days) to be seen. In some instances, these fractures are only seen on cross sectional imaging.
There are normal variations in the bone contour that can occur in the distal phalanx. Extensor process may have several different configurations, including a single point or double point and may rounded or pointed. The appearance of the distal phalanx as well as in the size of the crena marginalis may vary. Care should be taken not miss a centrally located keratoma. Contralateral radiographs may be beneficial in these cases. Ossification of the ungual cartilages can occur without clinical significance. However, recent study with MRI have demonstrated soft tissue abnormalities that can accompany ossification of ungual cartilages or fractures that may be difficult to see on radiographs. It is important to understand and know the attachments of commonly injury soft tissue on the bone. Bone can give indications that there is concurrent soft tissue injury.
The collateral ligaments of the distal interphalangeal joint on the middle and distal phalanges can be identified and should be closely evaluated. At the origin, there are smooth protuberances present on dorsomedial and dorsolateral P2. These also serves as the region of attachment for the chondrocoronal ligaments. The distal insertion of the collateral ligaments is an elongated oval lucency outlined at the 9 and 3 o’clock position on P3. Bone resorption/lysis in addition to regional sclerosis indicates enthesopathy of the collateral ligaments. Similar other enthesopathic injury, clinical significance needs to be correlated with physical exam and or advance imaging.
As with many sites, radiographic abnormalities in the foot may not correlate with lameness. The navicular bone in particular is often scrutinized. The shape, cortico-trabecular bone definition, presence of enthesophytes, size and number distal border synovial invaginations, and distal border fragmentation are often described. The clinical relevance of each of these findings is variable. Sound horses can have a trapezoidal or slightly elongated along the flexor cortex looking similar to a chef’s hat. Navicular bone distal margin fragmentation can be seen as an incidental finding. Synovial invaginations are often debated as degenerative changes in the navicular bone. Warmbloods have some normal variation of larger and more numerous synovial invaginations at the distal border compared to other breeds. When considered as a single entity, synovial invaginations may have minimal association with lameness. However, in the author’s opinion, enlarged synovial invaginations thinning the cortex have the propensity to erode through and become a flexor cortex defect and should be considered a risk. Radiographic findings considered clinically relevant are defect in the flexor cortex, loss of cortico-trabecular bone definition, medullary sclerosis and to a lesser extent proximomedial and lateral enthesophytes. Often when interpreting findings, it is several points of pathologic change in the foot/navicular region that mount and are assessed as a potential risk.
Ossified ungual cartilages were often thought of as a benign process with little clinical consequence, unless they fractured. There has a been more recent work to inspect this area further on purchase exams. The ungual cartilages can ossify in two patterns. The more common is from the base extending proximal and the second is with a separate ossification center proximal. The junction between the separate center and the base can be irregular and sclerotic. When reviewing this area is important to evaluate the ossified ungual cartilage and palmar process for sclerosis. This can precede a fracture of the distal phalanx. Ossified ungual cartilages have also been associated with collateral ligament injury. Ossified ungual cartilages will not likely preclude the horse from doing it’s intended purpose but should be scrutinized as a potential (low) risk.
The distal phalanx is uncommonly subject to osseous cyst like lesions but typically are regarded at clinically relevant and a source of lameness. These lesions are more often than not unilateral and in the forelimb. Mild signs of osteoarthritis are often encountered in the distal interphalangeal joint, especially in horses that have a long campaign record. In these cases, it is the author’s opinion that symmetry and history are important considerations to convey impressions of risk. Advanced demineralization and irregularity along the solar margins (anatomic diagnosis of pedal osteitis without an underlying cause) in thoroughbreds may be a consideration as these may be lame, whereas warmbloods tend to be less affected by lameness with similar radiographic findings.
There are often small osteophytes at the dorsoproximal aspect of the (fore) middle phalanx that are incidental. Osseous cyst like lesions are common seen associated with the hind proximal interphalangeal joint (distal aspect of the proximal phalanx) and are important to include in the radiographic views of the hind fetlock joint. When these cysts are located along the weight bearing surface, the author considers these a risk.
In conclusion, patient preparation, knowledge of regional anatomy, normal variation and careful interpretation of bone margins can help identify clinically relevant changes and direct future imaging to help diagnose the source of lameness.