Imaging of the Fetlock and Palmar/Plantar Metacarpus and Metatarsus

Lameness and poor performance are common problems afflicting equine athletes. After the clinical lameness examination has localized the site of lameness, imaging is often the next step in  the diagnostic plan. The fetlock and palmar/plantar metacarpal/tarsal regions are common sites  of lameness in sport horses. Specific injuries may include desmopathy of the suspensory ligament, distal accessory ligament, or distal sesamoidean ligaments, superficial or deep digital  flexor tendonopathies, osteoarthritis, and osteochondral chip fractures or major fractures.  Investigation of a problem may require only a single imaging modality, or may require a  multimodal approach.  

Radiography is often the first technique used when investigating lameness problems isolated to  the distal limb. Computed and direct digital radiography have become more prevalent over the  past decade and are now widely available on an ambulatory basis. These technologies can  enhance the diagnostic value of radiographs, but do not compensate for poor positioning or inappropriate technique.  

In the fetlock region, four views (dorsal-palmar/plantar with 20-30 degrees of elevation, lateral medial, dorsolateral-palmaro/plantaromedial oblique with 10-15 degrees of elevation, and  dorsomedial-palmaro/plantarolateral oblique with 10-15 degrees of elevation) may be sufficient. These images may reveal signs of osteoarthritis, including osteophyte formation or joint space  narrowing (Fig. 1). Enthesiophytes may suggest the presence of ligamentous disease. Fractures  can range from small osteochondral chip fractures to comminuted phalangeal or  metacarpal/tarsal bone fractures. Subtle variations in the angulation of the projection may be  necessary to identify nondisplaced fracture lines. Additional views may be necessary to  investigate specific locations within the joint.¹ For example, flexed lateral-medial and flexed  dorsal-palmar/plantar views can highlight pathology on the distal or palmar/plantar condyles of  the third metacarpal/tarsal bone.²  

Radiography can also be valuable in assessing pathology associated with the soft tissue  structures of the palmar/plantar metacarpus/tarsus. Four standard views (dorsal-palmar/plantar, lateral-medial, dorsolateral-palmaro/plantaromedial oblique, and dorsomedial-palmaro/  plantarolateral oblique) are generally obtained. Horses with suspensory ligament desmitis may  have irregularity of the palmar/plantar or endosteal surface of the proximal third  metacarpal/tarsal bone or avulsion fragments at the origin of the suspensory ligament.³ The  second and fourth metacarpal/tarsal bones can be assessed for fractures or exostoses that may impinge upon the suspensory ligament.⁴ 

Ultrasonography is an excellent choice to assess both soft tissue structures and bony contours. A  linear transducer operating between 8-12 megahertz is typically used for the fetlock and palmar/plantar metacarpal/tarsal regions. A microconvex transducer may allow more thorough assessment of some structures, such as the proximal suspensory ligament. A standoff pad can  improve image quality for superficial structures and contours to the horse’s leg. Comparison to  the opposite limb is encouraged, as ultrasonographic changes can be subtle. Some soft tissue  structures (e.g, the distal sesamoidean ligaments and the suspensory ligament) may normally  have some heterogenicity, so comparison to the normal limb can help reduce errors in  interpretation. Subtle irregularities of the bony margins are often better appreciated  ultrasonographically than radiographically.⁵ 

Ultrasonography is often the first diagnostic modality used when pathology of the suspensory  ligament is suspected. Findings may include enlargement, irregularity of the fiber pattern, and decreases in echogenicity (Fig. 2).⁶ Large core lesions are less common in the sport horse than is  a more chronic desmitis characterized by a more subtle decrease in echogenicity and  enlargement of the ligament. Irregularity or avulsion fractures of the proximal palmar/plantar  aspect of the third metacarpal/tarsal bone can be identified using ultrasonography as well.

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