Dorsal Aspect

Figure 17-1 shows normal ultrasonographic anatomy.^4,5 In normal fetlock joints the articular capsule is echogenic (except if too relaxed), and the articular margins of the proximal phalanx and the condyles of the third metacarpal bone (McIII) are smooth.

Ultrasonography is a useful technique for the differential diagnosis of soft tissue injuries on the dorsal aspect of the fetlock joint. These lesions include subcutaneous swelling or abscess, bursitis of the subtendonous bursa of the extensor tendons, extensor tendonitis, capsulitis, synovial fluid distention of the dorsal recess of the metacarpophalangeal joint (Figure 17-2), and chronic proliferative synovitis of the proximodorsal synovial fold of this joint (Figure 17-3). Thinning, fibrillation, and fissures of the articular cartilage of the dorsal and distal aspects of the condyle of the McIII can be identified with high-resolution transducers. Subchondral bone lesions can sometimes be detected before they are visible radiologically. Ultrasonography may be more sensitive than radiography for detection of the site and number of osteochondral fragments.⁷

Dorsomedial and Dorsolateral Aspects

Examination of the dorsomedial and dorsolateral aspects of the joint is especially useful for complete evaluation of the articular margins, which are smooth in a normal joint. The most common abnormal finding is the presence of periarticular osteophytes. Other injuries are subcutaneous lesions (fibrosis, swelling) and capsulitis.

Medial and Lateral Aspects

The medial and lateral collateral ligaments have superficial and deep layers.⁴ If the transducer is parallel to the skin (the ultrasound beam is perpendicular to fiber interface), the superficial layer of the collateral ligament is echogenic and the deep layer is hypoechogenic. Either layer may be damaged (Figure 17-4). If both layers are affected, joint instability, subluxation, or luxation occurs.

Palmar Aspect

A normal fetlock joint has a small amount of anechogenic synovial fluid in the proximopalmar recess of the fetlock joint. Abnormal findings observed in affected horses include synovial fluid distention, often associated with enlarged synovial folds, indicative of synovitis (Figure 17-5).

Echogenic material may be observed in the synovial fluid in any view. Small echogenic spots may represent fibrin or cartilaginous debris. Larger echogenic masses are compatible with osteochondral fragments. These abnormal findings are more easily seen after movement of the joint. A homogeneous increase in echogenicity of the fluid is compatible with hemarthrosis or infectious synovitis.

Femoropatellar Joint

The femoropatellar joint is examined using 5- to 10-MHz linear transducers with a thin standoff pad. The normal patellar ligaments are homogeneously echogenic.^1-3 The articular cartilage is thicker over the lateral trochlear ridge compared with the medial. Each femoral trochlea has a regular hyperechogenic subchondral bone surface.

Medial patellar desmopathy usually is iatrogenic (i.e., caused by medial patellar desmotomy or desmoplasty). Desmopathy of the intermediate (middle) patellar ligament is an injury that occurs in athletic horses.⁸ Lateral patellar desmopathy often is caused by trauma (Figure 17-6).

Osteochondrosis of the trochlear ridges is easy to diagnose, and ultrasonography provides information on the extent of the lesion (especially in a lateromedial direction), the size and location of the osteochondral fragments, and the severity of alteration of the subchondral bone.⁹ Evaluation of the synovial fluid and membrane may also be useful to assess the consequences of ligamentous and osteochondral lesions in the joint.

Medial and Lateral Femorotibial Joints

Examination of the femorotibial joint requires 5- to 10-MHz linear transducers and a thin standoff pad.^10-13 In normal joints the medial recess of the medial femorotibial joint is less than 3 cm long in a craniocaudal direction, and the synovial fluid is totally anechogenic. The normal medial collateral ligament and medial meniscus are homogeneously echogenic,^1-3 provided that the ultrasound beam is perpendicular to the orientation of the fibers. The normal articular margins of the medial femoral condyle and tibial plateau are smooth and regular.

The medial recess of the medial femorotibial joint can be considered a mirror of the joint, because its size and content are influenced by all joint lesions. Abnormalities include synovial effusion (Figure 17-7); chronic proliferative synovitis; hemarthrosis (Figure 17-8); echogenic spots compatible with fibrin, cartilaginous, or meniscal debris (Figure 17-9); osteochondral fragments; and calcinosis circumscripta. The caudal recess of the medial femorotibial joint can be examined with 2.5- to 5-MHz sector or convex array probes. Distention of this recess always indicates severe femorotibial disease. Distention of the subextensorius recess on the craniolateral aspect of the lateral femorotibial joint can be observed in lateral femorotibial arthropathy or femoropatellar joint lesions.

Desmopathy of the medial collateral ligament may be identified as disruption in the normal parallel fiber pattern (Figure 17-10).

Meniscal injuries can be observed alone or with other ligamentous or bone injuries (Figure 17-11). In horses, approximately 75% of these lesions are found in the medial meniscus and 25% in the lateral meniscus. A wide range of type and severity of medial meniscal injuries may be seen (see Figures 17-11 and 17-12).

If a transducer is held vertically and moved in a caudocranial direction, complete examination of the articular margins of the femoral condyles is possible. Ultrasonography may be more sensitive than radiography in detecting periarticular osteophytes. If the stifle is examined in flexion, the cartilage and subchondral bone surface of the femoral condyles can be imaged.^1-3 Alterations of the subchondral bone surface and echogenicity may be detected.