Ultrasonographic Assessment of the Stifle in the Field

Stifle lameness is common and in-field radiography and ultrasonography are useful primary diagnostic tools for lameness investigation. Radiography often indicates chronic disease and there is minimal correlation between radiographic findings and arthroscopic findings [1,2]. Ultrasonography provides greater diagnostic sensitivity for soft tissue injury and is more sensitive than radiography to detect osteochondrosis, especially of the trochlear ridges [3].

How are images optimised?

Ensuring that the patient is clean, clipped if required, and prepping with alcohol only can achieve excellent image quality. Scrubbing with chlorhexidine, alcohol and the application of a coupling gel will all improve image quality.

Equipment

A high frequency (7–15 mHz) linear probe is preferred by the author. The use of a curvilinear probe with a lower frequency range can be utilised when assessing the caudal aspect of the joint. The use of a microconvex probe has also been described [4].

Positioning for ultrasonography

Ultrasonography of the equine stifle will initially be performed weightbearing with the joint extended. Beginning on the cranial aspect of the stifle with a transverse probe orientation, the middle patellar ligament is visualised. Moving medially on the limb and switching to a vertical probe orientation, the medial femorotibial joint is assessed, including the degree of synovial effusion, quality and nature of synovial fluid, synovial hypertrophy and foreign material within the joint. The medial meniscus is assessed next to include shape, echogenicity, size and position of the meniscus. Pathology such as modifications in shape, calcification, tears and enthesophytes, fibroplasia, extrusion or fragmentation, separation from the collateral ligaments, and collapse of the meniscus leading to loss of the joint space can be seen [5,6]. Assessment of periarticular margins of the medial femorotibial joint, such as the presence of osteophytes as well as the medial collateral ligament, are assessed next.

Returning to the cranial aspect of the limb, the middle, medial and lateral patellar ligaments are then examined; small deviations in the normal architecture of the ligaments are unlikely to be a cause of significant lameness. However, avulsion of the middle patellar ligament at the point of insertion on to the tibial crest, sometimes associated with fracture of the tibial crest, may be present. Remodelling of the apex of the patella or the medial parapetallar fibrocartilaginous attachment on to the medial patellar ligament are indications of previous trauma or osteoarthritis of the joint.

Assessment of the anechoic cartilage of the medial and lateral trochlear ridges of the femur is performed. Moving laterally on the limb the lateral pouch of the femoropatellar joint can be identified just caudal to the lateral trochlear ridge.

With the probe in a transverse orientation, the lateral meniscus is identified just proximal to the tibial crest; angling the probe caudally optimises contact and thus image quality. Normally there will be very little effusion visible within the lateral femorotibial joint, and the presence of effusion at this site should be considered as pathognomic for the presence of pathology or recent diagnostic analgesia.

Flexed views of the stifle are useful, with the leg resting on the toe and slightly raised from the ground. A small wooden block may be used. The cranial attachment of both the medial and lateral meniscotibial ligaments can be visualised. Tearing of the ligaments and remodelling within the surface of the tibial attachment can be assessed. The weightbearing surface of the medial femoral condyle can also be visualised allowing assessment of the articular cartilage and the underlying subchondral bone.

Caudal ultrasonographic views of the stifle can be performed with a low frequency curvilinear probe. Injury within the attachment of the caudal cruciate ligament and meniscofemoral ligaments may be seen.

What are the indications for arthroscopy?

Arthroscopy not only provides an opportunity to diagnose soft tissue injuries such as cruciate ligament and meniscal tears, but it also has the advantage of providing treatment and assessment of prognosis [7,8]. However, not all of the stifle joint is accessible via arthroscope; for example, only the cranial and caudal aspects of the menisci are accessible surgically [9].

Advanced imaging

Computed tomography (CT), CT arthrography and magnetic resonance imaging provide diagnostic information about soft tissue and osseous injuries of the equine stifle that are not possible with other modalities. General anaesthesia is currently required for these procedures but CT imaging can be combined with arthroscopy in some settings.