Radiographs may be unremarkable with soft tissue injuries of the stifle; however, 48% to 83% of cases of meniscal injury have been reported to have associated radiographic changes. Changes may include new bone formation at the cranial aspect of the medial intercondylar eminence of the tibia, mineralization of the meniscus, and generalized osteoarthritic changes (Figure 193-3). If the meniscus is protruding from the joint space or is severely torn, narrowing of the femorotibial joint space may be evident.

Ultrasound is invaluable for diagnosis and prognostication of soft tissue injuries, and its value in evaluating meniscal tears is ever increasing. A 7- to 14-MHz linear transducer should be used to evaluate meniscal injuries in two perpendicular planes. Some meniscal lesions may be more apparent when the limb is non–weight bearing, or when a flexed cranial view is used. Ultrasound of the meniscus is challenging because the tissue fibers cannot be kept perpendicular to the probe at all sites, and a hypoechoic appearance is created in certain regions (i.e., the cranial medial meniscus). Additionally, because of the large soft tissue mass caudal to the stifle, the caudal aspect of the meniscus is very difficult to diagnostically image, even when a 4- to 6-MHz curvilinear probe is used. Abnormal ultrasonographic findings with meniscal injury may include hypoechoic regions of fiber disruption, core lesions, protrusion of the meniscus from the joint space, and generalized synovitis or arthritis changes (Figure 193-4). The sensitivity and specificity of ultrasound for identifying meniscal injuries are 79% and 56%, respectively, compared with arthroscopic findings. The apparent high rate of false-positive diagnoses may occur in part because a large portion of the meniscus is not visible on arthroscopic exam (specifically, the medial and lateral aspects of the joint) and because horizontal tears within the meniscus may not be visible on arthroscopic exam of the menisci. Thus combining findings from ultrasonographic and arthroscopic evaluation of the meniscus will yield the greatest diagnostic information.

Advanced imaging modalities for the equine patient are available at select referral hospitals. Magnetic resonance imaging (MRI), considered a gold standard for soft tissue evaluation, is used extensively in humans to evaluate meniscal injury. Magnetic resonance imaging allows for excellent evaluation of bone and soft tissue components of the entire stifle joint. Given the large size of the equine stifle joint and its proximity to the abdomen, MRI of the equine stifle is limited to the large or open-bore magnets. The wide (70 cm) and ultrashort (125 cm) bore of the Siemens Magnetom Espree 1.5 T magnet¹ is still only able to accommodate adult horses with relatively long limbs and narrow hips. General anesthesia is required for MRI and computed tomography (CT) evaluation of the equine stifle joint.

Computed tomography is helpful in identifying soft tissue injuries, particularly when contrast arthrography is used to enhance CT imaging. The most significant advantage of MRI and CT imaging is that they enable evaluation of the entire equine stifle, which is not possible with ultrasound or arthroscopic examination. This will lead to a greater understanding of meniscal injuries and concurrent pathologic processes in the stifle joint.

Nuclear scintigraphy identifies lesions based on their physiologic characteristics (blood flow and osteoblastic activity). Scintigraphy has been inconsistent in revealing stifle lesions, including subchondral bone cysts, and although it has the potential to identify meniscal lesions and associated lesions (osteoarthritis or cysts), it appears to have a relatively poor sensitivity and specificity for soft tissue lesions of the stifle.