CHAPTER 110 Diagnostic Imaging and Surgical Management of Conditions Involving the Stifle Joint
The equine stifle joint is anatomically complex and technically challenging to evaluate fully because of the large size of the joint itself and the surrounding muscle mass. Isolation of lameness to the stifle joint is typically based on a thorough physical examination, including palpation of accessible anatomic structures: the three joint compartments (femoropatellar, medial femorotibial, and lateral femorotibial), patella, distal patellar ligaments, and collateral ligaments. In addition, a thorough lameness examination with intra-articular anesthesia of all three joint compartments is frequently necessary to localize lameness to the stifle joint.
Several structures within the stifle joint can be a source of lameness: the articular cartilage and underlying subchondral bone of the femur, patella, or tibia; synovial membrane; menisci and their attachments to the femur or tibia; the cranial and caudal cruciate ligaments; and collateral ligaments. Therefore, lameness localized to the stifle joint should be further evaluated using imaging techniques to provide a more detailed anatomic diagnosis of the lameness. Frequently, several imaging modalities will be required to evaluate completely the potential source of lameness within the stifle joint. Radiographs without localization of lameness to the stifle joint should be interpreted with some caution because many radiographic changes are not associated with diminished performance. Conversely, horses with normal radiographic and ultrasonographic findings may have soft tissue injuries that are a source of the lameness, but the areas cannot be imaged with presently available tools. When a lesion is identified, a thorough three-dimensional understanding of the anatomic topography is imperative to determine whether the lesion is surgically accessible for treatment. Structures accessible during arthroscopy of the cranial and caudal pouches of the femorotibial joints have been described.
Radiography is typically the first imaging method used to evaluate the bony and soft tissue structures of the stifle joint. When possible, images should be obtained using digital radiography because they have significantly better contrast, elucidation of bone structure, and diagnostic value compared with images obtained using conventional radiography. A minimum of three survey projections should initially be used to obtain images, including caudocranial (with x-ray tube angled 0 to 15 degrees distad, depending on the conformation of the horse; Figure 110-1), lateromedial, and caudal 60 degrees lateral-craniomedial oblique views (Figure 110-2). A detailed map of the soft tissue attachments that includes tendons, ligaments, and fibrous portion of the joint capsules has been drawn for these three standard projections, and they can be useful when interpreting stifle radiographs. Only the caudolateral to craniomedial oblique view (and not a caudomedial or craniolateral oblique) is included in the survey set because the medial femoral condyle and the lateral trochlear ridge of the femur, which are often affected in the horse, are projected. This projection can provide important information about the depth of an osteochondritis desiccans (OCD) lesion of the femoral lateral trochlear ridge or the configuration of a medial femoral condyle subchondral bone cyst. When indicated, additional views, including a caudal 60-degree medial-craniolateral projection, flexed lateromedial, and skyline projection of the patella (Figure 110-3), should be obtained.
Figure 110-2 The radiographic projection used to obtain a caudocranial oblique view of the stifle joint. The x-ray tube is 50 to 60 degrees lateral from caudal, and the film cassette is positioned perpendicular to the x-ray beam. This view is often used to project the lateral trochlear ridge of the femur and the medial femoral condyle.
A complete ultrasonographic examination of the stifle joint can provide important information regarding both soft tissue and bony structures. A comprehensive comparative cross-sectional ultrasonographic atlas of the normal medial femorotibial joint and the related structures has been published and provides a very useful protocol for standardized ultrasonographic examination of the medial femorotibial joint. Significant experience is necessary to gain expertise in diagnostic ultrasonography of the equine stifle. Cartilage thickness and surface regularity, and the underlying subchondral bone on the trochlear ridges and femoral condyles can be evaluated for signs of osteoarthritis (OA), such as irregular, thin cartilage. The medial and lateral menisci and collateral ligaments can consistently be imaged. Imaging of the cruciate ligaments and the caudal meniscotibial ligaments and their attachments is very difficult and often incomplete. Flexion and extension of the stifle joint during imaging, as well as use of curvilinear and linear transducers in longitudinal, transverse, and oblique planes, can help to optimize imaging of the various structures.
Assessment of the musculoskeletal system using magnetic resonance imaging (MRI) has advanced considerably in recent years. Although not widely available for imaging the equine stifle, MRI is the most effective imaging modality for assessing the entire joint, including cartilage, meniscus, ligaments, and subchondral bone sclerosis and edema (Figure 110-4