Femoropatellar Joint

The femoropatellar joint is best imaged with the joint extended, as most of the trochlea is hidden by the patella when the limb is flexed. In the extended position the patella points craniolaterally, and the large medial trochlear ridge is palpable under the skin cranially. Starting in transverse planes, the probe is placed cranioproximally over the quadriceps muscles and examination is continued from proximal to distal, down to the tibial tuberosity. The quadriceps muscle is relatively homogeneous with a typical hypoechogenic muscular pattern. There is no insertion tendon as such, so that there is a sharp interface between the muscle belly and the proximal surface of the patella (Figure 7.1). The cranial surface of the patella forms a sharp, smooth hyperechogenic interface. Distally, it forms a shallow groove at the origin of the middle patellar ligament (PL) (see below). Medially, it is prolonged by the hypoechogenic, crescent-shaped parapatellar fibrocartilage, which curves around the medial trochlear ridge and terminates via the thin medial collateral patellar ligament (Figure 7.2). It also gives rise to the medial PL (see below).

Distal to the apex of the patella, the trochlear groove appears as a wide, U-shaped bone interface covered by anechogenic cartilage (Figure 7.2). In many horses, the latter is irregular in the center of the groove, which is considered to be a normal feature. The medial trochlear ridge is broad, smooth, and rounded, and is covered by relatively thin cartilage (0.8–1 mm thickness). This becomes irregular medially. The lateral trochlear ridge is much narrower and triangular in cross-section. Its cartilage is thicker (2–2.5 mm). In longitudinal sections (parasagittal), the ridges form a smooth, convex bone interface, topped by anechogenic cartilage, which should be perfectly regular in thickness. The lateral PL is seen as a striated structure lying directly over the lateral ridge. The capsule is tightly applied against the cartilage surface, and no fluid is normally visible over the trochlea.

The patellar ligaments actually anatomically correspond to the quadriceps tendon and are therefore similar in appearance to digital flexor tendons. Their borders are, however, ill defined due to poor contrast with the surrounding infrapatellar fat pad. The middle PL is round to oval in cross-section (Figure 7.3) and is the largest of the three PLs. It runs from the apex of the patella to the cranial-most part of the tibial tuberosity, within the fat pad and in the center of the trochlear groove. Distally, it becomes more triangular and often contains thin, hypoechogenic lines, giving it a webbed appearance. This should not be mistaken for a tear. Tilting of the probe to create an off-incidence artifact will cause the ligament to become hypoechogenic, enhancing its outline within the fat pad. The medial PL is triangular in cross-section and, in the extended limb, lies over the medial aspect of the medial trochlear ridge, 4 or 5 cm caudal to the apex of the ridge (Figure 7.4). Proximally it becomes more heterogeneous as its fibers spread out into the parapatellar fibrocartilage. Distally, it inserts approximately 2 cm medial to the middle PL and receives a tendinous branch from the sartorius muscle. The lateral PL is thinner and crescent shaped in cross-section (Figure 7.2). It caps the apex of the lateral trochlear ridge, outlining the cartilage. It inserts on the tibial tuberosity, immediately lateral to the middle PL’s insertion. There is often a small to moderate amount of anechogenic synovial fluid in the lateral and medial joint recesses caudal to the respective PLs, over the surface of the lateral and medial trochlear ridges.

Medial Femorotibial Joint: Medial Aspect

A linear transducer is best, as the joint is very close to the skin. The topography of the medial aspect of the joint is reviewed in Figure 7.5. The proximal edge of the tibial condyle and the round, smooth surface of the medial femoral condyle form a triangular space filled by the echogenic medial meniscus (Figure 7.6). Linear, anechogenic artifacts are caused by refraction at the insertion of the capsule on the outer surface of the meniscus. These are easily mistaken for tears but they should remain perpendicular to the probe when the latter is tilted. The capsule inserts over the whole abaxial aspect of the meniscus, so that fluid can only accumulate proximal to it, over the abaxial surface of the medial femoral condyle. Discrete distension of the joint is common but the synovial membrane should remain very thin. Small villi may be seen in the pouch in normal horses. Transverse plane images of the menisci are obtained by rotating the probe 90°. These images are useful to better evaluate the configuration of certain tears.

The medial collateral ligament (MCL) is a strong, flattened structure, approximately 5–6 mm in thickness. As in most joints it is made up of two poorly differentiated branches (Figure 7.7). It is necessary to rotate the probe to image each branch individually, as they run obliquely to each other, crossing over the meniscus. The superficial branch runs vertically; it originates on the femoral epicondyle several centimeters proximal to the joint, and inserts on the abaxial surface of the tibia. It receives part of the insertion of the adductor muscle proximally. The deep branch originates caudal to it, and runs in a craniodistal direction to insert on the tibia, cranial to the superficial branch.

Lateral Femorotibial Joint: Lateral Aspect

The general topography is presented in Figure 7.8. The joint is covered by a thicker layer of muscle and the bone surfaces are very oblique to the skin. It is consequently difficult to obtain images using a linear transducer. A micro-convex probe provides better images. Craniolaterally, the tendon of origin of the long digital extensor and peroneus tertius muscles originates on the lateral femoral epicondyle and runs within the deep extensor groove of the proximal tibia (Figure 7.9). The groove is covered by cartilage and a synovial recess extends from the lateral femorotibial joint (LFT), between the bone surface and the tendon. Similarly to the MCL, the lateral collateral ligament (LCL) is divided into two branches, although they are more difficult to tell apart. They both insert on the tibia and fibular head. The popliteal tendon originates immediately cranial to the LCL. It runs obliquely in a caudodistal direction, over the meniscus and underneath the LCL. It is triangular in cross-section and should not be mistaken for a torn portion of the meniscus. It fans out over the caudal proximal tibia as a hypoechogenic muscle. Fluid may be seen proximal to the meniscus and cranial or caudal to the collateral ligament, but this is less common than in the medial compartment.

Cranial Aspect of the Femorotibial Joints

Imaging the cranial aspect of the femorotibial articulation requires that the stifle be flexed, in order to expose the intercondylar space and the cruciate ligaments (Figure 7.10). A micro-convex transducer must be used. The surface of the femoral condyles, overlying cartilage, and cranial horns of the menisci are easily visualized deep to the fat pad. The round condylar surfaces can be assessed for cartilage or subchondral defects.

The cranial cruciate ligament (CrX) can be identified with the ultrasound beam perpendicular to the ligament fibers (Figure 7.11): the probe is placed immediately distal to the patella, over or medial to the middle PL. The probe is angled downward in a sagittal plane to image the surface of the tibial eminence. The beam is then rotated 15–20° clockwise in the left limb and anticlockwise in the right limb until the linear pattern of the ligament is recognized. The CrX originates caudally on the axial (medial) aspect of the lateral femoral condyle, and inserts in a dip between the medial and lateral prominences of the tibial eminence. Cross-sectional images can be obtained by rotating the probe 90° in the same position. The CrX is hyperechogenic with a regular striated pattern.