Diagnostic ultrasonography has more recently been applied to the assessment of other less traditional musculoskeletal problems such as evaluation of bone, joints, muscle and nerves. The soft tissue structures of the joints, back, and foot can be evaluated ultrasonographically yielding important diagnostic information that cannot be obtained radiographically.

Normal Sonographic Anatomy of the Joints of the Hindlimb

Back

The sacroiliac ligament has a dorsal portion that originates from the tuber sacrale and inserts on the tops of the dorsal spinous processes of the sacrum. The dorsal portion is a thick echoic structure that is partially covered by the gluteus medius muscle. It appears oval in transverse section and measures 6-10 mm in thickness. It also has a ventral portion that is a thick triangular sheet of ligamentous tissue that originates from the tuber sacrale and the caudomedial border of the ilial wing and inserts on the lateral sacral crest that is formed by the fused transverse processes. This ligament also appears echoic and is approximately 5 mm thick and 5 mm wide. The ventral sacroiliac ligament is attached to the ventral aspect of the articular surfaces, close to the joint. This ventral sacroiliac ligament blends ventrally and caudally with the sacrosciatic ligament.

Hip

The greater trochanter of the femur and the outer portion of the acetabulum can be imaged but the deeper portions of the hip joint are not visible sonographically. Transrectal sonographic evaluation of the pelvis can also be performed.

Stifle

The medial, middle and lateral patellar ligaments can be imaged in their entirety from origin to insertion. They are homogeneous echoic with a parallel fiber pattern. They vary in shape from a triangular medial patellar ligament to a round middle patellar ligament to a flattened and wide lateral patellar ligament. The proximal portion of the lateral patellar ligament extends over the lateral trochlear ridge of the femur. There is a large fat pad in the distal portion of the stifle separating the distal portion of the patellar ligaments from the synovial membrane. A small amount of joint fluid is usually imaged in the normal femoropatellar joint caudal to the medial patellar ligament and caudal to the lateral patellar ligament. Long and thick synovial villi are usually imaged in the medial recess of the femoropatellar joint.

The medial and lateral collateral ligaments extend from a recess in the distal femur to the proximal tibia. The medial collateral ligament is attached to the medial meniscus while the lateral collateral ligament is separated from its meniscus by the popliteal tendon. The medial and lateral meniscus both have a triangular radial section. Their attachment to the proximal tibia (cranial meniscotibial ligaments) can be imaged only in the flexed stifle. With flexion the insertion of the cranial cruciate onto the proximal tibia is imaged and the body of the caudal cruciate can also be imaged deep to the cranial cruciate ligament. From the caudal approach, the caudal attachments of the menisci are visible as are the cruciate ligaments. There is normally a small amount of fluid in the medial femorotibial joint that is located between the medial patellar ligament and the medial collateral ligament with little or no synovial villi visible. There is no synovial fluid normally imaged in the subextensorius recess of the lateral femorotibial joint.

The distal articular margins of the patella and the articular margins of the femorotibial joint are normally smooth and regular. The articular cartilage of the medial femoral trochlea is normally 1-2 mm thick while the lateral articular cartilage is 2-3 mm thick. The subchondral bone in the trochlear groove may appear irregular. With flexion, the distal articular margins of the femoral condyles can be imaged. With the caudal approach to the stifle joint, the caudal articular margins of the femoral condyles are visible.