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
The supraspinous ligament is visible as an echoic band superficially, where its fibers are parallel to the skin and hypoechoic
deeper due to the obliquity of the ligament near the interspinous spaces. The deeper portion of the supraspinous ligament
is continuous with the interspinous ligament, which also appears hypoechoic. The more superficial portion is continuous with
the thick echoic thoracolumbar fascia. The insertion of the supraspinal ligament onto the dorsal spinous process can be imaged.
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.
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.
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.