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 head, neck, joints of the forelimb
and foot can be evaluated ultrasonographically; yielding important diagnostic information that cannot be obtained radiographically.
Normal Sonographic Anatomy of the Joints of the Forelimb
The temporomandibular joint contains an echogenic fibrocartilagenous disc that separates the larger dorsal compartment from
the smaller ventral compartment. No free fluid is visible in the normal joint. The disc appears as a homogeneously echogenic
triangle with its base laterally. The disc is attached to the thin echogenic joint capsule and measures 2 cm in thickness
caudally and caudolaterally, thinning to 0.5 cm rostrolaterally. The hypoechoic layer of articular cartilage adjacent to the
disc measured up to 3 mm in foals but was difficult to measure in adults.
Ultrasonographic examination of the neck is best performed following radiographic and scintigraphic examination of the neck
to help localize the areas of potential pathology. Ultrasonographic evaluation of the normal atlanto-occipital articulation
and the cervical region has been described in horses. Ultrasound guided injections of the atlanto-occipital joint have been
performed for myelograms with the injection of the contrast material into the subarachnoid space. The detection of the "chair"
sign is an excellent guide for performing ultrasound guided injections of the cervical facet joints.
The muscles of the shoulder girdle are the stabilizers the shoulder joint and thus, all the musculature on the craniolateral
aspect of the shoulder girdle should be evaluated. The biceps brachii tendon originates on the supraglenoid tubercle of the
scapula and occupies the intertuberal groove of the humerus. The biceps tendon is imaged coursing over the cranial aspect
of the shoulder. A bursa lies below and around the tendon in the groove. There is normally a small amount of fluid visualized
in the bicipital bursa. A tendinous band extends from the greater to the lesser tubercle, anchoring the biceps tendon. It
is also possible to image the insertion of the infraspinatus muscle in the caudal eminence of the greater tubercle from the
lateral aspect of the shoulder. The thin joint capsule is best imaged from a lateral and caudolateral view of the shoulder
joint. Synovial fluid is not normally imaged in the shoulder joint. The articular margins of the lateral and caudal portions
of the humeral head are the only visible articular cartilage and are best imaged from the lateral and caudolateral views.
The collateral ligaments of the elbow joint are easily imaged sonographically as homogeneous echoic structures with parallel
fiber alignment from their origins on the medial and lateral epicondyle of the humerus to their insertions on the medial and
lateral tuberosity of the radius. There is a long superficial part of the medial collateral ligament that inserts on the medial
border of the radius immediately distal to the interosseous space.