Chapter 17 Ultrasonographic Examination of Joints
Ultrasonography has become an essential imaging technique for assessing joint lesions.1-6 It provides information complementary to radiography but does have some limitations. Ultrasonography requires a precise knowledge of anatomy, not only bone anatomy,4 and a systematic approach. Examining the joints of the distal part of the limb is physically uncomfortable for the imager and is time-consuming: complete examination of a joint may take up to 30 minutes. Quick examination of joints requires several years of daily practice.
The indications for ultrasonographic examination of equine joints include synovial fluid distention, local swelling, pain on passive manipulation of the joint, improvement in lameness after intraarticular or perineural analgesia, and positive radiological or scintigraphic findings. The area to be examined should be clipped, not shaved, and washed with hot water. High-resolution transducers (7.5- to 13-MHz linear probes) and a standoff pad are used for superficial structures. Convex array 2.5- to 5-MHz transducers are preferable for deeper structures. Both left and right joints should be examined to improve sensitivity and specificity. The image quality depends not only on the frequency of the transducer but also on the quality of the machine (from treatment of the signal to display on the monitor) and the skill of the operator in placing and orienting the transducer.
A comprehensive description of all joints cannot be given in this book. The fetlock joint is a model for a general approach to ultrasonography of joints because of its simple anatomy. Some aspects of examination of the stifle and hock also are presented.
Figure 17-1 shows normal ultrasonographic anatomy.4,5 In normal fetlock joints the articular capsule is echogenic (except if too relaxed), and the articular margins of the proximal phalanx and the condyles of the third metacarpal bone (McIII) are smooth.
Fig. 17-1 Sagittal ultrasonographic image of the dorsal aspect of the fetlock, normal appearance. Proximal is to the left. 1, Skin; 2, dorsal joint capsule; 3, proximal synovial fold; 4, subchondral bone of the condyle of the third metacarpal bone; 5, articular cartilage of the condyle of third metacarpal bone; 6, proximal phalanx.
Ultrasonography is a useful technique for the differential diagnosis of soft tissue injuries on the dorsal aspect of the fetlock joint. These lesions include subcutaneous swelling or abscess, bursitis of the subtendonous bursa of the extensor tendons, extensor tendonitis, capsulitis, synovial fluid distention of the dorsal recess of the metacarpophalangeal joint (Figure 17-2), and chronic proliferative synovitis of the proximodorsal synovial fold of this joint (Figure 17-3). Thinning, fibrillation, and fissures of the articular cartilage of the dorsal and distal aspects of the condyle of the McIII can be identified with high-resolution transducers. Subchondral bone lesions can sometimes be detected before they are visible radiologically. Ultrasonography may be more sensitive than radiography for detection of the site and number of osteochondral fragments.7
Fig. 17-2 Sagittal ultrasonographic image of the dorsal aspect of the fetlock joint. An abnormal synovial fluid accumulation appears between the joint capsule and the articular surface. The diagnosis is synovial fluid effusion, indicative of synovitis. Proximal is to the left. 1, Skin; 2, dorsal joint capsule; 3, proximal synovial fold; 4, subchondral bone of the condyle of the third metacarpal bone; 5, articular cartilage of the condyle of the third metacarpal bone; 6, synovial fluid; 7, proximal phalanx.
Fig. 17-3 Sagittal ultrasonographic image of the dorsal aspect of the fetlock joint. An abnormal hypoechogenic mass appears between the capsule and the condyle of the third metacarpal bone. The proximal aspect of this condyle is irregular, indicating bone lysis (arrowhead). The diagnosis is chronic proliferative synovitis. 1, Skin; 2, dorsal joint capsule; 3, proximal synovial fold tremendously thickened and echogenic; 4, condyle of the third metacarpal bone; 5, synovial fluid.
Examination of the dorsomedial and dorsolateral aspects of the joint is especially useful for complete evaluation of the articular margins, which are smooth in a normal joint. The most common abnormal finding is the presence of periarticular osteophytes. Other injuries are subcutaneous lesions (fibrosis, swelling) and capsulitis.
The medial and lateral collateral ligaments have superficial and deep layers.4 If the transducer is parallel to the skin (the ultrasound beam is perpendicular to fiber interface), the superficial layer of the collateral ligament is echogenic and the deep layer is hypoechogenic. Either layer may be damaged (Figure 17-4). If both layers are affected, joint instability, subluxation, or luxation occurs.
Fig. 17-4 A, Physical appearance of an injured right hind fetlock joint (arrowheads show medial enlargement). Radiography found no bony abnormalities. B, Transverse ultrasonographic images of the lateral (L) and medial (M)