Page 359 - Adams and Stashak's Lameness in Horses, 7th Edition

Diagnostic Imaging 325

sound transmission to the deeper aspects of the joint. have developmental orthopedic disease that, when
However lower‐frequency probes may need to be used, involving a joint, frequently involves the trochlear ridges
VetBooks.ir mised image quality. Microconvex or macroconvex the trochlear ridges of the talus; Warmbloods often have
of the stifle, the distal intermediate ridge of the tibia, or
which have poorer resolution and somewhat compro­
a similar distribution of osteochondrosis lesions, and
probes have the added advantage of a divergent beam
that allows image acquisition from a smaller skin con­ they also tend to develop periarticular changes in the
tact point. coffin, pastern, and fetlock joints. In each case, it is
Joint ultrasound requires a specific knowledge of the important to perform as complete an examination of the
anatomy of the joint. Unlike the palmar/plantar metacar­ joint as possible because many joint conditions involve
pal exam where the transverse images are typically multiple structures and/or multiple areas of the joint.
acquired first, it is helpful to begin the examination of For example, osteoarthritis tends to cause cartilage
many joints in the longitudinal scan plane. Identifying the degeneration and thinning in the loaded areas of the
articular surfaces of a joint provides orientation for the articular surface. Along with these cartilage changes,
rest of the examination. Global assessment of a joint can periarticular remodeling occurs in the form of osteo­
be time consuming requiring multiple scan plane. The phyte and/or enthesophyte formation. Joint capsule
extensor and flexor surfaces (and the corresponding soft thickening, with or without metaplasia, is also a com­
tissues such as the extensor/flexor tendons that cross mon feature of osteoarthritis. Many of the osteochon­
these areas) and the collateral ligaments and the articular dral fractures that occur in the joints of performance
surfaces (weight‐bearing and non‐weight‐bearing areas) horses occur on the dorsal surface due to hyperexten­
of the joint all should be examined. For example, exami­ sion of the joint. But hyperextension may also cause
nation of the fetlock, which is considered a simple joint, damage to the palmar/plantar soft tissues (flexor ten­
is divided into quadrants. The quadrants of the fetlock dons, SL, and distal sesamoidean ligaments), which can
are dorsal, palmar/plantar, medial/lateral, and a dorsome­ significantly alter the prognosis.
dial/dorsolateral scan plane. 23,27 In more complex joints Dynamic examination of the joint as described for
additional scan planes may be indicated due to additional the examination of the weight‐bearing cartilage allows
structures associated with the joint (i.e. the stifle has the the evaluation of redundant parts of the joint capsule in
patellar ligaments, long digital extensor tendon, popliteal high‐motion joints. For example, the dorsal surface of
tendon, etc.). This emphasizes that the examiner must the fetlock joint has redundancy of the joint capsule that
have developed experience in the use of ultrasound is relaxed when the horse is bearing weight on the limb.
and knowledge of the anatomy of the joint being Flexion of the fetlock tenses the dorsal aspect of the
examined. 3,9,12,13,15,16,25–27,32,42,44,64,66,71,83,84,86,88,89,93,95–98,100,103 joint capsule, allowing better evaluation of this part of
All tendinous or ligamentous structures should be the joint. Tearing of the dorsal joint capsule is more
evaluated in both transverse and longitudinal scan accurately imaged if the structure is under tension while
planes. A description of the scan plane used to acquire the joint is in flexion. Flexion and extension of the joint
the image should be documented on the stored image. during ultrasonographic examination can also be help­
Scan planes should be labeled similar to the nomencla­ ful in demonstrating mobility of an osteochondral frag­
ture utilized for radiographic projections. These oblique ment and in evaluating fluid movement within the joint.
projections are described by the direction that the cen­ The same joint on the contralateral limb should serve
tral ray of the primary beam penetrates the body part as a comparison when evaluating articular and periar­
of interest, from the point of entrance to the point of ticular structures. However, there are two precautions:
exit. Soft tissue structures should be examined first fol­ (1) the examiner must make certain to use the same ori­
lowed by the cartilage and subchondral bone surfaces. entation of the transducer and image the contralateral
Much of the cartilage and subchondral bone surface in joint at exactly the same location when making
the non‐weight‐bearing areas can be imaged in the comparisons, and (2) some joint conditions, in particu­
standing horse. However, dynamic examination of the lar osteochondrosis in young horses and osteochondral
cartilage and subchondral bones surfaces of the weight‐ fragmentation in racehorses, can be bilateral.
bearing areas of the joint requires lifting the limb, plac­ The major obstacle to the effective use of diagnostic
ing the probe on the dorsal joint surface all the while ultrasound in the examination of joints is the need to
attempting to place the probe 90° to the subchondral understand the unique anatomy of each joint and appre­
surface while flexing and extending the joint. While ciate how the various tissue types are influenced by
this maneuver requires considerable practice, it can be sound–tissue interactions. It is also important to under­
expedited by having someone hold the limb (or placing stand the information and misinformation (artifacts)
the limb in a stand) while the examiner positions the created during any ultrasound evaluation (see limita­
transducer while manipulating the limb. Maximally tions of ultrasound earlier in this chapter). There are
flexing the leg is necessary to access the major weight‐ many texts that describe the anatomy of specific joints
bearing areas of a joint. Unfortunately, flexion of an in the horse 31,46,85,90 and the use of diagnostic ultrasound
inflamed joint will likely be resented making it more in the examination of joints. 2,17,18,20,21,38,39,58,68,69,78,79,101,102
difficult to evaluate the weight‐bearing surface. It is recommended that these textbooks be available as
The joint examined and the area of interest within the reference materials to assist with the examination.
joint are, in many cases, directed by the signalment (and Descriptions of the ultrasonographic examination of the
by any prior radiographic findings). For example, race­ specific joints are beyond the scope of this chapter and
horses have a high incidence of problems involving the will not be discussed. However, a discussion of some
dorsal articular surfaces of the high‐motion joints, general features unique to the use of ultrasound in the
particularly the fetlock and carpus; younger horses may examination of joints follows.